Method for manufacturing steel sheet for cold rolling and method for manufacturing cold rolled steel sheet

Abstract

To provide a method for producing a steel sheet for cold-rolling in an intermediate process of producing a high-tensile cold-rolled steel sheet, capable of suppressing edge cracking of the steel sheet in later cold-rolling.SOLUTION: A method for producing a steel sheet for cold rolling includes: hot-rolling a slab having a prescribed chemical composition so that an exit temperature of a finishing mill is 800°C or more and 940°C or less; cooling at least a portion of the hot-rolled steel sheet at a water density of 100 L / min / m2 or more for 0.1 seconds or more 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 delivered on a runout table, and coiling the cooled hot-rolled steel sheet at a coiling temperature of 550°C or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a steel sheet for cold rolling in the intermediate process of manufacturing a high-tensile cold-rolled steel sheet having a tensile strength of 980 MPa or more, and a method for manufacturing a cold-rolled steel sheet using the steel sheet manufactured by the method. [Background technology]

[0002] When a hot-rolled steel sheet is cold-rolled, cracks may occur at the edges in the sheet width direction (hereinafter also referred to as "width-direction edges" or "width-direction both ends") and at the edges parallel to the rolling direction (hereinafter also referred to as "longitudinal leading edge" or "longitudinal tail edge"). These edge cracks are likely to occur during the production of high-tensile cold-rolled steel sheets containing large amounts of elements that improve hardenability, such as Mn. These edge cracks may cause fractures of the steel sheet originating from these edge cracks during the cold rolling process and in subsequent processes such as the annealing process and the plating process. Therefore, in order to reduce the risk of such edge cracks, the portions of the hot-rolled steel sheet where edge cracks are likely to occur are removed, resulting in a problem of reduced yield.

[0003] On the other hand, during the cooling process of hot-rolled steel sheets after coiling, the cooling rate at the widthwise ends of the coiled steel sheet is faster than that at the center of the hot-rolled steel sheet in the widthwise direction (hereinafter also referred to as the "widthwise center"). Therefore, in hot-rolled steel sheets containing large amounts of elements that improve hardenability, such as Mn, the ferrite-pearlite transformation at both widthwise ends of the steel sheet does not progress sufficiently, and these widthwise ends become hard structures containing a relatively large amount of martensite. The same is true for the longitudinal leading and trailing ends of the steel sheet. For these reasons, it is thought that edge cracking of steel sheets is more likely to occur during cold rolling of high-tensile cold-rolled steel sheets.

[0004] As a method for suppressing the above-mentioned end cracks of steel plate, for example, Patent Document 1 describes a method for cold rolling a strip-shaped hot-rolled steel plate that has been wound into a coil and cooled, the cold-rolling method including a payout step of paying out the hot-rolled steel plate from the coil, a heating step of heating both widthwise end portions of the paid-out hot-rolled steel plate to a temperature of 400°C or more that is below the A1 point of the hot-rolled steel plate material, a pickling step of cleaning the hot-rolled steel plate after the heating step with acid, and a cold-rolling step of cold-rolling the hot-rolled steel plate after the pickling step. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-141888 Summary of the Invention [Problem to be solved by the invention]

[0006] In the method described in Patent Document 1, martensite in the microstructure at both widthwise ends of the steel sheet is transformed into tempered martensite by heating, and edge cracking of the steel sheet is suppressed by appropriately softening the steel sheet.

[0007] However, heating steel sheets to temperatures 400°C or more below the A1 point requires equipment capable of high-temperature heating and the associated costs of installing such equipment. Furthermore, the production line for cold-rolled steel sheets requires a large amount of power, which in turn increases costs. Therefore, a new method is needed that can suppress edge cracking during cold rolling without requiring the equipment and running costs of such an additional high-temperature heating process.

[0008] Therefore, an object of the present invention is to provide a method for manufacturing a steel sheet for cold rolling, which is an intermediate process in the production of a high-tensile cold-rolled steel sheet, and which can suppress edge cracking of the steel sheet during subsequent cold rolling. [Means for solving the problem]

[0009] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. That is, the present invention includes the following preferred embodiments.

[0010] A method for producing a steel sheet for cold rolling according to a first aspect of the present invention comprises the steps of: C: 0.15% by mass or more, 0.25% by mass or less, Si: 0.8% by mass or more, 3.0% by mass or less, Mn: 1.8% by mass or more, 3.0% by 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% by mass) Hot rolling the slab containing the above-mentioned compound so that the exit temperature of the finishing rolling 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 has passed through the final stand of the finishing mill and is delivered onto a runout table, at least a portion of the steel sheet is subjected to a flow rate of 100 L / min / m 2 Cooling for 0.1 seconds or more at a water density of 0.1 seconds or more; and coiling the cooled hot-rolled steel sheet at a coiling temperature of 550°C or higher.

