Hot-stamped steel sheet and method of manufacturing same, and hot-stamped component and method of manufacturing same

Abstract

This invention relates to hot-stamping steel sheets and methods for manufacturing the same, as well as hot-stamping components and methods for manufacturing the same. The hot-stamping steel sheet of this invention has a predetermined composition and a microstructure comprising ferrite and cementite, wherein the Mn concentration of the ferrite and cementite is set to Mnα and Mnθ, respectively, and the Mnθ / Mnα ratio is 1.4 or higher.

Description

[0001] This application is a divisional application of the invention patent application filed on January 13, 2017, with application number 201780019325.6 (international application number PCT / JP2017 / 001110) and entitled "Hot stamping steel plate and manufacturing method thereof and hot stamping component and manufacturing method thereof". Technical Field

[0002] This invention relates to steel sheets for hot pressing and methods for manufacturing the same, as well as hot-stamped components and methods for manufacturing the same. It should be noted that hot-stamped components refer to stamped components formed by hot pressing steel sheets. Background Technology

[0003] In recent years, from the perspective of protecting the Earth's environment, there has been a strong desire to improve the fuel efficiency of automobiles. Therefore, there is a demand for lightweight automobile bodies. Consequently, in order to maintain safety even when reducing the thickness of automotive components, there is a requirement for high-strength steel sheets used as raw materials for these components.

[0004] However, generally speaking, as the strength of steel sheets increases, their formability decreases. Therefore, in the manufacturing of automotive components using high-strength steel sheets as raw materials, problems such as difficulty in forming or poor shape retention arise.

[0005] To address this problem, a technology for manufacturing high-strength automotive components using hot stamping has been proposed. Here, hot stamping refers to the following process: Steel sheets are heated to the austenitic region and then transported to a stamping press. Inside the press, i.e., in a die, the component is formed into the desired shape while being rapidly cooled. Furthermore, during the cooling process (rapid cooling) within the die, the microstructure of the component transforms from austenitic to martensite, thereby obtaining a high-strength component of the desired shape. It should be noted that "hot stamping process (forming)" is also known as "hot forming," "hot stamping," or "die hardening," etc.

[0006] Furthermore, recently, from the perspective of ensuring the safety of passengers, there is a desire to improve the impact resistance of automotive components. To meet this desire, it is necessary to improve the ability to absorb energy during a collision (impact energy absorption capacity). From this perspective, to prevent automotive components from breaking during a collision and thus reducing impact energy absorption capacity, improving the uniform elongation of automotive components is effective. Therefore, there is a strong desire to develop hot-stamped components with high strength and excellent uniform elongation.

[0007] To address such expectations, for example, Patent Document 1 discloses a hot-stamped formed article, which is a hot-stamped formed article obtained by stamping a thin steel sheet by a hot-stamping method. This article has a composition, by mass percent, containing C: 0.15–0.35%, Si: 0.5–3%, Mn: 0.5–2%, P: less than 0.05%, S: less than 0.05%, Al: 0.01–0.1%, Cr: 0.01–1%, B: 0.0002–0.01%, Ti: (N content) × 4–0.1%, N: 0.001–0.01%, with the balance being Fe and unavoidable impurities. It also has a microstructure, by area percent, containing 80–97% martensite, 3–20% retained austenite, and 5% or less of the remaining microstructure. The document states that, according to this technology, by appropriately controlling the forming conditions, the microstructure of the hot-stamped part can be made into a metallic microstructure with an appropriate amount of retained austenite, thereby obtaining a hot-stamped part with further improved inherent ductility of the formed product.

[0008] Furthermore, Patent Document 2 discloses a hot-stamped component with excellent ductility, comprising, by mass percent, C: 0.20–0.40%, Si: 0.05–3.0%, Mn: 1.0–4.0%, P: less than 0.05%, S: less than 0.05%, Al: 0.005–0.1%, N: less than 0.01%, with the balance being Fe and unavoidable impurities; and a microstructure in which the ferrite phase occupies an area fraction of 5–55%, the martensite phase occupies an area fraction of 45–95%, and the average grain size of the ferrite and martensite phases is 7 μm or less. It is described that, according to this technology, a hot-stamped component with high strength (TS: 1470–1750 MPa) and high ductility (E1: 8% or more) can be obtained.

[0009] Furthermore, with the increase in component strength, there are concerns about hydrogen-induced damage, i.e., hydrogen embrittlement, which also necessitates improved resistance to delayed failure.

[0010] To address such requirements, for example, Patent Document 3 discloses a hot stamping steel sheet, characterized in that, in the hot stamping steel sheet containing S: 0.001 to 0.005%, REM: 0.005 to 0.03%, and O: 0.003 to 0.007% by mass, spherical inclusions with a diameter of less than 0.1 μm containing two or more of S, O (oxygen), and REM are dispersed.

[0011] In addition, Patent Document 4 discloses a steel sheet for hot stamping, which contains, by mass %: C: 0.1-0.5%, Si: 0.05-2%, Mn: 0.1-3%, Al: 0.003-2%, and is limited to P: less than 0.05%, S: less than 0.03%, N: less than 0.01%, and Si+Al≥1.0%, while appropriately adjusting the hardenability index that depends on the chemical composition.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2013-79441

[0015] Patent Document 2: Japanese Patent Application Publication No. 2010-65293

[0016] Patent Document 3: Japanese Patent Application Publication No. 2012-237048

[0017] Patent Document 4: Japanese Patent Application Publication No. 2012-41613 Summary of the Invention

[0018] The problem that the invention aims to solve

[0019] However, in the technologies described in Patent Documents 1 and 2, the high strength of the hot-stamped component is achieved through the martensitic strengthening brought about by C. Therefore, when it is desired to further improve the tensile strength using this, from the viewpoint of improving the impact energy absorption capacity, the required uniform elongation is sometimes not obtained. In addition, these technologies do not consider the characteristics of resistance to delayed failure.

[0020] Furthermore, the technology described in Patent Document 3 also has the following problem: from the point of view of improving the ability to absorb impact energy, it may not be possible to obtain the required uniform elongation.

[0021] Furthermore, regarding the technology described in Patent Document 4, if the tensile strength of the hot-stamped component is to be increased to more than 1800 MPa, the retained austenite cannot be fully generated. As a result, there are problems such as not obtaining the desired resistance to delayed failure and the desired uniform elongation.

[0022] The present invention was developed in view of the above-mentioned situation, and its object is to provide a hot-stamped steel sheet and an advantageous manufacturing method thereof that can simultaneously obtain high strength with tensile strength TS: 1800 MPa or more, high ductility with uniform elongation uEl: 6.0% or more and excellent resistance to delayed failure in hot-stamped components obtained by hot stamping.

[0023] In addition, the present invention aims to provide a hot-stamped component and a method thereof that have both high strength with tensile strength TS: 1800MPa or more, high ductility with uniform elongation uEl: 6.0% or more, and excellent resistance to delayed failure.

[0024] It should be noted that the "excellent resistance to delayed failure" mentioned here means that no cracks are produced even after immersion in the test solution for 100 hours (preferably 200 hours) in the evaluation of resistance to delayed failure (four-point bending test method) described later.

[0025] Methods for solving problems

[0026] In order to obtain hot-stamped components that have both high strength (tensile strength TS: 1800 MPa or more), high ductility (uniform elongation uEl: 6.0% or more), and excellent resistance to delayed failure, the inventors studied various factors, especially those affecting uniform elongation uEl and resistance to delayed failure, and obtained the following insights.

[0027] (A) To achieve a tensile strength TS of 1800 MPa or higher and a uniform elongation uEl of 6.0% or higher, a microstructure with an appropriate amount of retained austenite is required. Furthermore, to form a microstructure with an appropriate amount of retained austenite at a C content of less than 0.300% by mass, a Mn content of 3.50% by mass or higher is required. It should be noted that Mn also contributes to the strength increase, ensuring the desired strength even when the C content is less than 0.300% by mass.

