Maoqiu Wang

Effect of tensile deformation of austenite on the morphology and strength of lath martensite

A hot-rolled steel, 22SiMn2TiB, was employed to study the effect of austenite deformation on the microstructure and strength of the subsequently formed lath martensite. It was revealed that the sizes of the martensite packet, block and lath were refined by the tensile deformation of austenite at temperatures above 850 °C. With the increase of the deformation temperature, the packet size increased, whereas the block size decreased. The width of the lath was independent of the prior austenite grain size and the deformation temperature. The refinement of martensite blocks was considered to strengthen the ausformed martensite.

Effect of Sulfur Content and Sulfide Shape on Fracture Ductility in Case Hardening Steel

The effects of sulfur content and calcium addition on smooth axisymmetric tensile fracture ductility of case hardening steel DIN 18CrNiMo7-6 have been investigated. The quantitative metallographic analysis of sulfide inclusions and the correlations between sulfide inclusions and fracture ductility were examined. Sulfide inclusions were found to have deleterious effect on fracture ductility, whereas the effect can be offset to some extent by calcium-treatment due to less easily deforming of sulfides during hot-working. The product (AA · λAW) of sulfide inclusion area fraction (AA) and its area-weighted aspect ratio (λAW) can be used as a parameter to describe the effect of sulfide inclusions on fracture true strain.

The 3rd Generation Automobile Sheet Steels Presenting with Ultrahigh Strength and High Ductility

In this study, research and development on the 3rd generation automobile steel, with targets of R m × A no less than 30 GPa% at R m level of 1–1.5 GPa, was carried out to fabricate high strength and high ductility steel by two methodologies, one is the medium manganese steels fabricated by intercritical annealing through austenite reverted transformation (ART-annealing) and another is the conventional carbon steels processed by quenching and partitioning(Q&P). The ultrafine grain sized austenite-ferrite duplex microstructure and the tempered martensite-fresh martensite-austenite multiphase microstructure were demonstrated based on the microstructure characterization in ART-annealed medium manganese steels and Q&P processed conventional carbon steels. In both heat treatment conditions, substantially enhanced ductility (30–40%) at ultrahigh tensile strength level (1–1.5 GPa) was obtained, which results in a significant improvement of the product of tensile strength to total elongation about 30–40 GPa%. Analysis on the work hardening behaviors and the relationship between microstructures and mechanical properties of the studied steels indicates that the greatly improved ductility results from the aid of the phase transformation induced plasticity (TRIP effects) and the ultrahigh strength stems from the hard matrix, such as the ultrafine grained duplex structure in ART-annealed steels and the martensite matrix in Q&P processed carbon steels. It is interesting to find that a strong dependence of the product of tensile strength to total elongation on the fraction of retained austenite phase of steels produced by both ART-annealing and Q&P processing techniques. It was proved that both ART-annealing and Q&P processes can be applied to fabricate the third generation automobile sheet steels offering ultrahigh strength and high ductility.

On the Performance Improvement of Steels through M3Structure Control

Steels are now considered to be a category of new materials, which have progressed year by year to meet the market requirements for high performance. Although it is well known that the performance of steel is dependent upon microstructure, the potential of microstructure evolution is really unknown by people at present due to the steadily developing steel metallurgy. The study is needed to investigate the potential microstructure evolution phenomena in steels and to develop new technology to improve the properties through microstructure control, by which the safety and reliability of steels in service could be eventually improved remarkably. The issues such as steel metallurgy for ultra cleanliness and ultra homogeneity, phase transformation mechanism of meta-stable austenite subject to temperature and load changes, carbon diffusion and partitioning during transformation, multi-scale characterization of structures, and microstructure stability subject to temperature and load changes are thought to be the key points to clarify the microstructure evolution at the existing circumferences. As a result, it can be expected that the fundamentals of microstructure control featured by multi-phase, meta-stable and multi-scale (M3) could be established, and then the technologies to process high performance steels could be developed: the third generation HSLA steels with improved toughness and/or ductility (A KV(−40°C) ≥ 200 J and/or A ≥ 20% as Rp0.2 in 800–1,000 MPa), the third generation advanced high strength steels (Rm × A ≥ 30 GPa% as Rm from 1,000 to 1,500 MPa) for automobiles with improved ductility and low cost, and the third generation heat resistant martensitic steels with improved creep strength (σ 10,000 650 ≥ 90 MPa). It can be expected that the new technology developed will improve the safety and reliability of steel products in service remarkably for infrastructures, automobiles and fossil power station in the future.

Long Life High Strength Steels to Resist Fatigue Failure and Delayed Fracture

In recent years, economic and environmental considerations have increased the need to safely extend the service life of components and structures beyond their original design life. It well known that fatigue and delayed fracture are the two important mechanisms for the failure of steel components and structures in service. Based on our systematic studies of the fatigue failure and delayed fracture behaviours of high strength steels in the last 10 years, we proposed a new kind of concept to improve both fatigue failure resistance and delayed fracture resistance of high strength steels, which comprises the combination of inclusion modification, structure controlling, hydrogen trap controlling and grain boundary strengthening, and it is called IST & GST technology in this paper. Firstly, basic considerations for the improvement of service life of high strength steels are introduced. Secondly, methods such as inclusion modification, structure controlling and hydrogen trapping controlling are discussed for improve the fatigue failure and delayed fracture resistance of high strength steels. Finally, based on IST & GST technology, examples of development of long life high strength steels such as a newly developed 2,000 MPa grade high strength spring steel with excellent fatigue failure resistance and 1,500 MPa grade high strength bolt steel with superior delayed fracture resistance are introduced.

A novel method for endpoint temperature prediction in RH

ABSTRACT Based on Cloud Model, a novel method was proposed to predict the endpoint temperature in Ruhrstahl Heraeus (RH) for Interstitial-free (IF) steel production, considering the starting temperature, scrap and refining cycle. 300 sets of RH production data was collected, mined and reasoned by Cloud Model. The prediction results of the Cloud Model are compared with BP neural network methods. The final results show that in the error scope from −10 to 10°C, Cloud Model acquired the 93.32% hit rate, BP neural network acquired the 89.33% hit rate. Compared with the BP neural network, the Cloud Model has higher accuracy and stronger generalisation ability.