Stock hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet and method for producing same, and methods for producing stock cold-rolled steel sheet for high-strength hot-dip galvanized steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength alloyed hot-dip galvanized steel sheet

Abstract

The purpose of the present invention is to provide: an intermediate product with which it is possible to produce a hot-dip galvanized steel sheet that has excellent plating appearance and adhesion amount stability in addition to high strength and high ductility; a method for producing the same; and methods for producing a high-strength hot-dip galvanized steel sheet and a high-strength alloyed galvannealed steel sheet.　Provided is a stock hot-rolled steel sheet for a high-strength hot-dip galvanized steel sheet, which has a specific component composition, wherein: the steel structure at the position of 1 / 4 the sheet thickness contains a total of 60-100% by area of ferrite and bainite, 40% by area or less of martensite, and 5% by area or less of pearlite; at the position of 1 / 4 the sheet thickness, the correlation coefficient between the concentrations of C and Mn is 0.40 or less as measured in the sheet thickness direction by line analysis of EPMA; and the ratio expressed by (prior austenite grain size at a depth of 100 µm from the surface of the steel sheet) / (prior austenite grain size at the position of 1 / 4 the sheet thickness) is 0.8-1.2.

Description

Material hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet, its manufacturing method, material cold-rolled steel sheet for high-strength hot-dip galvanized steel sheet, high-strength hot-dip galvanized steel sheet, and manufacturing method of high-strength alloyed hot-dip galvanized steel sheet

[0001] The present invention relates to a high-strength steel sheet excellent in surface quality and adhesion amount stability, an intermediate product suitable for manufacturing the high-strength steel sheet, and manufacturing methods thereof, which are used for applications that require both high strength and high formability such as steel sheets for automobiles.

[0002] In recent years, from the perspective of global environmental conservation, improving the fuel efficiency of automobiles has become an important issue. For this reason, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets used as materials for automobile members. Also, from the perspective of the rust prevention performance of the vehicle body, zinc-based plating may be applied to steel sheets used as materials for automobile members. Therefore, for the main structural parts that form the framework of an automobile cabin for the purpose of increasing the vehicle body strength, the development of hot-dip galvanized steel sheets with a tensile strength (TS) of 590 MPa or more is required.

[0003] Such steel sheets are manufactured by a continuous hot-dip galvanizing line (CGL). The cold-rolled sheet is annealed by heating, then cooled to near the temperature of the hot-dip galvanizing bath (450 - 600 °C), and directly immersed in the hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet. Generally, the adhesion amount of the plating is controlled to a desired amount by wiping with gas after hot-dip galvanizing. Hereinafter, unless otherwise specified, "adhesion amount" means "adhesion amount of the plating".

[0004] However, in the process from cooling in the CGL to the hot-dip galvanizing process, shape defects that are considered to be caused by steel sheet transformation often occur, and warping occurs in the width direction of the steel sheet. When passing through wiping in that state, the distance from the nozzle of the wiping to the latter half becomes different in the width direction. As a result, non-uniformity of the adhesion amount occurs in the front and back and width directions. Places with a large adhesion amount cause powdering during processing, and places with a small adhesion amount cause a shortening of the time to rust during use. Therefore, uniformization of the adhesion amount is desired.

[0005] Regarding technology for stabilizing adhesion amounts, there are patents that address this from the perspective of equipment, such as Patent Documents 1 and 2, but stabilizing the adhesion amount of material with shape defects in the width direction caused by the steel plate has been difficult.

[0006] Furthermore, especially in the case of hot-rolled and cold-rolled steel sheets, which are intermediate products of high-strength hot-dip galvanized steel sheets, the stability of adhesion caused by shape stability during sheet feeding has never been investigated before.

[0007] Patent No. 6912981 Patent No. 6702519

[0008] This invention was developed in view of the above-mentioned circumstances. Its purpose is to provide an intermediate product that can produce hot-dip galvanized steel sheets with high strength, specifically a tensile strength of 590 MPa or more, high ductility, and excellent plating appearance and adhesion stability, as well as a method for producing the same. Furthermore, this invention also aims to provide a cold-rolled steel sheet for high-strength hot-dip galvanized steel sheets, a high-strength hot-dip galvanized steel sheet, and a method for producing a high-strength alloyed hot-dip galvanized steel sheet using the intermediate product.

[0009] The present inventors have diligently investigated methods to improve adhesion stability by enhancing shape stability in CGL-threaded steel sheets during the manufacturing process, while simultaneously producing steel that possesses both high strength and high ductility. As a result, they have obtained the following findings.

