JPH0762179B2 - Method for producing A-1 killed cold-rolled steel sheet excellent in workability and surface properties - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an Al-killed cold-rolled steel sheet, and specific conditions for a finishing temperature of hot rolling of the Al-killed steel and a subsequent cooling pattern. The present invention provides a method for producing a steel sheet having workability comparable to box annealing by continuous annealing and having excellent surface properties.

[Prior Art] When using an Al killed steel as a material of a cold rolled steel sheet and a continuous annealing method for producing good workability, Japanese Patent Publication No.
After hot rolling as disclosed in 55-22533, 735 ℃
There is a method of winding at the above temperature. However, when winding is performed at such a high temperature, coarse crystal grains of 50 to 300 μm or more tend to be generated near the surface. When the coarse crystal grains are generated, the mechanical properties of the product plate become inhomogeneous, and the surface of the product plate becomes rough when processed by press molding or the like, which causes a problem that the appearance of the product is significantly deteriorated. It also causes a decrease in ductility.

Currently, as a method for preventing the formation of coarse crystal grains in this hot rolled sheet, for example, as disclosed in Japanese Patent Publication No. 58-5971, a steel sheet is placed on a runout table between the final stand of hot rolling and the winder. There is a method in which water cooling is stopped on the steel plate while the plate temperature is in the range of 880 to 840 ° C while running, or water is poured only on the lower surface of the steel plate. However, in order to improve the workability
In the steel having a composition in which the amount of C, Mn, and S is further reduced with respect to the composition shown in the example of the −5971 publication, the MnS number, Fe ₃ C
The decrease in the number of crystals and the decrease in the amount of solid solution C reduce the crystal grain growth suppressing action, so that the movement of crystal grain boundaries is facilitated and coarse grains are easily generated. Particularly, in the range of C ≦ 0.038% and S ≦ 0.020%, there is a problem that coarse grains are generated even if the cooling and winding conditions of Japanese Patent Publication No. 58-5971 are performed. Also,
When water is injected on the lower surface, problems such as coarse particles tend to be generated on the lower surface remain.

[Problems to be Solved by the Invention] Therefore, in the present invention, it is possible to suppress surface roughening during processing of a steel sheet produced by continuous annealing using an Al-killed steel containing C: 0.01 to 0.038%, and to provide excellent workability. The purpose is to produce cold rolled steel sheet.

[Means for Solving the Problems] The present inventors heat the slab to a specific temperature range during hot rolling of Al-killed steel, terminate hot rolling in a specific finishing temperature range, and then wind it. By cooling the specific temperature range up to a specific cooling rate at a specific cooling rate, even at a coiling temperature of 650 ° C or higher and 800 ° C or lower, coarse grains of the hot-rolled sheet are not generated, and roughening does not occur even after continuous annealing. Moreover, they have found that a cold rolled steel sheet having good workability and aging characteristics equivalent to those of the box annealing method can be manufactured. By utilizing this, it is possible to suppress the surface roughness during the processing of the steel sheet produced by continuous annealing using Al killed steel and to produce the cold rolled steel sheet having excellent workability.

The gist of the invention is as follows.

% By weight: C: 0.010 to 0.038%, Si: 0.040% or less, Mn: 0.05
~ 0.25%, P: 0.080% or less, S: 0.002-0.020%, Al: 0.020 ~
A slab containing 0.10%, N: 0.0060% or less and the balance of Fe and unavoidable impurities is heated to 1000 to 1200 ° C.
840 ~ after hot rolling at a finishing temperature of ℃ to 960 ℃
Cool down to a temperature range of 870 ℃ at a cooling rate of 25 ℃ / S or more and 100 ℃ / S or less, and from this temperature to a temperature range of 720-800 ℃, 5
Cool at a cooling rate of ℃ / S or more and 20 ℃ / S or less, and then 650 ~ 8
A method for producing an Al-killed cold-rolled steel sheet excellent in workability and surface property aging characteristics, which comprises rolling at 00 ° C, cold rolling at a rolling reduction of 60% or more, and then performing continuous annealing.

First, the reasons for limiting the chemical composition of steel to which the method of the present invention is applied will be described.

When C is less than 0.010%, aging deterioration after continuous annealing is large, which is not desirable. If it exceeds 0.038%, the workability of the product deteriorates. Therefore, the C content is limited to 0.010% to 0.038%.

Although there is no problem with a small amount of Si, if the content of Si increases, the workability decreases. Therefore, it must be 0.040% or less.

Mn is a component necessary to prevent hot brittleness, but 0.
If it is less than 05%, FeS is generated and its effect is not obtained. Also, 0.25
If it exceeds%, the deep drawability deteriorates. Therefore, set the amount of Mn to 0.
Limited to 05-0.25%.

P is an element that does not significantly affect the aging characteristics, but if its content increases, it lowers the workability. Therefore, the lower the content, the better the upper limit must be 0.080%.

