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1. Introduction

Since the beginning of the 21st century, the industrial revolution of automatic production has greatly developed. The annual output of automobiles has been further increased under the stimulus of economic globalization while bringing convenience to people’s travel, and it has also brought about three major problems: energy consumption, exhaust emissions, and traffic safety [1,2]. Experiments and research show that fuel consumption and emissions will be reduced by 6–8% for every 10% reduction in a vehicle’s dead weight [2]. An effective way to solve the problem of automobile energy consumption and the environment is to be light-weight; however, being light-weight has an important relationship with steel plate life and vehicle crash safety [3]. Since the 1990s, many automobile light weight projects have been carried out worldwide, such as the Partnership for a New Generation of Vehicles (PNGV) project of the United States, the Super Light Cars (SLC) project of the European Union, the “Ultralight Steel” series research projects organized by the International Iron and Steel Association, ArcelorMittal’s S-in motion project, etc. [4] These projects are aimed at solving the requirements of automobile light weight and safety from the aspects of high-strength materials, structural design, and advanced manufacturing.

New energy vehicles (NEVs) have been vigorously developed against the background of emission peaks and carbon neutrality. Due to limitations of battery weight gain and battery power ratio, light weight is more important in the NEVs industry [5,6]. In terms of collision safety, the higher the strength of the part, the greater the collision force it can withstand. Advanced high-strength steel (AHSS) and ultra-high-strength steel (UHSS) are cost-effective automotive manufacturing materials that can not only ensure light weight of automobiles but also improve and ensure safety of automobiles, which are difficult to replace by other materials [7,8]. For BIW, the CO2 emissions of automobile parts made of AHSS steels are less than half the equivalent aluminum parts; compared with carbon-fiber-reinforced plastic parts, the emissions are less than one third [9]. In addition, steel can be 100% recycled. Steel is still an important material in the sustainable circular economy because of its life cycle analysis (LCA) benefits [9,10]. High-performance AHSS and UHSS with good formability and weldability will occupy mainstream automotive steel from the perspectives of safety, energy conservation, and environmental protection [11].

AHSS for automobiles mainly includes cold-formed martensitic steel (MS), press-hardening steel (PHS), complex-phase (CP) steel, quenching and partitioning (Q&P) steel, dual-phase (DP) steel, medium-manganese steel, and low-density steel, among which cold-formed MS and PHS are martensitic steel and can reach grades with ultra-high-strength of above 1.5 GPa [9,12,13,14,15,16,17]. Ultra-high-strength cold-formed MS and PHS have high strength, good formability, and excellent collision resistance and have good application prospects in automobile door guard beam, battery pack support frame, frame, A-pillar, seat guide rail, and bumper [9]. Cold roll forming technology is used for parts with complex sections, and the utilization rate of raw materials is close to 100% compared with other forming methods, such as stamping, which can reduce tooling costs by 40% and tooling preparation time by 50% [18]. However, it is difficult to produce ultra-high-strength parts with complex cross sections and shapes by cold stamping and roll forming. Hot stamping can solve the problem of ultra-high-strength steel parts forming by stamping fully austenitized steel plates and quenching the parts at the same time [15,19,20]. At present, global research and development (R&D) and production of ultra-high-strength martensitic steels are mainly concentrated in Sweden Steel, ArcelorMittal, Baowu Steel, etc. The application technology of cold-formed MS and PHS below 1.5 GPa is relatively mature, while development of martensitic steels with higher strength is still in an incomplete stage, its production technology and quality control are unstable, and problems such as ultra-fast cooling processes, welding, molding, and hydrogen embrittlement urgently need to be solved [21,22,23,24,25]. In order to improve the environment, save energy, and improve the overall quality of ultra-high-strength automotive steel, it is an important research direction in the field of steel materials to reduce the alloy content, optimize the heat treatment and forming process, and reduce production costs as much as possible. This paper focuses on grades with tensile strengths above 1.5 GPa, systematically discusses the latest development and application, alloying design, and production process of ultra-high-strength martensitic steels for automobiles, and discusses some existing problems.

2. Progress of Ultra-High-Strength Martensitic Steels

2.1. Ultra-High-Strength Cold-Formed Martensitic Steels

Ultra-high-strength cold-formed MS is usually based on C-Si-Mn according to the main composition, and a small amount of microalloying elements are added to refine the grain to improve the performance, and martensite structure and high strength are obtained through rapid quenching process. Low alloy content leads to low hardness before final quenching and low rolling deformation resistance, so the rolling cost of ultra-high-strength cold-formed MS is lower than that of other third-generation advanced high-strength automotive steel [26,27]. The ultra-high-strength cold-formed MS also has a certain crack resistance, decarburization deformation resistance, excellent welding performance, and cold quenching performance. In the automotive industry, ultra-high-strength cold-formed MS has good application prospects in door guard beam, battery shell, and frame of electric vehicles, bumper, and other parts due to its high strength, good welding performance, and excellent collision resistance. At present, ultra-high-strength cold-formed MS has incomparable advantages, such as high strength and light weight, but also has the disadvantage of insufficient formability, which is mainly used in relatively simple roll forming structural parts.

Sweden Steel (SSAB) and ArcelorMittal are among the first enterprises to develop cold-rolled ultra-high-strength martensitic steels. ArcelorMittal [28,29] successfully developed MartINsite®1500 and MartINsite®1700, an ultra-high-strength cold-formed MS product with a strength of 1500–1750 and 1700–2000 MPa in approximately 2014. These two UHSS can ensure that the total elongation reaches at least 3.5%, which has a better weight reduction effect than traditional martensitic steels with a strength of 1100–1500 MPa. It can be supplied in the form of cold-rolled plates, strips, or coils, and the ultra-high-strength can make it possible for the thickness of the plates to be less than or equal to 1 mm. This series of cold-rolled martensite UHSS can contain 0.22 wt.% to 0.36 wt.% carbon, 0.5 wt.% to 2.0 wt.% manganese, 0.2 wt.% silicon, and one or more trace elements in Nb, Ti, B, Al, N, S, and P. This kind of martensite UHSS with simple alloy composition has good roll formability, weldability, stamping property, and delayed fracture resistance. ArcelorMittal applied MartINsite®1500 to seven groups of parts on the whole body in the S-in motion® light-weight automotive steel solution of Class D vehicle and the body weight was reduced by 13 kg. ArcelorMittal recommends that this product be applied to impact resistant automotive safety parts for protection of side impact and rollover vehicles, such as front and rear bumper beams, door beams, side beam reinforcement, roof beams, and new energy vehicle battery pack support frames (as shown in Figure 1), which provides more diversified choices to the new generation of light-weight design. SSAB [30] released Docol1700M at the International Automobile Technology Exhibition in 2015, with a tensile strength of more than 1700 MPa. The Docol1700M is a new generation product evolved from its original Docol1300M and Docol1500M by adding C, Si, Mn, and other alloy contents. The Docol series high-strength martensitic steels produced by SSAB can compete with PHS in terms of parts with simple forming process. Although the latter also has UHSS grade, the higher carbon equivalent of PHS leads to worse welding performance compared with the simpler alloy design of the Docol series. Benefiting from the development of roll forming technology, ultra-high-strength cold-formed MS has also started its market application. In 2017, Shape Group [31] used 3D roll forming technology to design 1700 MPa ultra-high-strength cold-formed MS as car roof rail pipe, and used Docol1700M cold-rolled martensitic steel to provide a unique light-weight solution for A-pillar and car roof rail pipe (as shown in Figure 2), which increased its weight to strength ratio by 50%, reduced the total vehicle weight by 2.8–4.5 kg, and was cost-effective. These components have been successfully applied to Ford’s 2020 Explorer and 2020 Escape models [30,31].

