Starting material hot-rolled steel sheet for high-strength hot-dip galvanized steel sheet and method for manufacturing same, starting material cold-rolled steel sheet for high-strength hot-dip galvanized steel sheet, high-strength hot-dip galvanized steel sheet, and method for manufacturing high-strength alloyed hot-dip galvanized steel sheet

Abstract

An objective of the present invention is to provide: an intermediate product that makes it possible to manufacture a hot-dip galvanized steel sheet having outstanding plating appearance and adhesion amount stability as well as high strength and high ductility; a method for manufacturing same; and a method for manufacturing a high-strength hot-dip galvanized steel sheet and a high-strength alloyed galvanized steel sheet.　The present invention pertains to a starting material hot-rolled steel sheet for a high-strength hot-dip galvanized steel sheet having a prescribed component composition, wherein the steel structure is such that: at a 1 / 4 plate thickness position, the areal percentage of ferrite and bainite is a total of 50% to 100%, the martensite areal percentage is 50% or less, and the pearlite areal percentage is 15% or less; and at the 1 / 4 plate thickness position, the correlation coefficient between the concentrations of C and Mn as measured in the plate thickness direction by EPMA line analysis is 0.40 or less, and a ratio represented by [prior austenite grain size at a position at a depth of 100 μm from the surface of the steel sheet] / [prior austenite grain size at the 1 / 4 position] is 0.8 to 1.2.

Description

Hot-rolled steel sheet as a material for high-strength hot-dip galvanized steel sheet, its manufacturing method, cold-rolled steel sheet as a material 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 high-strength steel sheets excellent in surface quality and adhesion amount stability, intermediate products suitable for manufacturing the high-strength steel sheets, manufacturing methods thereof, which are used for applications that require both high strength and high formability such as steel sheets for automobiles, cold-rolled steel sheets as materials for high-strength hot-dip galvanized steel sheets, high-strength hot-dip galvanized steel sheets, and manufacturing methods of high-strength alloyed hot-dip galvanized steel sheets.

[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 thinning the steel sheets used as materials for automobile members. Also, from the viewpoint of the rust prevention performance of the vehicle body, zinc-based plating may be applied to the steel sheets used as materials for automobile members. Therefore, for the main structural parts that form the skeleton of the 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 780 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 immersed in the hot-dip galvanizing bath as it is 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, during the cooling and hot-dip galvanizing process within CGL, shape defects often occur, which are thought to be caused by steel sheet transformation, resulting in warping in the width direction of the steel sheet. If the sheet passes through the wiping process in this state, the distance from the wiping nozzle to the latter half of the process will differ in the width direction. As a result, uneven adhesion occurs on both the front and back surfaces, and in the width direction. Areas with high adhesion lead to powdering during processing, while areas with low adhesion lead to faster rust formation during use. Therefore, uniform adhesion 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] Furthermore, another challenge is that high-strength steel sheets often contain alloying elements such as Si, Mn, and Cr to increase their strength, but these elements are easily oxidized. As a result, oxides form on the surface during annealing in CGL. This reduces wettability with hot-dip galvanizing, leading to defects such as unplated surfaces.

[0008] To address such defects, for example, Patent Document 3 uses oxidizing and reducing burners to perform oxidation and reduction treatment on the Fe surface of the steel sheet, thereby suppressing the formation of oxidized areas on the surface. Patent Document 4 suppresses oxidation on the steel sheet surface by lowering the dew point inside the annealing furnace to -45°C or lower. Furthermore, Patent Document 5 suppresses oxidation on the steel sheet surface by raising the dew point inside the annealing furnace to -10°C or higher.

[0009] However, the methods described in Patent Document 3 or Patent Document 4 require special equipment such as burners and equipment to reduce water vapor in the furnace, necessitating the introduction of new equipment for general CGL manufacturing. On the other hand, furnace humidification as described in Patent Document 5 can be achieved by incorporating water vapor into the gas introduced into the annealing furnace using methods such as bubbling, and is less expensive than other methods. However, the method described in Patent Document 5 does not suppress non-plating of the appearance, and the effect of improving the plating appearance is insufficient.

[0010] Patent No. 6912981 Patent No. 6702519 Patent No. 5915569 Patent No. 5982905 Patent No. 5982906

[0011] This invention was developed in view of the above-mentioned circumstances. It aims to provide an intermediate product that can produce hot-dip galvanized steel sheets with high strength, specifically a tensile strength of 780 MPa or more, high ductility, and excellent plating appearance and adhesion stability, as well as a method for producing the same. For example, this intermediate product is produced in a CGL equipped with humidification equipment for the annealing furnace. 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.

