Method for manufacturing cold-rolled steel sheets and method for manufacturing cold-rolled steel sheets

Abstract

According to the present invention, as a method for manufacturing a steel sheet in an intermediate process of manufacturing a high-strength cold-rolled steel sheet, a method for manufacturing a steel sheet for cold rolling can be provided that suppresses end cracking of the steel sheet during subsequent cold rolling. The method for manufacturing a steel sheet for cold rolling comprises hot rolling a slab containing a predetermined chemical composition such that the exit temperature of a finishing rolling mill is 800°C or higher and 940°C or lower; cooling at least a portion of the steel sheet after hot rolling for at least 0.1 seconds at a water density of 100 L / min / m2 or higher within 3.0 seconds after at least a portion of the steel sheet is sent onto a run-out table after passing through the final stand of the finishing rolling mill; and winding the hot-rolled steel sheet after cooling at a winding temperature of 550°C or higher.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a cold-rolled steel sheet 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 for manufacturing a cold-rolled steel sheet using a steel sheet manufactured by the method. Background Technology

[0002] When hot-rolled steel sheets are cold-rolled, cracks may occur at the ends in the width direction (hereinafter referred to as "width direction ends" or "both width direction ends") and at the ends parallel to the rolling direction (hereinafter referred to as "long direction leading ends" or "long direction trailing ends"). These end cracks are prone to occur during the manufacture of high-strength cold-rolled steel sheets using steel containing a large amount of elements that improve hardenability, such as Mn. Such end cracks can cause the steel sheet to fracture originating from these cracks during the cold-rolling process, and furthermore, during subsequent processes such as annealing and plating. Therefore, to reduce the risk caused by such end cracks, the parts of the hot-rolled steel sheets prone to end cracking are removed. However, the resulting decrease in yield is a problem.

[0003] Meanwhile, during the cooling process of hot-rolled steel sheets after coiling, the cooling rate at the ends of the coil-shaped steel sheet in the width direction is faster compared to the center of the steel sheet in the width direction (hereinafter also referred to as the "center in the width direction"). Consequently, in hot-rolled steel sheets using steel containing a large amount of elements that improve hardenability, such as Mn, the ferrite-pearlite transformation at both ends in the width direction does not proceed sufficiently, resulting in a hard structure containing a relatively large amount of martensite at both ends of the steel sheet. The same applies to the leading and trailing ends in the long direction of the steel sheet. For this reason, it is believed that end cracks are prone to occur during cold rolling and other processes when manufacturing high-strength cold-rolled steel sheets.

[0004] As a method for suppressing end cracks of the aforementioned steel plate, for example, Patent Document 1 describes a cold rolling method comprising a strip-shaped hot-rolled steel plate that is wound into a coil shape and cooled, the method comprising: a coiling process for coiling the hot-rolled steel plate from the coil; a heating process for heating both ends of the coiled hot-rolled steel plate in the width direction to a temperature of 400°C or higher below the A1 point of the hot-rolled steel plate material; a pickling process for cleaning the hot-rolled steel plate after the heating process with acid; and a cold rolling process for cold rolling the hot-rolled steel plate after the pickling process. Prior art literature

[0005] Japanese Patent Publication No. 2019-141888

[0006] The present invention aims to provide a method for manufacturing a steel sheet for cold rolling that is capable of suppressing end cracking of the steel sheet during subsequent cold rolling, as a method for manufacturing a steel sheet during an intermediate process of manufacturing a high-strength cold-rolled steel sheet.

[0007] The inventors arrived at the present invention as a result of conducting careful examinations to solve the above problem.

[0008] That is, the method for manufacturing a cold-rolled steel sheet according to the first aspect of the present invention, in terms of chemical composition,

[0009] C: 0.15 mass% or more, 0.25 mass% or less,

[0010] Si: 0.8 mass% or more, 3.0 mass% or less,

[0011] Mn: 1.8 mass% or more, 3.0 mass% or less,

[0012] Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%),

[0013] Ti, Nb, V: 1.0 mass% or less (including 0 mass%), and

[0014] B: 0.01 mass% or less (including 0 mass%)

[0015] Hot rolling a slab containing [the substance] such that the exit temperature of the finishing rolling mill is 800℃ or higher and 940℃ or lower, and

[0016] Within 3.0 seconds after at least a portion of the steel sheet after the above hot rolling passes through the final stand of the above finishing rolling mill and is fed onto the runout table, at least a portion of the steel sheet at 100 L / min / m 2 Cooling for 0.1 seconds or longer at a water volume density greater than or equal to the above, and

[0017] It includes winding the hot-rolled steel sheet after cooling at a winding temperature of 550℃ or higher.

[0018] The method for manufacturing a cold-rolled steel sheet according to the second phase of the present invention further comprises cold-rolling the steel sheet manufactured by the method according to the first phase described above at a reduction rate of 30% to 80%. Brief explanation of the drawing

[0019] FIG. 1 is a schematic diagram showing an example of a method for manufacturing a cold-rolled steel sheet in the present embodiment. FIG. 2 is a schematic diagram showing the position of the test specimen of the steel plate for hardness measurement in the present embodiment. Specific details for implementing the invention

[0020] As described above, in the method described in Patent Document 1, by heating, the martensite in the microstructure of both ends in the width direction of the steel sheet is modified into tempered martensite. As a result, the ends in the width direction of the steel sheet are appropriately softened, thereby suppressing end cracking of the steel sheet.

