JPH045733B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing Al-killed cold rolled steel sheets, in which a continuously cast slab is kept in a specific temperature range for a specific period of time and then directly rolled and continuously annealed. The present invention provides a method for improving the deep drawability of cold rolled steel sheets. (Prior art) Conventionally, when producing Al-killed steel sheets for deep drawing by continuous annealing, it is common to heat cold slab slabs to 1050 to 1300°C and then hot-roll and cold-roll them. However, heating a cold piece slab to a high temperature in this way requires a large amount of energy. On the other hand, from the viewpoint of energy saving, a direct hot rolling process is being developed in which continuously cast slabs are directly hot rolled without being turned into cold pieces. Prior art techniques that improve the workability of cold-rolled steel sheets manufactured by a direct rolling process as in the present invention include JP-A-60-43432 and JP-A-60-228617.
There is one mentioned in the publication. However, the former relates to a method for manufacturing a material for box annealing, and is different from the present invention, which manufactures a cold rolled sheet by continuous annealing. Furthermore, the latter method relates to a method of manufacturing a cold-rolled sheet by continuous annealing, but it is aimed at improving aging characteristics and does not have sufficient deep drawability. As a steel plate for deep drawing, it is necessary to have high value and ductility, and low yield strength. However, cold-rolled sheets manufactured by this method have finer precipitates compared to those manufactured by reheating cold slabs, and these properties are inferior with low-temperature annealing, so high-temperature annealing is essential. . However, high-temperature annealing tends to cause hearth roll build-up, poor shape, etc. during sheet threading, resulting in poor sheet threading properties, resulting in surface defects and increased costs. Currently, in order to avoid this problem, ultra-low carbon steel with added Ti or Nb is used for deep drawing steel sheets, and low-temperature annealing is performed. This will result in a significant increase in material costs compared to As described above, the energy savings achieved by the direct rolling process are currently not fully utilized due to the material costs and high temperature annealing. (Problems to be Solved by the Invention) Therefore, the present invention aims to use low-carbon Al-killed steel to produce a cold-rolled steel sheet for deep drawing through a direct rolling process and continuous low-temperature annealing. (Means for Solving the Problems) The present inventors have discovered that the above object can be achieved by hot rolling after giving a specific temperature history to the slab immediately after continuous casting. That is, the gist of the present invention is that C: 0.010-0.04%, Mn: 0.10-0.25%, S:
0.020% or less, Al: 0.010-0.080% and N:
When continuously casting steel containing 0.0030% or less and the balance consisting of Fe and unavoidable impurities to form a slab, the slab after solidification is T ₁ (°C) = {12800/(6.9−) for the above amount of S. log(%S))}-303 or less than 1050°C or more.
A method for producing a cold-rolled steel sheet with excellent deep drawability, which is characterized by directly hot-rolling after staying for at least a minute, coiling at a temperature of 650° C. or higher, followed by cold-rolling and continuous annealing. First, the reason for limiting the chemical composition of steel to which the method of the present invention is applied will be explained. If C is less than 0.010%, it is not desirable because aging deterioration after continuous annealing is large. Moreover, if it exceeds 0.040%, deep drawability deteriorates. Therefore, the amount of C should be 0.010~
Limited to 0.040%. Mn is a necessary component to prevent hot embrittlement, but if it is less than 0.10%, FeS will be generated and there will be no effect. Moreover, if it exceeds 0.25%, deep drawability deteriorates. Therefore, the amount of Mn was limited to 0.10-0.25%. If S exceeds 0.020%, it causes hot embrittlement, so it must be less than this. In addition, in order to utilize MnS as precipitation nuclei of Fe ₃ C during over-aging during continuous annealing, it is preferable that S is contained in an amount of 0.004% or more. Al needs to be at least 0.010% in order to precipitate N as AlN after deoxidation and winding. However, if it exceeds 0.080%, processability deteriorates. Furthermore, since AlN also deteriorates workability, it is better to have less AlN, and the amount of N was set to 0.0030% or less. The present inventors melted steel within the above composition range,
After that, it was cast, held isothermally at various temperatures, and continuously cooled in various temperature ranges, then hot-rolled, cold-rolled, and continuously annealed at 800°C to the recrystallization temperature, and the material properties were investigated. Representative results are shown in FIGS. 1 and 2. Figure 1 shows 0.010% and 0.020% S by weight ratio.
Continuously cast slab immediately after solidification containing the amount [{12800/(6.9~log(%S))}-303]℃~
This figure shows the effect on the value of slab residence time when hot rolling, cold rolling and continuous annealing are performed after residence in the temperature range of 1050°C. Figure 2 is
This figure shows the effect on the retention temperature when steel containing 0.010% and 0.020% S is isothermally retained at each temperature for 20 minutes or more and then hot-rolled, cold-rolled, and continuously annealed. From these results, it was found that in order to obtain a steel plate with good deep drawability, the slab temperature history must satisfy the following conditions. That is, for the above amount of S, [{12800/(6.9-log(%S))}-303]℃ or less