[0011] In the above-mentioned method for manufacturing a steel sheet for cold rolling, the slab is P: 0.1% by mass or less (including 0% by mass), S: 0.01% by mass or less (including 0% by mass), Al: 0.10 mass% or less (including 0 mass%), and N: 0.01% by mass or less (including 0% by mass) It is preferred that the compound further contains:

[0012] A method for producing a cold-rolled steel sheet according to a second aspect of the present invention further comprises cold-rolling the steel sheet produced by the method according to the first aspect described above at a reduction ratio of 30% to 80%. [Effects of the Invention]

[0013] According to the present invention, a method for manufacturing a steel sheet for cold rolling can be provided, which is an intermediate process in the production of a high-tensile cold-rolled steel sheet, and which can suppress edge cracking of the steel sheet during subsequent cold rolling. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing an example of a method for producing a steel sheet for cold rolling according to this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the positions of the steel plate test pieces for hardness measurement in this example. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted extensive research into a new method for manufacturing a steel sheet that can suppress edge cracking during subsequent cold rolling, and have completed the present invention by focusing in particular on the delivery temperature of a finishing mill in hot rolling and the water cooling control process after passing through the final stand of the finishing mill.

[0016] Specifically, in the method for producing a steel sheet for cold rolling according to this embodiment, a slab having a predetermined chemical composition is hot-rolled so that the exit temperature of the finishing mill is within a predetermined temperature range. The steel sheet is then water-cooled under predetermined conditions after passing through the final stand of the finishing mill, and then coiled at a temperature equal to or higher than the predetermined temperature. This method can promote ferrite-pearlite transformation at both widthwise ends, or the leading or trailing end, of the hot-rolled steel sheet, thereby softening these ends appropriately. As a result, the produced steel sheet can suppress edge cracking during subsequent cold rolling. The produced steel sheet can then be subjected to cold rolling, optional heat treatment, and the like to produce a high-tensile cold-rolled steel sheet, particularly a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more.

[0017] Hereinafter, embodiments of the present invention will be described in detail. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0018] 1. Manufacturing method of steel sheets for cold rolling Fig. 1 shows a schematic diagram of an example of a method for producing a steel sheet for cold rolling according to this embodiment. As shown in Fig. 1, for example, in the method for producing a steel sheet for cold rolling according to this embodiment, in rolling equipment 1, first, a slab containing a specific chemical composition is charged into a heating furnace 2, and hot-rolled by a hot rolling mill 3 while controlling the outlet temperature of a finishing mill 32 to be within a specific temperature range. Next, the hot-rolled steel sheet is sent out onto a runout table 4 and water-cooled by cooling equipment 5 under specific conditions. Thereafter, the steel sheet is coiled while adjusting the coiling temperature to be equal to or higher than a specific temperature.

[0019] These steps and optional steps are described in detail below.

[0020] (Slab preparation) First, a slab having a predetermined chemical composition is prepared. The slab can be prepared by any known method. One method for producing the slab is to produce a slab by melting steel having the chemical composition described below and continuously casting it. If necessary, the cast material obtained by ingot making or continuous casting may be subjected to blooming to obtain a slab.

[0021] The chemical composition of the slab used in the method for producing a steel sheet for cold rolling in this embodiment contains: C: 0.15% by mass or more and 0.25% by mass or less; Si: 0.8% by mass or more and 3.0% by mass or less; Mn: 2.0% by mass or more and 3.0% by mass or less; Ni, Cu, Cr, Mo: 1.0% by mass or less (including 0% by mass); Ti, Nb, V: 1.0% by mass or less (including 0% by mass); and B: 0.01% or less (including 0% by mass). Preferably, the slab further contains P: 0.1% by mass or less (including 0% by mass), S: 0.01% by mass or less (including 0% by mass), Al: 0.10% by mass or less (including 0% by mass), and N: 0.01% by mass or less (including 0% by mass).

[0022] The chemical composition of the slabs is described in more detail below.

[0023] [C: 0.15% by mass or more and 0.25% by mass or less] C is an important element for improving the strength of steel sheets. By increasing the C content to 0.15% by mass or more, the strength can be improved, ultimately resulting in a high-tensile cold-rolled steel sheet with a strength of 980 MPa or more. By increasing the C content to 0.25% by mass or less, the hardenability is improved, preventing insufficient promotion of the ferrite-pearlite transformation and suppressing a decrease in the weldability of the steel sheet due to an excessive C content. The C content is preferably 0.16% by mass or more, more preferably 0.17% by mass or more, and even more preferably 0.18% by mass or more. The C content is preferably 0.23% by mass or less, more preferably 0.21% by mass or less, and even more preferably 0.19% by mass or less.

