JP7047516B2 - Cooling method for slabs for high-strength steel sheets, manufacturing method for high-strength hot-rolled steel sheets, manufacturing method for high-strength hot-dip galvanized steel sheets, and manufacturing method for high-strength alloyed hot-dip galvanized steel sheets. - Google Patents

Description

The present invention relates to a method for cooling a slab for a high- strength steel sheet, a method for manufacturing a high- tensile hot-rolled steel sheet, a method for manufacturing a high- strength hot-dip galvanized steel sheet, and a method for manufacturing a high- strength alloyed hot-dip galvanized steel sheet.

High- strength hot-rolled steel sheets, high- strength hot-dip galvanized steel sheets, and high-tensile alloyed hot-dip galvanized steel sheets, which are expected to be applied to undercarriage, are required to have both high component resistance and fatigue characteristics, so Ti is added. However, high YP and TS up are achieved by precipitating as TiC during hot rolling. On the other hand, these alloying elements also form precipitates when the slabs of these high- strength steel plates are cooled after steelmaking and casting. In particular, the crystal grain size of the cast slab at high temperature is extremely large. In general, the slab undergoes transformation while cooling to room temperature, resulting in a smaller particle size. However, since Ti tends to precipitate even at high temperatures, it precipitates as coarse sulfides and carbides at the coarse austenite grain boundaries. In particular, the precipitated Ti precipitates are precipitated so as to cover the austenite grain boundaries, and are hard and brittle, so that the slab is embrittled.

On the other hand, when the slab is cooled, stress is generated due to the temperature difference between the surface of the slab and the inside. When this stress is high, cracks occur when the slab is cooled to room temperature, and in the worst case, cracks occur as shown in FIG. Alternatively, it opens during hot rolling and the coil during hot rolling breaks. Alternatively, cracks that are extremely small appear as quality defects called hege when reheated by hot rolling. Fine cracks in the slab can be removed by grinding the slab with a grinding machine, but large cracks cannot be removed. In addition, cracks may be overlooked, which may result in defects in the product board. Therefore, it is necessary to suppress slab cracking.

FIG. 2 shows the fracture surface of the slab crack portion. The fracture surface has the appearance of a grain boundary fracture surface along the old austenite grain boundaries, and when the fracture surface is observed by SEM and the components of the precipitates present on the fracture surface are analyzed, Ti is found along the grain boundaries. The presence of inclusions was observed (Fig. 3). From this, it was considered that Ti added for increasing the tension had an adverse effect on slab cracking. That is, it is considered that Ti precipitates as a precipitate even when the slab is cooled, and also precipitates as a coarse precipitate at the austenite grain boundaries, making the slab embrittlement.

On the other hand, when the slab is cooled, stress is generated due to the temperature difference between the surface of the slab and the inside. When this stress is high, when the slab is cooled to room temperature, the brittle Ti precipitates present at the old γ grain boundaries become the cracking starting point and slab cracking occurs. Since the cracks generated in this way remain even after hot rolling, they become surface defects such as shavings, which leads to yield drop or cracks in the slabs, so improvement is required. rice field. As described in Patent Document 1, measures against cracking of the slab have been conventionally studied.

Japanese Unexamined Patent Publication No. 2007-832743

By the way, while examining measures against cracking of Ti-added steel, an attempt was made to confirm the effectiveness of the method described in Patent Document 1, but even if controlled within the range described in Patent Document 1, Ti It was found that cracking of the added steel may not be suppressed.

The present invention has been made in such a background, and the subject of the present invention is not only cracking of the slab during cooling of the slab, but also shaving during hot spreading, etc., even if the slab contains Ti. It is to provide a cooling method for a slab for a high- strength steel plate that does not cause quality defects. Another object of the present invention is to provide a method for manufacturing a high- strength hot-rolled steel sheet, a high-tensile hot -dip galvanized steel sheet, and a high- tensile alloyed hot-dip galvanized steel sheet using the cooling method.

