| The iron and steel industry, as the traditional industry, has experienced its best period in the world. However, the intrinsic advantages of steel (good processing property, high recycle utilization and low price) leads to the more developing interspace, the iron and steel industry in our country has good developing foreground. Although china is the biggest country for the steel production in the world, under the resource, energy source and human go-on development requirements, the iron and steel material is facing many challenges such as the pressure of the cost for production increasing and steel price decreasing., facing the challenge and opportunity, the countermeasure for the iron and steel industry is to gain the comparison superiority in market.The improvement in the properties of the structural materials is the eternal subject to gain this superiority. A rapid, economic and accurate design of the high performance structural materials with the aid of the information technology is not only the important way to gain this superiority but also the only way for the iron and steel enterprises to survive and develop. The important marks of the high performance structural materials include good application and fine processing and shaping properties, wide resource, low-energy consumption in manufacturing, recycle utilization, little pollution of environment, satisfying go-on development requirement and low cost of production. By way of the optimum design, low carbon low-alloy steels can attain the optimum combination of composition, microstructure and properties, and achieve high production of strength and plasticity (σ×δ) as well as high proportion of performance to cost, and thus possess a large development space for the application as the high performance structural materials.The aim of adopting the information technology is to collect the information of composition of material, microstructure, synthesized method, and then treated to obtain the best results which demands the properties of materials. By the method, it could design the new material rapidly, simply and accurately. It can be seen from the analysis of application status of low carbon-low alloy-high strength steel that it could obtain many kinds and many phases steel if low carbon and low alloy combined with the advanced technology, such as ferrite and pearlite, ferrite and martensite, air-cooling banite, lath-shaped martensite, ferrite and banite and martensite and remnant austenite, banite and martensite and remnant austenite which possess high strength and high plasticity. Namely, the duplex-phase microstructure of low carbon and low alloy steel possess the best combination of high strength and high plasticity.On the basis of reviewing and summarizing the data pertinent to the critical phase transformation point and transformation kinetics, the calculation formulae for the starting point and ending point in the phase transformation kinetics of the duplex-phase steels were put forward. Using the existing research achievement and the phase transformation strengthening theory and following the property requirement, the calculation formulae for the design of the composition and microstructure of the low-carbon low-alloy high-strength steels were suggested. Moreover, the preceding formulae were testified by experiments. The experimental results basically coincide with the calculated TTT and CCT curves of this steel, which satisfies the design requirement. The transformation kinetic characteristic of the overcooled austenite in this steel, namely, the real cooling rate, Vr, after forging satisfies: V_B > Vr≥V_F>V_P; The ratio of the decreasing value of Bs to that of Ms (namely Bs / Ms) caused by the alloying element is better to be high.Aiming at solving the problem and shortage appeared in manufacturing the automobile roof beam with quenched and tempered steels, we used the above formulae and designed a novel duplex-phase steel, 20Mn2SiVB, which is easy to produce bainite by air-cooling after forging, yet without the requirement of tempering. The tensile strength,σ_b, is over 900MPa. Through the industrial smelting, rolling and then forging, the steel was used to make the automobile roof beam. The major chemical compositions (in weight percent) of this steel are 0.18C, 2.05Mn, 0.84Si, 0.12V, 0.0043B, 0.057S and Fe balance. The microstructure after forging at 11250℃and air-cooling treatments consists of bainite and a small amount of remnant austenite, ferrite and martensite, and the main mechanical properties areσ_b=963.4,MPa,σ_s=635.4MPa,δ=13.9%,ψ=43%, HB=290, a_k= 67J/cm~2, N_f = 200×10~4, which meet the design requirement.The microstructure and rupture section of 20Mn2SiVB were analyzed by the method of TEM, SEM XRD and EDS; the properties of strength, hardness, plasticity and fatigue-limit were test and the relationship of microstructure with properties was studied theoretically.The examination and analysis on the microstructure and properties of the 20Mn2SiVB steel showed that the morphology of the granular bainite or the non-carbide bainite is the major factor affecting the strength and toughness properties of the steel. The smaller of the size and the more quantities of the islands in the granular bainite (M-A), the higher of the strength and toughness of the steel obtained. In the duplex-phase microstructure, the原始remnant austenite distributes with film forms between the bainite laths or exists with the fixed angle on the bainite lath, separating the lath into large numbers of the smaller piece, which cuts the stress peak of crack tips and thus makes the tips passivation. This effect, together with the deformation strengthening effect, reduces crack propagation speed and raises the deformation resistance of the material. Moreover, through controlling the morphology, dimension and quantity of inclusions, the toughness of the steel can be improved and the fatigue-limit value can reach higher. The existence of a small amount of ferrite (no more than 6%) in the steel is profitable to the improvement in the plasticity and toughness of the steel and to the guarantee of the good cutting performance.The analysis on the cracks of the 20Mn2SiVB steel show that the fatigue-limit decreased significantly when the oxidation of Al, Si, Ca existed in the steel and their dimension were larger than 10μm, conversely, it had any effects when their dimension were smaller than 10μm.The effect of heating and cooling technology on the microstructure and properties of 20Mn2SiVB was also studied. The high transformation temperature of austenite leads to rapid cooling rate which makes the quantities of remnant austenite increase. The mechanic properties of steel increase when the quantities of remnant austenite increase, conversely, it decreased.Using the 20Mn2SiVB steel to substitute for the quenched and tempered steel to make the automobile roof beam, and adopting the treatments of 1250℃forging and then air-cooling, one can obtain the following relative contents of the phases in the microstructure: 9% remnant austenite plus 6% ferrite, and the others are bainite and a small amount of martensite (or tempered martensite). If so, the manufacturing and application requirements of the automobile roof beam can be satisfied. For the simple manufacture technology of automobile roof beam and lower cost of production, it could make better economic and social benefits.
|
PDF Full Text Request