Hot press components and steel plates for hot pressing

Abstract

Provided are: a hot-pressed member capable of achieving both high strength and cut part corrosion resistance and hydrogen embrittlement resistance; and a steel sheet for hot pressing for forming the hot-pressed member. The present invention includes a steel sheet and a coating layer disposed on at least one surface of the steel sheet. The coating layer has a composition containing, in mass %, at least one of 0.1 to 5.0% of Mg and 0.005 to 1.0% of Ca, 3.0 to 15.0% of Si, and 55.0% or less of Fe, the balance being Al and unavoidable impurities. The coating layer satisfies Ltc / Lw≤1, where Ltc is a total crack length index of a crack observed within a range of a length Lw in a direction parallel to the surface of the steel sheet in a cross section perpendicular to the surface of the steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a hot-pressed member and a hot-pressed steel sheet for manufacturing the hot-pressed member, and more particularly to a hot-pressed member and a hot-pressed steel sheet that are high in strength and have excellent corrosion resistance at the cut portion and resistance to hydrogen embrittlement. [Background technology]

[0002] To reduce the weight of automobiles and improve crash safety, there has been a trend towards increasing the strength of automotive steel sheets. In recent years, cold-rolled steel sheets with a tensile strength of 1.5 GPa have been developed, and their application is being considered. However, such cold-rolled steel sheets have presented challenges, such as forming defects during pressing and reduced dimensional accuracy due to springback, which are associated with increased strength.

[0003] Therefore, the manufacture of components using hot pressing technology, which involves press-forming steel sheets at a hot temperature rather than a cold temperature, is being considered. Hot pressing is a forming method in which a steel sheet is heated to the austenite temperature range, and then press-formed while still at a high temperature, while simultaneously being rapidly cooled by contact with a die. With hot pressing, high strength can be achieved by heating the steel sheet to improve its formability, performing press-forming, and then rapidly cooling it. As a result, hot-pressed components with superior strength can be manufactured with high dimensional accuracy.

[0004] Hot-pressed components are primarily used in automotive parts, particularly structural components for the frame where strength is required, i.e., inner panel frames. In recent years, they have also been used as so-called semi-exterior components, such as the pillar components visible when the car door is opened. Therefore, hot-pressed components are required to be suitable for painting and to have excellent corrosion resistance at the cut edges after painting.

[0005] Furthermore, while hot-pressed components have predominantly been those with a tensile strength of 1.5 GPa after hot-press forming, there is a demand for high-strength components with a tensile strength of 1.8 GPa or higher in order to further reduce the weight of automobile bodies.

[0006] However, it is known that high-strength hot press members are highly sensitive to hydrogen and tend to embrittle when hydrogen penetrates. Therefore, in order to increase the strength of hot press members, more advanced measures against hydrogen embrittlement are required. Hydrogen that penetrates during the manufacturing process of hot press members includes hydrogen introduced during the production of hot press steel sheets, hydrogen that penetrates during the hot press process, hydrogen that penetrates during the painting process, and the like.

[0007] In particular, as a hot press steel sheet suitable for the production of hot press members, a steel sheet provided with a coating composed of an Al-based plating layer on the surface of a steel base material has been conventionally proposed. However, in hot press members using Al-based plated steel sheets, the amount of hydrogen that penetrates during the hot press process is extremely large, so it is important to reduce the amount of hydrogen that penetrates during this process.

[0008] In contrast, in Patent Document 1, it is proposed to contain 0.1 to 0.5% by mass of an alkaline earth metal or a transition metal and form an oxide of the alkaline earth metal or the transition metal on the outer surface of the Al-based plating layer during hot pressing. In this method, since the Al-based plating layer is covered with an oxide film of an element having a high oxygen affinity such as Mg, the reaction between the Al-based plating layer and moisture is suppressed in the hot press process, and thereby the penetration of hydrogen can be reduced.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] Incidentally, when an aluminum-plated steel sheet is hot-pressed to form a hot-pressed member, the coating layer of the hot-pressed member becomes an Fe-Al alloy layer. Because the Fe-Al alloy layer is very hard and brittle, numerous cracks can occur that expose the base material, and these cracks can become the starting point for corrosion, potentially leading to a deterioration in corrosion resistance.

[0011] In the above-mentioned Patent Document 1, numerous cracks occur in the coating layer of the hot-pressed member, exposing the base material, and corrosion of these cracks becomes the starting point for a decrease in the corrosion resistance of the cut portion, which presents a problem. Furthermore, since the oxides of the alkaline earth metal or transition metal are formed on the outermost surface of the coating layer during the heating process before hot pressing, there is a problem that the protective effect against moisture does not work sufficiently on the cross-section of the coating layer exposed at the cracks.

