Steel sheet for hot pressing, hot-pressed member, and method for producing hot-pressed member

Abstract

Provided is a steel sheet for hot pressing, which has excellent adaptability to high-speed heating, prevents embrittlement cracking of a liquid metal, and has excellent coating adhesion after hot pressing. A steel sheet for hot pressing, which comprises a base steel sheet and coating layers that are provided on both surfaces of the base steel sheet and have a thickness of 0.5-6.0 [mu] m, the coating layers comprising Ni or a Ni-based alloy, and the Zn content in the coating layers being 0-30 mass%.

Description

Technical Field

[0001] The present invention relates to a hot-pressing steel plate, a hot-pressing component and a method for manufacturing the hot-pressing component. Background Art

[0002] In recent years, in the automotive field, in order to meet the conflicting demands of improving vehicle body strength and reducing weight, high-strength steel sheets are used as material for parts, and the strength required for the high-strength steel sheets tends to become higher year by year.

[0003] However, generally speaking, if the strength of the steel sheet is increased, the press formability decreases, so it is difficult to obtain a complex component shape. As complex shaped components in automotive applications, for example, suspension components such as chassis, structural components for frames such as B-pillars, etc. can be cited.

[0004] Against this background, the application of hot pressing technology, which performs hot forming instead of cold forming, is increasing. Hot pressing refers to a forming method in which a steel plate is heated to the austenite temperature range, press-formed at high temperature, and rapidly cooled by contact with a die. In hot pressing, press-forming is performed when the strength of the blank steel plate is low, and high strength is achieved by subsequent rapid cooling, so that both high strength and press formability can be achieved.

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

[0006] For example, Patent Document 1 proposes a hot-pressing steel sheet in which an aluminum plating layer is provided on the surface of a steel sheet containing 0.15 to 0.5% of carbon to suppress oxidation of the steel sheet during heating.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2000-038640 Summary of the invention

[0010] However, the conventional hot-pressing steel sheet proposed in Patent Document 1 has the following problems.

[0011] One problem is the low adaptability to high-speed heating. In order to perform hot pressing, the hot pressing steel plate needs to be preheated. Methods for heating the hot pressing steel plate include, for example, atmosphere furnace heating, direct electric heating, induction heating, etc. Among them, direct electric heating and induction heating are excellent in energy efficiency compared with atmosphere furnace heating, so that carbon dioxide emissions can be reduced. In addition, according to direct electric heating and induction heating, the hot pressing steel plate can be heated at a high speed, so the productivity can be improved.

[0012] Therefore, during hot pressing, it is desirable to heat the hot pressing steel sheet at a high speed by direct electric heating or induction heating. However, if the conventional Al-plated steel sheet as proposed in Patent Document 1 is heated at a high speed by direct electric heating or induction heating, the plated metal flows due to the magnetic field generated by the current, and the film thickness of the coating in the hot pressed part finally obtained will deviate. Moreover, if the deviation of the film thickness of the coating is large, the appearance quality and corrosion resistance after coating will also be reduced. Therefore, when hot pressing the Al-plated steel sheet, an atmosphere furnace with a slow heating speed has to be used for heating. Due to such a situation, a hot pressing steel sheet with excellent adaptability to high-speed heating is required.

[0013] Another problem is liquid metal embrittlement cracking. If tensile stress is applied to the surface of liquid metal and solid metal while the liquid metal is in contact, the solid metal will become embrittled. This phenomenon is called liquid metal embrittlement (LME). In hot pressing, the metal contained in the coating is melted by heating. If press forming is performed in this state, liquid metal embrittlement cracking will occur in the bending part subjected to tensile stress. Therefore, it is also required that the steel sheet for hot pressing is not prone to liquid metal embrittlement cracking.

[0014] Furthermore, the steel sheet for hot pressing is generally used in a painted state after hot pressing. Therefore, the steel sheet for hot pressing is also required to have excellent paint adhesion of the hot pressed part finally obtained.

[0015] The present invention has been made in view of the above-mentioned actual situation, and an object of the present invention is to provide a hot-pressing steel sheet and a hot-pressed component that can satisfy the following requirements (1) to (3).

[0016] (1) Excellent adaptability to high-speed heating.

[0017] (2) Liquid metal embrittlement cracks are not easily generated during hot pressing.

[0018] (3) Excellent coating adhesion after hot pressing.

[0019] The present invention has been made in order to solve the above-mentioned problems, and the gist of the present invention is as follows.

[0020] 1. A hot-pressing steel sheet comprising a base steel sheet and a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel sheet,

[0021] The coating layer is made of Ni or a Ni-based alloy, and a Zn content in the coating layer is 0 to 30 mass %.

[0022] 2. The steel sheet for hot pressing according to item 1 above, wherein a Zn content in the coating layer is 0.5 to 30% by mass.