In order to solve the above problems, in terms of mass %, C: 0.020 to 0.600%, Si: 0.01 to 3.00%, Mn: 1.00 to 3.00%, Ti: 0.030. -0.200%, P: 0.100% or less, S: 0.0001 to 0.0100%, Al: 0.005 to 1.000%, N: 0.0100% or less , in the balance For continuously cast slabs for high- strength steel plates containing Fe and unavoidable impurities , the average cooling rate of the slabs at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr or lower. Use the cooling method.

Further, the slab is further mass%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, W: 0.005 to 0.100%, B: 0.0005 to 0.0100%, REM : 0.0003 to 0.0300%, Ca: 0.0003 to 0.0300%, Ce: 0.0003 to 0.0300%, Mg: 0.0003 to 0.0300%, one or more. It is preferable that the composition contains it.

Further, for the high-strength steel plate slab, the slab temperature is secured at 700 ° C. or higher for at least 10 hr or more after the casting is completed, and then the average cooling rate of the slab at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr. The following is preferable.

Further, it is preferable to control the cooling rate of the slab by sandwiching the slab with a plurality of other slabs.

Further, it is preferable that the slabs are sandwiched by the other plurality of slabs at the same time.

Further, when cooling the slab for high-strength steel plate, it is preferable to cover it.

Further, using the slab for high-strength steel plate cooled by the cooling method, the slab heating temperature is heated in the range of 1100 ° C. or higher and 1300 ° C. or lower, and after rough rolling, the finish-rolled output side plate temperature is 800 ° C. or higher and 1100 ° C. or lower. It is preferable to perform finish rolling in a high- strength hot-rolled steel sheet by winding in a temperature range of room temperature or higher and 700 ° C. or lower.

Further, using the high- tensile hot-rolled steel sheet manufactured by the manufacturing method, after pickling the steel sheet, and if the plate thickness is to be further thinned, after pickling and then cold rolling with a reduction ratio of 30% or more and 80% or less. Reheated to a temperature range of 430 ° C or higher and 800 ° C or lower, immersed in a hot-dip galvanizing bath, hot-dip galvanized, and tempered and rolled at a rolling reduction of 0.2% or higher and 2.0% or lower. It is preferable to manufacture a high- tensile hot-dip galvanized steel sheet.

Further, it is preferable to manufacture a high- tensile alloyed hot-dip galvanized steel sheet by heating the hot-dip galvanizing to 470 ° C. or higher and 600 ° C. or lower to alloy the hot-dip galvanizing between the hot-dip galvanizing and tempering rolling.

According to the present invention, a method for cooling a slab for a high- strength steel plate, which does not cause not only slab cracking during cooling of the slab but also quality defects such as shavings during hot spreading, even if the slab contains Ti, is used. Can be provided. Further, it is possible to provide a method for manufacturing a high- strength hot-rolled steel sheet, a high-tensile hot -dip galvanized steel sheet, and a high- tensile alloyed hot-dip galvanized steel sheet using the cooling method.

This is a photograph of a slab with cracks. It is an image obtained by SEM observation of the fracture surface of the slab crack part. It is a figure which shows the result of SEM observation of the fracture surface, and the component analysis of the precipitate present in the fracture surface. However, the image obtained by SEM observation has a larger magnification than that in FIG. It is a figure which shows the temperature measurement position of a slab. It is a figure which shows the cooling mode of a slab. However, three types of cooling are described: normal cooling, cooling in which the slab to be cooled is sandwiched between two slabs of other components, and cooling in which the slab to be cooled is covered with a cover. It is a figure which shows the cooling history which concerns on the elapsed time and the slab surface temperature. However, it relates to the conditions H-1, H-2, the condition H-6, the H-8, and the condition H-10. This is a picture of a slab with cracks. It is a schematic diagram of FIG. 7.

The embodiment for carrying out the invention is shown below. The cooling method of the slab 1 for high- strength steel plate of the present embodiment is C: 0.020 to 0.600 %, Si: 0.01 to 3.00%, Mn: 1.00 to 3. 00%, Ti: 0.030 to 0.200%, P: 0.100% or less, S: 0.0001 to 0.0100%, Al: 0.005 to 1.000%, N: 0.0100% It relates to a continuously cast slab 1 for a high-strength steel plate containing the following and containing Fe and unavoidable impurities in the balance, and the average cooling rate of the slab 1 at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr or lower. Is to be. As a result, even if the slab 1 contains Ti, not only the slab 1 is cracked during cooling, but also the slab 1 for high- strength steel plate is cooled so that quality defects such as shavings during hot spreading do not occur. can do.