If S is less than 0.002%, the amount of MnS produced is small, and the crystal grains of the hot-rolled sheet are likely to become coarse, causing rough skin. If it exceeds 0.02%, it causes hot brittleness. Therefore, the amount of S is 0.00
Limited to 2 to 0.020%.

Al is an element necessary for controlling the amounts of oxygen and nitrogen in steel, and at least 0.020% is necessary for precipitating N as AlN after winding the hot-rolled sheet. But 0.10%
If it exceeds, workability is deteriorated. Therefore, 0.020 ~
Limited to 0.10%. In particular, the range of 0.035 to 0.085% is desirable in order to coarsen the size of precipitated AlN to the extent that roughening does not occur and to improve workability.

In addition, the precipitated AlN also deteriorates the workability, so the amount is preferably small, and therefore the amount of N must be 0.0060% or less.

The present inventors produced hot-rolled sheets with various slab heating temperatures and various temperatures T1, T2, T3, and cooling rates CR1, CR2, CR3 after hot rolling shown in FIG. 1 for steels of various compositions. Then, this was subjected to cold rolling and continuous annealing, and the workability and surface properties of the cold rolled steel sheet were examined. Typical results are shown in Table 3 and FIGS. 3 to 5.

Table 3 shows that a slab having the composition shown in Table 1 was heated to 1100 ° C,
Hot rolling is performed at a finishing temperature (T1) of 920 ° C, then T2, T3, CR1, CR2, CR3 shown in Fig. 1 are performed as shown in Table 2, followed by winding at 720 ° C and It shows the workability and surface properties of the steel sheet after continuous annealing when cold rolling at 80% and continuous annealing with a heat pattern as shown in FIG. 2 were performed.

FIG. 3 shows that a slab having the composition shown in Table 1 is heated to 1100 ° C.
Hot rolling was carried out at various finishing temperatures of 870 to 980 ° C, and then T2, T3, CR1, CR2 and CR3 shown in Fig. 1 were cooled under the conditions as shown in C of Table 2, followed by 720 It shows the effect of the finishing temperature on the workability and surface properties when the coil is wound at 80 ° C. and cold-rolled at 80% and the heat pattern as shown in FIG. 2 is continuously annealed.

FIG. 4 shows that a slab having the composition shown in Table 1 is reheated to a temperature of 950 ° C. to 1350 ° C., hot rolling is performed at a finishing temperature of 920 ° C., and then T2, T3 shown in FIG. , CR1, CR2, CR3 were cooled under the conditions shown in C of Table 2, followed by winding at 720 ° C and cold rolling at 80%, and continuous heat pattern as shown in Fig. 2. It shows the effect of the heating temperature of the slab on the workability and surface texture when annealed.

Fig. 5 shows that a slab having the composition shown in Table 1 is reheated to 1100 ° C and hot-rolled at a finishing temperature of 920 ° C, after which CR1 = 50 ° C /
Cooled under the condition of S, T2 = 870 ℃, and then to 750-850 ℃ for 15
Chilled at ℃ / S and wound at this cooling end temperature, and
CR1 = 50 ° C / S, T2 = 870 ° C, CR2 = 15 ° C / S, T3 = 750 ° C, CR3 = 6
When cooled at 0 ° C / S and wound at 600 ° C to 850 ° C, 80% cold rolling was performed, and continuous annealing of the heat pattern as shown in Fig. 2 was performed. It shows the effect of temperature on workability and surface properties.

From the above survey results, in order to make the workability of the steel sheet after continuous annealing and the good surface properties compatible with each other, the slab is 1000 to 1200.
After being heated to ℃ and hot-rolled at a finishing temperature of 900 ℃ to 960 ℃, 25 ℃ / S or more 100 ℃ / S in the temperature range of 840 ~ 870 ℃
Cool at the following cool speed, and then cool from this temperature to the temperature range of 720-800 ℃ at a cool speed of 5 ℃ / S or more and 20 ℃ / S or less,
It turns out that it is then necessary to wind at 650-800 ° C.

Similar results were obtained even when the composition, the winding temperature, the annealing temperature of continuous annealing, the cooling rate and the overaging treatment conditions were changed within the scope of the present invention in the above investigation.

The hot-rolled sheet produced under the above various conditions was sampled, and the structure was observed with an optical microscope. While it has a grain structure,
If one or all of the composition, the finishing temperature of hot rolling or the cooling condition after hot rolling is out of the scope of the present invention, it may be a mixed grain structure in which fine grain regions are seen near the surface layer. all right.

The reduction ratio of cold rolling is normally 60% or more, but in order to develop the (111) texture after continuous annealing and to improve the deep drawability, cold rolling under high pressure of 70% or more is required. Rate is preferred.

