JPH0349967B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a hot-rolled steel sheet with excellent formability. Conventionally, cold-rolled steel sheets with excellent formability have been generally used as steel sheets for processing, but recently there has been a demand for hot-rolled steel sheets with excellent formability for reasons such as cost reduction. The most problematic forming property of conventional hot rolled steel sheets compared to cold rolled steel sheets is poor deep drawability. Deep drawability is better as the number of crystallographic {111} planes parallel to the surface of the steel sheet increases, and on the other hand, the fewer {100} planes there are, the better the deep drawability is. ) is determined. In the conventional manufacturing method of hot-rolled steel sheets, it is common sense to roll at ₃ or more Ar points, but in this case, the texture becomes random during the γ → α transformation, and therefore the hot-rolled steel sheets manufactured by the conventional method are Deep drawability is significantly inferior to cold-rolled steel sheets. Recently, attempts have also been made to produce hot-rolled steel sheets by warm rolling at temperatures below Ar ₃ , but hot-rolled steel sheets produced by this method generally have a {100} axial density parallel to the sheet surface. is high, and therefore the deep drawing characteristics are not good. On the other hand, carbon equivalents equal to or greater than the carbon equivalent in steel
Hot-rolled steel sheets manufactured by hot-rolling steel added with Ti, Nb, or Zr at a finishing temperature of 750°C or less have an axial density ratio of {111}/{100} at the center of the sheet thickness of 7. However, even with the above method, it is difficult to obtain a hot rolled steel sheet with good deep drawability with a value of 1.1 or higher. difficult. The main reason for this is that in all conventional hot-rolled steel sheet manufacturing methods, the texture differs greatly in the thickness direction. Even in steel sheets, the axial density ratio is quite low near the surface, and unlike cold-rolled steel sheets with excellent deep drawability, the axial density ratio is high even near the surface and the texture is not nearly the same in the thickness direction. It will be done. The present invention provides that a hot rolled steel sheet manufactured by a conventional method is
It advantageously solves the problem of inferior deep drawability compared to cold-rolled steel sheets, and its main points are: C: 0.05% by weight or less, N: 0.01% by weight or less, and the addition of C and N. The amount is the addition amount of one or both of Ti and Nb and C/12+N/14<1.2 (Ti/48+
Steel related to Nb/93) is rolled with lubrication at a temperature range of 500℃ or higher and lower than the Ar ₃ transformation point, with a total reduction of 50% or higher, followed by cooling, rolling or annealing. This is a method for producing a hot-rolled steel sheet with excellent formability, which is characterized by recrystallizing the steel sheet. The manufacturing method of the present invention will be explained in detail below. In addition, % in the following description is weight %. In the method of the present invention, C/12+N/14<1.2(Ti/48
The reason why the composition of the steel is limited by the conditional expression (+Nb/93) is that by satisfying this condition, the axial density ratio of {111}/{100} becomes high after recrystallization. Metallurgically, steel that satisfies this condition is Ti,
It is thought that carbon/nitride is formed by Nb, and this formation reduces solid solution C and N, which promotes the formation of a texture that is advantageous for deep drawability. Also, the reason why we limited the amount of C to 0.05% or less and the amount of N to 0.01% or less is because if more C or N is added, the processability will not only deteriorate, but also because Ti, This is because the required amount of Nb increases and it becomes expensive. In addition, other components of the steel of the present invention include components normally included in workable hot-rolled steel sheets, that is, Mn<
0.5%, Si<0.5%, P<0.03%, S<0.02%, Al
<0.1% etc. are added. Next, the rolling conditions are limited for the following reasons. As mentioned above, the texture of conventional hot-rolled steel sheets differs greatly in the thickness direction. According to recent experimental results conducted by the inventors, Ar ₃ transformation point or below (here Ar ₃ point is Ar ₃ (℃) = 916-509C (%) +
27Si (%) - 64 Mn (%))), by making the strain distribution in the thickness direction of the hot rolled steel sheet uniform, the center of the thickness of the hot rolled steel sheet after recrystallization treatment is It was confirmed that the difference between the texture of the steel and the texture near the surface became smaller and the deep drawability improved. In order to make the strain distribution uniform in the plate thickness direction, lubricated rolling that reduces the coefficient of friction is the most effective method, so it was decided to perform rolling while applying lubricating oil. In addition, the result of back calculation from the measured value of the advance ratio revealed that the above-mentioned effect of deep drawability becomes noticeable when the friction coefficient is approximately 0.2 or less. Next, the lower limit of the finishing temperature of rolling was limited to 500° C. because the deformation resistance during rolling increases and the rolling force must be increased, which is not practical. On the other hand, the reason for limiting the total rolling reduction to 50% or more in the temperature range of 500℃ or higher and lower than the Ar ₃ transformation point is that if sufficient rolling texture is not formed immediately after rolling, the axial density of {100} will decrease after recrystallization treatment. This is because it is relatively high and good deep drawability cannot be obtained. In addition, rolling in the above temperature range may be performed by heating to ₃ or more Ar points and as an extension of rolling at ₃ or more Ar points, or by heating at 500℃ or more.
It may be heated to a temperature range below Ar ₃ transformation and then rolled. Regarding the recrystallization treatment, recrystallization may be performed during the winding process including the cooling process on the runout table after rolling, or recrystallization may be caused by heating above the recrystallization temperature after winding. Good too. In the latter case, if necessary, performing some cold rolling prior to heating does not detract from the gist of the present invention and may contribute to improving the properties. Next, embodiments of the present invention will be described. Steel with the chemical composition shown in Table 1 is melted in a converter, and a slab manufactured by the usual process is approximately
It was heated to 1200°C, then rolled by continuous hot rolling under the conditions shown in Table 2, and then rolled up under the same conditions shown in Table 2. Some of the materials are then heated to 700℃.
Continuous annealing was performed for 4 hours or 3 minutes at 850°C. The values of these materials are shown in Table 2, and the axial densities of {111} and {100} at the surface and center are shown in Table 3. Hot-rolled steel sheets produced by the method of the present invention, No. 5, 9, 12, 13
As can be seen from Table 2, the value is 1.1 or more, and as is clear from Table 3, it shows a high axial density ratio of {111}/{100} at the surface and center of the plate thickness.