In June 2021, Advanced Materials Development of Canada [32] developed a patented cold-rolled high-strength steel for cold stamping for its customers. Its ultimate tensile strength can reach 1520–1860 MPa, and its elongation is guaranteed to be no less than 4%. It can be used to produce car body structures and safety parts, including battery housings of electric vehicles. The carbon equivalents of the two designed components are approximately 0.975 and 0.61, respectively, which is significantly lower than the carbon equivalent (approximately 1.26) of Docol 1700M steel of SSAB and can better improve the welding quality. Compared with the traditional high-strength aluminum alloy used as EV battery shell material, the cost of this cold-rolled martensitic UHSS even after galvanizing or aluminizing is still only half that of aluminum alloy, which has a great cost advantage.

In addition, POSCO Iron in South Korea [33] also developed 1.8 and 2.0 GPa ultra-high-strength cold-formed MS. Its alloy design does not contain expensive V and Nb but improves the strength by increasing the carbon content to 0.3–0.5 wt.% and adding 0.03 wt.% Ti to refine the grains and precipitate the second phase for strengthening. POSCO also studied the control factors of quenching crack sensitivity of this ultra-high-strength cold-formed MS. The research shows that the samples with higher carbon content show greater lattice distortion and higher dislocation density during quenching and may be more prone to crack along the previous grain boundary. They put forward effective technical strategies to reduce the quenching rate and diffusive hydrogen concentration to improve the toughness of super strong steel with a tensile strength of 2.0 GPa and mitigate quenching cracking.

Baowu Steel began its research and development of cold-formed MS in 2006 and commercially supplied some products in 2009. By 2015, Baowu Steel had commercially supplied 980–1400 MPa grade cold-formed MS, 1500 MPa grade commercial cold-formed MS, and successfully manufactured 1700 MPa grade ultra-high-strength cold-formed MS [34]. Baowu Steel [35] also carried out a series of studies on martensitic steels containing 0.27 wt.% C, 1.52 wt.% Mn, and Ti and Nb microalloying. This cold-formed MS was austenitized at 850 °C for 80 s, cooled to 720 °C at a cooling rate of 3 K/s, and then water-quenched to obtain yield strength and tensile strength of 1364 and 1855 MPa, respectively, and elongation of 4.5%. In 2015, Beijing University of Science and Technology and CITIC Microalloying Technology Center [12] jointly studied Nb free and Nb containing martensite UHSS. They found that the UHSS containing 0.021 wt.% Nb had higher strength and impact toughness after the same tempering process, mainly because the addition of Nb played an important role in inhibiting austenite grain growth during hot rolling, leading to refinement of the martensite microstructure. In 2021, the cooperation team between Shanghai University and An Steel [36] designed a cold-formed MS with strength grade up to 1.5–1.7 GPa and better economic alloy composition. After quenching at 800 °C and 880 °C, the cold-formed MS can reach ultra-high-strength of 1726 and 1646 MPa and form film-like residual austenite between the lath martensite to improve the toughness of the material, resulting in 6.9% and 6.6% elongation, respectively. They realized the refinement of the microstructure of ultra-high-strength cold-formed MS through the coupling of Ti microalloying and heat treatment process conditions and chose a lower quenching temperature to prevent excessive growth of grains and obtained lath martensite and film-like residual austenite microstructure; the transmission electron microscope (TEM) microstructure is shown in Figure 3. The dispersed nano-TiC particles further reduced the C content in the matrix, thereby improving the material toughness. Then, dislocation strengthening, grain refinement, and nanoprecipitation are combined to ensure ultra-high tensile strength to avoid deterioration of toughness and elongation of hot-formed steel due to single strengthening mechanism that is too high.

The microstructure of martensitic steel is mainly composed of martensite, a small amount of precipitation, and a small amount of retained austenite. Affected by the high strength, low plasticity, and low toughness of martensite, it is difficult to greatly improve its formability and toughness through traditional processes. In recent years, obvious progress has been made in the research of increasing metastable austenite content in martensitic steels through the partitioning process and improving properties by using the transformation-induced plasticity (TRIP) effect during deformation [37,38,39,40]. Figure 4 shows the performance comparison between TRIP-enhanced martensitic (TM) steels and traditional martensitic steels [28,35,36,37,38,39,40,41,42]. The formability, toughness, and other key properties of TM steel are significantly higher than those of ordinary martensitic steels. In the low-temperature partition process after quenching, the supersaturated carbon in the martensitic laths diffuses to the residual austenite, thus improving the room temperature stability of metastable austenite. In the subsequent tempering process, the wide lath martensite precipitates fine carbides. By changing the structure of the traditional martensitic steels, the bending property and impact toughness of the new TM steel with the mixed structure of martensite + MA island (martensite + retained austenite mixed structure) + retained austenite are significantly improved compared with the traditional martensitic steels, which may be the new development direction of the traditional cold-formed MS to achieve better properties.

2.2. Ultra-High-Strength Press-Hardening Steels

At present, PHS components account for more than 10% of mainstream cars in Europe and the United States, approximately 5% of Japanese and Korean models, and even 38% of the VOLVO XC90 [43,44]. With the continuous improvement in vehicle light weight and safety requirements, application of ultra-high-strength PHS in the new generation of vehicles will further increase [44]. At present, PHS and its hot stamping parts are mainly used in automobile B-pillar body reinforcement, B-pillar reinforcement, A-pillar reinforcement, front door ring hinge pillar reinforcing plate body, front door ring hinge pillar inner plate, door sill reinforcement, mounting plat front wall, carrier understructure, tunnel, front bumper, rear bumper, side member reinforcement, rocker panel reinforcement, roof rail reinforcement, etc. (as shown in red in Figure 5) [44].

