Patent Application
20240216967
The present invention relates to a method for manufacturing a steel sheet for cold rolling in an 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.
When a hot-rolled steel sheet is cold-rolled, an end portion in a sheet width direction (hereinafter, also referred to as “width-direction end portion” or “both width-direction end portions”) and an end portion in a direction parallel to a rolling direction (hereinafter, also referred to as “longitudinal-direction distal end” or “longitudinal-direction tail end”) may crack. The end portion cracking often occurs during the manufacturing of a high-tensile cold-rolled steel sheet in which steel containing a large amount of an element such as Mn that improves hardenability is used. The end portion cracking thus generated may cause fracture of the steel sheet starting from the end portion cracking during the cold rolling, and further during subsequent steps such as an annealing step and a plating step. Therefore, in order to reduce the risk caused by the cracking of the end portion, a portion where end portion cracking is apt to occur in the hot-rolled steel sheet is removed. However, as a result, there is a problem that the yield decreases.
Meanwhile, in the cooling process of the hot-rolled steel sheet after coiling, the cooling speed of the width-direction end portion of the coiled steel sheet is faster than that of a central portion in the sheet width direction of the steel sheet (hereinafter, also referred to as “width-direction central portion”). Therefore, in a hot-rolled steel sheet in which steel containing a large amount of an element such as Mn that improves hardenability is used, ferrite/pearlite transformation does not sufficiently proceed at both the width-direction end portions of the steel sheet, and both the width-direction end portions of the steel sheet have a hard structure containing a relatively large amount of martensite. The same applies to the longitudinal-direction distal end and the longitudinal-direction tail end of the steel sheet. For this reason, it is considered that the end portion of the steel sheet is apt to crack during cold-rolling or the like during the manufacturing of the high-tensile cold-rolled steel sheet.
As the method for preventing the end portion of the steel sheet from cracking, for example, Patent Literature 1 discloses a method for cold-rolling a band-shaped hot-rolled steel sheet which is coiled into a coil shape and cooled. The method includes: a feeding step of feeding the hot-rolled steel sheet from a coil; a heating step of heating both the width-direction end portions of the fed hot-rolled steel sheet to a temperature of 400° C. or higher and an A1 point or lower of a hot-rolled steel sheet material; an acid washing step of washing the hot-rolled steel sheet after the heating step with an acid; and a cold-rolling step of cold-rolling the hot-rolled steel sheet after the acid washing step.
It is an object of the present invention to provide a method for manufacturing a steel sheet for cold rolling in an intermediate process of manufacturing of a high-tensile cold-rolled steel sheet, the method capable of preventing an end portion of the steel sheet from cracking during the subsequent cold-rolling.
As a result of intensive studies to solve the above problems, the present inventors have accomplished the present invention.
That is, a method for manufacturing a steel sheet for cold rolling according to a first aspect of the present invention comprises:
A method for manufacturing a cold-rolled steel sheet according to a second aspect of the present invention further comprises cold-rolling the above-described steel sheet manufactured by the method according to the first aspect at a rolling rate of 30% to 80%.
As described above, in a method described in Patent Literature 1, by heating, martensite in a microstructure at each of both the width-direction end portions of a steel sheet is modified to tempered martensite. As a result, both the width-direction end portions of the steel sheet are appropriately softened, so that the end portion of the steel sheet is prevented from cracking.
However, in order to heat the steel sheet to a temperature equal to or higher than 400° C. and equal to or lower than a point A1, a device that enables heating at a high temperature and cost for installing the device are required. Furthermore, power required for the production line of a cold-rolled steel sheet is also increased, and thus the cost thereof is also increased. Therefore, there is a demand for a novel method that does not require facility cost and running cost due to such an additional high-temperature heating step and can prevent the end portion from cracking during cold-rolling.
Therefore, the present inventors have conducted various studies on a novel method for manufacturing a steel sheet for cold rolling capable of preventing the end portion of the steel sheet from cracking during cold-rolling. In particular, the present inventors have completed the present invention by focusing on the outlet temperature of a finishing rolling mill in hot-rolling and a water-cooling controlling step after passing through a final stand of the finishing rolling mill.
