Hot-pressed member and steel sheet for hot pressing

Abstract

Provided is a hot-pressed member provided with an Al-Zn-based coating layer, the hot-pressed member having both cut part corrosion resistance and spot weldability at a high level without the problem of chemical conversion treatability, which is an Al-plated steel sheet, and the problem of LME cracks, which is a Zn-based plated steel sheet. This hot-pressed member is provided with a steel sheet, a coating layer disposed on at least one surface of the steel sheet, and an oxide layer disposed on the coating layer, and is characterized in that the coating layer has a component composition containing, in mass%, 25.0-55.0% of Zn, 1.1-8.0% of Si, 0.01-5.0% of Sr + Ca, and 55.0% or less of Fe, the remainder being Al and unavoidable impurities, and the coating layer contains a metal Zn phase.

Description

Technical Field

[0001] The present invention relates to a hot-pressed component, and particularly to a hot-pressed component having excellent corrosion resistance and spot weldability in a cut portion after painting. In addition, the present invention relates to a steel sheet for hot pressing. Background Art

[0002] For the purpose of improving the weight reduction and collision safety of automobiles, the high-strength of steel sheets for automobiles is being developed. In recent years, cold-rolled steel sheets with a tensile strength of 1500 MPa class have been developed, and their applications are being studied. However, with the high-strength of the steel sheet, poor formability, springback, etc. during pressing have become problems of dimensional accuracy.

[0003] Therefore, the application of hot pressing technology, which is performed under hot conditions rather than cold conditions, is gradually increasing. Hot pressing refers to a forming method in which a steel sheet is heated to the austenite temperature range and then hot-formed under high temperature conditions, while being rapidly cooled by contact with a metal mold. In hot pressing, hot forming is performed in a state where formability is improved by heating, and high strength is achieved by subsequent rapid cooling. Therefore, hot-pressed components with excellent strength can be manufactured with high dimensional accuracy.

[0004] Therefore, as a steel sheet for hot pressing suitable for manufacturing hot-pressed components, steel sheets having films such as an Al-based plating layer, a Zn-based plating layer, and an Al-Zn-based plating layer on the surface have been proposed (Patent Documents 1 to 5).

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-049256

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-073774

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 2005-113233

[0010] Patent Document 4: International Publication No. 2017 / 195269

[0011] Patent Document 5: International Publication No. 2019 / 180853 Summary of the Invention

[0012] The hot-pressed components obtained by hot-pressing the above-mentioned steel sheet for hot pressing are used mainly for automobile components, particularly for structural components (inner panel skeletons) that require strength. However, in recent years, they have also been used as so-called quasi-exterior panel components such as components around pillars that are visible when opening the door. Therefore, for hot-pressed components, suitability for painting and excellent corrosion resistance in the cut portion after painting are required.

[0013] In addition, since hot stamping parts are usually used with spot welding, excellent spot weldability is also required.

[0014] However, in the conventional technologies proposed in Patent Documents 1 to 5, as described below, all of the above requirements cannot be satisfied.

[0015] For example, in the technology proposed in Patent Document 1, in order to prevent oxidation scale and improve corrosion resistance, a molten Al-coated steel sheet is used as the hot stamping steel sheet. However, if the Al-coated steel sheet is hot stamped, Fe diffuses from the base steel sheet to the surface layer part of the Al coating layer to form a FeAl-based alloy layer. Generally, in hot stamping parts, before electrodeposition coating is performed, a chemical conversion treatment film is formed to ensure paintability. However, the FeAl-based alloy layer does not react with the chemical conversion treatment liquid, so a chemical conversion treatment film cannot be formed. In addition, the FeAl-based alloy layer does not have sacrificial corrosion protection, so the corrosion resistance such as the corrosion resistance of the cut part is insufficient.

[0016] On the other hand, in the technology proposed in Patent Document 2, in order to ensure film adhesion and corrosion resistance, a Zn-based coated steel sheet is used as the hot stamping steel sheet. However, if the Zn-based coated steel sheet is hot stamped, a thick oxide layer is formed on the surface of the coating layer, so the spot weldability is insufficient. In addition, since the melting point of Zn is low, if a steel sheet having a Zn-based coating layer as the main body is hot stamped, liquid metal embrittlement (LME) cracks are generated and sufficient fatigue resistance cannot be obtained.