[0010] The dimensional stability of CGL (Continuously Glazed Steel) is due to the microstructure of the steel sheet during cooling. We have shown that dimensional stability decreases when the hard phase (martensite) and soft phase (ferrite and bainite) of GA (Gas-Aged) steel sheets are linked in the L direction. We have also shown that controlling the segregation of C and Mn in hot-rolled steel sheets, specifically suppressing the cosegregation of Mn and C, is effective in controlling such L-direction linkage of hard and soft phases. Furthermore, we have shown that in order to obtain such a segregation morphology, it is necessary to add Nb to the steel sheet, appropriately control the conditions during hot rolling, and suppress ferrite transformation and C diffusion at high hot-rolling temperatures.

[0011] The improvement in shape due to the elimination of L-direction connectivity between the hard and soft phases is thought to be for the following reasons: After CGL annealing, transformation from austenite to ferrite and then to bainite occurs during cooling and immersion. At this time, the L-direction connectivity that occurs between the hard and soft phases of the final structure means that the amount of transformation from austenite to the soft phase differs greatly depending on the thickness direction of the sheet. Since the transformation from austenite to the soft phase is accompanied by volume expansion, this means that the amount of volume expansion differs depending on the thickness direction of the sheet. It is presumed that this generates stress inside the steel sheet, resulting in shape defects.

[0012] The present invention is based on the above findings, and the gist of the present invention is as follows: [1] The component composition is, in mass%, C: more than 0.050% and 0.130% or less, Si: less than 0.30%, Mn: more than 1.90% and less than 2.70%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and Nb: 0.005% or more and 0.100% or less, with the remainder being Fe and unavoidable impurities, and at the 1 / 4 position of the plate thickness, the steel structure is such that the total area ratio of ferrite and bainite is 60% or more and 100% or less. A hot-rolled steel sheet material for high-strength hot-dip galvanized steel sheets, wherein the martensite area ratio is 40% or less, the pearlite area ratio is 5% or less, the correlation coefficient between the concentrations of C and Mn measured in the thickness direction by EPMA line analysis at the 1 / 4 thickness position is 0.40 or less, and the ratio of the prior austenite grain size at a depth of 100 μm from the surface of the steel sheet to the prior austenite grain size at the 1 / 4 thickness position, i.e., [100 μm from the surface of the steel sheet] / [1 / 4 thickness position] is 0.8 or more and 1.2 or less. [2] The component composition further contains, in mass%, at least one element selected from Ti: 0.10% or less, B: 0.0050% or less, Mo: 1.00% or less, Ta: 0.10% or less, W: 0.10% or less, V: 0.10% or less, Cr: 1.00% or less, Co: 0.500% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, as described in [1].A method for manufacturing a base hot-rolled steel sheet for high-strength hot-dip galvanized steel sheets as described in [3], [1], or [2], wherein the slab is heated to a temperature of 1200°C or higher, the reduction ratio is 20% or more at the stand immediately preceding the final stand of the finishing rolling mill, the reduction ratio at the final stand is 2% to 15%, the rolling completion temperature is 850°C to 1000°C, the cooling time from the rolling at the stand immediately preceding the final stand of the finishing rolling mill to the final stand is 3 seconds or less, the cooling is further performed from 800°C to 650°C for 5 seconds to 20 seconds, the holding time at 550°C to 500°C is 1 second or less, and then the sheet is wound at 350°C to 500°C. [4] A method for manufacturing a cold-rolled steel sheet for high-strength hot-dip galvanized steel sheet, comprising pickling and cold-rolling a hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet as described in [1] or [2]. [5] A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising pickling, cold-rolling, annealing, and plating a hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet as described in [1] or [2]. [6] A method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet, comprising pickling, cold-rolling, annealing, plating, and alloying the plating layer of a hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet as described in [1] or [2].

[0013] According to the present invention, when CGL is passed through to produce a hot-dip galvanized steel sheet, it is possible to provide a hot-rolled steel sheet and a cold-rolled steel sheet for high-strength hot-dip galvanized steel sheets that exhibit excellent shape stability, resulting in excellent plating adhesion stability, high strength, and a superior appearance. Furthermore, the present invention can provide a method for manufacturing the above-mentioned hot-rolled steel sheet and cold-rolled steel sheet. The high-strength hot-dip galvanized steel sheet produced from the hot-rolled and cold-rolled steel sheets of the present invention allows for appropriate control of the plating adhesion when applied, for example, to structural members or energy-absorbing members of automobiles. This also reduces zinc consumption, giving it significant industrial value.

[0014] The present invention will be described based on the following embodiments. However, the present invention is not limited to the following embodiments.

[0015] [Component Composition] First, the component composition of the alloyed hot-dip galvanized steel sheet / base steel sheet for the hot-dip galvanized steel sheet, i.e., the hot-rolled steel sheet / cold-rolled steel sheet for use as a base material for a high-strength (alloyed) hot-dip galvanized steel sheet, according to one embodiment of the present invention will be described. Note that all units in the component composition are "mass%", but unless otherwise specified, they will be simply shown as "%".

