Title of the invention: coated steel member, coated steel sheet and method for manufacturing them.
Technical field
[0001]
The present invention relates to coated steel members, coated steel sheets, and methods for manufacturing them.
This application claims priority based on Japanese Patent Application No. 2019-068658 filed in Japan on March 29, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
In the field of steel sheets for automobiles, steel sheets with high tensile strength (high-strength steel sheets) have been applied in order to improve both fuel efficiency and collision safety against the background of recent stricter environmental regulations and collision safety standards. It is expanding. However, as the strength increases, the press formability of the steel sheet decreases, so that it becomes difficult to manufacture a product having a complicated shape.
[0003]
Specifically, the ductility of the steel sheet decreases as the strength increases, and there is a problem that the steel sheet breaks at a high-processed part when it is processed into a complicated shape. Further, as the strength of the steel sheet is increased, the residual stress after processing causes springback and wall warpage, which causes a problem that the dimensional accuracy is deteriorated. Therefore, it is not easy to press-mold a steel sheet having a high strength, particularly a tensile strength of 780 MPa or more, into a product having a complicated shape. Roll forming rather than press forming makes it easier to process high-strength steel sheets, but its application is limited to parts with a uniform cross section in the longitudinal direction.
[0004]
Therefore, in recent years, for example, as disclosed in Patent Documents 1 to 3, a hot stamping technique has been adopted as a technique for press-molding a material that is difficult to form, such as a high-strength steel plate. The hot stamping technique is a hot stamping technique in which a material to be molded is heated and then molded.
[0005]
With this technology, the material is heated before molding. Therefore, at the time of molding, the steel material is soft and has good moldability. As a result, even a high-strength steel sheet can be accurately formed into a complicated shape. Further, in the hot stamping technique, since quenching is performed at the same time as molding by a press die, the steel member after molding has sufficient strength.
[0006]
For example, according to Patent Document 1, it is disclosed that it is possible to impart a tensile strength of 1400 MPa or more to a steel member after forming by a hot stamping technique.
[0007]
On the other hand, higher fuel efficiency targets are currently set in various countries around the world. Along with this, weight reduction of the vehicle body has been studied, and further high strength is required for the steel member used for the vehicle body. For example, there is a need for a higher strength steel member that exceeds 1.5 GPa, which is the strength of a member generally used as a hot stamping member at present.
[0008]
Also, as mentioned above, automobiles are also required to have collision safety. The collision safety of an automobile is evaluated by the crush strength and absorbed energy in a collision test of the entire vehicle body or some members. In particular, since the crushing strength largely depends on the material strength, the demand for ultra-high strength steel members as automobile members is increasing dramatically.
However, in general, the fracture toughness and deformability of steel members decrease as the strength increases. Therefore, it may break at an early stage at the time of collision crushing, or it may break at a site where deformation is concentrated, and the crushing strength commensurate with the material strength may not be exhibited, and sufficient absorbed energy may not be obtained. Therefore, in order to improve the collision safety of automobiles, it is required to improve not only the material strength but also the fracture toughness and deformability, that is, the toughness and bendability of the steel members used.
[0009]
When the steel member has higher strength than the conventional hot stamping member, specifically, when the tensile strength exceeds 1.5 GPa, the bendability and toughness further deteriorate. Therefore, in order to apply a high-strength steel member with a tensile strength exceeding 1.5 GPa to a vehicle body, a steel member having toughness and bendability higher than before and exhibiting sufficient absorbed energy even in the event of a collision accident is required. We need the technology we provide.
[0010]
Regarding a high-strength steel material having a tensile strength of more than 1.5 GPa, for example, Patent Document 2 discloses a press-molded product which is excellent in toughness and hot press-formed with a tensile strength of 1.8 GPa or more. Patent Document 3 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more, and further having good toughness and ductility. Patent Document 4 discloses a steel material having a high tensile strength of 1.8 GPa or more and further having good toughness. Patent Document 5 discloses a steel material having an extremely high tensile strength of 2.0 GPa or more and further having good toughness.
However, Patent Documents 2 to 5 do not describe bendability, and in the use of high-strength steel materials having a tensile strength exceeding 1.5 GPa as an automobile member, it may not be possible to sufficiently meet higher demands.
[0011]
Regarding bendability, for example, Patent Documents 6 to 10 disclose hot stamp molded products having excellent bendability. However, Patent Documents 6 to 10 do not describe toughness, and in the use of high-strength steel materials having a tensile strength of more than 1.5 GPa as an automobile member, it may not be possible to sufficiently meet higher demands.
Prior art literature
Patent documents
[0012]
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-102980
Patent Document 2: Japanese Patent Application Laid-Open No. 2012-180594
Patent Document 3: Japanese Patent Application Laid-Open No. 2012-1802
Patent Document 4: International Publication No. 2015/182596
Patent Document 5: International Publication No. 2015/182591
Patent Document 6: International Publication No. 2015/033177
Patent Document 7: International Publication No. 2018/151333
Patent Document 8: International Publication No. 2018/151330
Patent Document 9: International Publication No. 2018/151332
Patent Document 10: International Publication No. 2018/151325
Outline of the invention
Problems to be solved by the invention
[0013]
The present invention has been made to solve the above-mentioned problems, and is a coated steel member having high tensile strength and excellent toughness and bendability, and a coated steel sheet suitable as a material for this steel member. And to provide a method for producing them.