[0021] However, in order to heat the steel plate to a temperature of 400°C or higher below the A1 point, a device capable of heating at high temperatures and the cost of installing said device are required. Furthermore, since the power required for the cold-rolled steel plate production line increases, the associated costs are also incurred. Therefore, a new method is required that can suppress end cracks during cold rolling without requiring such additional equipment costs and running costs for the high-temperature heating process.

[0022] Therefore, the inventors have conducted various studies on a new method for manufacturing steel sheets for cold rolling that can suppress end cracking of the steel sheet during cold rolling. In particular, by focusing on 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, the present invention was completed.

[0023] Specifically, the method for manufacturing a steel sheet for cold rolling according to the present embodiment comprises using a slab satisfying a predetermined chemical composition to perform hot rolling so that the exit temperature of the finishing rolling mill is within a predetermined temperature range, then water cooling the steel sheet under predetermined conditions after passing through the final stand of the finishing rolling mill, and then winding the steel sheet at a temperature above a predetermined temperature. According to this method, ferrite-pearlite transformation can be promoted at both ends, leading ends, or trailing ends in the width direction of the hot-rolled steel sheet, and these ends can be appropriately softened. As a result, the manufactured steel sheet can suppress end cracking during subsequent cold rolling. By continuing cold rolling and optionally heat treatment on the manufactured steel sheet, a high-strength cold-rolled steel sheet, particularly a high-strength cold-rolled steel sheet with a tensile strength (TS) of 980 MPa or more, is obtained.

[0024] That is, according to the present invention, as a method for manufacturing a steel sheet in an intermediate process of manufacturing a high-strength cold-rolled steel sheet, a method for manufacturing a steel sheet for cold rolling can be provided that suppresses end cracking of the steel sheet during subsequent cold rolling.

[0025] 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 modifications may be made within the scope that does not impair the spirit of the present invention.

[0026] 1. Method for manufacturing cold-rolled steel sheets

[0027] FIG. 1 shows a schematic diagram of an example of a method for manufacturing a cold-rolled steel sheet according to the present embodiment. In FIG. 1, each symbol represents a rolling facility (1), a heating furnace (2), a hot rolling mill (3), a run-out table (4), a cooling facility (5), a rough rolling mill (31), a final stand of the rolling mill (311), a finishing rolling mill (32), and a final stand of the finishing rolling mill (321). In the method for manufacturing a cold-rolled steel sheet according to the present embodiment, for example, as shown in FIG. 1, a slab containing a specific chemical composition is first loaded into a heating furnace (2) in the rolling facility (1). Afterward, 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 rolling mill (32) to be within a specific temperature range. Next, the hot-rolled steel plate sent onto the run-out table (4) is water-cooled by the cooling facility (5) under specific conditions. Then, the steel plate is wound while adjusting the winding temperature to be above a specific temperature.

[0028] The following describes these processes and optionally included processes in detail.

[0029] (Preparation of the slab)

[0030] First, a slab satisfying a specified chemical composition is prepared. The slab may be prepared by any known method. As a method for producing the slab, an example is to produce the slab by melting steel having the chemical composition described below and then continuously casting it. If necessary, the slab may also be obtained by breaking down and rolling a cast material obtained by ingot or continuous casting.

[0031] The slab used in the method for manufacturing a cold-rolled steel sheet according to the present embodiment contains, 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, 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: 0.01 mass% or less (including 0 mass%). In addition, it is preferable that the slab further contains 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%).

[0032] The chemical composition of the slab is described in more detail below.

[0033] [C: 0.15 mass% or more, 0.25 mass% or less]

[0034] Carbon (C) is an important element for improving the strength of steel sheets. By setting the C content to 0.15 mass% or more, a strength-enhancing effect can be achieved, and ultimately, a high-strength cold-rolled steel sheet with a strength of 980 MPa or higher can be obtained. By setting the C content to 0.25 mass% or less, hardenability is improved, preventing insufficient promotion of ferrite-pearlite transformation. Furthermore, the decrease in weldability of the steel sheet caused by an 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. Additionally, 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.

[0035] [Si: 0.8 mass% or more, 3.0 mass% or less]

[0036] Si is an element that contributes to the increase in strength of steel sheets as a solid solution strengthening element. By setting the Si content to 0.8 mass% or more, a strength-enhancing effect can be achieved, and ultimately, a high-strength cold-rolled steel sheet with a strength of 980 MPa or higher can be obtained. By setting the Si content to 3.0 mass% or less, a significant decrease in the weldability of the steel sheet caused by an excess amount of Si 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. In addition, 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.

[0037] [Mn: 1.8 mass% or more, 3.0 mass% or less]

[0038] Mn is an element that contributes to increasing the strength of steel sheets as a solid solution strengthening element, and it is also an effective element for improving the strength of steel sheets by increasing hardenability. By setting the Mn content to 1.8 mass% or more, a strength-enhancing effect can be exerted, and ultimately, a high-tensile cold-rolled steel sheet of 980 MPa or higher can be obtained. By setting the Mn content to 3.0 mass% or less, hardenability is improved, and it is possible to prevent insufficient promotion of ferrite-pearlite transformation. 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. In addition, 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.

