Hot stamping steel sheet, hot stamping member, and method for manufacturing hot stamping member

Abstract

Provided is a steel sheet for hot pressing that has excellent rapid heating compatibility, that can prevent liquid metal embrittlement cracking, and that has excellent post-hot-pressing coating adhesion. The steel sheet for hot pressing comprises: a base steel sheet; and a coating layer which is provided on both surfaces of the base steel sheet and which has a thickness of 0.5 to 6.0 µm. The coating layer is formed of Ni or an Ni-based alloy, and the Zn content in the coating layer is 0-30 mass%.

Description

Technical Field

[0001] The present invention relates to a hot press steel sheet, a hot press member, and a method for manufacturing a hot press member.

Background Art

[0002] In recent years, in the automotive field, in order to meet the conflicting requirements of improving vehicle body strength and weight reduction, high-strength steel sheets have been used as materials for parts, and the strength required for such high-strength steel sheets has been increasing year by year.

[0003] However, generally, when the strength of a steel sheet is improved, the press formability decreases, making it difficult to obtain complex part shapes. Examples of members with complex shapes for automotive applications include underbody members such as chassis and skeletal structural members such as B-pillars.

[0004] Against this background, the application of hot press technology, which performs forming hot rather than cold, has been increasing. Hot press is a forming method in which a steel sheet is heated to the austenite temperature range and then press-formed while still hot, and simultaneously quenched by contact with a mold. In hot press, press forming is performed while the strength of the material steel sheet is relatively low, and then high strength is achieved by subsequent quenching, so it is possible to achieve both high strength and ensuring press formability.

[0005] However, in hot press, since the steel sheet is heated to a high temperature as described above, there is a problem that the surface of the steel sheet oxidizes and scale is generated. Therefore, it has been proposed to use a steel sheet having a coating layer such as an Al-based plating layer, a Zn-based plating layer, and an Al-Zn-based plating layer as a hot press steel sheet.

[0006] For example, in Patent Document 1, a hot press steel sheet that suppresses oxidation of the steel sheet during heating by providing an aluminum plating layer on the surface of a steel sheet containing 0.15 to 0.5% carbon has been proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the conventional hot press steel sheets proposed in Patent Document 1 had the following problems.

[0009] One is the problem of low compatibility with high-speed heating. In order to perform hot pressing, it is necessary to pre-heat the hot press steel sheet. As methods for heating the hot press steel sheet, for example, there are atmosphere furnace heating, direct electric heating, induction heating, etc. Among them, direct electric heating and induction heating are superior in energy efficiency compared to atmosphere furnace heating, so the carbon dioxide emissions can be reduced. Also, according to direct electric heating and induction heating, the hot press steel sheet can be heated at high speed, so the productivity is improved.

[0010] Therefore, during hot pressing, it is desirable to heat the hot press steel sheet at high speed by direct electric heating or induction heating. However, when a conventional Al-based plated steel sheet proposed in Patent Document 1 is heated at high speed by direct electric heating or induction heating, due to the magnetic field generated by the current, the plated metal flows, and finally the film thickness of the plating layer in the obtained hot press member varies. And when the variation in the film thickness of the plating layer is large, the appearance quality and corrosion resistance after painting are also degraded. Therefore, when hot pressing an Al-based plated steel sheet, it was inevitable to use atmosphere furnace heating with a slow heating rate. For these reasons, a hot press steel sheet with excellent compatibility with high-speed heating is required.

[0011] Another problem is liquid metal embrittlement cracking. When tensile stress is applied to the surface of a solid metal in a state where liquid metal is in contact with it, the solid metal becomes embrittled. This phenomenon is called liquid metal embrittlement (LME). Also in hot pressing, when the metal contained in the plating layer melts due to heating and press forming is performed in that state, liquid metal embrittlement cracks occur in the bending process part that receives tensile stress. Therefore, hot press steel sheets are also required to be less likely to cause liquid metal embrittlement cracks.

[0012] Furthermore, hot press steel sheets are generally used in a painted state after hot pressing. Therefore, hot press steel sheets are also required to have excellent paint adhesion of the finally obtained hot press member.

[0013] The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot press steel sheet and a hot press member that can satisfy the following requirements (1) to (3). (1) Excellent compatibility with high-speed heating. (2) Less likely to cause liquid metal embrittlement cracks during hot press forming. (3) Excellent paint adhesion after hot pressing.

Means for Solving the Problems

[0014] The present invention has been completed based on solving the above problems, and the gist thereof is as follows.

[0015] 1. A hot press steel sheet including a base steel sheet and a coating layer with a thickness of 0.5 to 6.0 μm provided on both sides of the base steel sheet, wherein the coating layer is made of Ni or a Ni-based alloy, and the Zn content in the coating layer is 0 to 30% by mass.

[0016] 2. The hot press steel sheet according to the above 1, wherein the Zn content in the coating layer is 0.5 to 30% by mass.

