KR20240000569A - Manufacturing method of cold rolling steel sheet and manufacturing method of cold rolling steel sheet - Google Patents

Abstract

According to the present invention, it is possible to provide a method of manufacturing a steel sheet for cold rolling that can suppress end cracks of the steel sheet during subsequent cold rolling, as a method of manufacturing a steel sheet in an intermediate process of manufacturing a high-strength cold rolled steel sheet. there is. A method of manufacturing a steel sheet for cold rolling involves hot rolling a slab containing a predetermined chemical composition so that the exit temperature of a finishing mill is 800°C or more and 940°C or less, and at least a portion of the steel sheet after the hot rolling is in the finish rolling mill. Cooling at least a portion of the steel sheet for 0.1 second or more at a water density of 100 L/min/m ² or more within 3.0 seconds after passing through the final stand and being delivered to the run-out table, and hot rolling after the cooling at a coiling temperature of 550°C or more. It involves winding steel plates.

Description

Manufacturing method of cold rolling steel sheet and manufacturing method of cold rolling steel sheet

The present invention relates to a method of manufacturing a steel sheet for cold rolling in an intermediate process of manufacturing a high-strength cold rolled steel sheet with a tensile strength of 980 MPa or more, and a method of manufacturing a cold rolled steel sheet using a steel sheet manufactured by the method.

When a hot-rolled steel sheet is cold-rolled, an end portion in the width direction of the sheet (hereinafter referred to as “width direction end portion” or “width direction both ends”) and an end portion in a direction parallel to the rolling direction (hereinafter referred to as “longitudinal tip”) are formed. )” or “the lower end of the longitudinal direction”) may develop cracks. These end cracks are likely to occur during the production of high-strength cold-rolled steel sheets in which steel containing a large amount of elements that improve hardenability, such as Mn, is used. End cracks formed in this way may cause fracture of the steel sheet starting from the end cracks during cold rolling and subsequent processes such as an annealing process and plating process. Therefore, in order to reduce the risk caused by such end cracks, portions in the hot rolled steel sheet where end cracks are likely to occur are removed. However, as a result, a decrease in yield has become a problem.

On the other hand, in the cooling process of the hot rolled steel sheet after coiling, the cooling rate of the width direction end portion of the coil-shaped steel sheet is faster compared to the central portion in the width direction of the steel sheet (hereinafter also referred to as the “width direction center portion”). Therefore, in hot-rolled steel sheets containing a large amount of elements that improve hardenability, such as Mn, the ferrite/pearlite transformation does not sufficiently progress at both ends in the width direction of the steel sheet, and the both ends in the width direction of the steel sheet are martenized. It becomes a hard tissue containing a relatively large amount of zite. The same applies to the longitudinal end and longitudinal end of the steel plate. For this reason, it is thought that cracks at the ends of the steel sheet are becoming more likely to occur during cold rolling during the production of high-strength cold rolled steel sheet.

As a method of suppressing end cracking of the above-described steel sheet, for example, Patent Document 1 describes a method of cold rolling a strip-shaped hot-rolled steel sheet wound into a coil and cooled, wherein the hot-rolled steel sheet is fed from the coil ( a feeding process of heating the fed hot-rolled steel sheet to a temperature of 400° C. or lower, the A1 point of the hot-rolled steel sheet material, and washing the hot-rolled steel sheet with acid after the heating process. A cold rolling method comprising a pickling process and a cold rolling process of cold rolling the hot rolled steel sheet after the pickling process is described.

Japanese Patent Publication No. 2019-141888

The purpose of the present invention is to provide a method of manufacturing a steel sheet for cold rolling that can suppress end cracks of the steel sheet during subsequent cold rolling, as a method of manufacturing a steel sheet in an intermediate process of manufacturing a high-strength cold rolled steel sheet. Do it as

The present inventors conducted intensive studies to solve the above problems and arrived at the present invention.

That is, the method for manufacturing a steel sheet for cold rolling according to the first aspect of the present invention has a chemical composition,

C: 0.15 mass% or more, 0.25 mass% or less,

Si: 0.8 mass% or more, 3.0 mass% or less,

Mn: 1.8 mass% or more, 3.0 mass% or less,

Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%),

Ti, Nb, V: 1.0 mass% or less (including 0 mass%), and

B: 0.01% or less (including 0 mass%)

Hot rolling the slab containing so that the exit temperature of the finishing mill is 800°C or more and 940°C or less,

Within 3.0 seconds after at least a portion of the steel sheet after the hot rolling passes through the final stand of the finishing mill and is sent out on the runout table, at least a portion of the steel sheet is heated at a water density of 100 L/min/m ² or more for 0.1 second. Over cooling,

It includes winding the cooled hot-rolled steel sheet at a coiling temperature of 550°C or higher.

The method for manufacturing a cold rolled steel sheet according to the second aspect of the present invention further includes cold rolling the steel sheet manufactured by the method according to the first aspect described above at a reduction ratio of 30% to 80%.

1 is a schematic diagram showing an example of a method for manufacturing a steel sheet for cold rolling in this embodiment.
Figure 2 is a schematic diagram showing the position of a steel plate test piece for hardness measurement in this example.

As described above, in the method described in Patent Document 1, martensite in the microstructure of both ends in the width direction of the steel sheet is transformed into tempered martensite by heating. As a result, both ends in the width direction of the steel sheet are moderately softened, thereby suppressing cracks at the ends of the steel sheet.