[0017] In Patent Document 3, it is proposed to improve the spot weldability by forming an oxide layer containing Mn on the surface of the Zn-based coated steel sheet. However, in Patent Document 3, since a Zn-based coating layer is used as the main body, the fatigue resistance is insufficient due to LME cracks.

[0018] Therefore, it is proposed to use an Al-Zn-based coated steel sheet instead of the Al-coated steel sheet having a chemical conversion treatment problem and the Zn-based coated steel sheet having an LME crack problem.

[0019] For example, in Patent Document 4, it is proposed to form an interface layer with a specific composition at the interface between the base steel sheet and the coating layer by using an Al-Zn-based coated steel sheet to manufacture a hot stamping part. According to Patent Document 4, by providing the above interface layer on the hot stamping part, LME cracks can be prevented and the fatigue resistance can be improved. However, the spot weldability and the corrosion resistance of the cut part of this hot stamping part are still insufficient.

[0020] In Patent Document 5, by incorporating Mg into an Al-Zn-based plated steel sheet, an Mg oxide layer is formed during hot pressing, thereby obtaining a hot-pressed component in which Zn is not oxidized and a metallic Zn phase remains in the coating layer, resulting in high corrosion resistance. However, in this hot-pressed component, since the Mg oxide layer becomes an insulating film, sufficient spot weldability cannot be obtained.

[0021] Thus, in the technology using an Al-Zn-based plated steel sheet, in reality, it is still impossible to achieve a hot-pressed component that combines spot weldability and corrosion resistance at the cutting part at a high level.

[0022] The present invention has been completed in view of the above actual situation, and its object is to achieve high-level corrosion resistance at the cutting part and spot weldability in a hot-pressed component having an Al-Zn-based coating layer without the problems of chemical conversion treatment of Al-plated steel sheets and LME cracks of Zn-based plated steel sheets.

[0023] The present invention has conducted research to solve the above problems and as a result, obtained the following insights.

[0024] (1) By adding a specific amount of Si to the Al-Zn-based plating layer, alloying during heating is suppressed. As a result, the spot weldability and corrosion resistance at the cutting part of the hot-pressed component can be improved.

[0025] (2) When a trace amount of Sr or Ca is contained in the Al-Zn-based plating layer, Sr and Ca are preferentially oxidized to form a surface barrier. As a result, the formation of the oxide layer is suppressed, and the spot weldability of the hot-pressed component is improved. In addition, the oxidation of Zn is suppressed, and it remains as a metallic Zn phase in the coating layer, thereby improving the corrosion resistance.

[0026] (3) Therefore, a hot-pressed component obtained using an Al-Zn-based plated steel sheet containing Si and at least one of Sr and Ca in a specific amount has excellent spot weldability and corrosion resistance at the cutting part.

[0027] The present invention has been completed based on the above insights, and its gist is as follows.

[0028] 1. A hot-pressed component, comprising: a steel sheet, a coating layer disposed on at least one side surface of the steel sheet, and an oxide layer disposed on the coating layer, wherein the coating layer has the following composition: containing Zn: 25.0 to 55.0% by mass, Si: 1.1 to 8.0%, Sr + Ca: 0.01 to 5.0%, and Fe: 55.0% or less, the balance being composed of Al and unavoidable impurities, and the coating layer contains a metallic Zn phase.

[0029] 2. The hot-pressed component according to the above 1, wherein the thickness of the oxide layer is 0.3 μm or less.

[0030] 3. A steel sheet for hot pressing, comprising: a steel sheet and a plating layer disposed on at least one surface of the steel sheet, wherein the plating layer has the following composition: containing Zn: 30.0 to 70.0% by mass, Si: 1.1 to 8.0% by mass, Sr + Ca: 0.01 to 5.0% by mass, and the balance is composed of Al and inevitable impurities.

[0031] According to the present invention, it is possible to provide a hot-pressed component that combines high-level corrosion resistance and spot weldability of the cutting part. Detailed Embodiments

[0032] Hereinafter, embodiments of the present invention will be described. It should be noted that the following description shows preferred embodiments of the present invention and is not limited by any of the following descriptions. In addition, unless otherwise specified, the unit of content, i.e., "%" means "% by mass".