[0023] 3. The steel sheet for hot pressing according to 1 or 2 above, wherein the coating layer contains at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in an amount of 50 mass % or less in total.

[0024] 4. A hot-pressed component comprising a base steel plate and a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel plate,

[0025] The coating layer is made of Ni or a Ni-based alloy, and a Zn content in the coating layer is 0 to 30 mass %.

[0026] 5. The hot-pressed component according to 4 above, wherein a Zn content in the coating layer is 0.5 to 30% by mass.

[0027] 6. The hot-pressed component according to 4 or 5 above, wherein the coating layer contains at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in a total amount of 50 mass % or less.

[0028] 7. The hot-pressed component according to any one of 4 to 6 above, further comprising 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,

[0029] The total ratio of Mn and Fe to all metal elements contained in the oxide layer is 1 to 50 atomic %.

[0030] 8. A method for producing a hot pressed component, comprising hot pressing the steel sheet for hot pressing according to any one of 1 to 3 above to produce a hot pressed component.

[0031] According to the present invention, the above-mentioned problems can be solved. That is, the hot pressing steel sheet of the present invention has excellent adaptability to high-speed heating, and even when high-speed heating is performed by direct electric heating or induction heating, the thickness deviation of the coating layer is suppressed. In addition, the hot pressing steel sheet of the present invention suppresses liquid metal embrittlement cracks during hot pressing. Furthermore, the hot pressing parts obtained by hot pressing the hot pressing steel sheet of the present invention have excellent coating adhesion. DETAILED DESCRIPTION

[0032] Hereinafter, the mode for implementing the present invention will be described in detail. It should be noted that the present invention is not limited to this embodiment. In addition, unless otherwise specified, the unit "%" of the content in the component composition of the coating layer and the steel sheet represents "mass %".

[0033] [Steel sheets for hot pressing]

[0034] A hot-pressing steel sheet according to one embodiment of the present invention comprises a base steel sheet and a coating layer having 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 concentration in the coating layer is 0 to 30%.

[0035] [[Coating layer]]

[0036] The coating layer may be a coating layer composed of Ni (Ni coating layer), or a coating layer composed of a Ni-based alloy (Ni-based alloy coating layer). Here, "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.

[0037] In the heating process before hot pressing, the surface layer of the steel sheet is oxidized by oxygen and water vapor in the atmosphere. However, since the hot pressing steel sheet of the present invention has a Ni-based coating layer with a high melting point and oxidation resistance on the surface, the coating layer does not melt during heating, and a thick oxide layer can be prevented from forming on the surface. As a result, excellent coating adhesion can be obtained.

[0038] From the viewpoint of improving the above-mentioned effect, 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%.

[0039] Zn: 0~30%

[0040] If the above-mentioned coating layer contains a large amount of Zn, the oxidation resistance decreases. In addition, Zn melts due to heating, and liquid metal embrittlement cracks are generated during hot forming. Therefore, the Zn content in the above-mentioned coating layer is less than 30%, preferably less than 20%, more preferably less than 10%, and further preferably less than 5%. On the other hand, even if the above-mentioned coating layer does not contain Zn, the above-mentioned problem can be solved. Therefore, the lower limit of the Zn content is 0%. However, if the above-mentioned coating layer contains Zn, the chemical conversion treatment film formed in the chemical conversion treatment process as a pre-treatment process for coating becomes denser, and the coating adhesion is further improved. Therefore, the Zn content in the above-mentioned coating layer is preferably more than 0.5%, more preferably more than 1%, and further preferably more than 2%.

[0041] The above-mentioned coating layer may contain at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in a total of less than 50%. By adding at least one of Al, Ti, V, Cr, Mn, Co, Mo and W, better oxidation resistance can be obtained. Since these elements are elements that can be added arbitrarily, the lower limit of the total content can be 0%. However, in order to enhance the oxidation resistance of the coating layer, the coating layer preferably contains a total of more than 1% of these elements. In addition, when the above-mentioned coating layer is formed by electroplating, Fe dissolved from the parent steel plate into the plating bath sometimes enters the coating layer. However, if the content of these elements is excessive, the Ni content in the coating layer decreases relatively, thereby impairing the function of the coating layer. Therefore, the total content of the above-mentioned elements is less than 50%, preferably less than 40%, more preferably less than 30%, and further preferably less than 20%. It should be noted that the Fe content in the coating layer is preferably less than 20%, more preferably less than 5%, and further preferably less than 1%.

[0042] The coating layer in one embodiment of the present invention has the following composition:

[0043] Contains Zn by mass %: 0 to 30%,

[0044] At least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W: 0-50% in total,

[0045] The remainder consists of Ni and inevitable impurities.

[0046] In the above embodiment, the preferred ranges of the contents of the above components are also used. For example, from the viewpoint of further improving coating adhesion, the Zn content is preferably 0.5% or more, more preferably 1% or more, and even more preferably 2% or more.

