New Technology Progress: Precision Microstructure Control, Manufacturing Technology & Application of Low‑Cost High‑Performance GPa‑Grade Cold‑Stamping Steel Sheets

Driven by China’s dual‑carbon strategy and rapid new‑energy vehicle transformation, synergistic improvement of automotive lightweighting and crash‑safety performance has become a core global objective for the automobile industry. GPa‑grade cold‑stamping steel sheets (tensile strength ≥1 GPa) gain wide attention among global steelmakers and OEMs for high strength, good corrosion resistance, low forming cost and low energy consumption. Nevertheless, three major industrial bottlenecks have long restricted the manufacturing and application of GPa‑grade cold‑stamping steel.

First, multi‑performance synergy is hard to achieve. Ultra‑high strength, formability, hydrogen‑embrittlement resistance and liquid‑metal‑embrittlement (LME) resistance are difficult to obtain simultaneously. Conventional solutions rely on costly alloying elements such as Mo and Cr, pushing up material cost. The fundamental mechanism for designing low‑cost high‑performance GPa‑grade steel via microstructure engineering remains unclear.

Second, stable high‑efficiency production is challenging. Hot‑rolled GPa‑grade steel forms dense Si‑rich oxide scale, leading to poor pickling efficiency and inferior surface quality. Heterogeneous soft‑hard microstructure in hot bands causes violent fluctuation of cold‑rolling force, triggering edge cracking and strip breakage. High‑alloy hot‑rolled coils exhibit extremely high strength approaching cold‑mill load limits. Besides, dynamic hardening and non‑uniform friction during cold rolling make conventional rolling models inaccurate for shape prediction, resulting in poor dimensional accuracy and large scrap loss.

Third, application and evaluation systems are incomplete. GPa‑grade steel suffers severe springback during stamping and LME risk during spot welding, threatening component forming and joint reliability. Valid evaluation standards for hydrogen embrittlement and fatigue performance are insufficient for material qualification and selection. Lacking integrated “material‑process‑component” forward‑design methodology, GPa‑grade steel application is limited, with its share in automotive BIW below 5%.

This project adopts full‑chain collaborative innovation logic: “theory‑technology‑product‑application”. Targeting high‑cost poor‑performance products, unstable production and limited‑application scenarios, the research clarifies microstructure effects on formability, hydrogen embrittlement and LME for low‑alloy C‑Mn‑Si GPa‑grade steel, establishes quantitative multi‑phase microstructure construction methodology and develops precise multi‑phase & gradient‑stabilized microstructure control technologies. On manufacturing side, a multi‑process control system covering “surface regulation‑matrix softening reconstruction‑intelligent collaborative rolling” is built. For application & evaluation, springback and spot‑welding LME are solved; test & evaluation standards for hydrogen embrittlement and fatigue are set up. An EVI‑embedded lightweight design system delivers complete vehicle‑level lightweight solutions.

Key Innovative Achievements

1. Quantitative multi‑phase microstructure construction method for low‑alloy GPa‑grade steel with balanced multi‑properties The project reveals coupling effects of ferrite, gradient‑stabilized retained austenite and fresh martensite on formability, hydrogen‑embrittlement crack initiation and LME propagation. Heterogeneous deformation‑induced (HDI) strengthening and transformation‑induced plasticity (TRIP) from gradient‑stabilized austenite jointly improve formability. The optimal microstructure window is confirmed: 10%‑30% ferrite, ≥12% gradient‑stabilized austenite and zero fresh martensite. Ferrite suppresses zinc segregation at grain boundaries to retard LME crack growth and blunts hydrogen‑crack tips; highly stable austenite relieves local stress and delays hydrogen‑induced fracture. Based on above findings, a new quantitative multi‑phase construction theory is proposed: quantified ferrite + gradient‑stabilized austenite + zero fresh martensite.

Quantitative correlations between continuous‑annealing parameters and multi‑phase microstructures are studied. The influence of orientation‑dependent ferrite on element gradient distribution inside austenite and multi‑stage martensitic transformation is clarified. Effects of cooling rate and quenching temperature on phase transformation kinetics are quantified. Supported by ultra‑fast‑cooling continuous annealing & galvanizing lines, multi‑phase collaborative gradient‑stabilization control technology is developed. Two low‑cost GPa‑grade cold‑stamping steel grades (980 MPa and 1180 MPa) are successfully developed. Without expensive Mo / Cr alloy addition, the steel reduces ton‑steel cost by more than 1100 CNY. Compared with international benchmarks, hole‑expansion ratio rises by 46 %, hydrogen‑embrittlement resistance improves by over 2 times, spot‑welding LME cracks are fully eliminated with 100 % spot‑weld acceptance rate.