However, in order to heat a steel sheet to a temperature of 400°C or higher below the A1 point, a device that enables heating at high temperature and the cost of installing the device are required. Moreover, since the power required for the cold rolled steel production line increases, the resulting cost also increases. Therefore, there is a need for a new method that can suppress end cracks during cold rolling that does not require equipment costs and running costs due to such additional high-temperature heating processes.

Therefore, the present inventors have conducted various studies on a new method of manufacturing a steel sheet for cold rolling that can suppress end cracks of the steel sheet during cold rolling. In particular, the present invention was completed by paying attention to the exit temperature of the finishing mill during hot rolling and the water cooling control process after passing through the final stand of the finishing mill.

Specifically, the method of manufacturing a steel sheet for cold rolling according to the present embodiment involves performing hot rolling using a slab that satisfies a predetermined chemical composition so that the exit temperature of the finish rolling mill is within a predetermined temperature range, and then performing hot rolling at the finish rolling mill. This includes water-cooling the steel sheet under predetermined conditions after passing through the final stand, and then winding the steel sheet to a predetermined temperature or higher. According to this method, the ferrite/pearlite transformation at both ends, leading end, or tail end of the hot rolled steel sheet in the width direction can be promoted, and these ends can be appropriately softened. As a result, the manufactured steel sheet can suppress end cracks during subsequent cold rolling. By continuously subjecting the manufactured steel sheet to cold rolling, optional heat treatment, etc., a high tensile cold rolled steel sheet, particularly a high tensile strength cold rolled steel sheet with a tensile strength (TS: Tensile Strength) of 980 MPa or more, is obtained.

That is, according to the present invention, as a method of manufacturing a steel sheet in an intermediate process of manufacturing a high-strength cold rolled steel sheet, a method of manufacturing a steel sheet for cold rolling that can suppress end cracks of the steel sheet during subsequent cold rolling is provided. can do.

Hereinafter, embodiments of the present invention will be described in detail. Meanwhile, the scope of the present invention is not limited to the embodiments described herein, and various changes can be made without impairing the spirit of the present invention.

1. Manufacturing method of steel sheet for cold rolling

Figure 1 shows a schematic diagram of an example of a method for manufacturing a steel sheet for cold rolling in this embodiment. In FIG. 1, each symbol refers to rolling equipment 1, heating furnace 2, hot rolling mill 3, run-out table 4, cooling equipment 5, rough rolling mill 31, and final rolling mill 3. A stand 311, a finishing mill 32, and a final stand 321 of the finishing mill are shown. In the method of manufacturing a steel sheet for cold rolling in the present embodiment, for example, as shown in FIG. 1, in the rolling equipment 1, first, a slab containing a specific chemical composition is charged into the heating furnace 2. do. Thereafter, the slab extracted from the heating furnace 2 is hot rolled by the hot rolling mill 3 while controlling the exit temperature of the finishing mill 32 to be within a specific temperature range. Next, the hot-rolled steel sheet sent out on the run-out table 4 is water-cooled by the cooling facility 5 under specific conditions. Thereafter, the steel sheet is wound while adjusting the coiling temperature to become a specific temperature or higher.

Hereinafter, these processes and optionally included processes will be described in detail.

(Preparation of slab)

First, prepare a slab that satisfies a predetermined chemical composition. The slab can be prepared by any known method. A method of producing a slab includes melting a steel having the chemical composition described below and then continuously casting the slab. If necessary, a slab may be obtained by crushing and rolling a cast material obtained by ingot or continuous casting.

The slab used in the method for manufacturing a cold rolling steel sheet according to the present embodiment has, in its chemical composition, C: 0.15 mass% or more, 0.25 mass% or less, Si: 0.8 mass% or more, 3.0 mass% or less, and Mn. : 2.0 mass% or more, 3.0 mass% or less, Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%), Ti, Nb, V: 1.0 mass% or less (including 0 mass%) , and B: Contains 0.01% or less (including 0 mass%). In addition, the slab has P: 0.1 mass% or less (including 0 mass%), S: 0.01 mass% or less (including 0 mass%), Al: 0.10 mass% or less (including 0 mass%), and N: 0.01% by mass or less (including 0% by mass).

Hereinafter, the chemical composition of the slab will be described in more detail.

[C: 0.15 mass% or more and 0.25 mass% or less]

C is an important element for improving the strength of steel sheets. By setting the C content to 0.15% by mass or more, the strength improvement effect can be exhibited, and ultimately, a high-strength cold rolled steel sheet of 980 MPa or more can be obtained. By setting the C content to 0.25% by mass or less, hardenability is improved and insufficient acceleration of ferrite/pearlite transformation can be prevented. Furthermore, a decrease in the weldability of the steel sheet due to excessive C content can be suppressed. The C content is preferably 0.16 mass% or more, more preferably 0.17 mass% or more, and even more preferably 0.18 mass% or more. Moreover, the C content is preferably 0.23 mass% or less, more preferably 0.21 mass% or less, and even more preferably 0.19 mass% or less.

[Si: 0.8 mass% or more and 3.0 mass% or less]

Si is an element that contributes to increasing the strength of a steel sheet as a solid solution strengthening element. By setting the Si content to 0.8% by mass or more, a strength improving effect can be exhibited, and ultimately, a high-strength cold rolled steel sheet of 980 MPa or more can be obtained. By setting the Si content to 3.0% by mass or less, a significant decrease in the weldability of the steel sheet due to excessive Si content can be suppressed. The Si content is preferably 1.0 mass% or more, more preferably 1.5 mass% or more, and even more preferably 1.8 mass% or more. Moreover, the Si content is preferably 2.5 mass% or less, more preferably 2.1 mass% or less, and even more preferably 1.9 mass% or less.