[0047] Thickness: 0.5~6.0μm

[0048] It should be noted that if the above-mentioned hot pressing steel plate is heated, the Ni in the coating layer and the Fe in the base steel plate diffuse with each other, and a Ni-based alloy layer with an increased Fe concentration is generated on the surface of the hot pressing part. If the thickness of the coating layer of the hot pressing steel plate is less than 0.5 μm, when heated at a high temperature exceeding 1000°C, the Fe diffused to the surface is oxidized to form a thick and brittle Fe-containing oxide layer. If there is a thick and brittle Fe-containing oxide, the coating adhesion decreases. Therefore, the thickness of the coating layer is 0.5 μm or more. In addition, since the thicker the coating layer remains after heating, the better the corrosion resistance can be obtained, 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 larger, and the coating adhesion decreases instead. 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.

[0049] It should be noted that the hot-pressing steel sheet of the present invention has coating layers on both sides thereof, but 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.

[0050] [[Base Steel Plate]]

[0051] The base steel plate is not particularly limited, and any steel plate can be used. The base steel plate can be either a hot-rolled steel plate or a cold-rolled steel plate.

[0052] From the viewpoint of obtaining a hot-pressed component having a tensile strength exceeding 980 MPa after hot pressing, it is preferred to use a steel plate having the following composition as the above-mentioned base steel plate, wherein the above-mentioned composition contains, by mass%, C: 0.05-0.50%, Si: 0.1-1.0%, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.10% or less, and N: 0.01% or less, and the remainder is composed of Fe and unavoidable impurities. The reasons why the above-mentioned composition is preferred are described below.

[0053] C: 0.05~0.50%

[0054] By adding C, a hard structure such as martensite is formed, which can improve the strength of the steel plate. In order to obtain a high strength exceeding 980MPa, it is preferred that the C content in the base steel plate is 0.05% or more, more preferably 0.10% or more. On the other hand, if the C content exceeds 0.50%, the toughness of the spot weld decreases. Therefore, the C content is preferably 0.50% or less, more preferably 0.45% or less, further preferably 0.43% or less, and most preferably 0.40% or less.

[0055] Si: 0.1-1.0%

[0056] Si is an element effective in strengthening steel to obtain good material quality. In order to obtain the above-mentioned effect, it is preferred that the Si content is 0.1% or more, and more preferably 0.2% or more. On the other hand, if the Si content exceeds 1.0%, ferrite is stabilized, so hardenability decreases. Therefore, the Si content is preferably 1.0% or less, more preferably 0.4% or less, and further preferably 0.3% or less.

[0057] Mn: 0.5~3.0%

[0058] Mn is an element that helps to improve the strength of the steel sheet in a wide cooling rate range. In order to obtain the above effect, the Mn content is preferably 0.5% or more, more preferably 0.7% or more, and further preferably 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the effect of Mn is saturated. Therefore, the Mn content is preferably 3.0% or less, more preferably 2.5% or less, further preferably 2.0% or less, and most preferably 1.5% or less.

[0059] P: 0.1% or less

[0060] If the P content is higher than 0.1%, P segregates to the austenite grain boundaries during casting and causes grain boundary embrittlement. Moreover, 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, the lower limit of the P content is not particularly limited, but from the viewpoint of refining cost, the P content is preferably 0.01% or more.

[0061] S: 0.01% or less

[0062] S forms inclusions such as MnS, which deteriorates impact resistance and causes cracks due to metal flow along the weld. Therefore, it is desirable to reduce the S content as much as possible, preferably 0.01% or less. In addition, from the viewpoint of ensuring good flangeability, it is more preferably 0.005% or less, and further preferably 0.001% or less. On the other hand, the lower limit of the S content is not particularly limited, but from the viewpoint of refining cost, the S content is preferably 0.0002% or more.

[0063] Al: 0.10% or less

[0064] If the Al content is higher than 0.10%, the punching 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 further preferably 0.04% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of ensuring the effect as a deoxidizer, the Al content is preferably 0.01% or more.

[0065] N: 0.01% or less

[0066] If the N content is higher than 0.01%, AlN is formed during hot rolling and heating before hot pressing, and the punching workability and hardenability of the steel sheet are reduced. Therefore, the N content is preferably 0.01% or less. On the other hand, the lower limit of the N content is not particularly limited, but from the viewpoint of refining cost, it is preferred that the N content be 0.001% or more.

[0067] In order to further improve the characteristics, the component composition of the above-mentioned base steel plate may further contain at least one selected from Nb: 0.10% or less, Ti: 0.10% or less, B: 0.0002-0.010%, Cr: 0.1-1.0%, and Sb: 0.003-0.10%.

[0068] Nb: 0.10% or less

[0069] Nb is an element effective for strengthening steel, but if contained in excess, the rolling load increases. Therefore, when Nb is contained, the Nb content is set to 0.10% or less, preferably 0.06% or less, and 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.