[0024] [Si: 0.8 mass% or more and 3.0 mass% or less] Si is a solid solution strengthening element that contributes to increasing the strength of steel sheets. By making the Si content 0.8% by mass or more, the strength-improving effect can be exerted, and ultimately, a high-tensile cold-rolled steel sheet of 980 MPa or more can be obtained. By making the Si content 3.0% by mass or less, a significant decrease in the weldability of the steel sheet due to an excessive Si content can be suppressed. The Si content is preferably 1.0% by mass or more, more preferably 1.5% by mass or more, and even more preferably 1.8% by mass or more. In addition, the Si content is preferably 2.5% by mass or less, more preferably 2.1% by mass or less, and even more preferably 1.9% by mass or less.

[0025] [Mn: 1.8 mass% or more and 3.0 mass% or less] Mn is a solid-solution strengthening element that contributes to increasing the strength of steel sheets, and is also an effective element for improving hardenability and thereby improving the strength of steel sheets. By increasing the Mn content to 1.8% by mass or more, the strength-improving effect can be exerted, and a high-tensile cold-rolled steel sheet with a strength of 980 MPa or more can ultimately be obtained. By reducing the Mn content to 3.0% by mass or less, hardenability is improved and insufficient promotion of the ferrite-pearlite transformation can be prevented. The Mn content is preferably 2.0% by mass or more, more preferably 2.3% by mass or more, and even more preferably 2.5% by mass or more. Furthermore, the Mn content is preferably 2.9% by mass or less, more preferably 2.8% by mass or less, and even more preferably 2.7% by mass or less.

[0026] [Ni, Cu, Cr, or Mo: 1.0 mass% or less (including 0 mass%) Ni, Cu, Cr, or Mo is an element that contributes to increasing the strength of steel sheets as a solid solution strengthening element. These elements are also effective in improving the hardenability and 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 exert the strength improving effect, Ni, Cu, Cr, or M o'sThe content of each element is preferably 0.05 mass % or more. or M o's The content of each element is 1.0 mass% or less (including 0 mass%) to improve hardenability and prevent insufficient promotion of the ferrite-pearlite transformation. or M o's The content of each element is more preferably 0.1 mass % or more. or M o's The content of each element is preferably 0.5 mass % or less.

[0027] [Ti, Nb, or V: 1.0% by mass or less (including 0% by mass)] Ti, Nb, or V is a precipitation strengthening element that contributes 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, Ti, Nb, or V's The content of each element is preferably 0.01 mass % or more. or V's The content of each element is 1.0 mass % or less to avoid the aforementioned effect of increasing strength becoming saturated and the cost becoming unnecessary. or V's The content of each element is preferably 0.02 mass % or more. or V's The content of each element is more preferably 0.5 mass % or less.

[0028] [B: 0.01% by mass or less (0% by mass or more)] B is an element effective in improving hardenability and improving the strength of steel sheet, and may therefore be included in the chemical composition of the slab. To effectively exert hardenability, the B content is preferably 0.0001% by mass or more. Furthermore, to prevent the hardenability from being improved and the ferrite-pearlite transformation from being insufficiently promoted, the B content is 0.01% by mass or less, preferably 0.005% by mass or less.

[0029] [P: preferably 0.1% by mass or less (including 0% by mass)] P is an element that is inevitably present as an impurity element. P contributes to increasing strength through solid solution strengthening, but it segregates at prior austenite grain boundaries, embrittling the grain boundaries and causing edge cracking. Therefore, the P content is preferably limited to 0.1% by mass or less, and more preferably to 0.05% by mass or less.

[0030] [S: preferably 0.01% by mass or less (including 0% by mass)] S is an element that is inevitably present as an impurity element. S forms MnS inclusions, which become the starting point of cracks and cause edge cracking. Therefore, the S content is preferably limited to 0.01% by mass or less, and more preferably to 0.005% by mass or less.

[0031] [Al (S-Al): preferably 0.10 mass% or less (including 0 mass%)] Al is added as a deoxidizer. To effectively exert its function as a deoxidizer, the Al (S-Al) content is preferably 0.001 mass% or more. Since Al may deteriorate the cleanliness of steel, the Al (S-Al) content is preferably 0.10 mass% or less, and more preferably 0.05 mass% or less.

[0032] [N: preferably 0.01% by mass or less (including 0% by mass)] N is an element that is inevitably present as an impurity element. N forms coarse nitrides that become the starting point of cracks and cause edge cracking. Therefore, the N content is preferably limited to 0.01% by mass or less, and more preferably limited to 0.005% by mass or less.

[0033] In addition, the chemical composition of the slab in this embodiment may further contain, in addition to the above components, other well-known optional components (e.g., Zr, Hf, Ca, Mg, REM (rare earth elements), etc.) within a range that does not impair the promotion of ferrite-pearlite transformation, the required strength, sufficient workability, etc.

