JPS6041689B2 - Manufacturing method of low yield point cold rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a new method for producing a low-descending-point cold-rolled steel sheet using a hot-rolling-cold-rolling-annealing process.

Since cold-rolled steel sheets are formed into various parts by press forming or the like, the drop point or yield strength (hereinafter referred to as drop point) must be sufficiently low to facilitate processing and forming.

The precipitation point decreases as the grain size increases and as the amount of solute C and N decreases. Many techniques have been provided for implementation in the extension process. For example, hot-rolled high-temperature winding (Tokuko 49-196 Gin, Tokko 50-1341
This is due to coarsening of crystal grains (see Japanese Patent Publication No.

However, such high-temperature winding has the disadvantages of thickening the scale of the hot-rolled steel sheet and reducing the subsequent pickling efficiency, so it is not possible to lower the falling point of the product without adopting high-temperature winding. Ideally if possible. Another idea was to use the strain during hot rolling to cause grain growth and enlarge the crystal grains, but when this was actually implemented industrially, the crystal grains became mixed and a stable product was obtained. The situation was hopeless. The present invention provides a completely new technique for coarsening the grains of final products carried out in the hot rolling process, and is particularly effective in reducing solute C when annealing is carried out by continuous annealing. It also includes the law.

In the present invention, when hot rolling a cold-rolled steel sheet material containing 0.005 to 0.03% C, the hot rolling is finished in a temperature range of 600 to 820'C, and then heated to 300 to 4500 C for 12 to 8
The gist is a method for producing a cold-rolled steel sheet with a low descending point, which is characterized by rapidly cooling at a cooling rate of 50 C/sec or more, winding it into a coil at that temperature, allowing it to cool, and then cold-rolling and annealing it. be.
The present invention will be explained in detail below.

First, the present invention focused on utilizing strain addition and crystal grain growth during hot rolling.

Therefore, the hot rolling finishing temperature' was set to 820°C or lower. 820
This is because if the hot rolling is finished at a temperature exceeding .degree. C., the strain will recover during hot rolling and sufficient strain cannot be applied.

On the other hand, if the hot rolling finishing temperature is less than 6000C, sufficient strain can be applied, but the temperature is too low and sufficient grain growth will not occur, so the hot rolling finishing temperature must be 600°C or higher. Furthermore, when the C content of the material exceeds 0.03%, grains with extremely large and extremely small grain sizes coexist (mixed grains),
Since the crystal grain size does not increase uniformly, the C content is 0.
It is necessary to keep it below 0.03%. If the grain size of the hot rolled sheet is increased in this manner, the grain size of the cold rolled steel sheet will also be increased. Another factor that controls the grain size of cold-rolled steel sheets is the cleanliness of the hot-rolled steel sheet and the distribution of carbides.

As a result of detailed research, the present inventor found that the lower the solid solution C in a hot rolled steel sheet and the higher the cleanliness of the base steel, and the sparser the distribution of carbides, the larger the crystal grain size of the cold rolled steel sheet. Confirmed. Although the hot rolling high temperature winding method is one method for industrially implementing this, it also has the drawbacks mentioned above, so the present inventors invented a new technology using low temperature winding, which is the complete opposite.
That is, after the hot rolling is completed, the cooling rate is 12 to 85°C/second to 300°C.
It is rapidly cooled to ~4500C, then wound around a coil at that temperature and slowly cooled.

This is a condition for maintaining an appropriate degree of supersaturation of C, controlling the number of carbide precipitation nuclei, and quickly precipitating solid solution C as carbide to clean the steel base. When the cooling rate exceeds 85° C/sec, the degree of supersaturation of C becomes high and the distribution of carbides becomes too dense, resulting in a decrease in the crystal grain size of the cold rolled steel sheet. Furthermore, if the cooling rate is less than 17 C/sec, the degree of supersaturation of C is too low, and it takes too much time for solid solution C to precipitate as carbide after winding into a coil. For this reason,
After hot rolling, it is necessary to maintain the quenching rate from 300 to 450°C at 12 to 85°C/sec. In addition, if the crystal grain size is small, C will be trapped in the grain boundaries during quenching, and the degree of supersaturation of C will be small, so this appropriate cooling rate range is set in relation to the appropriate value of the hot rolling finishing temperature. It is something. If the quenching end temperature exceeds 450° C., the solid solubility limit of C is high, so the degree of supersaturation of C cannot be sufficiently increased, and the efficiency of pickling is also poor. On the other hand, if the quenching end temperature is less than 300° C., the diffusion rate of C is slow, so carbides are precipitated, and the steel substrate is not cleaned sufficiently. In this case, if the C content of the material is less than 0.005%, the steel base is in a substantially cleaned state even without applying the present invention. In addition, if the carbides are distributed sparsely in the hot-rolled sheet using the above method, and then a so-called continuous annealing process of rapid annealing and rapid cooling and one-pass aging is performed after cooling, a considerable amount of the carbides precipitated in the hot-rolled sheet during annealing can be removed. Since the carbide remains undissolved and serves as a hot spot for C precipitation during overaging, the time for overaging treatment can be shortened. The present invention is effective regardless of the steel type, and is effective in both batch annealing and continuous annealing.