[0017] 3. The hot press steel sheet according to 1 or 2 above, wherein the coating layer contains at least one selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Mo, and W, in a total amount of 50% by mass or less.

[0018] 4. A hot press member having a base steel sheet and a coating layer with a thickness of 0.5 to 6.0 μm provided on both sides of the base steel sheet, wherein the coating layer is made of Ni or a Ni-based alloy, and the Zn content in the coating layer is 0 to 30% by mass.

[0019] 5. The hot press member according to 4 above, wherein the Zn content in the coating layer is 0.5 to 30% by mass.

[0020] 6. The hot press member according to 4 or 5 above, wherein the coating layer contains at least one selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Mo, and W, in a total amount of 50% by mass or less.

[0021] 7. Further, on the coating layer, there is an oxide layer containing one or both of Mn and Fe and having a thickness of 0.1 to 5 μm, wherein the total ratio of Mn and Fe to all metal elements contained in the oxide layer is 1 to 50 atomic%.

[0022] 8. A method for manufacturing a hot press member, comprising hot pressing the hot press steel sheet according to any one of 1 to 3 above to obtain a hot press member.

Advantages of the Invention

[0023] According to the present invention, the above problems can be solved. That is, the hot press steel sheet of the present invention is excellent in compatibility with high-speed heating, and even when heated at high speed by direct electric heating or induction heating, the variation in the film thickness of the coating layer is suppressed. Further, the hot press steel sheet of the present invention suppresses liquid metal embrittlement cracking during hot press forming. Furthermore, the hot press member obtained by hot pressing the hot press steel sheet of the present invention has excellent paint adhesion.

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments for carrying out the present invention will be specifically described. Note that the present invention is not limited to this embodiment. In addition, the unit “%” of the content in the component composition of the coating layer and the steel sheet represents “mass %” unless otherwise specified.

[0025] [Hot Press Steel Sheet] The hot press steel sheet in one embodiment of the present invention includes a base steel sheet and a coating layer with a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel sheet. And the coating layer is made of Ni or a Ni-based alloy, and the Zn concentration in the coating layer is 0 to 30%.

[0026] [[Coating Layer]] The coating layer may be a coating layer made of Ni (Ni coating layer) or a coating layer made of a Ni-based alloy (Ni-based alloy coating layer). Here, the “Ni-based alloy” refers to an alloy having a Ni content of 50% or more. In other words, the coating layer of the present invention is a coating layer having a Ni content of 50% or more.

[0027] In the heating process prior to hot pressing, the surface layer of the steel sheet is oxidized by oxygen and water vapor in the atmosphere. However, since the hot press steel sheet of the present invention is provided with a Ni-based coating layer having a high melting point and oxidation resistance on the surface, the coating layer does not melt during heating, and the formation of a thick oxide layer on the surface can be prevented. As a result, excellent paint adhesion can be obtained.

[0028] From the viewpoint of enhancing the above effects, the Ni content in the coating layer is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more. On the other hand, the upper limit of the Ni content in the coating layer is not particularly limited and may be 100%.

[0029] Zn: 0 to 30% When the coating layer contains a large amount of Zn, the oxidation resistance decreases. In addition, Zn melts upon heating, and liquid metal embrittlement cracks occur during hot forming. Therefore, the Zn content in the coating layer is 30% or less, preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. On the other hand, the above problems can be solved even if the coating layer does not contain Zn. Therefore, the lower limit of the Zn content is 0%. However, when the coating layer contains Zn, the chemical conversion coating film formed in the chemical conversion treatment step, which is a pre-painting treatment step, becomes denser, and the painting adhesion is further improved. Therefore, the Zn content in the coating layer is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more.

[0030] The coating layer can optionally contain at least one selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Mo, and W, with a total content of 50% or less. By adding at least one of Al, Ti, V, Cr, Mn, Co, Mo, and W, further excellent oxidation resistance can be obtained. Since these elements are optionally added elements, the lower limit of the total content may be 0%. However, in order to enhance the oxidation resistance of the coating layer, it is preferable that the coating layer contains these elements in a total amount of 1% or more. Also, when the coating layer is formed by electroplating, Fe eluted from the base steel plate into the plating bath may be incorporated into the coating layer. However, when the content of these elements becomes excessive, the Ni content in the coating layer relatively decreases, thus impairing the function of the coating layer. Therefore, the total content of the above elements is 50% or less, preferably 40% or less, more preferably 30% or less, and even more preferably 20% or less. The Fe content in the coating layer is preferably 20% or less, more preferably 5% or less, and even more preferably 1% or less.

[0031] In one embodiment of the present invention, the coating layer, in mass%, Zn: 0 to 30%, at least one selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Mo, and W: contained in a total amount of 0 to 50%, and the balance consisting of Ni and unavoidable impurities.

[0032] Also in the above embodiment, the preferable ranges of the contents of the respective components described above are applicable. For example, from the viewpoint of further improving the paint adhesion, the Zn content is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more.

