Hot-dip galvanized steel sheet and method for manufacturing the same

Abstract

The present invention provides a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality. The composition of the base steel sheet is set within a specified range, the steel structure of the base steel sheet is a composite structure of ferrite, martensite, and bainite, and the amount of oxygen present as an oxide in the surface layer portion of the base steel sheet is set to 0.05 g / m 2 to 0.50 g / m 2 , and the Fe content in the hot-dip galvanized layer is set to 0.40 mass% or more.

Description

Technical Field

[0001] The present invention relates to a hot-dip galvanized steel sheet suitable for automobile parts and the like and a method for producing the same. Background Art

[0002] In recent years, from the perspective of global environmental protection, improving the fuel efficiency of automobiles has become an important issue. Therefore, activities to reduce the weight of automobile bodies have been actively developed by increasing the strength and reducing the thickness of steel sheets used as materials for automobile parts. In addition, since steel sheets used for automobile parts are formed into complex shapes, good processability is also required.

[0003] In response to such a demand, for example, Patent Document 1 discloses “a high-strength cold-rolled steel sheet having excellent uniform deformation ability and local deformation ability, characterized in that it contains, by mass%, C: 0.01% or more, 0.4% to Si: 0.001% or more, 2.5% or less, Mn: 0.001% or more, 4.0% or less, P: 0.001% or more, 0.15% or less, S: 0.0005% or more, 0.03% or less, Al: 0.001% or more, 2.0% or less, N: 0.0005% or more, 0.01% or less, O: 0.0005% or more, 0.01% or less, and the remainder is iron and unavoidable impurities. The structure is a group of {112}<110>~{113}<110> orientations from at least the surface of the steel plate to the plate surface in 5 / 8 to 3 / 8 of the plate thickness, and the average value of the X-ray random intensity ratio of the crystal orientation of {112}<131> is 5.0 or less and the X-ray random intensity ratio of the crystal orientation of {001}<110> is 4.0 or less, and the r value between the rolling direction and the right angle direction (rC) is 0.70 or more, and the r value between the rolling direction and 30° (r30) is 1.10 or less, and as the steel plate structure, the total amount of ferrite and bainite is 50% or more, and the amount of martensite is 1% to 50% or less in terms of area ratio.

[0004] In addition, Patent Document 2 discloses "a high-strength steel plate having little deterioration in aging properties and excellent sintering hardening properties, characterized in that it contains, by mass%, C: 0.05% to 0.20%, Si: 0.3 to 1.50%, Mn: 1.3 to 2.6%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 0.0005 to 0.1%, N: 0.0005 to 0.0040%, O: 0.0015 to 0.007%, the remainder being iron and unavoidable impurities, the steel plate structure being mainly composed of ferrite and bainite, the BH after sintering being 60 MPa or more, and the maximum tensile strength being 540 MPa or more".

[0005] Prior art literature

[0006] Patent Document

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2012-172159

[0008] Patent Document 2: Japanese Patent Application No. 2009-249733 Summary of the Invention

[0009] However, from the viewpoint of the rust prevention performance of the vehicle body, there is a case where a steel sheet as a material for automobile parts is subjected to zinc-based plating such as hot-dip galvanizing.

[0010] However, if hot-dip galvanizing is performed on the steel sheets disclosed in Patent Documents 1 and 2, there are cases where plating quality such as plating appearance and plating adhesion cannot be sufficiently obtained. Therefore, there is a need to improve this aspect at present.

[0011] The present invention has been developed in view of the above-mentioned situation, and an object thereof is to provide a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality. Another object of the present invention is to provide a method for manufacturing the above-mentioned hot-dip galvanized steel sheet.

[0012] In addition, in order to achieve the above object, the inventors repeated in-depth research and obtained the following situation.

[0013] (a) In order to obtain good workability, it is necessary to improve the hole expansion property and elongation of the steel sheet. In addition, from the viewpoint of preventing cracking during forming processing, it is effective to increase the yield ratio YR (=yield strength (YS) / tensile strength (TS)) of the steel sheet.

[0014] (b) In order to obtain high strength, it is effective to utilize martensite. On the other hand, in order to obtain excellent elongation, it is effective to utilize ferrite. In addition, in order to obtain excellent hole expansion property, it is necessary to reduce the hardness difference between ferrite as a soft phase and martensite as a hard phase. For this, it is effective to utilize bainite as an intermediate product phase. In addition, by utilizing bainite, the yield ratio is also increased.

[0015] (c) That is, by controlling the steel structure to a composite structure of ferrite, martensite, and bainite having a specified area ratio (hereinafter, also simply referred to as a composite structure), high strength and good workability can be achieved at the same time.

[0016] (d) In addition, in order to obtain good plating quality, the following method is effective.

[0017] · Before the plating treatment, internal oxidation is generated in the surface layer portion of the base steel sheet, oxides of Si and Mn are formed in the surface layer portion of the base steel sheet, and

[0018] · The hot-dip galvanized layer contains an appropriate amount of Fe.

[0019] That is, from the viewpoint of high strength of the steel sheet, it is effective to use Si and Mn. However, elements such as Si and Mn are easily oxidizable elements, and they combine with oxygen to form oxides on the surface of the steel sheet. If oxides of such Si and Mn exist on the surface of the base steel sheet during the plating treatment, the wettability of the base steel sheet by the plating bath (hot dip galvanizing) is reduced, resulting in poor plating appearance such as non-plating and a decrease in plating adhesion.

[0020] In contrast, if internal oxidation is generated in the surface layer portion of the base steel sheet before the plating treatment to form oxides of Si and Mn, these oxides existing in the surface layer portion of the base steel sheet become obstacles and inhibit the formation of oxides (hereinafter also referred to as external oxidation) on the surface of the base steel sheet. As a result, the plating quality such as plating appearance and plating adhesion is improved.

[0021] In addition, by containing an appropriate amount of Fe in the hot dip galvanized layer, the plating quality, particularly the plating adhesion, can be improved.

[0022] (e) In addition to the above, in order to form the above-mentioned composite structure, generate internal oxidation in the surface layer portion of the base steel sheet to form oxides of Si and Mn in the surface layer portion of the base steel sheet, and further contain an appropriate amount of Fe in the hot dip galvanized layer, it is important to appropriately control the annealing conditions and plating treatment conditions carried out before the plating treatment. Particularly important is to control the atmosphere during the holding of annealing and the immersion plate temperature in the plating bath during the plating treatment.

[0023] Specifically, the dew point is set in the range of -20°C to 5°C, and a certain amount of oxygen is ensured in the holding atmosphere of annealing to promote internal oxidation in the surface layer portion of the base steel sheet. On the other hand, the hydrogen concentration is set to 3 mass% to 20 mass%, and the oxides formed on the surface of the base steel sheet (and formed during the holding of annealing) are reduced. It is important to sufficiently introduce oxygen in the atmosphere into the interior (surface layer portion) of the base steel sheet while suppressing external oxidation. In addition, it is important to promote the diffusion of Fe from the base steel sheet into the plating layer by setting the immersion plate temperature in the plating bath to be 10°C or more higher than the plating bath temperature.

[0024] The present invention has been completed through further research based on the above insights.

[0025] That is, the gist configuration of the present invention is as follows.

