JPH0543768B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention uses C-Si-Mn steel or C-Mn steel with a small amount of Nb added, and improves the current hot rolling process to achieve precipitation strengthening using conventional special element addition. This invention relates to a method for manufacturing a high-strength hot-rolled steel sheet that has a better strength-ductility balance than a type high-strength hot-rolled steel sheet, and has better workability with less thinning of the weld area than DP, which has a better strength-ductility balance. . [Prior art] In recent years, in the automobile industry, one of the measures to improve the fuel efficiency of automobiles is to reduce the weight of automobile bodies, and from the viewpoint of thinner steel plates and safety, high-tensile hot-rolled steel sheets with excellent workability are being used. Demand is increasing. Conventionally, in order to obtain hot rolled steel sheets with excellent workability,
A manufacturing method that improves strength through solid solution hardening and precipitation strengthening through carbonitride formation by adding special elements such as Nb, Ti, V... and Dual, which has recently begun to be manufactured.
There is a manufacturing method that improves the strength-ductility balance by using Phase (hereinafter referred to as DP) steel sheets. In the former, additive elements such as Nb, Ti, V, etc. are expensive and cause an increase in cost, and resource constraints are expected in the future. The latter DP steel sheet has a good strength-ductility balance, but when applied to wheels, which are automobile parts, it has poor stretch flangeability, so for example, when processing wheel discs, it is difficult to use.
Cracks occur when forming the hub hole by burring, making it cheaper. There are two problems: the softening phenomenon of the weld heat-affected zone is large, so when the wheel rim and mold are straightened, the wall thickness of that part is greatly reduced, and no improvement in fatigue properties is observed. C-Si-
It is possible to manufacture high-strength hot-rolled steel sheets with a composite structure of ferrite and bainite using Mn steel. For example, as shown in Japanese Patent Application Laid-Open No. 57-145965, C-Si-
A high-tensile hot-rolled steel sheet with a composite structure of ferrite and bainite is produced using Mn steel with cooling control and coiling temperature control after the final rolling pass.
It is also inexpensive and has improved the problem of cracks caused by burring when forming hub holes during wheel disk processing. However, although the softening phenomenon of the weld heat-affected zone is not as great as that of DP steel, there is a decrease in wall thickness during wheel rim and mold straightening, and there are some problems with fatigue properties. Therefore, when applying to wheels, which are automobile parts, it is necessary to improve the reduction in wall thickness due to the softening phenomenon of the weld heat affected zone. [Object of the Invention] In view of the current situation, the present invention provides general C-Si
- By adding a small amount of Nb to a steel with similar composition to Mn-based steel, we have created a high-strength hot-rolled steel sheet with a good strength-ductility balance and excellent workability with little reduction in wall thickness in the weld heat-affected zone. This provides a new manufacturing method for manufacturing at low cost. [Structure and operation of the invention] The basic components of the present invention are C: 0.02 to 0.20%,
A steel piece is used in which the content is limited to Mn: 0.3 to 1.5%, Si: 1.5% or less, P: 0.02% or less, S: 0.01% or less, Nb: 0.008 to 0.025%, and the balance is Fe and unavoidable elements. C and Mn are essential elements for securing the necessary strength and obtaining a composite structure of ferrite and bainite. If C: less than 0.02% and Mn: less than 0.3%, ferrite and bainite will not form at the cooling speed of the normal hot rolling process. It is difficult to obtain a steel plate with a composite structure, and C: 0.20
%, Mn: more than 1.5%, the deterioration of ductility is large and the weldability is also impaired, so C: 0.02 to 0.20%, Mn:
Set at 0.3% to 1.5%. When Si is preferably added in an amount of 0.2% or more, the solid solution [C] within the ferrite grains decreases.
Since it promotes the concentration of C element in untransformed austenite grains, it has the function of making it easier to obtain a suitable composite structure of ferrite and bainite, and improves the strength-ductility balance of the steel sheet. Si: If it exceeds 1.5%, the concentration of C element in untransformed austenite grains becomes saturated, which is economically disadvantageous and impairs weldability. Therefore, Si: 1.5% or less, preferably 0.2 to 1.5%. Since P impairs weldability, the P content should be 0.02% or less. S forms MnS-based inclusions and reduces stretch-flangeability, so in order to reduce MnS-based inclusions and improve stretch-flangeability, S:
0.01% or less. Ca has the function of controlling the shape of inclusions to form fine spheres and improves stretch flangeability, so it is preferably contained up to 0.01%. Nb is normally used as an element to increase strength through solid solution hardening and precipitation hardening, but here, Nb is used to suppress the softening phenomenon of the weld heat affected zone rather than to increase strength. The purpose is to prevent thickness reduction, and as shown in Figure 1, the reduction in wall thickness during flat butt welding is effective from 0.008% or more, and is saturated at 0.025%. shall be 0.025%. The effect of Nb is thought to be to prevent softening of bainite during tempering through precipitation hardening. The heating temperature of the steel plate during hot rolling may be 1100°C or less. This is because there is no need to form a solid solution of Nb, so if the heating temperature is high, the rolling speed must be reduced to ensure the final rolling pass temperature, or there is a delay time on the entry side of the final rolling, which reduces productivity.
This is to prevent it. The reason why the rolling reduction ratios for the first three passes of finish rolling were set at high rolling reduction ratios of 40% or more in each pass was because the passing time of the steel plate between passes was considered to be sufficient time for the austenite grains to recrystallize after rolling. Therefore, the objective is to set the rolling reduction rate as high as possible to increase the generation of recrystallized nuclei and to make the austenite grains finer. After the 4th pass, since the time between passes is short, the effect of cumulative reduction (there is almost no recovery after each pass and it can be evaluated as the accumulation of reduction) is considered, and the total reduction ratio (finish rolling The front and rear rolling reduction ratio) shall be 80% or more.
The temperature of the final rolling pass was set to Ar ₃ +50℃ because the temperature range of the final rolling pass was set to (Ar ₃ +50℃) when manufacturing thin (approximately 2.5% or less) wide steel sheets with a composite structure of ferrite and bainite. ) ~ (Ar ₃ −50
For products that are difficult to manufacture because the rolling load exceeds the limit value of the rolling mill at (Ar 3 +50°C), the final pass temperature is set to exceed (Ar ₃ +50°C) to ensure that the temperature in the two-phase region is maintained after the final rolling pass. Cool slowly at a cooling rate of less than 45°C/s until then, and then at a cooling rate of 45°C/s or more and around 100°C/s.
Cool to a temperature of 300-550°C. Figure 2 summarizes the strength-ductility balance of this product, and it shows that because there are not enough fine grains, the strength-ductility balance is inferior to hot-rolled steel sheets that have a composite structure of fine-grained ferrite and bainite. It can be seen that the strength-ductility balance is equivalent to that of reinforced high-tensile hot-rolled steel sheets. Examples Examples according to the present invention are shown in Table 1. Invention example 1
~9 is a steel having a predetermined composition heated according to the present invention.