[0033] (1) Hot-pressed Component

[0034] The hot-pressed component in one embodiment of the present invention includes a steel sheet, a coating layer disposed on at least one surface of the steel sheet, and an oxide layer disposed on the coating layer. Hereinafter, each part will be described.

[0035] [Steel Sheet]

[0036] In the present invention, based on controlling the composition of the coating layer as described later, a metal Zn phase is contained in the coating layer to solve the above problems. Therefore, as the steel sheet, there is no particular limitation, and any steel sheet can be used.

[0037] It should be noted that the hot-pressed component of the present invention is manufactured by hot pressing a steel sheet for hot pressing as described later. Therefore, the above steel sheet can also be referred to as a steel sheet formed by hot pressing. The steel sheet can be either a cold-rolled steel sheet or a hot-rolled steel sheet.

[0038] From the viewpoint of use as an automotive component or the like, it is preferable that the hot-pressed component has high strength. In particular, in order to obtain a hot-pressed component with a strength exceeding 1470 MPa grade, it is preferable to use a steel material having the following composition.

[0039] The composition contains C: 0.20 to 0.35% by mass, Si: 0.1 to 0.5% by mass, Mn: 1.0 to 3.0% by mass, P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01% or less, and the balance is composed of Fe and inevitable impurities.

[0040] Hereinafter, the effects and appropriate contents of each element in the above preferred composition will be described.

[0041] C: 0.20 - 0.35%

[0042] C is an element that has the effect of increasing strength by forming structures such as martensite. From the perspective of obtaining a strength exceeding 1470 MPa grade, it is preferable to set the C content to 0.20% or more. On the other hand, if the C content exceeds 0.35%, the toughness of the spot welding part deteriorates. Therefore, the C content is preferably 0.35% or less.

[0043] Si: 0.1 - 0.5%

[0044] Si is an element effective for strengthening steel to obtain good material properties. To achieve the above effects, it is preferable to set the Si content to 0.1% or more. On the other hand, if the Si content exceeds 0.5%, ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 0.5% or less.

[0045] Mn: 1.0 - 3.0%

[0046] Mn is an element effective for high-strengthening of steel. From the perspective of ensuring excellent mechanical properties and strength, it is preferable to set the Mn content to 1.0% or more. On the other hand, if the Mn content exceeds 3.0%, the amount of Mn enriched on the steel plate surface during annealing increases, and as a result, the plating adhesion decreases. Therefore, the Mn content is preferably 3.0% or less.

[0047] P: 0.02% or less

[0048] If the P content is higher than 0.02%, due to grain boundary embrittlement accompanied by the segregation of P to the austenite grain boundaries during casting, the local ductility deteriorates. And as a result, the balance between the strength and ductility of the steel plate decreases. Therefore, from the perspective of improving the balance between the strength and ductility of the steel plate, it is preferable to set the P content to 0.02% or less. On the other hand, from the above perspective, the lower the P content, the better, so the lower limit of the P content is not particularly limited and can be 0%. From the perspective of refining cost, the P content is preferably 0.0005% or more.

[0049] S: 0.01% or less

[0050] S forms inclusions such as MnS, resulting in deterioration of impact resistance and cracks along the metal flow of the welded part. Therefore, it is preferable to minimize the S content, specifically, preferably 0.01% or less. Additionally, from the perspective of ensuring good flange stretching property, it is more preferably 0.005% or less. On the other hand, from the above perspective, the lower the S content, the better, so the lower limit of the S content is not particularly limited and can be 0%. However, from the perspective of refining cost, the S content is preferably 0.0002% or more.

[0051] Al: 0.1% or less

[0052] Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.1%, the hardenability decreases. Therefore, the Al content is preferably 0.1% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of improving the effect as a deoxidizer, the Al content is preferably 0.01% or more.

[0053] N: 0.01% or less

[0054] If the N content exceeds 0.01%, AlN is generated during heating before hot pressing, and the hardenability decreases. 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, and from the viewpoint of refining cost, the N content is preferably 0.001% or more.

[0055] In addition, the above composition may further optionally contain at least one selected from Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 - 0.0050%, Cr: 0.1 - 0.3%, and Sb: 0.003 - 0.03%.