[0026] 1. A hot dip galvanized steel sheet, which is a hot dip galvanized steel sheet having a base steel sheet and a hot dip galvanized layer on the surface of the base steel sheet,

[0027] The base steel sheet has the following composition:

[0028] By mass%,

[0029] C: 0.09% to 0.17%,

[0030] Si: 0.3% to 1.1%,

[0031] Mn: 1.9% to 2.7%,

[0032] P: below 0.10%,

[0033] S: below 0.050%,

[0034] Al: 0.01% to 0.20% and

[0035] N: below 0.10%,

[0036] The balance is Fe and inevitable impurities,

[0037] In addition, it has the following steel structure:

[0038] Based on the area ratio of the whole steel structure,

[0039] Ferrite is 30% to 85%,

[0040] Martensite is 5% to 30%,

[0041] Bainite is 10% to 60% and

[0042] Other metal phases are below 15%,

[0043] And, the amount of oxygen present as an oxide in the surface layer of the base steel plate is 0.05 g / m per single side 2 ~0.50 g / m 2 , in addition, the surface layer is the area from the surface of the base steel plate to a position with a depth of 100 μm,

[0044] The Fe content in the hot-dip galvanized layer is 0.40 mass% or more.

[0045] 2. The hot-dip galvanized steel plate according to item 1 above, wherein the area ratio of the above-mentioned other metal phases is 5% or less.

[0046] 3. The hot-dip galvanized steel plate according to item 1 or 2 above, wherein the Fe content in the above-mentioned hot-dip galvanized layer is 8.0 mass% or less.

[0047] 4. The hot-dip galvanized steel plate according to any one of items 1 to 3 above, wherein the plating adhesion amount per single side of the above-mentioned hot-dip galvanized layer is 20 g / m 2 or more.

[0048] 5. The hot-dip galvanized steel sheet according to any one of 1 to 4 above, wherein the composition of the base steel sheet further contains, by mass%, one or more of Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, and V: 0.065% or less.

[0049] 6. The hot-dip galvanized steel sheet according to any one of 1 to 5 above, wherein the composition of the base steel sheet further contains, by mass%, one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr: 0.1% or less in total.

[0050] 7. A method for manufacturing a hot-dip galvanized steel sheet, comprising the following steps:

[0051] A hot rolling step of hot rolling a steel slab having the composition described in 1, 5, or 6 above to obtain a hot rolled steel sheet,

[0052] A cold rolling step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet,

[0053] An annealing step of heating the cold rolled steel sheet to an annealing temperature, holding at this annealing temperature, and then cooling, and

[0054] Subsequently, a plating step of subjecting the cold rolled steel sheet to a hot-dip galvanizing treatment;

[0055] In this annealing step,

[0056] The average heating rate in the temperature range from 500 °C to the annealing temperature is 1 °C / second to 7 °C / second,

[0057] The annealing temperature is (A C1 point + 50 °C) to (A C3 point + 20 °C),

[0058] The holding time during this holding is 1 second to 40 seconds,

[0059] The dew point of the atmosphere during this holding is -20 °C to 5 °C, and the hydrogen concentration is 3% by mass to 20% by mass,

[0060] The average cooling rate in the temperature range from the annealing temperature to the primary cooling stop temperature is 10 °C / second or more,

[0061] The primary cooling stop temperature is 450 °C to 600 °C,

[0062] The secondary cooling time is 20 seconds to 100 seconds,

[0063] The secondary cooling stop temperature is 400 °C to 500 °C,

[0064] In this plating process,

[0065] The temperature of the immersion plate in the plating bath is 10 °C or more higher than the temperature of the plating bath.

[0066] According to the present invention, a hot-dip galvanized steel sheet having both high strength and good workability and excellent plating quality can be obtained.

[0067] Moreover, by applying the hot-dip galvanized steel sheet of the present invention to automotive parts, it can greatly contribute to the high performance of the automotive body. Detailed Embodiments

[0068] The present invention will be described based on the following embodiments.

[0069] First, the composition of the base steel sheet of the hot-dip galvanized steel sheet according to an embodiment of the present invention will be described. It should be noted that the units of the composition are all "mass%", and hereinafter, unless otherwise specified, only "%" will be used.

[0070] C: 0.09% to 0.17%

[0071] C is an element that improves hardenability. In addition, C also plays a role in increasing the strength of ferrite. Therefore, C needs to ensure a desired tensile strength (TS) of 750 MPa or more. Here, when the C content is less than 0.09%, the desired tensile strength cannot be obtained. Therefore, the C content is 0.09% or more. The C content is preferably 0.10% or more, more preferably 0.11% or more. On the other hand, if the C content exceeds 0.17%, the stability of austenite increases, and bainite is not easily formed. In addition, the strength of martensite increases excessively, and the yield ratio decreases. Therefore, the C content is 0.17% or less. The C content is preferably 0.16% or less, more preferably 0.15% or less.

[0072] Si: 0.3% to 1.1%

[0073] Si is a strengthening element based on solid solution strengthening. In addition, Si plays a role in increasing the yield ratio by increasing the strength of ferrite. From the viewpoint of obtaining such an effect, the Si content is 0.3% or more. The Si content is preferably 0.4% or more, more preferably 0.5% or more. On the other hand, if Si is contained excessively, Si accumulates on the surface of the base steel sheet to cause external oxidation, deteriorating the plating quality such as the plating appearance. Therefore, the Si content is 1.1% or less. The Si content is preferably 1.0% or less, more preferably 0.9% or less.

[0074] Mn: 1.9% to 2.7%

[0075] Mn is an element that improves the hardenability of steel. Therefore, Mn is required to ensure the desired tensile strength. Here, when the Mn content is less than 1.9%, the desired tensile strength cannot be obtained. Therefore, the Mn content is 1.9% or more. The Mn content is preferably 2.0% or more, more preferably 2.1% or more. On the other hand, if Mn is excessively contained, Mn accumulates on the surface of the base steel plate and external oxidation occurs, deteriorating the plating quality such as the plating appearance. In addition, during stress relief annealing or the like, Mn easily accumulates in austenite, and the strength of martensite transformed from austenite increases excessively. As a result, the yield ratio decreases. Therefore, the Mn content is 2.7% or less. The Mn content is preferably 2.6% or less, more preferably 2.5% or less.

[0076] P: 0.10% or less

[0077] P is an element that strengthens steel. However, if P is excessively contained, P segregates at grain boundaries, deteriorating the hole expansion property. Therefore, the P content is 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.03% or less. It should be noted that the lower limit of the P content is not particularly limited, and from the viewpoints of cost and the like, it is preferably 0.001% or more. The P content is more preferably 0.003% or more, further preferably 0.005% or more.

[0078] S: 0.050% or less

[0079] S is an element that deteriorates the elongation by forming MnS or the like. In addition, when S and Ti are contained, the hole expansion property may deteriorate due to the formation of TiS, Ti(C, S), etc. Therefore, the S content is 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.020% or less, further preferably 0.015% or less. It should be noted that the lower limit of the S content is not particularly limited, and from the viewpoints of cost and the like, it is preferably 0.0002% or more. The S content is more preferably 0.0005% or more.

[0080] Al: 0.01% - 0.20%

[0081] Al is an element added as a deoxidizer. In addition, Al also plays a role in reducing coarse inclusions in steel and improving the hole expansion property. Here, if the Al content is less than 0.01%, the above effects cannot be obtained sufficiently. Therefore, the Al content is 0.01% or more. The Al content is preferably 0.02% or more. On the other hand, if the Al content exceeds 0.20%, precipitates such as AlN nitride coarsen and the hole expansion property decreases. Therefore, the Al amount is 0.20% or less. The Al content is preferably 0.17% or less, more preferably 0.15% or less.