22MnB5 steel with a tensile strength of 1500 MPa developed by ArcelorMittal is the most widely used safety component of hot stamping bodies in industry [45]. In recent years, with the rapid development of automobile light weight, the world’s major steel companies have carried out a series of studies on higher-strength PHS. Germany ThyssenKrupp Company has developed 1900 MPa PHS named MBW1900 based on 34MnB5 composition; Sweden SSAB Group introduced 2000 MPa hot PHS Docol2000Bor based on 37MnB4 composition; POSCO of South Korea developed HPF2000 with strength of 2000 MPa; ArcelorMittal also developed a PHS called USIBOR2000 with strength grade of 2000 MPa [46]. Northeastern University [47] designed a new PHS steel 34MnB5V with high vanadium, high carbon, and ultra-high strength. They realized refinement of the PHS microstructure through coupling of vanadium microalloying and hot stamping process conditions and reduced the C content in the martensite through dispersion precipitation of nano-sized VC in the martensite matrix so as to avoid the brittleness of martensite caused by high C content and improve the material toughness. The tensile strength and yield strength of 34MnB5V are 1971 and 1558 MPa, and the uniform elongation and total elongation are 5.9% and 8.3%, respectively, by the combined strengthening mechanism of martensite strengthening, grain refining, and nano VC precipitation. The strength of 34MnB5V is much higher than that of 22MnB5. At the same time, its uniform elongation and total elongation are also improved, which is mainly due to the composite strengthening method to avoid deterioration of the toughness and elongation of PHS due to the high single strengthening mechanism.

Since Speer et al. [48] first proposed the Q&P treatment process in 2003, this process has been widely applied to a variety of high-strength steels and has obtained excellent comprehensive mechanical properties. In the past 10 years, some scholars have begun to study the application of the Q&P process to hot-formed steel to obtain high-strength and high-toughness hot-formed steel; the hot stamping, quenching, and partitioning (HS-Q&P) process is shown schematically in Figure 6 [15]. In 2011, Liu et al. [49,50] realized the coupling of Q&P treatment and hot stamping deformation with the help of a Gleeble 3500 thermo-mechanical simulator according to the design idea of “deformation + phase transformation + carbon partition”. They deformed 0.22C-1.58Mn-0.81Si steel during the austenitizing process to refine the grain, then quenched it to 300 °C for isothermal carbon distribution, and finally obtained the mixed structure of lath martensite and nano-sized residual austenite; a total elongation of 14.8% was obtained at a tensile strength of 1510 MPa. The martensitic transformation of PHS occurs continuously in the quenching process. If the precipitation of self-tempered carbide from martensite formed at high temperature (close to Ms) can be restrained, then the supersaturated C atoms in this part of martensite may be partitioned to the adjacent untransformed austenite, thereby improving the stability of retained austenite and keeping it at room temperature. Cai et al. [51] developed the concept of quenching and flash partitioning (Q&FP) based on the above theory. They have designed and developed a PHS with a strength of 1660 MPa and a total elongation of 10.5%, in which 1.5 wt.% Si is added to effectively inhibit precipitation of cementite during the die pressing quenching process of the hot stamping process and inhibit the self-tempering phenomenon of martensite, thus promoting partitioning of C into untransformed austenite and improving the C content and stability of retained austenite, and Ms is increased to more than 400 °C through alloy design. Approximately 7% of the retained austenite is obtained in the Q&FP PHS, and the retained austenite is distributed between martensitic laths in the form of thin films.

Linke et al. [52] found that the traditional 22MnB5 steel has a low Si element content (approximately 0.22 wt.%), which cannot effectively inhibit precipitation of cementite in the distribution process. Therefore, the C content from the martensite partition to the austenite distribution is too low to make austenite exist stably at room temperature. In addition, they designed three new steel grades with different Si contents based on the composition of 22MnB5 steel, containing 0.5 wt.%, 0.8 wt.%, and 1.5 wt.% Si, respectively, and adopted one-step and two-step Q&P treatment processes. It is found that only when the Si content is greater than 0.5% can the retained austenite (volume fraction of 2–8%) be obtained in the final microstructure. Elongation of the new design steel after the Q&P treatment is higher than that of the traditional 22MnB5 steel. Seo et al. [53] studied the feasibility of applying the Q&P process to a newly designed PHS containing 1.58% Si and 0.97% Cr based on 22MnB5. The microstructure of the steel after the two-step Q&P treatment contains martensite, carbon-free bainite, and retained austenite. It is found that addition of Si can inhibit formation of carbides, and addition of Cr can stabilize austenite. Based on TRIP-assisted steel (Fe-0.23C-1.23Si-1.50Mn, wt.%), Ariza et al. [54] studied the influence of hot pressing process on the partition of C atoms from martensite to austenite and the final microstructure and properties during Q&P treatment using advanced characterization equipment, such as a three-dimensional atom probe (3DAP), using a process similar to that in the literature [49]. It is found that deformation at high-temperature stage is beneficial to the Q&P process and can improve the stability of retained austenite.

Compared with typical hot stamping steel 22MnB5, it is found that the coupling of the Q&P process and hot stamping process has the potential to improve the plasticity of hot stamping steel. However, it should be noted that the Q&P process involves two crucial isothermal steps, namely isothermal heating in the quenching process and isothermal distribution, while the continuous quenching link in the hot stamping process does not easily achieve accurate control. Although isothermal quenching and isothermal distribution can be achieved in the hot stamping process by heating dies or using additional thermal insulation equipment, the production cycle, production cost, and energy consumption of hot stamping parts will increase accordingly. Because of this, the coupling of these two processes has obvious limitations in practical industrial applications.

3. Alloy Design and Strengthening

3.1. C Mn Si Alloying

For the purpose of high performance and low cost, cold-rolled ultra-high-strength martensitic steel is mainly composed of “cheap elements”, such as C, Mn, Si, etc. C is the cheapest and most effective element to control the martensite property, and it can significantly improve the strength of martensite steel by solution strengthening and transformation strengthening after the quenching process. The tensile strength and carbon content show a linear relationship in low-alloy high-strength steel with a carbon content of 0.2–0.5% after low-temperature tempering [33]. In addition, the strength of martensitic steel with a basic composition of C-Si-Mn is most affected by the carbon content, while it is relatively less affected by other alloying elements, such as Mn and Si, as shown in Figure 7 [35,55]. Too high of a carbon content will reduce the plasticity and welding performance of the steel. However, a low carbon content design will make the steel have a higher Ar3 temperature and the range of the austenite zone under the equilibrium phase diagram will be reduced. Therefore, other alloying elements should be added to reduce Ar3 to expand the austenite recrystallization processing area. Therefore, lower carbon equivalent design shall be adopted as much as possible to obtain better weldability under the condition of ensuring strength [12]. The calculation formula of the carbon equivalent is as follows [12]:

(1)Ceq=C+(Mn+Si)/6+(Cr+Mo+V+W+Ti)/5+(Ni+Cu)/15

where Ceq is the carbon equivalent, and C, Mn, Cr, Mo, V, Ni, and Cu are calculated by the mass fraction of elements in the alloy. The alloy composition and mechanical properties of several typical ultra-high-strength cold-formed MS and PHS are shown in Table 1. It can be found that the cold-formed MS is simpler than the PHS alloy design and has a lower carbon equivalent, i.e., better welding performance under the same strength grade.