[0016] C: Greater than 0.050% and less than or equal to 0.130% Carbon (C) is an effective element for increasing the strength of steel, and in particular, it contributes to increased strength by forming martensite, one of the hard phases in the steel structure. From the viewpoint of obtaining the desired high strength, specifically a tensile strength of 590 MPa or more, the C content should be greater than 0.050%. Preferably, the C content should be 0.055% or more, and more preferably 0.060% or more. On the other hand, if the C content is too high, the ductility will decrease. For this reason, the C content should be 0.130% or less, preferably 0.120% or less, and more preferably 0.100% or less.

[0017] Si: Less than 0.30% If Si is added in excess, it generates Si oxide on the surface of the steel sheet, reducing the plating properties. For this reason, the Si content should be less than 0.30%, preferably 0.25% or less, and more preferably 0.20% or less. The lower limit of the Si content is not particularly limited, for example, 0.01%, but it may also be 0 (zero).

[0018] Mn: Greater than 1.90% and less than 2.70%. Mn is an element that contributes to increasing the strength of steel through solid solution strengthening and martensite formation. To obtain these effects, the Mn content should be greater than 1.90%. A Mn content of 2.00% or more is preferable, and 2.20% or more is more preferable. On the other hand, if the Mn content is too high, Mn oxide will be generated on the surface of the steel sheet, reducing the plating properties. For this reason, the Mn content should be less than 2.70%, preferably 2.60% or less, and more preferably 2.50% or less.

[0019] P: 0.100% or less. P segregates at prior austenite grain boundaries, embrittles them, and reduces the ultimate deformability of the steel sheet. Therefore, if the P content is too high, the ductility decreases. For this reason, the P content should be 0.100% or less. Preferably, the P content should be 0.070% or less, and more preferably 0.040% or less. On the other hand, there is no particular lower limit to the P content. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. From this viewpoint, a P content of 0.001% or more is preferable, 0.003% or more is more preferable, and 0.005% or more is even more preferable.

[0020] S: 0.0200% or less. S exists as a sulfide and reduces the ultimate deformability of the steel sheet. If the S content is too high, the ductility decreases. For this reason, the S content should be 0.0200% or less. Preferably, the S content should be 0.0120% or less, and more preferably 0.0050% or less. On the other hand, there is no particular lower limit to the S content. However, due to constraints in production technology, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0021] Al: 0.100% or less. Al is added as a deoxidizing agent. From the viewpoint of obtaining its effect, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more. However, if the Al content is too high, it may cause surface defects in the plating layer. For this reason, the Al content is set to 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0022] N: 0.0100% or less. N exists as a nitride and reduces the ultimate deformability of the steel sheet. If the N content is too high, the ductility decreases. For this reason, the N content should be 0.0100% or less. Preferably, the N content should be 0.0070% or less, and more preferably 0.0050% or less. On the other hand, there is no particular lower limit to the N content. However, due to production technology constraints, the N content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0023] Nb: 0.005% or more and 0.100% or less. Nb segregates at grain boundaries during hot rolling, suppressing ferrite-pearlite transformation at high temperatures where carbon (C) is prone to segregation, and thus suppressing C segregation. To obtain this effect, the Nb content must be 0.005% or more. If the Nb content is too high, recrystallization is significantly suppressed during CGL annealing, and ductility decreases. For this reason, the Nb content is preferably 0.100% or less, and more preferably 0.070% or less.

[0024] The remainder of the component composition consists of Fe and unavoidable impurities. Preferably, the remainder of the component composition consists only of Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, and As. The total content of unavoidable impurities is preferably 0.100% or less. Unavoidable impurities may be introduced from raw materials such as scrap. Such inclusion is acceptable as long as it does not hinder the objectives of the present invention.

[0025] The composition of the steel sheet may further contain the elements listed below, as needed.

[0026] Ti: 0.10% or less. Ti improves ductility at high temperatures and improves castability in continuous casting. To obtain this effect, it is preferable to add 0.005% or more, more preferably 0.02% or more of Ti. However, if the amount of Ti exceeds 0.10%, the amount of carbonitride increases significantly, and ductility decreases. Therefore, when adding Ti, the amount of Ti should be in the range of 0.10% or less. The amount of Ti is preferably in the range of 0.09% or less for the effect of the present invention to be better.

[0027] B: 0.0050% or less. B is contained in the Si and Mn oxides formed on the surface of the steel plate, and improves the wettability between these oxides and molten zinc, thereby improving the appearance of the plating layer. From the viewpoint of obtaining this effect, the B content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the other hand, if the B content is too high, coarse B compounds may be formed, which may reduce ductility. For this reason, the B content is preferably 0.0050% or less, more preferably 0.0045% or less, and even more preferably 0.0040% or less.