[0039] [Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%)]

[0040] Ni, Cu, Cr, or Mo are elements that contribute to increasing the strength of steel sheets as solid solution strengthening elements. Additionally, these elements are also effective elements for improving the strength of steel sheets by increasing hardenability. Therefore, one or more elements selected from these elements may be included in the chemical composition of the slab. To effectively exert a strength-enhancing effect, it is preferable that the content of each of one or more elements selected from Ni, Cu, Cr, and Mo be 0.05 mass% or more. Furthermore, the content of each of one or more elements selected from Ni, Cu, Cr, and Mo is 1.0 mass% or less (including 0 mass%) to prevent insufficient promotion of ferrite-pearlite transformation due to improved hardenability. More preferably, the content of each of one or more elements selected from Ni, Cu, Cr, and Mo is 0.1 mass% or more. Additionally, the content of each of one or more elements selected from Ni, Cu, Cr, and Mo is preferably 0.5 mass% or less.

[0041] [Ti, Nb, V: 1.0 mass% or less (including 0 mass%)]

[0042] Ti, Nb, or V are elements that contribute to the increase in strength of steel sheets as precipitation strengthening elements. 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, it is preferable that the content of each of one or more elements selected from Ti, Nb, and V be 0.01 mass% or more. Furthermore, the content of each of one or more elements selected from Ti, Nb, and V is 1.0 mass% or less to avoid wasting costs by saturating the aforementioned effect of strength increase. The content of each of one or more elements selected from Ti, Nb, and V is preferably 0.02 mass% or more. Furthermore, the content of each of one or more elements selected from Ti, Nb, and V is more preferably 0.5 mass% or less.

[0043] [B: 0.01 mass% or less (0 mass% or more)]

[0044] B is an effective element for improving the strength of steel sheets by increasing hardenability. Therefore, B may be included in the chemical composition of the slab. In order to effectively exhibit hardenability, it is desirable that the B content be 0.0001 mass% or more. In addition, the B content is 0.01 mass% or less, and preferably 0.005 mass% or less, in order to prevent insufficient promotion of ferrite-pearlite transformation due to improved hardenability.

[0045] [P: Preferably 0.1 mass% or less (including 0 mass%)]

[0046] P is an element that inevitably exists as an impurity element. Although P contributes to an increase in strength through solid solution strengthening, it segregates at the grain boundaries of the austenite and causes end cracking by embrittlement of the grain boundaries. Therefore, it is desirable to keep the P content at 0.1 mass% or less, and more desirable to keep it at 0.05 mass% or less.

[0047] [S: Preferably 0.01 mass% or less (including 0 mass%)]

[0048] S is an element that inevitably exists as an impurity element. S forms MnS inclusions. These MnS inclusions become the starting point of cracks, thereby causing end cracks. Therefore, it is desirable to keep the S content at 0.01 mass% or less, and more desirable to keep it at 0.005 mass% or less.

[0049] [Al(S-Al): Preferably 0.10 mass% or less (including 0 mass%)]

[0050] Al is added as a deoxidizing agent. In order to effectively exert its function as a deoxidizing agent, it is preferable that the Al(S-Al) content be 0.001 mass% or more. In addition, Al may degrade the cleanliness of the steel. Therefore, it is preferable that the Al(S-Al) content be 0.10 mass% or less, and more preferable that it be 0.05 mass% or less.

[0051] [N: Preferably 0.01 mass% or less (including 0 mass%)]

[0052] N is an element that inevitably exists as an impurity element. N forms coarse nitrides. These coarse nitrides become the initiation point of cracks, thereby causing end cracks. Therefore, it is desirable to keep the N content at 0.01 mass% or less, and it is more desirable to keep it at 0.005 mass% or less.

[0053] In addition, the chemical composition of the slab in this embodiment may additionally contain other known optional components in addition to the above components, provided that such additional components do not impede the promotion of ferrite-pearlite transformation, the strength deemed necessary, or sufficient processability. Examples of other known optional components include Zr, Hf, Ca, Mg, and REM (rare earth elements).

[0054] [Remaining]

[0055] The remainder consists of Fe and unavoidable impurities. Unavoidable impurities include trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the conditions of raw materials, materials, manufacturing equipment, etc., and the incorporation of these trace elements is permitted. On the other hand, regarding P, S, and N as mentioned above, it is generally preferable to have lower content. Therefore, these elements can also be referred to as unavoidable impurities. However, since the present invention can exhibit its effects more effectively by suppressing the content of these elements to a specific range, they are defined as above. For this reason, in this specification, "unavoidable impurities" constituting the remainder are a concept excluding elements whose compositional range is specified.

[0056] (Treatment of cracks in slabs)

[0057] After that, as a general pre-rolling process, the prepared slab is loaded into a heating furnace.

[0058] When heating the slab, it is preferable to set the furnace extraction temperature of the slab to 1180°C or higher and 1280°C or lower. By setting the furnace extraction temperature of the slab to 1280°C or lower, the coarsening of the microstructure of the steel sheet can be suppressed. As a result, the suppression of ferrite-pearlite transformation can be prevented, thereby preventing it from becoming a cause of end hardening. By setting the furnace extraction temperature of the slab to 1180°C or higher, it is possible to prevent the rolling load from becoming excessively large and making hot rolling difficult. In this specification, the furnace extraction temperature is the temperature calculated by the method described in the following examples.