Here, the flow leading to the present invention will be described. The present inventors, in terms of mass %, C: 0.020 to 0.600%, Si: 0.01 to 3.00%, Mn: 1.00 to 3.00%, Ti: 0.030 to Contains 0.200%, P: 0.100% or less, S: 0.0001 to 0.0100%, Al: 0.005 to 1.000%, N: 0.0100% or less , and Fe in the balance. In examining the cracking of the continuously cast slab 1 for high-strength steel plates containing unavoidable impurities , the causes of the slab cracking are the formation of Ti-based precipitates at the old austenite grain boundaries and the temperature unevenness in the slab 1. It was found that the thermal stress was caused by the above. Since the steel is increased in tension by utilizing the precipitation strengthening of Ti, it is indispensable to add Ti. That is, since it is necessary to add Ti to generate Ti-based precipitates, by focusing on the reduction of thermal stress caused by the temperature unevenness of the slab 1 (particularly, the temperature difference between the surface and the inside), the slab 1 can be used. I wondered if it would be possible to suppress cracking.

Therefore, as shown in FIG. 4, a thermocouple is installed in the center of the upper surface of the wide surface of the slab 1, and various cooling conditions and slab cracking, and subsequent cracking during slab cooling or hot rolling, and a surface called hege. The presence or absence of defects was investigated. The temperature at this position is defined as the slab temperature. It is considered that the cracks formed by hot rolling are caused by the cracks 11 formed during cooling of the slab, which are opened by hot rolling heating or rolling, leading to cracks. On the other hand, even if the cracks do not occur, the opened cracks 11 may be detected as defects such as shavings, and therefore the evaluation was also carried out. For some slabs 1, the cooling rate was controlled by sandwiching and cooling with slabs 2 having different components, or by covering the slabs 1 with a cover 3 (see FIG. 5). In addition, the slab temperature was measured by these methods, and the cooling rate and the cooling start temperature were variously changed.

The chemical composition of the steel used in the study is shown in Table 1, and Tables 2-1 and 2-2 show the slab cooling conditions and the presence or absence of slab cracking and hot rolling. Further, FIG. 6 shows the elapsed time of each condition and the cooling history related to the slab surface temperature.

Condition H-1 was such that within 4 hours from the start of casting, a thermocouple was attached to the slab 1 as described above, a cover 3 was installed, and cooling was started. The average cooling rate at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr or lower (4.2 ° C./hr or lower in the illustrated range), and the holding time at 700 ° C. or higher is 10 hr or more (44 hr). , I was able to cool to room temperature without cracking. Then, it was hot rolled and rolled up and cooled to room temperature. After that, a tensile test piece was taken from a hot-rolled steel sheet and a tensile test was carried out to secure a tensile strength of 980 MPa or more.

Condition H-2 is that within 4 hours from the start of casting, different steel plate slabs 2 are laid on the bottom stage, three slabs 1 of steel plate component H are stacked, and then a thermocouple is attached as described above, and further, the steel plate component. After stacking two slabs 1 of H, different steel plate slabs 2 were stacked on the uppermost stage to start cooling. The average cooling rate at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr or lower (4.9 ° C./hr), and the holding time at 700 ° C. or higher is 10 hr or more (31 hr), which causes cracking to room temperature. I was able to cool it without. After that, the slab 1 was reheated, hot-rolled, and a tensile test piece was collected from the hot-rolled steel sheet, and a tensile test was carried out. As a result, a tensile strength of 980 MPa or more was secured.

