Hot-pressed member and steel sheet 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

Hot-pressed parts and hot-pressed steel plates

[0001] The present invention relates to a hot-pressed member and a hot-press steel plate for manufacturing the hot-pressed member, and more particularly to a hot-pressed member and a hot-press steel plate that have high strength and excellent cut corrosion resistance and hydrogen embrittlement resistance.

[0002] In order to reduce the weight of automobiles and improve their crashworthiness, efforts are being made to increase the strength of steel sheets for automobiles. 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 had problems such as poor forming during press work and reduced dimensional accuracy due to springback, which are associated with the increased strength.

[0003] For this reason, the application of hot pressing technology, in which steel sheets are press-formed hot rather than cold, to the manufacture of components is being considered. Hot pressing is a forming method in which steel sheets are heated to the austenite temperature range, press-formed while still in a high-temperature state, and then rapidly cooled by contact with a mold. In hot pressing, steel sheets are press-formed in a heated state to improve formability, and then rapidly cooled to increase strength. This makes it possible to manufacture hot-pressed components with excellent strength and high dimensional accuracy.

[0004] Hot-pressed components are primarily used for automotive components, particularly for structural components that require high strength, i.e., for the interior panel frame. In recent years, they have also been used as semi-exterior panel components, such as the components around the pillars that are visible when the car door is opened. Therefore, hot-pressed components are required to be suitable for painting and to have excellent corrosion resistance in the cut areas after painting.

[0005] In addition, while hot-pressed components have typically had 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-pressed components are highly sensitive to hydrogen and easily embrittled by hydrogen penetration. Therefore, more advanced measures against hydrogen embrittlement are required to increase the strength of hot-pressed components. Examples of hydrogen that penetrates during the manufacturing process of hot-pressed components include hydrogen introduced during the production of steel sheets for hot pressing, hydrogen that penetrates during the hot pressing process, and hydrogen that penetrates during the painting process.

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

[0008] In response to this, Patent Document 1 proposes that an alkaline earth metal or a transition metal be contained in an amount of 0.1 to 0.5 mass % and that an oxide of the alkaline earth metal or the transition metal be formed on the outer surface of the Al-based plating layer during hot pressing. In this method, the Al-based plating layer is covered with an oxide film of an element having a high oxygen affinity, such as Mg, which suppresses the reaction between the Al-based plating layer and moisture during the hot pressing step, thereby reducing the penetration of hydrogen.

[0009] Special table 2017-536472 publication

[0010] When an Al-based plated steel sheet is hot-pressed to form a hot-pressed member, the coating layer of the hot-pressed member is an Fe-Al alloy layer. Because the Fe-Al alloy layer is very hard and brittle, many cracks that expose the base material may occur, and these cracks may become the starting points for corrosion, resulting in a deterioration of corrosion resistance.

[0011] The above-mentioned Patent Document 1 also has a problem in that many cracks that expose the base material occur in the coating layer of the hot-pressed member, and corrosion at the cracked parts starts, reducing the corrosion resistance of the cut part. Furthermore, since the above-mentioned alkaline earth metal or transition metal oxide is formed in the outermost layer of the coating layer in the heating step before hot pressing, there is a problem in that the cross section of the coating layer exposed in the cracked parts does not have sufficient protection against moisture.

[0012] From the above, it is considered important to minimize the occurrence of cracks in the coating layer of the hot-pressed member in order to suppress the deterioration of the corrosion resistance of the hot-pressed member and the penetration of hydrogen.

[0013] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a hot-pressed part that achieves both high strength and cut corrosion resistance and hydrogen embrittlement resistance, and a hot-press steel plate for manufacturing the hot-pressed part.

[0014] As a result of investigations to solve the above problems, the inventors have arrived at the present invention relating to the following essential configuration: That is, the present invention provides a steel sheet and a coating layer disposed on at least one surface of the steel sheet, wherein the coating layer has a component composition containing, in mass %, at least one of Mg: 0.1 to 5.0% and Ca: 0.005 to 1.0%, Si: 3.0 to 15.0%, and Fe: 55.0% or less, with the balance 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 the cracks observed within the range of L ｔｃ In this case, L ｔｃ / L ｗ ≦1.

