Revolutionizing car body manufacturing using a unified steel metallurgy concept.

Numerous high-performance steels with various compositions and mechanical properties were developed to enable a safe and light-weight automotive body-in-white (BIW). However, this multisteel scheme creates substantial challenges, including the resistance spot welding of dissimilar steels, processing optimization, and recycling. Here, we propose a revolutionary unified steel (UniSteel) concept, i.e., using a single chemistry to produce multiple steel grades for the entire BIW. The tensile strengths of various UniSteel grades are ranging from 600 to 1680 MPa, encompassing the strengths of typical commercial counterparts while exhibiting competent ductility. The prototype parts made of UniSteel press-hardened steel (PHS) grade demonstrate superior side-intrusion resistance over the commercial PHS, and the satisfactory weldability is verified. The UniSteel reduces the resistivity difference within the sheet stack-ups, allowing the simplification of welding processes. The UniSteel concept could potentially revolutionize the manufacturing of BIW for the global automotive industry and contribute to carbon neutrality.

INTRODUCTION

The manufacturing of a steel car body, or body-in-white (BIW), has an evolving history. Until the 1960s, the whole car body was made of mild steels with a similar chemical composition and microstructure (). Driven by the requirements of crash safety and fuel economy since the 1970s, various high-performance steels have been developed to enable lightweight car bodies (–). High-strength low-alloy (HSLA) steels were first used in the 1980s (), and later, dual-phase (DP) steels were widely used in the 1990s (, ). Around 2000, ultrahigh-strength AlSi-coated press-hardened steels (PHSs) () and, later in the 2010s, quenching and partitioning (Q&P) steels with improved formability were applied (). Covering a wide range of mechanical properties, these steel grades are now used for different structural parts of BIW to accomplish lightweighting while simultaneously maintaining the desired performances, such as stiffness, fatigue, and intrusion resistance during a crash event.

The booming of steel grades offers a great opportunity for engineering design, but the manufacturing of BIW has become more complicated than ever before. It is now an integration of a few hundreds of shape-formed components, which are made from more than 10 steel materials with very different chemistries and mechanical performances (16 steel grades in a typical BIW as exemplified in fig. S1), involving a range of joining technologies. Such a multisteel scheme in BIW manufacturing creates substantial engineering and commercial challenges. One remarkable issue is the joining of different steel grades through resistance spot welding (RSW), which is the dominant welding method for joining steel BIW components. The difference in chemistry among steel components leads to a large contrast in electrical resistivity (table S1), and the resistivity ratio between two steel grades can exceed 3.0 (fig. S2A). The resistivity ratio is the decisive factor in the heat balance during RSW, determining the location and shape of the nugget (fig. S2B) and then the performance of the weld. Hence, a specific welding schedule must be developed for each combination of dissimilar steel stack-ups. Therefore, the number of such combinations in a BIW is usually larger than 10. In addition, the production of multiple steel grades amplifies the difficulties in processing optimization in chemical metallurgy, casting, inclusion control, and various rolling steps. Last, there are challenges associated with material qualifications, product revalidation due to material change, and sorting and recycling of steel scraps from end-of-life vehicle bodies for sustainability (–).

A reduced material category is essential in simplifying the BIW manufacturing and in improving metal recycling from the sustainability perspective. After reexamining the common wisdom of “the right material for the right application,” it might be better to rephrase it in a more explicit and elaborative way as “the right microstructure leading to the right property and hence for the right application.” Therefore, a low-cost and mass-production feasible “unified steel” (UniSteel) concept is proposed, i.e., a single-steel chemistry that can derive various microstructures to offer a wide range of mechanical properties. Compared to mild steels produced in the 1960s, the current UniSteel concept is technologically different because it has to be designed to satisfy the stringent safety regulations and fuel economy mandate through vehicle lightweighting. Essentially, various concepts of recycling-friendly metals and uni-alloys have been proposed from the perspective of unifying the cast or wrought aluminum alloys (, ). However, the feasibility of these concepts has not been validated, and achieving a wide range of mechanical properties using a single Al alloy composition is very challenging when compared with steels (). The recent proposal of “material planification,” as explained in (, ), relies on the control of strain-induced dislocations and interfaces to achieve the required range of mechanical properties in metals of lean alloying composition. However, this approach relies on the techniques of severe plastic deformation, which are not compatible with the existing industrial procedure for the production of sheet steels in large volumes. Therefore, they cannot be readily scaled up to serve the automotive industry for making BIW.