[0056] Nb: 0.05% or less

[0057] Nb is a component effective for strengthening steel, but if it is contained excessively, the shape freezing property decreases. Therefore, when Nb is contained, the Nb content is preferably 0.05% or less. On the other hand, the lower limit of the Nb content is not particularly limited and can be 0%. From the viewpoint of the strength improvement effect, the Nb content is preferably 0.005% or more.

[0058] Ti: 0.05% or less

[0059] Ti, like Nb, is a component effective for strengthening steel. If it is contained excessively, the shape freezing property decreases. Therefore, when Ti is added, the Ti content is preferably 0.05% or less. On the other hand, the lower limit of the Ti content is not particularly limited and can be 0%. However, from the viewpoint of the strength improvement effect, the Ti content is preferably 0.005% or more.

[0060] B: 0.0002 - 0.0050%

[0061] B has the effect of suppressing the formation and growth of ferrite from the austenite grain boundary. When B is added, in order to obtain the above effect, the B content is preferably 0.0002% or more. On the other hand, excessive addition of B reduces the formability. Therefore, when B is added, the B content is preferably 0.0050% or less.

[0062] Cr: 0.1 - 0.3%

[0063] Cr is an element useful for strengthening steel and improving hardenability. When adding Cr, in order to obtain the above effects, the Cr content is preferably 0.1% or more. On the other hand, since Cr is a high-valence element, adding too much Cr will cause a significant increase in cost. Therefore, when adding Cr, the Cr content is more preferably 0.3% or less.

[0064] Sb: 0.003 - 0.03%

[0065] Sb is an element having the effect of suppressing decarburization of the steel plate surface during hot pressing. When adding Sb, in order to obtain the above effects, the Sb content is preferably 0.003% or more. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, and thus the productivity decreases. Therefore, when adding Sb, the Sb content is preferably 0.03% or less.

[0066] [Coating layer]

[0067] The hot-pressed component of the present invention has a coating layer on at least one side surface of the above steel plate. The above coating layer may be provided only on one side surface of the steel plate, but is preferably provided on both sides.

[0068] In the hot-pressed component of the present invention, a metallic Zn phase is included in the above coating layer. In a corrosive environment, this metallic Zn phase plays a sacrificial anti-corrosion role, and thus high corrosion resistance can be obtained. When Zn exists only in a solid-solution state in the above coating layer, the desired corrosion resistance cannot be obtained. Therefore, it is necessary to exist as a metallic Zn phase in the coating layer. The presence or absence of the metallic Zn phase in the coating layer can be evaluated by X-ray diffraction.

[0069] The above coating layer has a composition containing at least one of Zn, Si, Sr, and Ca in a specific amount, and Fe, and the remaining part is composed of Al and inevitable impurities. Hereinafter, each component will be described.

[0070] Zn: 25.0 - 55.0%

[0071] If the Zn content of the coating layer is less than 25.0%, it is impossible to obtain the desired corrosion resistance of the cutting part without a metallic Zn phase in the coating layer. Therefore, the Zn content is set to 25.0% or more. When the Zn content is 30.0% or more, more metallic Zn phases exist in the coating layer, and thus the corrosion resistance of the cutting part is further improved. Therefore, the Zn content is preferably 30.0% or more. On the other hand, if the Zn content exceeds 55.0%, the amount of Zn oxide generated increases, and as a result, the desired spot weldability cannot be obtained. Therefore, the Zn content is 55.0% or less, preferably 50.0% or less.

[0072] Si: 1.1 - 8.0%

[0073] Si is an element that inhibits the alloying of the plating layer during the plating process and the heat treatment process before hot pressing. It can be seen that during the heat treatment process, as Fe diffuses into the plating layer, Zn in the plating layer is pushed to the surface layer. That is, if the alloying of the plating layer proceeds excessively, Sr and Ca are oxidized, and an oxide layer is formed by Zn before the surface barrier is formed. As a result, the desired weldability cannot be obtained, and the Zn in the coating layer decreases, resulting in a reduction in corrosion resistance. Therefore, the Si content in the finally obtained coating layer is 1.1% or more, preferably 1.3% or more. On the other hand, if the Si content is too high, the amount of Si-based oxides generated increases, resulting in impaired chemical conversion treatment properties and inferior corrosion resistance. Therefore, the Si content is 8.0% or less, preferably 4.0% or less.

