JP4781836B2 - Ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, its manufacturing method, manufacturing method of ultra-high strength hot-dip galvanized steel sheet, and manufacturing method of ultra-high-strength galvannealed steel sheet - Google Patents

Ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, its manufacturing method, manufacturing method of ultra-high strength hot-dip galvanized steel sheet, and manufacturing method of ultra-high-strength galvannealed steel sheet Download PDF

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JP4781836B2
JP4781836B2 JP2006031131A JP2006031131A JP4781836B2 JP 4781836 B2 JP4781836 B2 JP 4781836B2 JP 2006031131 A JP2006031131 A JP 2006031131A JP 2006031131 A JP2006031131 A JP 2006031131A JP 4781836 B2 JP4781836 B2 JP 4781836B2
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康治 佐久間
直樹 吉永
和彦 本田
昌史 東
鉄生 西山
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新日本製鐵株式会社
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  The present invention relates to an ultra-high-strength steel sheet excellent in hydrogen embrittlement resistance suitable for use in automobiles, building materials, home appliances, etc., a manufacturing method thereof, a manufacturing method of an ultra-high-strength hot-dip galvanized steel sheet, and an ultra-high-strength alloyed hot-dip galvanizing. The present invention relates to a method for manufacturing a steel sheet.

Conventionally, high-strength steel is often used for applications such as bolts, PC steel wires, and line pipes, and it is known that hydrogen embrittlement occurs due to hydrogen intrusion into steel when the strength becomes 980 MPa or more. . On the other hand, (1) the thin steel plate is thin, so that hydrogen is released in a short time even if hydrogen enters, and (2) there is almost no use of a steel plate of 980 MPa or more in terms of workability. The hydrogen embrittlement was hardly regarded as a problem.
In recent years, due to the need to reduce the weight of automobiles and improve collision safety, the use of ultra-high strength thin steel sheets of 980 MPa or higher for reinforcing materials such as bumpers and impact beams has been rapidly increasing. There is an urgent need to develop ultra-high-strength thin steel sheets equipped with.
The technology for improving the hydrogen embrittlement resistance has been developed for steel materials that are often used as products such as bolts, strips, and thick plates, and are often used below the yield strength or yield stress.

For example, in steel bars and bolt steels, development has been centered on tempered martensite, and additive elements exhibiting temper softening resistance such as Cr, Mo and V are effective in improving hydrogen embrittlement resistance. It has been reported (Non-Patent Document 1). This is a technique for precipitating alloy carbides and using them as hydrogen trap sites to shift the hydrogen embrittlement form from grain boundaries to intragranular fracture.
There has also been a proposal that an oxide mainly composed of Ti and Mg is effective in preventing hydrogen defects (Patent Document 1).

On the other hand, with regard to hydrogen embrittlement of thin steel sheets, for example, it has been reported about the promotion of hydrogen embrittlement due to work-induced transformation of the amount of retained austenite (Non-Patent Document 2). Although this considers the forming process of a thin steel plate, the regulation of the amount of retained austenite that does not deteriorate the hydrogen embrittlement resistance has been reported.
In addition, as a thin steel plate considering hydrogen trapping ability and formability, a steel plate for a hollow container excellent in resistance to tearing has been proposed (Patent Document 2). This is intended to suppress a surface defect called a trap generated after enamelling by trapping hydrogen that enters the steel sheet during production with an oxide contained in the steel sheet. Therefore, a large amount of oxide is contained inside the steel plate.
"New development of hydrogen brittleness elucidation", Japan Iron and Steel Institute, published in January 1997 Yamazaki et al., "Effect of microstructure on workability and delayed fracture properties of ultra-high strength cold-rolled steel sheet (CAMP-ISIJ)", Japan Iron and Steel Institute, October 1992, Vol. 6, No. 6, 1839-1842 page JP-A-11-293383 Japanese Patent Laid-Open No. 11-100638

By the way, the conventional steel of Non-Patent Document 1 has a C content of 0.4% or more and contains a large amount of alloy elements, so that the workability and weldability required for a thin steel sheet are inferior, and alloy carbide is precipitated. Therefore, since it is necessary to heat-treat for several hours or more, there is a problem that the manufacturing process becomes very long and the manufacturing cost increases.
In addition, the steel plate of Patent Document 1 is a thick steel plate, and although hydrogen embrittlement after welding with particularly high heat input is taken into consideration, both high formability and hydrogen embrittlement required for a thin steel plate are achieved. No point is taken into account.
The thin steel sheet of Non-Patent Document 2 is a high-strength thin steel sheet having a specific structure, and is not a measure for improving the fundamental hydrogen embrittlement resistance.