[0028] (B) Retained austenite acts as a hydrogen absorption site, contributing to improved resistance to delayed failure. Therefore, to improve resistance to delayed failure, it is important to obtain a microstructure with appropriately sized retained austenite in the hot-stamped component obtained by hot stamping.

[0029] (C) In addition, in order to obtain a microstructure with a suitable size of residual austenite in hot-stamped components, it is important to increase the amount of Mn in the hot-stamped steel sheet as the raw material, and to perform heat treatment after cold rolling, heating to a temperature range above Ac1-150°C and below Ac1 point and holding it within this temperature range for a predetermined time, so that Mn is enriched in cementite.

[0030] Furthermore, the cementite enriched with Mn acts as a site for the formation of retained austenite, resulting in a microstructure with appropriately sized retained austenite being obtained in hot-stamped components made from such hot-stamped steel sheets.

[0031] This invention was completed after further research based on the above insights.

[0032] That is, the main structure of the present invention is as follows.

[0033] 1. A hot-stamping steel sheet having the following composition: by mass % containing, C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01-1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005-0.1%, and N: less than 0.01%, with the balance consisting of Fe and unavoidable impurities.

[0034] It also has the following structure: it contains ferrite and cementite, and when the Mn concentration of the ferrite and the cementite is set to Mnα and Mnθ respectively, the Mnθ / Mnα ratio is 1.4 or higher.

[0035] 2. The hot stamping steel sheet as described in 1 above, wherein the above composition, by mass percent, further contains one or more groups selected from groups A to E below.

[0036] Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%.

[0037] Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, V: 0.005–3.0%, and W: 0.005–3.0%.

[0038] Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%.

[0039] Group D: Sb: 0.002–0.03%

[0040] Group E: B: 0.0005%

[0041] 3. The hot-stamping steel sheet as described in 1 or 2 above, wherein a coating is present on the surface.

[0042] 4. The hot stamping steel sheet as described in 3 above, wherein the coating is a Zn-based coating or an Al-based coating.

[0043] 5. The hot-stamping steel sheet as described in 4 above, wherein the Zn-based coating contains 10 to 25% by mass of Ni.

[0044] 6. A method for manufacturing a hot-stamping steel sheet, wherein a steel billet having a composition comprising, by mass percent, C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01 to 1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005 to 0.1%, and N: less than 0.01%, with the balance being Fe and unavoidable impurities, is heated and hot-rolled to produce a hot-rolled steel sheet; the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet; and the cold-rolled steel sheet is further subjected to annealing by heating the cold-rolled steel sheet to a temperature range of Ac1-150°C or more and less than Ac1, holding it in the temperature range for more than 1 hour, and then cooling it.

[0045] 7. The method for manufacturing hot-stamped steel sheet as described in 6 above, wherein the above-mentioned composition, by mass percent, further contains one or more groups selected from groups A to E below.

[0046] Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%.

[0047] Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, V: 0.005–3.0%, and W: 0.005–3.0%.

[0048] Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%.

[0049] Group D: Sb: 0.002–0.03%

[0050] Group E: B: 0.0005%

[0051] 8. The method for manufacturing hot-stamped steel sheet as described in 6 or 7 above, wherein, after the annealing, a coating is formed on the surface of the hot-stamped steel sheet.

[0052] 9. The method for manufacturing hot-stamping steel sheet as described in 8 above, wherein the coating is a Zn-based coating or an Al-based coating.

[0053] 10. The method for manufacturing hot-stamping steel sheet as described in 9 above, wherein the Zn-based coating contains 10 to 25% by mass of Ni.

[0054] 11. The method for manufacturing hot-stamping steel sheet as described in any one of 8 to 10 above, wherein the coating amount is 10 to 90 g / m² per single side. 2.

[0055] 12. A hot-stamped component having the following composition: by mass % containing C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01-1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005-0.1%, and N: less than 0.01%, with the balance consisting of Fe and unavoidable impurities.

[0056] And it has the following structure: containing more than 70.0% martensite by volume and more than 3.0% and less than 30.0% retained austenite by volume, and with a content of 2.0 × 10⁻⁶. 5 pcs / mm 2 The above contains residual austenite with a diameter of 0.3 μm or more, based on the equivalent circle diameter.

[0057] 13. The hot-stamped component as described in 12 above, wherein the above-mentioned component composition, by mass percent, further contains one or more groups selected from groups A to E below.

[0058] Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%.

[0059] Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, V: 0.005–3.0%, and W: 0.005–3.0%.

[0060] Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%.

[0061] Group D: Sb: 0.002–0.03%

[0062] Group E: B: 0.0005%

[0063] 14. The hot-stamped component as described in 12 or 13 above, wherein a coating is provided on the surface.

[0064] 15. The hot-stamped component as described in 14 above, wherein the coating is a Zn-based coating or an Al-based coating.

[0065] 16. The hot-stamped component as described in 15 above, wherein the Zn-based coating contains 10 to 25% by mass of Ni.

[0066] 17. A method for manufacturing a hot-stamped component, wherein,

[0067] The raw material is a steel sheet containing, by mass percent, 0.180% or more and less than 0.300% C, 3.50% or more and less than 11.0% Mn, 0.01% to 1.5% Si, 0.05% or less P, 0.05% or less S, 0.005% to 0.1% Al and 0.01% or less N, with the balance being Fe and unavoidable impurities, and having a microstructure containing ferrite and cementite, wherein the Mn concentration of the ferrite and the cementite is set as Mnα and Mnθ respectively, and the Mnθ / Mnα ratio is 1.4 or more.

[0068] The steel plate is heated to a temperature range above Ac3 and below 1000°C, held within this temperature range for 10 seconds to 900 seconds, and then simultaneously stamped and quenched using a forming die.

[0069] A mixture containing at least 70.0% martensite by volume and at least 3.0% and less than 30.0% retained austenite by volume, and with a content of 2.0 × 10⁻⁶... 5 pcs / mm 2 The above refers to hot-stamped components with a residual austenite structure of 0.3 μm or more in terms of the equivalent circle diameter.

[0070] 18. The method for manufacturing a hot-stamped component as described in 17 above, wherein the above-mentioned component composition, by mass percent, further contains one or more groups selected from groups A to E below.

[0071] Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%.

[0072] Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, V: 0.005–3.0%, and W: 0.005–3.0%.

[0073] Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%.

[0074] Group D: Sb: 0.002–0.03%

[0075] Group E: B: 0.0005%

[0076] 19. The method for manufacturing a hot-stamped component as described in 17 or 18 above, wherein the steel plate has a coating on its surface.

[0077] 20. The method for manufacturing a hot-stamped component as described in 19 above, wherein the coating is a Zn-based coating or an Al-based coating.

[0078] 21. The method for manufacturing a hot-stamped component as described in 20 above, wherein the Zn-based coating contains 10 to 25% by mass of Ni.

[0079] Invention Effects

[0080] By using the hot-stamped steel sheet of the present invention as raw material, it is possible to manufacture hot-stamped components that have both high strength (tensile strength TS: 1800 MPa or more), high ductility (uniform elongation uEl: 6.0% or more), and excellent resistance to delayed failure.

[0081] Furthermore, by applying such hot-stamped components to automotive components, it is possible to design a vehicle body structure that absorbs collision energy during a collision and improve fuel efficiency through vehicle body lightweighting, thus having significant industrial benefits. Attached Figure Description

[0082] Figure 1 This is a schematic diagram of the test fixture used in the evaluation of resistance to delayed failure. Detailed Implementation

[0083] The present invention will now be described in detail.

[0084] First, the following explains the reasoning for limiting the composition of the hot-stamping steel sheet according to one embodiment of the present invention. It should be noted that the unit in the composition is "mass%", and unless otherwise specified, it will be referred to as "%".