In general, alloying elements such as Mn, Cr, Ni, Mo, V, Nb, Si, etc., often added in martensitic steels, can improve the hardenability of the steel and ensure that the steel is easy to obtain martensite structure after quenching. For low-alloy steel, the strength and hardness can only be guaranteed if a large amount of martensite is obtained. Therefore, the first function of alloy element is to improve the hardenability. Mn is second only to Ni in its ability to stabilize the austenite, but its cost is far lower than Ni, and Mn is often the most added alloy element in martensitic steels. While taking into account the cost, Mn also strongly enhances the hardenability of steel and reduces the martensite transformation temperature. At the same time, the strengthening effect of Mn solution is less than that of C, so it has less influence on toughness while increasing strength. However, too high content of Mn will affect the carbon equivalent of steel and reduce the welding performance of steel. The addition of Si is mainly used to inhibit the precipitation of carbides, such as cementite. The content of Si element in existing cold-formed MS is generally less than 1.5 wt.% [28]. However, in the new Q&P process for producing ultra-high-strength martensitic steels, residual austenite can be obtained in the final microstructure only when the content of Si element is greater than 0.5 wt.% [52]. However, too high Si content will introduce a series of problems, such as surface oxidation and excessive rolled iron oxide scale [58].

3.2. Nb V Ti Microalloying

Nb, V, Ti, and other strong carbide elements have different functions according to their different forms of existence. Some of them can be dissolved in the iron matrix to play the role of solid solution strengthening, and some of them can form stable carbides, nitrides, or carbonitrides in the process of controlled rolling and controlled cooling and refine grains in the process of rolling [59,60,61].

Xiao et al. [62] studied the influence of Nb solute and NbC precipitation on recrystallization of microalloyed steel and found that part of NbC is insoluble during high-temperature heating, which plays a role in preventing growth of austenite grains in the holding stage before rolling, which is also conducive to further refining austenite grains and ferrite grains after rolling. Both Nb and NbC can inhibit recrystallization behavior, but Nb solute is effective in retarding dynamic recrystallization, while NbC is more effective in retarding static recrystallization. The Nb solution in austenite can also reduce the temperature of Ac3 and improve the hardenability. Under the same conditions, especially at a high cooling rate, Nb can promote formation of more low-temperature transformation products, such as acicular ferrite and bainitic ferrite [63]. The delaying effect of Nb, V, and Ti microalloying elements on austenite grain growth and recrystallization is shown in Figure 8 and Figure 9, respectively [64,65]. It can be seen that Ti plays an incomparable role in preventing austenite grain growth at high temperature, while Nb plays a slightly lesser role than Ti. However, with the development of controlled rolling and controlled cooling technology and upgrading of rolling production lines, the entire rolling process does not need to be held at a very high solution temperature for a long time, so the role of Nb will be equal to that of Ti. Nb can precipitate carbides to prevent recrystallization of deformed austenite during rolling at higher temperatures. Its unique mechanism has been successfully applied to the un-recrystallized controlled rolling technology, which is irreplaceable by titanium and vanadium. The dispersion distribution of Nb (C, N) precipitation can be achieved by strain-induced precipitation and controlling the cooling rate during rolling, making the size of precipitation particles very small. Therefore, even if the volume fraction of the precipitation phase is small, a strong precipitation strengthening effect can be achieved, and the strength and toughness level of steel can be adjusted over a wide range [66]. The expected strength level can be achieved by adding a small amount of niobium to steel, and 0.02–0.03 wt.% Nb can be equivalent to 0.04–0.06 wt.% V or 0.06–0.10 wt.% Ti [67].

Compared with the single addition of microalloying elements, the composite addition of Nb, V, and Ti microalloying has more obvious strengthening advantages. Nb and V are fully solution in austenite in the soaking stage before rolling, and TiN with a higher solution temperature can inhibit austenite grain growth. The carbide or carbonitride precipitated from part of Nb during high-temperature rolling can inhibit austenite recrystallization and prevent recrystallized grain growth. In the subsequent controlled cooling and coiling stages, a large amount of fine Nb, V, Ti carbide, or carbonitride can be precipitated. Steel has high strength and toughness by comprehensively utilizing precipitation strengthening and fine-grain strengthening mechanisms [68]. However, when microalloying elements Nb, V, and Ti are added at the same time, they can be mutually soluble or interchangeable with each other and easily form coarse composite precipitates because the carbonitrides of the three elements are all B1 type face-centered cubic structures, thus affecting the precipitation strengthening effect.

3.3. B Microalloying

At present, the hot-formed steel commonly used in the world contains B elements, and PHS is also referred to as B steel. Therefore, the effect of B on the microstructure and properties of PHS is particularly important. The role of B in steel was first proposed by R. Walter, mainly to improve the hardenability of steel [69]. Researchers found that adding B to steel can also stabilize the lath martensite structure and strengthen the grain boundary [70,71]. Some studies also hold that, because B easily segregates at the austenite grain boundary, delaying the nucleation of ferrite [72,73], addition of 0.0010–0.0030% B in steel can significantly improve the hardenability of steel [46], and addition of B can delay the appearance of ferrite and pearlite, which is beneficial to the actual hot stamping process. However, when the content of B increases to 0.5–0.8%, it will not be beneficial and may even play a softening role [46]. In addition, the existence of B carbide will lead to an increase in the ductile brittle transition temperature (DBTT) of hot-formed parts, resulting in poor toughness of hot-formed parts at lower temperatures.

4. Hydrogen Embrittlement

In recent years, to achieve energy conservation, emission reduction, and high safety of automobiles, people strongly need higher-strength steel plates to realize automobile light weight. From another point of view of improving crash safety performance, structural members for automobiles, such as A-pillar, require ultra-high-strength up to 1 GPa or above. In this ultra-high-strength steel plate, hydrogen embrittlement will occur due to hydrogen permeation generated by corrosion reactions in the environment, including water and hydrogen sulfide [74]. In addition, in galvanized steel sheets, especially in ultra-high-strength areas with tensile strength of 1000 MPa or higher, hydrogen embrittlement becomes particularly prominent due to hydrogen blockage during pickling after hot rolling [75,76].

4.1. Hydrogen Embrittlement Mechanism

Hydrogen embrittlement is a kind of environmental embrittlement caused by the interaction of material environmental stress, which is a form of hydrogen-induced material deterioration (hydrogen damage or hydrogen embrittlement) [74]. Hydrogen embrittlement has become a technical bottleneck that restricts development and application of ultra-high-strength steel for automobiles. At present, generally recognized hydrogen embrittlement mechanisms include hydrogen pressure theory, hydrogen reducing surface energy theory, hydrogen weakening bonding force (weak bond) theory, hydrogen promoting plastic deformation theory, and hydrogen promoting strain-induced vacancy formation theory [25,77].