[0082] N: 0.10% or less

[0083] N is an element that generates precipitates of nitride systems such as AlN that pin the grain boundaries and contributes to the improvement of the hole expansion property. However, if the N content exceeds 0.10%, the precipitates of nitride systems such as AlN coarsen, and conversely, the hole expansion property decreases. Therefore, the N content is 0.10% or less. The N content is preferably 0.05% or less, more preferably 0.010% or less. It should be noted that the lower limit of the N content is not particularly limited, and from the viewpoints of cost and the like, the N content is preferably 0.0006% or more. The N content is more preferably 0.0010% or more.

[0084] The base steel sheet of the hot-dip galvanized steel sheet according to an embodiment of the present invention contains the above elements and has a composition including the remaining Fe (iron) and inevitable impurities. In particular, the base steel sheet of the hot-dip galvanized steel sheet according to an embodiment of the present invention preferably has a composition including the above elements and the remaining part being composed of Fe and inevitable impurities.

[0085] As described above, the basic composition of the base steel sheet of the hot-dip galvanized steel sheet according to an embodiment of the present invention has been described. However, as optional additive elements, it may contain one or more of the following: Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, and V: 0.065% or less.

[0086] Moreover, as optional additive elements, it may contain one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr in a total amount of 0.1% or less.

[0087] It should be noted that when the above optional additive elements are contained in an amount less than the preferred lower limit value described later, they are regarded as being contained as inevitable impurities.

[0088] Nb: 0.040% or less

[0089] Nb contributes to the increase in strength through the refinement of old γ grains and the generation of fine precipitates. In addition, through the fine precipitates, the strength of ferrite increases, which also contributes to the increase in the yield ratio. In order to obtain such effects, the Nb content is preferably 0.0010% or more. The Nb content is more preferably 0.0015% or more, and further preferably 0.0020% or more. On the other hand, if Nb is excessively contained, the amount of precipitates of the carbonitride system is excessive, and the hole expansion property decreases. Therefore, when Nb is contained, the content is preferably 0.040% or less. The Nb content is more preferably 0.035% or less, and further preferably 0.030% or less.

[0090] Ti: 0.030% or less

[0091] Similar to Nb, Ti contributes to the increase in strength through the refinement of old γ grains and the formation of fine precipitates. In addition, through the fine precipitates, it also contributes to the increase in the strength of ferrite and the increase in the yield ratio. To obtain such an effect, the Ti content is preferably 0.0010% or more. The Ti content is more preferably 0.0015% or more, and further preferably 0.0020% or more. On the other hand, if Ti is contained excessively, the amount of precipitates in the carbonitride system is excessive, and the hole expansion property decreases. Therefore, when Ti is contained, the content is preferably 0.030% or less. The Ti content is more preferably 0.025% or less, and further preferably 0.020% or less.

[0092] B: 0.0030% or less

[0093] B is an element that improves the hardenability of steel. When B is contained and the Mn content is low, the desired tensile strength can be ensured. To obtain such an effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content is 0.0030% or more, the amount of precipitates in the nitride system such as BN is excessive, and the hole expansion property decreases. Therefore, when B is contained, the content is preferably 0.0030% or less. The B content is more preferably 0.0025% or less, and further preferably 0.0020% or less.

[0094] Cr: 0.3% or less

[0095] Cr is an element that improves the hardenability of steel. To obtain such an effect, the Cr content is preferably 0.005% or more. However, if Cr is contained excessively, there may be a case of an oxide formation reaction accompanied by the generation of hydrogen ions, which may cause the plating quality to deteriorate. In addition, if the amount of precipitates such as carbides is excessive, the hole expansion property decreases. Therefore, when Cr is contained, the content is preferably 0.3% or less. The Cr content is more preferably 0.2% or less, and further preferably 0.1% or less.

[0096] Mo: 0.2% or less

[0097] Similar to Cr, Mo is an element that improves the hardenability of steel. To obtain such an effect, the Mo content is preferably 0.005% or more. However, if Mo is contained excessively, there may be a case of an oxide formation reaction accompanied by the generation of hydrogen ions, which may cause the plating quality to deteriorate. In addition, if the amount of precipitates such as carbides is excessive, the hole expansion property decreases. Therefore, when Mo is contained, the content is preferably 0.2% or less. The Mo content is more preferably 0.1% or less, and further preferably 0.04% or less.

[0098] V: 0.065% or less

[0099] V is an element that improves the hardenability of steel in the same way as Cr. To obtain such an effect, the V content is preferably 0.005% or more. However, if V is contained in excess, there may be a case of an oxide formation reaction accompanied by the generation of hydrogen ions, which may cause plating quality. In addition, the amount of precipitates such as carbides is excessive, and the reaming property decreases. Therefore, when V is contained, the content is preferably 0.065% or less. The V content is more preferably 0.050% or less, and further preferably 0.035% or less.

[0100] One or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr: 0.1% or less in total

[0101] Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are elements that improve strength without deteriorating plating quality. To obtain such an effect, the content of these elements is preferably 0.0010% or more individually or in total. Among them, if the total content of these elements exceeds 0.1%, the above effect saturates. Therefore, when one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr are contained, the total content of these elements is preferably 0.1% or less.

[0102] The remainder other than the above elements is Fe and inevitable impurities.

[0103] Next, the steel structure of the base steel plate of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0104] The steel structure of the base steel plate of the hot-dip galvanized steel sheet according to one embodiment of the present invention is as follows: in terms of the area ratio with respect to the entire steel structure, ferrite is 30% to 85%, martensite is 5% to 30%, and bainite is 10% to 60% or less. It should be noted that the area ratio refers to the proportion of the area of each metal phase in the area of the entire steel structure.

[0105] Area ratio of ferrite: 30% to 85%

[0106] Ferrite is a phase required from the viewpoint of obtaining the desired elongation. Therefore, the area ratio of ferrite is 30% or more. The area ratio of ferrite is preferably 35% or more, and more preferably 40% or more. On the other hand, if ferrite is excessively large, the area ratio of martensite required to ensure strength decreases, and it is difficult to ensure strength. In addition, the generation of bainite is also suppressed, and the reaming property and yield ratio decrease. Therefore, the area ratio of ferrite is 85% or less. The area ratio of ferrite is preferably 80% or less.

[0107] It should be noted that the ferrite here is a structure composed of grains with a BCC lattice, and is formed by a phase transformation from austenite at a relatively high temperature.

[0108] Area ratio of martensite: 5% - 30%

[0109] Martensite helps to improve strength and is the phase required to ensure the desired tensile strength. Therefore, the area ratio of martensite is 5% or more. The area ratio of martensite is preferably 8% or more, more preferably 10% or more. On the other hand, if there is too much martensite, the elongation rate decreases. Therefore, the area ratio of martensite is 30% or less. The area ratio of martensite is preferably 28% or less, more preferably 25% or less.

[0110] It should be noted that the martensite here refers to a hard structure formed from austenite at a temperature below the martensite transformation point (abbreviated as Ms point), including both the so-called fresh martensite in the as-quenched state and the so-called tempered martensite obtained by reheating and tempering the fresh martensite.

[0111] Area ratio of bainite: 10% - 60%

[0112] Bainite is the phase required to improve the hole expansion property and increase the yield ratio. Therefore, the area ratio of bainite is 10% or more. The area ratio of bainite is preferably 15% or more, more preferably 20% or more. On the other hand, if there is too much bainite, the elongation rate decreases. Therefore, the area ratio of bainite is 60% or less. The area ratio of bainite is preferably 55% or less, more preferably 50% or less.

[0113] It should be noted that the bainite here refers to a hard structure in which fine carbides are dispersed in acicular or plate-like ferrite, and is formed from austenite at a relatively low temperature (above the martensite transformation point).