[0085] C: Above 0.180% and below 0.300%

[0086] Carbon (C) is an element that increases the strength of steel. From the viewpoint of achieving this effect and ensuring a tensile strength (TS) of 1800 MPa or higher, the C content is set to 0.180% or higher. On the other hand, when the C content is 0.300% or higher, the solid solution strengthening effect produced by C becomes excessive, making it difficult to achieve a uniform elongation (uEl) of 6.0% or higher in the hot-stamped component, and making it difficult to adjust the tensile strength (TS) to below 2300 MPa. Therefore, the C content is set to 0.180% or higher and below 0.300%. Preferably, it is 0.200% or higher. Furthermore, it is preferably 0.285% or lower.

[0087] Mn: 3.50% or higher and less than 11.0%

[0088] Mn is an important element that increases the strength of steel and improves the stability of retained austenite in hot-stamped components. To achieve this effect while simultaneously ensuring a tensile strength (TS) of 1800 MPa or more and a uniform elongation (uEl) of 6.0% or more in the hot-stamped components, the Mn content needs to be set to 3.50% or more. On the other hand, when the Mn content is 11.0% or more, the solid solution strengthening effect produced by Mn becomes excessive, making it difficult to adjust the uniform elongation (uEl) of the hot-stamped components to 6.0% or more, and difficult to adjust the tensile strength (TS) to below 2300 MPa. Therefore, the Mn content is set to 3.50% or more and below 11.0%. Preferably, it is 4.00% or more, more preferably 4.50% or more, and even more preferably 5.00% or more. Furthermore, it is preferably 10.0% or less, more preferably 8.00% or less, and even more preferably 7.00% or less.

[0089] Si: ≥0.01% and ≤1.5%

[0090] Si is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, the Si content is set to 0.01% or more. On the other hand, when the Si content exceeds 1.5%, the precipitation of cementite during the annealing process in the manufacture of hot-stamped steel sheets is suppressed. Therefore, in hot-stamped components made from such hot-stamped steel sheets, it is difficult to form a microstructure with appropriately sized retained austenite, and the desired resistance to delayed failure is not obtained. Therefore, the Si content is set to 0.01% or more and 1.5% or less. Preferably, it is 0.02% or more. Furthermore, it is preferably 1.2% or less.

[0091] P: below 0.05%

[0092] Phosphorus (P) is an element that inevitably exists in steel as an impurity, segregating at grain boundaries and causing adverse effects such as reduced toughness of hot-stamped components. Therefore, P is preferably reduced as much as possible, but can be as low as 0.05%. Thus, the P content is set to 0.05% or less, preferably 0.02% or less. However, excessive P removal treatment leads to increased refining costs; therefore, the P content is preferably set to 0.0005% or more.

[0093] S: less than 0.05%

[0094] Sulfur (S) is inevitably present in steel, existing as sulfide inclusions that reduce the ductility and toughness of hot-stamped components. Therefore, it is preferable to reduce S as much as possible, but it is permissible to be as low as 0.05%. Thus, the S content is set to 0.05% or less, preferably 0.005% or less. However, excessive desulfurization treatment leads to increased refining costs; therefore, the S content is preferably set to 0.0005% or more.

[0095] Al: 0.005–0.1%

[0096] Al is an element that functions as a deoxidizer. To achieve this effect, the Al content is set to 0.005% or more. On the other hand, when the Al content exceeds 0.1%, it combines with nitrogen to form a large amount of nitrides, which reduces the workability and hardenability of the hot-stamping steel sheet used as a raw material. Therefore, the Al content is set to 0.005% or more and 0.1% or less. Preferably, it is 0.02% or more. Furthermore, it is more preferably 0.05% or less.

[0097] N: less than 0.01%

[0098] Nitrogen (N) is unavoidably present in steel, but when the N content exceeds 0.01%, nitrides such as AlN are formed during hot rolling and hot stamping, reducing the workability and hardenability of the hot-stamping steel sheet used as raw material. Therefore, the N content is set to 0.01% or less, preferably 0.0050% or less.

[0099] It should be noted that, without special adjustments and where nitrogen is unavoidably present, the nitrogen content is approximately 0.0025%. Excessive nitrogen removal would lead to a significant increase in refining costs. Therefore, the nitrogen content is preferably set to 0.0025% or higher, and more preferably 0.0030% or higher.

[0100] In addition, based on the above basic ingredients, it may further contain one or more groups selected from groups A to E below.

[0101] Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%.

[0102] Ni, Cu, Cr, and Mo are all elements that increase the strength of steel and improve its hardenability. One or more of these elements can be selected as needed. To achieve this effect, the content of each element is set to 0.01% or more. On the other hand, from the viewpoint of avoiding excessive cost increases, the content of Ni, Cu, and Cr is set to 5.0% or less, and the content of Mo is set to 3.0% or less. Therefore, when Ni, Cu, Cr, and Mo are present, their contents are set as follows: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%. Each element is preferably 0.01% or more. Furthermore, each element is preferably 1.0% or less.

[0103] Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, V: 0.005–3.0%, and W: 0.005–3.0%.

[0104] Ti, Nb, V, and W are all elements that contribute to the increase of steel strength through precipitation strengthening and also contribute to the improvement of toughness through grain refinement. One or more of them can be selected as needed.

[0105] Here, Ti has the effect of increasing strength and improving toughness. Furthermore, Ti preferentially forms nitrides compared to B, thereby improving hardenability due to the solid solution of B. From the viewpoint of achieving such effects, the Ti content is set to 0.005% or more. On the other hand, when the Ti content exceeds 3.0%, the rolling load during hot rolling increases drastically, and the toughness of the hot-stamped component decreases. Therefore, in the case of containing Ti, its content is set to 0.005% or more and 3.0% or less. Preferably, it is 0.01% or more. Furthermore, it is preferably 1.0% or less.

[0106] Furthermore, from the viewpoint of achieving the aforementioned effects of increased strength and improved toughness, the Nb content is set to 0.005% or more. On the other hand, when the Nb content exceeds 3.0%, the amount of Nb carbonitrides increases, and the ductility and resistance to delayed fracture decrease. Therefore, in the case of containing Nb, its content is set to 0.005% or more and 3.0% or less. Preferably, it is 0.01% or more. Furthermore, it is more preferably 0.05% or less.

[0107] In addition to increasing strength and improving toughness, V also improves resistance to hydrogen embrittlement by precipitating as a precipitate or crystal and acting as a hydrogen trapping site. From the viewpoint of achieving this effect, the V content is set to 0.005% or more. On the other hand, when the V content exceeds 3.0%, the amount of V carbonitrides increases significantly, and ductility decreases. Therefore, in the case of V, its content is set to 0.005% or more and 3.0% or less. Preferably, it is 0.01% or more. Furthermore, it is preferably 2.0% or less.

[0108] In addition to increasing strength and improving toughness, W also improves resistance to hydrogen embrittlement. From the viewpoint of achieving this effect, the W content is set to 0.005% or more. On the other hand, when the W content exceeds 3.0%, ductility decreases. Therefore, when W is present, its content is set to 0.005% or more and 3.0% or less. Preferably, it is 0.01% or more. Furthermore, it is preferably 2.0% or less.

[0109] Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%.

[0110] REM, Ca, and Mg are all elements that improve ductility and resistance to hydrogen embrittlement by controlling the morphology of inclusions, and one or more can be selected as needed. From the viewpoint of achieving this effect, the content of each element is set to 0.0005% or more. On the other hand, from the viewpoint of not reducing hot workability, the REM and Ca contents are set to 0.01% or less. Furthermore, from the viewpoint of avoiding a decrease in ductility due to the formation of coarse oxides or sulfides, the Mg content is set to 0.01% or less. Therefore, when REM, Ca, and Mg are contained, their contents are set as follows: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%, respectively. Each element is preferably 0.0006% or more. Furthermore, each element is preferably 0.01% or less.