[0028] Mo: 1.00% or less, Cr: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less Mo, Cr, Ni, and Cu increase the hardenability of the steel sheet and contribute to improved strength. To obtain such effects, each is preferably at 0.01% or more. Furthermore, if the content is appropriate, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not decrease. For this reason, the content of Cr, Ni, Cu, and Mo is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0029] Ta: 0.10% or less, W: 0.10% or less, V: 0.10% or less. Ta, W, and V contribute to increased strength by forming alloy carbides and alloy carbonitrides. To obtain such effects, each is preferably at 0.01% or more. Furthermore, if the content is appropriate, large amounts of coarse precipitates and inclusions will not be formed, and the ductility of the steel sheet will not decrease. For this reason, the content of Ta, V, and W is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less.

[0030] Sn: 0.200% or less, Sb: 0.200% or less. Sn and Sb are elements that suppress decarburization, denitrification, etc., and are effective in suppressing the decrease in steel strength. To obtain such effects, it is preferable that each be at least 0.001%. If each exceeds 0.200%, it leads to a decrease in grain boundary strength and a decrease in ductility. For this reason, the content of Sn and Sb is preferably 0.200% or less, more preferably 0.180% or less, and even more preferably 0.160% or less.

[0031] Furthermore, in the present invention, other elements may include 0.500% or less of Co, 0.0100% or less of Ca, 0.0100% or less of Mg, 0.0100% or less of REM, 0.100% or less of Zr, 0.100% or less of Te, 0.10% or less of Hf, and 0.200% or less of Bi.

[0032] If the Co content is 0.500% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not decrease. For this reason, it is preferable to keep the Co content at 0.500% or less. There is no specific lower limit for the Co content, but since it is an element that improves hardenability, it is more preferable to have a Co content of 0.001% or more. Therefore, if Co is included, it is preferable to keep its content at 0.500% or less. More preferably, the Co content should be 0.001% or more. An even more preferable Co content is 0.080% or less.

[0033] If Ca, Mg, and REM are present in amounts of 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not decrease. Therefore, it is preferable to keep the content of Ca, Mg, and REM at 0.0100% or less. Although there is no specific lower limit for the content of Ca, Mg, and REM, it is more preferable to keep the content of Ca, Mg, and REM at 0.0005% or more, as these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, if Ca, Mg, and REM are present, their content should be 0.0100% or less. A more preferable content is 0.0005% or more. An even more preferable content is 0.0050% or less. REM is a collective term for Sc, Y, and 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content referred to here is the total content of these elements.

[0034] If the content of Zr and Te is 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not decrease. Therefore, it is preferable to keep the content of Zr and Te to 0.100% or less. There is no specific lower limit for the content of Zr and Te, but these elements spheroidize the shape of nitrides and sulfides, improving the ultimate deformability of the steel sheet. Therefore, it is more preferable that the content of Zr and Te be 0.001% or more. Therefore, if Zr and Te are included, their content should be 0.100% or less. A more preferable content is 0.001% or more. An even more preferable content is 0.080% or less.

[0035] If Hf is present at a concentration of 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the ductility of the steel sheet does not decrease. Therefore, it is preferable to keep the Hf content at 0.10% or less. Although there is no specific lower limit for the Hf content, it is more preferable to have an Hf content of 0.01% or more, as Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, if Hf is present, its content is preferably 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0036] If the Bi content is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the ductility of the steel sheet will not decrease. Therefore, it is preferable to keep the Bi content at 0.200% or less. There is no specific lower limit for the Bi content, but since it is an element that reduces segregation, it is more preferable to have a Bi content of 0.001% or more. Therefore, if Bi is included, its content should be 0.200% or less. A more preferable Bi content is 0.001% or more. An even more preferable Bi content is 0.100% or less.

[0037] Furthermore, regarding the above-mentioned Ti, B, Mo, Ta, W, Cr, Co, Ni, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

[0038] [Steel Structure (Microstructure)] Next, the steel structure (microstructure) of the hot-rolled steel sheet of the present invention will be described. In the present invention, in order to suppress the transformation difference due to the sheet thickness position of the CGL, it is important to control the structure at the position of 1 / 4 of the sheet thickness and the position of 100 μm from the surface of the steel sheet. The area ratio of each structure is based on 100% of the total steel structure.

[0039] The total of the ferrite area ratio and the bainite area ratio is 60% or more and 100% or less, and the martensite area ratio is 40% or less. In cold rolling, it is important to introduce strain uniformly in order to control the transformation during cooling in the subsequent annealing of the CGL. For this purpose, the total of the ferrite area ratio and the bainite area ratio needs to be 60% or more and 100% or less, and the martensite area ratio needs to be 40% or less. The lower limit of the martensite area ratio may be 0%.

[0040] The pearlite area ratio is 5% or less. Pearlite significantly causes the following co-segregation of C and Mn, increases the L-direction connectivity of the structure after CGL annealing, and reduces the shape stability in CGL through-thickness. To avoid this, it is necessary to set the pearlite area ratio to 5% or less. The lower limit of the pearlite area ratio may be 0%.