[0114] Area ratio of other metal phases: 15% or less

[0115] In addition, the steel structure of the base steel plate of the hot-dip galvanized steel sheet according to an embodiment of the present invention may contain other metal phases other than martensite, ferrite, and bainite. Here, when the total area ratio of other metal phases is 15% or less, it is allowed. Therefore, the area ratio of other metal phases is 15% or less. The area ratio of other metal phases is preferably 10% or less, more preferably 5% or less. It should be noted that the area ratio of other metal phases can be 0%.

[0116] As other metal phases, for example, pearlite, retained austenite, and unrecrystallized ferrite can be cited. Among them, since pearlite and unrecrystallized ferrite deteriorate workability (El and λ), the total area ratio of pearlite and unrecrystallized ferrite is 5% or less. The area ratio of pearlite and unrecrystallized ferrite can each be 0%. Since retained austenite does not deteriorate workability (El and λ), there is no problem as long as the area ratio of retained austenite is 15% or less. The area ratio of retained austenite is preferably 10% or less, more preferably 5% or less. The area ratio of retained austenite can be 0% or less.

[0117] It should be noted that the pearlite here is a structure composed of ferrite and acicular cementite. Retained austenite refers to austenite that remains without undergoing martensitic transformation. Unrecrystallized ferrite refers to ferrite that does not undergo recrystallization and has subgrain boundaries within the grains.

[0118] Among them, the area ratio of each phase is measured as follows.

[0119] That is, a test piece is taken from the base steel plate of the hot-dip galvanized steel sheet such that the L-section parallel to the rolling direction becomes the test surface. Next, the test surface of the test piece is mirror-polished, and the structure appears in a sodium nitrate liquid. The test surface of the test piece where the structure appears is observed by SEM at a magnification of 1500 times, and the area ratios of martensite, ferrite, and bainite at the 1 / 4 position of the plate thickness of the base steel plate are measured by the point counting method.

[0120] It should be noted that in the SEM image, martensite is a white structure. In addition, in tempered martensite among martensite, fine carbides are precipitated inside. Ferrite is a black structure. Bainite has white carbides precipitated in the black structure. Based on these aspects, each phase is identified in the SEM image. Among them, depending on the plane orientation of the grains and the degree of etching, there are cases where carbides inside are not easily visible, so in such cases, sufficient etching is required for confirmation.

[0121] The total area ratio of other metal phases is calculated by subtracting the area ratios of martensite, ferrite, and bainite from 100%.

[0122] In addition, among other metal phases, pearlite is a structure composed of ferrite and acicular cementite as described above, and pearlite is identified from this point by the above SEM image, and the area ratio of pearlite is measured. Unrecrystallized ferrite has subgrain boundaries within the grains as described above, and unrecrystallized ferrite is identified from this point by the above SEM image, and the area ratio of unrecrystallized ferrite is measured.

[0123] The area ratio of retained austenite is measured as follows.

[0124] The base steel sheet of the hot-dip galvanized steel sheet is ground in the thickness direction (depth direction) to the position of 1 / 4 of the sheet thickness, and the surface ground by chemical grinding by 0.1 mm is used as the observation surface. Subsequently, the observation surface is observed by X-ray diffraction method. The incident X-ray uses the Kα ray of Mo, and the ratio of the diffraction intensities of the (200), (220), and (311) planes of the fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of the bcc iron is obtained, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensities of each plane. And, the retained austenite is regarded as homogeneous in three dimensions, and thus the volume fraction of the retained austenite is used as the area fraction of the retained austenite.

[0125] The amount of oxygen present as an oxide in the surface layer portion of the base steel sheet (hereinafter, also referred to as the amount of oxygen in the oxide form in the surface layer portion of the base steel sheet): 0.05 g / m per single side 2 ~0.50 g / m 2

[0126] As described above, from the viewpoint of increasing the strength of the steel sheet, it is effective to use Si and Mn. However, elements such as Si and Mn are easily oxidizable elements, combine with oxygen, and form oxides on the surface of the steel sheet. When such oxides of Si and Mn are present on the surface of the base steel sheet during the plating treatment, it will become a cause for reducing the wettability of the plating bath (molten zinc) to the base steel sheet, resulting in poor plating appearance such as non-plating and reducing the plating adhesion.

[0127] In this regard, if internal oxidation occurs in the surface layer portion of the base steel sheet before the plating treatment to form oxides of Si and Mn, these oxides present in the surface layer portion of the base steel sheet will become obstacles and inhibit the formation of oxides on the surface of the base steel sheet (hereinafter, also referred to as external oxidation). As a result, the plating quality such as the plating appearance and the plating adhesion is improved. Therefore, the amount of oxygen in the oxide form in the surface layer portion of the base steel sheet is 0.05 g / m per single side 2 or more (it should be noted that the amount of oxygen described below is all for a single side). The amount of oxygen in the oxide form in the surface layer portion of the base steel sheet is preferably 0.06 g / m 2 or more. On the other hand, if the amount of oxygen in the oxide form in the surface layer portion of the base steel sheet exceeds 0.50 g / m 2 , the fracture due to the oxides develops, and the elongation and the hole expansion property decrease. Therefore, the amount of oxygen in the oxide form in the surface layer portion of the base steel sheet is 0.50 g / m 2 or less. The amount of oxygen in the oxide form in the surface layer portion of the base steel sheet is preferably 0.45 g / m 2 or less.

[0128] Here, the surface layer portion refers to the region from the surface of the base steel sheet to the position of a depth of 100 μm.

[0129] In addition, the oxide refers to a compound of O and elements such as Si, Mn, Fe, P, Al, Nb, Ti, B, Cr, Mo, and V contained in the base steel plate, and is mainly composed of Si oxide and Mn oxide.

[0130] It should be noted that since the internal oxidation amount and the external oxidation amount are inversely related, when external oxidation occurs on the base steel plate, the oxygen amount in the oxide form in the surface layer part of the base steel plate will be less than 0.05 g / m 2 .

[0131] In addition, the oxygen amount in the oxide form in the surface layer part of the base steel plate is measured by the "pulse furnace - infrared absorption method".

[0132] That is, first, the hot-dip galvanized layer is removed from the hot-dip galvanized steel plate. The method for removing the hot-dip galvanized layer is not particularly limited as long as it can sufficiently remove the hot-dip galvanized layer. For example, pickling, alkali stripping, mechanical grinding, etc. can be cited.

[0133] Next, the oxygen amount in the steel of the base steel plate is measured. And this measured value is used as the total oxygen amount OI (g) contained in the base steel plate.

[0134] Next, on both sides of the base steel plate, at least the surface layer part (the area from the surface of the base steel plate to a position with a depth of 100 μm) is ground and removed, and the oxygen amount in the steel of the base steel plate after removing the surface layer part is measured. And this measured value is set as OH (g).

[0135] Furthermore, the oxygen amount in the oxide form in the surface layer part of the base steel plate is calculated by the following formula.

[0136] [Oxygen amount in the oxide form in the surface layer part of the base steel plate] = {OI (g) - OH (g) × ([Thickness of the base steel plate before grinding (mm)] / [Thickness of the base steel plate after grinding (mm)])} ÷ ([Area of the surface of the base steel plate (per single side) (m 2 )] ÷ 2

[0137] It should be noted that in the above formula, by dividing ([Thickness of the base steel plate before grinding (mm)] / [Thickness of the base steel plate after grinding (mm)]) by OH (g), the oxygen amount in the solid solution state contained in the base steel plate is calculated;

[0138] Next, the oxygen amount in the solid solution state contained in the base steel plate is subtracted from the total oxygen amount OI (g) contained in the base steel plate;

[0139] Furthermore, by dividing this value by [Area of the surface of the base steel plate (per single side) (m 2)] and 2, from which the amount of oxygen in the oxide form of the surface layer of the base steel plate is calculated.