[0034] [Remainder] The balance is Fe and unavoidable impurities. A trace element (e.g., As, Sb, Sn, etc.) that is introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. is permitted as an unavoidable impurity. As mentioned above, P, S, and N are generally preferable as their contents are lower, so they can also be considered unavoidable impurities. However, these elements are specified as above because the present invention can achieve its effects by limiting their contents to a specific range. Therefore, in this specification, the "unavoidable impurities" that make up the balance are a concept that excludes elements whose composition ranges are specified.

[0035] (Slab soaking treatment) Thereafter, as a typical pre-rolling step, the prepared slab is loaded into a heating furnace.

[0036] When heating the slab, it is preferable to set the slab extraction temperature at 1180°C or higher and 1280°C or lower. By setting the slab extraction temperature at 1280°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, which can prevent edge hardening. By setting the slab extraction temperature at 1180°C or higher, it is possible to prevent the rolling load from becoming excessively large, which makes hot rolling difficult. In this specification, the extraction temperature of the heating furnace is the temperature calculated by the method described in the Examples below.

[0037] (hot rolling) Next, the slab removed from the heating furnace is hot-rolled to obtain a hot-rolled steel sheet. Hot-rolling is performed so that the temperature at the outlet of the finishing 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 this embodiment.

[0038] Generally, hot rolling includes rough rolling and finish rolling, each of which will be described below.

[0039] Rough rolling can be performed using, for example, a roughing mill 31 shown in Fig. 1. Rough rolling is preferably performed so that the temperature on the outlet side of the roughing mill 31 shown in Fig. 1, specifically the temperature of the steel sheet on the outlet side of the final stand 311 of the roughing mill, is 1000°C or higher and 1200°C or lower.

[0040] By setting the outlet temperature of the roughing mill to 1200°C or less, coarsening of the microstructure of the steel sheet can be suppressed. As a result, ferrite-pearlite transformation is suppressed, preventing edge hardening. By setting the outlet temperature of the roughing mill to 1000°C or more, it is possible to prevent the rolling load from becoming excessively large, making hot rolling difficult. In this specification, the outlet temperature of the roughing mill can be measured by the method described in the Examples below. The radiation thermometer can be located 0.1 m to 20 m from the final stand of the roughing mill.

[0041] Furthermore, the time from the heating furnace extraction to the completion of rough rolling (time between extraction and rough rolling) is preferably 240 seconds or less. By setting the time from the heating furnace extraction to the completion of rough rolling to 240 seconds or less, coarsening of the microstructure of the steel sheet can be suppressed. As a result, ferrite-pearlite transformation is suppressed, which can prevent edge hardening. In this specification, the outlet temperature of the roughing mill can be measured by the method described in the Examples below.

[0042] Finish rolling can be performed using, for example, a finish rolling mill 32 shown in Fig. 1. Finish rolling is performed so that the temperature on the outlet side of the finish rolling mill 32 shown in Fig. 1, specifically the temperature of the steel sheet measured on the outlet side of the final stand 321 of the finish rolling mill, is 800°C or higher and 940°C or lower.

[0043] When finish rolling is performed at high temperatures, the processed structure formed during hot rolling recovers, recrystallizes, and / or grows in grain, suppressing the ferrite-pearlite transformation after coiling and causing edge hardening of the steel sheet. Therefore, by setting the exit temperature of the finishing mill to 940°C or less, austenite recovery, recrystallization, and / or grain growth can be suppressed, thereby suppressing edge hardening of the steel sheet. By setting the exit temperature of the finishing mill to 800°C or higher, it is possible to prevent the rolling load from increasing, which makes hot rolling difficult.

[0044] The exit temperature of the finishing rolling mill is preferably 930°C or less, more preferably 920°C or less. The exit temperature of the finishing rolling mill is preferably 850°C or more, more preferably 870°C or more. In this specification, the exit temperature of the finishing rolling mill can be measured by the method described in the Examples below. The radiation thermometer may be located 0.1 m to 10 m from the final stand of the finishing rolling mill.

[0045] Furthermore, the time from the final stand of the roughing mill to the first stand of the finishing mill (the time between roughing and finishing rolling) is preferably 50 seconds or less. By setting the time from the final stand of the roughing mill to the first stand of the finishing mill to 50 seconds or less, recovery, recrystallization, and / or grain growth of the worked structure formed in hot rolling can be suppressed, and the suppression of ferrite-pearlite transformation after coiling can be more reliably prevented. In this specification, the time from the final stand of the roughing mill to the first stand of the finishing mill can be determined by the method described in the Examples below.

[0046] The hot-rolled steel sheet that has left the final stand of the finishing mill is sent out onto a runout table 4, as shown in Fig. 1, for example. At this time, the speed of the hot-rolled steel sheet 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.