[0059] (Hot rolling)

[0060] Next, hot rolling is performed using the slab extracted from the heating furnace to obtain a hot-rolled steel sheet. Hot rolling is performed such that the exit temperature of the finishing rolling mill is 800°C or higher and 940°C or lower, and other conditions are not particularly limited and can be appropriately set within a range that does not impair the effects of the present embodiment.

[0061] Generally, hot rolling includes rough rolling and finish rolling. Each rolling step is described below.

[0062] Rough rolling can be performed, for example, using a rough rolling mill (31) shown in FIG. 1. It is preferable to perform rough rolling such that the temperature of the steel plate at the exit side of the rough rolling mill (31) shown in FIG. 1, specifically at the exit side of the final stand (311) of the rough rolling mill, is 1000°C or higher and 1200°C or lower.

[0063] By keeping the exit temperature of the roughing mill at 1200°C or lower, the coarsening of the microstructure of the steel sheet can be suppressed. As a result, the suppression of ferrite-pearlite transformation can be prevented, thereby preventing it from becoming a cause of end hardening. By keeping the exit temperature of the roughing mill at 1000°C or higher, the rolling load can be prevented from becoming excessively large, which would make hot rolling difficult. In this specification, the exit temperature of the roughing mill can be measured by the method described in the following examples. The position where the radiation thermometer is placed should be 0.1m to 20m from the final stand of the roughing mill.

[0064] In addition, it is preferable that the time from performing furnace extraction until the completion of rough rolling (time between extraction and rough rolling) be 240 seconds or less. By making the time from performing furnace extraction until the completion of rough rolling 240 seconds or less, the coarsening of the microstructure of the steel sheet can be suppressed. As a result, the suppression of ferrite-pearlite transformation can be prevented, thereby preventing it from becoming a cause of end hardening. In this specification, the time between extraction and rough rolling can be measured by the method described in the following examples.

[0065] Finish rolling can be performed, for example, using a finish rolling mill (32) shown in FIG. 1. Finish rolling is performed such that the temperature of the steel plate measured at the exit side of the finish rolling mill (32) shown in FIG. 1, specifically at the exit side of the final stand (321) of the finish rolling mill, is 800°C or higher and 940°C or lower.

[0066] If finish rolling is performed at a high temperature, the processing structure formed by hot rolling recovers, recrystallizes, and / or grows grains. As a result, the ferrite-pearlite transformation after coiling is suppressed, causing end hardening of the steel sheet. Therefore, by keeping the exit temperature of the finish rolling mill at 940°C or lower, the recovery, recrystallization, and / or grain growth of austenite can be suppressed, thereby preventing end hardening of the steel sheet. By keeping the exit temperature of the finish rolling mill at 800°C or higher, the rolling load can be increased, which prevents hot rolling from becoming difficult.

[0067] The exit temperature of the finishing rolling mill is preferably 930°C or lower, more preferably 920°C or lower. Additionally, the exit temperature of the finishing rolling mill is preferably 850°C or higher, more preferably 870°C or higher. In this specification, the exit temperature of the finishing rolling mill can be measured by the method described in the following examples. The position where the radiation thermometer is placed should be 0.1m to 10m from the final stand of the finishing rolling mill.

[0068] In addition, it is preferable that the time from passing the final stand of the roughing mill to reaching the first stand of the finishing mill (time between roughing and finishing rolling) be 50 seconds or less. By making the time from passing the final stand of the roughing mill to reaching the first stand of the finishing mill 50 seconds or less, the recovery, recrystallization, and / or grain growth of the processing structure formed by hot rolling can be suppressed. As a result, the suppression of ferrite-pearlite transformation after coiling can be more reliably prevented. In this specification, the time between roughing and finishing rolling can be obtained by the method described in the following examples.

[0069] The hot-rolled steel sheet coming out of the final stand of the finishing rolling mill is fed onto a run-out table (4), for example, as shown in FIG. 1. At this time, the plate speed of the hot-rolled steel sheet on the run-out table (4) varies depending on the position in the long direction of the steel sheet, but is approximately 300 m / min to 1000 m / min.

[0070] (Cooling control on the runout table)

[0071] Next, within 3.0 seconds after at least a portion of the steel sheet after hot rolling passes through the final stand of the finishing rolling mill and is fed onto the runout table, at least a portion of the steel sheet is at 100 L / min / m² 2 Cooled for more than 0.1 seconds with a quantity density above.

[0072] If the steel sheet is maintained at a high temperature during cooling on a runout table, the processing structure formed by hot rolling recovers, recrystallizes, and / or grows grains. As a result, the ferrite-pearlite transformation after coiling is suppressed, causing end hardening of the steel sheet. Therefore, by controlling the cooling on the runout table in this manner, the recovery, recrystallization, and / or grain growth of austenite can be suppressed, thereby preventing end hardening of the steel sheet.

[0073] The thickness of the steel plate after cooling from hot rolling is not particularly limited, and it can be about 1.0 mm to 5.0 mm, which is the thickness of a hot-rolled steel plate common in the field of this technology.