[0111] Group D: Sb: 0.002–0.03%

[0112] Sb suppresses the formation of a decarburized layer on the surface of steel plates during heating and cooling; therefore, it can be included as needed. From the viewpoint of achieving this effect, the Sb content is set to 0.002% or more. On the other hand, an Sb content exceeding 0.03% leads to an increase in rolling load and a decrease in productivity. Therefore, when Sb is present, its content is set to 0.002% or more and 0.03% or less. Preferably, it is 0.002% or more. Furthermore, it is more preferably 0.02% or less.

[0113] Group E: B: 0.0005%

[0114] Boron (B) contributes to improved hardenability during hot stamping and increased toughness after hot stamping; therefore, it can be included as needed. From the viewpoint of achieving this effect, the B content is set to 0.0005% or more. On the other hand, when the B content exceeds 0.05%, cracks may sometimes occur in the steel sheet due to increased rolling load during hot rolling and the formation of martensite and bainite after hot rolling. Therefore, when B is included, its content is set to 0.0005% or more and 0.05% or less. Preferably, it is 0.0005% or more and 0.01% or less.

[0115] It should be noted that the components other than those mentioned above are Fe and unavoidable impurities. Additionally, unavoidable impurities include, for example, O (oxygen), which is permissible at concentrations below 0.0100%.

[0116] Next, the structure of the hot stamping steel sheet of the present invention will be described.

[0117] The hot stamping steel sheet of the present invention has a microstructure consisting of ferrite and cementite, wherein the Mn concentration of the ferrite and the cementite is set to Mnα and Mnθ respectively, and the Mnθ / Mnα ratio is 1.4 or higher.

[0118] Here, the ferrite content, in terms of volume fraction, is preferably set to 92.0% or more and 99.0% or less. This is because when the ferrite volume fraction is less than 92.0%, the tensile strength exceeds 590 MPa, leading to a decrease in formability; on the other hand, when it is greater than 99.0%, the tensile strength is less than 340 MPa, resulting in a decrease in the strength of the stamped article. More preferably, it is 95.0% or more, and even more preferably, it is 96.0% or more. Furthermore, more preferably, it is 99.0% or less, and even more preferably, it is 98.4% or less.

[0119] Furthermore, the cementite content, by volume fraction, is preferably set to 1.0% or more and 5.0% or less. This is because when the cementite volume fraction is less than 1.0%, the elongation exceeds 45%, the sheet thickness at the portion subjected to stamping stress becomes extremely thin, and the unevenness of the sheet thickness after stamping increases. On the other hand, when it is greater than 5.0%, the elongation is less than 25%, resulting in a decrease in formability. More preferably, it is 1.6% or more. Even more preferably, it is 4.4% or less.

[0120] It should be noted that the microstructure of the hot stamping steel sheet of the present invention is basically composed of the ferrite and cementite described above, but may contain trace amounts of bainite and pearlite as surplus microstructures other than ferrite and cementite, and the total volume fraction of these surplus microstructures may be less than 5.0%.

[0121] In addition, the microstructure of hot-stamped steel sheets and the determination of their volume fraction are carried out as follows.

[0122] Specifically, test pieces for microstructure observation are cut from hot-stamped steel sheets with the observation surface parallel to the rolling direction and perpendicular to the rolling plane. The observation surface is ground, etched with a 3% (v / v) nitric acid ethanol solution to expose the microstructure, and the microstructure at a position representing 1 / 4 of the sheet thickness is observed and photographed using a scanning electron microscope (magnification: 1500x). The microstructure is identified through image analysis based on the obtained photographs. Here, the phase observed as a relatively smooth and dark surface is identified as ferrite; the phase observed as a film or blocky and relatively white is identified as cementite; the phase where ferrite and cementite form layers is identified as pearlite; and the phase consisting of carbides formed between laths and bainitic ferrite without carbides within the grains is identified as bainite. Next, the area fraction occupied by each phase in the microstructure photograph is calculated, these phases are considered three-dimensionally homogeneous, and the area fraction of each phase is taken as the volume fraction.

[0123] It should be noted that the "volume fraction of the remaining structure" is obtained by subtracting the "volume fraction of ferrite" and "volume fraction of cementite" mentioned above from 100%.

[0124] The Mn concentration ratio of cementite to ferrite (Mnθ / Mnα) is greater than 1.4.

[0125] Cementite in the microstructure of hot-stamped steel sheets serves as a site for the formation of retained austenite in hot-stamped components. Furthermore, to ensure a predetermined amount of retained austenite and to form a microstructure with retained austenite of a predetermined size or larger in the hot-stamped component, it is important to increase the enrichment of Mn in cementite relative to that in ferrite, the parent phase. Here, when the Mn concentrations in ferrite and cementite are set as Mnα and Mnθ, respectively, if Mnθ / Mnα is less than 1.4, the enrichment of Mn in cementite is low. Therefore, the desired volume fraction of retained austenite cannot be obtained in the hot-stamped component, or a microstructure with appropriately dispersed retained austenite of a predetermined size or larger cannot be formed.

[0126] Therefore, Mnθ / Mnα is set to 1.4 or higher. Preferably, it is 1.5 or higher, and more preferably 1.8 or higher. It should be noted that there is no particular upper limit to Mnθ / Mnα, and it is actually around 10.0.

[0127] It should be noted that the ratio of Mn concentration in cementite to Mn concentration in ferrite, Mnθ / Mnα, is calculated as follows.

[0128] Specifically, test pieces for microstructure observation are cut from hot-stamped steel plates with the observation surface parallel to the rolling direction and perpendicular to the rolling plane. The observation surface is ground, and the microstructure is exposed by etching with a 3% (v / v) nitric acid-ethanol solution. The microstructure at a position representing 1 / 4 of the plate thickness is observed using an EPMA (Electron Probe Micro Analyzer). Quantitative analysis of Mn is performed on 30 particles each of ferrite and cementite. Based on the quantitative analysis results of Mn, the Mn concentration of each grain in ferrite and cementite is averaged, and these average values ​​are taken as Mnα and Mnθ, respectively. Then, the value obtained by dividing Mnθ by Mnα is taken as Mnθ / Mnα.

[0129] Next, the manufacturing method of the hot stamping steel sheet of the present invention will be described.

[0130] The method for manufacturing hot-stamping steel sheet of the present invention comprises: a step of heating a steel billet having the above-described composition and hot-rolling it to obtain a hot-rolled steel sheet; a step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; and an annealing step of heating the cold-rolled steel sheet to a temperature range of Ac1-150°C or higher and lower than Ac1, holding it in the temperature range for more than 1 hour, and then cooling it.

[0131] <Processes for obtaining hot-rolled steel sheets>

[0132] There are no particular restrictions on the process for obtaining hot-rolled steel sheets; it can be carried out using conventional methods.

[0133] For example, when molten steel with the above-mentioned composition is smelted in a converter or similar furnace, it is preferable to produce steel billets using continuous casting to prevent macroscopic segregation. It should be noted that ingot casting or thin slab continuous casting can also be used instead of continuous casting.

[0134] It should be noted that the steel billet is temporarily cooled to room temperature before being loaded into the heating furnace for reheating. However, energy-saving processes such as loading the steel billet into the heating furnace in a warm state without cooling it to room temperature, or hot rolling the steel billet immediately after a short period of heat preservation, can also be used.

[0135] The steel billet obtained in this way is heated to a predetermined heating temperature and then hot-rolled to produce hot-rolled steel sheet. The preferred heating temperature is 1000–1300°C. The heated steel billet is typically hot-rolled at a finishing mill inlet temperature of 1100°C or less and a finishing mill outlet temperature of 800–950°C. It is then cooled at an average cooling rate of 5°C / second or higher and coiled at a coiling temperature of 300–750°C to produce hot-rolled steel sheet.

[0136] Furthermore, regarding finish rolling, there are no particular limitations on conditions other than those mentioned above, but it is preferable to set the cumulative reduction rate within the temperature range of 950°C or higher to 40% or more. When the cumulative reduction rate within the temperature range of 950°C or higher increases, austenite recrystallization occurs, resulting in grain refinement. This austenite refinement reduces the original austenite grain size in the final hot-stamped component, thereby improving resistance to delayed failure. More preferably, the cumulative reduction rate within the temperature range of 950°C or higher is 60% or more.