[0140] Further, the thickness of the base steel plate of the hot-dip galvanized steel sheet according to one embodiment of the present invention is preferably 0.2 mm to 3.2 mm.

[0141] Next, the hot-dip galvanized layer of the hot-dip galvanized steel sheet according to one embodiment of the present invention will be described.

[0142] Fe content in the hot-dip galvanized layer: 0.40% by mass or more

[0143] In order to improve the plating adhesion, it is preferable that the Fe content in the hot-dip galvanized layer is large. Therefore, the Fe content in the hot-dip galvanized layer is 0.40% by mass or more. The Fe content in the hot-dip galvanized layer is preferably 0.50% by mass or more. On the other hand, if the Fe in the hot-dip galvanized layer is excessively large, a hard Fe-Zn alloy phase is formed in the hot-dip galvanized layer. As a result, the plating itself is liable to be damaged, and conversely, there is a case where the plating adhesion is reduced. Therefore, the Fe content in the hot-dip galvanized layer is preferably 8.0% by mass or less. The Fe content in the hot-dip galvanized layer is more preferably 7.5% by mass or less, and further preferably 7.0% by mass or less.

[0144] Plating adhesion amount in the hot-dip galvanized layer: 20 g / m per single side 2 or more

[0145] In order to improve the corrosion resistance, it is preferable that the plating adhesion amount is large. Therefore, the plating adhesion amount is preferably 20 g / m per single side 2 or more (it should be noted that the plating adhesion amounts described below are all values per single side). The plating adhesion amount is more preferably 25 g / m 2 or more, and further preferably 30 g / m 2 or more. The upper limit of the plating adhesion amount is not particularly limited. If the plating adhesion amount exceeds 120 g / m 2 , the above effects are saturated. Therefore, the plating adhesion amount is preferably 120 g / m 2 or less.

[0146] Here, the Fe content and the plating adhesion amount of the hot-dip galvanized layer are measured according to the following procedures.

[0147] That is, after degreasing the surface of the hot-dip galvanized steel sheet used as the test piece, the mass of the test piece is weighed once. Next, after adding 2 to 3 drops of an inhibitor (inhibitor) for Fe corrosion to 30 cc of a 1:3 HCl aqueous solution (HCl aqueous solution with a concentration of 25 vol%), the test material is immersed in this solution to dissolve the hot-dip galvanized layer of the test material. After dissolving the hot-dip galvanized layer (after the generation of H2 gas on the surface of the test piece ends), this solution is collected. In addition, after recovering the test piece and drying it, the mass of the test piece is weighed twice.

[0148] Furthermore, the plating adhesion amount is calculated by the following formula.

[0149] [Plating adhesion amount (g / m 2 )] = ([Mass of the test piece weighed for the first time (g)] - [Mass of the test piece weighed for the second time (g)]) ÷ [Area of the plated part of the test piece (the area of the part covered by the hot-dip galvanized layer in the test piece before dissolving the hot-dip galvanized layer) (m 2 )]

[0150] In addition, by ICP (Inductively Coupled Plasma) method, the masses of Fe, Zn, and Al dissolved in the collected solution (hereinafter, also referred to as Fe dissolution amount, Zn dissolution amount, and Al dissolution amount) are measured, and the Fe content of the hot-dip galvanized layer is obtained by the following formula.

[0151] [Fe content of the hot-dip galvanized layer (mass%)] = [Fe dissolution amount (g)] / ([Fe dissolution amount (g)] + [Zn dissolution amount (g)] + [Al dissolution amount (g)]) × 100

[0152] It should be noted that the hot-dip galvanized layer has Zn as the main component and is basically composed of Zn and the above-mentioned Fe. In addition, depending on the plating bath composition, the hot-dip galvanized layer sometimes contains 0.30 mass% or less, especially 0.15 to 0.30 mass% of Al. The remaining part other than Zn, Fe, and Al is inevitable impurities. In addition, the hot-dip galvanized layer can be provided only on one surface of the base steel sheet or on both surfaces.

[0153] Next, the mechanical properties of the hot-dip galvanized steel sheet based on one embodiment of the present invention will be described.

[0154] The tensile strength (TS) of the hot-dip galvanized steel sheet based on one embodiment of the present invention is 750 MPa or more. The tensile strength (TS) is preferably 780 MPa or more. It should be noted that the upper limit of the tensile strength is not particularly limited, and from the viewpoint of easily obtaining a balance with other properties, the tensile strength is preferably less than 980 MPa.

[0155] In addition, from the perspective of workability, TS×El is 18,000 MPa·% or more, TS×λ is 40,000 MPa·% or more, and the yield ratio YR (=YS / TS) is 0.55 or more.

[0156] TS×El is preferably 19,000 MPa·% or more, more preferably 20,000 MPa·% or more.

[0157] TS×λ is preferably 45,000 MPa·% or more, more preferably 50,000 MPa·% or more.

[0158] YR is preferably 0.60 or more, more preferably 0.65 or more.

[0159] Here, the tensile strength (TS), yield strength (YS), and elongation (El) are measured as described below.

[0160] That is, from the central part of the plate width of the hot-dip galvanized steel sheet, a JIS No. 5 test piece with a gauge length of 50 mm and a gauge width of 25 mm is taken in such a way that the rolling direction becomes the long side direction. Then, using the taken JIS No. 5 test piece, a tensile test is carried out based on the provisions of JIS Z2241 (2011) to measure the tensile strength (TS), yield strength (YS), and elongation (El). It should be noted that the tensile speed is 10 mm / minute.

[0161] In addition, λ is the limiting hole expansion ratio (%), and is measured as follows.

[0162] That is, from the central part of the plate width of the hot-dip galvanized steel sheet, a test piece with a size of 100 mm square is taken. Then, using the taken test piece, a hole expansion test is carried out based on the Japan Iron and Steel Federation standard JFST1001 to measure λ. Specifically, after a hole with a diameter of 10 mm is punched in the test piece, a 60° conical punch is pressed into the hole under the condition of restricting the periphery, and the diameter of the hole at the limit of crack generation is measured. And the limiting hole expansion ratio λ (%) is calculated by the following formula.

[0163] Limiting hole expansion ratio λ (%) = {(D f −D0) / D0}×100

[0164] Here, D f is the diameter (mm) of the hole at the limit of crack generation, and D0 is the diameter (mm) of the initial hole (before pressing the punch).

[0165] In addition, "excellent plating quality" means that through the ball impact test under the following conditions, peeling of the hot-dip galvanized layer does not occur and there are no non-plated defects in the hot-dip galvanized layer (preferably no uneven plating appearance) through visual observation. It should be noted that non-plated defects refer to areas with a size of about several μm to several mm where the base steel plate is exposed without the hot-dip galvanized layer.

[0166] · Ball impact test conditions

[0167] Ball mass: 2.8 kg, Drop height: 1 m

[0168] (Under the above conditions, drop the ball, after the ball collides with the hot-dip galvanized steel plate, based on the tape (tape with JIS Z 1522 (2009) peel the ball collision part, and the adhesion force is 8 N per 25 mm width), visually judge the presence or absence of peeling of the hot-dip galvanized layer.)

[0169] Next, a method for manufacturing a hot-dip galvanized steel plate according to an embodiment of the present invention will be described.