[0033] Thickness: 0.5 to 6.0 μm When the hot press steel sheet is heated, Ni in the coating layer and Fe in the base steel sheet diffuse into each other, and an Ni-based alloy layer with an increased Fe concentration is formed on the surface layer of the hot press member. If the thickness of the coating layer of the hot press steel sheet is less than 0.5 μm, when heated at a high temperature exceeding 1000°C, Fe that has diffused and reached the surface layer is oxidized, and a thick and brittle Fe-containing oxide layer is formed. The presence of a thick and brittle Fe-containing oxide reduces the paint adhesion. Therefore, the thickness of the coating layer should be 0.5 μm or more. Also, since better corrosion resistance can be obtained as the coating layer remains thick after heating, the thickness of the coating layer is preferably 1.0 μm or more, and more preferably 2.0 μm or more. On the other hand, if the thickness of the coating layer exceeds 6.0 μm, the surface unevenness after heating becomes large, and conversely, the paint adhesion decreases. Therefore, the thickness of the coating layer is 6.0 μm or less, preferably 5.0 μm or less, and more preferably 4.0 μm or less.

[0034] Note that the hot press steel sheet of the present invention has coating layers on both sides thereof. The thickness of the coating layer on one side may be the same as or different from the thickness of the coating layer on the other side, as long as the thickness of the coating layer on each side satisfies the above conditions.

[0035] [[Base steel sheet]] As the base steel sheet, any steel sheet can be used without particular limitation. The base steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet.

[0036] From the viewpoint of obtaining a hot press member having a tensile strength exceeding 980 MPa class after hot pressing, as the base steel sheet, in mass%, C: 0.05 to 0.50%, Si: 0.1 to 1.0%, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.10% or less, and N: 0.01% or less, and It is preferable to use a steel sheet having a component composition consisting of the remainder and Fe and inevitable impurities. The reasons for the preferable component composition will be described below.

[0037] C: 0.05 to 0.50% By adding C, a hard structure such as martensite is formed, and the strength of the steel sheet can be increased. In order to obtain a high strength exceeding 980 MPa class, it is preferable that the C content in the base steel sheet is 0.05% or more, and more preferably 0.10% or more. On the other hand, when the C content exceeds 0.50%, the toughness of the spot weld portion decreases. Therefore, the C content is preferably 0.50% or less, more preferably 0.45% or less, still more preferably 0.43% or less, and most preferably 0.40% or less.

[0038] Si: 0.1 to 1.0% Si is an effective element for strengthening steel and obtaining good material properties. In order to obtain the above effects, it is preferable that the Si content is 0.1% or more, and more preferably 0.2% or more. On the other hand, when the Si content exceeds 1.0%, ferrite is stabilized, so the hardenability decreases. Therefore, the Si content is preferably 1.0% or less, more preferably 0.4% or less, still more preferably 0.3% or less.

[0039] Mn: 0.5 to 3.0% Mn is an element that contributes to the strength improvement of the steel sheet in a wide cooling rate range. In order to obtain the above effects, it is preferable that the Mn content is 0.5% or more, more preferably 0.7% or more, and still more preferably 1.0% or more. On the other hand, when the Mn content exceeds 3.0%, the effect of Mn saturates. Therefore, the Mn content is preferably 3.0% or less, more preferably 2.5% or less, still more preferably 2.0% or less, and most preferably 1.5% or less.

[0040] P: 0.1% or less If the P content is higher than 0.1%, P segregates to the austenite grain boundaries during casting, resulting in grain boundary embrittlement. As a result, the local ductility decreases, and the balance between the strength and ductility of the steel sheet deteriorates. Therefore, the P content is preferably 0.1% or less. On the other hand, although the lower limit of the P content is not particularly limited, from the perspective of refining cost, the P content is preferably 0.01% or more.

[0041] S: 0.01% or less S forms inclusions such as MnS, causing deterioration of impact resistance and cracks along the metal flow in the welded part. Therefore, it is desirable to reduce the S content as much as possible, and it is preferably 0.01% or less. Also, from the perspective of ensuring good elongation flanging properties, it is more preferably 0.005% or less, and even more preferably 0.001% or less. On the other hand, although the lower limit of the S content is not particularly limited, from the perspective of refining cost, the S content is preferably 0.0002% or more.

[0042] Al: 0.10% or less If the Al content is higher than 0.10%, the blanking workability and hardenability of the steel sheet decrease. Therefore, the Al content is preferably 0.10% or less, more preferably 0.07% or less, and even more preferably 0.04% or less. On the other hand, although the lower limit of the Al content is not particularly limited, from the perspective of ensuring the effect as a deoxidizer, the Al content is preferably 0.01% or more.

[0043] N: 0.01% or less If the N content is higher than 0.01%, AlN is formed during hot rolling or heating before hot pressing, and the blanking workability and hardenability of the steel sheet decrease. Therefore, the N content is preferably 0.01% or less. On the other hand, although the lower limit of the N content is not particularly limited, from the perspective of refining cost, the N content is preferably 0.001% or more.