[0074] In this specification, "at least a portion of the steel plate" refers to a portion of the steel plate after hot rolling that is subject to water cooling. Specifically, "at least a portion of the steel plate" may be any of the entire surface of the steel plate, a specific area of ​​the steel plate, or a specific location of the steel plate. From the perspective of ease of cooling control, it is preferable that "at least a portion of the steel plate" be the entire steel plate. Alternatively, if the focus is on the area where end cracks are prone to occur in the steel plate, it is preferable that "at least a portion of the steel plate" include one or more areas selected from the area near both ends in the width direction, the area near the leading end in the long direction, and the area near the trailing end in the long direction. In other words, by making these areas the portions of the steel plate subject to water cooling, the method for manufacturing a steel plate for cold rolling according to the present embodiment can be applied more effectively.

[0075] In this specification, the phrase “at least a portion of the steel plate is cooled within 3.0 seconds after passing through the final stand of the finishing rolling mill and being sent onto the runout table” strictly means the following: The portion of the steel plate to be cooled is cooled within 3.0 seconds after being sent onto the runout table, based on the point in time when it passes through the final stand of the finishing rolling mill of the hot rolling mill (i.e., the 0-second point). Specifically, for example, in FIG. 1, it means that the portion of the steel plate to be cooled reaches the cooling facility (5) and is cooled within 3.0 seconds from the point in time when it passes through the final stand (321) of the finishing rolling mill. The time until such cooling begins is hereinafter also referred to as the “water cooling start time.” Meanwhile, the plate speed of the steel plate varies depending on its position in the long direction. Therefore, in this specification, the water cooling start time is defined as the time obtained by the method described in the later examples, that is, the time calculated from the minimum value of the plate speed.

[0076] By setting the water cooling start time to within 3.0 seconds, it is possible to avoid the steel sheet after hot rolling remaining at a high temperature for a long time on the runout table. If the steel sheet after hot rolling remains at a high temperature for a long time, the processing structure in the steel sheet formed by hot rolling recovers, recrystallizes, and / or grows grains. As a result, the ferrite-pearlite transformation after coiling is suppressed, causing end hardening 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.

[0077] Water flow density during cooling is 100 L / min / m³ 2By doing so, insufficient cooling of the steel sheet on the runout table can be avoided. If the cooling of the steel sheet is insufficient, the processed structure formed by hot rolling recovers, recrystallizes, and / or grows grains. As a result, the ferrite-pearlite transformation after coiling is suppressed, causing end hardening of the steel sheet.

[0078] The water density during cooling is preferably 200 L / min / m³ 2 Ideally, 250 L / min / m 2 That concludes the explanation. Meanwhile, although the upper limit of the quantity density is not specifically restricted, from the perspective of ensuring the permeability of the steel sheet, for example, 3000 L / min / m² 2 It is preferable that the following be true. In this specification, the water density can be obtained by dividing the amount of water flow (L / min) used for cooling in the portion of the steel plate to be cooled by the length (m) and width (m) of the cooling facility, etc., in the same way as the method described in the following examples. Meanwhile, in the following examples, the water density is calculated when the portion of the steel plate to be cooled is located at a position 4 / 5 of the total length of the steel plate from the leading edge in the long direction of the steel plate. In addition, the amount of water flow used for cooling can be controlled by adjusting the valve, etc. provided by the cooling facility.

[0079] By making the cooling time, specifically the total water cooling time within 3 seconds from passing through the final stand of the finishing rolling mill (hereinafter also referred to as the "total water cooling time within 3 seconds"), 0.1 seconds or more, insufficient cooling of the steel sheet can be avoided. If the cooling of the steel sheet is insufficient, the processing structure formed by hot rolling will recover, recrystallize, and / or grow grains. As a result, ferrite-pearlite transformation after coiling is suppressed, causing end hardening of the steel sheet.

[0080] The total water cooling time within 3 seconds is preferably 0.2 seconds or more, more preferably 0.4 seconds or more. Meanwhile, the upper limit of the total water cooling time within 3 seconds is not specifically limited and is less than 3 seconds. The total water cooling time within 3 seconds may also vary depending on the position along the longitudinal direction of the steel plate, just like 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 following examples, that is, the time calculated from the maximum value of the plate speed.

[0081] In addition, when the total length of the runout table is set to 1, the temperature measured at a position 1 / 4 to 3 / 4 from the final stand of the finishing rolling mill (hereinafter also referred to as the "intermediate temperature") is preferably 650°C or higher, more preferably 700°C or higher, and even more preferably 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, thereby preventing difficulty in securing the coiling temperature. In this specification, such an intermediate temperature is defined as a temperature measured by the same method as shown in the following examples. Specifically, the intermediate temperature is defined as the temperature of the center of the coil in the width direction, measured by a radiation thermometer installed at a position 1 / 4 to 3 / 4 from the final stand of the finishing rolling mill when the total length of the runout table is set to 1.

[0082] Such cooling may be achieved by applying any known method and is not particularly limited. For example, for water cooling, an upper laminar system or a lower spray system may be applied.

[0083] (Recording)

[0084] After that, the hot-rolled steel sheet is wound after cooling at a winding temperature of 550°C or higher.

[0085] By setting the coiling temperature to 550°C or higher, sufficient time can be secured to hold the steel sheet in the temperature range where ferrite-pearlite transformation proceeds after coiling. As a result, end hardening of the steel sheet can be suppressed.