Specifically, a method for manufacturing a steel sheet for cold rolling according to the present embodiment includes: hot-rolling a slab satisfying predetermined chemical composition so that the outlet temperature of a finishing rolling mill falls within a predetermined temperature range; water-cooling the steel sheet under a predetermined condition after passing through a final stand of the finishing rolling mill; and coiling the steel sheet at a predetermined temperature or higher. Such a method makes it possible to promote ferrite/pearlite transformation at both the width-direction end portions, distal end, or tail end of the hot-rolled steel sheet, and to appropriately soften the end portions. As a result, the manufactured steel sheet can prevent the end portion from cracking during the subsequent cold-rolling. The manufactured steel sheet is subsequently subjected to cold-rolling, and optional heat treatment or the like, thereby obtaining 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.
That is, the present invention can provide a method for manufacturing a steel sheet for cold rolling in an intermediate process of manufacturing of a high-tensile cold-rolled steel sheet, the method making it possible to prevent an end portion of the steel sheet from cracking during the subsequent cold-rolling.
Hereinafter, an embodiment of the present invention will be described in detail. Note that, the scope of the present invention is not limited to the embodiment described herein, and various modifications can be made without departing from the spirit of the present invention.
Hereinafter, these steps and optionally included steps will be described in detail.
First, a slab satisfying predetermined chemical composition is prepared. The slab can be prepared by any known method. Examples of a method for producing the slab include a method in which steel having chemical composition described below is smelted, and then subjected to continuous casting to produce the slab. If necessary, a cast material obtained by ingot-making or continuous casting may be subjected to blooming milling to obtain the slab.
The slab used in the method for manufacturing a steel sheet for cold rolling in the present embodiment has chemical composition containing: C: 0.15% by mass or more and 0.25% by mass by mass, 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, and Mo: 1.0% by mass or less (including 0% by mass), Ti, Nb, and V: 1.0% by mass or less (including 0% by mass), and B: 0.01% or less (including 0% by mass). The slab preferably 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).
Hereinafter, the chemical composition of the slab will be described in more detail.
C is an important element for improving the strength of the steel sheet. By setting the content of C to 0.15% by mass or more, a strength improving action can be exerted, and finally, a high-tensile cold-rolled steel sheet of 980 MPa or more can be obtained. By setting the content of C to 0.25% by mass or less, the hardenability of the steel sheet is improved, and therefore the promotion of ferrite/pearlite transformation can be prevented from becoming insufficient. Furthermore, it is possible to suppress a decrease in the weldability of the steel sheet due to an excessive content of C. The content of C is preferably 0.16% by mass or more, more preferably 0.17% by mass or more, and still more preferably 0.18% by mass or more. The content of C is preferably 0.23% by mass or less, more preferably 0.21% by mass or less, and still more preferably 0.19% by mass or less.
Si is an element that contributes to an increase in the strength of the steel sheet as a solid solution strengthening element. By setting the content of Si to 0.8% by mass or more, a strength improving action can be exerted, and finally, a high-tensile cold-rolled steel sheet of 980 MPa or more can be obtained. By setting the content of Si to 3.0% by mass or less, it is possible to suppress a significant decrease in the weldability of the steel sheet due to an excessive amount of Si. The content of Si is preferably 1.0% by mass or more, more preferably 1.5% by mass or more, and still more preferably 1.8% by mass or more. The content of Si is preferably 2.5% by mass or less, more preferably 2.1% by mass or less, and still more preferably 1.9% by mass or less.
Mn is an element that contributes to an increase in the strength of the steel sheet as a solid-solution strengthening element, and is also an element effective for enhancing the hardenability of the steel sheet to improve the strength thereof. By setting the content of Mn to 1.8% by mass or more, a strength improving action can be exerted, and finally, a high-tensile cold-rolled steel sheet of 980 MPa or more can be obtained. By setting the content of Mn to 3.0% by mass or less, the hardenability of the steel sheet is improved, and therefore the promotion of ferrite/pearlite transformation can be prevented from becoming insufficient. The content of Mn is preferably 2.0% by mass or more, more preferably 2.3% by mass or more, and still more preferably 2.5% by mass or more. The content of Mn is preferably 2.9% by mass or less, more preferably 2.8% by mass or less, and still more preferably 2.7% by mass or less.
