JP2002256339A - Method for manufacturing steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel plate which is subjected to accelerated cooling or quenching after hot rolling and has excellent cutting crack resistance and causes no cutting crack in the cutting face even if subjected to cutting using a shearing machine. SOLUTION: In the method for manufacturing the steel plate which is subjected to accelerated cooling or quenching after hot rolling, reheating to 200-<500 deg.C is performed before the steel plate is cooled to <=150 deg.C steel-plate temperature, an induction heating furnace is used at reheating; and further, at reheating, reheating is done by setting heating equipment on the line identical with that of either or both of rolling-mill equipment and cooling equipment. By carrying out reheating before the steel-plate temperature falls down to the prescribed value or below, hydrogen in the steel can be throughly removed and cracking can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used for machine structures, construction civil engineering, line pipes, etc., which does not crack when cut by a shearing machine.
The present invention relates to a method for producing a steel sheet having excellent cut crack resistance.

[0002]

2. Description of the Related Art Thick steel sheets rolled to a predetermined thickness by hot rolling are subjected to measuring work after being cooled on a cooling floor.
The measured thick steel plate is cut into a predetermined width and length. This cutting is performed by cutting with a shearing machine or gas cutting. Usually, a thick steel plate having a thickness of less than about 50 mm is cut by a shearing machine, and when it exceeds the thickness, gas cutting is performed. The shearing machine includes a crop shear for cutting a top portion and a bottom portion of a steel plate, a side shear for cutting an ear portion, a slitter for dividing a width direction of the steel plate into two, and an end shear for cutting a length direction to a predetermined dimension. Are passed through a cutting line constituted by these shearing machines to cut into a thick steel plate having a predetermined size.

[0003] Defects in the cut surface caused by the shearing machine include sagging, burrs, mechanical cracks, cuts, steps, and the like. If these occur, they cannot be used as a product as they are, and care or re-cutting such as grinder grinding etc. And so on, which leads to a decrease in yield and an increase in manufacturing cost. Various proposals have been made as a countermeasure for these cut surface defects.

[0004] For example, in the Iron and Steel Handbook, Vol. 3 (1) Rolling foundation and steel plate (edited by the Iron and Steel Institute of Japan, 3rd edition, p. 285), these cut surface defects are largely determined by the equipment conditions of the shearing machine. It has been proposed to manage equipment conditions of a shearing machine to an appropriate value. Japanese Patent Application Laid-Open No. H6-190627 proposes a method for reducing the sagging, in which a crop section including a cutting line is preheated and then cut by a shearing machine.

However, in the case of high-toughness high-strength steel sheets having a tensile strength of about 550 MPa or more, which are used for machine structures, building civil engineering, line pipes, etc., even if the equipment conditions of the shearing machine are controlled to appropriate values, Although the frequency of occurrence is low, cracks (hereinafter referred to as “cut cracks”) may occur on the cut surface along the center of the plate thickness. When such shear cutting cracks occur, it is necessary to remove the cracks, and the yield decreases. Furthermore, in order to prevent cutting cracks, it is necessary to use a gas cutting machine, which has led to a significant decrease in productivity as compared with shear cutting.

[0006] The prevention of cutting cracks by such a shearing machine,
That is, for the purpose of improving the cutting crack resistance, JP-A-2000-30984
No. 5 proposes a high-toughness high-strength steel plate having a defined chemical composition. In this technology, C content, S content,
By keeping the Pcm value low and adding Ca, cutability by a shearing machine has been greatly improved.

On the other hand, Japanese Patent Application Laid-Open No. 2000-119744 describes a method for preventing hydrogen cracking during shearing of a high-strength steel sheet in which a hot-rolled steel sheet is subjected to dehydrogenation treatment while passing through a high-temperature heat treatment apparatus. .

[0008]

However, these prior arts have the following problems. For example, in the technique described in Japanese Patent Application Laid-Open No. Hei 6-90627, it is necessary to preheat the crop portion before cutting with a shearing machine, and there is a problem that the efficiency of the shearing operation is reduced. In addition, the technique described in Japanese Patent Application Laid-Open No. 2000-309845 has a problem that it is difficult to apply the technique to steel materials for a wide range of uses because the chemical components are limited, and further increases the material cost. And JP 2000-11974
In the technique described in Japanese Patent Application Laid-Open No. 4 (1999) -1995, the dehydrogenation treatment is performed at a high temperature of 550 to 700 ° C., which results in a decrease in strength and deterioration of materials such as DWTT characteristics.

The present invention has been made in view of the above circumstances,
After hot rolling, in a steel sheet that is accelerated cooling or quenching,
An object of the present invention is to provide a method for producing a steel sheet having excellent cut crack resistance, in which cut cracks do not occur on the cut surface even when cut by a shearing machine.