[0012] Based on the above, it is considered important to minimize the occurrence of cracks within the coating layer of hot-pressed members in order to suppress the deterioration of corrosion resistance and hydrogen intrusion of hot-pressed members.

[0013] This invention has been made in view of the above circumstances, and aims to provide a hot-pressed member that enables both high strength and corrosion resistance and hydrogen embrittlement resistance of the cut portion, and a hot-pressed steel sheet for manufacturing the hot-pressed member. [Means for solving the problem]

[0014] As a result of our investigations to solve the above problems, we have come up with the present invention relating to the following gist of the structure. In other words, the present invention comprises a steel sheet and a coating layer disposed on at least one surface of the steel sheet, wherein the coating layer has a composition in mass% of at least one of Mg: 0.1 to 5.0% and Ca: 0.005 to 1.0%, Si: 3.0 to 15.0%, Fe: 55.0% or less, with the remainder being Al and unavoidable impurities, and the coating layer has a length L in a direction parallel to the surface of the steel sheet in a cross section perpendicular to the surface of the steel sheet. w The total crack length index of cracks observed within the range is L.tc In that case, L tc / L w This is a hot-pressed member characterized by satisfying the condition ≤ 1.

[0015] Furthermore, the present invention comprises a steel sheet and a plating layer disposed on at least one surface of the steel sheet, wherein the plating layer comprises an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, wherein the metal layer has a composition in mass% of at least one of Mg: 0.2 to 7.0% and Ca: 0.01 to 1.5%, Si: 1.0 to 10.0%, Fe: 10% or less, with the remainder being Al and unavoidable impurities, and the metal layer has a maximum length L of the crystal grains contained in the metal layer in a direction parallel to the surface of the steel sheet in a cross section perpendicular to the surface of the steel sheet. c The thickness of the metal layer is set to L h In that case, L c ≥L h This is a steel sheet for hot pressing characterized in that the area ratio of the crystal grains is 50% or more. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a hot-press member and a hot-press steel sheet equipped with an Al-based plating layer that achieve both high strength and corrosion resistance and hydrogen embrittlement resistance at the cut portion. [Brief explanation of the drawing]

[0017] [Figure 1] This figure schematically shows a cross-section of the hot-pressed member of the present invention, perpendicular to the surface of the steel plate. [Figure 2] This figure schematically shows a cross-section perpendicular to the steel sheet of the hot-pressing steel sheet of the present invention. [Figure 3] This graph shows the relationship between cooling time and steel sheet temperature when manufacturing steel sheets for hot pressing according to the present invention. [Modes for carrying out the invention]

[0018] (1) Hot-pressed member Embodiments of the present invention will now be described. Note that the following describes a preferred embodiment, and the hot-pressed member of the present invention is not limited to this embodiment. Furthermore, unless otherwise specified, the unit of content, [%], represents "mass%".

[0019] A hot-pressed member in one embodiment of the present invention comprises a steel plate and a coating layer provided on at least one surface of the steel plate. The parts will be described below.

[0020] [Steel plate] In this embodiment, the above problems are solved by controlling the structure of the coating layer, as will be described later. Therefore, the steel sheet is not particularly limited and any steel material can be used, and either cold-rolled steel sheet or hot-rolled steel sheet may be used.

[0021] The hot-pressed member of this embodiment is manufactured by hot-pressing a hot-pressed steel sheet, which will be described later.

[0022] Hot-pressed members are preferably made with high strength from the viewpoint of use as automotive components, etc. In particular, in order to obtain hot-pressed members with a tensile strength exceeding 1.8 GPa, it is preferable to use steel sheets having the following composition: C: 0.05~0.50%, Si: 0.1~1.5%, Mn: 0.5~5.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.10% or less, and N: 0.01% or less, with the remainder being Fe and unavoidable impurities.

[0023] The following describes the effects of each element and their suitable content in the above preferred composition of steel sheets.

[0024] C: 0.05~0.50% Carbon (C) is an element that improves strength by forming structures such as martensite. From the viewpoint of obtaining strength exceeding 1.8 GPa, it is preferable to have a C content of 0.05% or more, and more preferably 0.10% or more. On the other hand, if the C content exceeds 0.50%, the toughness of the spot weld deteriorates. Therefore, it is preferable to have a C content of 0.50% or less, more preferably 0.45% or less, even more preferably 0.43% or less, and most preferably 0.40% or less.

[0025] Si: 0.1~1.5% Si is an effective element for strengthening steel and obtaining good material properties. To obtain this effect, the Si content is preferably 0.1% or more, and more preferably 0.2% or more. On the other hand, if the Si content exceeds 1.5%, the ferrite is stabilized, which reduces the hardenability. Therefore, the Si content is preferably 1.5% or less, more preferably 1.3% or less, and even more preferably 1.1% or less.