The condition H-6 is that within 9 hours from the start of casting, six different steel plate slabs 2 are stacked, then the slab 1 of the steel plate component H is stacked on the uppermost stage, and then a thermocouple is attached as described above, and the thermocouple is mounted on the slab 1. Cooling was started without stacking another slab. Although the holding time in the temperature range of 700 ° C. or higher could be secured at 10 hr or more (15 hr), the average cooling rate from 700 ° C. to 500 ° C. was 29 ° C./hr exceeding 20 ° C./hr. After that, when hot rolling was carried out, cracks 11 were found after slab heating (see FIGS. 7 and 8). These cracks 11 are likely to be opened by rolling, and rolling was stopped immediately.

In condition H-8, after the start of casting, the slab 1 of the steel plate component H was placed at the bottom, a thermocouple was installed as described above, and then four slabs 1 of the steel plate component H were stacked and cooled. By placing the slab 1 at the bottom, it was not possible to secure 10 hr or more at 700 ° C or higher (7 hr), and the average cooling rate between 700 ° C and 500 ° C was 29 ° C / hr exceeding 20 ° C / hr. It became. As a result, cracks occurred in the slab 1 cooled to room temperature (see FIG. 1). Further, since a plurality of cracks 11 were found, hot rolling could not be performed.

In the condition H-10, after the start of casting, the slab 2 having a different component is placed at the bottom, then the slab 1 having the steel plate component H is placed, and then the thermocouple is installed as described above, and further, the slab having the steel plate component H is placed. After stacking 5 of 1's, the cover 3 was put on and cooled. The holding time of the slab 1 at 700 ° C. or higher was 5 hr, and the average cooling rate between 700 ° C. and 500 ° C. was 10 ° C./hr, which was less than 20 ° C./hr. An average cooling rate of less than 20 ° C / hr could be ensured between 700 ° C and 500 ° C, and although the slab cooled to room temperature did not crack, a small heddle was observed at the end of the coil after hot rolling. Was done. However, since it was a small amount and could be shipped by cutting and removing the end of the product, no nonconforming product was generated.

The slab 1 according to the present invention is transshipped depending on various conditions. When transshipment occurs, the cooling rate of the slab 1 may temporarily exceed 20 ° C / hr, for example 120-170 ° C / hr. However, the slab 1 has at least 10 tons or more, its thermal inertia is large, and it seems that the heat shock can be absorbed if the handling time is about transshipment (1 to 2 hours at the longest), and cracks and shavings occur. Does not reach. Although not shown in FIG. 6, in consideration of this in the present invention, the cooling rate is not 20 ° C./hr or less throughout all of 500 ° C. and above and 700 ° C. or less, but the average cooling rate in the temperature range is 20 ° C./hr. It was found that if the following is the case, it does not lead to cracking or swelling.

More specifically, the average cooling rate (V: ° C./hour) of the slab 1 is 20 when the slab temperature (the surface temperature ((T: ° C.)) at the center of the upper and lower surfaces of the wide surface of the slab 1 is between 500 ° C. and 700 ° C.). It was found that slab cracking can be suppressed by cooling at ° C./hr or less. Further, at least 10 hr or more after the completion of casting was caused by minute cracking of slab 1 by ensuring the slab temperature at 700 ° C. or higher. It was found that hege can also be suppressed.

Next, the reason for limiting the chemical composition of the high- strength hot-rolled steel sheet, which is the premise of the present invention, will be described. The% of the content is mass%.

The reason for setting C to 0.020 to 0.600% is as follows. C is an element added to increase the strength of the steel sheet. Specifically, it is an element that contributes to high tension by precipitating as TiC. If C is less than 0.020%, the required tension cannot be obtained, so the lower limit of the addition amount is 0.020%. On the other hand, if it exceeds 0.600%, the weldability and workability become insufficient. Therefore, it is set to 0.020 to 0.600%.

The reason for setting Si to 0.01 to 3.00% is as follows. Si may be added to increase the strength of the steel sheet. However, the addition of more than 3.00% not only saturates the effect, but also produces a strong scale on the hot rolled plate. As a result, the appearance and pickling property are deteriorated, so that the upper limit is 3.00% or less. On the other hand, if it is less than 0.01%, not only the effect is saturated, but also productivity and economic efficiency are adversely affected. For this reason, it is desirable to add 0.01% or more. Therefore, it is set to 0.01 to 3.00%. However, if productivity and economy are not a concern, it may be less than 0.01%.