[Mn: 1.8 mass% or more and 3.0 mass% or less]

Mn is an element that contributes to increasing the strength of a steel sheet as a solid solution strengthening element, and is also an effective element for improving the strength of a steel sheet by increasing hardenability. By setting the Mn content to 1.8% by mass or more, the strength improving effect can be exhibited, and ultimately, a high-strength cold rolled steel sheet of 980 MPa or more can be obtained. By setting the Mn content to 3.0% by mass or less, hardenability is improved and insufficient acceleration of ferrite/pearlite transformation can be prevented. The Mn content is preferably 2.0 mass% or more, more preferably 2.3 mass% or more, and even more preferably 2.5 mass% or more. Additionally, the Mn content is preferably 2.9 mass% or less, more preferably 2.8 mass% or less, and even more preferably 2.7 mass% or less.

[Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%)]

Ni, Cu, Cr, or Mo are solid solution strengthening elements that contribute to increasing the strength of the steel sheet. Additionally, these elements are also effective elements for improving hardenability and improving the strength of steel sheets. Therefore, one or more elements selected from these elements may be included in the chemical composition of the slab. In order to effectively exhibit the strength improving effect, the content of one or more elements selected from Ni, Cu, Cr, and Mo is preferably 0.05% by mass or more. In addition, the content of one or more elements selected from Ni, Cu, Cr, and Mo is 1.0% by mass or less (including 0% by mass) in order to improve hardenability and prevent insufficient acceleration of ferrite/pearlite transformation. It is). The content of one or more elements selected from Ni, Cu, Cr, and Mo is more preferably 0.1% by mass or more. Additionally, the content of one or more elements selected from Ni, Cu, Cr, and Mo is preferably 0.5% by mass or less.

[Ti, Nb, V: 1.0 mass% or less (including 0 mass%)]

Ti, Nb, or V are precipitation strengthening elements that contribute to increasing the strength of the steel sheet. Therefore, one or more elements selected from these elements may be included in the chemical composition of the slab. In order to effectively exert the precipitation strengthening effect, the content of each element selected from Ti, Nb, and V is preferably 0.01% by mass or more. Additionally, the content of each element selected from Ti, Nb, and V is 1.0% by mass or less in order to avoid saturating the effect of increasing strength described above and wasting costs. The content of one or more elements selected from Ti, Nb, and V is preferably 0.02% by mass or more. Additionally, the content of each element selected from Ti, Nb, and V is more preferably 0.5% by mass or less.

[B: 0.01 mass% or less (0 mass% or more)]

B is an effective element for improving hardenability and improving the strength of steel sheets. Therefore, B may be included in the chemical composition of the slab. In order to effectively exhibit hardenability, the B content is preferably 0.0001% by mass or more. In addition, the B content is 0.01% by mass or less, and preferably 0.005% by mass or less, in order to improve hardenability and prevent insufficient acceleration of ferrite/pearlite transformation.

[P: Preferably 0.1 mass% or less (including 0 mass%)]

P is an element that inevitably exists as an impurity element. Although P contributes to the increase in strength through solid solution strengthening, it segregates at the old austenite grain boundaries and embrittles the grain boundaries, causing end cracks. Therefore, it is preferable to suppress the P content to 0.1 mass% or less, and more preferably to 0.05 mass% or less.

[S: Preferably 0.01 mass% or less (including 0 mass%)]

S is an element that inevitably exists as an impurity element. S forms MnS inclusions. These MnS inclusions become the starting point of cracks and cause end cracks. Therefore, it is preferable to suppress the S content to 0.01 mass% or less, and more preferably to 0.005 mass% or less.

[Al(S-Al): Preferably 0.10 mass% or less (including 0 mass%)]

Al is added as a deoxidizer. In order to effectively function as a deoxidizing agent, the Al(S-Al) content is preferably 0.001% by mass or more. Additionally, Al may worsen the cleanliness of the river. Therefore, the Al(S-Al) content is preferably 0.10 mass% or less, and more preferably 0.05 mass% or less.

[N: Preferably 0.01% by mass or less (including 0% by mass)]

N is an element that inevitably exists as an impurity element. N forms coarse nitride. This coarse nitride becomes the starting point of cracks and causes end cracks. Therefore, it is preferable to suppress the N content to 0.01 mass% or less, and more preferably to 0.005 mass% or less.

In addition, the chemical composition of the slab in the present embodiment further contains, in addition to the above components, other well-known optional components within a range that does not impede acceleration of ferrite/pearlite transformation, required strength, sufficient workability, etc. You may. Other well-known optional components include, for example, Zr, Hf, Ca, Mg, and REM (rare earth elements).

[Remaining balance]

The remainder is Fe and inevitable impurities. Inevitable impurities include trace elements (eg, As, Sb, Sn, etc.) that are brought in depending on the circumstances of raw materials, materials, manufacturing facilities, etc., and the mixing of these trace elements, etc. is permitted. On the other hand, the smaller the content of P, S and N as described above, the more preferable it is usually. Therefore, these elements can also be said to be inevitable impurities. However, since the present invention can exhibit its effects better by suppressing the content of these elements to a specific range, they are stipulated as above. For this reason, in this specification, the “inevitable impurities” constituting the remainder are a concept excluding elements whose composition range is specified.