[0026] Mn: 0.5~5.0% Mn is an effective element for obtaining high strength regardless of the cooling rate. From the viewpoint of ensuring excellent mechanical properties and strength, it is preferable to have a Mn content of 0.5% or more, more preferably 0.7% or more, and even more preferably 1.0% or more. On the other hand, if the Mn content exceeds 5.0%, the cost increases, and the effect of adding Mn saturates. Therefore, it is preferable to have a Mn content of 5.0% or less, more preferably 4.5% or less, and even more preferably 4.0% or less.

[0027] P:0.1% or less If the phosphorus (P) content is excessive, local ductility deteriorates due to grain boundary embrittlement caused by P segregation at austenite grain boundaries during casting. As a result, the balance between the strength and ductility of the steel sheet decreases. Therefore, from the viewpoint of improving the balance between the strength and ductility of the steel sheet, it is preferable to keep the P content at 0.1% or less. On the other hand, from the viewpoint of refining costs, it is preferable to keep the P content at 0.01% or more.

[0028] S: 0.01% or less S (sulfur) acts as an inclusion such as MnS, causing deterioration of impact resistance and cracking along the metal flow in the weld. Therefore, it is desirable to reduce the S content as much as possible, and specifically, it is preferable to keep it to 0.01% or less. Furthermore, from the viewpoint of ensuring good elongation flange properties, it is more preferable to keep it to 0.005% or less, and even more preferable to keep it to 0.001% or less. On the other hand, from the viewpoint of refining costs, it is preferable to keep the S content to 0.0002% or more.

[0029] Al: 0.10% or less Al is an element that acts as a deoxidizing agent. However, if the Al content exceeds 0.10%, the blanking processability and hardenability of the steel sheet deteriorate. Therefore, it is preferable to keep the Al content at 0.10% or less, more preferably at 0.07% or less, and even more preferably at 0.04% or less. On the other hand, from the viewpoint of ensuring the effectiveness as a deoxidizing agent, it is preferable to keep the Al content at 0.01% or more.

[0030] N: 0.01% or less If the nitrogen content exceeds 0.01%, AlN nitrides are formed during hot rolling and heating before hot pressing, reducing the blanking and hardenability of the steel sheet. Therefore, it is preferable to keep the nitrogen content below 0.01%. On the other hand, from the viewpoint of refining costs, it is preferable to keep the nitrogen content above 0.001%.

[0031] Furthermore, the above component composition may optionally contain at least one selected from Nb: 0.10% or less, Ti: 0.05% or less, B: 0.0002-0.005%, Cr: 0.1-1.0%, and Sb: 0.003-0.03%.

[0032] Nb: 0.10% or less Nb is an effective component for strengthening steel, but if it is present in excess, the rolling load increases. Therefore, when adding Nb, it is preferable to keep the Nb content at 0.10% or less, and more preferably at 0.05% or less. On the other hand, there is no particular lower limit to the Nb content, and it may be 0%, but when used for strengthening steel, it is preferable to keep the Nb content at 0.005% or more.

[0033] Ti: 0.05% or less Ti, like Nb, is an effective component for strengthening steel, but if present in excess, it reduces shape retention. Therefore, when adding Ti, it is preferable to keep the Ti content at 0.05% or less, and more preferably at 0.03% or less. On the other hand, there is no particular lower limit to the Ti content, and it may be 0%, but when used for strengthening steel, it is preferable to keep the Ti content at 0.005% or more.

[0034] B: 0.0002~0.005% B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. Therefore, when adding B, it is preferable to have a B content of 0.0002% or more, and more preferably 0.0010% or more. On the other hand, excessive addition of B reduces moldability, so it is preferable to have a B content of 0.005% or less, and more preferably 0.003% or less.

[0035] Cr: 0.1~1.0% Cr, like Mn, is a useful element for strengthening steel and improving its hardenability. Therefore, when adding Cr, it is preferable to have a Cr content of 0.1% or more, and more preferably 0.2% or more, in order to obtain the aforementioned effects. On the other hand, since Cr is an expensive element, adding too much Cr would lead to a significant increase in costs, so it is preferable to have a Cr content of 1.0% or less, and more preferably 0.6% or less.

[0036] Sb: 0.003~0.03% Sb is an element that inhibits decarburization of the surface layer of steel sheets during the annealing process in the manufacturing of base steel sheets. When adding Sb, it is preferable to have an Sb content of 0.003% or more, and more preferably 0.005% or more, in order to obtain the above effect. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, which reduces productivity. Therefore, it is preferable to have an Sb content of 0.03% or less, more preferably 0.02% or less, and even more preferably 0.01% or less.