[0041] The steel structure of the present invention may contain carbides such as retained austenite and cementite or other known structures of the steel sheet as the remaining structure. The upper limit of the area ratio of the remaining structure is not particularly limited, but 5.0% or less is preferable. The lower limit of the area ratio of the remaining structure is not particularly limited and may be 0.0%.

[0042] Here, the total area ratio of ferrite and bainite, the martensite area ratio, and the pearlite area ratio are obtained as follows.

[0043] First, a test piece is taken from the hot-rolled steel sheet so that the cross-section parallel to the rolling direction of the steel sheet (base steel sheet) and parallel to the sheet thickness direction becomes the observation surface. The black skin (iron oxide) on the surface of the test piece is dissolved and removed using hydrochloric acid added with an inhibitor. Next, the observation surface of the test piece is mirror-polished and then corroded with 3 vol% nital to make the structure appear on the observation surface.

[0044] After that, using a scanning electron microscope (SEM), observe a desired range on the observation surface of the test piece at a magnification of 3,000 times. More specifically, observe five fields each at the 1 / 4 position of the steel plate thickness (the range including the 1 / 4 position of the plate thickness) and at a depth of 100 μm from the surface of the steel plate so that the center of the image is formed, and obtain SEM images respectively. For the obtained SEM images, perform coloring for each structure, and determine the area ratio (average value of five fields) of each structure from the number of pixels. Note that each area ratio is calculated at the 1 / 4 position of the plate thickness.

[0045] In the SEM image, ferrite and bainite are black or dark gray structures, and martensite is a white or light gray structure. Pearlite is a structure in which black or dark gray and white or light gray are arranged in streaks and is distinguishable from ferrite, bainite, and martensite. Martensite includes autotempered martensite containing carbide.

[0046] When the correlation coefficient of the concentrations of C and Mn measured in the plate thickness direction by EPMA line analysis at the 1 / 4 position of the plate thickness is 0.40 or less, the co-segregation of C and Mn in the hot-rolled sheet reduces the shape stability in CGL through-plate. Also, it increases the L-direction connectivity of the structure after CGL annealing. To avoid these, it is necessary to make the correlation coefficient of the concentrations of C and Mn measured by EPMA 0.40 or less.

[0047] Here, we will explain the method for measuring the correlation coefficient. First, a test piece is taken from the hot-rolled steel sheet so that the cross-section parallel to the rolling direction of the steel sheet (base steel sheet) and parallel to the thickness direction becomes the observation surface. Next, a line analysis is performed in the thickness direction to a depth of 100 μm. The 1 / 4 thickness position means that the center of the line analysis is at the 1 / 4 thickness position. Under conditions of a beam diameter of 1 μm and a measurement interval of 0.5 μm, the concentrations of C and Mn are measured at a total of 5 locations at random positions and lengths of 100 μm, and the correlation coefficient between the C concentration and the Mn concentration is calculated using all the measured points. Here, the correlation coefficient between X and Y is expressed by the following formula: Correlation coefficient between X and Y = Covariance of X and Y / (Standard deviation of X × Standard deviation of Y) Here, X and Y in the formula correspond to the C concentration and Mn concentration, respectively. The covariance of X and Y is the average product of the deviations of X and Y, obtained by first calculating the mean values ​​of X and Y, and then calculating the deviations of X and Y respectively. The covariance is calculated from all five measurement points for C concentration and Mn concentration within the 100 μm line analysis. A large covariance value means that when the C concentration is large, the Mn concentration is also large, indicating strong co-separation between C and Mn. On the other hand, a small covariance value means that when the C concentration is large, the Mn concentration is small, or when the C concentration is small, the Mn concentration is large. The standard deviation of X and the standard deviation of Y refer to the average of the standard deviations of C concentration and Mn concentration within the 100 μm line analysis, calculated at the five points mentioned above. A positive sign for the covariance means that X and Y are correlated, and a negative sign means that they are inversely correlated. A covariance of zero means there is no correlation, i.e., no correlation is observed between X and Y.

[0048] The ratio of the prior austenite grain size at a depth of 100 μm from the surface of the steel sheet to the prior austenite grain size at 1 / 4 of the sheet thickness, [100 μm from the surface of the steel sheet] / [1 / 4 of the sheet thickness], should be between 0.8 and 1.2. In hot-rolled steel sheets, the grain size ratio between the position 100 μm from the surface of the steel sheet and the position 1 / 4 of the sheet thickness results in a difference in transformation timing during subsequent CGL annealing, leading to shape defects. To suppress this, the ratio of the prior austenite grain size at a depth of 100 μm from the surface of the steel sheet to the prior austenite grain size at 1 / 4 of the sheet thickness, [100 μm from the surface of the steel sheet] / [1 / 4 of the sheet thickness], should be between 0.8 and 1.2.

[0049] Furthermore, the prior austenite grain size is measured as follows.