[0070] Ti: 0.10% or less

[0071] Ti is also an element effective for strengthening steel like Nb. However, if Ti is contained in excess, shape fixity decreases. Therefore, when Ti is contained, the Ti content is set to 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.

[0072] B: 0.0002~0.010%

[0073] B is an element that has the effect of inhibiting the formation and growth of ferrite from austenite grain boundaries. When B is contained, in order to obtain the above-mentioned effect, the B content is preferably 0.0002% or more, and more preferably 0.0010% or more. On the other hand, excessive B will greatly impair formability. Therefore, when B is contained, the B amount is 0.010% or less, and preferably 0.005% or less.

[0074] Cr: 0.1~1.0%

[0075] Cr is an element that improves hardenability and helps to improve the strength of the steel sheet. When Cr is added, in order to obtain the above-mentioned effect, 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 lead to a significant increase in cost. Therefore, when Cr is contained, the Cr content is set to 1.0% or less, preferably 0.5% or less, and more preferably 0.3% or less.

[0076] Sb: 0.003~0.10%

[0077] Sb is an element that has the effect of suppressing decarburization of the surface layer during the annealing process when manufacturing the base steel plate. When Sb is contained, in order to obtain the above-mentioned effect, the Sb content is set to 0.003% or more, preferably 0.005% or more. On the other hand, if the Sb content is higher than 0.10%, the rolling load increases and the productivity decreases. Therefore, when Sb is contained, the Sb content is set to 0.10% or less, preferably 0.05% or less, and more preferably 0.03% or less.

[0078] [Method for producing hot-pressing steel sheet]

[0079] The hot pressing steel sheet of the present invention is not particularly limited and may be produced by any method. Preferred production conditions are described below.

[0080] First, a base steel plate is manufactured. Typically, the base steel plate can be manufactured by rolling a steel slab obtained by casting. As the steel slab, a steel slab having the above-mentioned component composition is preferably used.

[0081] In rolling, the hot slab obtained by casting can be directly (without reheating) supplied to hot rolling, and in addition, after casting, the temperature can be lowered to a cold slab and then reheated and supplied to hot rolling. There is almost no difference in the properties of the steel plate obtained when the hot slab is directly rolled and when the cold slab is rolled after reheating. When the cold slab is reheated before hot rolling, the reheating temperature is not particularly limited, but considering the productivity, it is preferably in the range of 1000°C to 1300°C.

[0082] The hot rolling may be a conventional hot rolling process or a continuous hot rolling process in which slabs are joined and rolled in finish rolling. The rolling end temperature in the hot rolling is not particularly limited, but is preferably above the Ar3 transformation point from the viewpoint of productivity and plate thickness accuracy.

[0083] Next, the hot rolled steel sheet obtained by the hot rolling is cooled according to a conventional method. From the viewpoint of productivity, the coiling temperature at this time is preferably 550°C or higher. In addition, when the coiling temperature is too high, the pickling property deteriorates, so the coiling temperature is preferably 750°C or lower. After the above cooling, it is preferably pickled according to a conventional method.

[0084] When a cold-rolled steel sheet is used as the base steel sheet, it may be further cold-rolled according to a conventional method after the pickling.

[0085] Next, a coating layer is formed on the surface of the obtained steel plate. The method for forming the coating layer is not particularly limited, and the coating layer can be formed by any method such as plating, PVD, clad rolling, etc. As the above-mentioned plating, for example, electroplating can be mentioned. As the above-mentioned PVD, for example, vacuum evaporation, sputtering, and ion plating can be mentioned. In addition, when clad rolling is used, layers having a desired composition can be stacked on both sides of the base steel plate and rolled.

[0086] 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, it is preferably formed by electroplating, but it can also be formed by other methods. When the coating layer is a composition that is difficult to electrodeposit from an aqueous solution, such as a Ni-Ti alloy, it is preferably formed by PVD.

[0087] It should be noted that when the coating layer is formed by any method, the conditions can be adjusted so that the coating layer on one side (surface) of the steel plate and the other side (back side) of the steel plate has a desired thickness. For example, in the case of electroplating, the thickness of the coating layer on each side can be adjusted by changing either or both of the current density and the power-on time on each side.

[0088] [Hot-pressed parts]

[0089] A hot-pressed component according to one embodiment of the present invention comprises a base steel plate and a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel plate. The coating layer is made of Ni or a Ni-based alloy, and the Zn content of the coating layer is 0 to 30%.

[0090] The above-mentioned base steel plate and coating layer are subject to the description of the base steel plate and coating layer in the above-mentioned hot pressing steel plate. That is, as the base steel plate of the hot pressing component, the same steel plate as the base steel plate of the above-mentioned hot pressing steel plate can be used. In addition, as the coating layer of the hot pressing component, the same coating layer as the coating layer of the above-mentioned hot pressing steel plate can be used. For example, the Zn content in the coating layer of the above-mentioned hot pressing component is also preferably 0.5% or more, more preferably 1% or more, and further preferably 2% or more. In addition, the above-mentioned coating layer may also contain at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in a total of 50 mass % or less.

