Materials Science Forum Vol. 1105

Abstract: Extensive efforts have been undertaken worldwide to develop new high strength steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. These “3rd Generation” Advanced High Strength Steels (AHSS) are being implemented, and spot-welding has been found to present new challenges for these steels when Zn-based corrosion resistant coatings are involved, wherein zinc liquid metal embrittlement (LME) can occur. Some recent work is highlighted here that was designed to examine the separate effects of prior microstructure and alloy composition on LME sensitivity. LME behavior was assessed by comparing hot-ductility of steels with and without a Zn coating tested under conditions simulating spot-weld thermal cycles. Effects of prior microstructure on LME susceptibility were assessed with a single AHSS alloy composition, using annealing modifications to produce martensitic, Q&P, TBF and dual-phase substrates. The dual-phase steel exhibited less sensitivity to LME, perhaps because the Zn penetration and cracking are unable to follow (prior) austenite boundaries in this microstructure. With respect to alloy composition, carbon and manganese variations did not lead to noticeable effects on LME sensitivity, while silicon clearly leads to increased LME sensitivity. Addition of 1.3 wt. pct. aluminum to a 0.5 wt. pct silicon-containing AHSS steel further increased LME sensitivity at some test temperatures. The effects of alloying are interpreted in terms of the propensity to form an intermetallic reaction layer that consumes liquid and physically separates the substrate and liquid zinc.

Abstract: Double soaking (DS) has been proposed as an alternate processing route for medium manganese steels. DS consists of soaking in the intercritical annealing region to stimulate manganese enrichment of austenite by depletion of ferrite followed by secondary soaking at a higher temperature and cooling to room temperature to obtain a martensite/austenite microstructure. DS is different from more traditional medium manganese heat treating which usually involves a single soaking step in the intercritical region to generate a ferrite/austenite microstructure. DS has been shown effective at generating attractive tensile properties notably tensile strength levels in excess of typically observed levels in medium manganese steels. A review of properties and microstructural evolution obtained by DS of medium manganese steels is presented here.

Abstract: The consumption of steel for fasteners in China is about 9 million tons annually, accounting for approximately one-fifth of worldwide usage. High strengthening is the main development trend of high performance of fasteners. As strength increase, the sensitivity of both fatigue failure and delayed fracture increases, and the processing technology varies. These are the main problems need to be addressed. Fasteners of property classes 8.8-12.9 are currently the most widely used high-strength fasteners. The match between fatigue life with strength is commonly the paramount consideration. We have developed a series of steels involving (i) SCM435 steel wire for key fasteners of engine with ultra-narrow composition (ΔC≤0.01%), slight hardness variation (ΔHRC≤ 2) and a qualified fatigue performance of 1×10⁷ cycles; (ii) 10.9 and 12.9 fastener steels with large size, low cost and high hardenability for wind tower; (iii) A286 alloy wire with desirable 650 °C durability. In addition, we developed ML35 wire free of spheroidizing and microalloyed steels for slender bolts free of quenching & tempering process. The use of ultra-high strength fasteners ≥13.9 is necessary to achieve equipment light weighting. We have developed steels for 13.9 and 15.9 fasteners with an excellent resistance to delayed fracture. An austempering process was applied to 14.8-17.8 fasteners, which enables the reduction of the alloy content. A novel strengthening method needs to be introduced to develop fasteners in excess of class 19.8 fastener. We have developed 19.8 fastener steel with good fatigue and delayed fracture resistance. Secondary hardening plays a key role in the improvement of strength.

Abstract: Carbide free bainitic microstructures of steels in hot rolled condition have high potential for automotive and structural applications, where both high elongation and toughness at a high strength level are needed. However, achieving a combination of these properties remains a challenge due to difficulties in ensuring a high stability of retained austenite while maintaining industrial processability. Therefore, an attempt has been made in this work to achieve combined high toughness and high elongation in hot rolled carbide free bainitic steels.

Abstract: A 7 wt.% Mn steel was designed and was cast and processed to 1.5 mm sheets. The sheets were continuous annealed and coated with a Zn alloy by hot dip galvanizing before subjecting them to hot press forming cycles. The final microstructure was characterized by ultrafine ferrite grains and a high fraction of retained austenite. Excellent combinations of in-service strength-ductility-bendability were achieved for hot forming in the temperature range of 530-675 °C. The use of the low temperature hot forming minimized the liquid metal embrittlement induced cracking with Zn coating during hot forming and spot welding. Various application properties such as oxidation resistance, corrosion resistance, and springback were found at optimum levels for hot forming at 675 °C.