[0015] The present invention also provides a steel sheet and a plating layer disposed on at least one surface of the steel sheet, the plating layer comprising an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, the metal layer having a component composition containing, by mass%, 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 balance being Al and unavoidable impurities, and the metal layer has a maximum length L of a crystal grain 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 and the thickness of the metal layer is L ｈ In this case, L ｃ ≧L ｈ The steel sheet for hot pressing is characterized in that the area ratio of crystal grains having the above crystal grain structure is 50% or more.

[0016] According to the present invention, it is possible to provide a hot-pressed member that achieves both high strength, corrosion resistance in cut portions, and hydrogen embrittlement resistance, and a steel sheet for hot-pressing that is provided with an Al-based plating layer.

[0017] 1 is a diagram showing a cross section perpendicular to the surface of a steel plate of a hot-pressed member according to the present invention. 2 is a diagram showing a cross section perpendicular to the steel plate of a steel plate for hot press-forming according to the present invention. 3 is a graph showing the relationship between cooling time and steel plate temperature when producing a steel plate for hot press-forming according to the present invention.

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

[0019] A hot-pressed member according to one embodiment of the present invention includes a steel plate and a coating layer provided on at least one surface of the steel plate. Each part will be described below.

[0020] [Steel Sheet] In this embodiment, the above-mentioned problem is solved by controlling the structure of the coating layer as described below. Therefore, the steel sheet is not particularly limited and any steel material can be used, and either a cold-rolled steel sheet or a hot-rolled steel sheet can be used.

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

[0022] Hot-pressed members preferably have high strength from the viewpoint of use as automotive parts, etc. In particular, to obtain hot-pressed members having a tensile strength of more than 1.8 GPa, it is preferable to use a steel sheet having the following composition: C: 0.05 to 0.50%, Si: 0.1 to 1.5%, Mn: 0.5 to 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 balance being Fe and unavoidable impurities.

[0023] The effects and preferred contents of each element in the above-mentioned preferred composition of the steel sheet will be described below.

[0024] C: 0.05 to 0.50% C is an element that improves strength by forming a structure such as martensite. From the viewpoint of obtaining strength exceeding 1.8 GPa, the C content is preferably 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 welds deteriorates. Therefore, the C content is preferably 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 to 1.5% Si is an element that is effective in strengthening steel to obtain 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%, ferrite is stabilized, resulting in a decrease in 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 to 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, the Mn content is preferably 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%, not only will the cost increase, but the effect of adding Mn will saturate. Therefore, the Mn content is preferably 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 P content is excessive, local ductility deteriorates due to grain boundary embrittlement caused by P segregation to austenite grain boundaries during casting. As a result, the balance between strength and ductility of the steel sheet deteriorates. Therefore, from the viewpoint of improving the balance between strength and ductility of the steel sheet, it is preferable that the P content be 0.1% or less. On the other hand, from the viewpoint of refining costs, it is preferable that the P content be 0.01% or more.

[0028] S: 0.01% or less S becomes inclusions such as MnS, which causes deterioration of impact resistance and cracking along the metal flow of welds. Therefore, it is desirable to reduce the S content as much as possible, specifically, it is preferable to set it to 0.01% or less. Furthermore, from the viewpoint of ensuring good stretch flangeability, it is more preferable to set it to 0.005% or less, and even more preferable to set it to 0.001% or less. On the other hand, from the viewpoint of refining costs, it is preferable to set the S content to 0.0002% or more.

[0029] Al: 0.10% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.10%, the blanking workability and hardenability of the steel sheet deteriorate. Therefore, the Al content is preferably 0.10% or less, more preferably 0.07% or less, and even more preferably 0.04% or less. On the other hand, from the viewpoint of ensuring the effect as a deoxidizer, the Al content is preferably 0.01% or more.