The reason for setting Mn to 1.00% to 3.00% is as follows. Mn is an element added to increase the strength of the steel sheet. Specifically, it is an element added to control the strength of the steel sheet through transformation control by hot rolling. If it is less than 1.00%, it cannot be sufficiently strengthened, so it is necessary to add 1.00% or more. On the other hand, addition of more than 3.00% is not desirable because the effect is saturated and the economy is poor. Therefore, it is set to 1.00% to 3.00%.

The reason for setting Ti to 0.030 to 0.200% is as follows. Ti is an element that contributes to high tension by binding to C and forming TiC. Since this effect becomes remarkable at 0.030% or more, it is necessary to add 0.030% or more. On the other hand, if the addition of more than 0.200%, the Ti precipitate formed during casting becomes too stable and cannot be melted during slab heating in hot rolling. Therefore, the effect of increasing the tension , which is the target, cannot be exhibited. Therefore, it is necessary to add 0.030 to 0.200%.

The reason for setting P to 0.100% or less is as follows. P is an element that segregates in the central portion of the thickness of the steel sheet and is also an element that embrittles the welded portion. A lower value is preferable, but the upper limit is preferably 0.100% because of the productivity of de-P and the costly effect. A more preferable upper limit is 0.050%. The effect of the present invention is exhibited without specifying the lower limit, but reducing P to less than 0.001% is further economically disadvantageous, so the lower limit is set to 0.001%.

The reason for setting S to 0.0001 to 0.0100% is as follows. Since S exists as a sulfide and causes slab embrittlement and deteriorates the formability of the product plate, it is preferable to limit the content in the steel sheet. For this reason, it is preferable to set the upper limit to 0.0100%. However, it is more economically disadvantageous to reduce S to less than 0.0001% from the viewpoint of productivity and cost of de-S, so it is preferable to set the lower limit to 0.0001%.

The reason for setting Al to 0.005 to 1.000% is as follows. Al is added in an amount of 0.005% or more for tissue control and deoxidation by hot rolling. If it is less than 0.005%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are present in the steel sheet. On the other hand, addition of more than 1.000% is not preferable because it causes slab embrittlement. Therefore, the addition amount needs to be 0.005 to 1.000%.

The reason for setting N to 0.0100% or less is as follows. N is an element that forms a coarse nitride and deteriorates bendability and hole expansion property. If N exceeds 0.0100%, the bendability and hole expansion property are significantly deteriorated, so the upper limit is set to 0.0100%. It should be noted that N is preferably a small amount because it causes blow holes during welding. The lower limit of N does not need to be set in particular, but if it is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so 0.0001% is a substantial lower limit. N is preferably 0.0005% or more from the viewpoint of manufacturing cost.

In addition, other unavoidable elements may be contained in a trace amount. For example, O forms an oxide and exists as an inclusion.

The steel sheet of the present invention further contains one or more of the following elements in the following proportions, if necessary. Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%.

Ni, Cu, Cr and Mo are elements that affect the strength. These elements bring about higher strength through microstructural control in hot rolling. This effect becomes remarkable when one or more of Ni, Cu, Cr, and Mo are added in an amount of 0.01% or more, respectively, and therefore it is necessary to add 0.01% or more. If the amount of each element exceeds the upper limit of each element, weldability, hot workability and the like deteriorate, so the upper limit of Ni, Cu, Cr and Mo is set to 2.00%.

The steel sheet of the present invention further contains one or more of the following elements in the following proportions, if necessary. Nb: 0.005 to 0.100%, V: 0.005 to 0.100%, W: 0.005 to 0.100%.

Nb, V, and W may be added because they affect the strength of the steel sheet through precipitation strengthening. Since this effect becomes remarkable when 0.005% or more is added, it is desirable to add 0.005% or more. On the other hand, addition of more than 0.100% is not desirable. It is preferably in the range of 0.005 to 0.090%.