For continuous annealing, as is usually done, a continuous annealing method for cold-rolled steel sheet with overaging treatment to improve the aging characteristics after recrystallization annealing may be used, but in order to obtain higher values, the annealing temperature Is preferably 750 ° C. or higher. Also,
50 to 25 after recrystallization annealing for better aging properties
It is better to perform overaging by cooling to 200 to 450 ° C at a cooling rate of 0 ° C / S. The temperature history of overaging treatment is 200-450.
If the temperature is within the range of ° C, either the isothermal holding process or the temperature changing process with the aging time may be performed. In order to use MnS in steel more effectively as Fe ₃ C precipitation nuclei and to finish overaging in a short time, the primary cooling end temperature is set to 350 ° C or lower, and then reheated to 320 to 450 ° C. Overaging treatment that lowers the overaging temperature with time and finishes overaging at 200 to 300 ° C is desirable.

As described above, the present invention can provide the cold rolled steel sheet with excellent surface properties and workability by providing a specific cooling pattern from hot rolling to winding.

Hereinafter, examples of the present invention will be shown together with comparative examples.

[Examples] Example-1 Steels having chemical compositions shown in Table 4 were subjected to hot rolling conditions and subsequent cooling conditions as shown in Table 5, respectively. Samples A, B,
C, D, E, F, G and H are within the composition range of the present invention, and are sample I,
Any of the components of J, K, L, M, N, O and P is outside the scope of the present invention. The hot rolling conditions and the subsequent cooling conditions are within the scope of the present invention. 80% of these hot rolled plates
Was subjected to cold rolling and subjected to continuous annealing in a heat pattern as shown in FIGS. 2 and 6. The cold-rolled steel sheet obtained as a result was subjected to a tensile test using JIS No. 5 tensile test pieces to investigate surface properties, yield strength, and elongation.
The results of the material investigation are shown in Table 6. It can be seen from Tables 4 to 6 that the components and conditions falling within the scope of the present invention are superior in both surface properties and processability to those outside the scope of the present invention.

Example-2 Steels having the chemical composition shown in Table 7 were subjected to hot rolling conditions and cooling conditions as shown in Table 8, respectively. The composition of the steel used for the test is within the scope of the present invention. Further, among the hot rolling conditions and the subsequent cooling conditions, ~ is within the range of the present invention, and any of ~ 16 is outside the range of the present invention. Regarding the cold rolled steel sheet obtained as a result, JI
A tensile test was carried out using a No. S5 tensile test piece to investigate surface properties, yield strength, and elongation. Table 9 shows the results of the material inspection.

From Tables 7 to 9, it can be seen that those whose components and conditions are within the scope of the present invention are superior in both surface properties and processability to those outside the scope of the present invention.

[Effects of the Invention] As described above, the method of the present invention heats a slab to a specific temperature range when hot rolling an Al-killed steel, finishes hot rolling in a specific finishing temperature range, and then 650 by cooling the temperature range at a specific cooling rate
Cold rolling that does not generate coarse grains of hot rolled sheet even at coiling temperature of ℃ or more and 800 ° C or less, does not cause surface roughening even after processing after continuous annealing, and has good workability and aging characteristics equivalent to the box annealing method This is the method by which steel can be produced.

[Brief description of drawings]

FIG. 1 is a diagram showing a cooling pattern on a runout table after hot rolling, FIGS. 2 and 6 are diagrams showing a heat pattern of continuous annealing, and FIG. 3 is a surface texture at a finishing temperature. Fig. 4 shows the effect on workability, Fig. 4 shows the effect of heating temperature of the slab on the surface properties and workability of the steel sheet after continuous annealing, and Fig. 5 shows continuous annealing at coiling temperature. It is the figure which showed the influence which acts on the surface property and workability of the subsequent steel plate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koji Fujii, 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Prefecture Nippon Steel Co., Ltd. Hirohata Works (72) Inventor, Ryoji Akazawa 1 Fuji-cho, Hirohata-shi, Himeji-shi, Hyogo Address: Nippon Steel Co., Ltd. inside the Hirohata Works (72) Inventor Yuzo Nishimoto Himeji City, Hyogo Prefecture Fujimachi, Hirohata-ku 1 No. 1 Nippon Steel Co., Ltd. inside the Hirohata Works (72) Ino Tonohiro Himeji City, Hyogo Prefecture 1st Fuji-machi, Hirohata-ku Shin Nippon Steel Co., Ltd. inside Hirohata Works

Claims (1)

[Claims] 1. In wt%, C: 0.010 to 0.038% Si: 0.040% or less Mn: 0.05 to 0.25% P: 0.080% or less S: 0.002 to 0.020% Al: 0.020 to 0.10% N: 0.0060% or less Then, the slab consisting of the balance Fe and unavoidable impurities is heated to 1000 to 1200 ° C and hot-rolled at a finishing temperature of 900 ° C to 960 ° C, and then 25 ° C to a temperature range of 840 to 870 ° C.
/ S or more and 100 ℃ / S or less at a low cooling rate, and then from this temperature to a temperature range of 720 to 800 ℃, cool at 5 ℃ / S or more and 20 ℃ / S or less, then 650 to 800 Winding at ℃, rolling reduction 60
% Or more, after cold rolling, continuous annealing is performed, and a method for producing an Al-killed cold-rolled steel sheet having excellent workability and surface properties.