Lin Xiu Du

Research of High Grade HIC-Resistant Pipeline Steel

Carry out research on the processes, structures and properties of the X70 corrosion-resistant pipeline steel and analyze the mechanism of the hydrogen induced cracking. The results show that: the developed steel not only has excellent mechanical properties, but also has good resistance to H2S corrosion. Microstructure of the X70 corrosion-resistant pipeline is mainly the acicular ferrite and granular bainite, with well-closed formation and fine grains. Hydrogen Induced Cracking (HIC) begins with the appearance of the hydrogen blistering on the specimen surface, and cracks generated by the hydrogen blistering gradually extend inward from the surface along the grain boundary, diffuse inside the specimen gradually, and finally form hydrogen induced cracking.

Effect of TMCP Parameters on Microstructure and Mechanical Properties of Cargo Oil Tanks Steel

The effect of TMCP parameters on the microstructure and mechanical properties of cargo oil tanks (COT) steel is investigated. The microstructure characteristics are performed by optical microscope, and the mechanical properties are researched by tensile test at ambient temperature and impact test at different temperatures (-20°C, -40°C). The results show that the microstructures of COT steel are a mixed microstructure consisting of ferrite, granular bainite and pearlite, which are resulted from the special TMCP parameters. The COT steel has good comprehensive mechanical properties by analyzing the experimental data, satisfying the international standard. Furthermore, impact fracture surfaces appear small dimples and a few of large ones, most of the dimple with a certain direction, and the inclusion with the fracture is mainly composed of MnS, CaS and Al2O3 composite product.

Dynamic Growth of Ultra-Fine Austenite Grains Deformed above Ad3 Temperature in a Nb-V-Ti Steel

Generally, three typical behaviors are recognized in hot-deformation of austenite. However, considering that those austenite grains involved in single-pass deformation are mostly on the scale of dozens of micrometers or even much larger than that, it is meaningful to investigate hot-deformation behaviors of austenite grains smaller than 10μm. In the current paper, austenite grains of different sizes were prepared through repetitive treatment of rapid reheating and quenching with changing the holding temperature and/or holding time. Kinds of true stress-true strain curves at 900oC and 950oC indicate that austenitic deformation can be gradually coordinated by grain boundary behaviors, such as grain boundary sliding and/or diffusion. Simultaneously, the macroscopic deformation is more likely to be dominated through co-operation grain boundary sliding (CGBS).

Investigation on a Mn-Ti High Strength Steel

By controlled rolling and cooling process, a Mn-Ti high strength steel with yield strength on the order of 700MPa had been developed successfully. The microstructure consists of fine lath shaped bainite and ferrite. The strengthening factors of this steel are attributed to the tiny interlocked distribution of bainite lath and titanium carbide precipitation. When the amount of bainite is more than 70%, the yield strength of steel is higher than 700MPa, and good mechanical performances are obtained. As a result of presence of finely dispersed bainite slabs the plasticity of steel is improved, with the steel’s elongation being about 19%. Many tiny TiC precipitates are observed under transmission electron micrograph, and these particles result in precipitation strengthening.

Study on Microstructures and Mechanical Properties of B510L Steel by TMCP

TMCP treatments were carried out on B510L steel followed by using a pilot rolling mill. Effects of finish rolling temperature and coiling temperature on mechanical properties and microstructures of the steel were analyzed. The tensile and impact properties were measured and the microstructures were observed by OM, TEM and SEM. With a proper control of rolling and cooling conditions, the yield strength of 500MPa was obtained, which was much higher than that in normal production. The yield ratio and ductility of the experimental steel were also reasonable. It was revealed that a good combination of ferrite with bainite microstructure was ideal for good mechanical properties. It was also concluded that the strengthening mechanisms included solution hardening, fine ferrite grain hardening, bainite hardening and precipitation hardening. This work can provide an experimental basis for industrial productions.

Mesoscopic Simulation of the Ferrite Nucleation on Austenitic Grain Boundary for Nanograined Steel

We use the cellular automaton (CA) modeling to investigate the ferrite nucleation on the austenite grains. On the basis of the thermodynamics and kinetics of phase transformation from austenite to ferrite, the CA modeling demonstrates that the size of nucleated ferrite grains is increased with increasing of cooling rates, and nucleation process is finished instantly at a given cooling rate. The initial austenite grain size plays an important role in the obtained ferrite nucleation number, and the potential nucleation cells are increased.