In recent years, many studies have not reached a consensus on the micro mechanism of hydrogen embrittlement, and more involve several fracture mechanisms that occur simultaneously. They have gradually tended to believe that its essence lies in diffusion and aggregation of hydrogen atoms in the material to the location defects (inclusions, dislocations, micro-pores, vacancies, etc.) where the stress is concentrated. Hydrogen accumulates at these defects and generates hydrogen pressure. Under the conditions of external stress and internal residual stress, the number of internal defects in the matrix is changed, which further affects its dynamic characteristics, promotes local plastic deformation, and finally leads to delayed fracture of the material [78].

4.2. Cold-Formed MS Hydrogen Embrittlement Study

Due to the complex shape of auto parts and large deformation in the production process, auto manufacturers pay more attention to the delayed fracture performance, which has become one of the most important items in the performance certification of automotive high-strength materials [79]. Steel enterprises in various countries have always included evaluation and improvement of hydrogen embrittlement performance in the process of developing high-strength automotive steel. At the end of the 20th century, Japan carried out much research on the evaluation methods and mechanisms of the delayed fracture properties of UHSS and set up the “Research Society for Delayed Fracture” in 2000. Since 2011, Europe has also held several special meetings on steel and hydrogen. Since 2017, China has also organized the “Research on Hydrogen Induced Delayed Fracture of China Automotive EVI and High Strength Steel” conference, which has been successfully held for three sessions [78]. Compared with extensive international research and good progress achieved, research in the field of delayed fracture of high-strength steel by Chinese enterprises and scientific research institutes started late, limiting further development of high-strength steel, and some basic problems involved need to be solved [79].

ArcelorMittal [80] studied the effect of Nb and other alloying elements on the hydrogen-induced delayed fracture resistance of ultra-high-strength cold-formed MS with 2000 MPa in 2015. The addition of Nb to low-alloy martensitic steels leads to often-reported grain refinement of the prior austenite grain size. Nb microalloyed steel contains a lower diffusive hydrogen content than the base steel during the thermal desorption analysis and has higher hydrogen capture after charging. This study suggests that the reason why Nb improves the delayed fracture resistance of steel is mainly due to hydrogen capture and grain refinement. However, the effect of grain refinement on hydrogen embrittlement sensitivity is a controversial issue. Martinez et al. [81] found that the sensitivity of fine grains to hydrogen embrittlement is low because the hydrogen concentration in the unit grain boundary of fine grains is low. Some researchers have also reported similar results in other alloys, such as martensitic steel [82,83], Fe Ni alloy [84], maraging steel [85], and high-strength low-alloy steel [86]. In contrast, Ghosh et al. did not observe significant differences in the hydrogen embrittlement sensitivity of microalloyed steels with different grain sizes [84]. The controversial results may be related to the differences in material microstructure, strength level, residual stress state, hydrogen exposure environment, hydrogen concentration in materials, diffusivity, trap location, etc., as well as the differences in test methods.

ArcelorMittal [80] also studied the effect of other alloy compositions on hydrogen embrittlement, as shown in Table 2. The results show that Si improves the delayed fracture resistance of steel. On the one hand, it inhibits formation of carbides during tempering; on the other hand, Si reduces the diffusion coefficient of hydrogen (the solubility and permeability of hydrogen are also reduced) and slows the formation of hydrogen molecules. The view on the effect of Cr is that it is generally believed that Cr (especially low content) will increase the sensitivity of hydrogen-induced delayed fracture. However, ArcelorMittal’s research shows that increasing the Cr content can inhibit the occurrence of fracture, thus improving the delayed fracture resistance of materials because chromium carbide can be used as an H trap to reduce the diffusion rate. The precipitation temperature of Nb (C, N) is much lower than that of TiN, so the size of these particles is smaller, forming more uniformly distributed niobium-based precipitates as strong traps for hydrogen (as shown in Figure 10) and significantly reducing the mobility of hydrogen, thereby improving the hydrogen embrittlement resistance of steel. Wei and Tsuzaki [87] believed that, in tempered martensite structure, coherent and semi-coherent NbC have better hydrogen capture ability than TiC precipitation, which is a better choice to improve the hydrogen embrittlement resistance of UHSS.

Baowu Steel and the University of Queensland [88] studied the effect of hydrogen on 1700 MPa martensitic high-strength steel and compared the hydrogen embrittlement sensitivity of low-strength martensitic high-strength steel MS980, MS1180, MS1300, and MS1500 studied in previous studies [59,60,61]. MS1700 shows typical hydrogen embrittlement fracture when conducting cathodic hydrogen charging in NaOH and HCl but shows the lowest hydrogen sensitivity when conducting hydrogen charging in 3.5 wt.% NaCl at open-circuit potential and zinc potential, and the measured hydrogen diffusion coefficient is 3.24 × 10−7 cm2s−1. The influence of hydrogen in MS1700 is shown by a reduction in ductility and the occurrence of shear fracture, which has little effect on the tensile strength. The heat treatment process related to baking will increase the hydrogen embrittlement sensitivity. The high hydrogen sensitivity of MS1700 is attributed to higher carbon and martensite content, which enhances hydrogen capture. With the increase in the strength of cold-formed MS (MS980, MS1100, MS1300, MS1500, and MS1700), the hydrogen embrittlement sensitivity increases, which is attributed to the increase in hydrogen traps and is related to the strength increase caused by the increase in carbon and martensite content.

4.3. PHS Hydrogen Embrittlement Study

Hydrogen diffuses into PHS when the steel is in the process of smelting, hot rolling, pickling, austenitizing, and welding [89]. Diffusive hydrogen atoms will permeate into the PHS without coating and Al–Si coating, and the latter has greater hydrogen absorption. In the process of high-temperature austenitization, the hydrogen generation reaction of uncoated PHS is mainly the reduction reaction of water vapor molecules and iron. In addition to the above reactions, the PHS with Al–Si coating will also undergo the reduction reaction of Al and H2O, and even the reaction of Si and H2O generates hydrogen [90]. The Al–Si coating after the reaction acts as a hydrogen atom diffusion barrier to prevent hydrogen atoms from escaping from the steel matrix [89,91]. Therefore, hydrogen-induced delayed fracture under actual working conditions mostly occurs on PHS auto parts with Al–Si coating, so hydrogen embrittlement prevention is needed. The probability of hydrogen embrittlement of 1.5 GPa uncoated hot stamping formed parts is small, but the embrittlement of 1.7 GPa and above for uncoated PHS and parts is high, and the hydrogen embrittlement sensitivity is high. The hydrogen embrittlement behavior must be considered in the design of parts, materials, and processes. The 1.8 GPa hot stamping bumper beam of Mazda CX5 has earlier applied microalloying technology to restrain hydrogen embrittlement [43], becoming a model for PHS development of 1.7 GPa and above. At present, both uncoated and Al–Si-coated PHS are almost microalloyed to prevent hydrogen embrittlement when the strength level is 1.7 GPa or above.