[0047] (Cooling control on the runout table) Next, within 3.0 seconds after at least a portion of the hot-rolled steel sheet has passed through the final stand of the finishing mill and been sent onto the runout table, at least a portion of the steel sheet is subjected to a flow rate of 100 L / min / m 2 Cool for 0.1 seconds or more at a water density of 1000 or more.

[0048] If the steel plate is held at a high temperature during cooling on the runout table, the processed structure formed during hot rolling will recover, recrystallize, and / or grow grains, which will suppress the ferrite-pearlite transformation after coiling and cause edge hardening of the steel plate. Therefore, by controlling the cooling on the runout table in this way, it is possible to suppress the recovery, recrystallization, and / or grain growth of austenite, thereby suppressing edge hardening of the steel plate.

[0049] The thickness of the steel sheet after being cooled after hot rolling is not particularly limited, and may be about 1.0 mm to 5.0 mm, which is the thickness of a typical hot rolled steel sheet in this technical field.

[0050] In this specification, "at least a portion of the steel plate" refers to a portion of the steel plate after hot rolling that is to be water-cooled, and may be any of the entire steel plate, a specific region of the steel plate, and a specific location of the steel plate. From the viewpoint of ease of cooling control, "at least a portion of the steel plate" preferably refers to the entire steel plate. Alternatively, when focusing on the region of the steel plate where edge cracking is likely to occur, "at least a portion of the steel plate" preferably includes one or more regions selected from the regions near both ends of the steel plate in the width direction, the region near the leading end in the longitudinal direction, and the region near the trailing end in the longitudinal direction. In other words, by designating these regions as the portions of the steel plate that are to be water-cooled, the method for manufacturing a steel plate for cold rolling according to this embodiment can be applied more effectively.

[0051] In this specification, "cooling at least a portion of the steel sheet within 3.0 seconds after it passes through the final stand of the finishing mill and is delivered onto the runout table" strictly means that the portion of the steel sheet to be cooled is cooled within 3.0 seconds after it is delivered onto the runout table, with the point in time when the portion of the steel sheet to be cooled passes through the final stand of the finishing mill in hot rolling being used as the reference (i.e., time 0). Specifically, for example, in FIG. 1 , this means that the portion of the steel sheet to be cooled reaches the cooling equipment 5 and is cooled within 3.0 seconds after it passes through the final stand 321 of the finishing mill. The time until such cooling starts is hereinafter also referred to as the "water cooling start time." Note that the sheet speed of the steel sheet varies depending on its position in the longitudinal direction, so in this specification, the water cooling start time is defined as the time determined by the method described later in the Examples, i.e., the time calculated from the minimum value of the sheet speed.

[0052] By setting the water cooling start time to within 3.0 seconds, it is possible to prevent the hot-rolled steel sheet from being held at high temperatures on the runout table for a long time. If the hot-rolled steel sheet is held at high temperatures for a long time, the processed structure formed in the hot-rolled steel sheet will recover, recrystallize, and / or grow grains, ultimately suppressing the ferrite-pearlite transformation after coiling and causing edge 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.

[0053] The water flow rate during cooling is 100 L / min / m 2 This prevents the strip from cooling too quickly on the runout table, which can lead to recovery, recrystallization, and / or grain growth of the processed structure formed during hot rolling, suppressing the ferrite-pearlite transformation after coiling and causing edge hardening.

[0054] The water flow rate during cooling is preferably 200 L / min / m 2 More than 250 L / min / m2 The upper limit of the water flow rate is not particularly limited, but from the viewpoint of ensuring the ease of passing the steel sheet, it is set to, for example, 3000 L / min / m 2 It is preferable that the water flow rate is equal to or less than 1 / 2 of the total length of the steel plate. In this specification, the water flow rate density can be determined by dividing the water flow rate (L / min) used for cooling in the portion of the steel plate to be cooled (calculated as a position from the leading end in the longitudinal direction to 4 / 5 of the total length of the steel plate in the examples described later), in the same manner as in the method described later in the examples. The water flow rate used for cooling can be controlled by adjusting valves and the like provided in the cooling equipment.

[0055] Insufficient cooling of the steel sheet can be avoided by setting the cooling time, specifically the total water cooling time within 3 seconds after passing through the final stand of the finishing mill (hereinafter also referred to as "total water cooling time within 3 seconds"), to 0.1 seconds or more. If the steel sheet is not cooled sufficiently, the processed structure formed during hot rolling will recover, recrystallize, and / or grow, suppressing the ferrite-pearlite transformation after coiling and causing hardening at the edges of the steel sheet.

[0056] The total water cooling time of 3 seconds or less is preferably 0.2 seconds or more, more preferably 0.4 seconds or more. The upper limit of the total water cooling time of 3 seconds or less is not particularly limited, and is less than 3 seconds. As with the water cooling start time described above, the total water cooling time of 3 seconds or less may vary depending on the position in the longitudinal direction of the steel sheet. Therefore, in this specification, the total water cooling time of 3 seconds or less is defined as the time determined by the method described in the Examples below, i.e., the time calculated from the maximum sheet speed.