Effect of Cooling Procedure on Tensile and Charpy Impact Properties of Cr-Mo Ultra-High Strength Steel

The effect of cooling procedure on the transformation behavior of low-carbon Cr-Mo microalloyed steel was investigated by using microstructural observations, mechanical properties and impact fractographs. Three steel plates were adopted under three different cooling rates, and their microstructure, tensile and impact properties were evaluated. The results indicated that the strength of experimental steels was increased and the impact toughness was decreased with decreasing the coiling temperature. Steel A consisted of granular bainite, coarse bainitic ferrite lath and M/A constituent subjected to a coiling temperature of 560 oC. The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20 oC were 740MPa, 1020MPa, and 33.5J, respectively, which were imperfect in strength. The effects of coiling temperature were potent on the refinement of microstructure and the size of M/A constituents. Steel B consisted of a small amount of lath bainite, fine M/A constituents and bainitic ferrite lath subjected to a lower coiling temperature of 520 oC. The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20°C were 840MPa, 1030MPa, and 30.7J, respectively. However, steel C was composed of lath bainite and lath martensite subjected to the lowest coiling temperature of 380 oC (slightly above Ms point). The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20 oC were 985MPa, 1200MPa and 22.5J, respectively, which could meet the demand of ultra high strength structural steel applications.

Microstructure and Mechanical Properties of X70 Pipeline Steel with High H2S Resistance

Anti-H2S X70 pipeline steel was developed. The microstructure of X70 pipeline steel was studied by the analysis of OM, SEM and TEM. The precipitation behavior was discussed. The comprehensive mechanical properties, HIC and SCC performance were systematically studied. The results indicated that the microstructure of the experimental steel was mainly acicular ferrite and granular bainite. The second phase precipitates dispersedly distributed in the matrix. The experimental steel possessed excellent strength, plasticity, low temperature toughness and low yield ratio. And therefore, the X70 pipeline steel in the study is suitable for sour service with the high strength, excellent toughness and low HIC&SSC susceptibility.

Influence of Sulfur and Manganese Contents on Texture and Fish-Scale Resistance of DC03EK Cold-Rolled Enamel Steel

In the this paper, the compositions design with low carbon and addition of sulfur and manganese which can enhance fish-scale resistance by MnS as hydrogen trap was adopted to produce DC03EK cold-rolled enamel steel. The effect of sulfur and manganese contents on formability and fish-scale resistance of DC03EK cold-rolled enamel steel was investigated. Results show that yield strength, ultimate tensile strength, elongation, work hardening exponent and average plastic strain ratio of the steel with sulfur content of 0.024wt% and manganese content of 0.22wt% is 133 MPa, 310 MPa, 36.8%, 0.22 and 1.46, respectively, showing the well formability. And its hydrogen penetration time and diffusion coefficient is higher than 12 min and lower than 9.26×10-7 cm2/s, respectively, showing excellent fish-scale resistance. However, for the steel with sulfur content of 0.012wt% and manganese content of 0.12wt%, even though better formability is obtained, the fish-scale resistance is poor with hydrogen penetration time of 4.7 min and hydrogen diffusion coefficient of 2.36×10-6 cm2/s which is far from the expectation of steel for enamel.

Development of Nano-Scale Precipitation Strengthened Hot-Rolled 590MPa Grade Wheel Steel

In order to promote the production of lightweight automobiles and enhance the strength grade of the automobile wheel steel, a nanoscale precipitation strengthened hot-rolled 590MPa grade wheel steel was developed to replace the Q235 steel, which was originally use for the wheel. The new wheel steel type was based on a C-Mn steel composition, microalloyed with Nb, V and Ti, thus making full use of the strengthening from both grain refinement as well as nanoscale precipitation. The microstructure of the wheel steel was composed of fine-grained ferrite and pearlite and carbides distributed along the ferrite grain boundaries. The yield and tensile strengths of the strip were about 550 MPa and 620 MPa, respectively. The value of hole expansion rate was 90%. The strip has shown high low temperature impact toughness and cold formability. The strengthening mechanisms of the strip were mainly governed by grain refinement and nanoscale precipitation (Nb,V,Ti)C, and the amount of the contribution from precipitation strengthening alone was about 215 MPa. The finishing rolling start temperature was about 950°C and the finish rolling temperature was between 790~830°C. Coiling temperature of 650°C gave the best combination of mechanical properties.