The steel sheet of the present invention further contains B in a proportion of 0.0005 to 0.0100%, if necessary. B may be added because it affects the strength through tissue strengthening in order to control the transformation due to hot rolling. Since this effect becomes remarkable at 0.0005% or more, it is necessary to add 0.0005% or more. On the other hand, addition of more than 0.0100% is not preferable because not only the effect is saturated but also the precipitation of iron-based boride is caused and the quenching effect of B is lost. The desirable range is 0.0005 to 0.0080%, and the more desirable range is 0.0005 to 0.0050%.

The steel sheet of the present invention further contains one or more of the following elements in the following proportions, if necessary. REM: 0.0003 to 0.0300%, Ca: 0.0003 to 0.0300%, Ce: 0.0003 to 0.0300%, Mg: 0.0003 to 0.0300%.

REM, Ca, Ce, and Mg are elements that affect the strength and contribute to the improvement of the material. If the total of one or more of REM, Ca, Ce, and Mg is less than 0.0003%, a sufficient addition effect cannot be obtained, so the lower limit of the total is set to 0.0003%. If the total of one or more of REM, Ca, Ce, and Mg exceeds 0.0300%, castability and hot workability will deteriorate, so the upper limit is 0.0300%. REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM is often added as a misch metal, and may contain a lanthanoid-series element in a complex in addition to Ce. The effect of the present invention is exhibited even if the steel sheet of the present invention contains elements of the lanthanoid series other than La and Ce as unavoidable impurities, and the effect of the present invention is exhibited even if a metal is added. ..

If the slab 1 is in accordance with the cooling method according to the present invention, it can be used for high- tensile hot-rolled steel sheets, high- tensile hot-dip galvanized steel sheets, and high- tensile alloyed hot-dip galvanized steel sheets that do not crack slabs after casting or generate heddle during hot-spreading. It can be manufactured. These manufacturing methods are as follows.

First, the slab 1 according to the cooling method of the present invention is used. The following are the same as the manufacturing conditions for ordinary high- strength steel sheets. In hot rolling, the slab heating temperature is heated in the range of 1100 ° C to 1300 ° C, and after rough rolling, finish rolling is performed at a finish rolling output side plate temperature of 800 ° C to 1100 ° C, and the rolling is performed in the temperature range of room temperature to 700 ° C. Roll out. Rapid cooling, plate temperature maintenance / heat retention, and air cooling may be performed between the finishing rolling side and winding. In this way, a high- strength hot-rolled steel sheet is manufactured.

In the case of a high- tensile cold-rolled steel sheet, the high- tensile hot-rolled steel sheet is pickled, and in the case of further thinning, after pickling, cold rolling with a reduction ratio of 30% to 80% is performed, and then 750 ° C. It is reheated to a temperature range of 900 ° C. and tempered and rolled at a reduction rate of 0.2 to 2.0% to obtain a cold-rolled steel sheet. When plating is applied, after the above heat treatment, it is immersed in a hot-dip galvanizing bath, hot-dip galvanized, tempered and rolled at a reduction rate of 0.2% to 2.0%, and high- tensile cold-rolled hot-dip zinc. It shall be a plated steel plate.
In the case of a high- tensile hot-rolled hot-dip galvanized steel sheet, the high- tensile hot-rolled steel sheet is pickled, reheated to a temperature range of 750 ° C to 900 ° C, immersed in a hot-dip galvanized bath, and hot-dip galvanized. High- tensile hot-dip galvanized steel sheet is obtained by temper rolling at a rolling reduction of 0.2% to 2.0%.

In the case of a high - tensile alloyed hot - dip galvanized steel sheet, the temperature is 470 ° C. to 600 ° C. A high- tensile alloyed hot-dip galvanized steel sheet is obtained by heating to ℃ and alloying the hot-dip galvanizing.

According to the present invention, a crack-free slab 1 containing a large amount of Ti can be manufactured without cracking, which can contribute to an improvement in yield. Further, since a large amount of Ti can be added, it becomes possible to manufacture a hot-rolled high- strength steel plate (for example, 1180 MPa class) for undercarriage of an automobile having higher tension .