[0044] The component composition of the base steel plate may further optionally contain at least one selected from the group consisting of Nb: 0.10% or less, Ti: 0.10% or less, B: 0.0002 to 0.010%, Cr: 0.1 to 1.0%, and Sb: 0.003 to 0.10% for further improvement of properties.

[0045] Nb: 0.10% or less Nb is an element effective for strengthening steel, but when contained in excess, the rolling load increases. Therefore, when Nb is contained, the Nb content is 0.10% or less, preferably 0.06% or less, more preferably 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%, but from the viewpoint of refining cost, it is preferably 0.005% or more.

[0046] Ti: 0.10% or less Ti is also an element effective for strengthening steel, similar to Nb. However, when Ti is contained in excess, the shape freezing property deteriorates. Therefore, when Ti is contained, the Ti content is 0.10% or less, preferably 0.06% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but from the viewpoint of refining cost, it is preferably 0.003% or more.

[0047] B: 0.0002 to 0.010% B is an element that has the effect of suppressing the formation and growth of ferrite from the austenite grain boundary. When B is contained, in order to obtain the above effect, the B content is preferably 0.0002% or more, more preferably 0.0010% or more. On the other hand, the inclusion of excessive B greatly impairs the formability. Therefore, when B is contained, the amount of B is 0.010% or less, preferably 0.005% or less.

[0048] Cr: 0.1 to 1.0% Cr is an element that improves hardenability and contributes to the strength improvement of the steel plate. When adding Cr, in order to obtain the above effects, the Cr content is set to 0.1% or more, preferably 0.2% or more. On the other hand, since Cr is expensive, adding more than 1.0% will cause a significant cost increase. Therefore, when containing Cr, the Cr content is set to 1.0% or less, preferably 0.5% or less, more preferably 0.3% or less.

[0049] Sb: 0.003 - 0.10% Sb is an element that has the effect of suppressing decarburization of the surface layer in the annealing process when manufacturing the base steel plate. When containing Sb, in order to obtain the above effects, the Sb content is set to 0.003% or more, preferably 0.005% or more. On the other hand, when the Sb content is higher than 0.10%, the rolling load increases and productivity decreases. Therefore, when containing Sb, the Sb content is set to 0.10% or less, preferably 0.05% or less, more preferably 0.03% or less.

[0050] [Manufacturing Method of Hot Press Steel Plate] The hot press steel plate of the present invention can be manufactured by any method without particular limitation, but the following preferred manufacturing conditions will be described.

[0051] First, a base steel plate is manufactured. The base steel plate can typically be manufactured by rolling a steel slab obtained by casting. As the steel slab, it is preferable to use a steel slab having the above-described component composition.

[0052] In rolling, the hot slab obtained by casting may be directly subjected to hot rolling (without reheating), or the cold slab whose temperature has decreased after casting may be reheated and then subjected to hot rolling. There is almost no difference in the characteristics of the obtained steel plate between the case of directly rolling the hot slab and the case of rolling after reheating the cold slab. When reheating the cold slab before hot rolling, the reheating temperature is not particularly limited, but it is preferably in the range of 1000°C to 1300°C in consideration of productivity.

[0053] The hot rolling can be either a normal hot rolling process or a continuous hot rolling process in which slabs are joined and rolled in finish rolling. The finishing temperature in hot rolling is not particularly limited, but it is preferably set to be equal to or higher than the Ar3 transformation point from the viewpoints of productivity and sheet thickness accuracy.

[0054] The hot rolled steel sheet obtained by the above hot rolling is then cooled according to a conventional method. From the viewpoint of productivity, the coiling temperature is preferably 550°C or higher. Further, since pickling property deteriorates when the coiling temperature is too high, the coiling temperature is preferably 750°C or lower. After the cooling, pickling is preferably performed according to a conventional method.

[0055] When a cold rolled steel sheet is used as the base steel sheet, cold rolling may be further performed according to a conventional method after the above pickling.

[0056] Next, a coating layer is formed on the surface of the obtained steel sheet. The method for forming the coating layer is not particularly limited, and it can be formed by any method such as plating, PVD, clad rolling, etc. Examples of the plating include electroplating. Examples of the PVD include vacuum evaporation, sputtering, and ion plating. When clad rolling is used, layers having a desired composition may be laminated on both sides of the base steel sheet and then rolled.

[0057] The method for forming the coating layer is preferably selected according to the composition of the coating layer to be formed. For example, when the coating layer is a Ni layer, a Ni-Cr alloy layer, or a Ni-Zn alloy layer, film formation by electroplating is preferably used, but film formation can also be performed without problems by other methods. When the coating layer has a composition that is difficult to electrodeposit from an aqueous solution, such as a Ni-Ti alloy, film formation by PVD is preferably used.

[0058] In any case of forming the coating layer, the conditions may be adjusted so that the coating layers on one surface (front surface) and the other surface (back surface) of the steel sheet have a desired thickness. For example, in the case of the electroplating method, the thickness of the coating layer on each surface can be adjusted by changing either or both of the current density and the energization time on each surface.