[0057] Furthermore, when the total length of the runout table is taken as 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 "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, making it difficult to ensure the coiling temperature. In this specification, such intermediate temperature is defined as the temperature at 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 taken as 1, as in the method shown in the examples below.

[0058] Such cooling may be performed by any known method, and is not particularly limited. For example, water cooling can be performed using an upper surface laminar device, a lower surface spray device, or the like.

[0059] (winding) Thereafter, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 550°C or higher.

[0060] By setting the coiling temperature to 550°C or higher, it is possible to ensure that the steel sheet is held in the temperature range where the ferrite-pearlite transformation progresses after coiling for a sufficient period of time, and this makes it possible to suppress hardening of the steel sheet at the edges.

[0061] The coiling temperature is preferably 600°C or higher, more preferably 630°C or higher. 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, it is possible to prevent the ferrite-pearlite transformation after coiling from being inhibited by recovery, recrystallization, and / or grain growth of the processed structure formed by hot rolling. In this specification, the coiling temperature can be measured by the method described in the Examples below. The radiation thermometer is located at a position 1 / 5 of the way from the winder side, assuming that the total length of the runout table is 1.

[0062] After being wound, the coiled hot-rolled steel sheet may be naturally cooled to room temperature.

[0063] By going through the above-described steps and any other steps that are optionally included, a coiled steel sheet for cold rolling according to this embodiment can be obtained. The steel sheet for cold rolling according to this embodiment obtained in this manner can suppress edge cracking of the steel sheet during subsequent cold rolling. In this case, no equipment costs or running costs are required for additional high-temperature heating. In addition, the method for manufacturing a steel sheet for cold rolling according to this embodiment can solve the problem of reduced yield due to removal of portions that are prone to edge cracking during subsequent cold rolling.

[0064] 2. Manufacturing method of cold rolled steel sheet The method for producing a cold-rolled steel sheet according to the present embodiment further includes cold-rolling the steel sheet produced by the method according to the above-described embodiment. Hereinafter, an example of the method for producing a cold-rolled steel sheet according to the present embodiment will be described.

[0065] (pickling) Before cold rolling, the steel sheet for cold rolling manufactured by the method of the above-described embodiment may be pickled. The pickling method is not particularly limited, and any known method may be applied. For example, the steel sheet may be immersed in hydrochloric acid or the like to remove scale.

[0066] (cold rolling) The cold rolling method is not particularly limited, and any known method may be applied. For example, to obtain a desired plate thickness, cold rolling can be performed at a reduction rate of 30% to 80%. The plate thickness of the cold-rolled steel plate is not particularly limited.

[0067] By undergoing the above-described steps and any other steps that may be optionally included, a cold-rolled steel sheet can be obtained that can be used to manufacture a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more. The cold-rolled steel sheet of this embodiment obtained in this manner can suppress edge cracking during cold rolling, thereby reducing the risk of subsequent breakage of the steel sheet due to edge cracking. Therefore, by applying annealing using any method, a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more can be suitably manufactured. [Example]

[0068] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0069] In this example, a steel plate for cold rolling was actually manufactured using the method of this embodiment, and the risk of edge cracking of the steel plate during subsequent cold rolling was calculated from the hardness of a test piece near the edge of the manufactured steel plate.

[0070] [Manufacturing steel sheets for cold rolling] The chemical composition shown in Table 1 below (target chemical composition) (The balance includes Fe and unavoidable impurities.) ) steel was melted in a converter and then continuously cast into slabs. The slabs produced by continuous casting were directly charged into a heating furnace with a surface temperature between 200°C and 900°C, heated to a high temperature, removed from the heating furnace, and hot-rolled through rough rolling and finish rolling. The final thickness was 2.3 mm. Following hot rolling, the hot-rolled steel sheet was fed directly onto a runout table and cooled using a top-surface laminar and / or bottom-surface spray system. The cooled hot-rolled steel sheet was then coiled and cooled to produce steel sheet for cold rolling. The total length of the runout table, which stretched from the final stand of the finishing mill to the steel sheet winder, was 188.3 m.