Ni, Cu, Cr, or Mo is an element that contributes to an increase in the strength of the steel sheet as a solid solution strengthening element. These elements are also effective elements for enhancing the hardenability of the steel sheet to improve the strength thereof. Therefore, one or more elements selected from these elements may be included in the chemical composition of the slab. In order to effectively exert a strength improving action, the content of each of one or more elements selected from Ni, Cu, Cr, and Mo is preferably 0.05% by mass or more. The content of each of one or more elements selected from Ni, Cu, Cr, and Mo is 1.0% by mass or less (including 0% by mass) in order to improve the hardenability of the steel sheet to prevent the promotion of ferrite/pearlite transformation from becoming insufficient. The content of each of one or more elements selected from Ni, Cu, Cr, and Mo is more preferably 0.1% by mass or more. The content of each of one or more elements selected from Ni, Cu, Cr, and Mo is preferably 0.5% by mass or less.
Ti, Nb, or V is an element that contributes to an increase in the strength of the steel sheet as a precipitation strengthening element. Therefore, one or more elements selected from these elements may be included in the chemical composition of the slab. In order to effectively exert a precipitation strengthening action, the content of each of one or more elements selected from Ti, Nb, and V is preferably 0.01% by mass or more. The content of each of one or more elements selected from Ti, Nb, and V is 1.0% by mass or less in order to avoid the above-described effect of increasing the strength from being saturated and the cost from being wasted. The content of each of one or more elements selected from Ti, Nb, and V is preferably 0.02% by mass or more. The content of each of one or more elements selected from Ti, Nb, and V is more preferably 0.5% by mass or less.
B is an element effective for enhancing the hardenability of the steel sheet to improve the strength thereof. Therefore, B may be included in the chemical composition of the slab. In order to effectively exert the hardenability of the steel sheet, the content of B is preferably 0.0001% by mass or more. The content of B is 0.01% by mass or less, and preferably 0.005% by mass or less in order to improve the hardenability of the steel sheet to prevent the promotion of ferrite/pearlite transformation from becoming insufficient.
P is an element inevitably present as an impurity element. P contributes to an increase in the strength of the steel sheet due to solid solution strengthening. However, P segregates at prior austenite grain boundaries and embrittles the grain boundaries, thereby causing an end portion of the steel sheet to crack. Therefore, the content of P is preferably suppressed to 0.1% by mass or less, and more preferably suppressed to 0.05% by mass or less.
S is an element inevitably present as an impurity element. S forms MnS inclusions. The MnS inclusions serve as starting points of cracks, thereby causing an end portion of the steel sheet to crack. Therefore, the content of S is preferably suppressed to 0.01% by mass or less, and more preferably suppressed to 0.005% by mass or less.
Al is added as a deoxidizing material. In order to effectively exert the action of the deoxidizing material, the content of Al(S—Al) is preferably 0.001% by mass or more. Al may deteriorate the cleanliness of steel. Therefore, the content of Al(S—Al) is preferably 0.10% by mass or less, and more preferably 0.05% by mass or less.
N is an element inevitably present as an impurity element. N forms coarse nitrides. The coarse nitrides serve as starting points of cracks, thereby causing an end portion of the steel sheet to crack. Therefore, the content of N is preferably suppressed to 0.01% by mass or less, and more preferably suppressed to 0.005% by mass or less.
The chemical composition of the slab in the present embodiment may further contain, in addition to the above components, other known optional components as long as promotion of ferrite/pearlite transformation, required strength, and sufficient workability and the like are not hindered. Examples of the other known optional components include Zr, Hf, Ca, Mg, and REM (rare earth element).