To improve the hydrogen brittleness resistance of martensitic PHS steel, microalloying with Mo, V, Ti, and Nb is usually used to obtain the best combination of grain refinement, precipitation strengthening, and hydrogen brittleness resistance. Yoo et al. [92] found that, after adding 0.15 wt.% Mo to 32MnB5 steel, it showed a large ductility loss (50–79%) after hydrogen charging, while the addition of Mo significantly reduced the plastic loss (17–26%), with sufficient post elongation, indicating that the addition of Mo made 32MnB5 more resistant to hydrogen embrittlement. This is mainly because the solute Mo reduces the diffusivity of H, which is due to the high H affinity and repulsive strain field caused by the large atomic size of Mo. The solute Mo in the matrix reduces the H-localization and strain-localization on the original austenite grain boundary, and the Mo segregation enhances the grain boundary cohesion. These two factors comprehensively change the H-induced crack path in the grain from the original austenite grain boundary to the H-enhanced slip surface, thereby improving the hydrogen resistance brittleness of Mo steel. Chen et al. [57] studied the interaction between dislocations, precipitates, and hydrogen atoms in 2000 MPa PHS through microstructure characterization and hydrogen embrittlement sensitivity tests. They found that the hydrogen embrittlement sensitivity index of 2000 MPa PHS increased with increasing hydrogen charging current density, and the corresponding HE mechanism changed from hydrogen-enhanced local plasticity (HELP) to hydrogen-enhanced debonding (HEDE). In addition, they also found that dispersed V-rich (Ti, V)C precipitates can refine grains to increase the number of reversible hydrogen traps. Pinned dislocations can inhibit the H -dislocation interaction and capture hydrogen atoms as irreversible hydrogen traps, thus improving the resistance to hydrogen embrittlement.

Kim et al. [93] studied the effect of titanium content on hydrogen embrittlement of hot stamping boron steel. With the increase in Ti content in hot-stamped boron steel from 0.02% to 0.03%, the hydrogen embrittlement resistance is significantly improved, and the elongation is slightly reduced due to the existence of small titanium carbide deposits. Yoo Jisung et al. [94] further studied the competitive effect of Ti alloying on the hydrogen embrittlement resistance of (Nb + Mo)-alloyed 32MnB5 PHS. It was found that the composite addition of Nb, Mo, and Ti promoted formation of coarse Ti(C, N) particles and nanocomposite precipitation of (Nb, Ti)C and (Nb, Mo, Ti)C. Increasing the Ti content to 0.03 wt.% increases the volume fraction of nanometer-sized precipitates, which effectively refined the original austenite grain size and interface incoherence, thus providing a stable hydrogen capture site with higher activation energy for hydrogen desorption. Although the increase in Ti also promotes formation of brittle coarse Ti(C, N) particles, due to the interaction with Nb and Mo, the decrease in particle size can offset the negative impact of coarse Ti(C, N) particles on hydrogen embrittlement.

Chen et al. [95] used the isotope deuterium (D) of H as a tracer element to diffuse hydrogen into the sample by the electrochemical hydrogen charging method and then used low-temperature transfer atom probe tomography (APT) to observe the distribution of hydrogen atoms in niobium containing ferritic steel and niobium containing PHS under specific microstructural characteristics. The results show that hydrogen atoms are pinned at different interfaces of dislocations, grain boundaries, and Nb and V precipitates in PHS (22MnBNb) (as shown in Figure 11). Therefore, these nano-sized carbides can be used as hydrogen traps to reduce the concentration of diffusible hydrogen in the matrix. They also observed hydrogen directly at carbon-rich dislocations and grain boundaries, and hydrogen was also observed at the incoherent interface between the NbC precipitated phase and steel matrix (as shown in Figure 12), which directly proved that the incoherent interface can be a hydrogen trap. Zhang et al. [96] studied the effect of Nb on the hydrogen-induced delayed fracture resistance of high-strength hot-formed steel (22MnB5) through constant load and hydrogen permeation test methods. The research results show that Nb can effectively refine the original austenite grains of the test steel and increase the grain boundary and phase interface, and a large number of NbC precipitates can pin dislocations, while grain boundaries, phase interfaces, and dislocations are hydrogen traps, which can effectively bind hydrogen atoms and prevent their diffusion and aggregation. On the other hand, the NbC precipitate itself is an irreversible hydrogen trap, and the dispersed NbC precipitate can adsorb hydrogen atoms uniformly distributed in the test steel. Therefore, the hydrogen-induced delayed fracture resistance of high-strength hot-formed steel can be effectively improved by introducing appropriate Nb (0.05 wt.%) microalloying elements into the traditional chemical composition system of high-strength hot-formed steel. Their recent research results show that, in low-alloy high-strength steel, reasonable improvement in the heat treatment process and realization of coupling of the Nb element precipitation peak temperature and hot forming process can increase the low-angle grain boundary, refine the original austenite grain, reduce the dislocation density, and change the grain boundary [97]. The hydrogen-induced delayed fracture properties of low-alloy high-strength steel can be effectively improved by reasonably controlling the size, quantity, and distribution of precipitation and grains. JO et al. [56] found through a slow-strain-rate tensile test that the true fracture strain of Nb-free hot-formed steel is 0.103, that of Nb containing hot-formed steel is 0.160, and that of Nb–Mo-composite-microalloyed steel is 0.223. The two microalloyed PHS showed lower elongation loss and strength loss. Therefore, Nb + Mo composite microalloying improves the hydrogen embrittlement resistance of 1.9 GPa hot-formed steel.

4.4. Hydrogen Trap and Hydrogen Embrittlement Improvement

Darken et al. [98] first proposed the concept of hydrogen traps in steel in 1949. Pressouyre et al. [99] found in 1978 that there is a binding energy between the hydrogen trap and hydrogen, that is, the binding energy of the hydrogen trap. According to the binding energy of the hydrogen trap, hydrogen traps can be divided into reversible hydrogen traps and irreversible hydrogen traps. If the binding energy of the hydrogen trap is greater than 50 kJ/mol, the hydrogen trap is irreversible and can trap hydrogen until saturation [100]. If the binding energy of the hydrogen trap is less than 30 kJ/mol, the hydrogen trap is reversible. Even at room temperature, hydrogen can escape from the trap and enter the lattice gap [101,102]. There are many types of hydrogen traps in steel, mainly including solute atoms, vacancies, dislocations, grain boundaries, precipitates, inclusions, cracks, voids, and retained austenite [103]. Table 3 shows the binding energy of some hydrogen traps [78]. The delayed fracture sensitivity of steel is mainly affected by reversible hydrogen traps because many high-strength steels have obvious delayed fracture behavior near room temperature. In addition, the reversibility of the hydrogen trap is also related to temperature. With increasing temperature, the binding energy of the hydrogen trap will also change. The irreversible hydrogen trap at normal temperature can become a reversible hydrogen trap at high temperature. Hydrogen traps are also potential defects, which may be a part of hydrogen crack nucleation in high-strength steel and may also play a role in strengthening and toughening by preventing crack growth [104]. However, whether hydrogen-delayed fracture occurs depends on the capacity of the hydrogen trap, the critical concentration value, and the hydrogen concentration reaching the defect position after being stressed; if the hydrogen concentration exceeds the critical concentration, crack nucleation will occur. In fact, there are many hydrogen traps that will not lead to crack nucleation. This trap is a useful hydrogen trap because it will bind hydrogen around itself and prevent more hydrogen from accumulating in the potential danger area to cause crack nucleation.