[0059] [Hot press member] The hot press member in one embodiment of the present invention is a hot press member having a base steel sheet and coating layers with a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel sheet. The coating layer is made of Ni or a Ni-based alloy, and the Zn content in the coating layer is 0 to 30%.

[0060] Regarding the above base steel sheet and coating layer, the descriptions of the base steel sheet and coating layer in the above-described hot press steel sheet are applicable. That is, as the base steel sheet of the hot press member, the same steel sheet as the base steel sheet of the above-described hot press steel sheet can be used. Also, as the coating layer of the hot press member, the same coating layer as the coating layer of the above-described hot press steel sheet can be used. For example, the Zn content in the coating layer of the hot press member is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more. Further, the coating layer can also contain at least one selected from the group consisting of Al, Ti, V, Cr, Mn, Fe, Co, Mo, and W, in a total amount of 50% by mass or less.

[0061] [Oxide layer] The hot press member in another embodiment of the present invention further has an oxide layer containing one or both of Mn and Fe and having a thickness of 0.1 to 5 μm on the coating layer. That is, the hot press member of this embodiment has a base steel sheet, coating layers with a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel sheet, and an oxide layer with a thickness of 0.1 to 5 μm provided on the coating layer.

[0062] The above oxide layer is formed by the components contained in the coating layer or the base steel sheet reacting with oxygen or water vapor in the atmosphere during the hot press process. The composition and thickness of the oxide layer vary depending on heating conditions such as heating temperature, heating time, and atmosphere. When the thickness of the oxide layer exceeds 5 μm, the coating adhesion decreases, and as a result, sufficient corrosion resistance after painting cannot be obtained. Therefore, when the oxide layer is present, the thickness of the oxide layer should be 5 μm or less, preferably 3 μm or less, more preferably 1 μm or less. On the other hand, from the perspective of coating adhesion, the thinner the oxide layer is better, but in order to make the thickness of the oxide layer less than 0.1 μm, it is necessary to perform hot pressing under an atmosphere with an extremely low oxygen partial pressure or to provide a step of removing the oxide after hot pressing, which increases the manufacturing cost. Therefore, the thickness of the oxide layer is preferably 0.1 μm or more.

[0063] The above oxide layer contains one or both of Mn and Fe. When the oxide layer contains these elements, these components elute into the chemical conversion treatment liquid in the chemical conversion treatment process. As a result, the formation of the chemical conversion coating is promoted, and better coating adhesion can be obtained. In order to obtain the above effects, the total ratio of Mn and Fe to all the metals contained in the oxide layer is set to 1 to 50 atomic %.

[0064] The thickness of the oxide layer can be measured by observing the cross-section of the hot press member with a scanning electron microscope (SEM). More specifically, it can be measured by the method described in the examples.

[0065] Note that the ratio of Mn and Fe to all the metals contained in the oxide layer can be measured by EPMA (electron probe microanalyzer) on the cross-section of the hot press member. More specifically, it can be measured by the method described in the examples.

[0066] [Manufacturing Method of Hot Press Member] In one embodiment of the present invention, a hot press steel sheet is hot pressed to manufacture a hot press member. The method of performing the hot press is not particularly limited and can be carried out according to a conventional method. Typically, the hot press steel sheet is heated to a predetermined heating temperature (heating step), and then the hot press steel sheet heated in the heating step is hot pressed (hot press step). Hereinafter, preferable hot press conditions will be described.

[0067] If the heating temperature in the heating step is lower than the Ac3 transformation point of the base steel sheet, the strength of the final hot press member will be low. Therefore, the heating temperature is preferably equal to or higher than the Ac3 transformation point of the base steel sheet, more preferably 860°C or higher. On the other hand, if the heating temperature exceeds 1000°C, the oxide layer generated by oxidation of the base material and the coating layer will become excessively thick, which may deteriorate the paint adhesion of the obtained hot press member. Therefore, the heating temperature is preferably 1000°C or lower, more preferably 960°C or lower, and even more preferably 920°C or lower. Note that the Ac3 transformation point of the base steel sheet varies depending on the steel components and is determined by a Formaster test.

[0068] The temperature at which the heating starts is not particularly limited, but generally it is room temperature.

[0069] The time required for the temperature rise (temperature rise time) from the start of heating to reaching the heating temperature is not particularly limited and can be any time. However, if the temperature rise time exceeds 300 seconds, the time exposed to high temperature becomes long, so the oxide layer generated by oxidation of the base material and the plating layer becomes excessively thick. Therefore, from the viewpoint of suppressing the decrease in paint adhesion due to oxides, the temperature rise time is preferably 100 seconds or less, more preferably 80 seconds or less, and even more preferably 60 seconds or less. On the other hand, if the temperature rise time is less than 3 seconds, it is difficult to stably raise the temperature. Therefore, the temperature rise time is preferably 3 seconds or more, more preferably 4 seconds or more, and even more preferably 5 seconds or more.

