JPH0132293B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing medium-strength hot-rolled steel sheets with excellent workability ^. The purpose is to provide a method for manufacturing the same. It is between soft hot-rolled steel sheet and high-strength hot-rolled steel sheet.
Medium-strength hot-rolled steel sheets with a TS of 35 to 50 Kgf/mm ² have received less attention than the above-mentioned two, and there is little to be seen in terms of improvements in their workability. In other words, conventional materials of this class have a C of 0.10~
Manufactured with 0.20% rimmed steel or Al-killed steel, and when workability is required, Al-killed steel is used.
Depending on the degree of processing, hot-rolling and high-temperature winding or strong desulfurization have been used to cope with the problem. However, in recent years, the applications for this class of materials have shifted to those that are more difficult to process, and there are cases where it is not possible to handle them simply by high-temperature winding or strong desulfurization, such as automobile suspension parts and propane gas containers. In such cases, the main problems are cracks in the stretch flange formed part and vertical cracks in the deep drawn part. The present invention was created after repeated studies in view of the above-mentioned circumstances, and includes, in weight% (hereinafter simply referred to as %), C: 0.08% or less, Si: 0.1 to 1.0%,
Mn: 0.5-1.4%, P: 0.020% or less, S: 0.010%
Hereinafter, steel with sol, Al: 0.020 to 0.100%, N: 0.0060% or less, the balance consisting of Fe and unavoidable impurities, is heated at a slab heating temperature of 1100℃ or higher and a finishing temperature of
This paper proposes a method for producing medium-strength hot-rolled steel sheets with excellent workability, characterized by hot rolling at ₃ or more Ar points and at a coiling temperature of 600 to 740°C. 0.1% or less of one or more of the group consisting of Zr, Ca, and REM or (and) B
It is proposed to hot-roll a steel containing 0.0025% or less under the same conditions as above. That is, to further explain the present invention, in view of the above-mentioned circumstances, the present inventors have conducted various studies to obtain a medium-strength hot-rolled steel sheet with excellent stretch flange formability and longitudinal cracking resistance. As a result, there is a correlation between the amount of C and the properties described above, and by lowering the amount of C and combining appropriate reinforcing elements as described above, it is possible to create a medium with much better workability than conventional ones. This confirms that it is possible to produce high-strength hot-rolled steel sheets. To explain the reason for limiting each condition as described above, first, the chemical composition is as follows. C is an element having an important meaning in the present invention, and has a large influence on the stretch flange formability and longitudinal cracking resistance of the product. In other words, Figure 1 shows the relationship between the notch elongation in the C direction of a hot rolled sheet and the amount of C when TS is kept constant. It is well known that the influence of the S content is large and that reducing the S content improves notch elongation, but as shown in Figure 1, notch elongation can also be improved by lowering the C content. Moreover, as shown in the figure, the effect is large in the region where the amount of C is low, and in that case, the effect of desulfurization becomes small. For example, 0.005% S and 0.15 C
When lowering from % to 0.08%, S is 0.005 at 0.15%C.
The same effect as lowering C from % to 0.001% can be obtained, and if C is further lowered, notch elongation is further improved.Such an effect is thought to be caused by a decrease in notch sensitivity due to a decrease in carbides in the steel. It will be done. Furthermore, Figure 2 shows the relationship between the fracture surface transition temperature vTrs after deep drawing and the amount of C, which is an index of longitudinal cracking resistance.
After deep drawing, the lower the C content, the lower the temperature of vTrs.
Excellent vertical cracking resistance. However, as shown with hatching in Figures 1 and 2, these effects gradually decrease when C exceeds 0.08%, and spot weldability also deteriorates, so the upper limit of C was set at 0.08%. , more preferably 0.06% or less. Si is useful as a solid solution strengthening element and 0.1%
The strength can be increased by the above additions.
However, if it exceeds 1%, the longitudinal cracking resistance will deteriorate, so it is set in the range of 0.1 to 1.0%. It is also necessary to add Mn in an amount exceeding 0.5% to ensure product strength, but if it is added in an amount exceeding 1.4%, the dependence of strength on the winding temperature increases and the stability of the material is impaired, so add 0.5%. It is necessary to limit it to 1.4%. Since P is an element that deteriorates longitudinal cracking resistance and is preferably lower, it is set to 0.020% or less. As shown in Figures 1 and 2 above, like C, S has a large effect on workability, so it is preferable to have a low content. Good workability can be obtained. In other words, the upper limit is 0.010%. Since sol and Al can be within the range of normal aluminum killed steel, they were set to 0.020 to 0.100%. Since N precipitates in the steel as AlN and reduces the ductility of the product, the lower the content, the lower the content is set to 0.0060% or less, but preferably 0.0040% or less. Zr, Ca, and REM improve the processability of the product through their sulfide shape control effects, but even if they are combined at 0.1% or more, the effect will be saturated and it is not economical, so one or more of them should be kept at 0.1% or less. And so. Furthermore, B has the effect of suppressing grain coarsening that tends to occur in ultra-low C materials and high-temperature materials, but if it exceeds 0.0025%, this effect will be saturated and at the same time, the ductility will decrease, so it should be kept at 0.0025% or less. . Next, the slab heating temperature was set to 1100°C or higher, which is the standard practice, but if the rolling mill capacity is available, there is no problem in terms of the material even if the heating temperature is lower than this.
Moreover, there is no problem in terms of material quality even if hot piece charging or direct rolling is applied. If the finishing temperature is lower than Ar ₃ , the material will deteriorate overall, so it is necessary to set it to Ar ₃ or higher. Further, since there is no need to specify the winding temperature in particular, it was set to 600 to 740°C, which is a normal range. Specific examples of the invention are as follows. That is, Table 1 below shows the chemical composition according to the present invention and that of comparative steels. Steels A to I are within the scope of the present invention, whereas steels J to M are comparative steels, and Steels J and K have C outside the scope of the present invention, steel L has Si outside the scope of the present invention, and steel M has S outside the scope of the present invention. Note that the same effect can be obtained even if Cr in Steel H is replaced with Ni.