[0091] [Oxide layer]

[0092] The hot pressed part 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 above-mentioned coating layer. That is, the hot pressed part of this embodiment has a base steel plate, a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the above-mentioned base steel plate, and an oxide layer having a thickness of 0.1 to 5 μm provided on the above-mentioned coating layer.

[0093] The above-mentioned oxide layer is formed by the reaction of the components contained in the coating layer or the base steel plate with oxygen or water vapor in the atmosphere during the hot pressing process. The composition and thickness of the above-mentioned oxide layer vary according to the heating conditions such as heating temperature, heating time, and atmosphere. If the thickness of the above-mentioned oxide layer exceeds 5μm, the adhesion of the coating film decreases, and as a result, sufficient corrosion resistance after painting cannot be obtained. Therefore, when an oxide layer is present, the thickness of the oxide layer is 5μm or less, preferably 3μm or less, and more preferably 1μm or less. On the other hand, from the viewpoint of the adhesion of the coating film, the thinner the thickness of the above-mentioned oxide layer, the better, but in order to make the thickness of the oxide layer less than 0.1μm, it is necessary to perform hot pressing in an atmosphere with an extremely reduced oxygen partial pressure, or to set a process for removing the oxide after hot pressing, which increases the manufacturing cost. Therefore, the thickness of the oxide layer is preferably 0.1μm or more.

[0094] The oxide layer contains one or both of Mn and Fe. Since the oxide layer contains these elements, these components are dissolved into the chemical conversion treatment solution in the chemical conversion treatment process. As a result, the formation of the chemical conversion treatment film is promoted, and better coating adhesion can be obtained. In order to obtain the above effect, the total ratio of Mn and Fe contained in the oxide layer to all metals is 1 to 50 atomic %.

[0095] The thickness of the oxide layer can be measured by observing the cross section of the hot-pressed component using a scanning electron microscope (SEM). More specifically, it can be measured by the method described in the examples.

[0096] The ratio of Mn and Fe contained in the oxide layer to the total metal can be measured by EPMA (electron probe microanalyzer) on the cross section of the hot-pressed part. More specifically, it can be measured by the method described in the examples.

[0097] [Method for manufacturing hot-pressed parts]

[0098] In one embodiment of the present invention, a hot-pressed steel sheet is hot-pressed to produce a hot-pressed component. The method for hot-pressing is not particularly limited and can be performed according to a conventional method. Typically, the hot-pressed steel sheet is heated to a predetermined heating temperature (heating process), and then the hot-pressed steel sheet heated by the heating process is hot-pressed (hot-pressing process). The preferred hot-pressing conditions are described below.

[0099] If the heating temperature in the above-mentioned heating process is lower than the Ac3 transformation point of the base steel plate, the strength of the final hot-pressed component will be reduced. Therefore, the above-mentioned heating temperature is preferably above the Ac3 transformation point of the base steel plate, and more preferably above 860°C. On the other hand, if the above-mentioned heating temperature exceeds 1000°C, the oxide layer produced by oxidation of the base material and the coating layer becomes too thick, and the paint adhesion of the hot-pressed component obtained thereby may deteriorate. Therefore, the above-mentioned heating temperature is preferably below 1000°C, more preferably below 960°C, and further preferably below 920°C. It should be noted that the Ac3 transformation point of the base steel plate varies depending on the steel composition and can be obtained by the Formastor test.

[0100] The temperature at which the heating is started is not particularly limited, but is usually room temperature.

[0101] The time (heating time) required for heating up to the above-mentioned heating temperature is not particularly limited and can be any time. However, if the above-mentioned heating time exceeds 300 seconds, the time of exposure to high temperature becomes long, so the oxide layer produced by oxidation of the parent material and the coating becomes too thick. Therefore, from the viewpoint of suppressing the decrease in the adhesion of the coating caused by the oxide, it is preferred that the above-mentioned heating time is less than 100 seconds, more preferably less than 80 seconds, and more preferably less than 60 seconds. On the other hand, if the above-mentioned heating time is less than 3 seconds, it is difficult to stably heat up. Therefore, the above-mentioned heating time is preferably more than 3 seconds, more preferably more than 4 seconds, and more preferably more than 5 seconds.

[0102] After reaching the above-mentioned heating temperature, it can be maintained at the heating temperature. In the case of the above-mentioned maintenance, the holding time is not particularly limited, and it can be maintained for any length. However, if the holding time exceeds 100 seconds, the oxide layer produced by oxidation of the base material and the coating layer becomes too thick, and the coating adhesion of the hot-pressed part obtained thereby may deteriorate. Therefore, the holding time is preferably less than 100 seconds, more preferably less than 60 seconds, and further preferably less than 20 seconds. On the other hand, the lower limit of the holding time is also not limited, but from the viewpoint of homogenizing the base material steel plate, it is preferably more than 1 second.