[0074] Sr + Ca: 0.01 - 5.0%

[0075] Sr and Ca are elements that have the effect of improving weldability by inhibiting the formation of an oxide layer. That is, since Sr and Ca are preferentially oxidized to form a surface barrier, the formation of the oxide layer is inhibited as a result, and the weldability is improved. Furthermore, since the formation of the oxide layer is inhibited, Zn contained in the coating layer is not oxidized and remains in the coating layer, resulting in improved corrosion resistance. Therefore, in the present invention, it is important that the coating layer contains one or both of Sr and Ca. However, if the total content of Sr and Ca is less than 0.01%, the desired effect cannot be obtained. Therefore, the total content of Sr and Ca is 0.01% or more, preferably 0.1% or more. On the other hand, if the content of Sr and Ca is too high, the oxides of Sr and Ca themselves are excessively generated, and as a result, the spot weldability deteriorates. Therefore, the total content of Sr and Ca is 5.0% or less, preferably 1.0% or less. It should be noted that in the present invention, the total content of Sr and Ca is expressed as "Sr + Ca".

[0076] Fe: 55.0% or less

[0077] Due to the heating before hot pressing, Fe diffuses from the steel plate into the plating layer. Therefore, Fe is inevitably contained in the coating layer of the hot-pressed component. However, if the Fe content increases, the solid solubility limit of Zn in the coating layer decreases. As a result, Zn is enriched on the surface layer of the coating layer. And, as a result, the formation of Zn oxide becomes excessive, and the weldability deteriorates. Therefore, the Fe content is 55.0% or less, preferably 50.0% or less. On the other hand, the lower limit of the Fe content is not particularly limited, and in the case of manufacturing under general conditions, the Fe content can be 20.0% or more.

[0078] The composition of the coating layer can be measured by SEM (scanning electron microscope) - EDX (energy dispersive X-ray analysis).

[0079] The coating amount of the above coating layer is not particularly limited. From the viewpoint of corrosion resistance, the coating amount is preferably 60 g / m on one side of the steel sheet. 2 On the other hand, from the viewpoint of manufacturing cost, the coating amount is preferably 400 g / m or less on one side of the steel sheet. 2 The coating amount of the coating layer can be obtained by dissolving and removing the coating layer from the surface of the hot-pressed part using an acid solution, and subtracting the weight of the hot-pressed part after removal from the weight before removal. An inhibitor for suppressing the dissolution of the base steel sheet is added to the above acid solution.

[0080] [Oxide layer]

[0081] When hot pressing the hot-pressing steel sheet, if Fe in the base iron diffuses into the plating layer to form the above coating layer, at the same time, the components in the plating layer combine with oxygen present in the heating atmosphere gas to form an oxide layer on the surface of the coating layer.

[0082] The thickness of the above oxide layer is not particularly limited. However, since the oxide layer is insulating, if the oxide layer is too thick, the spot weldability may sometimes decrease. Therefore, from the viewpoint of further improving the spot weldability, the thickness of the oxide layer is preferably 0.6 μm or less. In addition, when the thickness of the oxide layer is sufficiently thin, the oxide layer can be broken by the pressing force of the electrode during spot welding, thereby further improving the spot weldability. From the above viewpoints, the thickness of the oxide layer is more preferably 0.3 μm or less. On the other hand, from the viewpoint of spot weldability, the thinner the thickness of the oxide layer, the better. Therefore, the lower limit of the thickness of the oxide layer is not particularly limited and can be 0 μm.

[0083] The thickness of the above oxide layer can be measured by observing the cross section of the hot-pressed part with SEM.

[0084] (2) Hot-pressing steel sheet

[0085] The hot-pressing steel sheet in one embodiment of the present invention includes a steel sheet and a plating layer disposed on at least one side surface of the above steel sheet.

[0086] [Steel sheet]

[0087] As the above steel sheet, there is no particular limitation, and any steel sheet can be used. The above steel sheet can be either a cold-rolled steel sheet or a hot-rolled steel sheet. There is no particular limitation on the composition of the above steel sheet, but it is preferably a steel sheet having the composition described in the above description of the hot-pressed part.

[0088] [Plating layer]

[0089] The steel sheet for hot pressing of the present invention has a plating layer on at least one side surface of the above-mentioned steel sheet. The above-mentioned plating layer can be provided only on one side surface of the steel sheet, and is preferably provided on both sides.