[0170] The method for manufacturing a hot-dip galvanized steel plate according to an embodiment of the present invention includes:

[0171] A hot rolling process of hot rolling a steel billet having the above composition to form a hot rolled steel plate;

[0172] A cold rolling process of cold rolling the hot rolled steel plate to form a cold rolled steel plate;

[0173] An annealing process of heating the cold rolled steel plate to the annealing temperature, holding at the annealing temperature and then cooling; and

[0174] Next, a plating process of performing hot-dip galvanizing treatment on the cold rolled steel plate.

[0175] It should be noted that in the following description, unless otherwise specified, the temperature is the surface temperature of the steel plate or steel billet. The surface temperature of the steel plate or steel billet is measured using a radiation thermometer, for example.

[0176] · Hot rolling process

[0177] This process is a process of hot rolling a steel billet material (steel billet) having the above composition to form a hot rolled steel plate.

[0178] It should be noted that the used steel billet material is preferably manufactured by continuous casting to prevent microsegregation of components. The steel billet material can be manufactured by blooming method or thin slab casting method.

[0179] Hereinafter, the suitable manufacturing conditions of the hot rolling process will be described.

[0180] Heating temperature of the steel billet: 1200 °C or higher

[0181] When the heating temperature of the steel billet is less than 1200 °C, precipitates such as AlN cannot be fully dissolved. Therefore, precipitates such as AlN coarsen during hot rolling, which may lead to poor hole expansion performance. Therefore, the heating temperature of the steel billet is preferably 1200 °C or higher. The heating temperature of the steel billet is more preferably 1230 °C or higher, and further preferably 1250 °C or higher. It should be noted that the upper limit of the heating temperature of the steel billet is not particularly limited, and is preferably 1400 °C or lower. The heating temperature of the steel billet is more preferably 1350 °C or lower.

[0182] Finish rolling temperature: 840 °C to 900 °C

[0183] When the finish rolling temperature is less than 840 °C, inclusions and coarse carbides may be generated, resulting in poor hole expansion performance. In addition, the internal quality of the base steel plate may also be reduced. Therefore, the finish rolling temperature is preferably 840 °C or higher. The finish rolling temperature is more preferably 860 °C or higher. On the other hand, if the holding time at a high temperature becomes longer, coarse inclusions may be generated, resulting in poor hole expansion performance. Therefore, the finish rolling temperature is preferably 900 °C or lower. The finish rolling temperature is more preferably 880 °C or lower.

[0184] Coiling temperature: 450 °C to 650 °C

[0185] After rolling into a steel billet as described above, a hot-rolled steel plate is obtained by coiling. Here, if the coiling temperature exceeds 650 °C, decarburization of the surface of the matrix iron occurs. In this case, a tissue difference is generated inside and on the surface of the base steel plate, which may lead to uneven alloy concentration. In addition, coarse carbides and nitrides may be generated, resulting in poor hole expansion performance. Therefore, the coiling temperature is preferably 650 °C or lower. The coiling temperature is more preferably 630 °C or lower. On the other hand, in order to prevent the deterioration of cold rolling properties, the coiling temperature is preferably 450 °C or higher. The coiling temperature is more preferably 470 °C or higher.

[0186] In addition, the hot-rolled steel plate after coiling can be pickled. The pickling conditions are not particularly limited and can be based on common methods. In addition, a heat treatment for softening the structure can be performed on the hot-rolled steel plate after coiling.

[0187] · Cold rolling process

[0188] This process is to cold-roll the hot-rolled steel sheet obtained in the hot-rolling process to produce a cold-rolled steel sheet. Here, if the target sheet thickness can be controlled, there is no limit on the cold-rolling rate. When the cold-rolling rate is excessively small, recrystallization is less likely to occur during the subsequent annealing process. That is, unrecrystallized ferrite may be produced, resulting in a decrease in tensile strength. Therefore, the cold-rolling rate is preferably 20% or more. More preferably, the cold-rolling rate is 30% or more. On the other hand, when the cold-rolling rate is high, due to excessive deformation, recrystallization is less likely to occur during the subsequent annealing process. That is, unrecrystallized ferrite may be produced, resulting in a decrease in tensile strength. Therefore, the cold-rolling rate is preferably 90% or less. More preferably, the cold-rolling rate is 80% or less.

[0189] · Annealing process

[0190] This process is to heat the cold-rolled steel sheet obtained in the cold-rolling process to the annealing temperature, hold at this annealing temperature, and then cool it.

[0191] Moreover, in this process, from the perspective of producing the above-mentioned composite structure, generating internal oxidation in the surface layer of the base steel sheet, forming oxides of Si and Mn in the surface layer of the base steel sheet, and further containing an appropriate amount of Fe in the hot-dip galvanized layer, the following conditions are important:

[0192] Set the average heating rate in the temperature range from 500°C during heating to the annealing temperature (hereinafter, also referred to as the average heating rate) to 1°C / second to 7°C / second;

[0193] Set the annealing temperature to (A C1 point + 50°C) to (A C3 point + 20°C), set the holding time during holding (hereinafter, also referred to as the annealing time) to 1 second to 40 seconds, set the atmosphere during holding to dew point: -20°C to 5°C, hydrogen concentration: 3 mass% to 20 mass%, and set the average cooling rate in the temperature range from the annealing temperature during cooling to the primary cooling stop temperature (hereinafter, also referred to as the primary cooling rate) to 10°C / second or more;

[0194] Set the primary cooling stop temperature to 450°C to 600°C;

[0195] Set the secondary cooling time (the time from reaching the primary cooling stop temperature to reaching the secondary cooling stop temperature (in the case where the primary cooling stop temperature = secondary cooling stop temperature, the residence time at this temperature from reaching the primary cooling stop temperature)) to 20 seconds to 100 seconds;

[0196] Set the secondary cooling stop temperature to 400°C to 500°C.

[0197] Average heating rate: 1°C / second to 7°C / second

[0198] In order to recrystallize ferrite and ensure the desired area ratio of ferrite, the average heating rate is preferably slow. Therefore, the average heating rate is 7 °C / second or less. The average heating rate is preferably 6 °C / second or less, and more preferably 5 °C / second or less. On the other hand, if the average heating rate becomes slow, Mn with a slow diffusion rate also accumulates in austenite, stabilizing austenite. As a result, bainite transformation is not likely to occur, and the desired composite structure cannot be obtained. Therefore, the average heating rate is 1 °C / second or more. The average heating rate is preferably 2 °C / second or more, and more preferably 3 °C / second or more.

[0199] Annealing temperature: (A C1 point + 50 °C) to (A C3 point + 20 °C)

[0200] When the annealing temperature is less than (A C1 point + 50 °C), since coarse Fe-based precipitates are formed, the strength and hole expansion property decrease. Therefore, the annealing temperature is (A C1 point + 50 °C) or higher. The annealing temperature is preferably (A C1 point + 60 °C) or higher. On the other hand, if the annealing temperature exceeds (A C3 point + 20 °C), the area ratio of ferrite decreases and the elongation decreases. Therefore, the annealing temperature is (A C3 point + 20 °C) or lower. The annealing temperature is preferably (A C3 point + 10 °C) or lower.

[0201] It should be noted that the A C1 point and the A C3 point are calculated by the following formulas respectively. In addition, in the following formulas, (% element symbol) refers to the content (mass %) of each element in the composition of the base steel plate. Among them, when this element is not contained (including the case where it is inevitably contained), it is calculated as 0.

[0202] A C1 = 723 + 22 (%Si) - 18 (%Mn) + 17 (%Cr) + 4.5 (%Mo) + 16 (%V)

[0203] A C3 = 910 - 203√(%C) + 45 (%Si) - 30 (%Mn) - 20 (%Cu) - 15 (%Ni) + 11 (%Cr) + 32 (%Mo) + 104 (%V) + 400 (%Ti) + 460 (%Al)

[0204] In addition, the annealing temperature can be constant during holding. In addition, if the annealing temperature is within the above temperature range and the temperature variation range is within ±10 °C of the set temperature, it may not always be constant during holding.