The balance is Fe and inevitable impurities. Examples of the inevitable impurities include trace elements (for example, As, Sb, or Sn or the like) incorporated depending on the situations of raw materials, materials, and producing facilities and the like, and it is permitted to mix these trace elements and the like. The contents of P, S, and N as described above are usually preferably smaller. Therefore, these elements can also be said to be inevitable impurities. However, these elements are defined as described above since the present invention can more favorably exert the effect by suppressing the contents thereof to a specific range. Thus, in the present specification, “inevitable impurities” constituting the balance mean the concept excluding elements whose composition range is defined.
Thereafter, as a general pre-rolling step, the prepared slab is charged into a heating furnace.
When the slab is heated, the heating furnace extraction temperature of the slab is preferably 1180ºC or higher and 1280° C. or lower. By setting the heating 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, it is possible to prevent the suppression of ferrite/pearlite transformation and to prevent the occurrence of end portion hardening. By setting the heating furnace extraction temperature of the slab to 1180° C. or higher, it is possible to prevent difficult hot-rolling due to an excessively large rolling load. In the present specification, the heating furnace extraction temperature is a temperature calculated by a method described in the following Examples.
Then, the slab extracted from the heating furnace is hot-rolled to obtain a hot-rolled steel sheet. Other conditions are not particularly limited as long as the slab is hot-rolled so that the outlet temperature of the finishing rolling mill is 800° C. or higher and 940° C. or lower, and can be appropriately set as long as the effect of the present embodiment is not impaired.
Generally, the hot-rolling includes rough rolling and finishing rolling. Each rolling will be described below.
The rough rolling can be performed using, for example, the rough rolling mill 31 shown in
By setting the outlet temperature of the rough rolling mill to 1200° C. or lower, the coarsening of the microstructure of the steel sheet can be suppressed. As a result, it is possible to prevent the suppression of ferrite/pearlite transformation and to prevent the occurrence of end portion hardening. By setting the outlet temperature of the rough rolling mill to 1000° C. or higher, it is possible to prevent difficult hot-rolling due to an excessively large rolling load. In the present specification, the outlet temperature of the rough rolling mill can be measured by a method described in the following Examples. A radiation thermometer may be disposed at a position of 0.1 m to 20 m from the final stand of the rough rolling mill.
A time from extraction in the heating furnace to completion of the rough rolling (time between extraction and rough rolling) is preferably 240 seconds or less. By setting the time from extraction in the heating furnace to completion of the rough rolling to 240 seconds or less, the coarsening of the microstructure of the steel sheet can be suppressed. As a result, it is possible to prevent the suppression of ferrite/pearlite transformation and to prevent the occurrence of end portion hardening. In the present specification, the time between extraction and rough rolling can be measured by a method described in the following Examples.
Finishing rolling can be performed using, for example, the finishing rolling mill 32 shown in
When the finishing rolling is performed at a high temperature, a processed structure formed by hot-rolling is recovered, recrystallized, and/or grown. As a result, ferrite/pearlite transformation after coiling is suppressed, which causes the end portion hardening of the steel sheet. Therefore, by setting the outlet temperature of the finishing rolling mill to 940° C. or less, it is possible to suppress the recovery, recrystallization, and/or grain growth of austenite and to suppress the end portion hardening of the steel sheet. By setting the outlet temperature of the finishing rolling mill to 800° C. or higher, it is possible to prevent difficult hot-rolling due to a large rolling load.
The outlet temperature of the finishing rolling mill is preferably 930° C. or lower, and more preferably 920° C. or lower. The outlet temperature of the finishing rolling mill is preferably 850° C. or higher, and more preferably 870° C. or higher. In the present specification, the outlet temperature of the finishing rolling mill can be measured by a method described in the following Examples. A radiation thermometer may be disposed at a position of 0.1 m to 10 m from the final stand of the finishing rolling mill.