The slab is kept in a temperature range of 1050℃ or higher for 20 minutes or more. Further, within this temperature range, any one of isothermal holding, continuous cooling, reheating after the temperature has fallen to around 1050°C may be used, or a combination of these may be used. Although there is no particular upper limit to the residence time, it is usually preferable to stay for 20 to 120 minutes because a residence time of 120 minutes or more results in higher energy costs than reheating the cold slab. When the precipitates of cold-rolled sheets were investigated using a transmission electron microscope, the number of fine MnS was small in the slabs that followed the slab temperature history of the present invention, but the number of fine MnS was large in the slabs that underwent a temperature history that deviated from this. Therefore, it is thought that deep drawability is greatly influenced by these fine MnS particles. Furthermore, the slab used in the manufacturing method of the present invention is
It can contain Si, P and other elements. The winding temperature needs to be 650°C or higher in order to sufficiently precipitate AlN, but it may cause coarsening of AlN and
In order to coagulate Fe ₃ C, the temperature is preferably in the range of 680 to 780°C. The cold rolling reduction ratio may be the same as usual, but
In order to develop the 111 texture after continuous annealing and to improve deep drawability, a cold rolling reduction under high pressure of 70% or more is preferable. Next, the continuous annealing conditions will be described. The heating temperature needs to be higher than the recrystallization temperature. If hot rolling, cold rolling, etc. have been performed according to the present invention, sufficient deep drawability can be obtained even by cold annealing. Therefore, currently the steel is mainly heated to 700 to 830°C, but deep drawability is not impaired even if annealing is performed at a higher temperature than this temperature. In addition, in order to improve the aging characteristics, the primary cooling rate should be 50℃/sec or higher, and then the cooling rate should be 250℃/sec or higher.
It is best to carry out overaging in a temperature range of 350°C. this is,
Due to the slab temperature history conditions mentioned above, the distribution of MnS in the steel is just right for it to act as precipitation nuclei for Fe ₃ C during overaging, and this overaging condition makes this effect particularly effective. It is from. The cold-rolled sheet manufactured under the above conditions has excellent deep drawability compared to conventional directly hot-rolled materials. Example Steel having the composition shown in Table 1 was continuously cast into a slab. This was made into a hot rolled sheet under the conditions shown in Table 2. Samples A, B, C, D, and E are within the scope of the present invention; for F, G, H, and I, the underlined conditions are outside the scope of the present invention. The hot-rolling finishing temperature was 900°C and it was rolled to 4.0mm.
After pickling this hot-rolled sheet, it was cold-rolled to a thickness of 0.8 mm at a rolling reduction rate of 80%, and the cold-rolled sheet was further heated to a temperature of 800℃ for 1
Continuous annealing was performed under the condition of holding for minutes. At this time, the primary cooling rate was 100℃/sec, and the overaging was 55℃ at 320℃.
I went for a minute. The resulting cold-rolled steel sheet was subjected to a tensile test using a JIS No. 5 tensile test piece, and the yield strength, elongation, and aging index were measured. The results of these measurements are shown in Table 2. This result shows that if any of the slab retention temperature, residence time, and coiling temperature is out of the range of the present invention, a material with sufficient value, yield strength, and/or ductility for deep drawing can be obtained. However, if these conditions are within the scope of this invention, the material has sufficient value, yield strength, and ductility to perform deep drawing, and the aging properties are also sufficient. It's summery. When the MnS distribution in cold-rolled steel sheets was investigated using a transmission electron microscope, the following findings were made. When the slab holding temperature range and residence time are within the range of the present invention (samples A, B, C, D, E), as shown in Table 3-A, B, C, D, E.
There are few fine MnS particles smaller than 0.05μ. However, when either the slab holding temperature range or residence time is outside the range of the present invention (Samples F, G, H)
is fine MnS as shown in Table 3-F, G, H.
becomes the main subject. In this case, the total number of MnS in the steel is also very large compared to the sample within the scope of the invention with less fine MnS. In other words, by giving the slab after continuous casting the temperature history of the present invention, fine MnS
The number of MnS decreases, which leads to a decrease in the total number of MnS and improves deep drawability. In No. 1, the slab temperature history is within the range of the invention, but the coiling temperature is lower than the range of the invention, so AlN precipitation is insufficient and the deep drawability is degraded.

【table】

【table】

[Table] (Effects of the invention) According to the present invention, Al-killed steel is used to produce cold-rolled steel sheets, and even when subjected to direct hot rolling process and low-temperature continuous annealing, products with good deep drawability and aging properties can be obtained. can do. This technology is extremely valuable economically as it can reduce the cost of the raw material, hot rolling, and continuous annealing.

[Brief explanation of drawings]

FIG. 1 shows the influence of samples with different amounts of S on the value of residence time in the slab. Figure 2 shows the influence on the value of slab retention temperature.

Claims (1)

[Claims] 1. C: 0.010 to 0.04%, Mn: 0.10 to 0.04% by weight
When continuously casting steel containing 0.25%, S: 0.020% or less, Al: 0.010 to 0.080%, and N: 0.0030% or less, with the balance consisting of Fe and unavoidable impurities to form a slab, the slab after solidification is For the above amount of S, T ₁ (℃) = {12800/(6.9-log(%S))}-303 In the temperature range of 1050℃ or higher,
A method for producing a cold-rolled steel sheet with excellent deep drawability, characterized by directly hot-rolling the steel sheet after staying for at least a minute, coiling at a temperature of 650°C or higher, followed by cold-rolling and continuous annealing.