[0090] In order to make the composition of the coating layer after hot pressing satisfy the above conditions, the plating layer of the steel sheet for hot pressing needs to have the following composition.

[0091] It has a composition containing Zn: 30.0 to 70.0%, Si: 1.1 to 8.0%, and Sr + Ca: 0.01 to 5.0%, and the balance is composed of Al and inevitable impurities.

[0092] Here, the Zn content of the above-mentioned plating layer is preferably 35.0% or more. On the other hand, the above-mentioned Zn content is preferably 65.0% or less. In addition, the Si content of the above-mentioned plating layer is preferably 1.3% or more. On the other hand, the above-mentioned Si content is preferably 4.0% or less. Furthermore, the Sr + Ca of the above-mentioned plating layer is preferably 0.1% or more. On the other hand, the above-mentioned Sr + Ca is preferably 1.0% or less.

[0093] That is, the above-mentioned plating layer preferably has a composition containing Zn: 35.0 to 65.0%, Si: 1.3 to 4.0%, and Sr + Ca: 0.1 to 1.0%, and the balance is composed of Al and inevitable impurities.

[0094] It should be noted that, similar to the composition of the above-mentioned coating layer, in the composition of the plating layer, the total content of Sr and Ca is expressed as "Sr + Ca".

[0095] The composition of the plating layer can be measured by SEM (scanning electron microscope) - EDX (energy dispersive X-ray analysis).

[0096] The above-mentioned plating layer is not particularly limited and can be formed by any method, but is preferably formed by hot dip plating. In other words, the above-mentioned plating layer is preferably a hot dip plating layer.

[0097] The coating amount of the above-mentioned plating layer is not particularly limited. From the viewpoint of corrosion resistance, the coating amount is preferably 30 g / m on one side of the steel sheet 2 or more. On the other hand, from the viewpoint of manufacturing cost, the coating amount is preferably 200 g / m on one side of the steel sheet 2 or less. The coating amount of the plating layer can be obtained by dissolving and removing the plating layer from the surface of the steel sheet for hot pressing using an acid solution, and subtracting the weight of the steel sheet for hot pressing after removal from the weight before removal. An inhibitor for suppressing the dissolution of the base steel sheet is added to the above-mentioned acid solution.

[0098] (3) Manufacturing method of hot pressing component

[0099] Next, a suitable manufacturing method for the hot pressing member of the present invention will be described.

[0100] The hot pressing member of the present invention can be manufactured by hot pressing a hot pressing steel sheet that satisfies the above conditions.

[0101] The method of performing hot pressing is not particularly limited and can be carried out according to a conventional method. Typically, the hot pressing steel sheet is heated to a specified heating temperature (heat treatment step), and then, the hot pressing steel sheet heated in the above heat treatment step is hot pressed (hot pressing step). Hereinafter, preferred hot pressing conditions will be described.

[0102] [Heat Treatment]

[0103] In the above heat treatment step, the hot pressing steel sheet is heated to a heating temperature of Ac 3 transformation point to 980 °C. By making the heating temperature above the Ac 3 transformation point, the structure of the steel sheet can be austenitized. Austenite becomes a hard phase such as martensite phase through rapid cooling during subsequent hot pressing. As a result, the hot pressing member can be strengthened. If the heating temperature is lower than the Ac 3 transformation point, the austenite fraction in the heated steel sheet decreases. Therefore, the volume fraction of martensite after hot pressing is insufficient, and sufficient tensile strength cannot be ensured. On the other hand, if the heating temperature is higher than 980 °C, the alloying of the plating layer proceeds excessively, and the Fe content of the coating layer sometimes becomes too much. Even when the Fe content is not excessive, the oxidation of Zn proceeds excessively, and the metallic Zn phase cannot remain in the coating layer. Therefore, a hot pressing member that satisfies the conditions of the present invention cannot be obtained. In addition, if the heating temperature is higher than 980 °C, a thick oxide layer is formed, and furthermore, the crystal grain size becomes excessively large, so the bending crushability decreases.

[0104] It should be noted that the Ac 3 transformation point can be obtained by the following formula (1).

[0105] Ac 3 transformation point (°C) = 881 - 206C + 53Si - 15Mn - 20Ni - 1Cr - 27Cu + 41Mo... (1)

[0106] Among them, the element symbols in formula (1) represent the contents (mass %) of the respective elements. The content of the element not contained is calculated as 0. For example, in the case where Ni, Cu, and Mo are not contained, the following formula (2) can be used.