A time from the passage of the steel sheet at the final stand of the rough rolling mill to the arrival of the steel sheet at the first stand of the finishing rolling mill (time between rough rolling and finishing rolling) is preferably 50 seconds or less. By setting the time from the passage of the steel sheet at the final stand of the rough rolling mill to the arrival of the steel sheet at the first stand of the finishing rolling mill to 50 seconds or less, it is possible to suppress the recovery, recrystallization, and/or grain growth of the processed structure formed by hot-rolling. As a result, the suppression of ferrite/pearlite transformation after coiling can be more reliably prevented. In the present specification, the time between rough rolling and finishing rolling can be determined by a method described in the following Examples.
The hot-rolled steel sheet that has left the final stand of the finishing rolling mill is, for example, sent out on the run-out table 4 as shown in
Next, within 3.0 seconds after at least a portion of the hot-rolled steel sheet passes through the final stand of the finishing rolling mill and is sent out on the run-out table, the at least a portion of the steel sheet is cooled at a water volume density of 100 L/min/m2 or more for 0.1 seconds or more.
If the steel sheet is held at a high temperature during cooling on the run-out table, the processed structure formed by hot-rolling is recovered, recrystallized, and/or grown. As a result, ferrite/pearlite transformation after coiling is suppressed, which causes the end portion hardening of the steel sheet. Therefore, by performing cooling control on the run-out table, it is possible to suppress the recovery, recrystallization, and/or grain growth of austenite and to suppress the end portion hardening of the steel sheet.
The sheet thickness of the hot-rolled steel sheet to be cooled is not particularly limited, and may be about 1.0 mm to 5.0 mm, which is the sheet thickness of a hot-rolled steel sheet common in the present technical field.
In the present specification, the “at least a portion of the steel sheet” means a portion to be water-cooled in the hot-rolled steel sheet. Specifically, the “at least a portion of the steel sheet” may be any of the entire surface of the steel sheet, a specific region in the steel sheet, and a specific location in the steel sheet. From the viewpoint of the case of cooling control, the “at least a portion of the steel sheet” is preferably the entire steel sheet. Alternatively, when focusing on a region where the end portion of the steel sheet is apt to crack, the “at least a portion of the steel sheet” preferably includes one or more regions selected from a region in the vicinity of both the width-direction end portions of the steel sheet, a region in the vicinity of the longitudinal-direction distal end, and a region in the vicinity of the longitudinal-direction tail end. In other words, by setting these regions as portions to be water-cooled in the steel sheet, the method for manufacturing a steel sheet for cold rolling in the present embodiment can be more effectively applied.
In the present specification, “within 3.0 seconds after at least a portion of the steel sheet passes through the final stand of the finishing rolling mill and is sent out on the run-out table, the at least a portion of the steel sheet is cooled” strictly means the following. The portion to be cooled in the steel sheet is cooled within 3.0 seconds after being sent out on the run-out table as it is with reference to a time point at which the portion passes through the final stand of the finishing rolling mill for hot-rolling (that is, as a 0 second point). Specifically, for example, in
By setting the water-cooling start time to 3.0 seconds or less, it is possible to avoid the hot-rolled steel sheet from being held at a high temperature for a long time on the run-out table. If the hot-rolled steel sheet is held at a high temperature for a long time, the processed structure in the steel sheet formed by hot-rolling is recovered, recrystallized, and/or grown. As a result, finally, ferrite/pearlite transformation after coiling is suppressed, which causes the end portion hardening of the steel sheet. The water-cooling start time is preferably 2.5 seconds or less, more preferably 2.0 seconds or less, and still more preferably 1.5 seconds or less.
By setting the water volume density during cooling to 100 L/min/m2 or more, it is possible to avoid insufficient cooling of the steel sheet on the run-out table. If the steel sheet is insufficiently cooled, the processed structure formed by hot-rolling is recovered, recrystallized, and/or grown. As a result, ferrite/pearlite transformation after coiling is suppressed, which causes the end portion hardening of the steel sheet.