[0086] The coiling temperature is preferably 600°C or higher, more preferably 630°C or higher. Additionally, 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, the recovery, recrystallization, and / or grain growth of the processed structure formed by hot rolling can be suppressed, thereby preventing the suppression of ferrite-pearlite transformation after coiling. In this specification, the coiling temperature can be measured by the method described in the following examples. The position where the radiation thermometer is placed is set to 1 / 5 of the way from the coiler side, where the total length of the runout table is 1.

[0087] Hot-rolled steel sheets in coil form after winding may be naturally cooled to room temperature.

[0088] In the method for manufacturing a steel sheet, a cold-rolled steel sheet in the form of a coil according to the present embodiment can be obtained by undergoing the process described above and optionally included processes. The cold-rolled steel sheet according to the present embodiment obtained in this way can suppress end cracking of the steel sheet during subsequent cold rolling. At that time, there is no need for equipment costs and running costs for additional high-temperature heating. Furthermore, according to the method for manufacturing a cold-rolled steel sheet according to the present embodiment, the problem of yield reduction caused by the removal of parts prone to end cracking during subsequent cold rolling can be resolved.

[0089] 2. Method for manufacturing cold-rolled steel sheets

[0090] The method for manufacturing a cold-rolled steel sheet in the present embodiment further comprises cold-rolling the steel sheet manufactured by the method in the above-described embodiment. Hereinafter, an example of the method for manufacturing a cold-rolled steel sheet in the present embodiment will be described.

[0091] (Mountain terrain)

[0092] Prior to cold rolling, the steel sheet for cold rolling manufactured by the method in the aforementioned 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, etc.

[0093] (Cold rolled)

[0094] The method of cold rolling is not particularly limited, and any known method may be applied. For example, cold rolling can be performed with a reduction rate of 30% to 80% to achieve a desired plate thickness. The plate thickness of the cold-rolled steel sheet is not particularly limited.

[0095] In a method for manufacturing a cold-rolled steel sheet, by undergoing the process described above and optionally included processes, a cold-rolled steel sheet used for manufacturing a high-strength cold-rolled steel sheet with a tensile strength (TS) of 980 MPa or more can be obtained. In the cold-rolled steel sheet of the present embodiment obtained in this way, since end cracking during cold rolling is suppressed, subsequent risks such as fracture of the steel sheet caused by end cracking can be reduced. Therefore, by performing annealing on the cold-rolled steel sheet in any way, a high-strength cold-rolled steel sheet with a tensile strength (TS) of 980 MPa or more can be suitably manufactured.

[0096] Although the overview of the present invention has been described above, the method for manufacturing a cold-rolled steel sheet and the method for manufacturing a cold-rolled steel sheet in an embodiment of the present invention are summarized as follows.

[0097] A method for manufacturing a cold-rolled steel sheet according to the first aspect of the present invention, in terms of chemical composition,

[0098] C: 0.15 mass% or more, 0.25 mass% or less,

[0099] Si: 0.8 mass% or more, 3.0 mass% or less,

[0100] Mn: 1.8 mass% or more, 3.0 mass% or less,

[0101] Ni, Cu, Cr, Mo: 1.0 mass% or less (including 0 mass%),

[0102] Ti, Nb, V: 1.0 mass% or less (including 0 mass%), and

[0103] B: 0.01 mass% or less (including 0 mass%)

[0104] Hot rolling a slab containing [the substance] such that the exit temperature of the finishing rolling mill is 800℃ or higher and 940℃ or lower, and

[0105] Within 3.0 seconds after at least a portion of the steel sheet after the above hot rolling passes through the final stand of the above finishing rolling mill and is fed onto the runout table, at least a portion of the steel sheet at 100 L / min / m 2 Cooling for 0.1 seconds or longer at a quantity density greater than or equal to the above, and

[0106] It includes winding the hot-rolled steel sheet after cooling at a winding temperature of 550℃ or higher.

[0107] In the above-described method for manufacturing cold-rolled steel sheets, the slab is,

[0108] P: 0.1 mass% or less (including 0 mass%),

[0109] S: 0.01 mass% or less (including 0 mass%),

[0110] Al: 0.10 mass% or less (including 0 mass%), and

[0111] N: 0.01 mass% or less (including 0 mass%)

[0112] It is desirable to additionally contain .

[0113] The method for manufacturing a cold-rolled steel sheet according to the second phase of the present invention further comprises cold-rolling the steel sheet manufactured by the method according to the first phase described above at a reduction rate of 30% to 80%.

[0114] Examples

[0115] The present invention will be explained more specifically below by way of examples, but the present invention is not limited in any way by the examples.

[0116] In this embodiment, a steel sheet for cold rolling was actually manufactured using the method of this embodiment, and the risk rate of end cracking of the steel sheet during subsequent cold rolling was calculated from the hardness of a test specimen near the end of the manufactured steel sheet.