[0137] <Processes for obtaining cold-rolled steel sheets>

[0138] Then, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. To prevent abnormal grain growth during subsequent annealing and heating processes before hot stamping, the reduction rate during cold rolling is preferably set to 30% or more, more preferably 50% or more. In addition, since the rolling load increases and the productivity decreases, the reduction rate is preferably set to 85% or less.

[0139] It should be noted that before cold rolling, the hot-rolled steel sheet can undergo a scale removal process based on pickling, etc. Additionally, when the rolling load during cold rolling increases, a softening annealing process can be performed on the hot-rolled steel sheet.

[0140] <Annealing process>

[0141] The cold-rolled steel sheet obtained in this way is heated to a temperature range of Ac1-150°C or higher but lower than Ac1, and then held within this temperature range for at least 1 hour before cooling. The reasons for limiting the annealing temperature and other parameters in the annealing process are explained below.

[0142] Heating temperature: Ac1-150℃ or higher but below point Ac1

[0143] As described above, it is important to enrich Mn in the cementite during this annealing process. Here, when the heating temperature is below Ac1-150°C, the diffusion rate of Mn becomes insufficient, preventing adequate enrichment of Mn into the cementite. Consequently, the desired volume fraction of retained austenite and / or a microstructure with a properly dispersed retained austenite of a predetermined size cannot be obtained in the hot-stamped component. On the other hand, above Ac1, the region becomes a ferrite-austenite two-phase region, thus Mn cannot be sufficiently enriched in the cementite, and a microstructure with a properly dispersed retained austenite of a predetermined size still cannot be obtained in the hot-stamped component.

[0144] Therefore, the heating temperature is set to Ac1-150°C or higher and below Ac1. Preferably, it is Ac1-130°C or higher. Furthermore, it is preferably Ac1-10°C or lower.

[0145] Duration: Over 1 hour

[0146] Furthermore, when the holding time is less than 1 hour, the enrichment of Mn into the cementite becomes insufficient, and the desired Mnθ / Mnα ratio cannot be obtained. Consequently, the size of the cementite that serves as the formation site for retained austenite in the hot-stamped component decreases. Therefore, the desired volume fraction of retained austenite in the hot-stamped component decreases, and a microstructure with properly dispersed retained austenite of a size greater than the predetermined value cannot be obtained. As a result, the desired resistance to delayed fracture and uniform elongation are not achieved.

[0147] Therefore, the holding time (annealing time) is set to 1 hour or more. It should be noted that there is no specific upper limit, but from a productivity point of view, it is preferable to set it to 48 hours or less. More preferably, it is 1.5 hours or more. Furthermore, it is even more preferable to set it to 24 hours or less.

[0148] It should be noted that there are no particular limitations on the cooling after holding; appropriate static cooling (slow cooling) or controlled cooling can be carried out depending on the heating furnace used.

[0149] Furthermore, this annealing process is preferably performed using an intermittent annealing furnace. When annealing is performed using an intermittent annealing furnace, there are no particular limitations other than the conditions mentioned above. For example, from the viewpoint of enriching Mn into cementite, the average heating rate is preferably set to 10°C / hour or more and 150°C / hour or less, and the average cooling rate after holding is preferably set to 10°C / hour or more and 150°C / hour or less.

[0150] More preferably, the heating rate is 20°C / hour or higher. Furthermore, more preferably, the heating rate is 100°C / hour or lower.

[0151] Furthermore, a cooling rate of 20°C / hour or higher is more preferable. Additionally, a cooling rate of 100°C / hour or lower is even more preferable.

[0152] Furthermore, the Ac1 point (°C) mentioned above and the Ac3 point (°C) described later are calculated using the following formula.

[0153] Ac1 point (°C) = 751 - 16C + 11Si - 28Mn - 5.5Cu - 16Ni + 13Cr + 3.4Mo

[0154] Ac3 point (°C) = 910 - 203°C 1 / 2 +44.7Si-4Mn+11Cr

[0155] Here, C, Si, Mn, Cu, Ni, Cr and Mo in the formula represent the content (mass%) of each element. If the above elements are not present, the content of the element is calculated as zero.

[0156] <Plating Process>

[0157] Furthermore, a coating can be formed on the surface of the hot-stamping steel sheet obtained in the above manner. When using a hot-stamping steel sheet without a coating as raw material, it is necessary to perform oxide scale removal treatment such as shot peening on the hot-stamped component after the hot-stamping process. However, when a coating is formed on the surface of the hot-stamping steel sheet, oxide scale formation is suppressed during the heating process of hot stamping. Therefore, oxide scale removal treatment after the hot-stamping process is not required, and productivity is improved.

[0158] It should be noted that the coating is preferably a Zn-based or Al-based coating. When corrosion resistance is required, Zn-based coatings are superior to Al-based coatings. This is because the sacrificial corrosion protection of zinc reduces the corrosion rate of the steel substrate. Furthermore, it offers the following advantages: when hot-stamping the coated steel sheet, a zinc oxide film forms during the initial heating stage of the hot-stamping process, preventing Zn evaporation during subsequent processing of the hot-stamped components.

[0159] Furthermore, examples of Zn-based coatings include conventional hot-dip galvanized (GI) coatings, alloyed hot-dip galvanized (GA) coatings, and Zn-Ni-based coatings, among which Zn-Ni-based coatings are preferred. Zn-Ni-based coatings not only significantly suppress oxide scale formation during hot stamping heating but also prevent brittle fracture of the liquid metal. From the viewpoint of achieving this effect, Zn-Ni-based coatings preferably contain 10 to 25% by mass of Ni. It should be noted that even when containing more than 25% by mass of Ni, this effect becomes saturated.

[0160] It should be noted that, as an Al-based coating, examples include Al-10% by mass Si coating, etc.

[0161] Furthermore, the coating adhesion amount is preferably set to 10–90 g / m² per single side. 2 This is because the adhesion amount is 10g / m. 2 At the above levels, the formation of oxide scale during heating can be effectively suppressed, with an adhesion amount of 90 g / m². 2 The following values ​​will not hinder productivity. A preferred value is 30g / m³. 2 That's all. Furthermore, 70g / m² is more preferable. 2 the following.

[0162] It should be noted that pickling and leveling rolling can be carried out appropriately between the above-mentioned processes, which goes without saying.

[0163] Next, the hot-stamped component of the present invention will be described.

[0164] The hot-stamped component of the present invention has the composition of the hot-stamped steel sheet described above, and has the following microstructure: containing 70.0% or more martensite by volume and 3.0% or more and 30.0% or less retained austenite by volume, and having a microstructure of 2.0 × 10⁻⁶. 5 pcs / mm 2 The above contains residual austenite with a diameter of 0.3 μm or more, based on the equivalent circle diameter.

[0165] In addition, the hot-stamped component of the present invention can be obtained, for example, by hot-stamping the above-mentioned hot-stamping steel sheet under predetermined conditions.

[0166] The structure of the hot-stamped component of the present invention will be described below.

[0167] Martensite volume fraction: 70.0% or higher

[0168] To ensure a tensile strength TS of 1800 MPa or higher in hot-stamped components, martensite is required as the main phase; specifically, the martensite volume fraction should be 70.0% or higher, preferably 80.0% or higher. It should be noted that, to contain the desired amount of retained austenite, the martensite volume fraction should be 97.0% or lower.

[0169] Volume fraction of retained austenite: 3.0–30.0%

[0170] Retained austenite is an important microstructure for improving uniform elongation by utilizing the TRIP effect (transformation-induced plasticity) during deformation. To achieve a uniform elongation uEl of 6.0% or more, it is necessary to contain retained austenite at a volume fraction of 3.0% or more. On the other hand, when the volume fraction of retained austenite exceeds 30.0%, there is an excessive amount of hard martensite that undergoes phase transformation after exhibiting the TRIP effect, resulting in reduced toughness. Therefore, the volume fraction of retained austenite is set to be 3.0% or more and 30.0% or less. Preferably, it is 5.0% or more. Furthermore, it is more preferably 20.0% or less.