The water volume density during cooling is preferably 200 L/min/m2 or more, and more preferably 250 L/min/m2 or more. The upper limit of the water volume density is not particularly limited, but is preferably, for example, 3000 L/min/m2 or less from the viewpoint of securing the runnability of the steel sheet. In the present specification, the water volume density can be determined by dividing a water flow rate (L/min) used for cooling in the portion to be cooled of the steel sheet by the length (m) and width (m) of the section of the cooling facility or the like, as with to a method described in the following Examples. In Examples described later, the water volume density is calculated when the portion to be cooled of the steel sheet is located at a position of ⅘ of the total length of the steel sheet from the longitudinal-direction distal end of the steel sheet. The water flow rate used for cooling can be controlled by adjusting a valve or the like included in the cooling facility.
By setting a cooling time, specifically, a total water-cooling time (hereinafter, also referred to as “total water-cooling time within 3 seconds”) within 3 seconds after passing through the final stand of the finishing rolling mill to 0.1 seconds or more, it is possible to avoid insufficient cooling of the steel sheet. If the steel sheet is insufficiently cooled, the processed structure formed by hot-rolling is recovered, recrystallized, and/or grown. As a result, ferrite/pearlite transformation after coiling is suppressed, which causes the end portion hardening of the steel sheet.
The total water-cooling time within 3 seconds is preferably 0.2 seconds or more, and more preferably 0.4 seconds or more. The upper limit of the total water-cooling time within 3 seconds is not particularly limited, and is less than 3 seconds. The total water-cooling time within 3 seconds may also vary depending on the longitudinal-direction position of the steel sheet, as with the water-cooling start time described above. Therefore, in the present specification, the total water-cooling time within 3 seconds is defined as a time determined by a method described in the following Examples, that is, a time calculated from the maximum value of the sheet speed.
When the total length of the run-out table is 1, a temperature (hereinafter, also referred to as “intermediate temperature”) measured at a position of ¼ to ¾ from the final stand of the finishing rolling mill is preferably 650° C. or higher, more preferably 700° C. or higher, and still more preferably 750° C. or higher. By setting the intermediate temperature to 650° C. or higher, it is possible to avoid becoming the excessively fast cooling rate, which cannot prevent difficult securement of the coiling temperature. In the present specification, such an intermediate temperature is a temperature measured by the same method as that shown in the following Examples. Specifically, when the total length of the run-out table is 1, the intermediate temperature is a temperature of the width-direction center portion of the coil measured by a radiation thermometer installed at a position of ¼ to ¾ from the final stand of the finishing rolling mill.
Such cooling may be performed by any known method, and is not particularly limited. For example, for water-cooling, an upper surface laminar facility or a lower surface spray facility or the like can be applied.
Thereafter, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 550° C. or higher.
By setting the coiling temperature to 550° C.′ or higher, it is possible to sufficiently secure a time for holding the steel sheet in a temperature region where ferrite/pearlite transformation proceeds after coiling. As a result, the end portion hardening of the steel sheet can be suppressed.
The coiling temperature is preferably 600° C. or higher, and more preferably 630° C. or higher. The coiling temperature is preferably 750° C. or lower, and 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, and therefore the suppression of ferrite/pearlite transformation after coiling can be prevented. In the present specification, the coiling temperature can be measured by a method described in the following Examples. The radiation thermometer is disposed at a position of ⅕ from the coiling machine side when the entire length of the run-out table is 1.
The coiled hot-rolled steel sheet after coiling may be naturally cooled to room temperature.
In the method for manufacturing a steel sheet, the coiled steel sheet for cold rolling in the present embodiment can be obtained through the steps as described above and optionally included steps. The thus obtained steel sheet for cold rolling in the present embodiment can prevent the end portion of the steel sheet from cracking during the subsequent cold-rolling. In this regard, no facilities and running costs for additional high temperature heating are required. In addition, according to the method for manufacturing a steel sheet for cold rolling in the present embodiment, it is possible to solve the problem of a decrease in yield due to the removal of a portion where end portion cracking is apt to occur during the subsequent cold-rolling.
The method for manufacturing a cold-rolled steel sheet in the present embodiment further includes cold-rolling the steel sheet manufactured by the method in the above-described embodiment. Hereinafter, an example of the method for manufacturing a cold-rolled steel sheet in the present embodiment will be described.