[0071] [Table 1]

[0072] Using the above manufacturing method, steel sheets for cold rolling were manufactured under various conditions by changing the conditions of hot rolling, cooling, and coiling. The following Table 2 shows the heating furnace extraction temperature during hot rolling, the temperature at the outlet of the roughing mill, the time from heating furnace extraction to the completion of rough rolling (time between extraction and roughing rolling), the time from the last stand of the roughing mill to the first stand of the finishing mill (time between roughing and finish rolling), the temperature at the outlet of the finishing mill, the time from the last stand of the finishing mill to the start of water cooling on the runout table (water cooling start time), the total water cooling time within 3 seconds of passing through the last stand of the finishing mill (total water cooling time within 3 seconds), the water flow density during cooling, the temperature of the steel sheet near the middle of the runout table (intermediate temperature), the time from the outlet of the finishing mill to temperature measurement near the middle of the runout table (time between finish rolling and intermediate temperature measurement), and the coiling temperature during the manufacturing of each type of steel sheet. In Table 2 below, "-" indicates that the steel plate sent out onto the runout table is cooled by the installed upper surface laminar equipment and / or lower surface spray equipment, but the total water cooling time and water density within 3.0 seconds are 0 because 3.0 seconds have passed since the start of water cooling.

[0073] [Table 2]

[0074] The detailed measurement and calculation methods for each item in Table 2 above are as follows: Heating furnace extraction temperature: The extraction temperature was calculated by heat transfer calculation based on the slab temperature when charging the heating furnace, the ambient temperature inside the heating furnace, and the residence time inside the heating furnace. - Temperature at the exit of the roughing mill: The temperature at the center of the coil width direction was measured using a radiation thermometer installed at the exit of the roughing mill. The thermometer was installed 16.6 m from the final stand of the roughing mill. Time between extraction and rough rolling: The time between extraction and rough rolling was defined as the time from when the heating furnace extraction was carried out until the rough rolling of the longitudinal tail end of the steel plate was completed. Time between rough rolling and finish rolling: The time from the end of rough rolling of the longitudinal tail end of the steel plate to the start of finish rolling of the longitudinal front end of the steel plate was defined as the time between rough rolling and finish rolling. - Temperature at the exit of the finishing mill: The temperature at the center of the coil width direction was measured using a radiation thermometer installed at the exit of the finishing mill. The thermometer was installed 5.9 m from the final stand of the finishing mill. Water cooling start time: Because the plate speed at the exit of the finishing rolling mill varies depending on the position in the longitudinal direction of the steel plate, the water cooling start time was defined based on the position of the leading edge of the steel plate in the longitudinal direction, where the plate speed is the slowest in the longitudinal direction and where 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 in the longitudinal direction of the steel plate. Total water cooling time within 3 seconds: The total water cooling time within 3 seconds was calculated by dividing the length (m) of the cooling equipment section in which cooling was actually performed at a position 4 / 5 of the total length of the steel plate from the longitudinal leading edge (in other words, a position 1 / 5 of the total length of the steel plate from the longitudinal tail edge) by the maximum longitudinal plate speed of the steel plate. Water flow density: The water flow density was calculated by dividing the water flow rate (L / min) used for cooling at a position 4 / 5 of the total length of the steel plate from the longitudinal leading edge (in other words, a position 1 / 5 of the total length of the steel plate from the longitudinal tail edge) by the length (m) and width (m) of the cooling equipment section where the cooling was actually performed. Intermediate temperature: The temperature at the center of the coil width direction was measured using a radiation thermometer installed near the middle of the runout table. The thermometer was installed 56.1 m from the final stand of the finishing mill. Time between finish rolling and intermediate temperature measurement: The time from when the leading edge of the steel plate in the longitudinal direction reaches the radiation thermometer installed on the exit side of the finish rolling mill until when 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. Coiling temperature: The temperature at the center of the coil width direction was measured using a radiation thermometer installed near the end of the runout table. The thermometer was placed 180.1 m from the final stand of the finishing mill.

[0075] Furthermore, in the classifications of Table 2 above, the examples of the present invention are test pieces in which the finish rolling outlet temperature was 800°C or higher and 940°C or lower and the water cooling start time was 3.0 seconds or shorter. On the other hand, comparative example 1 is a test piece in which the finish rolling outlet temperature was higher than 940°C and the water cooling start time was 3.0 seconds or shorter. comparative example 2 is a test piece in which the finish rolling outlet temperature was 940°C or lower and the water cooling start time was longer than 3.0 seconds. comparative example 3 is a test piece in which the finish rolling outlet temperature was higher than 940°C and the water cooling start time was longer than 3.0 seconds.

[0076] [Hardness measurement of steel plate specimens for cold rolling] Test pieces were cut from both widthwise ends of each cold-rolled steel plate obtained by the above method using a shear cutter, so as to include a position 30 m from the longitudinal tail end. The size of the test piece was 10 mm (parallel to the rolling direction) × 20 mm (width direction) × 2.3 mm (thickness). Figure 2 is a schematic diagram showing the position of the steel plate test piece for hardness measurement. The position of the longitudinal tail end is indicated by arrow X. As shown in Figure 2, specifically, the test piece was cut so as to include a position (indicated by arrow Z) 1 mm from both widthwise ends of the steel plate at a position 30 m from the longitudinal tail end of the steel plate (indicated by dashed line Y). Vickers hardness was measured from the test pieces cut in this way at a position 1 mm from both widthwise ends of the steel plate 30 m from the longitudinal tail end of the steel plate for cold rolling, and at a position at one-quarter of the plate thickness. The Vickers hardness test was performed under a load of 9.807 N, and the maximum value at both ends in the width direction was used for evaluation. If the Vickers hardness thus obtained was greater than 290 HV, the steel sheet for cold rolling produced was evaluated as having hardened edges, and there was a risk of edge cracking of the steel sheet during cold rolling.