(Crack treatment of slab)

Thereafter, as a general pre-rolling process, the prepared slab is charged into the heating furnace.

When heating a slab, it is desirable to set the heating extraction temperature of the slab to 1180°C or more and 1280°C or less. By setting the heating and extraction temperature of the slab to 1280°C or lower, coarsening of the microstructure of the steel plate can be suppressed. As a result, suppression of ferrite/pearlite transformation can be prevented and it can be prevented from becoming a cause of edge hardening. By setting the heating and extraction temperature of the slab to 1180°C or higher, it is possible to prevent the rolling load from becoming excessively large, making hot rolling difficult. In this specification, the heating furnace extraction temperature is the temperature calculated by the method described in the later examples.

(Hot Rolled)

Next, hot rolling is performed using the slab extracted from the heating furnace to obtain a hot rolled steel sheet. Hot rolling is performed so that the exit temperature of the finish rolling mill is 800°C or more and 940°C or less. Other conditions are not particularly limited and can be appropriately set within a range that does not impair the effect in the present embodiment.

Generally, hot rolling includes rough rolling and finish rolling. Hereinafter, each rolling will be described.

Rough rolling can be performed, for example, using the rough rolling mill 31 shown in FIG. 1. Rough rolling is preferably performed so that the exit temperature of the rough rolling mill 31 shown in FIG. 1, specifically, the temperature of the steel sheet on the exit side of the final stand 311 of the rough rolling mill, is 1000°C or more and 1200°C or less.

By setting the exit temperature of the rough rolling mill to 1200°C or lower, coarsening of the microstructure of the steel sheet can be suppressed. As a result, suppression of ferrite/pearlite transformation can be prevented and it can be prevented from becoming a cause of edge hardening. By setting the exit temperature of the rough rolling mill to 1000°C or higher, it is possible to prevent the rolling load from becoming excessively large, making hot rolling difficult. In this specification, the exit temperature of the rough rolling mill can be measured by the method described in the later examples. The location where the radiation thermometer is placed can be anywhere from 0.1 m to 20 m from the final stand of the rough rolling mill.

In addition, the time from extraction by heating to completion of rough rolling (time between extraction and rough rolling) is preferably 240 seconds or less. By setting the time from extraction by heating until completion of rough rolling to 240 seconds or less, coarsening of the microstructure of the steel sheet can be suppressed. As a result, suppression of ferrite/pearlite transformation can be prevented and it can be prevented from becoming a cause of edge hardening. In this specification, the time between extraction and rough rolling can be measured by the method described in the later examples.

Finish rolling can be performed, for example, using the finish rolling mill 32 shown in FIG. 1. Finish rolling is performed so that the exit temperature of the finish rolling mill 32 shown in FIG. 1, specifically, the temperature of the steel sheet measured at the exit side of the final stand 321 of the finish rolling mill, is 800°C or more and 940°C or less.

When finish rolling is performed at a high temperature, the processed structure formed by hot rolling recovers, recrystallizes, and/or grains grow. As a result, ferrite-pearlite transformation after winding is suppressed, which causes hardening of the ends of the steel sheet. Therefore, by setting the exit temperature of the finish rolling mill to 940°C or lower, recovery, recrystallization, and/or grain growth of austenite can be suppressed, and end hardening of the steel sheet can be suppressed. By setting the exit temperature of the finishing mill to 800°C or higher, it is possible to prevent the rolling load from becoming large and hot rolling becoming difficult.

The exit temperature of the finishing mill is preferably 930°C or lower, more preferably 920°C or lower. Additionally, the exit temperature of the finishing mill is preferably 850°C or higher, and more preferably 870°C or higher. In this specification, the exit temperature of the finish rolling mill can be measured by the method described in the later examples. The position where the radiation thermometer is placed may be 0.1 m to 10 m from the final stand of the finishing mill.

In addition, the time from the steel sheet passing through the final stand of the rough rolling mill to reaching the first stand of the finishing mill (the time between rough rolling and finishing rolling) is preferably 50 seconds or less. By setting the time from passing the final stand of the rough rolling mill to reaching the first stand of the finishing mill to 50 seconds or less, recovery, recrystallization, and/or grain growth of the worked structure formed by hot rolling can be suppressed. As a result, the ferrite-pearlite transformation after winding can be more reliably prevented. In this specification, the time between rough rolling and final rolling can be obtained by the method described in the later examples.

The hot-rolled steel sheet that exits the final stand of the finishing mill is sent out onto the run-out table 4, for example, as shown in FIG. 1 . At this time, the speed of the steel sheet after hot rolling on the runout table 4 varies depending on the position in the longitudinal direction of the steel sheet, but is approximately 300 m/min to 1000 m/min.

(Cooling control on run-out table)

Next, within 3.0 seconds after at least a part of the steel sheet after hot rolling passes through the final stand of the finishing mill and is sent out on the runout table, at least a part of the steel sheet is cooled for 0.1 second or more at a water density of 100 L/min/m ² or more. do.

If the steel sheet is maintained at a high temperature while cooling on the runout table, the processed structure formed by hot rolling recovers, recrystallizes, and/or grain grows. As a result, ferrite/pearlite transformation after winding is suppressed, resulting in hardening of the ends of the steel sheet. Therefore, by performing such cooling control on the run-out table, recovery, recrystallization, and/or grain growth of austenite can be suppressed, and end hardening of the steel sheet can be suppressed.