[0205] Annealing time: 1 second to 40 seconds

[0206] The annealing time is an important condition for transforming austenite into bainite. Among them, from the perspective of not enriching Mn in austenite, that is, considering obtaining an appropriate amount of bainite by avoiding excessive stabilization of austenite, the annealing time is preferably short. Therefore, the annealing time is 40 seconds or less. The annealing time is preferably 30 seconds or less, more preferably 25 seconds or less. On the other hand, if the annealing time is less than 1 second, the recrystallization of ferrite will not be promoted, so the hole expansion property decreases. Therefore, the annealing time is 1 second or more. The annealing time is preferably 5 seconds or more. It should be noted that the annealing time refers to the holding time at the annealing temperature.

[0207] Dew point of the holding atmosphere: -20°C to 5°C

[0208] As described above, in order to generate internal oxidation in the surface layer of the base steel plate and form an appropriate amount of oxides of Si and Mn in the surface layer of the base steel plate, a certain amount of oxygen needs to be ensured in the holding atmosphere. In addition, from the perspective of ensuring an appropriate amount of Fe content in the hot-dip galvanized layer, the dew point needs to be increased to a certain extent. Therefore, the dew point of the holding atmosphere is -20°C or more. The dew point of the holding atmosphere is preferably -18°C or more, more preferably -15°C or more. On the other hand, when the dew point is too high, the internal oxidation in the surface layer of the base steel plate proceeds excessively, reducing the elongation and hole expansion property. In addition, if the dew point becomes too high, the diffusion of iron during the plating process is excessively promoted, and the amount of iron diffusion in the plating layer becomes excessive. Therefore, the dew point of the holding atmosphere is 5°C or less. The dew point of the holding atmosphere is preferably 0°C or less.

[0209] Hydrogen concentration of the holding atmosphere: 3 mass% to 20 mass%

[0210] In order to promote internal oxidation in the surface layer of the base steel plate and ensure the plating adhesion amount of the hot-dip galvanized layer, it is necessary to reduce the oxides formed on the surface of the base steel plate (and formed during the holding of annealing). Therefore, the hydrogen concentration of the holding atmosphere is 3 mass% or more. The hydrogen concentration of the holding atmosphere is preferably 5 mass% or more. On the other hand, if the hydrogen concentration of the holding atmosphere is too large, hydrogen will penetrate into the steel, reducing the elongation and hole expansion property. Therefore, the hydrogen concentration of the holding atmosphere is 20 mass% or less. The hydrogen concentration of the holding atmosphere is preferably 17 mass% or less.

[0211] Primary cooling rate: 10°C / second or more

[0212] During the cooling process in the temperature range from the annealing temperature to the primary cooling stop temperature, since bainite is formed, it is necessary to appropriately control the cooling rate. That is, if the primary cooling rate is slow, not only ferrite but also pearlite is formed, and an appropriate amount of bainite cannot be obtained. Therefore, the primary cooling rate is 10 °C / second or more. The primary cooling rate is preferably 12 °C / second or more, and more preferably 15 °C / second or more. It should be noted that in order to suppress the pearlite phase transformation, the primary cooling rate is preferably fast, so the upper limit of the primary cooling rate is not particularly limited. For example, there is no problem in setting the primary cooling rate to 2000 °C / second or more by water cooling or the like.

[0213] Primary cooling stop temperature: 450 °C to 600 °C

[0214] In order to suppress the pearlite phase transformation during primary cooling and ensure a specified amount of bainite during secondary cooling, the primary cooling stop temperature is set to 450 °C to 600 °C. That is, when the primary cooling stop temperature exceeds 600 °C, the pearlite phase transformation is promoted during secondary cooling. Therefore, the primary cooling stop temperature is 600 °C or less. The primary cooling stop temperature is preferably 580 °C or less, and more preferably 560 °C or less. On the other hand, when the primary cooling stop temperature is less than 450 °C, the bainite phase transformation during secondary cooling is suppressed, and it is difficult to ensure the specified bainite fraction. Therefore, the primary cooling stop temperature is 450 °C or more. The primary cooling stop temperature is preferably 460 °C or more, and more preferably 470 °C or more.

[0215] Secondary cooling time: 20 seconds to 100 seconds

[0216] Following the primary cooling process, during the secondary cooling process from the primary cooling stop temperature to the secondary cooling stop temperature, bainite is formed, so it is necessary to appropriately control the secondary cooling time. That is, the longer the secondary cooling time, the more the bainite phase transformation is promoted. Therefore, the secondary cooling time is 20 seconds or more. The secondary cooling time is preferably 25 seconds or more, and more preferably 30 seconds or more. On the other hand, if the secondary cooling time becomes too long, the amount of bainite is excessive, and the area ratio of martensite required for ensuring strength cannot be obtained. Therefore, the secondary cooling time is 100 seconds or less. The secondary cooling time is preferably 90 seconds or less, and more preferably 80 seconds or less.

[0217] Secondary cooling stop temperature: 400 °C to 500 °C

[0218] The secondary cooling stop temperature is set to 400°C to 500°C from the viewpoints of ensuring a specified bainite fraction and controlling the temperature of the plate immersed in the plating bath in the subsequent plating process within a specified range. That is, when the secondary cooling stop temperature exceeds 500°C, bainite phase transformation is promoted during secondary cooling, and the bainite fraction is excessive. Therefore, the secondary cooling stop temperature is 500°C or lower. The secondary cooling stop temperature is preferably 495°C or lower, more preferably 490°C or lower. On the other hand, when the secondary cooling stop temperature is less than 400°C, especially in the case of using a CGL (Continuous Galvanizing Line), it is difficult to set the temperature of the plate immersed in the plating bath to be 10°C or higher than the plating bath temperature even if a heat treatment is performed before the plating process. Therefore, the secondary cooling stop temperature is 400°C or higher. The secondary cooling stop temperature is preferably 420°C or higher, more preferably 440°C or higher.

[0219] · Plating process

[0220] This process is a process of performing hot-dip galvanizing on the cold-rolled steel sheet after the above annealing treatment.

[0221] Moreover, in this process, it is important to set the temperature of the plate immersed in the plating bath to be 10°C or higher than the plating bath temperature.

[0222] Temperature of the plate immersed in the plating bath: 10°C or higher than the plating bath temperature

[0223] In the hot-dip galvanized layer, in order to ensure an appropriate amount of Fe content, it is necessary to set the temperature of the plate immersed in the plating bath to be higher than the plating bath temperature, especially controlled to be 10°C or higher than the plating bath temperature. The temperature of the plate immersed in the plating bath is preferably 15°C or higher than the plating bath temperature, more preferably 20°C or higher than the plating bath temperature. The upper limit of the temperature of the plate immersed in the plating bath is not particularly limited, and it is preferably 500°C or lower.

[0224] It should be noted that the plating bath composition is basically composed of Zn, and there are cases where it contains 0.15 to 0.30 mass% of Al. It should be noted that the remaining part other than Zn and Al is an inevitable impurity.

[0225] In addition, the appropriate plating bath temperature is 440 to 500°C.

[0226] Moreover, the above annealing process and plating process can be carried out using a CAL (Continuous Annealing Line) or a CGL (Continuous Galvanizing Line). In addition, these processes can be carried out by batch processing.