[0070] After reaching the heating temperature, it may be maintained at that heating temperature. When performing the holding, the holding time is not particularly limited, and holding for an arbitrary length can be performed. However, if the holding time exceeds 100 seconds, the oxide layer generated by oxidation of the base material and the coating layer becomes excessively thick, which may deteriorate the paint adhesion of the resulting hot press member. Therefore, the holding time is preferably 100 seconds or less, more preferably 60 seconds or less, and even more preferably 20 seconds or less. On the other hand, the lower limit of the holding time is not particularly limited, but from the viewpoint of homogenously austenitizing the base steel plate, it is preferably 1 second or more.

[0071] The atmosphere in the heating step is not particularly limited. For example, heating can be performed under an air atmosphere or an atmosphere into which air flows. From the viewpoint of reducing the amount of diffusible hydrogen remaining in the member after hot pressing, it is preferable that the dew point of the atmosphere is 10°C or lower. The lower limit of the dew point is not particularly limited, but it may be, for example, -40°C or higher.

[0072] The method of heating the hot press steel plate is not particularly limited, and heating can be performed by any method. The heating can be performed, for example, by furnace heating, electric heating, induction heating, high-frequency heating, flame heating, etc. Among them, it is preferable to use electric heating, induction heating, or high-frequency heating, which can raise the temperature in a short time and have excellent energy efficiency. As the heating furnace, any heating furnace such as an electric furnace or a gas furnace can be used.

[0073] Next, the heated hot press steel sheet is hot pressed to obtain a hot press member. In the hot pressing, the steel sheet is cooled using a coolant such as a mold or water simultaneously with or immediately after the processing. In the present invention, the hot pressing conditions are not particularly limited. For example, pressing can be started within a general hot pressing temperature range of 600 to 800°C. Further, since the hot press steel sheet of the present invention does not have the risk of liquid metal embrittlement, it is also possible to perform forming at a higher temperature than general hot pressing. Therefore, the hot press starting temperature is preferably 600 to 1000°C.

Example

[0074] Hereinafter, the present invention will be specifically described based on examples.

[0075] · Fabrication of hot press steel sheet As the base steel sheet, a cold-rolled steel sheet with a thickness of 1.4 mm having a component composition containing, by mass%, C: 0.34%, Si: 0.25%, Mn: 1.20%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Ti: 0.02%, B: 0.002%, Cr: 0.18%, Sb: 0.008%, and the balance being Fe and inevitable impurities was used. The Ac3 transformation point of the base steel sheet was 783°C, and the Ar3 transformation point was 706°C.

[0076] Coating layers were formed on both surfaces of the above base steel sheet by the methods shown in Tables 1 and 2. Each of the methods used will be described below. For comparison, in Comparative Example No. 1, no coating layer was formed.

[0077] (Electroplating) The formation of the coating layer by electroplating was carried out under the following conditions. In any case, electrolysis was performed with the base steel sheet as the cathode and the iridium oxide-coated titanium plate as the anode, and the thickness of the coating layer was adjusted by changing the energization time.

[0078] (1) Ni plating · Plating solution composition: Nickel sulfate hexahydrate 240 g / L Boric acid 30 g / L · pH: 3.0 · Temperature: 50 °C · Current density: 40 A / dm 2

[0079] (2) Ni-Fe alloy plating · Plating solution composition: Nickel sulfate hexahydrate 192 g / L Iron(III) sulfate heptahydrate 48 g / L, Boric acid 30 g / L · pH: 3.0 · Temperature: 50 °C · Current density: 40 A / dm 2

[0080] (3) Ni-Co alloy plating · Plating solution composition: Nickel sulfate hexahydrate 180 g / L Cobalt(II) sulfate heptahydrate 60 g / L Boric acid 30 g / L · pH: 3.0, · Temperature: 50 °C · Current density: 40 A / dm 2

[0081] (4) Ni-Mo alloy plating · Plating solution composition: Nickel sulfate hexahydrate 13 g / L Sodium molybdate dihydrate 19 g / L Citric acid 88 g / L · pH: 3.5 · Temperature: 25 °C · Current density: 10 A / dm 2

[0082] (5) Ni-W alloy plating · Plating solution composition: Nickel sulfate hexahydrate 13 g / L Sodium tungstate dihydrate 30 g / L Citric acid 88 g / L · pH: 3.5 · Temperature: 25°C · Current density: 10 A / dm 2 at

[0083] (6) Ni-Zn alloy plating · Plating solution composition: Nickel sulfate hexahydrate 240 g / L Zinc sulfate heptahydrate · pH: 2.0 · Temperature: 50°C · Current density: 40 A / dm 2 In the above Ni-Zn alloy plating, the concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer would be the values shown in Tables 1 and 2.

[0084] (7) Ni-Fe-Zn alloy plating A plating solution obtained by further adding zinc sulfate heptahydrate to the plating solution for the above (2) Ni-Fe alloy plating was used. The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer would be the values shown in Tables 1 and 2. Other conditions were the same as those for the above (2) Ni-Fe alloy plating.

