MO is an important alloying element, which is a body-centered cubic crystal structure, which is widely used in alloy steels, and has a unique and irreplaceable role in steel because of its advantages of good high-temperature strength, high hardness, and strong corrosion resistance [1]. At the same time, due to the complex stress effects of various impacts and torsions during driving, the service conditions of the frame are quite harsh [2], coupled with the "carbon peak" and "carbon neutrality" goals and the strict restrictions on overload and overrun, the comprehensive performance requirements for automotive steel are getting higher and higher. At present, about 80% of modified vehicles in the market use high-strength beam steel with a strength of more than 700 MPa, and the popularization and application of this kind of precipitated strengthened high-strength steel in commercial vehicle girder steel proves that formability, low-temperature toughness, and weldability are important parameters to measure whether hot-rolled automotive structural steel can be manufactured [3]. However, in the current automobile manufacturing industry, stamping and roll forming processes are generally used for automobile beams, and their deformation methods are mainly cold formed, so high-strength beam steel must have good comprehensive properties [4 5].
A 200 kg vacuum induction furnace was used to melt the ingot of the experimental steel in the two furnaces, and the composition is shown in Table 1, and the difference between the two furnaces is whether MO element is added for microalloying. After that, the two experimental steel ingots were heated to 1200 °C, kept warm for 5 h, and then forged at 1150 °C to forge a slab with a thickness of 55 mm.
The Gleeble2000 thermal simulation test machine was used to carry out thermal simulation experiments on steel 1 and steel 2, that is, the samples were heated to an austenitizing temperature of 1200 °C at a speed of 10 °C s, insulated for 5 min, and then cooled to 850 °C at a cooling rate of 10 °C s, insulated for 30 s, and then treated separately after complete austenitization. The phase transition temperature at the main phase transition point can be measured according to the change in the size of the sample, and the continuous cooling transition (CCT) curves of the two experimental steels are plotted, as shown in Figure 1.
From the measured data and the two curves, it can be seen that the Mo element has the effect of increasing the austenite transition temperature, and the austenitization temperature is 89103 increased to 89412 °C, and has the effect of narrowing the austenite phase zone, while delaying the pearlite transition and reducing its transformation speed, the gestation period increases, and the curve shifts to the right. The bainite phase region is significantly enlarged, and the tissue is more easily obtained. At the same time, the MO element also increases the hardenability of 2 steel, and at the cooling rate of 5 10 °C, bainite structure will gradually appear, and the strength of the steel plate will be improved by phase change strengthening. According to the results of the simulation experiment, the hot rolling parameters of the experimental steel were formulated: heating temperature 1200 1250 °C, rolling thickness 60 mm, coiling temperature 600 °C, rolling passes 7. The final rolling temperature of 890 °C was selected to study its effect on the properties and microstructure of MO-containing high-strength beam steel.
Fig. 2 shows the metallographic structure of the experimental steel after rolling at different final rolling temperatures, and the metallographic structures are all finely distributed punctate martensite and austenitic islands (ma islands) in ferrite. The 1 steel structure without MO is mainly polygon ferrite + punctate ma island, and with the increase of final rolling temperature, the polygon ferrite has a tendency to coarse and uneven. The 2 steel structure with MO is mainly acicular ferrite + punctate ma island, the structure is more uniform, and when the final rolling temperature increases, it can have a certain refinement effect on the structure, and the roughening trend is not obvious. MO element effectively inhibits the formation of polygonal ferrite and pearlite, improves the hardenability of steel, and has a certain effect on microstructure refinement.
Table 2 shows the transverse mechanical properties of the experimental steel after rolling at different final rolling temperatures, and the final rolling temperature of 850 °C has the best comprehensive performance. When the final rolling temperature is 890 °C, the austenite grains are not fully refined, and the strengthening effect of fine grains is limited, so the strength and toughness are poor. When the final rolling temperature is 800 °C, although the structure has been further refined due to the presence of Nb elements, the deformation resistance during production is large, and the load of the rolling mill is increased, so it is difficult to produce at the final rolling temperature. The comprehensive properties of MO steel are better than those of non-MO steel, and the strength is increased by 7 14 MPa, and the elongation is increased by 1About 0%, mainly due to the difference in metallographic structure and the influence on carbide precipitation. There are relatively more acicular ferrite in MO steel, and acicular ferrite has good interlocking, which has a certain hindrance and containment effect on crack propagation, so the performance is better, especially in terms of toughness and plasticity, which can better meet the requirements of use. In Nb-containing steels, MO can improve the solubility product of Nb(C, N) in austenite, so that a large amount of Nb can be kept in the solid solution, so that it can be dispersed and precipitated in the low-temperature transition, which plays a good strengthening role. MO can also increase the nucleation position of carbides in steel, resulting in finer and more abundant carbides formed [6, 7].
Fig. 3 is the microstructure of steel 1 and 2 at the final rolling temperature of 850 °C, steel 1 is mainly composed of large polygonal ferrite structure, and steel 2 is mainly composed of long needle-like ferrite structure, which is consistent with the metallographic structure of microscope. Fig. 4 shows the fracture morphology of the impact fracture of 1 and 2 experimental steels at 40 °C (the tough-brittle transition temperature of this steel grade is about 40 °C) at the final rolling temperature of 890 °C. 2 steel dimples are more diffuse and smaller, among which the large dimples are large and deep, and many small dimples are clustered around [8], when the material is subjected to impact load, the ability to absorb plastic deformation work and fracture work is more dispersed, especially for the low temperature impact work below 40 °C, which can be increased by up to 20 J, so that the low temperature impact toughness of 2 steel with MO is better.
1) The CCT curves of the two components of steel are composed of ferrite, pearlite and bainite phase regions, and the MO element has the effect of delaying the transformation of pearlite and inhibiting the formation of polygonal ferrite and pearlite.
2) After hot rolling, the metallographic structure of MO steel is acicular ferrite + punctate MA island, and the metallographic structure of non-MO steel is polygonal ferrite + punctate MA island.
3) When the final rolling temperature is 850 °C and the coiling temperature is 600 °C, the comprehensive performance is the best and meets the requirements. In terms of strength and toughness, MO steel is generally better than non-MO steel, with a corresponding increase in strength of 7 14 MPa and an elongation of 1About 0%, the impact energy effect on the temperature below 40°C is obvious, and it can be increased by up to 20 J.
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Article** — Metal World.