Although the present invention has been described above with a focus on the embodiments, the present invention is not limited to the above embodiments and can be in various embodiments.

1 slab (cooling target )
2 slab (for cooling control)
3 cover 11 crack

Claims (11)

By mass%,
C: 0.020 to 0.600%,
Si: 0.01-3.00%,
Mn: 1.00 to 3.00%,
Ti: 0.030 to 0.200%,
P: 0.100% or less,
S: 0.0001 to 0.0100%,
Al: 0.005 to 1.000%,
N: 0.0100% or less,
For continuously cast slabs for high-strength steel sheets containing Fe and unavoidable impurities in the balance.
A method for cooling a slab for a high-strength steel plate, characterized in that the average cooling rate of the slab at 500 ° C. or higher and 700 ° C. or lower is 20 ° C./hr or lower. The slab is further in mass%
Ni: 0.01-2.00%,
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.01-2.00%,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.100%,
W: 0.005 to 0.100%,
B: 0.0005-0.0100%,
REM: 0.0003-0.0300%,
Ca: 0.0003-0.0300%,
Ce: 0.0003-0.0300%,
Mg: 0.0003-0.0300%,
The method for cooling a slab for a high-strength steel plate according to claim 1, wherein the slab for high-strength steel plate contains one or more of the above. The slab for high-strength steel plate having the component according to claim 1 or 2 has a slab temperature of 700 ° C. or higher for at least 10 hr or more after the completion of casting, and then the average of the slab at 500 ° C. or higher and 700 ° C. or lower. The method for cooling a slab for a high-strength steel plate according to claim 1 or 2, wherein the cooling rate is 20 ° C./hr or less. The method for cooling a slab for a high-strength steel plate according to any one of claims 1 to 3, wherein the cooling rate of the slab is controlled by sandwiching the slab between a plurality of other slabs. The method for cooling a slab for a high-strength steel plate according to claim 4, wherein a plurality of the slabs are simultaneously sandwiched by the other plurality of slabs. The method for cooling a slab for a high-strength steel plate according to any one of claims 1 to 5, wherein a cover is applied to cool the slab for the high-strength steel plate. Using the high-strength steel plate slab cooled by the cooling method according to any one of claims 1 to 6, the slab heating temperature is heated in the range of 1100 ° C. or higher and 1300 ° C. or lower, and after rough rolling, finish rolling is performed. A method for manufacturing a high-strength hot-rolled steel sheet, which comprises performing finish rolling at a temperature of the output side plate of 800 ° C. or higher and 1100 ° C. or lower, and winding in a temperature range of room temperature or higher and 700 ° C. or lower. A high-strength hot-rolled steel sheet manufactured by the manufacturing method according to claim 7 was used, and after pickling, cold rolling with a reduction ratio of 30 to 80% was performed after pickling to further reduce the thickness. After that, it is reheated to a temperature range of 700 to 900 ° C., annealed, and then tempered and rolled at a reduction rate of 0.2 to 2.0% to produce a high-strength cold-rolled steel sheet. Method. A high-tensile hot-rolled steel sheet manufactured by the manufacturing method according to claim 7 is used, pickled, reheated to a temperature range of 700 to 900 ° C., hot-dip galvanized, and then 0.2 to 0.2. A method for manufacturing a high-tensile hot-rolled hot-dip galvanized steel sheet, which comprises performing temper rolling at a rolling reduction of 2.0%. A high-tensile hot-rolled steel sheet manufactured by the manufacturing method according to claim 7 was used, and after pickling, cold rolling with a reduction ratio of 30 to 80% was performed after pickling to further reduce the thickness. After that, it is reheated to a temperature range of 700 to 900 ° C., hot-dip galvanized, and then temper-rolled at a reduction rate of 0.2 to 2.0%. Manufacturing method of galvanized steel sheet. Manufacture of a high-tensile alloyed hot-dip galvanized steel sheet, which comprises heating to 470 ° C. or higher and 600 ° C. or lower to alloy the hot-dip galvanizing between the hot-dip galvanizing of claim 9 or 10 and temper rolling. Method.

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