The sheet thickness of the steel sheet after hot rolling to be cooled is not particularly limited, and may be about 1.0 mm to 5.0 mm, which is the sheet thickness of a common hot rolled steel sheet in this technical field.

In this specification, “at least a portion of a steel sheet” means a portion of a steel sheet after hot rolling that is subject to water cooling. Specifically, “at least a portion of the steel sheet” may be any of the entire surface of the steel sheet, a specific area in the steel sheet, or a specific location in the steel sheet. From the viewpoint of ease of cooling control, it is preferable that “at least a portion of the steel sheet” is the entire steel sheet. Alternatively, when focusing on the area where end cracks of the steel sheet are likely to occur, “at least a portion of the steel sheet” is selected from the area near both ends of the steel sheet in the width direction, the area near the leading end in the longitudinal direction, and the area near the tail end in the longitudinal direction. It is desirable to include one or more regions. In other words, by making these areas the parts of the steel sheet that are subject to water cooling, the manufacturing method of the steel sheet for cold rolling in this embodiment can be applied more effectively.

In this specification, “cooling at least a part of the steel sheet within 3.0 seconds after the steel sheet passes through the final stand of the finishing mill and is sent out on the run-out table” strictly means the following. . The part of the steel sheet to be cooled is cooled within 3.0 seconds after being sent out onto the runout table as is, based on the time it passes the final stand of the hot rolling finish rolling mill (i.e., 0 second point). Specifically, for example, in Figure 1, the portion of the steel sheet to be cooled reaches the cooling facility 5 within 3.0 seconds from the time it passes the final stand 321 of the finishing mill and is cooled. it means. The time until such cooling starts is hereinafter also referred to as “water cooling start time.” On the other hand, the plate speed of a steel plate varies depending on its longitudinal position. Therefore, in this specification, the water cooling start time is defined as the time calculated by the method described in the later examples, that is, the time calculated from the minimum value of the plate speed.

By setting the water cooling start time to 3.0 seconds or less, it is possible to avoid the steel sheet after hot rolling being held at a high temperature for a long time on the run-out table. If the steel sheet after hot rolling is kept at a high temperature for a long time, the processed structure in the steel sheet formed by hot rolling recovers, recrystallizes, and/or grain grows. As a result, the ferrite-pearlite transformation after winding is ultimately suppressed, resulting in hardening of the ends of the steel sheet. The water cooling start time is preferably within 2.5 seconds, more preferably within 2.0 seconds, and even more preferably within 1.5 seconds.

By setting the water density during cooling to 100 L/min/m ² or more, insufficient cooling of the steel sheet on the run-out table can be avoided. When cooling of the steel sheet becomes insufficient, the processed structure formed by hot rolling recovers, recrystallizes, and/or grain grows. As a result, ferrite-pearlite transformation after winding is suppressed, causing hardening of the ends of the steel sheet.

The water density during cooling is preferably 200 L/min/m ² or more, more preferably 250 L/min/m ² or more. On the other hand, the upper limit of the water density is not particularly limited, but is preferably, for example, 3000 L/min/m ² or less from the viewpoint of ensuring the sheetability of the steel sheet. In this specification, similar to the method described in the later examples, the water density refers to the water flow rate (L/min) used for cooling in the part to be cooled of the steel plate, and the length of the section such as cooling equipment (m It can be obtained by dividing by ) and width (m). Meanwhile, in the examples described later, the water density is calculated when the portion of the steel sheet to be cooled is at a position 4/5 of the total length of the steel sheet from the tip in the longitudinal direction of the steel sheet. Additionally, the amount of water flow used for cooling can be controlled by adjusting valves, etc. provided in the cooling equipment.

Cooling of the steel sheet is achieved by setting the cooling time, specifically the total water cooling time within 3 seconds from passing the final stand of the finishing mill (hereinafter also referred to as “total water cooling time within 3 seconds”) to 0.1 second or more. You can avoid becoming inadequate. When cooling of the steel sheet becomes insufficient, the processed structure formed by hot rolling recovers, recrystallizes, and/or grain grows. As a result, ferrite-pearlite transformation after winding is suppressed, causing hardening of the ends of the steel sheet.

The total water cooling time within 3 seconds is preferably 0.2 seconds or more, more preferably 0.4 seconds or more. On the other hand, the upper limit of the total water cooling time within 3 seconds is not particularly limited and is less than 3 seconds. The total water cooling time within 3 seconds may also vary depending on the position in the longitudinal direction of the steel sheet, similar to the water cooling start time described above. Therefore, in this specification, the total water cooling time within 3 seconds is defined as the time obtained by the method described in the later examples, that is, the time calculated from the maximum value of the plate speed.

In addition, when the overall length of the runout table is set to 1, the temperature measured at a position of 1/4 to 3/4 from the final stand of the finishing mill (hereinafter also referred to as “intermediate temperature”) is preferably 650°C or higher, More preferably, it is 700°C or higher, and even more preferably, it is 750°C or higher. By setting the intermediate temperature to 650°C or higher, it is possible to prevent the cooling rate from becoming excessively fast, making it difficult to secure the coiling temperature. In this specification, such intermediate temperature is taken as the temperature measured by the same method as shown in the later examples. Specifically, the intermediate temperature is the temperature of the central portion in the width direction of the coil measured with a radiation thermometer installed at a position of 1/4 to 3/4 from the final stand of the finishing mill when the overall length of the runout table is set to 1. do.