A proper UniSteel chemistry does not exist at present, probably due to the challenges of meeting a complicated set of requirements, as described in Fig. 1. First, automotive steel grades must be cost-effective and easily recyclable. Second, steel grades must be feasible for conventional steelmaking process, i.e., it shall be readily produced by the currently available steelmaking process, which usually consists of continuous casting, hot rolling, pickling, cold rolling, and annealing. Then, a very strict requirement is that the new chemistry must fit all the appropriate heat treatments to produce a wide spectrum of microstructures, encompassing the phase constituents from the commercial grades, such as HSLA, DP, Q&P, and PHS, with the mechanical properties comparable to or outperforming the commercial counterparts. Thus far, the existing steel grades cannot offer such a flexibility. For example, the commercial HSLA and DP steels cannot be used to produce hot-stamped components due to their insufficient hardenability (). Even the hardenability of the commercial grade of Q&P steel is still lower than that of the conventional PHS grade 22MnB5 (), which restricts its application to cold forming. With the popular PHS grade 22MnB5, it is difficult to obtain enough retained austenite (RA) to produce a Q&P type of steel due to the lack of the Si addition to prohibit cementite formation during partitioning. Last, new steels should have a good resistance spot weldability, which sets certain limits on the maximum allowable carbon level or carbon equivalence.

Fig. 1.

General requirements for the new UniSteel concept to become a viable material solution for steel car body manufacturing.

Here, we propose a novel metallurgy solution to accomplish the goal of unifying the sheet steel grades for automotive BIW manufacturing. That is, a single chemistry can generate a few steel grades with different microstructures through various heat treatments, and all of the above considerations for adoption by the automotive industry can be fulfilled. To achieve such a flexibility in microstructure engineering of the UniSteel, carbon (C), manganese (Mn), silicon (Si), and chromium (Cr) were selected as the major alloying elements, and niobium (Nb) was used as a microalloying element. An industrial-scale trial of a 300–metric ton heat was successfully performed, and conventional steel production processes were used to fabricate hot-rolled and cold-rolled coils of various thicknesses. The findings subsequently demonstrate that the UniSteel can offer a wide range of mechanical properties. The PHS variant of the UniSteel was chosen as an example to produce prototype parts through the standard hot-stamping process at a production line. The component performance was superior to that of the conventional PHS grade 22MnB5, and similar weldability was maintained. The UniSteel concept exemplifies a novel engineering approach to greatly simplify the BIW manufacturing and the selection of sheet steels while offering a competitive mechanical performance to enable the mass reduction of the body structure.

RESULTS

The present steel chemistry allows the formation of various phase constituents, including nanosized precipitates, ferrite, martensite, and RA. The delicate combination of these constituents provides many possibilities to tailor the microstructure for achieving a wide range of mechanical properties. In this study, four microstructures with different combinations of constituent phases were obtained using four distinct heat treatment schedules, as illustrated in fig. S3. These microstructures correspond to the typical microstructures of the existing commercial-grade HSLA, DP, Q&P, and PHS. The UniSteel HSLA grade is a ferritic steel with dispersed spheroidized Cr-rich carbide and nanosized NbC (Fig. 2, A1 to A3). The microstructure of the UniSteel DP grade is featured with martensite islands embedded in the ferritic matrix (Fig. 2, B1 and B2), sometimes with a minor amount of RA [~0.5 volume % (vol %)]. The UniSteel Q&P grade has similar phase constituents and morphology with the DP steel (Fig. 2C1), but it contains a much higher volume fraction of RA (~16 vol %), as shown in Fig. 2 (C2 and C3). The metastable RA will transform to martensite during deformation, resulting in an enhancement of work hardening and hence a higher tensile elongation. The UniSteel PHS grade has a lath martensitic matrix, as highlighted in Fig. 2 (D1 and D2), with some dispersed carbides and approximately 4 vol % of RA. In particular, dynamic carbon partitioning also occurs (Fig. 2, D3 and D4) during the die quenching stage of the hot-stamping process, which is essential in stabilizing austenite via local carbon enrichment. Therefore, the UniSteel concept can be adopted to deliver a wide range of microstructures, which were previously offered by different steels with different chemistries. Furthermore, the conventional steelmaking processes can be used without any modification to existing steel production facilities.