[0171] It should be noted that the microstructure of the hot-stamped component of the present invention is basically composed of the above-mentioned martensite and retained austenite, but may contain trace amounts of bainite, ferrite, cementite and pearlite as surplus microstructures other than martensite and retained austenite. The total volume fraction of these surplus microstructures may be less than 10% (including 0%).

[0172] In addition, the volume fraction of the microstructure of the hot-stamped component was determined as follows.

[0173] First, an X-ray diffraction test piece was cut from the cap plate of the hot-stamped component. Mechanical and chemical grinding were performed, with the 1 / 4 wall thickness surface serving as the measurement surface. Then, X-ray diffraction was performed. CoKα rays were used as the incident X-rays. The integrated intensities of the peaks on the {200}, {220}, and {311} planes of retained austenite (γ) and the peaks on the {200} and {211} planes of ferrite (α) were measured. For six groups—α{200}-γ{200}, α{200}-γ{220}, α{200}-γ{311}, α{211}-γ{200}, α{211}-γ{220}, and α{211}-γ{311}—the residual γ volume fraction was calculated from the integrated intensity ratio. Their average value was taken as the "retained austenite volume fraction."

[0174] Next, a microstructure observation test piece was cut from the cap plate of the hot-stamped component, with the surface parallel to the rolling direction and perpendicular to the cap plate surface as the observation surface. The observation surface was ground, and the microstructure was exposed by etching with a 3% (v / v) nitric acid ethanol solution. The microstructure at the 1 / 4 thickness position was observed and photographed using a scanning electron microscope (magnification: 1500x). The microstructure was identified by image analysis based on the obtained microstructure photographs. Here, the phase that appears relatively smooth and dark was identified as ferrite; the phase that appears as film or blocky at the grain boundaries and is relatively white was identified as cementite; the phase in which ferrite and cementite form layers was identified as pearlite; and the phase in which carbides are formed between laths and the phase consisting of bainitic ferrite without carbides within the grains were identified as bainitic phase. Next, the area ratio of each phase in the microstructure photograph is calculated. These phases are considered as three-dimensional homogeneous, and the area ratio of each phase is taken as the volume ratio. Their sum is taken as the "volume ratio of the remaining microstructure other than martensite and retained austenite".

[0175] Then, the "volume percentage of martensite" is obtained by subtracting the "volume percentage of retained austenite" and the "volume percentage of the remaining structure other than martensite and retained austenite" from 100%.

[0176] Residual austenite with a diameter greater than 0.3 μm based on the equivalent circle diameter: 2.0 × 10⁻⁶ 5 pcs / mm 2 The aforementioned retained austenite acts as hydrogen absorption sites, enhancing resistance to delayed fracture. Specifically, to achieve excellent resistance to delayed fracture while maintaining a high tensile strength (TS: 1800 MPa or higher), the microstructure of the hot-stamped component is formed with a density of 2.0 × 10⁻⁶. 5 pcs / mm 2 The microstructure containing retained austenite with a relatively large size of 0.3 μm or more based on the equivalent circle diameter is essential. Here, the retained austenite with a size of 0.3 μm or more based on the equivalent circle diameter is less than 2.0 × 10⁻⁶. 5 pcs / mm 2 When hydrogen cannot be uniformly absorbed into the steel plate, the desired resistance to delayed failure cannot be obtained. Therefore, the retained austenite with a diameter of 0.3 μm or more (based on the equivalent circle diameter) is increased to 2.0 × 10⁻⁶. 5 pcs / mm 2 The above applies. It should be noted that the value equivalent to a retained austenite volume fraction of 30% is the upper limit, which is 4.3 × 10⁻⁶ when the equivalent circle diameter of the retained austenite is 0.3 μm. 6 pcs / mm 2 .

[0177] It should be noted that, as described above, in order to obtain a microstructure with an appropriate amount of retained austenite and a retained austenite of a predetermined size, it is important to use a steel sheet with a composition in which the Mn content is appropriately controlled and a microstructure in which Mn is enriched in cementite as the raw material steel sheet, and to perform hot stamping by heating the steel sheet under predetermined conditions.

[0178] It should be noted that the number of residual austenite particles per unit area with an equivalent circle diameter of 0.3 μm or more is calculated as follows.

[0179] That is, test pieces for tissue observation were cut in the same manner as described above. The observation surface was electrolytically polished, and images were taken at a position corresponding to 1 / 4 of the plate thickness using a scanning electron microscope (magnification: 1500x) equipped with EBSP (Electron Back-Scattering Pattern). Next, the EBSP data was image-processed to extract only the grains identified as retained austenite. Then, the equivalent circle diameter was calculated from the area of ​​each grain, and the number of retained austenite grains with an equivalent circle diameter of 0.3 μm or more was counted, calculating the number per 1 mm × 1 mm.

[0180] Furthermore, although not specifically limited, the original austenite grain size in the microstructure of the hot-stamped component is preferably set to 100 μm or less. This is because the finer the grain size, the greater the area of ​​the grain boundaries. As a result, the hydrogen coverage per unit grain boundary area decreases, and the delayed failure characteristics are improved. More preferably, it is in the range of 3 to 20 μm.

[0181] It should be noted that the determination of the original austenite grain size was carried out as follows.

[0182] That is, test pieces for microstructure observation are prepared in the same manner as described above. Next, the observation surface of the prepared test pieces is ground, and the microstructure is exposed by etching with picric acid and a surfactant. The microstructure at the location representing 1 / 4 of the plate thickness is observed and photographed using an optical microscope (magnification: 500x). Using the obtained microstructure photographs, the equivalent circle diameter is calculated for each original austenite grain based on the area occupied by the original austenite grains, and the average value is taken as the original austenite grain size.

[0183] By setting the composition and structure as described above, the hot-stamped component of the present invention has both high strength (tensile strength TS: 1800 MPa or more, preferably less than 2300 MPa) and high ductility (uniform elongation uEl: 6.0% or more, less than 20%), and can obtain excellent resistance to delayed failure.

[0184] Next, preferred stamping conditions for manufacturing the hot-stamped component of the present invention will be described.

[0185] <Hot stamping heating process>

[0186] The hot stamping steel plate described above is heated to a temperature range above Ac3 and below 1000°C, and held within this temperature range for more than 10 seconds and less than 900 seconds.

[0187] Heating temperature: above Ac3 and below 1000℃

[0188] When the heating temperature is below the Ac3 point, which is the single-phase region of austenite, austenitization becomes insufficient, and the desired amount of martensite cannot be ensured in hot-stamped components, thus failing to obtain the desired tensile strength.

[0189] Furthermore, during the heating process, cementite undergoes a reverse transformation into austenite. The austenite formed from this reverse transformation of cementite has a higher Mn concentration compared to the austenite formed from the reverse transformation of ferrite. Additionally, Mn is an element that stabilizes austenite. Therefore, the austenite formed from the reverse transformation of Mn-enriched cementite has a high Mn concentration and functions as a site for the formation of retained austenite in hot-stamped components.

[0190] On the other hand, when the heating temperature exceeds 1000°C, the Mn enriched in the cementite is homogenized, making it impossible to ensure the desired amount of retained austenite and thus failing to obtain the desired uniform elongation. Furthermore, it is impossible to form a microstructure with a properly dispersed retained austenite of a predetermined size, resulting in the failure to obtain the desired resistance to delayed fracture.

[0191] Therefore, the heating temperature is preferably set to above Ac3 and below 1000°C. More preferably, it is above (Ac3 + 30)°C. Furthermore, it is even more preferably below 950°C.