Means to solve problems
[0014]
The gist of the present invention is the following coated steel members, coated steel sheets, and methods for manufacturing them. Hereinafter, a steel sheet having no coating on the surface, which is a material of the coated steel sheet, is simply referred to as a "steel sheet".
[0015]
(1) The coated steel member according to one aspect of the present invention has C: 0.25 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00 in mass%. %, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, Nb: 0.02 to 0.10%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1 .00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, and It has a steel sheet base material containing REM: 0 to 0.30% and having a chemical composition in which the balance is Fe and impurities, and a coating formed on the surface of the steel sheet base material and containing Al and Fe. The coating is formed on the surface side of the low Al-containing region having an Al content of 3% by mass or more and less than 30% by mass and the low Al-containing region, and has an Al content of 30% by mass or more. The maximum C content of the high Al content region is 25% or less of the C content of the steel sheet base material, and the maximum C content of the low Al content region is the above. The maximum C content in the range from the interface between the steel sheet base material and the coating to the depth of 10 μm on the steel sheet base material side is 40% or less of the C content of the steel sheet base material. It is 80% or less with respect to the C content.
(2) In the coated steel member according to (1) above, the steel sheet base material contains Cr: 0.05 to 1.00% as the chemical composition, and the maximum Cr content in the high Al content region is , 80% or more of the Cr content of the steel sheet base material may be used.
(3) The coated steel sheet according to another aspect of the present invention has C: 0.25 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to 3.00 in mass%. %, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100%, Nb: 0.02 to 0.10%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1 .00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00%, and It has a steel sheet having a chemical composition containing REM: 0 to 0.30% and the balance being Fe and impurities, and a coating formed on the surface of the steel sheet and containing Al. A lower layer existing on the steel sheet side and containing 3% by mass or more and less than 70% by mass of Al and an upper layer containing 70% by mass or more and 95% by mass or less of Al are provided, and the lower layer is contained in the steel sheet in mass%. The steel sheet contains 1.2 times or more Cr of the Cr content, or the upper layer contains Si and Ni in a total amount of 5.0% by mass or more and 30.0% by mass or less, and the steel sheet is formed from the interface between the steel sheet and the coating. The maximum C content in the range up to a depth of 20 μm on the side is 80% or less of the average C content in the total thickness of the steel sheet.
(4) In the coated steel sheet according to (3) above, the steel sheet contains Cr: 0.05 to 1.00% as the chemical composition, and in the coating, the lower layer is by mass% and the steel sheet. The upper layer may contain Si and Ni in a total amount of 5.0% by mass or more and 30.0% by mass or less.
(5) The method for producing a coated steel sheet according to another aspect of the present invention is, in mass%, C: 0.25 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to. 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100% , Nb: 0.02 to 0.10%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00. % And REM: 0 to 0.30%, and a slab preparation step of melting and casting a steel having a chemical composition in which the balance is Fe and impurities to obtain a slab, and hot rolling on the slab. A hot rolling step of applying the hot-rolled steel sheet to obtain a hot-rolled steel sheet, a winding step of winding the hot-rolled steel sheet, and 450 to 800 in an atmosphere in which the hot-rolled steel sheet after the winding step contains 80% or more of nitrogen. A hot-rolled steel sheet annealing step in which the hot-rolled steel sheet is annealed at ° C. for 5 hours or more, and a cold-rolled step in which the hot-rolled steel sheet is descaled and cold-rolled to obtain a cold-rolled steel sheet, if necessary. The hot-rolled steel sheet or the cold-rolled steel sheet is annealed to obtain an annealed steel sheet, and the hot-rolled steel sheet, the cold-rolled steel sheet or the annealed steel sheet is formed with an Al-based coating to form a coated steel sheet. Covering process and
To be equipped.
(6) The method for producing a coated steel sheet according to another aspect of the present invention is, in mass%, C: 0.25 to 0.65%, Si: 0.10 to 2.00%, Mn: 0.30 to. 3.00%, P: 0.050% or less, S: 0.0100% or less, N: 0.010% or less, Ti: 0.010 to 0.100%, B: 0.0005 to 0.0100% , Nb: 0.02 to 0.10%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, V: 0 to 1.00%, Ca: 0 to 0.010%, Al: 0 to 1.00%, Sn: 0 to 1.00%, W: 0 to 1.00%, Sb: 0 to 1.00. % And REM: 0 to 0.30%, and the balance is Fe and impurities. Steel having a chemical composition is melted and cast to obtain a slab, and hot rolling is performed on the slab. A hot-rolling step of applying the hot-rolled steel sheet to obtain a hot-rolled steel sheet, a winding step of winding the hot-rolled steel sheet, and, if necessary, a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet, and if necessary. And the hot rolling