[0085] (8) Ni-Co-Zn alloy plating A plating solution obtained by further adding zinc sulfate heptahydrate to the plating solution for the above (3) Ni-Co alloy plating was used. The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer would be the values shown in Tables 1 and 2. Other conditions were the same as those for the above (3) Ni-Co alloy plating.

[0086] (9) Ni-Mo-Zn alloy plating A plating solution obtained by further adding zinc sulfate heptahydrate to the plating solution for the above (4) Ni-Mo alloy plating was used. The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer would be the values shown in Tables 1 and 2. Other conditions were the same as those for the above (4) Ni-Mo alloy plating.

[0087] (10) Ni-W-Zn alloy plating A plating solution in which zinc sulfate heptahydrate was further added to the plating solution of the above (5) Ni-W alloy plating was used. The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer would be the values shown in Tables 1 and 2. Other conditions were the same as those of the above (5) Ni-W alloy plating.

[0088] (11) Zn plating · Plating solution composition: Zinc sulfate heptahydrate 240 g / L · pH: 2.0 · Temperature: 50 °C · Current density: 40 A / dm 2

[0089] (PVD) The formation of the coating layer by PVD was carried out by ion plating using a batch-type high-frequency (RF) excited ion plating apparatus manufactured by Showa Vacuum Co., Ltd. The temperature of the base steel plate was 400 °C, the pressure was 3 Pa, and the bias voltage was -20 V. The composition of the coating layer was controlled by adjusting the composition of the metal used as the evaporation source. Also, the thickness of the coating layer was controlled by adjusting the evaporation time.

[0090] (Clad rolling) On both sides of a steel slab with a thickness of 30 mm having the same composition as the above base steel plate, a Ni-16%Cr-8%Fe alloy (Alloy 600) with a thickness of 300 μm was laminated and rolled to produce a hot press steel plate of Invention Example No. 14.

[0091] Furthermore, a hot press steel plate of Invention Example No. 30 was produced in the same manner as Invention Example No. 14 except that a Ni-16%Cr-8%Fe-0.5%Zn alloy was used as the alloy to be laminated. Similarly, a hot press steel plate of Invention Example No. 46 was produced using a Ni-16%Cr-8%Fe-2%Zn alloy as the alloy to be laminated.

[0092] (Molten plating) The formation of the coating layer by the molten plating method was carried out by immersing the base steel plate in a molten plating bath for 1 second, and then N 2It was carried out by performing gas wiping. The composition of the coating layer was controlled by adjusting the composition of the molten plating bath used.

[0093] The component composition and thickness of the coating layer of the obtained hot press steel sheet were measured by the following methods respectively. The measurement results are shown in Tables 1 and 2.

[0094] (Component composition of the coating layer) The hot press steel sheet to be evaluated was sheared, and a sample of 10 mm × 15 mm was taken and embedded in a conductive resin to prepare a cross-sectional sample of the hot press steel sheet. By EPMA, the average composition of the coating layer from the outermost surface to the interface with the base material was measured. The component composition of the coating layer was obtained by averaging the measured values of any 3 samples.

[0095] (Thickness of the coating layer) The thickness of the coating layer of the hot press steel sheet was measured by SEM observation using the above cross-sectional sample. The thickness of the coating layer was measured at any 10 locations within a field of view with a width of 100 μm or more. The thickness of the coating layer was obtained by averaging all the measured values of any 3 samples.

[0096] ·Fabrication of hot press member Next, the above hot press steel sheet was subjected to hot pressing. Specifically, a test piece of 200 mm × 1000 mm was taken from the hot press steel sheet, and the test piece was directly heated by an electric resistance heating device. The heating was carried out under the conditions of heating temperature: 950 °C, heating rate: 20 seconds, holding time: 5 seconds.

[0097] Then, a hot press in the shape of a hat was carried out at 2 spm (Strokes Per Minute) by a press device installed adjacent to the heating furnace. The forming start temperature was 800 °C. The shape of the obtained hot press member was such that the width of the flat part on the upper surface was 70 mm, the length of the flat part on the side surface was 30 mm, and the length of the flat part on the lower surface was 25 mm. Also, the bending R of the mold was 7R at both shoulders on the upper surface and both shoulders on the lower surface.

[0098] (Component composition and thickness of the coating layer) The component composition and thickness of the coating layer of the obtained hot press member were measured in the same manner as the component composition and thickness of the coating layer in the above-described hot press steel sheet. The cross-sectional sample used for the measurement was prepared by the following procedure. First, the flat portion at the top of the hot press member was cut out and sheared to collect a 10 mm × 15 mm sample. Next, the sample was embedded in a conductive resin to obtain a cross-sectional sample. The measurement results are shown in Tables 3 and 4.

[0099] Furthermore, the composition and thickness of the oxide layer in the hot press member were measured by the following methods, respectively. The measurement results are shown in Tables 3 and 4.