[0103] The atmosphere in the heating step is not particularly limited, and heating may be performed, for example, in an atmosphere of air or in an atmosphere into which air flows. From the viewpoint of reducing the amount of diffusible hydrogen remaining in the hot-pressed component, the dew point of the atmosphere is preferably set to 10° C. or less. The lower limit of the dew point is also not particularly limited, and may be, for example, -40° C. or more.

[0104] The method of heating the hot-pressing steel sheet is not particularly limited, and any method may be used. The above-mentioned heating may be performed, for example, by heating using furnace heating, electric heating, induction heating, high-frequency heating, flame heating, etc. Among them, it is preferred to use electric heating, induction heating or high-frequency heating that can heat up in a short time and has excellent energy efficiency. As the above-mentioned heating furnace, any heating furnace such as an electric furnace and a gas furnace may be used.

[0105] Next, the heated hot pressing steel plate is hot pressed to form a hot pressed component. In the above hot pressing process, the steel plate is cooled using a cooling medium such as a mold or water while or just after the process. In the present invention, the hot pressing conditions are not particularly limited. For example, pressing can be started at a general hot pressing temperature range of 600 to 800°C. In addition, since the hot pressing steel plate of the present invention does not have the risk of liquid metal brittleness, forming can also be carried out at a higher temperature than general hot pressing. Therefore, the hot pressing start temperature is preferably 600 to 1000°C.

[0106] Example

[0107] Hereinafter, the present invention will be specifically described based on examples.

[0108] Production of hot pressing steel plates

[0109] As the base steel plate, a cold-rolled steel plate with a thickness of 1.4 mm having the following composition was used, wherein the composition contained, 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 remainder was composed of Fe and inevitable impurities. The Ac3 transformation point of the base steel plate was 783°C, and the Ar3 transformation point was 706°C.

[0110] The coating layers were formed on both surfaces of the base steel plate by the methods shown in Tables 1 and 2. The methods used are described below. It should be noted that, for comparison, the coating layer was not formed in Comparative Example No. 1.

[0111] (plating)

[0112] The coating layer was formed by electroplating under the following conditions: In any case, electrolysis was performed with the base steel plate 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.

[0113] (1) Ni plating

[0114] Plating solution composition:

[0115] Nickel sulfate hexahydrate 240g / L

[0116] Boric acid 30g / L

[0117] pH: 3.0

[0118] Temperature: 50℃

[0119] Current density: 40A / dm 2

[0120] (2) Ni-Fe alloy plating

[0121] Plating solution composition:

[0122] Nickel sulfate hexahydrate 192g / L

[0123] Ferrous sulfate heptahydrate 48g / L,

[0124] Boric acid 30g / L

[0125] pH: 3.0

[0126] Temperature: 50℃

[0127] Current density: 40A / dm 2

[0128] (3) Ni-Co alloy plating

[0129] Plating solution composition:

[0130] Nickel sulfate hexahydrate 180g / L

[0131] Cobalt sulfate heptahydrate 60g / L

[0132] Boric acid 30g / L

[0133] pH: 3.0,

[0134] Temperature: 50℃

[0135] Current density: 40A / dm 2

[0136] (4) Ni-Mo alloy plating

[0137] Plating solution composition:

[0138] Nickel sulfate hexahydrate 13g / L

[0139] Sodium molybdate dihydrate 19g / L

[0140] Citric acid 88g / L

[0141] pH: 3.5

[0142] Temperature: 25℃

[0143] Current density: 10A / dm 2

[0144] (5) Ni-W alloy plating

[0145] Plating solution composition:

[0146] Nickel sulfate hexahydrate 13g / L

[0147] Sodium tungstate dihydrate 30g / L

[0148] Citric acid 88g / L

[0149] pH: 3.5

[0150] Temperature: 25℃

[0151] Current density: 10A / dm 2

[0152] (6) Ni-Zn alloy plating

[0153] Plating solution composition:

[0154] Nickel sulfate hexahydrate 240g / L

[0155] Zinc sulfate heptahydrate

[0156] pH: 2.0

[0157] Temperature: 50℃

[0158] Current density: 40A / dm 2

[0159] In the above-mentioned 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.

[0160] (7) Ni-Fe-Zn alloy plating

[0161] A plating solution was used in which zinc sulfate heptahydrate was further added to the plating solution for Ni—Fe alloy plating (2). The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer was the value shown in Tables 1 and 2. Other conditions were the same as those for Ni—Fe alloy plating (2).

[0162] (8) Ni-Co-Zn alloy plating

[0163] A plating solution was used in which zinc sulfate heptahydrate was further added to the above-mentioned Ni—Co alloy plating solution (3). The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer was the value shown in Tables 1 and 2. Other conditions were the same as those of the above-mentioned Ni—Co alloy plating solution (3).