[0030] N: 0.01% or less If the N content exceeds 0.01%, nitrides of AlN are formed during hot rolling or heating before hot pressing, which reduces the blanking workability and hardenability of the steel sheet. Therefore, the N content is preferably 0.01% or less. On the other hand, from the viewpoint of refining costs, the N content is preferably 0.001% or more.

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

[0032] Nb: 0.10% or less Nb is an effective component for strengthening steel, but excessive Nb content increases the rolling load. Therefore, when Nb is added, the Nb content is preferably 0.10% or less, and more preferably 0.05% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%, but when used to strengthen steel, the Nb content is preferably 0.005% or more.

[0033] Ti: 0.05% or less Like Nb, Ti is an effective component for strengthening steel, but if it is contained in excess, shape fixability decreases. Therefore, when Ti is added, the Ti content is preferably 0.05% or less, and more preferably 0.03% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but when used to strengthen steel, the Ti content is preferably 0.005% or more.

[0034] B: 0.0002 to 0.005% B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. Therefore, when B is added, the B content is preferably 0.0002% or more, and more preferably 0.0010% or more. On the other hand, since excessive addition of B reduces formability, the B content is preferably 0.005% or less, and more preferably 0.003% or less.

[0035] Cr: 0.1 to 1.0% Cr, like Mn, is a useful element for strengthening steel and improving its hardenability. Therefore, when adding Cr, in order to obtain the above-mentioned effect, the Cr content is preferably 0.1% or more, and more preferably 0.2% or more. On the other hand, since Cr is an expensive element, adding excessive Cr leads to a significant increase in costs, so the Cr content is preferably 1.0% or less, and more preferably 0.6% or less.

[0036] Sb: 0.003 to 0.03% Sb is an element that has the effect of suppressing decarburization of the surface layer of a steel sheet during the annealing process in the production of a base steel sheet. When Sb is added, in order to obtain this effect, the Sb content is preferably 0.003% or more, and more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, resulting in a decrease in productivity. Therefore, the Sb content is preferably 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 has 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 the coating layer be provided on both surfaces.

[0038] The coating layer has an Fe—Al alloy phase as a main layer and contains at least one of Mg and Ca, and Si. As described below, such a coating layer is obtained by hot pressing a steel sheet for hot pressing having an Al-based plating layer. In the hot pressing process, at least one of Mg and Ca contained in the coating layer is oxidized in the surface layer, and the coating layer is covered with an oxide film of at least one of the oxidized Mg and Ca, thereby suppressing the penetration of hydrogen from the surface layer of the coating layer. The effects and suitable contents of each element in the preferred component composition of the coating layer will be described below.

[0039] Mg: 0.1 to 5.0% If the Mg content in the coating layer is less than 0.1%, the Mg oxide film does not adequately cover the coating layer, and the hydrogen penetration suppression effect during the hot pressing process is not sufficiently achieved. Therefore, the Mg content is set to 0.1% or more. If the Mg content exceeds 5.0%, lumpy Mg oxides are formed in the outermost layer of the coating layer. The lumpy Mg oxides are likely to react with moisture during the hot pressing process, and the expected hydrogen penetration suppression effect is not achieved. Therefore, the Mg content is set to 5.0% or less.

[0040] Ca: 0.005 to 1.0% Like Mg, Ca forms an oxide film on the surface of the coating layer and suppresses hydrogen penetration. Therefore, in the present invention, the Ca content in the coating layer is set to 0.005% or more, at which point the expected hydrogen penetration suppression effect can be obtained. On the other hand, if the Ca content exceeds 1.0%, the reaction with moisture in the aggregated Ca oxides is accelerated during the hot pressing process, as in 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 to 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 poor cut corrosion resistance and hydrogen embrittlement resistance. Therefore, the Si content is set to 3.0% or more. On the other hand, if the Si content is excessive, the amount of Si-based oxides produced increases, resulting in poor coating adhesion and, as a result, poor corrosion resistance. Therefore, the Si content is set to 15.0% or less.