【table】

【table】

[Table] (Note) ○ indicates the steel of the present invention.
According to the method of the present invention, a texture with an extremely high axial density ratio of {111}/{100} parallel to the sheet surface is created not only at the center of the sheet thickness but also near the surface of the sheet, compared to hot rolled steel sheets produced by the conventional method. It also has a high value of 1.1 or higher, making it possible to manufacture steel sheets with properties comparable to cold-rolled steel sheets for deep drawing. Furthermore, due to the effect of lubricated rolling, the hot-rolled steel sheet produced by the method of the present invention has a surface quality close to that of cold-rolled steel sheet, and has the advantage that it can be used as a substitute for cold-rolled materials.

Claims (1)

[Claims] 1 C: 0.05% by weight or less, N: 0.01% by weight or less, and the amount of C and N added is equal to the amount of one or both of Ti and Nb added: C/12+N/14<1.2 (Ti/48+Nb/93) steel in the temperature range of 500℃ or higher and below the Ar ₃ transformation point, with a total reduction rate of 50% while being lubricated.
A method for producing a hot-rolled steel sheet with excellent formability, which comprises performing the above rolling process and recrystallizing it during the subsequent cooling, winding, or annealing process. 2. The method for producing a hot-rolled steel sheet with excellent formability according to claim 1, wherein the hot-rolled steel sheet is lubricated rolled in a temperature range of 500° C. or higher and lower than the Ar ₃ transformation point and with a friction coefficient of 0.2 or lower.