[0050] A test specimen is taken from a hot-rolled steel sheet so that the cross-section is parallel to the rolling direction of the steel sheet (base steel sheet) and parallel to the thickness direction, and this cross-section serves as the observation surface. The test surface of the specimen is then mirror-polished, and the prior austenite grain boundaries are exposed by picric acid corrosion. The SEM is set to a magnification of 1000x, and three fields of view are captured so that the center of the image is 100 μm from the surface of the steel sheet. A binarized image of the grain boundaries is created from these images, and the diameter of the approximate circle is calculated from the average area of ​​each crystal grain. This diameter is then taken as the prior austenite grain size.

[0051] [Method for Manufacturing Hot-Rolled Steel Sheets for High-Strength Hot-Dip Galvanized Steel Sheets] Next, a method for manufacturing hot-rolled steel sheets for high-strength hot-dip galvanized steel sheets according to this embodiment will be described. Hereinafter, unless otherwise specified, each temperature refers to the surface temperature of the slab or steel sheet (hot-rolled steel sheet, cold-rolled steel sheet, etc.). In this embodiment, hot rolling is generally carried out using a slab having the above-described component composition. With regard to hot rolling, there are no particular limitations other than the conditions described below, and conditions in accordance with conventional methods can be appropriately adopted. There are no particular limitations on the method for manufacturing the molten steel that will become the slab, and known methods using converters, electric furnaces, etc. can be adopted. Scrap may also be used as raw material.

[0052] This is a method for manufacturing hot-rolled steel sheets for high-strength hot-dip galvanized steel sheets with excellent adhesion stability. The slab is heated to a temperature of 1200°C or higher, and rolled at the stand immediately preceding the final stand of the finishing rolling mill with a reduction ratio of 20% or higher, and at the final stand with a reduction ratio of 2% to 15%. At this time, the finishing rolling temperature is 850 to 1000°C, and the time from rolling at the stand immediately preceding the final stand of the finishing rolling mill, through rolling at the final stand, to cooling down to 800°C is within 3 seconds. Furthermore, it is cooled from 800°C to 650°C for 5 seconds to 20 seconds, and then cooled again so that the holding time at 550 to 500°C is 1 second or less. After that, it is wound at 350°C to 500°C.

[0053] [Slab heating temperature: 1200°C or higher] In the present invention, it is important to solidify the added Nb during slab heating, suppress the ferrite-pearlite transformation during cooling, and prevent an increase in the correlation coefficient between C and Mn. For this reason, the slab heating temperature is 1200°C or higher, preferably 1210°C or higher, and more preferably 1220°C or higher. The upper limit of the slab heating temperature is not particularly limited, for example, 1350°C, preferably 1330°C, and more preferably 1310°C.

[0054] [Reduction ratio of 20% or more at the stand immediately preceding the final stand of the finishing rolling mill, reduction ratio of 2% to 15% at the final stand, and time from rolling at the stand immediately preceding the final stand of the finishing rolling mill, through rolling at the final stand, to cooling to 800°C within 3 seconds.] The strain introduced by rolling tends to be introduced more at the surface of the steel sheet than at the interior. Therefore, the surface of the steel sheet tends to become finer due to recrystallization. In order to suppress such a difference in particle size between the surface and interior of the steel sheet, it is necessary to reduce the amount of reduction at the final stand, where the temperature is lowest, after reducing the particle size significantly at the stand before the final stand to equalize the particle size. Also, if the time from the stand immediately preceding the final stand to 800°C is longer than 3 seconds, the desired microstructure cannot be obtained at the surface of the steel sheet and in the last quarter of the sheet thickness due to the strain distribution. Therefore, the reduction ratio at the stand immediately preceding the final stand of the finishing rolling mill should be 20% or more, and the reduction ratio at the final stand should be 2% to 15%. Furthermore, the time from rolling at the stand just before the final stand of the finishing rolling mill, through rolling at the final stand, to cooling and reaching 800°C should be within 3 seconds.

[0055] [Rolling completion temperature: 850°C to 1000°C] The rolling completion temperature can be set according to normal conditions, but if it is below 850°C, rolling becomes difficult due to the increased rolling load, and if it is above 1000°C, it becomes difficult to cool down to 800°C within 3 seconds. Therefore, the rolling completion temperature should be set to 850°C to 1000°C.

[0056] [Cooling from 800°C to 650°C for 5 to 20 seconds] If the cooling time from 800°C to 650°C is too long (more than 20 seconds), pearlite will form, causing shape defects in CGL. If the above cooling time is too short (less than 5 seconds), the area ratio of martensite will increase, the hot-rolled structure will harden, causing shape defects in cold rolling and subsequent shape defects in CGL. Therefore, the cooling time from 800°C to 650°C should be between 5 seconds and 20 seconds.