[0164] (9) Ni-Mo-Zn alloy plating

[0165] A plating solution was used in which zinc sulfate heptahydrate was further added to the plating solution for Ni-Mo alloy plating (4). The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer was the value shown in Tables 1 and 2. Other conditions were the same as those for Ni-Mo alloy plating (4).

[0166] (10) Ni-W-Zn alloy plating

[0167] A plating solution was used in which zinc sulfate heptahydrate was further added to the plating solution for Ni—W alloy plating (5). The concentration of zinc sulfate heptahydrate in the plating solution was adjusted so that the Zn content of the coating layer was the value shown in Tables 1 and 2. Other conditions were the same as those for Ni—W alloy plating (5).

[0168] (11) Zn plating

[0169] Plating solution composition:

[0170] Zinc sulfate heptahydrate 240g / L

[0171] pH: 2.0

[0172] Temperature: 50℃

[0173] Current density: 40A / dm 2

[0174] (PVD)

[0175] The coating layer formed by PVD is implemented by ion plating using an intermittent high frequency (RF) excitation ion plating device manufactured by Showa Vacuum Co., Ltd. The temperature of the base steel plate is 400°C, the pressure is 3Pa, and the bias voltage is -20V. The composition of the coating layer is controlled by adjusting the composition of the metal used as the evaporation source. In addition, the thickness of the coating layer is controlled by adjusting the evaporation time.

[0176] (Cladding rolling)

[0177] A 300 μm thick Ni-16% Cr-8% Fe alloy (Alloy 600) was laminated on both surfaces of a 30 mm thick steel billet having the same composition as the base steel plate, and rolled to produce a hot pressing steel plate of Invention Example No. 14.

[0178] Furthermore, a hot press steel sheet of Inventive Example No. 30 was produced by the same method as Inventive Example No. 14 except that Ni-16%Cr-8%Fe-0.5%Zn alloy was used as the laminated alloy. Similarly, a hot press steel sheet of Inventive Example No. 46 was produced using Ni-16%Cr-8%Fe-2%Zn alloy as the laminated alloy.

[0179] (Hot dip galvanizing)

[0180] The coating layer is formed by hot-dip coating by immersing the base steel sheet in a hot-dip coating bath for 1 second and then purging with N2 gas. The composition of the coating layer is controlled by adjusting the composition of the hot-dip coating bath used.

[0181] The component composition and thickness of the coating layer of the obtained hot-pressing steel sheet were measured by the following methods. The measurement results are shown in Tables 1 and 2.

[0182] (Composition of coating layer)

[0183] The hot-pressed steel sheet to be evaluated was sheared, and a 10 mm × 15 mm sample was taken and embedded with a conductive resin to prepare a cross-sectional sample of the hot-pressed steel sheet. The average composition of the coating layer from the outermost layer to the interface with the base material was measured using EPMA. The measured values ​​of any three samples were averaged to obtain the component composition of the coating layer.

[0184] (Thickness of coating layer)

[0185] The thickness of the coating layer of the hot-pressed steel sheet was measured by SEM observation using the cross-section 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 three samples.

[0186] Production of hot pressed parts

[0187] Next, the hot press steel plate was subjected to hot pressing. Specifically, a test piece of 200 mm×1000 mm was taken from the hot press steel plate and heated by a direct current resistance heating device. The heating was performed under the conditions of heating temperature: 950°C, heating time: 20 seconds, and holding time: 5 seconds.

[0188] Next, a hat-shaped hot press was performed at 2 spm (Strokes Per Minute) using a pressing device installed adjacent to the above-mentioned heating furnace. The molding start temperature was 800°C. It should be noted that the shape of the obtained hot-pressed part was 70 mm wide on the flat part of the upper surface, 30 mm long on the flat part of the side, and 25 mm long on the flat part of the lower surface. In addition, the curvature R of the mold was 7R on both shoulders of the upper surface and both shoulders of the lower surface.

[0189] (Composition and thickness of coating layer)

[0190] The composition and thickness of the coating layer of the obtained hot-pressed part are measured by the same method as the composition and thickness of the coating layer in the above-mentioned hot-pressed steel plate. The cross-sectional sample used in the above-mentioned measurement is made according to the following steps. First, the flat part of the top of the head of the above-mentioned hot-pressed part is cut off and sheared to take a 10mm×15mm sample. Then, the cross-sectional sample is made by embedding the above-mentioned sample in a conductive resin. The measurement results are shown in Tables 3 and 4.

[0191] Furthermore, the composition and thickness of the oxide layer in the hot-pressed component were measured by the following methods. The measurement results are shown in Tables 3 and 4.

[0192] (Composition of oxide layer)

[0193] The composition of the oxide layer in the hot pressed part was measured by EPMA. In the above measurement, the cross-section sample was used to perform point analysis at any 10 points in a field of view with a width of 100 μm or more. The content (atomic %) of each metal element relative to all metal elements contained in the oxide layer was calculated from the above measurement results.