[0010]

The above object is achieved by the following invention. The invention is a method for producing a steel sheet which is subjected to accelerated cooling or quenching after hot rolling, wherein the steel sheet temperature is 15
A method for producing a steel sheet, comprising reheating to a temperature of 200 ° C or more and less than 500 ° C before cooling to 0 ° C or less.

The present invention has been made based on the knowledge obtained by focusing on diffusible hydrogen which causes cutting cracks as a result of intensive studies on cutting cracks caused by a shearing machine. It is a finding obtained while studying a heat treatment method for preventing diffusible hydrogen present in steel from being fixed to various defects in steel, and after quenching, the steel sheet temperature is equal to or lower than a predetermined temperature. By reheating before the temperature is lowered, the fixation of diffusible hydrogen to various defects can be prevented.

Hereinafter, the components of the present invention and the reasons for the limitations will be described. First, the steel material targeted in the present invention is a steel sheet that is subjected to accelerated cooling or quenching after hot rolling.This is because steel sheets that have been accelerated cooled or quenched generally have high strength and are cut by a shearing machine. This is because there is a high possibility that cutting cracks will occur.

Generally, a steel sheet that has been accelerated cooled or quenched is cooled on a cooling floor or the like. In the present invention, reheating is performed before the steel sheet is cooled to a predetermined temperature. This facilitates the removal of diffusible hydrogen in steel. For reheating, the steel plate temperature is 15
Perform before cooling to below 0 ° C. The reason is that when cooled to 150 ° C. or less, diffusible hydrogen in steel is fixed to various defects such as dislocations, point defects, or crystal grain boundaries, and is difficult to be removed in a subsequent heat treatment. is there. Therefore, in the present invention, after accelerated cooling or quenching,
Reheat before cooling to below 150 ° C.

[0014] Thereafter, the diffusible hydrogen is removed by reheating to a temperature of 200 ° C or higher and lower than 500 ° C. 20 reheats
If the temperature is lower than 0 ° C. or higher, the diffusible hydrogen is not sufficiently removed, and cutting cracks are likely to occur when cut with a shearing machine. On the other hand, if the reheating is at a temperature of 500 ° C. or more, the diffusible hydrogen is sufficiently removed, so that cracking does not occur during shear cutting, but the amount of reduction in strength due to annealing rather than tempering becomes too large, and sufficient High strength cannot be obtained. Therefore, the reheating temperature is specified to be 200 ° C. or more and less than 500 ° C.

[0015] The invention of the reheating method based on the above invention may be a method for producing a steel sheet, wherein an induction heating furnace is used for reheating.

According to the present invention, by using the induction heating furnace, the heat treatment can be quickly performed before the diffusible hydrogen is fixed.

Further, in these inventions, when reheating, a heating device is installed on the same line as at least one of rolling equipment or cooling equipment, and reheating is performed. It can also be a method.

According to the present invention, by installing the induction heating furnace or other heating means on the same line as the rolling equipment or the cooling equipment, the reheating treatment can be performed promptly after the completion of rolling and cooling. It is possible to prevent a decrease in the temperature of the steel sheet after rolling and cooling. As a result, the diffusible hydrogen can be easily removed by heating, and the cutting cracks after shearing can be greatly reduced.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the present invention, molten steel produced by a converter, an electric furnace, or a combination of these refining furnaces and a secondary refining furnace such as an RH vacuum degassing apparatus is subjected to a normal ingot casting method. Or by a continuous casting method to obtain a slab slab. Then, it is processed into a steel sheet by hot rolling. As long as the heat treatment conditions for the steel sheet satisfy the present invention, the method for manufacturing the steel sheet does not affect the cutting property by the shearing machine, and the method for manufacturing the steel sheet up to hot rolling does not need to be particularly limited.

As described above, the steel material targeted in the present invention is a steel plate which is subjected to accelerated cooling or quenching after hot rolling, and corresponds to a steel plate having a tensile strength of 550 MPa or more. As a means of accelerated cooling or reheating after quenching, if it can be quickly heated after cooling, such as a normal gas combustion furnace,
The method does not matter. In particular, the induction heating furnace is capable of rapid heating and the equipment is compact, so that it can be installed relatively close to the cooling equipment, which is preferable for quickly heating the steel sheet after cooling. .

The mechanism by which the diffusible hydrogen can be easily removed by heating is considered as follows based on the results of research. Cutting by a shearing machine causes a steel material to be sheared and deformed in a cold state, so that residual stress is generated on the cut surface.
This residual stress has a higher value for a high-strength material. In general, steel materials contain a small amount of hydrogen, and hydrogen is accumulated on the cut surface by the stress field of the residual stress, so that the cut surface is cracked. Such hydrogen in steel can be removed by heat-treating the steel material, but the hydrogen that causes cutting cracks is called diffusible hydrogen and is easily affected by various types of defects such as dislocations, point defects, and grain boundaries. To be fixed.