[0057] [Cooling under conditions where the holding time at 550-500°C is 1 s or less] Bainitic ferrite is formed at 500-550°C. The bainitic ferrite transformation proceeds very rapidly, and since this is a region where the migration rate of carbon is high, suppressing the bainitic ferrite transformation is very important for suppressing the cosegregation of carbon and manganese. To suppress it, the holding time in the temperature range of 550-500°C must be 1 s or less. Therefore, the holding time at 550-500°C is set to 1 s or less.

[0058] [Winding at 350°C to 500°C] If the winding temperature is too high (above 500°C), pearlite transformation occurs, and if it is too low (below 350°C), shape defects occur due to a high martensite fraction. Therefore, the winding temperature should be between 350°C and 500°C.

[0059] [Pickling] The hot-rolled steel sheets obtained as raw material for high-strength hot-dip galvanized steel sheets may be pickled as needed. The pickling method is not particularly limited and can be any conventional method.

[0060] [Cold Rolling] Hot-rolled steel sheets used as base material for high-strength hot-dip galvanized steel sheets, which have been pickled, may be cold-rolled as needed to produce cold-rolled steel sheets. There are no particular restrictions on the cold-rolling conditions, but a reduction ratio of 20% or more is preferred.

[0061] [Annealing] Hot-rolled or cold-rolled steel sheets used as raw materials for high-strength hot-dip galvanized steel sheets after pickling may be annealed. The annealing conditions, such as heating temperature and holding time, can be determined according to conventional methods depending on the desired properties, but a heating temperature of 700°C or higher is preferred. It is preferable to carry out the annealing and plating treatment on a continuous annealing and plating line (CGL).

[0062] [Plating Treatment] Plating treatment may be performed on the hot-rolled or cold-rolled steel sheet material for high-strength hot-dip galvanized steel sheet after annealing, as needed. There are no special restrictions on the plating treatment conditions, and known hot-dip galvanizing treatments are acceptable. Furthermore, alloying treatment may be performed on the plating layer obtained as a result of the plating treatment, according to conventional methods. When performing alloying treatment, the alloying treatment can be carried out in a temperature range of, for example, 450 to 600°C.

[0063] [Temper Rolling] The obtained steel sheet may be subjected to temper rolling according to conventional methods as needed.

[0064] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0065] [Manufacturing of raw material hot-rolled steel sheet] Steel having the component composition shown in Table 1 below (the remainder consisting of Fe and unavoidable impurities) was melted in a converter, and a steel slab was obtained by continuous casting. Using the obtained steel slab, hot rolling was performed under the conditions shown in Table 2 below to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was cooled (primary and secondary cooling) and coiled under the conditions shown in the same table. In this way, a raw material hot-rolled steel sheet (coil) was obtained. The structure (microstructure) of the obtained raw material hot-rolled steel sheet was investigated using the method described above. The results are shown in the same table.

[0066]

[0067]

[0068] [Manufacturing of Cold-Rolled Steel Sheets] The obtained hot-rolled steel sheets were pickled, and then cold-rolled at a reduction ratio of 50%. Subsequently, CGL (Continuous Galvanizing) was performed at a maximum annealing temperature of 820°C and a zinc bath entry temperature of 480°C to obtain hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets. The obtained steel sheets were then temper-rolled at an elongation ratio of 0.2%. In Table 3, GI / GA refers to alloyed hot-dip galvanized steel sheet, and GI refers to hot-dip galvanized steel sheet.

[0069] [Evaluation] The obtained hot-dip galvanized steel sheets were evaluated as follows. The results are shown in Table 3.

[0070]

[0071] [Tensile Test] A JIS No. 5 test specimen was taken from the center of the width direction of the obtained hot-dip galvanized steel sheet, so that the tensile direction was perpendicular to the rolling direction of the steel sheet. Using the taken test specimen, a tensile test was performed in accordance with JIS Z 2241 (2011), and the TS (tensile strength) and EL (total elongation) were measured. Here, if TS ≥ 590 MPa, it can be said that the strength is high.

[0072] Furthermore, for TS:590MPa class, EL ≥ 20% is considered to indicate good ductility, and for TS:780MPa class, EL ≥ 14% is considered to indicate good ductility. Note that TS:590MPa class means TS is 590MPa or higher and less than 780MPa, and TS:780MPa class means TS is 780MPa or higher and less than 900MPa.

[0073] [Measurement of Adhesion Amount and Evaluation of Adhesion Amount Stability] A 1m length of steel sheet was taken from approximately 10m from the tip of the obtained hot-dip galvanized steel sheet. 30mm x 30mm test pieces were taken from 1 / 4, the center, and 3 / 4 of the width direction, and the amount of plating adhesion to the front and back surfaces of each piece was measured as follows.

