JP2002035805A - Method for manufacturing thin steel plate excellent in surface properties - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel plate with extremely less surface defect originating in cracks caused in the hot rolling, even in the straight forward rolling process and its manufacturing method. SOLUTION: A steel material containing C: 0.2% or less, Si: 1% or less, Mn: 0.4 to 2.0%, P: 0.8% or less, O: 0.005% or less, S: 0.03% or less, N: 0.01% or less, Sn: 0.004% or less (all in mass percent) is east in a continuous casting machine, and then the steel material is held for more than five minutes in the surface temperature of the slab: 950 to 1150 degrees C and by avoiding the temperature range wherein the surface temperature of the cast slab reaches more than 750 degree C and less than 850 degree C, a bending deformation is performed so that the distortion speed on the surface shows 1×10-4 to 1×10-1 and after a draft in the width direction in the roughing roll, plural drafts in the plate thickness direction are performed at the distortion speed of 1/second or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thin steel sheet having less surface defects due to cracks during hot rolling, and particularly to hot rolling after continuous casting, cooling a slab and reheating, or directly. The present invention also relates to a method for producing a thin steel sheet with less occurrence of cracks on the surface during hot rolling even in a direct rolling process for performing a hot rolling.

[0002]

2. Description of the Related Art When a thin steel sheet is manufactured by a direct-feed rolling process that directly performs from continuous casting to hot rolling, the occurrence of surface defects due to cracks in hot rolling often becomes a problem. This type of cracking occurs in carbon steel containing 0.2% or less of C.

It is considered that the cause of cracks in hot rolling is that cracks on the slab surface or under the slab surface caused by tensile stress when bending deformation is applied to the slab during continuous casting have been extended by rough rolling. ing.

In particular, in the direct rolling process, slab maintenance is not possible, and cracks generated in the slab remain in products such as hot-rolled steel strips and cold-rolled steel strips as they are, which significantly lowers the product yield. Was.

[0005] To deal with such a problem, Japanese Patent Publication No.
No. 75 discloses a cooling process after solidification of 1200 to 10
A hot rolling method is disclosed in which after a slab is held in a temperature region of 50 ° C. for a specific time, rolling is started at an Ar3 point or higher to prevent surface cracks of a billet.

[0006]

However, when the present technology is applied to an actual machine, the occurrence of surface defects cannot be sufficiently suppressed, and cracks occur in the direct rolling process due to the temperature conditions of hot rolling. However, in the method in which the slab is cooled and then reheated after casting, surface defects are generated depending on the reheating conditions. Incidentally, cracks in hot rolling are generally attributed to red hot embrittlement.

In view of the above, the present invention is directed to a direct-rolling rolling process in which continuous casting is performed directly from hot rolling to a continuous casting machine, and a process in which a slab is cooled and re-heated to perform rough rolling. Thin steel sheet excellent in surface properties with extremely few surface defects caused by cracks, especially TS270MP
An object of the present invention is to provide a method for producing a steel sheet of class a or higher.

[0008]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied the composition of steel materials and various conditions from casting by a continuous casting machine to hot rolling. After casting the steel, hold it for a certain period of time, then perform bending deformation while controlling the surface temperature, strain rate on the surface,
It has been found that, when the rolling conditions in the rough rolling are specified, a thin steel sheet excellent in surface properties with extremely few surface defects caused by cracks in hot rolling can be obtained. Even if cold rolling and surface treatment are performed after hot rolling, the effects of the present invention are not impaired, and the thin steel sheet includes a cold-rolled steel sheet and a surface-treated steel sheet.

The present invention has been made based on these findings and further studied. That is, the present invention provides: A method for producing a thin steel sheet having excellent surface properties, comprising the following steps.

(1) In mass%, C: 0.2% or less, S
i: 1% or less, Mn: 0.4 to 2%, P: 0.08% or less, O: 0.005% or less, S: 0.03% or less, N:
Casting a steel containing 0.01% or less and Sn: 0.004% or less by a continuous casting machine.

(2) After casting, the temperature of the slab surface: 950-
A step of holding at 1150 ° C. for 5 minutes or more.

(3) The surface temperature of the slab is less than 850 ° C.
A step of performing a bending deformation such that the strain rate on the surface is 1 × 10 ⁻⁴ to 1 × 10 ⁻¹ , avoiding a temperature range exceeding 750 ° C.

(4) In the rough rolling, after reduction in the width direction,
A step of reducing the thickness in the thickness direction at least once at a strain rate of 1 / second or more.

2. The casting speed in the continuous casting machine was set to 1.
3. The method for producing a thin steel sheet having excellent surface properties according to item 1, which is at least 5 m / min.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention specifies the steel composition and various conditions from continuous casting in a direct rolling process to hot rolling.

1. Steel composition C C is added to secure the strength of the steel sheet. If it exceeds 0.20%, the workability deteriorates, so the content is made 0.20% or less. A preferable C amount for reducing surface defects is 0.10% or less, more preferably 0.06% or less.

FIG. 1 shows the effect of the amount of carbon on hot workability. As the amount of C decreases, the degree of deterioration of the aperture value at around 800 ° C. decreases, and the hot workability is improved.

Si Si is a solid solution strengthening element and is added to secure the strength of the steel sheet. If it exceeds 1%, the surface properties will deteriorate,
1% or less.