[Table] For these steels A to M, the slab heating temperature was 1250°C, the finishing temperature was 880°C, and the coiling temperature was 640°C to obtain hot rolled steel sheets with a thickness of 2.6 mm. However, the results of testing and measuring the mechanical properties of the product thus obtained are summarized in Table 2 below.

【table】

[Table] That is, Fig. 3 shows the relationship between the hole expansion rate, vTrs after processing, and TS. Steels A to I according to the present invention are all distributed in a better range than the comparative steels, and are excellent. It is clear that it has both stretch flange formability and longitudinal cracking resistance. As explained above, according to the present invention, a medium-strength hot-rolled steel sheet with excellent workability can be effectively and appropriately manufactured by reducing the amount of C, combining appropriate strengthening elements, and selecting appropriate hot-rolling conditions. This is an invention with great industrial effects.

[Brief explanation of drawings]

The drawings show the technical relationship of the present invention, and FIG. 1 is a chart showing the relationship between notch elongation in the direction perpendicular to rolling and C amount when the components are varied while keeping TS approximately constant; Figure 2 is a diagram summarizing the relationship between vTrs and C content after deep drawing when the components are varied while keeping TS almost constant.
FIG. 3 is a chart summarizing the relationship between the hole expansion rate, vTrs after deep drawing, and TS for examples according to the present invention and comparative examples.

Claims (1)

[Claims] 1 C: 0.08wt% or less, Si: 0.1 to 1.0wt%, Mn: more than 0.5wt% to 1.4wt%, P: 0.020wt% or less, S: 0.010wt% or less, sol. Steel containing Al: 0.020 to 0.100wt%, N: 0.0060wt or less, and the balance consisting of Fe and unavoidable impurities, is heated at a slab heating temperature of 1100℃ or higher, a finishing temperature of Ar _{at 3} points or higher, and a coiling temperature of 600 to 740℃. A method for producing a medium-strength hot-rolled steel sheet with excellent workability, characterized by hot rolling under the following conditions. 2 C: 0.08wt% or less, Si: 0.1 to 1.0wt%, Mn: more than 0.5wt% to 1.4wt%, P: 0.020wt% or less, S: 0.010wt% or less, sol.Al: 0.020 to 0.100wt %, N: Contains 0.0060wt% or less and is one of the group consisting of Zr, Ca, and REM.
0.1wt% or less of species or more or (and) B
Steel containing 0.0025wt% or less with the remainder consisting of Fe and unavoidable impurities is heated to a slab heating temperature of 1100℃.
Above, finishing temperature Ar ₃ points or more, winding temperature 600~
A method for producing medium-strength hot-rolled steel sheets with excellent workability, characterized by hot rolling at 740°C.