The hydrogen embrittlement of ultra-high-strength steel is mainly manifested as cleavage fracture and brittle intergranular fracture. Improvement in the hydrogen embrittlement resistance of UHSS mainly starts from improving the effectiveness of hydrogen trapping and reducing the diffusible hydrogen concentration and improving the grain boundary strength to be closer to the grain interior to change the fracture form from intergranular brittle fracture to transgranular ductile fracture. At present, the common measures to improve the hydrogen embrittlement resistance are as follows.

  • (1). Grain refining: the crack source of UHSS hydrogen embrittlement fracture often appears at the original austenite grain boundary, and the crack extends along the grain boundary at the initial stage. With a decrease in intergranular fracture tendency, the delayed fracture resistance increases significantly [94]. Therefore, the addition of Al, Ti, Nb, V, and other elements to form dispersed carbonitrides to refine austenite grains can improve the strength and toughness as well as the delayed fracture property. However, some studies have shown that, when the grain size is less than 2 μ, refining the grain will increase the hydrogen embrittlement sensitivity [105,106].

  • (2). Improve grain boundary strength: by reducing the segregation of P, S, and other impurities at the original austenite grain boundary, improving the grain boundary strength, delaying the initiation of delayed fracture cracks at the grain boundary, and thus improving the delayed fracture resistance of steel [14]. To this end, Mo and Ti can be added to form compounds with P, and Al and Ti can be added to form nitrides. Impurities can be trapped in the crystal to inhibit its grain boundary segregation [107].

  • (3). Tempering precipitation: add elements with strong tempering softening resistance, such as Cr, Mo, V, etc., and precipitate carbides on the grain boundary during tempering, making the strength inside the grain closer to the grain boundary [108]. At the same time, precipitation of a large number of carbides in the grains reduces the solid solution strengthening of the matrix, and more three-dimensional reversible hydrogen traps are added to capture hydrogen [56,109].

  • (4). Improve surface corrosion resistance: the corrosion pit on the metal surface will increase the contact area between hydrogen in the air and the matrix, thus increasing its delayed fracture sensitivity. The amount of hydrogen intrusion on the steel surface can be reduced by adding alloying elements, such as Cr and Mo, that inhibit formation of corrosion pits [110].

  • (5). Harmless invading hydrogen: add an appropriate amount of microalloy elements V, Ti, Nb, etc., to form fine carbonitrides that can be used as hydrogen traps, inhibit the diffusion of hydrogen, and uniformly distribute hydrogen in steel [59].

In addition, the Max Planck Institute [111] recently proposed a strategy to improve the hydrogen embrittlement resistance of medium-manganese steel based on the chemical heterogeneity in the steel structure and composition. Use of alternate dual-phase microstructures and ultra-fine grain sizes and the heterogeneous distribution of Mn components in austenite help to locally enhance the crack resistance, thus forming a buffer zone to prevent hydrogen-induced micro-cracks. The hydrogen embrittlement resistance of composition heterogeneity samples has been improved by 122% compared with uniform components, which is also superior to the traditional H-resistance enhancing methods, such as carbide precipitation, Cu precipitation, and grain refinement, as shown in Figure 13. This kind of ingenious use of composition heterogeneity to improve the delayed fracture property may be a new development idea for improving the hydrogen embrittlement resistance of ultra-high-strength martensitic steels.

5. Summary and Perspective

With the background of emission peaks and carbon neutrality, automobile light weight is an important means and development trend of energy conservation and emission reduction, an inevitable way to adjust and upgrade the industrial structure of the automobile manufacturing industry, and a need for automobile manufacturers to achieve sustainable development and improve their international competitiveness. In the process of automobile light weight, an increasing number of AHSS and UHSS steel plates are used to achieve the purpose of automobile light weight and safety. Ultra-high-strength cold-formed MS and PHS have simple alloy design and good welding performance. Compared with medium-manganese steels and high-manganese steels, they have natural low-price advantages. The excellent strength performance can also cope with the fierce competition of aluminum, magnesium, plastics, and other materials. The R&D and production of automotive light weight steel in China started relatively late, and the research on ultra-high-strength martensitic steel in Baowu Steel, An Steel, Pan Steel, etc., in China is quite different from that abroad. Especially with the strong impetus of the “dual carbon” background and new energy vehicles, China’s steel enterprises should seize the opportunity for industrial upgrading, grasp the development trend of international AHSS and UHSS in a timely manner, and actively promote research regarding product development and application technology.

Author Contributions

H.C. and L.Z.: conceptualization, methodology, investigation, and writing of the manuscript; Z.C.: supervision and reviewing; T.W.: correcting of the manuscript; L.Z. and S.L.: funding acquisition; Z.L.: methodology and arranging illustrations. All authors have read and agreed to the published version of the manuscript.

 Institutional Review Board Statement

Not applicable.

 Informed Consent Statement

Not applicable.

 Data Availability Statement

Not applicable.

 Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables
Figure 1. Application of ultra-high-strength cold-formed MS recommended by ArcelorMittal in automobile parts [29]. (a) Front and rear bumper beam, door beam, side member reinforcement, roof beam. (b) Support framework and frame of battery pack for new energy vehicles. This figure is reproduced based on [29].
Figure 2. Application of Docol1700M in automobile roof rail pipe [30,31]. This figure is reproduced based on [30,31].
Figure 3. TEM analysis of martensitic UHSS [36]. (a,b) Lath martensite and nano-scale TiC carbides precipitated. (c,d) Retained austenite between lath martensites. Reprinted with permission from [36]. Copyright 2022 MDPI.
Figure 4. Comparison of tensile strength and elongation of TM steels [35,36,37,38], DP steels [39], TRIP steels [39], martensitic steels [26,33,34,39], Q&P steels [39], and Q-P-T steels [40]. This figure is reproduced based on [28,35,36,37,38,39,40,41,42].
Figure 5. Typical application of PHS (shown in red) in automobiles [44]. This figure is reproduced based on [44].
Figure 6. Schematics of hot stamping, quenching and partitioning (HS-Q&P) process, and microstructural evolution [15]. Reprinted with permission from [15]. Copyright 2022 CSPM.
Figure 7. Relationship between the strength, carbon content, and alloying elements of martensitic steel [35,55]. This figure is reproduced based on [35,55].
Figure 8. Effect of Nb, V, and Ti microalloy elements on austenite grain growth [64,65]. This figure is reproduced based on [64,65].
Figure 9. Effect of microalloying elements Nb, V, and Ti on austenite recrystallization stop temperature [64,65]. This figure is reproduced based on [64,65].
Figure 10. TEM characterization of martensite UHSS carbides with different compositions [80]. (a) Base. (b) Base–Si. (c) Base–Si–Cr. (d) Base–Si–Cr–Nb. Reprinted with permission from [80]. Copyright 2022 Springer.
Figure 11. APT analyses of D-charged martensitic steel samples containing (a–d) dislocations and deuterium (D, large red spheres), carbon (C, small blue spheres) and iron (Fe, small gray sphere), and (e–h) GBs and Fe, C, D elements [95]. Reprinted with permission from [95]. Copyright 2022 AAAS.
Figure 12. APT analysis of a D-charged ferritic steel sample containing NbC [95]. (a) Full reconstructed atom map showing atoms with mass-to-charge ratios that match D (large red spheres), C (small blue spheres), and niobium (Nb, small brown spheres). (b) Schematic describing the data in (C) to (F). (c–f) Local enlarged views of (a). Reprinted with permission from [95]. Copyright 2022 AAAS.
Figure 13. Improvement in the resistance to H embrittlement via designing chemical heterogeneity [111]. (a) Slow-strain-rate tensile properties. (b) Comparison between our approach and other H-resistance enhancing methods. This figure is reproduced based on [111].