[0194] (Thickness of oxide layer)

[0195] The thickness of the oxide layer in the hot pressed part was measured by SEM observation using the cross-section 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 all the measured values ​​were averaged to obtain the thickness of the oxide layer.

[0196] <Adaptability to high-speed heating>

[0197] In order to evaluate the adaptability of the hot-pressed steel sheet to high-speed heating, the thickness deviation of the coating layer in the obtained hot-pressed component was measured. Specifically, first, a cross section in the width direction (short side direction) of the hat-shaped hot-pressed component was cut and embedded with resin. Then, the cross section was observed using SEM to obtain the coating thickness t at the top of the head. C and the plating thickness t of the left and right flanges L ,t R For each example, three samples were observed, and the I value defined by the following formula (1) was calculated for each sample.

[0198] I=|t C -(t R +t L ) / 2| / t C …(1)

[0199] Set the maximum I value among the three samples as I max , as an indicator of the deviation in thickness of the coating layer in the hot-pressed component of this embodiment. max The evaluation was carried out according to the following criteria, and the cases of A and B were rated as acceptable. The evaluation results are shown in Tables 3 and 4.

[0200] A: I max ≤0.2

[0201] B: 0.2<I max ≤0.5

[0202] C:I max >0.5

[0203] <LME cracks during molding>

[0204] In order to evaluate the liquid metal embrittlement cracking during hot press forming, the cracks in the obtained hot pressed parts were measured. Specifically, first, the shoulder of the upper surface of the hat-shaped hot pressed part was cut off, and after embedding the resin, it was etched with 3% nitric acid. Next, the cross section was observed to measure the depth of the crack extending from the surface of the shoulder to the inside of the plate thickness. Three samples were observed for each embodiment, and the length of the longest crack was judged according to the following criteria, and the cases of A, B and C were rated as qualified. The evaluation results are shown in Tables 3 and 4.

[0205] A: Maximum crack length = 0 mm

[0206] B: 0mm<maximum crack length≤0.01mm

[0207] C: 0.01mm<maximum crack length≤0.1mm

[0208] D: Maximum crack length>0.1mm

[0209] <Painting Adhesion>

[0210] The electrodeposition coating adhesion of the obtained hot-pressed parts was evaluated according to the following steps. First, a test piece cut from the flat portion of the upper surface of the hot-pressed part was subjected to a zinc phosphate chemical conversion treatment and an electrodeposition coating to prepare a sample. Next, a peeling test of 25 squares of a 1 mm wide checkerboard test was performed using the adhesion evaluation based on the cross-hatch method specified in JIS K 5600-5-6 (1999) to evaluate the electrodeposition coating adhesion. The evaluation was made by rating, and the cases of A and B were rated as qualified. The evaluation results are shown in Tables 3 and 4.

[0211] A: Rating 5

[0212] B: Rating 4

[0213] C: Rating 3

[0214] D: Rating 2 or below

[0215] As shown in Tables 1 to 4, the hot pressing steel sheet satisfying the conditions of the present invention suppresses the variation in the thickness of the coating layer even in the case of high-speed heating, and thus has high adaptability to high-speed heating. In addition, the hot pressing steel sheet satisfying the conditions of the present invention prevents liquid metal embrittlement cracking during hot press forming. Furthermore, the hot-pressed parts obtained by hot-pressing the hot-pressing steel sheet of the present invention also have excellent coating adhesion.

[0216]

[0217]

[0218]

[0219]

Claims

1. A hot-pressing steel sheet comprising a base steel sheet and a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel sheet, The coating layer is composed of Ni or a Ni-based alloy, and the Zn content in the coating layer is 0 to 30 mass %.

2. The hot pressing steel sheet according to claim 1, wherein: The Zn content in the coating layer is 0.5 to 30 mass %.

3. The hot pressing steel sheet according to claim 1 or 2, wherein: The coating layer contains at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in an amount of 50 mass % or less in total.

4. A hot-pressed component comprising a base steel plate and a coating layer having a thickness of 0.5 to 6.0 μm provided on both surfaces of the base steel plate, The coating layer is composed of Ni or a Ni-based alloy, and the Zn content in the coating layer is 0 to 30 mass %.

5. The hot pressing component according to claim 4, wherein: The Zn content in the coating layer is 0.5 to 30 mass %.

6. The hot pressing component according to claim 4 or 5, wherein: The coating layer contains at least one selected from Al, Ti, V, Cr, Mn, Fe, Co, Mo and W in an amount of 50 mass % or less in total.

7. The hot press component according to any one of claims 4 to 6, wherein: The coating layer further comprises 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 %.

8. A method for producing a hot pressed part, comprising hot pressing the steel sheet for hot pressing according to any one of claims 1 to 3 to produce a hot pressed part.

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