Before the cold-rolling, the above-described steel sheet for cold rolling manufactured by the method in the embodiment may be pickled. The pickling method is not particularly limited, and any known method may be applied. For example, the scale may be removed by immersion using hydrochloric acid or the like.
The cold-rolling method is not particularly limited, and any known method may be applied. For example, in order to obtain a desired sheet thickness, the cold-rolling can be performed at a rolling rate of 30% to 80%. The sheet thickness of the cold-rolled steel sheet is not particularly limited.
In the method for manufacturing a cold-rolled steel sheet, a cold-rolled steel sheet to be used for manufacturing a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more can be obtained through the steps as described above and optionally included steps. In the cold-rolled steel sheet as described above in the present embodiment obtained, the end portion is prevented from cracking during cold-rolling, so that the risk after the fracture or the like of the steel sheet caused by the cracking of the end portion can be reduced. Therefore, by annealing the cold-rolled steel sheet by any method, a high-tensile cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more can be suitably manufactured.
Although the outline of the present invention has been described above, the method for manufacturing a steel sheet for cold rolling and the method for manufacturing a cold-rolled steel sheet according to the embodiment of the present invention are summarized as follows.
A method for manufacturing a steel sheet for cold rolling according to a first aspect of the present invention comprises:
In the method for manufacturing a steel sheet for cold rolling described above, the slab preferably further contains:
A method for manufacturing a cold-rolled steel sheet according to a second aspect of the present invention further includes cold-rolling the above-described steel sheet manufactured by the method according to the first aspect at a rolling rate of 30% to 80%.
Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited by the Examples at all.
In the present Example, a steel sheet for cold rolling was actually manufactured by a method in the present embodiment, and the risk rate of the cracking of the end portion of the steel sheet during the subsequent cold-rolling was calculated from the hardness of a test piece in the vicinity of the end portion of the manufactured steel sheet.
[Manufacturing of steel sheet for cold rolling] Steel having chemical composition (target chemical composition) shown in Table 1 below was smelted in a converter, and then a slab was produced by continuous casting. The slab produced by continuous casting was directly charged into a heating furnace in a state where the surface temperature of the slab was 200° C. or higher and 900° C. or lower, and heated to a high temperature. Thereafter, the slab was extracted from the heating furnace and hot-rolled by rough rolling and finishing rolling. The sheet thickness of the hot-rolled steel sheet was finally set to 2.3 mm. The hot-rolled steel sheet was directly sent out on a run-out table, and the steel sheet on the run-out table was cooled by an upper surface laminar facility and/or a lower surface spray facility installed in advance of the run-out table. Thereafter, the cooled hot-rolled steel sheet was coiled into a coil shape and cooled to manufacture a steel sheet for cold rolling. The total length of the run-out table continuing from a final stand of a finishing rolling mill to a coiling machine of the steel sheet was 188.3 m.
In the manufacturing method described above, steel sheets for cold-rolling were manufactured under various conditions by changing the conditions of hot-rolling, cooling, and coiling. In the manufacturing of various steel sheets, a heating furnace extraction temperature during hot-rolling, an outlet temperature of a rough rolling mill, a time from heating furnace extraction to completion of rough rolling (time between extraction and rough rolling), a time from passage at a final stand of the rough rolling mill to arrival at a first stand of a finishing rolling mill (time between rough rolling and finishing rolling), the outlet temperature of the finishing rolling mill, a time from passage at a final stand of the finishing rolling mill to start of water-cooling on a run-out table (water-cooling start time), a total water-cooling time within 3 seconds from passage at the final stand of the finishing rolling mill (total water-cooling time within 3 seconds), a water volume density during cooling, the temperature of the steel sheet in the vicinity of the middle of the run-out table (intermediate temperature), a time from passage at the outlet side of the finishing rolling mill to temperature measurement in the vicinity of the middle of the run-out table (time between finishing rolling and intermediate temperature measurement), and a coiling temperature are shown in Table 2 below. In Table 2 below, “-” indicates that since the water-cooling start time is over 3.0 seconds, the total water-cooling time within 3.0 seconds and the water volume density are 0.