[0012] 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 high strength, high ductility, and excellent appearance. As a result, they have obtained the following findings.

[0013] 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.

[0014] 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.

[0015] Furthermore, in an investigation into improving the appearance of CGL equipped with humidification equipment, it was found that adding certain amounts of Si, Cr, and Sb dramatically improved the appearance. This effect is thought to be because Si and Cr form internal oxides with Mn, suppressing surface oxide formation, and Sb further enhances these effects.

[0016] 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: 0.060% to 0.140%, Si: 0.30% to 1.50%, Cr: 0.30% to 1.50%, Sb: 0.005% to 0.100%, Mn: 2.00% to 3.30%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, Nb: 0.005% to 0.100%, with the remainder being Fe and unavoidable impurities, and at the 1 / 4 thickness position, the steel structure is such that the total area ratio of ferrite and bainite is A hot-rolled steel sheet material for high-strength hot-dip galvanized steel sheets, having a martensite area ratio of 50% or more and 100% or less, a martensite area ratio of 50% or less and a pearlite area ratio of 15% or less, a correlation coefficient of C and Mn concentrations measured in the thickness direction by EPMA line analysis at the 1 / 4 thickness position, of 0.40 or less, and a 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 depth from the surface of the steel sheet] / [1 / 4 thickness position] of 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, Co: 0.500% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 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].

[0017] According to the present invention, it is possible to provide hot-rolled steel sheets and cold-rolled steel sheets for hot-dip galvanized steel sheets that have a tensile strength of 780 MPa or more and are excellent in ductility, plating appearance, and plating adhesion stability.

[0018] 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.

[0019] The present invention will be described based on the following embodiments. However, the present invention is not limited to the following embodiments.

[0020] [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 "%".

[0021] C: 0.060% to 0.140% 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 780 MPa or more, the carbon content should be 0.060% or more. Preferably, the carbon content should be 0.075% or more, and more preferably 0.080% or more. On the other hand, if the carbon content is too high, the ductility will decrease. For this reason, the carbon content should be 0.140% or less, preferably 0.135% or less, and more preferably 0.130% or less.

[0022] Mn: 2.00% to 3.30%. Mn 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 high strength, specifically a tensile strength of 780 MPa or more, the Mn content should be 2.00% or more. A Mn content of 2.20% or more is more preferable, and 2.30% or more is even more preferable. On the other hand, if the Mn content is too high, the appearance of the plating deteriorates. For this reason, the Mn content should be 3.30% or less, preferably 3.20% or less, and more preferably 3.10% or less.

[0023] Si: 0.30% to 1.50%, Cr: 0.30% to 1.50%, Sb: 0.005% to 0.100% Si and Cr are elements that improve the hardenability of steel sheets and contribute to high strength, but their addition forms oxides on the surface of the steel sheet, reducing the plating properties. On the other hand, in annealing steel sheets, if the atmosphere is humidified to a dew point of -20°C or higher, adding the amounts listed above will form a composite oxide of Si, Cr, and Mn inside the steel sheet, suppressing the amount of oxide on the surface and improving the plating properties. Furthermore, by adding Sb, Sb segregates on the surface, further suppressing surface oxide formation and improving the plating properties. To obtain these effects, it is necessary to add 0.30% or more of Si, 0.30% or more of Cr, and 0.005% or more of Sb. On the other hand, if the Si and Cr content is too high, the appearance of the plating will deteriorate. Therefore, the Si and Cr content should be 1.50% or less, preferably 1.30% or less, and more preferably 1.10% or less. Furthermore, if the Sb content is too high, recrystallization will be suppressed during CGL annealing, and the ductility will decrease. Therefore, the Sb content should be 0.100% or less, preferably 0.08% or less, and more preferably 0.06% or less.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Nb: 0.005% to 0.100% Nb segregates at grain boundaries during hot rolling, suppressing ferrite transformation at high temperatures where carbon is prone to segregation, and thus suppressing carbon 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 should be 0.100% or less, preferably 0.070% or less.

[0029] 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, and their inclusion is permissible as long as it does not hinder the objective of the present invention.

[0030] The composition of the steel sheet may further contain the elements listed below, as needed.

[0031] 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 of Ti. However, if the amount of Ti exceeds 0.10%, the amount of carbonitride increases significantly, and ductility decreases. Therefore, in order to make the effects of the present invention even better, when adding Ti, the lower limit is preferably 0.005% or more, and more preferably 0.020% or more. To prevent a decrease in ductility, the upper limit of the amount of Ti is 0.10% or less, and it is preferable that it is in the range of 0.090% or less.