[0192] It should be noted that the average heating rate up to the heating temperature is not particularly limited, but is preferably set to 1 to 400°C / second. Here, an average heating rate of 1°C / second or higher will not impair productivity, while a rate below 400°C / second can prevent instability in temperature control. More preferably, it is 10°C / second or higher. Furthermore, even more preferably, it is 150°C / second or lower.

[0193] Duration: 10 seconds or more but less than 900 seconds

[0194] As the holding time increases, the enriched Mn diffuses and becomes homogenized. Therefore, when the holding time exceeds 900 seconds, the desired amount of retained austenite cannot be ensured, and the desired uniform elongation cannot be obtained. Furthermore, a microstructure with a properly dispersed retained austenite of a predetermined size cannot be formed, and the desired delayed failure characteristics cannot be obtained. Conversely, when the holding time is 10 seconds or less, the reverse transformation from cementite to austenite becomes insufficient, and a microstructure with a properly dispersed retained austenite of a predetermined size still cannot be formed, and the desired delayed failure characteristics cannot be obtained. Therefore, the holding time is preferably set to 10 seconds or more and 900 seconds or less.

[0195] It should be noted that there are no particular limitations on the heating method; common heating methods such as electric furnaces, gas furnaces, infrared heating, high-frequency heating, and direct electric heating can all be used. Furthermore, there are no particular limitations on the atmosphere; atmospheric conditions and inert gas atmospheres are both acceptable.

[0196] <Hot stamping forming process>

[0197] In the hot stamping process, a forming die is used to simultaneously stamp and quench the hot stamping steel sheet after the aforementioned hot stamping heating process to obtain a hot stamped component of a predetermined shape. Here, "hot stamping" is a process in which a heated steel sheet is stamped and quenched simultaneously using a die; it is also known as "hot forming," "hotstamping," or "die quenching."

[0198] It should be noted that there is no particular limitation on the forming start temperature within the stamping press, but it is preferably set above Ms point. When the forming start temperature is below Ms point, the forming load increases, and the load applied to the stamping press increases. It should also be noted that air cooling is generally used during the handling of the raw steel sheet up to the start of forming. Therefore, the upper limit of the forming start temperature is the heating temperature in the aforementioned heating process. Furthermore, when handling the material in an environment where cooling is accelerated using gas, liquid, or similar materials, it is preferable to use insulated fixtures such as heat-insulating boxes to reduce the cooling rate.

[0199] In addition, there is no particular limitation on the cooling rate inside the mold. From the point of view of productivity, it is preferable to set the average cooling rate up to 200°C to be 20°C / second or more, and more preferably 40°C / second or more.

[0200] There are no particular limitations on the time for removal from the mold or the cooling rate after removal. As a cooling method, for example, the punch mold can be held at bottom dead center for 1 to 60 seconds, and the stamped component can be cooled using both the die and the punch mold. Then, the stamped component is removed from the mold and cooled. Cooling within the mold and after removal can be achieved by combining cooling methods using refrigerants such as gases or liquids, thereby improving productivity.

[0201] Example

[0202] Molten steel with the composition shown in Tables 1 and 4 (balance: Fe and unavoidable impurities) was melted in a small vacuum melting furnace to produce steel billets. The billets were heated to 1250°C and further hot-rolled, including roughing and finishing, to obtain hot-rolled steel sheets. The hot-rolling conditions were set as follows: finishing mill inlet temperature: 1100°C; finishing mill outlet temperature: 850°C; and the cumulative reduction rate in the temperature range above 950°C was set as shown in Tables 2 and 5. After hot rolling, the average cooling rate in the temperature range of 800–650°C was set to 15°C / second for cooling, and the coiling temperature was set to 650°C for coiling.

[0203] The obtained hot-rolled steel sheet was pickled and then cold-rolled at a reduction rate of 54% to obtain a cold-rolled steel sheet (thickness: 1.6 mm). Next, it was heated to the heating temperature T1 shown in Tables 2 and 5 at an average heating rate of 40°C / hour, held for the times shown in Tables 2 and 5, and then cooled at an average cooling rate of 40°C / hour to obtain a hot-stamping steel sheet.

[0204] The obtained hot-stamping steel sheet was then subjected to microstructure identification, volume fraction determination, and Mnθ / Mnα derivation using the methods described above. The results are shown in Tables 2 and 5.

[0205] It should be noted that, as shown in Tables 2 and 5, some hot-stamping steel sheets undergo plating treatment. In Tables 2 and 5, "Zn-Ni coating" refers to a Zn-12% by mass Ni coating, and "Al-Si coating" refers to an Al-10% by mass Si coating. It should also be noted that the coating adhesion is 60 g / m² per single side. 2 .

[0206] Next, these hot-stamping steel sheets are heated under the conditions shown in Tables 3 and 6, and then hot-stamped to obtain a hot-stamped component with a hat-shaped cross-section. It should be noted that the hot stamping is performed using a punch die with a width of 70 mm and a shoulder radius R of 6 mm, and a forming depth of 30 mm.

[0207] It should be noted that when the above heating is performed in the atmosphere using an electric heating furnace, the average heating rate from room temperature to 750°C is set to 7.5°C / second, and the average heating rate from 750°C to the heating temperature is set to 2.0°C / second. Furthermore, when the above heating is performed in the atmosphere using a direct-current heating device, the average heating rate from room temperature to the heating temperature is set to 100°C / second. Then, after reaching the heating temperature, it is maintained at that temperature.

[0208] Furthermore, the forming start temperature in hot stamping is set to 750°C. The cooling within the die is performed as follows: the punch die is held at bottom dead center for 15 seconds, and then cooled to below 150°C using a combination of clamping with the die and punch mold, and air cooling on the open die. It should be noted that the average cooling rate from the forming start temperature to 200°C is 100°C / second.

[0209] JIS No. 5 tensile test specimens (parallel section width: 25 mm, parallel section length: 60 mm, GL = 50 mm) were cut from the cap plate portion of the hot-stamped component obtained in this way. Tensile tests were performed according to JIS Z 2241 to determine the yield stress YS, tensile strength TS, uniform elongation uEl, and total elongation tEl. The results are shown in Tables 3 and 6.

[0210] Furthermore, for the aforementioned hot-stamped components, the microstructure was identified and the volume fraction was determined, the original austenite grain size was measured, and the number of retained austenite particles with an equivalent circle diameter of 0.3 μm or larger was determined using the methods described above. The results are shown in Tables 3 and 6.

[0211] Furthermore, a rectangular test piece (20 mm wide and 115 mm long) for evaluating delayed failure characteristics was cut from the cap plate portion of the aforementioned hot-stamped component. The cut rectangular test piece was then subjected to... Figure 1 The four-point bending test fixture shown is immersed in a test solution (hydrochloric acid, pH: 1) under applied stress to evaluate its resistance to delayed failure. It should be noted that cases where no cracks appear even after immersion in the test solution for more than 200 hours are designated as ◎ (Pass, Excellent), cases where cracks appear after 100 to less than 200 hours are designated as ○ (Pass), and cases where cracks appear in less than 100 hours are designated as × (Fail).

[0212] The results are shown in Tables 3 and 6.

[0213] It should be noted that the additional stress σ (kg / mm) applied to the rectangular test piece 2The stress is set to 90% of the stress YS obtained from the tensile test described above, and the additional stress is adjusted by the screw insertion depth y (mm). It should be noted that the additional stress σ on the rectangular test piece can be calculated from the screw insertion depth y (mm) using the following formula.

[0214]

[0215] Where y: screwing depth (mm), σ: additional stress (kg / mm) 2 E: Young's modulus; t: plate thickness (mm); L1 = 30mm, L2 = 40mm. It should be noted that Young's modulus is expressed as 2.1 × 10⁻⁶. 4 (kg / mm 2 ) to calculate.

[0216] [Table 1]

[0217]

[0218] [Table 2]

[0219]

[0220] *B: Bainite, M: Martensite

[0221]

[0222] [Table 4]

[0223]

[0224] [Table 5]

[0225]

[0226]

[0227] As shown in Tables 3 and 6, the inventive examples all achieved high strength (TS) of 1800 MPa or more and high ductility (uEl) of 6.0% or more. Furthermore, they all exhibited excellent resistance to delayed failure. In contrast, the comparative examples did not satisfy at least one of these characteristics.