[0077] The position 30 m from the longitudinal tail end is closer to the tail end than the position 4 / 5 of the length of the steel plate from the longitudinal leading end, where the water flow rate was determined in the manufacturing process described above. It is assumed that the closer to the longitudinal leading end, the faster the steel plate speed becomes, and that edge hardening is generally more likely to occur. Therefore, if edge hardening is not observed at a position 30 m from the longitudinal leading end, it is likely that edge hardening will not be observed at a position 4 / 5 of the length of the steel plate from the longitudinal leading end. In addition, this result suggests that edge hardening can be suppressed throughout the entire longitudinal length of the steel plate, from the leading end to the tail end, by appropriately adjusting the portion of the steel plate to be cooled as needed.

[0078] Table 3 below shows the Vickers hardness (HV) measured on the test pieces of each steel plate and the evaluation results.

[0079] [Table 3]

[0080] The risk of edge cracking was calculated from the number of test pieces in each category in Table 3 and the results of the Vickers hardness evaluation. The calculation results are shown in Table 4 below.

[0081] [Table 4]

[0082] (Consideration) As shown in Table 4 above, all six test specimens of the invention examples had a Vickers hardness of 290 HV or less, and therefore the risk of edge cracking was 0. On the other hand, the edge hardening incidence rates for the test specimens of comparative examples 1 to 3 were 0.33, 0.5, or 1.0. These results show 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, it is possible to suppress austenite recovery, recrystallization, and / or grain growth, and thereby suppress edge hardening of the steel sheet.

[0083] The embodiments and examples disclosed herein should be understood to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0084] 1. Rolling equipment 2 Furnace 3. Hot rolling mill 4 Runout Table 5 Cooling equipment 31 Roughing mill 311 Roughing Mill Final Stand 32 Finishing Mill 321 Final stand of finishing mill

Claims

1. In chemical composition, C: 0.15% by mass or more, 0.25% by mass or less, Si: 0.8% by mass or more, 3.0% by mass or less, Mn: 1.8% by mass or more, 3.0% by mass or less, Ni: 1.0 mass% or less (including 0 mass%), Cu: 1.0 mass% or less (including 0 mass%), Cr: 1.0% by mass or less (including 0% by mass), Mo: 1.0 mass% or less (including 0 mass%), Ti: 1.0 mass% or less (including 0 mass%), Nb: 1.0 mass% or less (including 0 mass%), V: 1.0% by mass or less (including 0% by mass), P: 0.1% by mass or less (including 0% by mass), S: 0.01% by mass or less (including 0% by mass), N: 0.01% by mass or less (including 0% by mass), B: 0.01% by mass or less (including 0% by mass), and Al: 0.10 mass% or less (including 0 mass%) hot rolling the slab consisting of the slab containing the slab alloy and the remainder being Fe and inevitable impurities so that the temperature on the delivery side of a finishing rolling mill is 800°C or more and 940°C or less; Cooling at least a portion of the hot-rolled steel sheet; Coiling the cooled hot-rolled steel sheet at a coiling temperature of 550°C or higher, In the cooling, cooling of at least a portion of the steel plate after the hot rolling is started within 3.0 seconds after at least a portion of the steel plate passes through the final stand of the finishing mill and is delivered onto a runout table, and the cooling rate is 100 L / min / m ２ at least a portion of the steel sheet is cooled with a water flow density of at least 0.4 seconds within 3.0 seconds after passing through the final stand of the finishing rolling mill.

2. 2. The method for producing a steel plate for cold rolling according to claim 1, wherein at least a portion of the steel plate includes one or more regions selected from regions near both ends in a width direction of the steel plate, a region near a leading end in a longitudinal direction, and a region near a tail end in a longitudinal direction of the steel plate.

3. In the cooling, cooling of at least a portion of the steel plate is started within 1.5 seconds after being sent onto the runout table, and the cooling rate is 250 L / min / m ２ 3. The method for producing a steel sheet for cold rolling according to claim 1, wherein at least a portion of the steel sheet is cooled with a water flow density equal to or greater than this.

4. A method for producing a cold-rolled steel sheet, further comprising cold-rolling the steel sheet produced by the method according to any one of claims 1 to 3 at a reduction ratio of 30% to 80%.

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