[0032] 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.

[0033] Mo: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less Mo, Ni, and Cu increase the hardenability of the steel sheet and contribute to improved strength. To obtain such effects, each is preferably at least 0.01%. 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 Mo, Ni, and Cu is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0034] 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, a content of 0.01% or more is preferable. 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, W, and V is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.06% or less, respectively.

[0035] Sn: 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, a content of 0.001% or more is preferable. If the Sn content exceeds 0.200%, it leads to a decrease in grain boundary strength and a decrease in ductility. For this reason, the Sn content is preferably 0.200% or less, more preferably 0.180% or less, and even more preferably 0.160% or less.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] If Hf is 0.10% or less, no coarse precipitates or inclusions will increase, and the ductility of the steel sheet will not be reduced. Therefore, the content of Hf is preferably 0.10% or less. Although the lower limit of the content of Hf is not particularly defined, since it is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformation ability of the steel sheet, the content of Hf is more preferably 0.01% or more. Therefore, when Hf is contained, its content is preferably 0.10% or less, more preferably 0.01% or more, and still more preferably 0.08% or less.

[0041] If Bi is 0.200% or less, no coarse precipitates or inclusions will increase, and the ductility of the steel sheet will not be reduced. Therefore, the content of Bi is preferably 0.200% or less. Although the lower limit of the content of Bi is not particularly defined, since it is an element that reduces segregation, the content of Bi is more preferably 0.001% or more. Therefore, when Bi is contained, its content is 0.200% or less. The more preferable Bi content is 0.001% or more. The still more preferable Bi content is 0.100% or less.

[0042] Regarding Ti, B, Mo, Ta, W, Cr, Co, Ni, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi described above, when each content is less than the preferable lower limit value, since it will not impair the effects of the present invention, they are included as inevitable impurities.

[0043] [Steel Structure (Microstructure)] Next, the steel structure (microstructure) of the hot-rolled steel sheet of the material of the present invention will be described. In the present invention, in order to suppress the transformation difference due to the plate thickness position of CGL, it is important to control the structure at the position of 1 / 4 of the plate thickness and the position of 100 μm from the surface of the steel sheet. The area ratio of each structure is based on the total steel structure being 100%, and is measured at the position of 1 / 4 of the plate thickness respectively.

[0044] In cold rolling, it is important to uniformly introduce strain so that the total ferrite area ratio and bainite area ratio are 50% or more and 100% or less, and the martensite area ratio is 50% or less in order to control the transformation during cooling in subsequent CGL annealing. For this purpose, the total of the ferrite area ratio and bainite area ratio needs to be 50% or more and 100% or less, and the martensite area ratio needs to be 50% or less. The lower limit of the martensite area ratio may be 0%.

[0045] When the pearlite area ratio is 15% 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 decreases the shape stability in CGL sheet passing. To avoid this, it is necessary to set the pearlite area ratio to 15% or less. The lower limit of the pearlite area ratio may be 0%.

[0046] The steel structure of the present invention may contain carbides such as retained austenite and cementite or other substances known as the structure of the steel sheet as the remaining structure. The upper limit of the area ratio of the remaining structure is not particularly limited, but is preferably 5.0% or less. The lower limit of the area ratio of the remaining structure is not particularly limited and may be 0.0%.

[0047] Here, the total area ratio of ferrite and bainite, the martensite area ratio, and the pearlite area ratio are determined as follows.

[0048] 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 mill scale (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 volume% nital to cause the structure to appear on the observation surface.

[0049] Subsequently, a scanning electron microscope (SEM) is used to observe a desired area on the observation surface of the test specimen at a magnification of 3,000x. More specifically, five fields of view are observed at the 1 / 4 thickness position of the steel plate (including the 1 / 4 thickness position) and at a depth of 100 μm from the surface of the steel plate, with the image centered at each location. SEM images are obtained for each field of view. The obtained SEM images are then color-coded by tissue type, and the area percentage of each tissue (average value of the five fields of view) is determined from the number of pixels. Note that each area percentage is calculated at the 1 / 4 thickness position.

[0050] In SEM images, ferrite and bainite appear as black or dark gray structures, while martensite appears as white or light gray structures. Perlite is characterized by streaks of black or dark gray and white or light gray, and is distinguished from ferrite, bainite, and martensite. Martensite includes auto-tempered martensite containing carbides.