Fig. 2.

Microstructures of the UniSteel after various heat treatments.

The HSLA grade (A1) involves the dispersion of spheroidized Cr-carbide (A2) and NbC nanoprecipitate (A3). The DP steel (B1) is characterized as the necklace of martensite islands embedded in the ferritic matrix (B2). The Q&P grade (C1) contains ferrite, martensite, and retained austenite (RA). M/A refers to martensite/austenite. Nanosized film–like RA (C2) and submicron islands (C3) are shown here. In the EBSD mapping (C3), the red color indicates ferrite/martensite phase, and the green color indicates the RA. Black lines depict the high-angle grain boundaries (>15°). The PHS grade is a martensitic steel with a lath morphology (D1 and D2). The APT results (D3 and D4) capture the carbon partitioning from martensite to austenite during the die quenching of the hot-stamping process. at %, atomic %.

The tensile properties of the UniSteel variant grades (HSLA, DP, Q&P, and PHS) are presented in Fig. 3A. A wide spectrum of mechanical properties has been obtained, with ultimate tensile strength spanning from 600 to 1680 MPa and corresponding tensile elongation ranging from 25 to 9%. To highlight the viewpoint of “the right property for the right component,” the BIW parts are marked with different colors. Each color corresponds to individual UniSteel variant grades and their respective tensile strength. For example, the red tensile curve stands for the PHS grade, which is used to produce the red-colored parts in the BIW (e.g., B pillars and door beams), as shown in Fig. 3 (A and B). The UniSteel HSLA grade is used for the parts with a tensile strength lower than 600 MPa in BIW. The ultimate tensile strength and elongation of the UniSteel variant grades are compared to those of the commercial steel grades on the elongation–tensile strength plot, as shown in Fig. 3C. Excellent mechanical properties have been achieved for all steel grades with the present UniSteel chemistry. Some UniSteel variant grades have higher elongation than their commercial counterparts (HSLA, DP, and PHS) at the same strength level, whereas the elongation of the UniSteel Q&P grade is comparable to that of the commercial Q&P steel.

Fig. 3.

Mechanical performances of the UniSteel variant grades.

(A) BIW design concept using UniSteel variant grades HSLA, DP, Q&P, and PHS. (B) Tensile properties of the UniSteel variant grades. (C) Comparison of the tensile strength and elongation for UniSteel variant grades and some existing commercial steel grades.

To demonstrate that the UniSteel chemistry can be adopted to practical applications, the UniSteel PHS variant was taken as an example. As shown in Fig. 4A, after hot rolling and cold rolling, a coil was obtained, and blank cuts from this coil were hot-stamped to produce prototype front bumper beams. For automotive BIW applications, the bending performance is of great importance in addition to the standard tensile properties. The fracture of body structural components most likely initiates under the bending-type deformation during a crash event. Accordingly, a three-point bending test of the front bumper beams was conducted, and the resulting force versus displacement curves are shown in Fig. 4B. The UniSteel PHS variant has a higher peak force (41,297 N) and larger displacement (45.5 mm) at the peak load than the commercial PHS grade 22MnB5 (34,290 N and 34.9 mm, respectively). This condition indicates that the UniSteel PHS component has a higher resistance to the fracture, i.e., approximately 68% higher in energy absorption (which is defined as the product of the peak load and the displacement at the peak load) than that of the 22MnB5. Figure 4C shows the cross-tension performance, a critical assessment for RSW performance in practice (), of the UniSteel PHS variant. The performance of the UniSteel PHS weld is comparable with that of commercial PHS grade 22MnB5. A good overall welding performance with various UniSteel grades is expected, as the same chemistry is used and the maximum resistivity ratio among the UniSteel variants is approximately 1.2 (fig. S4). To demonstrate the welding microstructure among UniSteel variants, in this study, we performed a welding test between the UniSteel variants with the maximum resistivity ratio, i.e., UniSteel PHS (yield strength = 1400 MPa, the highest grade of UniSteel) and UniSteel HSLA (yield strength = 410 MPa, the lowest grade of UniSteel). The microstructure after welding, as shown in fig. S5, is similar to that of the conventional welding microstructure of existing steels with a similar resistivity ratio. There is almost no distortion in the welding nugget as the resistivity ratio is as low as 1.2. In comparison, the maximum resistivity ratio of commercial automotive steels is as high as 3.2. In particular, a low resistivity ratio leads to a less distorted nugget shape. Therefore, the welding of UniSteel variant grades should be simplified, and few welding schedules will be needed for various combinations of the UniSteel variants.