[0037] [Coating layer] The hot-pressed member of this embodiment is provided with a coating layer on at least one surface of the steel plate. The coating layer may be provided on only one surface of the steel plate, but it is more preferable that it be provided on both surfaces.

[0038] The coating layer consists mainly of an Fe-Al alloy phase and contains at least one of Mg and Ca, and Si. Such a coating layer is obtained by hot pressing a steel sheet for hot pressing that has an Al-based plating layer, as described later. During the hot pressing process, at least one of the Mg and Ca contained in the coating layer oxidizes on the surface, and the coating layer is covered by an oxide film of at least one of the oxidized Mg and Ca, thereby suppressing the penetration of hydrogen from the surface of the coating layer. The effects of each element in a preferred component composition of the coating layer and the preferred content will be described below.

[0039] Mg: 0.1~5.0% If the Mg content in the coating layer is less than 0.1%, the Mg oxide film cannot adequately cover the coating layer, and the hydrogen penetration suppression effect during the hot pressing process cannot be sufficiently obtained. Therefore, the Mg content should be 0.1% or more. If the Mg content exceeds 5.0%, lumpy Mg oxide will form on the outermost layer of the coating layer. Lumpy Mg oxide readily reacts with moisture during the hot pressing process, and the expected hydrogen penetration suppression effect cannot be obtained. Therefore, the Mg content should be 5.0% or less.

[0040] Ca: 0.005~1.0% Like Mg, Ca forms an oxide film on the surface of the coating layer, suppressing hydrogen penetration. Therefore, in this invention, the Ca content in the coating layer is set to 0.005% or more to obtain the expected hydrogen penetration suppression effect. On the other hand, if the Ca content exceeds 1.0%, the reaction with moisture in the lump-shaped Ca oxide is accelerated during the hot pressing process, similar to the case of Mg, and the expected hydrogen penetration suppression effect cannot be obtained. Therefore, the Ca content is set to 1.0% or less.

[0041] Si: 3.0~15.0% Si has the effect of suppressing alloying of the plating layer during the plating process and the heat treatment process before hot pressing. If the Si content in the plating layer is less than 3.0%, excessive diffusion of Fe into the coating layer occurs during the heating process of hot pressing, making the coating layer brittle and prone to cracking, resulting in a deterioration of the corrosion resistance and hydrogen embrittlement resistance of the cut area. Therefore, the Si content should be 3.0% or more. On the other hand, if the Si content is too high, the amount of Si-based oxides produced increases, resulting in a deterioration of coating adhesion and consequently inferior corrosion resistance. Therefore, the Si content should be 15.0% or less.

[0042] Fe:55.0% or less Since Fe diffuses from the steel sheet into the coating layer during the process of applying molten plating to the steel sheet and the heating before hot pressing, Fe is inevitably contained in the coating layer of the hot pressing member. However, when the Fe content increases, the Al concentration in the coating layer decreases, the coating layer becomes brittle and cracks are likely to occur, and the corrosion resistance and hydrogen embrittlement resistance of the cut portion deteriorate. Therefore, the Fe content should be 55.0% or less. On the other hand, the lower limit of the Fe content is not particularly limited, but when manufacturing under general conditions, the Fe content may be 20.0% or more.

[0043] Next, cracks generated in the coating layer will be described with reference to figures. FIG. 1 is a diagram schematically showing a cross-section perpendicular to the surface of the steel sheet 2 of the hot pressing member 1 according to the present embodiment. The hot pressing member 1 in FIG. 1 includes a steel sheet 2 and a coating layer 3 disposed on one surface of the steel sheet 2, and a plurality of cracks 4 are observed in the coating layer 3. Note that the coating layer 3 may be provided on both surfaces of the steel sheet 2.

[0044] In the present embodiment, in FIG. 1, the length of the coating layer 3 in the direction parallel to the surface of the steel sheet 2 is L w (hereinafter referred to as the parallel length L w [ ). When the total crack length index of the cracks observed within the range of the parallel length L w [ is L tc [ , it is necessary to satisfy L tc [ / L w [ ≤ 1.

[0045] This is because when L tc [ / L w [ > 1, a part of the steel sheet 2 serving as the base material is exposed without being covered by the coating layer 3, or the cross-section of the coating layer 3 not covered by the oxide film is exposed at the crack 4 portion, and thus the expected corrosion resistance and hydrogen embrittlement resistance of the cut portion cannot be obtained.

