| With the increasing requirement of vehicle weight reduction and energy conservation from automobile industry, the investigation and development of high strength steel sheet has been stressed extensively. BH steel, as an important kind of automotive body steel sheet, has been paied close attention from scientific and industry researcher. In the present work, project"research on ultra low carbon bake hardening steel"is used as research basic, mechanical properties and BH phenomenon is detected in BH steels after different heat treatment and especially different annealing processes; according to thermodynamic calculation and microanalysis results, microstructure, second phase precipitates and solute distributions were analyzed in hot-rolled, annealed and pre-deformed BH steels. The effect factors of mechanical properties, BH phenomenon and formability in BH steel was discussed; this provides theoretical guidance for preparing high performace BH steel. The main research result from the present work is as follows:Three kinds of BH steels (BH-0, BH-Mn, BH-P) were prepared according to thermodynamic calculation results and related theory; micro hardness in BH steels with different annealing temperatures were tested, the recrystalization temperature of BH steels were thereby determined; Ac1 and Ac3 temperatures were determined according to thermal expansion test result; thermal expansion test result also shows that no martensite phase transformation was happened during annealing process in the three BH steels, that is, the microstructure is exclusive ferrite phase in BH steels after annealing; two different annealing processes were designed in the temperature range between recrystalization and Ac1, annealing process 1 is water quenching, annealing process 2 is added an over time aging stage as compared with process 1.The effect law of annealing process on mechanical properties of BH steels is as follows: mechanical properties of BH steels were hardly affected by the annealing temperature except for n value; the general property of BH-Mn steel is better than BH-0 and BH-P steels. Over time aging process will induce increase of yield strength and tensile elongation as well as the slight decrease of tensile strength in BH steels. The effect law of over time aging on r value in the three BH steels is completely different, the r value in BH-0 steel changes little after over aging process, while it shows obvious decrease in BH-Mn steel and increase in BH-P steel. In order to explain the r value variation, micro texture in the three BH steels were detected after different annealing temperature and annealing processes. It is obtained that the completeness of recrystalization in BH steels is especially important for goodγtexture. Once after recrystalization, the increase of annealing temperature has slight effect on BH steel texture. According to the results of the three BH steels texture after two annealing processes, the effect law of over time aging on the three BH steels texture is quite different, the effect of texture on deep drawing property is induced by both advantageous and disadvantageous textures.The addition of Mn will increase MnS precipitation temperature by about 300℃. The addition of Mn in BH steel will induce the formation of complicated compound and decrease the solubility of C. Besides, dimensions of most vanadium carbides are tens of nanometers in BH-Mn steel, addition of Mn induces the clusters of V and C with size of several nanometers in the matrix. In BH-P steel, however, more vanadium carbides precipitate over manganese sulfide and titanium sulfide rather than exist alone. Much nanometer-sized particles of manganese sulfide precipitate in the steel, part of which precipitate together with TiS (or Ti4C2S2). In addition, all P is dissolved in BH-P steel and exists as segregation rather than precipitates as particles. No free Ti, N or S atoms are dissolved in BH-Mn and BH-P steels.According to 3DAP analysis and tensile test result, P segregates mainly in BH-P steel, and part of P segregates together with C, which strongly pin the dislocations and is the main reason that induces the yield point elongation and the worsening of steel sheet surface quality during tensile process. This also induces the weaker microstructure and lower r value in BH-P steel as compared with BH-Mn steel. In BH-Mn steel, Mn hardly segregates in the matrix and C segregates very little, so the yield strength of BH-Mn steel is lower than that of BH-P steel, whereas the plasticity of it is better than the latter. According to microanalysis of BH-Mn and BH-P steels with different annealing processes, it is found that C concentration decreases, whereas V increases during over time aging in the two BH steels. It is originally obtained by 3DAP that no vanadium carbide forms during overaging in the BH steels, this is in agreement with the thermodynamic calculation result. The decrease of C concentration is reasonably due to the diffusion of C to grain boundaries and/or dislocations. The pinning of dislocations by C is the key factor that induces the increase of yield strength in the two BH steels during over time aging. The together segregation of C and P is the main factor that induces the worsening of microstructure in BH-P steel during over time aging. This diffusion of C further induces the increase of yield point elongation and the worsening of steel sheet surface quality. Pre-deformation must be higher than the yield point elongation and totally drag away the Cottrell atmosphere formed during the over time aging in order to obtain the BH phenomenon. Results also reveal that once the pre-deformation is more than yield point elongation, BH value in the same BH steel will be affected slightly by the variation of pre-deformation.Solute distributions in BH steels with different pre-deformations were investigated by 3DAP. BH values of these steels were tested by tensile experiments. Results indicate that C distribution changes little with the increase of pre-deformation in BH-Mn steel. In BH-P steel however, C diffuses during pre-deformation and baking process, which forms more C clusters and induces the decrease of C concentration in the matrix. Distribution patterns and maximum separation distance method results prove that the C cluster is just C segregation or C together with P segregation rather than vanadium carbides precipitate. BH value in BH-P steel without pre-deformation is 20MPa, much higher than that in BH-Mn steel, which is induced by the much solute C and enough dislocations in the BH-P matrix. This is in agreement with the result that there is a long yield point elongation in BH-P steel during tensile test. |
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