[0074] The test specimens were immersed in 10% hydrochloric acid containing a corrosion inhibitor for iron ("Ibit" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to dissolve the plating layer. The amount of mass loss of the test specimen due to dissolution was measured, and this value was normalized by the surface area of ​​the steel plate to determine the plating adhesion amount (g / m²). ２ The amounts of Zn and Fe dissolved in hydrochloric acid were measured using ICP emission spectrometry, and the Fe concentration (mass%) of the plating layer was defined as {Amount of Fe dissolved / (Amount of Fe dissolved + Amount of Zn dissolved)} × 100. Using the obtained deposition amounts at six locations, the average deposition amount and standard deviation were calculated from the following formulas, and those with a standard deviation / average deposition amount of 0.15 or less were considered to have excellent deposition amount stability. Average deposition amount = Σ deposition amount / 6 Standard deviation = √(Deposition amount - Average deposition amount) ２ / 6 [Plating Appearance] One 1m length of steel sheet is taken from approximately 10m from the leading and trailing ends of the hot-dip galvanized steel sheet, and the front and back surfaces are inspected visually and evaluated using the following gauge scores. If multiple items are found to be unplated, the worst evaluation is adopted. A and B are considered acceptable. The diameter of the unplated area is considered to be the equivalent diameter of a circle. D: 10 points / 1m for unplated areas with a diameter of 0.3mm or more ２ The above is recognized. C: 10 unplated points / 1m with a diameter of 0.3 mm or more and less than 0.5 mm. ２ Ten unplated points with a diameter of less than 0.3 mm are permitted per meter. ２ The above is recognized. B: 10 unplated points with a diameter of less than 0.3 mm per meter. ２ Less than acceptable A: No unplated areas were observed. As shown in Table 3 above, all of the hot-dip galvanized steel sheets obtained from the inventive examples had a TS of 590 MPa or higher, excellent ductility, and excellent plating appearance and adhesion stability. In contrast, the comparative examples were insufficient in at least one of the following: TS, ductility, and adhesion stability.

[0075] The hot-rolled steel sheet material of the present invention makes it possible to manufacture high-strength hot-dip galvanized steel sheets with excellent adhesion stability and a tensile strength (TS) of 590 MPa or more. High-strength cold-rolled steel sheets manufactured from the hot-rolled steel sheet material of the present invention have great industrial value because, for example, they can be applied to structural members and energy-absorbing members of automobiles to improve fuel efficiency by reducing the weight of the vehicle body.

Claims

1. The composition is such that, in mass%, C: greater than 0.050% and 0.130% or less, Si: less than 0.30%, Mn: greater than 1.90% and less than 2.70%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and Nb: 0.005% or more and 0.100% or less, with the remainder being Fe and unavoidable impurities, and at the 1 / 4 thickness position, the steel structure has a total ferrite and bainite area ratio of 60% or more and 100% or less, a martensite area ratio of 40% or less, and a pearlite area ratio of 5% or less, and at the 1 / 4 thickness position, the correlation coefficient between the concentrations of C and Mn measured in the thickness direction by EPMA line analysis is 0.40 or less. A hot-rolled steel sheet material for high-strength hot-dip galvanized steel sheets, wherein the ratio of the prior austenite grain size at a depth of 100 μm from the surface of the steel sheet to the prior austenite grain size at a position 1 / 4 of the sheet thickness, i.e., [at a depth of 100 μm from the surface of the steel sheet] / [at a position 1 / 4 of the sheet thickness], is between 0.8 and 1.

2.

2. The above component composition is further defined in mass percent as follows: Ti: 0.10% or less, B: 0.0050% or less, Mo: 1.00% or less, Ta: 0.10% or less, W: 0.10% or less, V: 0.10% or less, Cr: 1.00% or less, Co: 0.500% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less. A hot-rolled steel sheet material for a high-strength hot-dip galvanized steel sheet according to claim 1, comprising at least one element selected from among the following.

3. A method for manufacturing a base hot-rolled steel sheet for a high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein the slab is heated to a temperature of 1200°C or higher, the reduction ratio is 20% or more at the stand immediately preceding the final stand of the finishing rolling mill, the reduction ratio at the final stand is 2% to 15%, the rolling completion temperature is 850°C to 1000°C, the cooling time from the rolling at the stand immediately preceding the final stand of the finishing rolling mill to the final stand is 3 seconds or less, the cooling is further performed from 800°C to 650°C for 5 seconds to 20 seconds, the holding time at 550°C to 500°C is 1 second or less, and then the sheet is wound at 350°C to 500°C.

4. A method for manufacturing a cold-rolled steel sheet for a high-strength hot-dip galvanized steel sheet, comprising pickling and cold-rolling a hot-rolled steel sheet for a high-strength hot-dip galvanized steel sheet according to claim 1 or 2.

5. A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising pickling, cold rolling, annealing, and galvanizing a hot-rolled steel sheet material for a high-strength hot-dip galvanized steel sheet according to claim 1 or 2.

6. A method for manufacturing a high-strength alloyed hot-dip galvanized steel sheet, comprising pickling, cold rolling, annealing, plating, and alloying the plating layer of a hot-rolled steel sheet material for a high-strength hot-dip galvanized steel sheet according to claim 1 or 2.

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