Fig. 4.

Assessment of the production of UniSteel parts made by hot stamping.

(A) Schematics showing the hot rolling, cold rolling, and press hardening processes to produce hot-stamped front bumper beams using UniSteel. The hot-rolled, cold-rolled coils and front bumper beams are listed from left to right. (B) Component level three-point bending of the prototype front bumper beam (punch speed, 15 mm/min). The setup of three-point bending is shown as an inset figure. (C) Cross-tension performance of UniSteel PHS (1680 MPa) to itself (2.0 mm) by resistant spot welding. The performance is the same with that of the existing PHS 22MnB5 of the same thickness. The inset in (C) shows the shape and geometry of the sample for cross tension test.

DISCUSSION

Multiple steel grades with a wide range of mechanical properties can be satisfactorily derived from a low-cost, single UniSteel chemistry using commercially available steel processing routes and component manufacturing techniques. Therefore, the UniSteel concept is a paradigm shift in material selection for automotive body engineering, from the conventional wisdom of “the right material for the right component” to the concept of “one material for all.” The flexibility in microstructure engineering and the capability of satisfying various design requirements of the UniSteel are enabled by the unique alloy design strategy. C, Mn, Si, and Cr are economically competitive as compared with other premium elements, such as nickel (Ni) and molybdenum (Mo). C is the most effective alloying element for increasing the strength of steels (). Si addition can prohibit the formation of cementite in martensite, which is crucial for the introduction of RA in Q&P steel (, ). Cr is a cost-effective alloying element to improve hardenability, which is important for DP steels and PHSs (). The satisfactory hardenability of the UniSteel can be revealed in the continuous cooling transformation diagram in fig. S6. The co-addition of Si, Mn, and Cr will increase the volume fraction of RA in the Q&P and PHS grades for better ductility (, ). The microalloying through Nb addition enables the NbC nanoprecipitation and the processing of the HSLA grade. NbC provides pinning at the austenite grain boundaries, and hence, Nb plays the role as a grain refiner. Although it contains a set of alloying elements, the UniSteel can still be considered a lean alloy. Therefore, it is not expected to bring any challenges to the existing steel manufacturing processes. The selected alloying elements are widely used by many commercial steels on the market, and their effects on thermodynamic and kinetics properties are well established in lean alloying systems. The existing thermomechanical processing routes can be directly applied to this steel to generate the corresponding microstructures with the aid of thermodynamics prediction and conventional metallographic procedures. With the combined benefits of these alloying additions, the UniSteel chemistry has successfully achieved its design goal as the material of choice for automotive body structure components.

The underlying deformation and strengthening mechanisms of the UniSteel variant grades are summarized as follows: (i) For the HSLA grade, the strengthening mechanisms include grain refinement hardening, solid solution strengthening, and precipitation hardening. The average ferrite grain size is approximately 8 μm, and the volume fraction and average size of the NbC nanoprecipitates are 0.015% and 2 nm, respectively. The strength attributing to the solid solution and grain size effect is estimated to be 302 MPa according to the empirical equation from the literature (). The nanosized NbC particles are presumably shearable, resulting in an additional strength increase of 84 MPa (). A summation of these quantities (386 MPa) agrees well with the measured yield strength of the UniSteel HSLA grade (410 ± 19 MPa). (ii) For the DP grade, the volume fraction of martensite in the DP980 is approximately 30.5 vol %, and the carbon content in martensite is estimated as 0.72 weight % (wt %), which corresponds to a strength of 1878 MPa (). The ferritic matrix of the DP grade is assumed to have similar properties of the HSLA grade with a tensile strength of 600 MPa. The deformation of the DP steels starts from the yielding of ferrite and then the progressive co-deformation of martensite (, ). Therefore, the tensile strength of the DP980 at the onset of necking can be assessed by the rule of mixture, providing an estimated tensile strength of 989 MPa that agrees with the experimental value (965 MPa). (iii) For the UniSteel Q&P1180 grade, the transformation of RA to martensite during tensile deformation is shown in fig. S7, which gradually occurs and continuously contributes to the strain hardening, through the so-called dynamic composite effect (). As a result, the UniSteel Q&P1180 grade has an excellent ductility, which cannot be achieved by the DP grade of the same strength. (iv) The plastic deformation in the martensitic PHS is approximated by the glide of dislocations in a highly dislocated microstructure with solute atoms and nanoprecipitates. The strength of the commercial PHS grade 22MnB5 (0.22% C–1.2% Mn–0.005% B), which is martensitic steel with a similar carbon level as the UniSteel PHS grade, is used as the baseline for comparison. The additional strengthening in the UniSteel PHS is originated from the nanoprecipitation (84 MPa) and solid solution strengthening (73 MPa). The total increase in strength (157 MPa) agrees well with the strength difference between the 22MnB5 (1500 MPa) and UniSteel PHS (1680 MPa). The details of the calculations are provided in the Supplementary Materials.