Once hydrogen in the steel is fixed to such various defects, it cannot be sufficiently removed by low-temperature heating such as ordinary tempering. Therefore, in the present invention, the steel sheet is reheated after the rapid cooling before the temperature of the steel sheet falls below a predetermined temperature, thereby preventing the diffusible hydrogen from sticking to the defects in the steel. Next, in the reheating, the hydrogen in the steel can be removed without reducing the strength by limiting the temperature range. As a result, it is possible to manufacture a steel sheet that does not generate cutting cracks when it is cut by a shearing machine even with a steel sheet that has been subjected to accelerated cooling or quenching.

[0023]

Embodiments of the present invention will be described below. Table 1 shows the chemical components of the test steel. These steels were melted by a combination of a converter and an RH vacuum degassing device, and were made into slab slabs by a continuous casting method. This slab was rolled into a steel plate having a thickness of 8 to 50 mm by hot rolling, and then cooled using a water-cooled accelerated cooling facility.

[0024]

[Table 1]

Table 2 shows the rolling end temperature and the cooling stop temperature at this time. After that, it was installed on the same line as the gas combustion furnace, induction heating furnace, or cooling equipment (in-line type)
Reheating was performed using an induction heating furnace. FIG. 1 is a schematic diagram of a facility arrangement when an in-line induction heating furnace is used. After the steel sheet 2 rolled by the hot rolling mill 3 arranged on the rolling line 1 is cooled by the accelerating cooling device 4, it is reheated by the induction heating furnace 5 installed on the same line 1. In the figure, 6 is a hot leveler, and 7 is a shearing machine. In addition, as the induction heating furnace 5, it is preferable in terms of heating efficiency that a plurality of solenoid type induction heating devices suitable for rapid heating are arranged in series. When using a gas combustion furnace or an induction heating furnace that is not on the same line, the steel sheet is transported to a heat treatment furnace installed on another line immediately after accelerated cooling. Table 2 also shows these reheating conditions. Here, the heating temperature is the set temperature in the furnace in the case of a gas fired furnace,
In the case of the induction heating furnace, it is the steel sheet surface temperature immediately after the end of the reheating. In each case, the heating time is the furnace time from when the steel sheet is charged into the heating furnace until when the heating is completed.

[0026]

[Table 2]

After the steel sheet after reheating was cooled to a temperature of 100 ° C. or less, each steel sheet was cut at 10 places by a shearing machine, and a cut crack test was performed. In the test, the cut end face of the steel sheet was inspected by magnetic particle flaw detection, and the number of cut portions (end face) where cut cracks were recognized was obtained. Here, even if a plurality of cracks can be confirmed in one end face, since one end face is one,
The number of occurrences of cutting cracks was 1. In addition, the tensile strength was measured by a tensile test using a full-thickness test piece. The results of the above test and measurement are also shown in Table 2.

In Examples Nos. 1 to 14 of the present invention, after cooling, the steel sheet was reheated to 200 ° C. or more before the temperature of the steel sheet fell to 150 ° C. or less, and no crack was observed at the cut end face. It has excellent cut cracking resistance. On the other hand, the comparative example N
o. 15 to 21, even if the steel sheet has a high cooling end temperature, the temperature before reheating has dropped to 150 ° C or less, which is outside the scope of the invention, and No. 17, 20, and 21 also have the reheating temperature. It is lower than the range of the invention, and cracks occur at the cut end face, and the cut crack resistance is inferior.

[0029]

According to the present invention, hydrogen is fixed to defects in steel by performing reheating before accelerated cooling or quenching of the steel sheet after rolling and before the temperature of the steel sheet falls below a predetermined temperature. Prevention and sufficient removal of hydrogen in steel. Therefore, hydrogen does not accumulate on the cut surface, and generation of cracks can be prevented. According to the present invention, it is possible to stably produce a steel sheet in which a cutting crack does not occur on the cut surface even when cutting with a shearing machine, and as a result, an industrially beneficial effect such as an improvement in yield and a reduction in manufacturing cost is brought. It is.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a facility arrangement according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rolling line 2 Steel plate 3 Hot rolling mill 4 Accelerated cooling device 5 In-line induction heating device 6 Hot leveler 7 Shearing machine

Claims (3)

[Claims] 1. A method for producing a steel sheet which is subjected to accelerated cooling or quenching after hot rolling, wherein the steel sheet is reheated to a temperature of 200 ° C. or more and less than 500 ° C. before being cooled to 150 ° C. or less. A method for producing a steel sheet, characterized in that: 2. The method for producing a steel sheet according to claim 1, wherein an induction heating furnace is used for reheating. 3. The reheating method according to claim 1, wherein when reheating, a heating device is installed on the same line as at least one of the rolling equipment and the cooling equipment, and the reheating is performed.
The method for producing a steel sheet according to the above.