[0107] Ac 3 transformation point (°C) = 881 - 206C + 53Si - 15Mn - 1Cr... (2)

[0108] Among them, the elemental symbols in formula (2) represent the content (mass %) of each element. The content of the element not contained is calculated as 0.

[0109] In the above heat treatment process, it can be held at the above heating temperature after being heated to the above heating temperature. The holding time at the above heating temperature is not particularly limited. If the above holding time is more than 5 minutes, the alloying of the plating layer proceeds excessively, and the Fe content of the coating layer is too much, and sometimes a hot pressing part that meets the conditions of the present invention cannot be obtained. In addition, the thickness of the oxide layer also becomes too thick. Therefore, the above holding time is preferably 5 minutes or less. On the other hand, since the above holding is an optional process, the holding time can also be 0 seconds. However, from the viewpoint of uniformly austenitizing the base steel plate, the holding time is preferably 10 seconds or more.

[0110] In the heat treatment process, the method of heating the hot pressing steel plate is not particularly limited, and any method can be used. The above heating can be performed, for example, by heating using a heating furnace, electric heating, induction heating, high-frequency heating, flame heating, etc. As the above heating furnace, any heating furnace such as an electric furnace or a gas furnace can be used.

[0111] [Hot pressing]

[0112] After the above heating, the hot pressing steel plate is hot pressed to form a hot pressing part. In the above hot pressing, cooling is performed using a refrigerant such as a metal mold or water simultaneously with or after the processing. In the present invention, the hot pressing conditions are not particularly limited. For example, pressing can be performed in a normal hot pressing temperature range of 600 to 800 °C.

[0113] Examples

[0114] In order to confirm the effects of the present invention, a hot pressing steel plate and a hot pressing part using the hot pressing steel plate were manufactured, and their characteristics were evaluated.

[0115] · Hot pressing steel plate

[0116] A plating layer was formed on the surface of the steel plate in the following order to manufacture a hot pressing steel plate. Specifically, plating layers were formed on both sides of a steel plate with a thickness of 1.4 mm by a continuous melting plating apparatus. As the above steel plate, a cold-rolled steel plate having the following composition was used: containing C: 0.24%, Si: 0.25%, Mn: 1.3%, P: 0.01%, S: 0.002%, Al: 0.03%, N: 0.005%, Cr: 0.16%, Ti: 0.03%, B: 0.002%, and Sb: 0.008%, and the balance being composed of Fe and unavoidable impurities. The Ac of the above cold-rolled steel plate 3The phase transition point is 825 °C. Additionally, the temperature of the plating bath is 600 °C, and the coating amount on one side of the steel plate is 100 g / m 2 , that is, the total for both sides is 200 g / m 2 .

[0117] (Composition of the coating layer)

[0118] The composition of the coating layer was determined by area analysis using SEM (scanning electron microscope) - EDX (energy dispersive X-ray analysis). In the above SEM-EDX analysis, an SEM (JSM-7200F) manufactured by JEOL Ltd. and an EDX detector (UltraDry) manufactured by Thermo Fisher were used, and the analysis was carried out at an acceleration voltage of 15.0 kV. The results obtained are shown in Tables 1 and 2.

[0119] · Hot-pressed parts

[0120] Next, hot pressing was performed on each of the obtained hot-pressing steel plates under the conditions shown in Tables 3 and 4 to produce hot-pressed parts. Specifically, first, the above hot-pressing steel plate was cut into a size of 70 mm × 150 mm and heat-treated using an electric furnace. The heating temperature and the holding time at this heating temperature in the above heat treatment are as described in Tables 3 and 4. Next, the hot-pressing steel plate was taken out of the electric furnace and hot-pressed using a flat metal mold. The forming start temperature was 700 °C.

[0121] Next, regarding each of the obtained hot-pressed parts, the presence or absence of the metallic Zn phase, the composition of the coating layer, and the thickness of the oxide layer were evaluated according to the following steps. The measurement results are shown in Tables 3 and 4.