[0046] The ratio of the total crack length index L tc [ of the coating layer 3 to the parallel length L w [ is L tc [ / L w [This can be determined by observing a cross-section of the hot-pressed member 1 perpendicular to the surface of the steel plate 2 using a scanning electron microscope (SEM) and analyzing the obtained image. Specifically, the cross-section of the coating layer 3 is observed for 10 randomly selected fields of view, and for each field of view, the total crack length index L, which is the total length of the cracks 4 generated within the cross-section of the coating layer 3, is determined. tc And, parallel length L w Find L tc / L w Calculate the calculated L. tc / L w The average value of the 10 fields of view is the L of the hot-pressed member. tc / L w The value shall be [value]. Note that the cross-section of the coating layer 3 is observed along the parallel length L. w The process should be carried out so that the particle size is 500 μm or larger.

[0047] The amount of coating layer 3 applied is not particularly limited, but from the viewpoint of corrosion resistance, the amount applied should be 60 g / m² per side of the steel plate 2. 2 It is preferable to keep the amount above. On the other hand, from the viewpoint of manufacturing cost, the amount of adhesion should be 400 g / m² per side of the steel plate 2. 2 The following is preferable. The amount of coating layer 3 can be determined by dissolving and removing the coating layer 3 from the surface of the hot press member 1 using an acid solution, and subtracting the weight after removal from the weight of the hot press member 1 before removal. An inhibitor that suppresses the dissolution of the steel plate 2 is added to the acid solution.

[0048] (2) Steel sheet for hot pressing Next, the hot-press steel sheet of the present invention will be described. The following is a preferred embodiment, and the hot-press steel sheet of the present invention is not limited to this embodiment. Also, unless otherwise specified, the unit of content [%] represents "mass%". The hot-pressed member described in (1) above can be manufactured by hot-pressing a hot-pressed steel sheet of this embodiment that has a plating layer.

[0049] The hot-press steel sheet of this embodiment comprises a steel sheet and a plating layer disposed on at least one surface of the steel sheet.

[0050] [Steel plate] Any steel material can be used as the steel plate without any particular limitations. The steel plate may be either cold-rolled or hot-rolled steel plate. The composition of the steel plate is also not particularly limited, but it is preferable to use a steel material having the component composition mentioned in the description of the hot-pressed member in (1) above.

[0051] [Plating layer] The hot-press steel sheet of this embodiment has a plating layer on at least one surface of the steel sheet. The plating layer comprises an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer. The plating layer may be provided on only one surface of the steel sheet, but it is more preferable that it be provided on both sides of the steel sheet.

[0052] [Interfacial alloy layer] Steel sheets for hot pressing are typically manufactured by applying hot-dip galvanizing to the steel sheets, as described later. During this process, components such as Fe and Mn contained in the steel sheet react with components such as Al and Si contained in the galvanizing bath, forming an interfacial alloy layer at the interface between the steel sheet and the metal layer. In this embodiment, the composition of the interfacial alloy phase is not particularly limited.

[0053] [Metal layer] In order for the composition of the coating layer of the hot-pressed member formed by hot-pressing the hot-pressed steel sheet of this embodiment to satisfy the above-mentioned conditions, the metal layer of the hot-pressed steel sheet must have the following component composition: that is, it contains at least one of Mg: 0.2 to 7.0% and Ca: 0.01 to 1.5%, Si: 1.0 to 10.0%, and Fe: 10% or less, with the remainder being Al and unavoidable impurities.

[0054] Furthermore, in the final hot-pressed member, the total crack length index L of the coating layer 3 in a cross-section perpendicular to the surface of the steel plate 2 shown in Figure 1 is... tc However, parallel length L w The following is, L tc / L wIn order to satisfy the condition ≤ 1, the following conditions must be met. That is, as shown in the schematic diagram of the cross-section of the hot-press steel sheet 5 in Figure 2, (a) The hot-press steel sheet 5 comprises a base steel sheet 2 and a plating layer 7 disposed on at least one surface of the steel sheet 2, wherein the plating layer 7 comprises an interface alloy layer 7A disposed on the steel sheet 2 and a metal layer 7B disposed on the interface alloy layer 7A, and further, (b) The metal layer 7B of the hot-press steel sheet 5, in a cross section perpendicular to the surface of the steel sheet 2, has a maximum length of L of the crystal grains 8 contained in the metal layer 7B in a direction parallel to the surface of the steel sheet 2. c Let the thickness of the metal layer 7B be L h In that case, L c ≥L h The area ratio occupied by the crystal grain 8 is 50% or more. That is the case.

[0055] This is because the number of cracks 4 occurring in the coating layer 3 of the hot-press member 1 tends to decrease as the crystal grains 8 in the metal layer 7B of the hot-press steel sheet 5 become larger.