The present UniSteel solution was thereafter compared with the medium-Mn steel, which can also derive a variety of mechanical properties through controlled phase transformations (–). First, the UniSteel chemistry has a moderate cost, which is cheaper than the medium-Mn steel with 5 to 12% Mn. Furthermore, the UniSteel’s commercial readiness is much better than that of the medium-Mn steel. The UniSteel concept only involves a lean alloying scheme with moderate Cr/Si/Mn addition, and it has been successfully validated by multiple industrial trials in China and the USA, including slab casting, hot and cold rolling, and annealing. Meanwhile, the medium-Mn steel family (e.g., FeMnC or FeMnSiC system) is more difficult to handle due to the much higher degree of alloying. For example, the mechanical properties of medium-Mn steels could be very different when the annealing temperature is slightly changed (e.g., by 20°C) (). Last, welding is still a persistent issue to be resolved for enabling the applications of medium-Mn steels (, ).

In conclusion, a novel UniSteel concept is proposed to substantially reduce the manufacturing complexity and system cost of the BIW. The UniSteel chemistry presents superior flexibility in tailoring microstructure to achieve a wide range of tensile strengths varying from 600 to 1680 MPa, which encompasses the strength spectrum of typical commercial steel grades (HSLA420, DP780, DP980, Q&P1180, Q&P1400, and PHS1600) with comparable or even better elongation. The success of the alloy design is also substantiated by the detailed microstructure characterization and the quantitative analysis of strengthening mechanisms. A 300–metric ton industry trial has been made and the UniSteel PHS grade has been successfully used to produce prototype body structure parts for component performance evaluation. The resistance to side intrusion is better than the commercial PHS grade 22MnB5, while spot weldability is similar. The unification of chemistry for all the steel grades used for BIW constructure reduces the resistivity difference within any given sheet stack-up, potentially leading to simplified and more robust welding processes. In addition, the UniSteel concept will greatly improve the sustainability of the vehicle body because (i) the recycling of the end-of-life body structure becomes much easier due to the unification of steel chemistry and (ii) of the possibility of using steel scrap to reduce dependency on natural iron ore. We anticipate that the UniSteel concept will revolutionize the manufacturing of automotive BIW once it becomes widely adopted by the global automotive industry.

MATERIALS AND METHODS

Material preparation

The proposed UniSteel chemistry is listed in Table 1. An industry-scale production with a 300–metric ton steel slab has been produced by Ma Steel in China to assess the manufacturability of the UniSteel concept from the steelmaking perspective. The steel slab was then reheated to 1200°C for homogenization for 2 hours before it was hot-rolled to 3 mm in thickness (see fig. S3 for the schematic illustration of the thermomechanical process steps). After removing the surface oxide, the hot-rolled coil was cold rolled to produce multiple sheet steel coils with thicknesses of 1.4 and 2.0 mm. The cold-rolled sheet was then subjected to different annealing processes, as shown in fig. S3, to produce the UniSteel variant grades as follows:

Table 1.

UniSteel chemistry (in wt %).

Element	C	Mn	Cr + Si	Nb	Balance
Concentration	0.22	1.1	≤4	0.02–0.05	Fe

1) UniSteel variant grades for cold stamping applications.