[0227] It should be noted that the conditions for the other processes other than the above are not particularly limited and can be based on common methods. In addition, temper rolling for shape adjustment can be performed after the annealing process.

[0228] Moreover, based on the above manufacturing method, a hot-dip galvanized steel sheet with high strength, good workability, and excellent plating quality can be obtained, and this hot-dip galvanized steel sheet can be suitably used for automotive parts.

[0229] Example

[0230] A steel billet having the composition shown in Table 1 (the balance being Fe and inevitable impurities) was melted in a vacuum melting furnace and then subjected to blooming rolling to obtain a blooming-rolled material with a thickness of 27 mm. The obtained blooming-rolled material was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet with a thickness of 4.0 mm. Subsequently, the obtained hot-rolled steel sheet was ground to a thickness of 3.0 mm and then cold-rolled under the conditions shown in Table 2 to produce a cold-rolled steel sheet with a thickness of 0.9 to 1.8 mm. Then, the obtained cold-rolled steel sheet was annealed and plated under the conditions shown in Table 2 to produce a hot-dip galvanized steel sheet having hot-dip galvanized layers on both sides. The blank in Table 1 indicates that the element was not intentionally added (not necessarily 0 mass%, and sometimes included as an inevitable impurity).

[0231] Next, using the obtained hot-dip galvanized steel sheet, the microstructure of the base steel sheet was identified, the amount of oxygen in the oxide morphology in the surface layer of the base steel sheet was measured, and the plating adhesion amount per side and the Fe content in the hot-dip galvanized layer were measured according to the above gist.

[0232] The results are shown in Table 3.

[0233] It should be noted that in the identification of the microstructure (point counting method) of the base steel sheet, 16×15 grids were evenly spaced in the observation area (82 μm×57 μm area) of the SEM. And the number of points of each phase at the grid points was output, and the ratio of the number of grid points occupied by each phase to the total number of grid points was set as the area ratio of each phase. In addition, the area ratio of each phase is the average value of the area ratios of each phase obtained from three SEM images.

[0234] In addition, using the obtained hot-dip galvanized steel sheet, the mechanical properties were measured according to the above procedure. The results are shown in Table 4.

[0235] It should be noted that the target tensile strength (TS) is 750 MPa or more.

[0236] In addition, from the perspective of workability, the target values are TS×El of 18000 MPa·% or more, TS×λ of 40000 MPa·% or more, and the yield ratio YR (=YS / TS) of 0.55 or more.

[0237] Moreover, using the obtained hot-dip galvanized steel sheet, the plating quality (plating adhesion and plating appearance) was investigated by the above-mentioned method and evaluated according to the following criteria. The evaluation results are shown in Table 4.

[0238] · Plating adhesion

[0239] 〇 (Qualified, excellent): In the ball impact test based on the above-mentioned method, there is no peeling of the hot-dip galvanized layer.

[0240] × (Unqualified): In the ball impact test based on the above-mentioned method, there is peeling of the hot-dip galvanized layer.

[0241] · Plating appearance

[0242] ◎ (Qualified, particularly excellent): There are no non-plating defects and uneven plating appearance of the hot-dip galvanized layer.

[0243] 〇 (Qualified, excellent): There is uneven plating appearance of the hot-dip galvanized layer, but there are no non-plating defects.

[0244] × (Unqualified): There are non-plating defects of the hot-dip galvanized layer.

[0245]

[0246] Table 2

[0247]

[0248] Table 3

[0249]

[0250] α: Area ratio of ferrite M: Area ratio of martensite B: Area ratio of bainite P: Area ratio of pearlite

[0251] Un-α: Area ratio of unrecrystallized ferrite Residual γ: Area ratio of retained austenite

[0252] Table 4

[0253]

[0254] As shown in Table 4, all the inventive examples have high strength and good workability, and the plating quality is also excellent.

[0255] On the other hand, in the comparative examples, at least one of the strength, workability, and plating quality is insufficient.

Claims

1. A hot-dip galvanized steel sheet having a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet, The base steel sheet has the following composition: by mass%, C: 0.09% to 0.17%, Si: 0.3% to 1.1%, Mn: 1.9% to 2.7%, P: 0.10% or less, S: 0.050% or less, Al: 0.01% to 0.20%, and N: 0.10% or less, with the balance being Fe and inevitable impurities, And has the following steel structure: based on the area ratio with respect to the entire steel structure, ferrite is 30% to 85%, martensite is 5% to 30%, bainite is 10% to 60%, and other metal phases are 15% or less, The amount of oxygen present as an oxide in the surface layer of the base steel plate is 0.05 g / m per single side 2 ~0.50 g / m 2 , and this surface layer is the region from the surface of the base steel plate to a position with a depth of 100 μm The Fe content in the hot-dip galvanized layer is 0.40 mass% or more.

2. The hot-dip galvanized steel sheet according to claim 1, wherein, The area ratio of the other metal phases is 5% or less.

3. The hot-dip galvanized steel sheet according to claim 1, wherein, The Fe content in the hot-dip galvanized layer is 8.0 mass% or less.

4. The hot-dip galvanized steel sheet according to claim 2, wherein, The Fe content in the hot-dip galvanized layer is 8.0 mass% or less.

5. The hot-dip galvanized steel sheet according to claim 1, wherein, The coating adhesion amount per single side of the hot-dip galvanized layer is 20 g / m 2 or more.

6. The hot-dip galvanized steel sheet according to claim 2, wherein, The coating adhesion amount on each single side of the hot-dip galvanized layer is 20 g / m 2 or more.

7. The hot-dip galvanized steel sheet according to claim 3, wherein, The coating adhesion amount on each single side of the hot-dip galvanized layer is 20 g / m 2 or more.

8. The hot-dip galvanized steel sheet according to claim 4, wherein, The coating adhesion amount on each single side of the hot-dip galvanized layer is 20 g / m 2 or more.

9. The hot-dip galvanized steel sheet according to any one of claims 1 to 8, wherein, The composition of the base steel sheet further contains at least one of the following (A) and (B): (A): by mass%, Nb: 0.040% or less, Ti: 0.030% or less, B: 0.0030% or less, Cr: 0.3% or less, Mo: 0.2% or less, and V: 0.065% or less, one or more of them; (B) by mass%, a total of 0.1% or less of one or more selected from Ta, W, Ni, Cu, Sn, Sb, Ca, Mg, and Zr.

10. A method for manufacturing a hot-dip galvanized steel sheet, having the following steps: A hot rolling step of hot rolling a steel billet having the composition according to claim 1 or 9 to form a hot rolled steel sheet, A cold rolling step of cold rolling the hot rolled steel sheet to form a cold rolled steel sheet, An annealing step of heating the cold rolled steel sheet to an annealing temperature, holding at this annealing temperature and then cooling, and Subsequently, a plating treatment step of performing a hot-dip galvanizing treatment on the cold rolled steel sheet; In this annealing step, The average heating rate in the temperature range from 500 °C to the annealing temperature is 1 °C / second to 7 °C / second, The annealing temperature is (A C1 point + 50°C) to (A C3 point + 20°C), The holding time during this holding is 1 second to 40 seconds, The dew point of the atmosphere during this holding is -20 °C to 5 °C, and the hydrogen concentration is 3 mass% to 20 mass%, The average cooling rate in the temperature range from the annealing temperature to the primary cooling stop temperature is 10 °C / second or more, The primary cooling stop temperature is 450 °C to 600 °C, The secondary cooling time is 20 seconds to 100 seconds, The secondary cooling stop temperature is 400 °C to 500 °C, In this plating treatment step, The immersion plate temperature into the plating bath is the plating bath temperature + 10 °C or more.

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