2. Multi‑process collaborative manufacturing technologies resolving pickling, strip‑breakage and shape‑control bottlenecks To tackle excessive cold‑rolling force from high‑strength hot bands, matrix homogenization & softening reconstruction technology for hot‑rolled coils is developed. Edge targeted heating, adaptive edge shielding and asymmetric U‑type coiling realize homogeneous hot‑rolled microstructure. Through high‑temperature phase transformation, recovery and static recrystallization, martensite‑bainite matrix is transformed into equiaxed coarsened ferrite‑pearlite, which uniformly decreases hot‑band strength. Head‑tail scrap length is shortened from 20 m to within 10 m, cutting ton‑steel cost by nearly 200 CNY and lowering cold‑rolling force by 45 %.

An innovative EPS mechanical descaling combined with turbulent‑flow pickling is applied. Dense Si‑rich scale is efficiently removed and micro‑channels for acid penetration are created. Pickling speed exceeds 100 m/min (25 % higher than international counterparts), and surface color‑difference defect rate drops below 0.18 %.

A thermo‑mechanical coupled dynamic deformation‑resistance model is constructed. Genetic‑algorithm optimized BP‑neural‑network, GWO‑KELM rolling‑force prediction and GSTSA global optimization algorithm realize multi‑objective co‑optimization for friction, strip shape and surface quality. Cold‑strip shape closed‑loop error ≤0.5 %, rolling strip‑breakage rate below 0.8 %, and process yield raised above 96 %.

3. Application‑evaluation technologies and proprietary lightweighting system for large‑volume automotive serial production To resolve stamping springback and spot‑welding LME, coupling influences of die geometry and process parameters on springback are revealed. Multi‑objective springback control based on die‑corner radius, stamping speed and draw‑bead design improves component dimensional accuracy. A novel “multi‑pulse + tempering” spot‑welding process widens stable process window from 0.8 kA to 1.6 kA.

For service‑performance evaluation gap: a deep‑drawing‑based hydrogen‑embrittlement test method is originally proposed for GPa‑grade cold‑stamping sheets to overcome drawbacks of conventional U‑bend test. A self‑developed anti‑buckling fixture solves buckling failure for thin‑gauge (<2 mm) fatigue specimens. Three group standards are drafted, winning first‑class CSTM Standard Innovation Award for two consecutive years.

An EVI‑full‑cycle lightweight design system is built with BIW multi‑objective sensitivity analysis for component‑level differentiated optimization. 1.0 mm & 1.4 mm thin‑gauge 980 MPa GPa‑grade steel replaces conventional 350‑650 MPa steels for seat‑frame applications. Products are adopted for 31 kinds of auto parts including B‑pillars and battery‑pack housings. GPa‑grade steel BIW application ratio rises from 5 % to 11 %, maximum single‑part weight reduction reaches 25 %, BIW mass‑reduction achieves 10 %, satisfying C‑NCAP 5‑star crash‑safety requirements. The materials have been mass‑produced for 28 mainstream vehicle models.

Application Outcomes & Social‑economic Benefits

Full‑chain innovations covering microstructure theory, manufacturing processes, evaluation methodology and vehicle application are accomplished. Adopting low‑alloy C‑Mn‑Si composition plus ultra‑fast cooling, hole‑expansion ratio exceeds 70 %; hydrogen‑embrittlement resistance is doubled; spot‑welding LME cracks are completely eliminated. Comprehensive properties surpass international benchmark products with ton‑steel cost cut over 1100 CNY.

Mechanical‑descaling & turbulent‑pickling technology lifts pickling efficiency by 25 %. Hot‑band homogenization‑softening reconstruction reduces scrap rate by 90 % and rolling force by 45 %, resolving pickling difficulty, strip breakage and poor shape control for high‑strength steel to enable stable mass production.

Engineering bottlenecks of stamping springback and spot‑welding LME are solved. Five group standards are formulated filling gaps in hydrogen‑embrittlement & fatigue evaluation system. Products are supplied to 11 major OEM brands including GAC, BYD, Mercedes‑Benz, BMW and Audi, covering 28 vehicle models with over 10 million vehicles equipped. Cumulative output reaches 810 000 tons in recent three years, bringing 4.7 billion CNY new output value and 680 million CNY economic benefit. Thirty‑six authorized invention patents and thirty‑eight academic papers are achieved.

The project greatly enhances domestic supply capacity of automotive advanced high‑strength steel, guides global technical trends of GPa‑grade cold‑stamping steel and promotes industrial upgrading of automotive steel sector.

Source: The Chinese Society for Metals