Mn Mn is an element that is effective in suppressing the occurrence of cracks on the slab surface or under the slab surface during continuous casting, and is added. 0.
If the content is less than 4%, the effect cannot be obtained. On the other hand, if the content exceeds 2%, the workability deteriorates.
(0.4% or more and 2% or less).

PP is a harmful element that easily causes cracks on the slab surface or under the slab surface during continuous casting. When the content exceeds 0.08%, cracks are remarkably generated on the slab surface or under the surface layer of the slab during continuous casting, and the frequency of occurrence of cracks during hot rolling increases. In addition, it is more preferably at most 0.02%.

O 2 O is set to 0.005% or less in order to suppress the occurrence of cracks on the slab surface or under the slab surface during continuous casting.

S S is a harmful element that tends to cause cracking on the slab surface or under the slab surface during continuous casting, and induces cracking during hot rolling even if the slab has no crack.

If the content exceeds 0.03%, cracks are remarkably generated on the slab surface or under the surface layer of the slab during continuous casting, and the frequency of cracks during hot rolling increases.
03% or less. Note that the content is more preferably 0.01% or less, and still more preferably 0.005% or less.

NN is set to 0.0% to suppress the occurrence of cracks during hot rolling.
1% or less. Sn Sn is a very harmful element that easily causes cracks on the slab surface or under the slab surface layer during continuous casting. If it exceeds 0.004%, the tendency is particularly strong, so that Sn is 0.004% or less. And

The basic composition of the steel according to the present invention is as described above. However, in order to further improve the strength characteristics, if necessary, T
One or more of i, Nb, V, and Mo, and
01-0.2%, Cu: 0.02-2%, B: 0.00
One or two or more of 0.01 to 0.01% can be added.

2. Manufacturing conditions: Step of maintaining the slab surface at 950 to 1150 ° C. for 5 minutes or more after casting In the present invention, in order to improve hot workability, the slab surface temperature is 950 after casting by a continuous casting machine.
The temperature is kept within a range of １1150 ° C. for 5 minutes or more.

When the surface temperature of the slab is less than 850 ° C. and 750
Exceeding the temperature range exceeding ℃, the strain rate on the surface is 1 ×
Step of performing bending deformation to be 10 ⁻⁴ to 1 × 10 ^{−1 In the} present invention, in order to suppress surface cracking due to bending deformation,
Avoiding a temperature range of less than 850 ° C. and a temperature exceeding 750 ° C., which is a hot brittleness range, the strain rate on the surface is 1 × 10 ⁻⁴ to 1 × 10
Bending deformation is performed so as to be ⁻¹ .

In the rough rolling, a step of performing rolling in the sheet thickness direction at least once at a strain rate of 1 / second or more after rolling in the width direction.

Before the finish rolling, the surface of the material, especially at the corners, is refined to suppress the occurrence of cracks. Reduce at least once. The width reduction is preferably set to a strain rate of 1 / second or more.

In the present invention, the type of the continuous casting machine is not specified, and the invention can be applied to a curved continuous casting machine, a vertical bending continuous casting machine and the like. In order to suppress the temperature drop of the slab at a high efficiency and enable direct rolling, the casting speed is 1.5 m / mi.
It is preferable to set it to n or more.

The rough rolling is not particularly limited, since it can be cooled once after bending deformation, and then can be performed after reheating, or can be directly performed without cooling.

[0032]

EXAMPLES Steels A, B, and C having the chemical components shown in Table 1 were melted in a converter and cast, rough-rolled, and finish-rolled to form hot-rolled coils. Thereafter, the presence or absence of surface defects was examined by surface inspection. Table 2 shows the manufacturing conditions and evaluation results.

In Table 2, No. 1 of the present invention was used. 1-3
No surface defects were detected, and excellent surface properties were obtained. On the other hand, in Comparative Example No. In Nos. 4 to 6, any of the holding time of the slab, the surface temperature at the time of bending deformation, and the rough rolling conditions were out of the range of the present invention, and generation of surface defects was recognized.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

According to the present invention, the holding time of the slab, the surface temperature during bending deformation, and the strain rate are adjusted, and further, in the rough rolling, after the reduction in the width direction, the reduction in the sheet thickness direction is achieved. A thin steel sheet excellent in surface properties, in which generation of surface defects due to cracks in cold rolling is extremely small, and a method for producing the same are obtained, and are extremely useful in industry.

[Brief description of the drawings]

FIG. 1 is a view showing the effect of the amount of carbon on hot workability (a drawn value in a high-temperature tensile test).

Claims (2)

[Claims] 1. A method for producing a thin steel sheet having excellent surface properties, comprising the following steps. (1) In mass%, C: 0.2% or less, Si: 1% or less,
Mn: 0.4 to 2%, P: 0.08% or less, O: 0.0
Casting a steel containing not more than 05%, not more than 0.03% of S, not more than 0.01% of N, not more than 0.004% of Sn by a continuous casting machine. (2) A step of holding the slab surface at 950 to 1150 ° C. for 5 minutes or more after casting. (3) The strain rate on the surface is 1 × 10, avoiding a temperature range in which the surface temperature of the slab is less than 850 ° C. or more than 750 ° C.
Step of performing bending deformation to be ⁻⁴ to 1 × 10 ⁻¹ . (4) In the rough rolling, after the reduction in the width direction, a step of performing the reduction in the thickness direction at least once at a strain rate of 1 / second or more. 2. The casting speed in a continuous casting machine is 1.5 m.
The method for producing a thin steel sheet having excellent surface properties according to claim 1, wherein the rate is not less than / min.