Such cooling may be performed by applying any known method and is not particularly limited. For example, water cooling can be done using a laminar equipment on the top or a spray equipment on the bottom.

(winding)

Thereafter, the cooled hot-rolled steel sheet is wound at a coiling temperature of 550°C or higher.

By setting the coiling temperature to 550°C or higher, it is possible to secure sufficient time to maintain the steel sheet in the temperature range where ferrite-pearlite transformation occurs after coiling. As a result, hardening of the ends of the steel plate can be suppressed.

The coiling temperature is preferably 600°C or higher, more preferably 630°C or higher. Moreover, the coiling temperature is preferably 750°C or lower, more preferably 700°C or lower. By setting the coiling temperature to 750°C or lower, recovery, recrystallization, and/or grain growth of the worked structure formed by hot rolling can be suppressed, and suppression of ferrite-pearlite transformation after coiling can be prevented. In this specification, the coiling temperature can be measured by the method described in the later examples. The position where the radiation thermometer is placed is 1/5 from the winder side when the total length of the run-out table is set to 1.

The coiled hot-rolled steel sheet may be naturally cooled to room temperature.

In the steel sheet manufacturing method, the coil-shaped steel sheet for cold rolling in this embodiment can be obtained by going through the above-described processes and optionally included processes. The steel sheet for cold rolling in this embodiment obtained in this way can suppress cracks at the ends of the steel sheet during subsequent cold rolling. At that time, there is no need for equipment cost and running cost for additional high temperature heating. Furthermore, according to the method for manufacturing a steel sheet for cold rolling in this embodiment, the problem of lowering the yield due to removal of portions where end cracks are likely to occur during subsequent cold rolling can be solved.

2. Manufacturing method of cold rolled steel sheet

The method for manufacturing a cold rolled steel sheet in the present embodiment further includes cold rolling the steel sheet manufactured by the method in the above-described embodiment. Hereinafter, an example of the manufacturing method of the cold rolled steel sheet in this embodiment will be described.

(sans)

Before cold rolling, the steel sheet for cold rolling manufactured by the method in the above-described embodiment may be pickled. The pickling method is not particularly limited, and any known method may be applied. For example, scale may be removed by immersion using hydrochloric acid or the like.

(cold rolled)

The method of cold rolling is not particularly limited, and any known method may be applied. For example, in order to achieve a desired plate thickness, cold rolling can be performed at a reduction ratio of 30% to 80%. The thickness of the cold rolled steel sheet is not particularly limited.

In the method of manufacturing a cold rolled steel sheet, a cold rolled steel sheet used for manufacturing a high tensile strength cold rolled steel sheet with a tensile strength (TS) of 980 MPa or more can be obtained by going through the processes described above and optionally included processes. In the cold rolled steel sheet of the present embodiment obtained in this way, end cracking during cold rolling is suppressed, and thus the risk of fracture of the steel sheet due to end cracking can be reduced. Therefore, by annealing the cold rolled steel sheet by any method, a high tensile strength cold rolled steel sheet with a tensile strength (TS) of 980 MPa or more can be suitably manufactured.

The outline of the present invention has been described above, but the manufacturing method of the cold rolling steel sheet and the manufacturing method of the cold rolling steel sheet in the embodiment of the present invention are summarized as follows.

The method for producing a steel sheet for cold rolling according to the first aspect of the present invention has a chemical composition,

C: 0.15 mass% or more, 0.25 mass% or less,

Si: 0.8 mass% or more, 3.0 mass% or less,

Mn: 1.8 mass% or more, 3.0 mass% or less,

Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%),

Ti, Nb, V: 1.0 mass% or less (including 0 mass%), and

B: 0.01% or less (including 0 mass%)

Hot rolling the slab containing so that the exit temperature of the finishing mill is 800°C or more and 940°C or less,

Within 3.0 seconds after at least a portion of the hot rolled steel sheet passes through the final stand of the finishing mill and is sent out on a runout table, cooling at least a portion of the steel sheet for 0.1 second or more at a water density of 100 L/min/m ² or more. And,

It includes winding the cooled hot-rolled steel sheet at a coiling temperature of 550°C or higher.

In the method of manufacturing the steel sheet for cold rolling described above, the slab is,

P: 0.1 mass% or less (including 0 mass%),

S: 0.01 mass% or less (including 0 mass%),

Al: 0.10 mass% or less (including 0 mass%), and

N: 0.01 mass% or less (including 0 mass%)

It is preferable to additionally contain.

The method for manufacturing a cold rolled steel sheet according to the second aspect of the present invention further includes cold rolling the steel sheet manufactured by the method according to the first aspect described above at a reduction ratio of 30% to 80%.

Example

Below, the present invention will be described in more detail by way of examples, but the present invention is in no way limited by the examples.

In this example, a steel sheet for cold rolling was actually manufactured by the method in this embodiment, and the risk rate of end cracking of the steel sheet during subsequent cold rolling was calculated from the hardness of the test piece near the end of the manufactured steel sheet. did.