[0042] Fe: 55.0% or less. Because Fe diffuses from the steel sheet into the plating layer during hot-dip plating and heating before hot pressing, Fe is inevitably contained in the coating layer of the hot-pressed member. However, as the Fe content increases, the Al concentration in the coating layer decreases, making the coating layer brittle and prone to cracking, and deteriorating the cut corrosion resistance and hydrogen embrittlement resistance. Therefore, the Fe content is set to 55.0% or less. On the other hand, the lower limit of the Fe content is not particularly limited, but when manufactured under general conditions, the Fe content may be 20.0% or more.

[0043] Next, cracks occurring in the coating layer will be explained using the drawings. FIG. 1 is a diagram schematically showing a cross section perpendicular to the surface of the steel plate 2 of a hot-pressed member 1 according to this embodiment. The hot-pressed member 1 in FIG. 1 comprises a steel plate 2 and a coating layer 3 disposed on one surface of the steel plate 2, and multiple cracks 4 are observed in the coating layer 3. The coating layer 3 may be provided on both sides of the steel plate 2.

[0044] In this embodiment, the length of the coating layer 3 in the direction parallel to the surface of the steel plate 2 in FIG. ｗ (Hereinafter, the parallel length L ｗ ) and its parallel length L ｗ The total crack length index of the cracks observed within the range of L ｔｃ In this case, L ｔｃ / L ｗ ≦1 must be satisfied.

[0045] This is L ｔｃ / Lｗ If the value is >1, a part of the steel plate 2 serving as the base material will be exposed without being covered by the coating layer 3, or a cross section of the coating layer 3 not covered by the oxide film will be exposed at the crack 4, making it impossible to obtain the expected corrosion resistance and hydrogen embrittlement resistance of the cut portion.

[0046] Total crack length index L of the coating layer 3 ｔｃ and parallel length L ｗ L, which is the ratio of ｔｃ / L ｗ can be obtained by observing a cross section of the hot-pressed member 1 perpendicular to the surface of the steel plate 2 with a scanning electron microscope (SEM) and analyzing the obtained image. That is, the cross section of the coating layer 3 is observed in 10 randomly selected visual fields, and the total crack length index L, which is the total length of the cracks 4 generated in the cross section of the coating layer 3, is calculated for each visual field. ｔｃ and the parallel length L ｗ and L ｔｃ / L ｗ Calculate the calculated L ｔｃ / L ｗ The average value of 10 fields of view is calculated as the L ｔｃ / L ｗ The cross section of the coating layer 3 is observed with a parallel length L ｗ The thickness is to be 500 μm or more.

[0047] The amount of coating layer 3 is not particularly limited, but from the viewpoint of corrosion resistance, the amount of coating layer 3 is set to 60 g / m per one side of the steel sheet 2. ２ On the other hand, from the viewpoint of manufacturing costs, the coating amount is preferably 400 g / m per one side of the steel sheet 2. ２ The amount of coating layer 3 applied can be determined by dissolving and removing the coating layer 3 from the surface of the hot-pressed member 1 using an acid solution, and subtracting the weight of the hot-pressed member 1 after removal from the weight of the hot-pressed member 1 before removal. An inhibitor that inhibits dissolution of the steel plate 2 is added to the acid solution.

[0048] (2) Steel Plate for Hot Pressing Next, the steel plate for hot pressing of the present invention will be described. The following describes one preferred embodiment, and the steel plate for hot pressing of the present invention is not limited to this embodiment. Furthermore, the unit of content, "%", represents "mass%" unless otherwise specified. The hot press member described in (1) above can be produced by hot pressing the steel plate for hot pressing of the present embodiment having a plating layer.

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

[0050] [Steel Plate] The steel plate is not particularly limited and any steel material can be used. The steel plate may be either a cold-rolled steel plate or a 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 listed in the description of the hot-pressed member in (1) above.

[0051] [Plating layer] The steel sheet for hot press use of this embodiment has a plating layer on at least one surface of the steel sheet. The plating layer has 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 the plating layer be provided on both surfaces of the steel sheet.