[0228] Industrial availability

[0229] The hot-stamped components of this invention are suitable for use as structural components in automobiles, such as anti-collision beams, center pillars, and bumpers, which require high collision energy absorption capacity and excellent resistance to delayed failure. Therefore, they are extremely useful in industry.

Claims

1. A hot-stamping steel sheet having the following composition: by mass % containing C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01-1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005-0.1%, N: less than 0.01%, Sb: 0.002-0.03%, and W: 0.005-3.0%, with the balance consisting of Fe and unavoidable impurities. It has the following structure: it consists of ferrite of 92.00% to 99.0% by volume, cementite of 1.0% to 5.0% by volume, and the balance of 5.0% or less, wherein the balance is selected from bainite and pearlite, and when the Mn concentration of the ferrite and the cementite is set as Mnα and Mnθ respectively, the Mnθ / Mnα ratio is 1.4 or more, and Mn is enriched in the cementite.

2. The hot-stamping steel plate as described in claim 1, wherein, The composition, by mass%, also contains one or more groups selected from groups A to D below. Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%. Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, and V: 0.005–3.0%. Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%. Group D: B: 0.0005~0.05%.

3. The hot-stamping steel plate as described in claim 1 or 2, wherein, It has a coating on the surface.

4. The hot-stamping steel plate as described in claim 3, wherein, The coating is a Zn-based coating or an Al-based coating.

5. The hot-stamping steel plate as described in claim 4, wherein, The Zn-based coating contains 10–25% by mass of Ni.

6. A method for manufacturing a hot-stamped steel plate, wherein, A steel billet containing, by mass percent, C: ≥0.180% and ≤0.300%, Mn: ≥3.50% and ≤11.0%, Si: 0.01–1.5%, P: ≤0.05%, S: ≤0.05%, Al: 0.005–0.1%, N: ≤0.01%, Sb: 0.002–0.03%, and W: 0.005–3.0%, with the balance consisting of Fe and unavoidable impurities, is heated, hot-rolled, and then subjected to a process at 950 °C. Hot-rolled steel sheets are produced by finishing with a cumulative reduction rate of 72% or more in a temperature range above ℃ and coiling them into coils at a coiling temperature of 650℃ to 750℃ above the martensitic transformation point. The hot-rolled steel sheets are then cold-rolled with a reduction rate of 30% or more and 85% or less to produce cold-rolled steel sheets. The cold-rolled steel sheets are further subjected to annealing by heating them to a temperature range of Ac1-150℃ or more and Ac1-10℃ or less, holding them in this temperature range for more than 1 hour, and then cooling them.

7. The method for manufacturing a hot-stamping steel plate as described in claim 6, wherein, The composition, by mass%, also contains one or more groups selected from groups A to D below. Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%. Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, and V: 0.005–3.0%. Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%. Group D: B: 0.0005~0.05%.

8. The method for manufacturing a hot-stamping steel sheet as described in claim 6 or 7, wherein, After the annealing, a coating is formed on the surface of the hot-stamped steel sheet.

9. The method for manufacturing a hot-stamping steel plate as described in claim 8, wherein, The coating is a Zn-based coating or an Al-based coating.

10. The method for manufacturing a hot-stamping steel sheet as described in claim 9, wherein, The Zn-based coating contains 10–25% by mass of Ni.

11. The method for manufacturing a hot-stamping steel sheet as described in claim 8, wherein, The coating adhesion amount is 10-90 g / m² per single side. 2 .

12. The method for manufacturing a hot-stamping steel plate as described in claim 9, wherein, The coating adhesion amount is 10-90 g / m² per single side. 2 .

13. The method for manufacturing a hot-stamping steel sheet as described in claim 10, wherein, The coating adhesion amount is 10-90 g / m² per single side. 2 .

14. A hot-stamped component having the following composition: by mass % containing C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01-1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005-0.1%, N: less than 0.01%, Sb: 0.002-0.03%, and W: 0.005-3.0%, with the balance consisting of Fe and unavoidable impurities. It also has the following microstructure: containing more than 70.0% martensite by volume and more than 3.0% and less than 30.0% retained austenite by volume, and with a particle size of 2.0 × 10⁻⁶. 5 pcs / mm 2 The above contains retained austenite with a diameter greater than 0.3 μm based on the equivalent circle diameter. The hot-stamped component is obtained by hot-stamping steel plates. The steel plate has the following microstructure: it consists of ferrite of 92.00% to 99.0% by volume, cementite of 1.0% to 5.0% by volume, and the balance of 5.0% or less, wherein the balance microstructure is selected from bainite and pearlite. When the Mn concentrations of the ferrite and the cementite are set as Mnα and Mnθ, respectively, the Mnθ / Mnα ratio is 1.4 or more, and Mn is enriched in the cementite.

15. The hot-stamped component as claimed in claim 14, wherein, The composition, by mass%, also contains one or more groups selected from groups A to D below. Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%. Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, and V: 0.005–3.0%. Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%. Group D: B: 0.0005~0.05%.

16. The hot-stamped component as claimed in claim 14 or 15, wherein, It has a coating on the surface.

17. The hot-stamped component as claimed in claim 16, wherein, The coating is a Zn-based coating or an Al-based coating.

18. The hot-stamped component as claimed in claim 17, wherein, The Zn-based coating contains 10–25% by mass of Ni.

19. A method for manufacturing a hot-stamped component, wherein, Using the following steel plate as raw material, the steel plate has the following composition: by mass % C: 0.180% or more and less than 0.300%, Mn: 3.50% or more and less than 11.0%, Si: 0.01-1.5%, P: less than 0.05%, S: less than 0.05%, Al: 0.005-0.1%, N: less than 0.01%, Sb: 0.002-0.03%, and W: 0.005-3.0%, with the balance being... It is composed of Fe and unavoidable impurities, and has the following microstructure: ferrite comprising 92.00% to 99.0% by volume, cementite comprising 1.0% to 5.0% by volume, and the balance comprising 5.0% or less, wherein the balance microstructure is selected from bainite and pearlite, and when the Mn concentrations of the ferrite and the cementite are set to Mnα and Mnθ respectively, the Mnθ / Mnα ratio is 1.4 or more, and Mn is enriched in the cementite. The steel plate is heated to a temperature range above Ac3 and below 1000°C, held within this temperature range for 10 seconds to 900 seconds, and then simultaneously stamped and quenched using a forming die. A hot-stamped component with a microstructure comprising 70.0% or more martensite by volume and 3.0% or more and 30.0% or less retained austenite by volume, and having a density of 2.0 × 10⁻⁶ ppm. 5 pcs / mm 2 The above contains residual austenite with a diameter of 0.3 μm or more, based on the equivalent circle diameter.

20. The method for manufacturing a hot-stamped component as described in claim 19, wherein, The composition, by mass%, also contains one or more groups selected from groups A to D below. Group A: Selected from one or more of the following: Ni: 0.01–5.0%, Cu: 0.01–5.0%, Cr: 0.01–5.0%, and Mo: 0.01–3.0%. Group B: Selected from one or more of the following: Ti: 0.005–3.0%, Nb: 0.005–3.0%, and V: 0.005–3.0%. Group C: Selected from one or more of the following: REM: 0.0005–0.01%, Ca: 0.0005–0.01%, and Mg: 0.0005–0.01%. Group D: B: 0.0005~0.05%.

21. The method for manufacturing a hot-stamped component as described in claim 19 or 20, wherein, The steel plate has a coating on its surface.

22. The method for manufacturing a hot-stamped component as described in claim 21, wherein, The coating is a Zn-based coating or an Al-based coating.

23. The method for manufacturing a hot-stamped component as described in claim 22, wherein, The Zn-based coating contains 10–25% by mass of Ni.

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