【table】

【table】

[Table] Inter-rolling was performed. Comparative Example 10 was hot-rolled in accordance with the present invention using a component system with a high C content, but the balance between strength and ductility was poor, and the drop in elongation was particularly large. Comparative Example 11 was a product with a low Nb content and was hot rolled in accordance with the present invention, but thinning of the weld heat affected zone was a major problem. Comparative Example 12, like 11, has a low Nb component system and a low cooling rate, resulting in a ferrite/pearlite structure with low strength and a serious problem of thinning of the weld heat affected zone. Comparative Example 13 has the prescribed components, but the winding temperature is not high enough, resulting in a ferrite/pearlite structure and a decrease in strength.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between Nb content and the amount of wall thinning (local strain at 5% tension) in the weld heat affected zone, and Fig. 2 is a diagram showing a comparison of strength-ductility balance.

Claims (1)

[Claims] 1 C0.02~0.20%, Mn0.3~1.5%, Si≦1.5%,
A steel billet containing P≦0.02%, S≦0.01%, and Nb0.008 to 0.025%, with the remainder consisting of Fe and unavoidable elements, is heated and rolled in each pass of the first three or more passes of continuous hot finish rolling. The rolling reduction rate is 40% or more, the total reduction rate of the final rolling is 80% or more, the temperature of the final rolling pass is over (Ar ₃ + 50℃), and after the completion, Ar ₃ ~ _{Ar 1}
After slow cooling at a cooling rate of less than 45℃/s until the temperature reaches 45
A method for producing a high tensile strength hot rolled steel sheet having a composite structure of ferrite and bainite and excellent workability, characterized by cooling at a cooling rate of ℃/s or more and winding at 300 to 550℃.