Because extremely low-temperature winding is used, the scale of the hot-rolled sheet is thin, so even if it is cold-rolled without pickling before cold-rolling, there is less chance of damage to the rolls, etc. It is also possible to efficiently pickle after cracking. In carrying out the present invention, hot rolling finishing at 600 to 700°C is preferred in order to obtain more uniform grain growth, and when continuous annealing is applied, dissolution of carbides precipitated in the hot rolled sheet is preferred. In order to avoid this, it is desirable that the residence time in the temperature range of 600° C. or higher during annealing is 4×2 or less.

Further, in the case of A1 killed steel, it is even more preferable to set the hot rolling heating temperature to 1100° C. or lower to sufficiently promote the precipitation of AlN, since slow aging of the final product can also be obtained at the same time. Example 1 C: 0.02%, Mn: 0.25%, S: 0.012%
, SOlAl: 0.032%, N: 0.0038% A1 killed steel continuous cast slab was heated at a low temperature of 1030°C, and the finish temperature was 2.8 TmI! at a finishing temperature of 500-880°C! L
Hot rolled to a thick thickness and then cooled to 350°C at a cooling rate of 35°C/sec.
It was cooled to C, then wound into a coil at that temperature, allowed to cool to room temperature, pickled, and cold rolled at a rolling reduction of 60%.

Batch annealing of this cold-rolled plate at 700'C x 3 hours,
As a typical example of continuous annealing, annealing is performed at 7100C for 20 seconds at a heating rate of 40°C/sec, followed by 450℃ annealing at 7100C for 20 seconds.
An overaging treatment was performed at ℃ for 1 minute. The residence time in the temperature range of 600° C. or higher during continuous annealing was 27 seconds. Thereafter, 0.8% temper rolling was performed to prepare a JIS No. 5 test piece, and a tensile test was performed to determine the yield strength at 0.2% strain as the descent point. The results are as shown in FIG. 1, and it was confirmed that the falling point was lower in the finishing temperature range of 600 to 820°C. Example 2 A slab with the same components as in Example 1 was heated to 1100°C and then finished at a finishing temperature of 685°C at a temperature of 1.677!77! Hot rolled to a thick thickness and rapidly cooled at a cooling rate of 42 to 480 C/sec to 100 to 590
! Wind it into a coil with C, let it cool to room temperature, and then add 5% HC.
Pickling was performed at 40° C. in an aqueous solution, and the time required for pickling was measured.

Thereafter, 60% cold rolling was performed and continuous annealing was performed under the same conditions as in Example 1, and the descending point was measured in the same manner. The results are shown in Figure 2. When the annealing temperature is 300 to 450°C, the time required for pickling is short and the product has a low precipitation point, clearly demonstrating the effects of the present invention. As explained above, according to the present invention, a low-descending-point cold-rolled steel sheet can be efficiently manufactured.

[Brief explanation of the drawings] Figure 1 is a chart showing the relationship between hot rolling finishing temperature and descending point, and Figure 2 is a chart showing the relationship between hot rolling winding temperature, pickling time, and descending point. be.

Claims (1)

[Claims] 1 C: In hot rolling a cold rolled steel sheet material containing 0.005 to 0.03%, the hot rolling is finished at a temperature range of 600 to 820°C, and then heated to a temperature of 300 to 450°C, 1
A method for producing a cold-rolled steel sheet with a low yield point, characterized in that it is rapidly cooled at a cold-rolling speed of 2 to 85° C./sec, wound into a coil at that temperature, allowed to cool, then cold-rolled, and further annealed.