[0117] [Manufacture of cold-rolled steel sheets]

[0118] After melting steel with the chemical composition (target chemical composition) shown in Table 1 below in a converter, a slab was manufactured by continuous casting. The slab manufactured by continuous casting was charged directly into a heating furnace while its surface temperature was between 200°C and 900°C and heated to a high temperature. Subsequently, the slab was removed from the heating furnace and hot-rolled by rough rolling and finish rolling. The final plate thickness was set to 2.3 mm. The steel plate after hot rolling was fed directly onto a run-out table, and the steel plate on the run-out table was cooled by an upper-surface laminating device and / or a lower-surface spraying device installed in front of it. Afterward, the cooled hot-rolled steel plate was wound into a coil shape and cooled to manufacture a steel plate for cold rolling. Meanwhile, the total length of the run-out table extending from the final stand of the finish rolling mill to the steel plate coiler was 188.3 m.

[0119]

[0120] In the above manufacturing method, cold-rolled steel sheets were manufactured under various conditions by changing the conditions of hot rolling, cooling, and coiling. In the manufacture of various steel sheets, the furnace extraction temperature during hot rolling, the exit side temperature of the roughing mill, the time from furnace extraction until the completion of roughing rolling (time between extraction and roughing rolling), the time from passing the final stand of the roughing mill to reaching the first stand of the finishing mill (time between roughing and finishing rolling), the exit side temperature of the finishing mill, the time from passing the final stand of the finishing mill until the start of water cooling on the run-out table (water cooling start time), the total water cooling time within 3 seconds from passing the final stand of the finishing mill (total water cooling time within 3 seconds), the water density during cooling, the temperature of the steel sheet near the middle of the run-out table (intermediate temperature), the time from passing the exit side of the finishing mill until the temperature measurement near the middle of the run-out table (time between finishing rolling and intermediate temperature measurement), and the coiling temperature are shown in Table 2 below. Meanwhile, in Table 2 below, "-" indicates that the total water cooling time and water quantity density within 3.0 seconds are 0 because the water cooling start time has elapsed 3.0 seconds.

[0121]

[0122] In Table 2 above, the detailed measurement and calculation methods for each item are as follows.

[0123] · Furnace extraction temperature: The furnace extraction temperature was calculated by heat transfer calculations based on the slab temperature at the time of furnace charging, the atmosphere temperature inside the furnace, and the residence time inside the furnace.

[0124] · Temperature at the exit side of the roughing mill: The temperature of the center of the coil in the width direction was measured by a radiation thermometer installed at the exit side of the roughing mill. The thermometer was installed at a position 16.6 m from the final stand of the roughing mill.

[0125] · Time between extraction and rough rolling: The time from the furnace extraction until the completion of rough rolling of the tail end in the long direction of the steel sheet was defined as the time between extraction and rough rolling.

[0126] · Time between rough rolling and finish rolling: The time from when the rough rolling of the tail end of the steel plate in the long direction is completed until the finish rolling of the leading end of the steel plate in the long direction begins is defined as the time between rough rolling and finish rolling.

[0127] · Temperature at the exit side of the finishing rolling mill: The temperature of the center of the coil in the width direction was measured by a radiation thermometer installed at the exit side of the finishing rolling mill. The thermometer was installed at a position 5.9 m from the final stand of the finishing rolling mill.

[0128] · Water Cooling Start Time: The plate speed at the exit side of the finishing rolling mill varies depending on the position along the longitudinal direction of the steel plate. Therefore, the water cooling start time was defined based on the plate speed at the leading edge position along the longitudinal direction, where the plate speed is slowest and grain growth is most likely. Specifically, the water cooling start time was calculated by dividing the distance from the final stand of the finishing rolling mill to the position on the runout table where water cooling is performed by the minimum plate speed along the longitudinal direction of the steel plate.

[0129] · Total water cooling time within 3 seconds: The total water cooling time within 3 seconds was calculated at a position 4 / 5 of the length of the steel plate from the leading edge in the long direction (in other words, at a position 1 / 5 of the length of the steel plate from the trailing edge in the long direction). Specifically, the total water cooling time within 3 seconds at this position was calculated by dividing the length (m) of the section of the cooling facility where cooling was actually performed by the maximum value of the steel plate speed in the long direction.

[0130] · Water density: The water density was calculated at a position 4 / 5 of the total length of the steel plate from the leading edge in the long direction of the steel plate (in other words, at a position 1 / 5 of the total length of the steel plate from the trailing edge in the long direction of the steel plate). Specifically, the water density at this position was calculated by dividing the amount of water flow (L / min) used for cooling by the length (m) and width (m) of the section of the cooling facility where cooling was actually performed.

[0131] · Intermediate temperature: The temperature of the center of the coil in the width direction was measured by a radiation thermometer installed near the middle of the runout table. The thermometer was installed at a position 56.1 m from the final stand of the finishing rolling mill.

[0132] · Time between finish rolling and intermediate temperature measurement: The time from when the leading edge of the steel sheet in the long direction reaches the radiation thermometer installed at the exit side of the finish rolling mill until it reaches the radiation thermometer installed near the middle of the runout table was defined as the time between finish rolling and intermediate temperature measurement.

[0133] · Winding temperature: The temperature of the center of the coil in the width direction was measured by a radiation thermometer installed near the end of the runout table. The thermometer was positioned at a distance of 180.1 m from the final stand of the finishing rolling mill.