Alloy composition, carbon equivalent (CE), and mechanical properties of typical ultra-high-strength cold-formed MS and PHS (tensile strength >1500 MPa).

C Mn Si Other Alloys Rm/MPa CE Ref.
Cold-formed MS 0.22 1.5 0.198 0.036 Al, 0.019 Nb 1737 0.50 [28]
0.25 1.99 0.201 0.025 Al 1858 0.62 [28]
0.28 0.988 0.201 0.038 Al, 0.024 Ti, 0.001 B, 0.028 Nb 1981 0.48 [28]
0.28 2.01 0.202 0.032 Al 1927 0.65 [28]
≤0.35 ≤3.0 ≤1.00 Nb + Ti ≤ 0.15, Cr + Mo ≤ 1.00, B ≤ 0.001, Cu ≤ 0.20 ≥1700 ≤1 [29]
0.30 1.0 0.28 0.50 Cr, 0.012 Ti 1752 0.51 [12]
0.30 1.0 0.30 0.49 Cr, 0.015 Ti, 0.021 Nb 1841 0.52 [12]
0.275 1.52 0.40 0.22 Ti, 0.025 Nb, 0.0022 B 1855 0.60 [35]
0.18 2.13 1.40 1.00 Cr, 0.012 Ti 1726 0.77 [36]
0.23~0.25 <1.0 <1.0 Cr, Ti 1520~1620 0.98 [32]
0.26~0.3 <1.0 <1.0 Cr, Ti 1520~1860 0.61 [32]
PHS 0.23 1.18 0.22 0.16 Cr, 0.040 Ti, 0.002 B 1478 0.46 [46]
0.25 1.24 0.21 0.34 Cr, 0.042 Ti, 0.002 B 1611 0.49 [46]
0.28 1.30 0.4 0.005 B 1740 0.56 [46]
0.34 1.30 0.4 0.005 B 1919 0.62 [46]
0.37 0.81 0.31 0.19 Cr, 0.046 Ti, 0.001 B 2040 0.56 [46]
0.32 1.2 0.25 0.12 Cr, 0.030 Ti, 0.002–0.003 B, 0.05 Nb 1933 0.56 [56]
0.32 1.2 0.25 0.12 Cr, 0.030 Ti, 0.002–0.003 B, 0.05 Nb, 0.1 Mo 1917 0.56 [56]
0.35 1.35 0.24 0.03–0.05 Ti, 0.0020 B, 0.25 Cr, 0.15–0.20 V 1977 0.62 [57]

Effect of alloying elements Si, Cr, and Nb on mechanical properties, delayed fracture, and microstructure [80].

YS, MPa TS, MPa Delayed Fracture Time, h Average Martensitic Lath Size, μm Average Misorientation, °
Base 1611 1991 <6 2.10 38.8
Base–Si 1647 2033 137 1.78 32.5
Base–Si–Cr 1610 1990 278 2.09 33.7
Base–Si–Cr–Nb 1684 2039 >600 1.92 33.1

Binding energy of different hydrogen traps [78].

Hydrogen Trap Type Binding Energy (kJ∙mol−1) Material
C atom 3.0 Pure iron
Mn atom 11.0 Pure iron
grain boundary 17.2 Pure iron
Ferrite/Fe3C interface 18.4 Medium-carbon steel
V and Cr atoms 26.0~27.0 Pure iron
dislocation 26.8 Pure iron
V4C3 (total grid) 30.0 Low-carbon-alloy steel
Micropore 35.2 Pure iron
TiC (total) 46.0~59.0 Mild steel
Single vacancy 46.0–79.0 Pure iron
retained austenite 55.0 Duplex steel
NbC 63.0~68.0 Mild steel
MnS 72.3 Low-carbon-alloy steel
Fe3C (incoherent) 84.0 Medium-carbon steel
TiC (incoherent) 86.9 Medium-carbon steel

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Abstract

With the background of emission peaks and carbon neutrality, light weight has become an irreversible trend in the development of the automobile industry. It is an inevitable choice to use a large amount of ultra-high-strength steels to realize light weight and safety of automobiles. Ultra-high-strength martensitic steels can be divided into hot-formed steels and cold-formed steels according to the forming process. In recent years, ultra-high-strength martensitic steels have been rapidly developed in automotive battery pack frameworks, door guard beams, bumpers, A-pillars, etc., depending on their good plasticity and advanced forming technology. In this paper, the recent progress of ultra-high-strength martensitic steels for automobiles is systematically reviewed, the mechanisms of alloying, strengthening, and toughening are emphatically expounded, and the hydrogen embrittlement problems in application are summarized. Finally, the prospects of manufacture and application of ultra-high-strength martensitic steels for automobiles in the future are forecasted.

Details

Title
Progress and Perspective of Ultra-High-Strength Martensitic Steels for Automobile
Author
Chen, Hao 1 ; Zhao, Linlin 2 ; Lu, Shenghai 3 ; Lin, Zhangguo 3 ; Tong, Wen 1 ; Chen, Zejun 1   VIAFID ORCID Logo 

1  College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China 
2  Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China; Collaborative Innovation Center, HBIS Group Technology Research Institute, Shijiazhuang 050023, China 
3  Collaborative Innovation Center, HBIS Group Technology Research Institute, Shijiazhuang 050023, China 
First page
2184
Publication year
2022
Publication date
2022
Publisher
MDPI AG
e-ISSN
20754701
Source type
Scholarly Journal
Language of publication
English
DOI
https://doi.org/10.3390/met12122184
ProQuest document ID
2756755824
Copyright
© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.