[0052] [Interface alloy layer] A steel sheet for hot pressing is typically produced by hot-dip galvanizing the steel sheet, as described below. During this process, Fe, Mn, etc. contained in the steel sheet react with components such as Al, Si, etc. contained in the galvanizing bath to form an interface alloy layer at the interface between the steel sheet and the metal layer. In this embodiment, the composition of the interface 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 steel sheet for hot press use of this embodiment to satisfy the above-mentioned conditions, the metal layer of the steel sheet for hot press use must have the following composition: Mg: 0.2 to 7.0% and / or Ca: 0.01 to 1.5%, Si: 1.0 to 10.0%, and Fe: 10% or less, with the balance being Al and unavoidable impurities.

[0054] Furthermore, in the finally obtained 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 FIG. ｔｃ is the parallel length L ｗ Below, that is, L ｔｃ / L ｗ In order to satisfy the condition of ≦1, the following conditions must be satisfied. That is, as shown in the diagram of Fig. 2 which schematically shows a cross section of a steel sheet for hot press use 5, (a) the steel sheet for hot press use 5 comprises a steel sheet 2 as a base material and a plating layer 7 disposed on at least one surface of the steel sheet 2, and 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 steel sheet for hot press use 5 has a maximum length L of crystal grains 8 contained in the metal layer 7B in a direction parallel to the surface of the steel sheet 2 in a cross section perpendicular to the surface of the steel sheet 2. ｃ and the thickness of the metal layer 7B is L ｈ In this case, L ｃ ≧L ｈ The area ratio of the crystal grains 8 having the above-mentioned crystal grain shape is 50% or more.

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

[0056] In the metal layer 7B, L ｃ ≧L ｈ The area ratio of the crystal grains 8, which is the area ratio of the steel sheet 5 for hot press work, can be determined by observing a cross section perpendicular to the surface of the steel sheet 2 with a scanning electron microscope (SEM) and analyzing the obtained image with electron beam scattering diffraction (EBSD). That is, the cross section of the coating layer 7 is observed in three randomly selected visual fields, and for each visual field, 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 calculated. ｃ and the thickness L of the metal layer 7B. ｈ The relationship with ｃ ≧L ｈ The area ratio occupied by the crystal grains 8 satisfying the above condition is calculated. The average value of the calculated area ratios in the three fields of view is used as the L value of the steel sheet for hot press use.ｃ ≧L ｈ The cross section of the plating layer 7 is observed so that the length in the direction parallel to the surface of the steel sheet 2 is 200 μm or more.

[0057] In order for the crystal grains 8 contained in the metal layer 7B of the hot-press steel sheet 5 to satisfy the above conditions, cooling under specific conditions is required during the production of the hot-press steel sheet 5. Specifically, as shown in FIG. 3 , after the steel sheet 2 is pulled up from the hot-dip galvanizing bath, a cooling and holding process is performed, in which the steel sheet 2 is held at a temperature ranging from 660°C to 500°C. By providing the cooling and holding process, the crystal grains 8 in the metal layer 7B can be coarsened during the solidification process. 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-pressed member 1 can be reduced. Note that the cooling and holding time is set to 15 seconds or more. This is because if the cooling and holding time is less than 15 seconds, the crystal grains 8 are not coarsened sufficiently, and as a result, the occurrence of cracks 4 in the coating layer 3 of the hot-pressed member 1 cannot be reduced. Furthermore, if the cooling and holding time is less than 500°C, the crystal grains 8 do not coarsen easily, requiring a longer holding time, which poses challenges in production efficiency and manufacturing costs.

[0058] (3) Manufacturing method of hot-pressed member Next, a preferred manufacturing method of 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 steel sheet for hot-pressing that satisfies the above-mentioned conditions. In particular, it is important that the steel sheet for hot-pressing used is one that has been cooled (cooling and holding process) under specific conditions after plating as described above, thereby coarsening the crystal grains in the metal layer.