[0134] Additionally, in the classification shown in Table 2 above, the example of the present invention is a test specimen in which the finish rolling exit side temperature is 800°C or higher and 940°C or lower, and the water cooling start time is 3.0 seconds or lower. Meanwhile, Comparative Example 1 is a test specimen in which the finish rolling exit side temperature is higher than 940°C and the water cooling start time is 3.0 seconds or lower. Comparative Example 2 is a test specimen in which the finish rolling exit side temperature is 940°C or lower and the water cooling start time is greater than 3.0 seconds. Comparative Example 3 is a test specimen in which the finish rolling exit side temperature is higher than 940°C and the water cooling start time is greater than 3.0 seconds.

[0135] [Measurement of Hardness of Test Specimens of Cold-Rolled Steel Sheets]

[0136] The hardness of each cold-rolled steel sheet test specimen obtained by the above method was measured. Test specimens were cut at both ends of the steel sheet in the width direction by shear cutting, including a position 30 m from the tail end in the long direction of the steel sheet. The size of the test specimen was set to 10 mm (direction parallel to the rolling direction) × 20 mm (plate width direction) × 2.3 mm (plate thickness). FIG. 2 is a schematic diagram showing the position of the steel sheet test specimen for hardness measurement. The position of the tail end in the long direction is indicated by arrow X. As shown in FIG. 2, specifically, the test specimen was cut to include a position 1 mm from both ends in the width direction of the steel sheet (indicated by arrow Z) at a position 30 m from the tail end in the long direction of the steel sheet (indicated by dashed line Y). Using the test specimens cut in this manner, Vickers hardness was measured at a position 30 m from the tail end in the long direction of the cold-rolled steel sheet, at a position 1 mm from both ends in the width direction of the steel sheet, and at a position at one-quarter of the thickness of the sheet. The Vickers hardness test was performed with a load of 9.807 N, and the maximum value among the measurements at both ends in the width direction was evaluated. If the Vickers hardness obtained in this way was greater than 290 HV, it was evaluated that the manufactured cold-rolled steel sheet was end-hardened and there was a risk of end cracking of the steel sheet during cold rolling.

[0137] The position 30m from the tail end in the longitudinal direction of the steel plate is closer to the tail end than the position 4 / 5 of the length of the steel plate from the leading end in the longitudinal direction, which is the position where the quantity density was calculated in the aforementioned manufacturing process. In the longitudinal direction of the steel plate, it is assumed that the plate speed of the steel plate becomes faster as it approaches the tail end, and that end hardening generally becomes more likely to occur. Therefore, if end hardening is not observed at the position 30m from the tail end in the longitudinal direction of the steel plate, it is assumed that end hardening will naturally not be observed at the position 4 / 5 of the length of the steel plate from the leading end in the longitudinal direction either. Furthermore, from this result, it is assumed that end hardening can be suppressed throughout the entire length from the leading end to the tail end of the steel plate by appropriately adjusting the part of the steel plate subject to cooling as needed.

[0138] Table 3 below shows the measured Vickers hardness (HV) and evaluation results for each steel plate test specimen.

[0139]

[0140] The risk rate of end cracking was calculated from the number of test specimens in each category of Table 3 above and the results of the hardness evaluation based on their Vickers hardness. The results of the calculation are shown in Table 4 below.

[0141]

[0142] (Consideration)

[0143] As shown in Table 4 above, since all six test specimens of the present invention had a Vickers hardness of 290 HV or less, the risk of end cracking was 0. On the other hand, the risk of end cracking of the test specimens of Comparative Examples 1 to 3 was 0.33, 0.5, or 1.0. From these results, it was found that by setting the exit temperature of the finishing rolling mill to 940°C or less and the water cooling start time to 3.0 seconds or less, the recovery, recrystallization, and / or grain growth of austenite can be suppressed, thereby suppressing end hardening of the steel sheet.

[0144] This application is based on Japanese Patent Application No. 2021-079218 filed on May 7, 2021, the contents of which are incorporated herein.

[0145] The embodiments and examples disclosed herein should be interpreted as illustrative in all respects and not restrictive. The scope of the invention is defined by the claims, not by the description above, and is intended to include all modifications within the meaning and scope equivalent to the claims. Industrial applicability

[0146] According to the present invention, a method for manufacturing a steel sheet for cold rolling can be provided that suppresses end cracking of the steel sheet during cold rolling. The manufactured steel sheet for cold rolling can be suitably used without causing a problem of reduced yield when manufacturing a high-strength cold-rolled steel sheet with a tensile strength of 980 MPa or more.

Claims

Claim 1 A slab containing, in terms of 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 mass% or less (including 0 mass%), hot-rolling the slab such that the exit side temperature of the finishing rolling mill is 800°C or more and 940°C or less; and 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 rolling mill and is fed onto a run-out table, at least a portion of the steel sheet is rolled at a rate of 100 L / min / m² 2 A method for manufacturing a steel sheet for cold rolling, comprising cooling for 0.4 seconds or more at a water density of the above amount and winding the hot-rolled steel sheet after cooling at a winding temperature of 550°C or higher. Claim 2 A method for manufacturing a cold-rolled steel sheet according to claim 1, wherein the slab further contains 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%). Claim 3 A method for manufacturing a cold-rolled steel sheet, further comprising cold-rolling the steel sheet manufactured by the method described in claim 1 or 2 at a reduction rate of 30% to 80%.

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