[0056] In metal layer 7B, L c ≥L h The area ratio occupied by the crystal grains 8 can be determined by observing a cross-section of the hot-press steel sheet 5 perpendicular to the surface of the steel sheet 2 using a scanning electron microscope (SEM), and analyzing the obtained image using electron beam scattering analysis (EBSD). Specifically, the cross-section of the plating layer 7 is observed in three randomly selected fields of view, and for each field of view, the maximum length L of the crystal grains 8 in the metal layer 7B in the direction parallel to the surface of the steel sheet 2 is determined. c And the thickness L of the metal layer 7B h The relationship is L c ≥L h The area ratio of crystal grains 8 that satisfy the condition is calculated. The average of the three fields of view of the calculated area ratio is used for the L of the hot press steel sheet. c ≥L h This value represents the area ratio occupied by the crystal grains 8. The cross-section of the plating layer 7 should be observed such that the length in the direction parallel to the surface of the steel plate 2 is 200 μm or more.

[0057] In the hot-press steel sheet 5, for the crystal grains 8 contained in the metal layer 7B to satisfy the above conditions, it is necessary to cool the hot-press steel sheet 5 under specific conditions during its manufacture. Specifically, as shown in Figure 3, after the steel sheet 2 is removed from the molten plating bath, a cooling and holding process is provided, and the steel sheet 2 is held in a range of 660°C to 500°C during this cooling and holding process. By providing a cooling and holding process, the crystal grains 8 in the metal layer 7B during the solidification process can be coarsened. As a result, a crystal grain distribution that satisfies the above conditions can be obtained. Furthermore, as a result, the occurrence of cracks 4 in the coating layer 3 of the hot-press member 1 can be reduced. The cooling and holding time should be 15 seconds or more. This is because if the cooling and holding time is less than 15 seconds, the coarsening of the crystal grains 8 will be insufficient, and as a result, the occurrence of cracks 4 in the coating layer 3 of the hot-press member 1 cannot be reduced. Also, if the cooling and holding temperature is below 500°C, the coarsening of the crystal grains 8 will not progress easily, requiring a longer holding time, which would be a challenge in terms of production efficiency and manufacturing costs.

[0058] (3) Method for manufacturing hot-pressed members Next, a preferred method for manufacturing the hot-pressed member of the present invention will be described. The hot-pressed member of the present invention can be manufactured by hot-pressing a hot-pressed steel sheet that satisfies the above conditions. In particular, it is important that the hot-pressed steel sheet used is one that has been cooled under specific conditions (cooling and holding process) after plating, as described above, to coarseen the crystal grains in the metal layer.

[0059] The hot pressing method for hot-press steel sheets is not particularly limited and can be carried out according to conventional methods. A typical method includes a heat treatment step of heating the hot-press steel sheet to a predetermined heating temperature, and a hot-press step of hot-pressing the hot-press steel sheet. Preferred hot-pressing conditions will be described below.

[0060] [Heat treatment process] In the heat treatment process, the steel sheet for hot pressing is heated to a temperature between the Ac3 transformation point (°C) and 980°C. This is the so-called austenite region. This heat treatment process causes the plating layer to alloy (Fe-Al alloy phase). By setting the heating temperature above the Ac3 transformation point (°C), the structure of the steel sheet that forms the base material for the hot pressing can be austenitized. The austenite then rapidly cools during hot pressing, transforming it into a hard phase such as martensite. As a result, the hot-pressed member can be made highly strong. If the heating temperature is lower than the Ac3 transformation point (°C), the austenite fraction in the heated steel sheet decreases. Therefore, the volume fraction of martensite becomes insufficient after hot pressing, and sufficient tensile strength cannot be secured. On the other hand, if the heating temperature is higher than 980°C, the Fe concentration in the plating layer (Fe-Al alloy phase) becomes excessive, and the expected corrosion resistance cannot be obtained. Furthermore, if the heating temperature is higher than 980°C, crack formation in the coating layer of the hot-pressed member becomes significant, and the expected hydrogen embrittlement resistance cannot be obtained. Therefore, a hot-pressed member that satisfies the conditions of the present invention cannot be obtained.

[0061] The Ac3 transformation point (°C) can be determined by the following equation (I). Ac3 transformation point (℃)=881-206C+53Si-15Mn-20Ni-1Cr-27Cu+41Mo...(I) However, the element symbols in equation (I) represent the content of each element expressed in mass percent. The content of elements that are not present is calculated as 0.