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

[0060] [Heat Treatment Step] In the heat treatment step, the steel sheet for hot pressing is ３ The plated layer is heated to a temperature between the transformation point (°C) and 980°C. This is called heating to the austenite region. This heat treatment process causes the plated layer to be alloyed (to form an Fe-Al alloy phase). ３ By heating the steel sheet at a temperature above the transformation point (°C), the structure of the steel sheet that will be the base material for the hot press steel sheet can be austenitized. The austenite then becomes a hard phase such as martensite by rapid cooling during the hot press. As a result, the strength of the hot press member can be increased. ３ If the heating temperature is lower than the transformation point (°C), the austenite fraction in the heated steel sheet decreases. As a result, the volume fraction of martensite becomes insufficient after hot pressing, making it impossible to ensure sufficient tensile strength. 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, making it impossible to obtain the expected corrosion resistance. Furthermore, if the heating temperature is higher than 980°C, cracks will occur significantly in the coating layer of the hot-pressed member, making it impossible to obtain the expected hydrogen embrittlement resistance. Therefore, it is not possible to obtain a hot-pressed member that meets the conditions of the present invention.

[0061] In addition, Ac ３ The transformation point (°C) can be calculated by the following formula (I): ３ Transformation point (°C) = 881 - 206C + 53Si - 15Mn - 20Ni - 1Cr - 27Cu + 41Mo (I) where the element symbols in formula (I) represent the content of each element expressed in mass%. The content of elements that are not contained is calculated as 0.

[0062] In the heat treatment process, the steel sheet is heated to the above-mentioned heating temperature and then 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, by providing a holding time, the alloying of the coating layer can be ensured. If the alloying of the coating layer is insufficient, a pure Al phase will remain in the coating layer, which may cause 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 obtained. Furthermore, hydrogen penetration into the steel sheet during heating may progress, which may prevent the desired hydrogen embrittlement resistance from being obtained. Therefore, the holding time in the heat treatment process is set to 5 minutes or less.

[0063] The method for heating the steel sheet for hot press use in the heat treatment step is not particularly limited, and any method can be used. For example, heating can be performed using a heating furnace, electrical heating, induction heating, high-frequency heating, flame heating, etc. Note that any heating furnace, such as an electric furnace or a gas furnace, can be used as the heating furnace.

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

[0065] In order to confirm the effects of the present invention, a steel plate for hot press-use and a hot press-formed member using the steel plate for hot press-use were produced, and their properties were evaluated.

[0066] <Steel Sheet for Hot Pressing> A plating layer was formed on the surface of a steel sheet serving as a base material by the following procedure, to prepare a steel sheet for hot pressing. Specifically, a plating layer was formed on both sides of a steel sheet with a thickness of 1.4 mm using a continuous hot-dip galvanizing facility. The steel sheet used was a cold-rolled steel sheet having a chemical composition containing 0.34% C, 0.25% Si, 1.20% Mn, 0.005% P, 0.001% S, 0.03% Al, 0.004% N, 0.02% Ti, 0.002% B, 0.18% Cr, and 0.008% Sb, with the balance being Fe and unavoidable impurities. The Ac content of the cold-rolled steel sheet was ３ The transformation point is 783°C. The temperature of the plating bath is 640°C, and the coating weight of the plating layer is 60 g / m per side of the steel sheet. ２ , i.e., 120 g / m on both sides in total ２ In forming the coating layer, a cooling and holding step was provided and cooling was carried out under the conditions shown in Table 1. After the steel sheet was pulled out of the hot dip coating bath, cooling was carried out with nitrogen gas.

[0067] (Composition of Metal Layer) The composition of the obtained metal layer was measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). In the 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 performed at an acceleration voltage of 15.0 kV. The results obtained are shown in Table 1.