[0122] (Metallic Zn phase)

[0123] The presence or absence of the metallic Zn phase in the coating layer was determined by X-ray diffraction. The above X-ray diffraction was analyzed using SmartLab manufactured by Rigaku Corporation, using X-ray: Cu-Kα, tube voltage: 40 kV, tube current: 30 mA, and scanning speed: 4° / min.

[0124] (Composition of the coating layer)

[0125] The composition of the coating layer was measured by performing area analysis using SEM-EDX. In the above SEM-EDX analysis, an SEM (JSM-7200F) manufactured by JEOL Ltd. and an EDX detector (UltraDry) manufactured by Thermo Fisher were used, and the analysis was carried out at an acceleration voltage of 15.0 kV.

[0126] (Thickness of the oxide layer)

[0127] A test piece for cross-sectional observation was collected from the flat part of the upper surface of the hot-pressed component, and cross-sectional observation was carried out to measure the thickness of the oxide layer. Specifically, the cross-section of the surface of the hot-pressed component was observed using SEM at a magnification of 500 times, the thicknesses of the oxide layers at 20 arbitrary locations were measured, and the average value thereof was taken as the thickness of the oxide layer.

[0128] Next, for each of the obtained hot-pressed components, the spot weldability and the corrosion resistance of the cut portion were evaluated according to the following steps. The measurement results are shown in Tables 3 and 4.

[0129] (Spot weldability)

[0130] Resistance spot welding was performed in a state where two test pieces collected from the obtained hot-pressed components were overlapped. The above resistance spot welding was performed using an AC spot welder equipped with a DR-type electrode (chromium copper) having a diameter of 6 mm at the front end curved surface portion and a curvature radius of the curved surface portion (front end R) of 40 mm. The shape of the test piece was a rectangle of 30×50 mm, and the pressing force was 3.5 kN. The current value was changed from 4 kA to the current value at which fracture occurred to perform resistance spot welding, and the range from the current value at which the nugget diameter became 4t 1 / 2 mm (t = plate thickness) to the current value at which fracture occurred was taken as the appropriate current range. The wider the appropriate current range, the more excellent the spot weldability. It should be noted that if the appropriate current range is 1.0 kA or more here, it is judged that there is a sufficient appropriate current range.

[0131] (Corrosion resistance of the cut portion)

[0132] Phosphoric acid-based chemical conversion treatment and electrodeposition coating were performed on the test piece collected from the hot-pressed component to produce a test piece for corrosion resistance evaluation. After setting a total of 160 mm of transverse cuts (angle 60°) each with a length of 80 mm in the center of the above test piece for corrosion resistance evaluation, it was subjected to a corrosion test (SAE-J2334). Based on the generation status of red rust after 30 cycles, the corrosion resistance of the cut portion was evaluated according to the following criteria.

[0133] Score 4: No red rust was generated in the cut portion.

[0134] Score 3: The length of the cut portion where red rust was generated was less than 2 mm.

[0135] Score 2: The length of the cut portion where red rust was generated was 2 mm or more and less than 4 mm.

[0136] Score 1: The length of the cut portion where red rust was generated was 4 mm or more.

[0137] It should be noted that if the score is 3 or more, it is judged that there is sufficient corrosion resistance of the cut portion.

[0138] As can be seen from the results shown in Table 3 and Table 4, the hot pressing parts that meet the conditions of the present invention have both excellent spot weldability and corrosion resistance of the cutting part.

[0139] [Table 1]

[0140]

[0141] [Table 2]

[0142]

[0143]

[0144]

Claims

1. A hot-pressed component, comprising: a steel plate, a coating layer disposed on at least one surface of the steel plate, and an oxide layer disposed on the coating layer. Wherein, The coating layer has the following composition: containing Zn: 25.0 to 55.0% by mass, Si: 1.1 to 8.0%, Sr + Ca: 0.01 to 5.0%, and Fe: 55.0% or less, and the balance consists of Al and inevitable impurities. And, the coating layer contains a metallic Zn phase.

2. The hot-pressed component according to claim 1, Wherein, The thickness of the oxide layer is 0.3 μm or less.

3. A steel plate for hot pressing, comprising: a steel plate and a plating layer disposed on at least one surface of the steel plate. Wherein, The plating layer has the following composition: containing Zn: 30.0 to 70.0% by mass, Si: 1.1 to 8.0%, Sr + Ca: 0.01 to 5.0%, and the balance consists of Al and inevitable impurities.

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