[0062] In the heat treatment process, after heating to the above-mentioned heating temperature, the sheet is held at that temperature for 5 minutes or less. By providing a holding time, the steel sheet can be reliably transformed into austenite, thereby increasing its strength. Furthermore, providing a holding time ensures that the coating layer is properly alloyed. If the alloying of the coating layer is insufficient, pure Al phase may remain in the coating layer, raising concerns about adhesion to the press die. On the other hand, if the holding time is too long, the alloying of the plating layer will progress, resulting in an excessive Fe content in the coating layer, which may prevent the desired corrosion resistance from being achieved. Also, hydrogen may penetrate into the steel sheet during heating, potentially preventing the desired hydrogen embrittlement resistance from being obtained. For this reason, the holding time in the heat treatment process should be 5 minutes or less.

[0063] The method for heating the steel sheet for hot pressing in the heat treatment process is not particularly limited, and any method can be used. For example, it can be done by heating in a heating furnace, electric heating, induction heating, high-frequency heating, or flame heating. Any heating furnace can be used, such as an electric furnace or a gas furnace.

[0064] [Hot pressing process] After the heat treatment process described above, a hot pressing process is performed to hot press the steel sheet for hot pressing to form a hot pressing member. In the hot pressing process, cooling is performed simultaneously with or immediately after processing using a mold or a coolant such as water. In this embodiment, the hot pressing conditions are not particularly limited. For example, pressing can be performed in the general hot pressing temperature range of 600 to 800°C. [Examples]

[0065] To confirm the effects of the present invention, a steel sheet for hot pressing and a hot-pressed member using the steel sheet were manufactured, and their properties were evaluated. [Example 1]

[0066] <Steel sheet for hot pressing> A plating layer was formed on the surface of a base steel sheet using the following procedure to create a steel sheet for hot pressing. Specifically, a plating layer was formed on both sides of a 1.4 mm thick steel sheet using a continuous hot-dip galvanizing system. The steel sheet used was a cold-rolled steel sheet with a composition of 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%, and Sb:0.008%, with the remainder being Fe and unavoidable impurities. The Ac3 transformation point of the cold-rolled steel sheet is 783°C. The plating bath temperature was 640°C, and the amount of plating layer deposited was 60 g / m² per side of the steel sheet. 2 That is, 120g / m² in total for both sides. 2 That's what I decided. Furthermore, during the formation of the plating layer, a cooling and holding process was carried out under the conditions shown in Table 1. After the steel sheet was removed from the molten plating bath, cooling was performed using nitrogen gas.

[0067] (Composition of the metal layer) The component composition of the obtained metal layer was measured by area analysis using SEM (scanning electron microscope)-EDX (energy-dispersive X-ray spectroscopy). For the SEM-EDX analysis, a JEOL SEM (JSM-7200F) and a Thermo Fisher EDX detector (UltraDry) were used, and the analysis was performed at an acceleration voltage of 15.0 kV. The results are shown in Table 1.

[0068] (metal layer crystal grain) Next, the grain distribution of the metal layer was measured using the EBSD method with a SEM. An SEM (JSM-7200F) manufactured by JEOL and an EBSD detector (Digiview iv) manufactured by TSL were used, and the analysis was performed at an acceleration voltage of 15.0 kV. Specifically, the cross-section of the hot-press steel sheet was observed at a magnification of 500x using the SEM, and the grain distribution was measured by analyzing three randomly selected fields of view using the EBSD method. Furthermore, in the grain distribution, the maximum length L in the direction parallel to the surface of the steel sheet was measured. c However, the thickness L of the metal layer hThe area ratio of the metal layer cross-section occupied by the crystal grains was measured, and the average value of the three fields of view was used for the L of the test steel plate. c ≥L h This was defined as the area ratio occupied by the crystal grains. The obtained area ratios were evaluated according to the following criteria. The evaluation results are shown in Table 1. ○: Area ratio 50% or more ×: Area ratio less than 50%

[0069] [Table 1] [Example 2]

[0070] <Hot-pressed components> Next, each hot-press steel sheet obtained in Example 1 was subjected to hot pressing under the conditions shown in Table 2 to produce hot-press members. Specifically, first, the hot-press steel sheet was cut to a size of 70 mm x 150 mm and heat-treated in an electric furnace. The heating temperature and holding time at the heating temperature during the heat treatment were as shown in Table 2. The heat treatment was carried out in an atmosphere with a dew point of 10°C. Then, the hot-press steel sheet was removed from the electric furnace and hot-pressed using a flat die. The forming start temperature was 700°C.

[0071] (Composition of the coating layer) The component composition of the coating layer of the obtained hot-pressed member was measured by area analysis using SEM (scanning electron microscope)-EDX (energy-dispersive X-ray spectroscopy). SEM-EDX analysis was performed using a JEOL SEM (JSM-7200F) and a Thermo Fisher EDX detector (UltraDry) at an acceleration voltage of 15.0 kV. The results are shown in Table 2.