• HSLA420: subcritical annealing at 650°C for 60 min (via a batch annealing process) or 780°C for 8 min (via a continuous annealing process).

• DP780: intercritical annealing at 790°C for 5 min and cooling to 300°C at a rate of 30°C/s, holding for 10 min and final air cooling to room temperature.

• DP980: intercritical annealing at 800°C for 5 min and cooling to 320°C at a rate of 30°C/s, holding for 10 min and final air cooling to room temperature.

• Q&P1180: intercritical annealing at 820°C for 6 min, followed by quenching at 30°C/s to 230°C and last, a partitioning step at 450°C for 1 min (the corresponding dilatometry curve is shown in fig. S8).

• Q&P1400: austenitization at 900°C for 6 min, followed by quenching at a cooling rate of 30°C/s to 280°C and last, a partitioning step at 450°C for 1 min (the corresponding dilatometry curve is shown in fig. S8).

2) UniSteel variant grade for hot forming.

• PHS1600: austenitization at 920°C for 6 min, followed by subsequent die quenching on a production PHS line and last, tempering at 170°C for 20 min to simulate the paint baking process used in the automotive industry.

Note that according to the common automotive nomenclature, the numeral in HSLA420 refers to a yield strength of 420 MPa, and its tensile strength is approximately 600 MPa. Meanwhile, the numerals in DP780, DP980, Q&P1180, Q&P1400, and PHS1600 all indicate the approximate tensile strength of these steel grades, respectively.

Microstructure characterization

Microstructures were characterized by a ZEISS-AURIGA scanning electron microscope and electron backscatter diffraction (EBSD) with a scanning step size of 50 nm. Data acquisition was performed with the AZtec-EBSD software. The three-dimensional atom probe tomography (APT) was used to investigate element distribution at the nanoscale. The APT specimens were cut from the bulk material using a focused ion beam. APT characterization was performed using a Cameca LEAP 5000X HR instrument at a temperature of ~−222.3°C, a pulse rate of 200 kHz, and a pulse fraction of 20%. The data were analyzed using the Cameca IVAS software version 3.8.4. The RA volume fractions of the samples were determined from the intensities of their diffraction peaks. The x-ray diffraction patterns of the samples were recorded on a diffractometer using Co Kα radiation. The RA volume fractions of the samples were calculated using the following equation ()where Vγ is the volume fraction of RA and Iγ and Iα are the average integral intensities of the austenite and martensite peaks, respectively. For austenite, the average integral intensities of the (200), (220), and (311) peaks were used, whereas for martensite, the average integral intensities of the (200) and (211) peaks were used for Eq. 1.

Mechanical tests

The mechanical properties were assessed through uniaxial tensile tests following the ASTM E8 standard (). All samples were prepared by electrical discharge machining. All the tensile samples have the same dimensions, i.e., 1.4 mm in thickness, 50 mm in gauge length, and 12.7 mm in width, with their longitudinal direction parallel to the sheet rolling direction. Tensile tests were performed on an Instron 5984 tensile machine at an initial engineering strain rate of 0.001/s. Component-level quasi-static three-point bending tests were conducted on hot-stamped prototype front bumper beams. A semi-cylindrical punch with a radius of 152.4 mm was used to apply load on the top face of the front bumper beam at a rate of 15 mm/min via displacement control. The distance between the two supports was 550 mm. The punch force and displacement were digitally recorded at a sampling frequency of 100 Hz.

Welding

RSW experiments were performed using a WTC medium-frequency resistance welding machine WT6000k with water-cooled Cu-Zr electrodes. To evaluate the mechanical behavior of spot welds, cross-tension tests were conducted at a crosshead speed of 2 mm/min using an Instron 5982 testing machine.

Resistivity

The resistivities of the UniSteel variants and existing commercial steels were measured at room temperature using LSR-3 Linseis Seebeck Coefficient and electric resistivity platform, which is based on a four-terminal measurement technique. The samples were 20 mm by 5 mm by 1.4 mm in size, and their surfaces were mechanically polished. Two terminals were attached to the ends of the 20-mm-long rectangular plate sample. Then, a constant direct current I was passed through the sample. The resulting voltage drop V over a length L of the sample was measured by the other two terminals. On the basis of the measured data, the resistivity ρ can be calculated according to the following formulaswhere S is the cross-sectional area of the sample.