[Manufacture of steel sheets for cold rolling]

Steel with the chemical composition (target chemical composition) shown in Table 1 below was melted in a converter, and then a slab was manufactured by continuous casting. The slab manufactured by continuous casting was directly charged into a heating furnace with a surface temperature of 200°C or more and 900°C or less, and heated to a high temperature. After that, the slab was extracted from the heating furnace and hot rolled by rough rolling and finish rolling. The plate thickness was finally set to 2.3 mm. The steel sheet after hot rolling was delivered as is on the run-out table, and the steel sheet on the run-out table was cooled by the upper surface laminar equipment and/or the lower surface spray equipment installed in front of it. After that, the cooled hot-rolled steel sheet was wound into a coil shape and cooled, thereby producing a steel sheet for cold rolling. On the other hand, the total length of the runout table extending from the final stand of the finishing mill to the steel plate winder was 188.3 m.

In the above manufacturing method, the conditions for hot rolling, cooling, and coiling were changed, and steel sheets for cold rolling were manufactured under various conditions. In the production of various steel sheets, the heating furnace extraction temperature during hot rolling, the exit temperature of the rough rolling mill, the time from heating furnace extraction until rough rolling is completed (time between extraction and rough rolling), and the rough rolling mill Time from passing the final stand to reaching the first stand of the finishing mill (time between rough rolling and finishing rolling), exit temperature of the finishing mill, time from passing the final stand of the finishing mill to the start of water cooling on the run-out table (start of water cooling) time), total water cooling time within 3 seconds from passing through the final stand of the finishing mill (total water cooling time within 3 seconds), water density during cooling, temperature of the steel sheet near the middle of the run-out table (intermediate temperature) ), the time from passage through the exit side of the finish rolling mill to the temperature measurement near the middle of the run-out table (time between finish rolling and intermediate temperature measurement), and the coiling temperature are shown in Table 2 below. Meanwhile, in Table 2 below, “-” indicates that the water cooling start time has passed 3.0 seconds, so the total water cooling time and water density within 3.0 seconds are 0.

In Table 2 above, detailed measurement and calculation methods for each item are as follows.

· Heating furnace extraction temperature: The heating furnace extraction temperature was calculated by heat transfer calculation from the slab temperature at the time of charging the heating furnace, the ambient temperature in the heating furnace, and the residence time in the heating furnace.

· Temperature on the exit side of the rough rolling mill: The temperature of the central portion in the width direction of the coil was measured using a radiation thermometer installed on the exit side of the rough rolling mill. The thermometer was installed at a position of 16.6 m from the final stand of the rough rolling mill.

· Time between extraction and rough rolling: The time between extraction by heating and the end of rough rolling of the lower end of the longitudinal direction of the steel sheet was taken as the time between extraction and rough rolling.

· Time between rough rolling and finish rolling: The time from the end of rough rolling of the bottom end in the longitudinal direction of the steel sheet until the start of finish rolling of the leading end of the steel sheet in the longitudinal direction is the time between rough rolling time and finish rolling. did.

· Exit temperature of the finish rolling mill: The temperature of the central portion in the width direction of the coil was measured using a radiation thermometer installed on the exit side of the finish rolling mill. The thermometer was installed at a position of 5.9 m from the final stand of the finishing mill.

· Water cooling start time: The sheet speed at the exit side of the finishing rolling mill varies depending on the position in the longitudinal direction of the steel sheet. Therefore, the water cooling start time was defined based on the sheet speed at the tip position in the longitudinal direction of the steel sheet, where the sheet speed is slowest and grain growth is most likely to occur. Specifically, the water cooling start time was obtained by dividing the distance from the final stand of the finishing mill to the position on the runout table where water cooling is performed by the minimum value of the sheet speed in the longitudinal direction of the steel sheet.

· Total water cooling time within 3 seconds: The total water cooling time within 3 seconds is the position of 4/5 of the total length of the steel sheet from the tip in the longitudinal direction of the steel sheet (in other words, the position of 1/5 of the total length of the steel sheet from the tail end of the longitudinal direction) ) was obtained. Specifically, the total water cooling time of less than 3 seconds at this position was obtained by dividing the length (m) of the section of the cooling facility where cooling was actually performed by the maximum value of the plate speed in the longitudinal direction of the steel plate.

· Quantity density: The quantity density was obtained at a position of 4/5 of the total length of the steel sheet from the tip in the longitudinal direction of the steel sheet (in other words, a position of 1/5 of the total length of the steel sheet from the tail end of the steel sheet in the longitudinal direction). Specifically, the water volume density at this location was obtained by dividing the water flow rate (L/min) used for cooling by the length (m) and width (m) of the section of the cooling facility where cooling was actually performed.

· Middle temperature: The temperature of the central portion in the width direction of the coil was measured using a radiation thermometer installed near the middle of the run-out table. The thermometer was installed at a position of 56.1 m from the final stand of the finishing rolling mill.

· Time between finish rolling and intermediate temperature measurement: The time from when the longitudinal tip of the steel sheet reaches the radiation thermometer installed on the exit side of the finish rolling mill to the time it reaches the radiation thermometer installed near the middle of the run-out table is the finish. The time between rolling and intermediate temperature measurements was taken.

· Winding temperature: The temperature of the central portion in the width direction of the coil was measured using a radiation thermometer installed near the end of the run-out table. The thermometer was placed at a position of 180.1 m from the final stand of the finishing mill.