[0072] (Evaluation of crack length) Furthermore, the total crack length index L of the cracks that occur in the coating layer tc And, parallel length L w and were measured by the above SEM, L tc / L wThe L was calculated. Specifically, the cross-section of the hot-pressed member was observed at a magnification of 500x using an SEM, and SEM images of 10 randomly selected fields of view were obtained. From the obtained SEM images, the L for each field of view was calculated. tc / L w Determine the average value of 10 fields of view, and the L value in the test member. tc / L w The value was set to [value]. The obtained results are shown in Table 2.

[0073] (Corrosion resistance of cut sections) Next, the corrosion resistance of the cut portion of each of the obtained hot-pressed members was evaluated using the following procedure. Test specimens for corrosion resistance evaluation were prepared by taking specimens from hot-pressed members and subjecting them to phosphate-based chemical conversion treatment and electrodeposition coating. Cross-cut scratches (at an angle of 60°) totaling 160mm in length, each 80mm long, were made in the center of the corrosion resistance evaluation specimens, and then subjected to a corrosion test (SAE-J2334). Based on the occurrence of red rust after 30 cycles, the corrosion resistance of the cut areas was evaluated according to the following criteria. The evaluation results are shown in Table 2. Rating 3: The length of the cut area where red rust has formed is less than 2mm. Rating 2: The length of the cut area where red rust has formed is between 2mm and 4mm. Rating 1: The length of the cut area where red rust has formed is 4mm or longer. In this context, a score of 2 or higher was considered to indicate sufficient corrosion resistance at the cut surface.

[0074] (Amount of diffusible hydrogen in steel) To evaluate the hydrogen embrittlement resistance of the obtained hot-pressed members due to hydrogen intrusion during the hot-pressing process, the amount of diffusible hydrogen in the steel immediately after pressing was measured. The amount of diffusible hydrogen in the steel of the obtained hot-pressed member immediately after hot pressing was measured using the following method. A 10 × 15 mm small piece was cut from the flat part of the hot-pressed member, and the coating layer on both sides was removed by grinding with a precision rotary tool. Subsequently, a temperature-induced desorption analysis was performed, and the cumulative value of the amount of hydrogen when heated to 200°C was defined as the amount of diffusible hydrogen in the steel. For the temperature-induced desorption analysis, a temperature-induced desorption analyzer manufactured by J-Science Co., Ltd. was used, with argon as the carrier gas and a heating rate of 200°C / s. The obtained amount of diffusible hydrogen in the steel was evaluated according to the following criteria. The evaluation results are shown in Table 2. ○: Less than 0.20 ppm by mass ×: 0.20 mass ppm or more

[0075] As can be seen from the results shown in Table 2, the hot-pressed member that satisfies the conditions of the present invention exhibits excellent corrosion resistance at the cut portion, as well as a low amount of diffusible hydrogen in the steel immediately after hot pressing, and possesses resistance to hydrogen embrittlement.

[0076] [Table 2] [Explanation of symbols]

[0077] 1 Hot-pressed member 2 steel plate 3 Covering layer 4 Cracks 5. Steel plates for hot pressing 7 Plating layer 7A interfacial alloy phase 7B Metal layer 8 crystal grains

Claims

1. It comprises a steel plate and a coating layer disposed on at least one surface of the steel plate, The coating layer is, by mass%, Mg: 0.1-5.0% and Ca: 0.005-1.0% (at least one of these) Si: 3.0 to 15.0%, Fe: Contains 41.0% to 55.0%, The composition has a remainder consisting of Al and unavoidable impurities. The coating layer has a length L in a direction parallel to the surface of the steel plate, in a cross-section perpendicular to the surface of the steel plate. ｗ The total crack length index of cracks observed within the range is L. ｔｃ In that case, L ｔｃ / L ｗ A hot-pressed member characterized by satisfying the condition ≤ 1.

2. The invention comprises a steel plate and a plating layer disposed on at least one surface of the steel plate, The plating layer comprises an interface alloy layer disposed on the steel plate and a metal layer disposed on the interface alloy layer. The aforementioned metal layer is, by mass%, Mg: 0.2-7.0% and Ca: 0.01-1.5% (at least one of these) Si: 1.0 to 10.0%, Fe: Contains 10% or less, The composition has a remainder consisting of Al and unavoidable impurities. In a cross-section perpendicular to the surface of the steel sheet, the metal layer has a maximum length L of the crystal grains contained within the metal layer in a direction parallel to the surface of the steel sheet. ｃ Let the thickness of the metal layer be L ｈ In that case, L ｃ ≥ L ｈ A steel sheet for hot pressing, characterized in that the area ratio of crystal grains is 50% or more.

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