[0068] (Metal Layer Crystal Grains) Next, the crystal grain distribution of the metal layer was measured by the EBSD method using an SEM. Analysis was performed at an acceleration voltage of 15.0 kV using an SEM (JSM-7200F) manufactured by JEOL Ltd. and an EBSD detector (Digiview iv) manufactured by TSL Ltd. Specifically, the cross section of the hot press steel sheet was observed at a magnification of 500 times using an SEM, and analysis was performed by the EBSD method from three randomly selected fields of view to measure the crystal grain distribution. Furthermore, in the crystal grain distribution, the maximum length L in the direction parallel to the surface of the steel sheet was measured. ｃ is the thickness of the metal layer L ｈ The area ratio of the cross section of the metal layer occupied by crystal grains having a grain size of 100% or more was measured, and the average value of the three fields of view was taken as the L value of the test steel sheet. ｃ ≧L ｈThe area ratios obtained 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] [Example 2]

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

[0071] (Component Composition of 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 analysis). 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 cracks occurring in the coating layer was calculated. ｔｃ and the parallel length L ｗ and were measured by the SEM. ｔｃ / L ｗ Specifically, the cross section of the hot-pressed member was observed at a magnification of 500 times using an SEM, and SEM images of 10 randomly selected fields were obtained. From the obtained SEM images, the L for each field was calculated. ｔｃ / L ｗ The average value of 10 fields of view was calculated as the L ｔｃ / L ｗ The results are shown in Table 2.

[0073] (Cut Corrosion Resistance) Next, the cut corrosion resistance of each of the obtained hot-pressed members was evaluated using the following procedure. Test specimens for corrosion resistance evaluation were prepared by subjecting test specimens taken from the hot-pressed members to a phosphate-based chemical conversion treatment and electrodeposition coating. Cross-cut scratches (60° angle), each 80 mm long and measuring a total of 160 mm, were made in the center of the corrosion resistance evaluation test specimens, and then subjected to a corrosion test (SAE-J2334). Cut corrosion resistance was evaluated based on the occurrence of red rust after 30 cycles using the following criteria. The evaluation results are shown in Table 2. Rating 3: The length of the cut scratch where red rust occurred was less than 2 mm. Rating 2: The length of the cut scratch where red rust occurred was 2 mm or more but less than 4 mm. Rating 1: The length of the cut scratch where red rust occurred was 4 mm or more. A rating of 2 or higher was considered to be sufficient cut corrosion resistance.

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

[0075] As can be seen from the results shown in Table 2, the hot-pressed parts satisfying the conditions of the present invention had excellent corrosion resistance in the cut portion, and also had a low amount of diffusible hydrogen in the steel immediately after hot pressing, and thus had resistance to hydrogen embrittlement.

[0076]

[0077] REFERENCE SIGNS LIST 1 Hot-pressed member 2 Steel plate 3 Coating layer 4 Crack 5 Steel plate for hot pressing 7 Plating layer 7A Interface alloy phase 7B Metal layer 8 Crystal grain

Claims

1. A steel plate and a coating layer disposed on at least one surface of the steel plate, the coating layer having a composition containing, by mass%, at least one of Mg: 0.1-5.0% and Ca: 0.005-1.0%, Si: 3.0-15.0%, and Fe: 55.0% or less, with the balance being Al and unavoidable impurities, the coating layer having 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 the cracks observed within the range of L ｔｃ In this case, L ｔｃ / L ｗ A hot-pressed member characterized in that it satisfies ≦1.

2. A steel sheet and a plating layer disposed on at least one surface of the steel sheet, the plating layer comprising an interface alloy layer disposed on the steel sheet and a metal layer disposed on the interface alloy layer, the metal layer having a composition containing, by mass%, at least one of Mg: 0.2-7.0% and Ca: 0.01-1.5%, Si: 1.0-10.0%, and Fe: 10% or less, with the balance being Al and unavoidable impurities, the metal layer having a maximum length in a direction parallel to the surface of the steel sheet of a crystal grain contained in the metal layer in a cross section perpendicular to the surface of the steel sheet, L ｃ The thickness of the metal layer is L ｈ In this case, L ｃ ≧L ｈ The steel sheet for hot pressing is characterized in that the area ratio of crystal grains having the above crystal grain shape is 50% or more.

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