[0100] (Composition of oxide layer) The component composition of the oxide layer in the hot press member was measured by EPMA. In the measurement, using the above cross-sectional sample, point analysis was performed at any 10 points within a field of view with a width of 100 μm or more. From the measurement results, the content (atomic %) of each metal element with respect to all metal elements contained in the oxide layer was determined.

[0101] (Thickness of oxide layer) The thickness of the oxide layer in the hot press member was measured by SEM observation using the above cross-sectional sample. The thickness of the coating layer was measured at any 10 locations within a field of view with a width of 100 μm or more, and the average of all the measured values was taken as the thickness of the oxide layer.

[0102] <Compatibility with high-speed heating> To evaluate the compatibility of the hot press steel sheet with high-speed heating, the variation in the thickness of the coating layer in the obtained hot press member was measured. Specifically, first, a cross-section in the width direction (short side direction) of the hat-shaped hot press member was cut out and embedded in resin. Next, the cross-section was observed by SEM, and the plating thickness t C at the top and the plating thicknesses t L and t R of the left and right flange portions were obtained. Observation was performed on 3 samples for each example, and the value of I defined by the following formula (1) was calculated for each sample. I = |t C -(tR +t L ) / 2| / t C …(1)

[0103] The maximum value of I among the 3 samples was taken as I max and used as an index for the variation in the thickness of the coating layer in the hot press member of that example. The obtained I max was used to make a determination according to the following criteria, and cases A and B were considered qualified. The evaluation results are shown in Tables 3 and 4. A: I max ≦0.2 B: 0.2 < I max ≦0.5 C: I max > 0.5

[0104] <LME cracking during forming> In order to evaluate liquid metal embrittlement cracking during hot press forming, cracks in the obtained hot press member were measured. Specifically, first, the shoulders on the upper surface of the hat-shaped hot press member were cut out, embedded in resin, and then etched with 3% nital. Next, the cross-section was observed, and the depth of the crack progressing from the surface of the shoulder into the plate thickness was measured. For each example, observations were made on 3 samples, and a determination was made according to the following criteria based on the length of the longest crack, and cases A, B, and C were considered qualified. The evaluation results are shown in Tables 3 and 4. A: Maximum crack length = 0 mm B: 0 mm < Maximum crack length ≦ 0.01 mm C: 0.01 mm < Maximum crack length ≦ 0.1 mm D: Maximum crack length > 0.1 mm

[0105] <Paint adhesion> The electrodeposition coating adhesion of the obtained hot press member was evaluated by the following procedure. First, a test piece cut out from the flat part of the upper surface of the hot press member was subjected to zinc phosphate chemical conversion treatment and electrodeposition coating to prepare a sample. Next, the electrodeposition coating adhesion was evaluated by performing a peel test of a 25-square 1-mm-wide checkerboard test according to the adhesion evaluation by the cross-cut method specified in JIS K 5600-5-6 (1999). The judgment was made based on the rating, and cases of A and B were regarded as passing. The evaluation results are shown in Tables 3 and 4. A: Rating 5 B: Rating 4 C: Rating 3 D: Rating 2 or less

[0106] As can be seen from the results shown in Tables 1 to 4, the hot press steel sheet satisfying the conditions of the present invention suppresses the variation in the thickness of the coating layer even when heated at high speed, and therefore has high compatibility with high-speed heating. Further, the hot press steel sheet satisfying the conditions of the present invention prevented liquid metal embrittlement cracking during hot press forming. Furthermore, the hot press member obtained by hot pressing the hot press steel sheet of the present invention was also excellent in coating adhesion.

[0107] [Table 1]

[0108] [Table 2]

[0109] [Table 3]

[0110] [Table 4]

Claims

1. A hot stamping steel sheet comprising a base steel sheet and a coating layer with a thickness of 0.5 to 6.0 μm provided on both sides of the base steel sheet, wherein the coating layer, by mass%, Zn: 0.5 to 5%, and at least one selected from the group consisting of Al, Ti, Cr, Mn, Fe, Co, Mo, and W: in total 0 to 50% is contained, the balance consists of a Ni-based alloy having a component composition consisting of Ni and inevitable impurities, and is directly provided on the surface of the base steel sheet.

2. A hot stamping member having a base steel sheet and a coating layer with a thickness of 0.5 to 6.0 μm provided on both sides of the base steel sheet, wherein the coating layer, by mass%, Zn: 0.5 to 5%, and at least one selected from the group consisting of Al, Ti, Cr, Mn, Fe, Co, Mo, and W: in total 0 to 50% is contained, the balance consists of a Ni-based alloy having a component composition consisting of Ni and inevitable impurities, and is directly provided on the surface of the base steel sheet.

3. Furthermore, on the coating layer, there is an oxide layer containing one or both of Mn and Fe and having a thickness of 0.1 to 5 μm, The total ratio of Mn and Fe to all metal elements contained in the oxide layer is 1 to 50 atomic%. The hot stamping member according to claim 2.

4. A method for manufacturing a hot stamping member, wherein the hot stamping steel sheet according to claim 1 is hot stamped to obtain a hot stamping member.

Images