[0051] At the 1 / 4 thickness position, the correlation coefficient between C and Mn concentrations measured in the thickness direction by EPMA line analysis is 0.40 or less. Cosegregation of C and Mn in hot-rolled steel sheets reduces the shape stability of CGL-processed sheets. It also increases the L-direction connectivity of the structure after CGL annealing. To avoid these problems, it is necessary to keep the correlation coefficient between C and Mn concentrations measured by EPMA at 0.40 or less.

[0052] 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.

[0053] 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.

[0054] Furthermore, the prior austenite grain size is measured as follows.

[0055] 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.

[0056] [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.

[0057] 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.

[0058] [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.

[0059] [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.

[0060] [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.

[0061] [Cooling to 800°C to 650°C for 5 to 20 seconds] If the residence time at 800 to 650°C is too long (more than 20 seconds), pearlite will form, causing shape defects in CGL. If the residence 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 residence time from 800°C to 650°C should be between 5 seconds and 20 seconds.

[0062] [Cooling under conditions where the holding time at 550-500°C is 1 second 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 second or less. Therefore, the holding time at 550-500°C is set to 1 second or less.

[0063] [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.

[0064] [Pickling] The hot-rolled steel sheets obtained as raw material for high-strength galvanized steel sheets may be pickled as needed. The pickling method is not particularly limited and can be any conventional method.

[0065] [Cold Rolling] Hot-rolled steel sheets used as base material for high-strength 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.

[0066] [Annealing] Hot-rolled or cold-rolled steel sheets used as raw materials for high-strength 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).

[0067] [Plating Treatment] Plating treatment may be performed on the hot-rolled or cold-rolled steel sheet material for high-strength 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.

[0068] [Temper Rolling] The obtained steel sheet may be subjected to temper rolling according to conventional methods as needed.

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0070] [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.

[0071]

[0072]

[0073] [Manufacturing of Hot-Dip Galvanized Steel Sheets] The obtained hot-rolled steel sheets were pickled, and then cold-rolled at a reduction ratio of 50%. Subsequently, CGL (Closed Galvanizing Line) was performed under the conditions of a maximum annealing temperature of 820°C, a zinc bath entry temperature of 480°C, humidification of the annealing furnace, and a dew point of -10°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.

[0074] [Evaluation] The obtained hot-dip galvanized steel sheets were evaluated as follows. The results are shown in Table 3.

[0075]

[0076] [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 ≥ 780 MPa, it can be said that the strength is high.

[0077] Furthermore, good ductility can be indicated if the EL (Electrical Strength) is ≥ 14% for TS:780MPa class, ≥ 11% for TS:980MPa class, and ≥ 8% for TS:1180MPa class. Note that TS:780MPa class means TS is 780MPa or more and less than 980MPa, TS:980MPa class means TS is 980MPa or more and less than 1180MPa, and TS:1180MPa class means TS is 1180MPa or more and less than 1380MPa.

[0078] [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.

[0079] 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 defined as the equivalent diameter of a circle. E: One unplated area with a diameter of 0.5mm or more per meter ２ The above is confirmed: D: 10 unplated points / 1m with a diameter of 0.3 mm or more and less than 0.5 mm. 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 780 MPa or higher, high 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.

[0080] The hot-rolled steel sheet material of the present invention enables the production of high-strength hot-dip galvanized steel sheets with a tensile strength (TS) of 780 MPa or higher and excellent plating appearance and adhesion stability in a CGL equipped with humidification facilities. High-strength cold-rolled steel sheets produced 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 as follows, by mass%, containing C: 0.060% to 0.140%, Si: 0.30% to 1.50%, Cr: 0.30% to 1.50%, Sb: 0.005% to 0.100%, Mn: 2.00% to 3.30%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, Nb: 0.005% to 0.100%, with the remainder being Fe and unavoidable impurities, and at the 1 / 4 thickness position, the steel structure is such that the total area ratio of ferrite and bainite is 50% or more and 100% or less, the martensite area ratio is 50% or less, and the pearlite area ratio is 15% or less, and at the 1 / 4 thickness position, A hot-rolled steel sheet material for high-strength hot-dip galvanized steel sheets, wherein the correlation coefficient between the concentrations of C and Mn measured in the thickness direction by EPMA line analysis 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 a depth of 1 / 4 of the sheet thickness, [100 μm from the surface of the steel sheet] / [1 / 4 of the sheet thickness], is 0.8 or more and 1.2 or less.

2. The hot-rolled steel sheet material for high-strength hot-dip galvanized steel sheet according to claim 1, wherein the component composition further contains, by 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, Co: 0.500% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 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.

3. A method for manufacturing a raw material 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 used as a base 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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