Additionally, in the categories shown in Table 2 above, the examples of the present invention are test specimens when the finish rolling exit temperature is 800°C or more and 940°C or less and the water cooling start time is 3.0 seconds or less. On the other hand, Comparative Example 1 is a test piece when the finish rolling exit temperature is higher than 940°C and the water cooling start time is 3.0 seconds or less. Comparative Example 2 is a test piece in which the finish rolling exit temperature is 940°C or lower and the water cooling start time is greater than 3.0 seconds. Comparative Example 3 is a test piece in which the finish rolling exit temperature is higher than 940°C and the water cooling start time is longer than 3.0 seconds.

[Measurement of hardness of test pieces of cold rolling steel sheets]

The hardness of each cold rolling steel sheet obtained by the above method was measured. Test pieces were cut from both ends in the width direction of the steel plate by shear cutting so as to include a position 30 m from the tail end in the longitudinal direction of the steel plate. The size of the test piece was 10 mm (direction parallel to the rolling direction) x 20 mm (plate width direction) x 2.3 mm (plate thickness). Figure 2 is a schematic diagram showing the position of a steel plate test piece for hardness measurement. The position of the tail end in the longitudinal direction is indicated by an arrow X. As shown in Figure 2, specifically, the test piece includes a position (indicated by arrow Z) 1 mm from both ends in the width direction of the steel sheet at a position of 30 m from the longitudinal end of the steel sheet (indicated by a broken line Y). I cut it out so that I could do it. Using the test piece cut in this way, the Vickers hardness at a position 30 m from the longitudinal end of the cold rolling steel sheet, 1 mm from both ends in the width direction of the steel sheet, and a position at one quarter of the sheet thickness was measured. The Vickers hardness test was measured with a load of 9.807N, and evaluated as the maximum value among the measured values at both ends in the width direction. When the Vickers hardness determined in this way was greater than 290 HV, the manufactured steel sheet for cold rolling was hardened at the ends, and it was evaluated that there was a risk of cracking at the ends of the steel sheet during cold rolling.

The position 30 m from the longitudinal end of the steel sheet is closer to the tail end than the position 4/5 of the total length of the steel sheet from the longitudinal end of the steel sheet, which is the position where the quantity density was calculated in the above-described manufacturing process. In the longitudinal direction of the steel sheet, it is assumed that the plate speed of the steel sheet becomes faster as it approaches the tail end, and that end hardening is generally more likely to occur. Therefore, if end hardening is not observed at a position of 30 m from the longitudinal end of the steel sheet, it is natural that end hardening is not observed even at a position 4/5 of the total length of the steel sheet from the longitudinal end of the steel sheet. Furthermore, from these results, it is assumed that end hardening can be suppressed even over the entire length of the steel sheet from the tip to the tail end in the longitudinal direction by appropriately adjusting the portion of the steel sheet that is subject to cooling as necessary.

Table 3 below shows the Vickers hardness (HV) measured on test pieces of each steel plate and the evaluation results.

The end crack risk rate was calculated from the number of test pieces in each category in Table 3 and the results of hardness evaluation based on their Vickers hardness. The calculation results are shown in Table 4 below.

(Review)

As shown in Table 4 above, the Vickers hardness of all six test pieces of the present invention example was 290 HV or less, so the risk rate of end cracking was 0. On the other hand, the risk ratio of end cracking of the test pieces of Comparative Examples 1 to 3 was 0.33, 0.5, or 1.0. From these results, by setting the exit temperature of the finish rolling mill to 940°C or lower and the water cooling start time to 3.0 seconds or less, recovery, recrystallization and/or grain growth of austenite can be suppressed, and end hardening of the steel sheet can be suppressed. could know that

This application is based on Japanese Patent Application No. 2021-079218 filed on May 7, 2021, the contents of which are included in this application.

The embodiments and examples disclosed this time should be construed as illustrative in all respects and not restrictive. The scope of the present invention is indicated by the claims rather than the above description, and is intended to include all changes within the meaning and scope equivalent to the claims.

According to the present invention, it is possible to provide a method for manufacturing a steel sheet for cold rolling that can suppress cracks at the ends of the steel sheet during cold rolling. The manufactured steel sheet for cold rolling can be suitably used without causing the problem of lower yield when manufacturing a high-tensile cold rolled steel sheet with a tensile strength of 980 MPa or more.

Claims (3)

In chemical composition,
C: 0.15 mass% or more, 0.25 mass% or less,
Si: 0.8 mass% or more, 3.0 mass% or less,
Mn: 1.8 mass% or more, 3.0 mass% or less,
Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%),
Ti, Nb, V: 1.0 mass% or less (including 0 mass%), and
B: 0.01% or less (including 0 mass%)
Hot rolling the slab containing so that the exit temperature of the finishing mill is 800°C or more and 940°C or less,
Within 3.0 seconds after at least a portion of the steel sheet after the hot rolling passes through the final stand of the finishing mill and is sent out on the runout table, at least a portion of the steel sheet is heated at a water density of 100 L/min/m ² or more for 0.1 second. Over cooling,
A method of manufacturing a steel sheet for cold rolling, comprising winding the cooled hot rolled steel sheet at a coiling temperature of 550°C or higher. According to claim 1,
The slab is,
P: 0.1 mass% or less (including 0 mass%),
S: 0.01 mass% or less (including 0 mass%),
Al: 0.10 mass% or less (including 0 mass%), and
N: 0.01 mass% or less (including 0 mass%)
A method of manufacturing a steel sheet for cold rolling, further comprising: A method for producing a cold rolled steel sheet, further comprising cold rolling the steel sheet manufactured by the method according to claim 1 or 2 at a reduction ratio of 30% to 80%.

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