In Table 2 above, detailed measurement and calculation methods of the items are as follows.
Furthermore, in the categories shown in Table 2 above, the present Invention Examples are test pieces in a case where the finishing rolling outlet temperature is 800° C. or higher and 940° C. or lower and the water-cooling start time is 3.0 seconds or less. Meanwhile, Comparative Example 1 is a test piece in a case where the finishing rolling outlet temperature is higher than 940° C. and the water-cooling start time is 3.0 seconds or less. Comparative Example 2 is a test piece in a case where the finishing rolling outlet temperature is 940° C. or lower and the water-cooling start time is longer than 3.0 seconds. Comparative Example 3 is a test piece in a case where the finishing rolling outlet temperature is higher than 940° C. and the water-cooling start time is longer than 3.0 seconds.
[Measurement of hardness of test piece of steel sheet for cold rolling] The hardness of the test piece of each steel sheet for cold rolling obtained by the above method was measured. Test pieces at both the width-direction end portions of the steel sheet were cut by shear cutting so as to include a position of 30 m from the longitudinal-direction tail end of the steel sheet. The size of the test piece was 10 mm (direction parallel to the rolling direction)×20 mm (sheet width direction)×2.3 mm (sheet thickness).
The position of 30 m from the longitudinal-direction tail end of the steel sheet is closer to the tail end than the position of ⅘ of the total length of the steel sheet from the longitudinal-direction distal end of the steel sheet, which is the position where the water volume density is obtained in the above-described manufacturing process. In the longitudinal direction of the steel sheet, it is assumed that the sheet speed of the steel sheet is faster as it is closer to the tail end, and in general, end portion hardening is more likely to occur. Therefore, if the end portion hardening is not observed at the position of 30 m from the longitudinal-direction tail end of the steel sheet, it is considered that the end portion hardening is not naturally observed even at the position of ⅘ of the total length of the steel sheet from the longitudinal-direction distal end of the steel sheet. In addition, from this result, by appropriately adjusting the portion to be cooled in the steel sheet if necessary, it is also assumed that the end portion hardening can be suppressed over the entire portion from the longitudinal-direction distal end to the tail end of the steel sheet.
Table 3 below shows the Vickers hardness (HV) measured in the test piece of each steel sheet and the evaluation result thereof.
From the number of test pieces in each category of Table 3 and the result of evaluation of hardness based on the Vickers hardness, the risk rate of the cracking of the end portion was calculated. The calculation results are shown in Table 4 below.
As shown in Table 4 above, all the six test pieces of the present Invention Examples had a Vickers hardness of 290 HV or less, and therefore the risk rate of the cracking of the end portion was 0. Meanwhile, the risk rate of the cracking of the end portion of the test piece of each of Comparative Examples 1 to 3 was 0.33, 0.5, or 1.0. From these results, it was found that by setting the outlet temperature of the finishing rolling mill to 940° C. or lower and the water-cooling start time to 3.0 seconds or less, the recovery, recrystallization, and/or grain growth of austenite can be suppressed, and the end portion hardening of the steel sheet can be suppressed.
This application is based on Japanese Patent Application No. 2021-079218 filed on May 7, 2021, the contents of which are incorporated herein.
It should be understood that the embodiments and examples disclosed herein are exemplary in all respects and do not pose any limitation. The scope of the present invention is indicated by the scope of claims instead of the above description, and is intended to include meanings equivalent to the scope of claims and all modifications within the scope.
The present invention can provide a method for manufacturing a steel sheet for cold rolling capable of preventing the end portion of the steel sheet from cracking during cold-rolling. The manufactured steel sheet for cold rolling can be suitably used without causing a problem of a decrease in yield when a high-tensile cold-rolled steel sheet having a tensile strength of 980 MPa or more is manufactured.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-079218 | May 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/017334 | 4/8/2022 | WO |