  Moreover, since the steel plate for enamel containers of patent document 2 contains a large amount of oxides in the steel plate in order to suppress the surface defect called the picking, these oxides are dispersed in the steel plate at high density. As a result, the formability is deteriorated, and there is a problem in application to automobile steel sheets that require high formability. In addition, this steel sheet is not intended to achieve both high strength and hydrogen embrittlement resistance.

Thus, delayed fracture in tempered martensitic steel is considered to be caused by the accumulation of hydrogen at the prior austenite grain boundaries and the like, resulting in fracture starting from that portion. Therefore, if the hydrogen trap sites are dispersed evenly and finely and hydrogen is trapped there, the concentration of diffusible hydrogen is lowered and the susceptibility to delayed fracture is lowered. As already described, it has been found that the delayed fracture resistance due to hydrogen is improved by controlling the oxide dispersion in the thick steel sheet to which Mg and Ti are added in combination. These oxides have poor moldability and deteriorate the moldability when subjected to strong processing. As a result, in thin steel sheets that require excellent workability, it is difficult to disperse oxides such as Ti and Mg at such a high density that hydrogen embrittlement can be suppressed.
Alternatively, since it takes a long time to disperse alloy carbides such as V in steel, it is difficult to utilize in continuous annealing lines and continuous plating lines, which are production lines for thin steel sheets.

  The present invention has been made in view of the above circumstances, and is an ultra-high-strength steel sheet that prevents hydrogen defects and has excellent resistance to hydrogen embrittlement at welds, its manufacturing method, and ultra-high-strength hot-dip galvanizing. It aims at providing the manufacturing method of a steel plate, and the manufacturing method of an ultra-high-strength galvannealed steel plate.

As a result of intensive studies, the present inventors have found a method for improving the hydrogen embrittlement resistance in consideration of the use environment in the thin steel sheet. That is, by adding Si, Mn, Al and Cr to the steel sheet and controlling the atmosphere in the continuous annealing process or the continuous hot dip galvanizing process, the surface layer within 10 μm from the surface of the steel sheet, in the plating layer, or with the steel sheet By forming an oxide in the crystal grain boundary in the surface layer within 10 μm from the interface with the hot dip galvanized layer, or in the crystal grain, or in the crystal grain boundary and the crystal grain, formability that is a necessary characteristic as a thin steel plate It has been found that an ultra-high strength steel sheet having a tensile maximum strength of 980 or more and excellent in hydrogen embrittlement resistance can be obtained without impairing corrosion resistance.
This invention is made | formed based on said knowledge, The place made into the summary is as follows.

(1) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total of each content is 0.3% or more, and any of crystal grain boundaries, crystal grains, crystal grain boundaries, and crystal grains in the surface layer within 10 μm from the surface of the steel plate, the balance being iron and inevitable impurities Or one or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO as the oxide , And Mn 2 SiO 4 and Cr 2 O 3 in total, an average content of 0.01 to 30 An ultra-high-strength steel sheet excellent in hydrogen embrittlement resistance, characterized by containing in mass% and having a maximum tensile strength of 980 MPa or more.

(2) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total content of each is 0.3% or more, the balance is less than 7% by mass of Fe on the surface of the steel plate made of iron and inevitable impurities, and the remainder is made of Zn, Al and inevitable impurities. A galvanized layer is formed, and any one of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the steel sheet within 10 μm from the interface between the steel sheet and the hot dip galvanized layer, or in the hot dip galvanized layer. one kind or two or more, as oxides, SiO 2, FeSiO 3, Fe 2 SiO 4, nSiO 3, Al 2 O 3, MnAl 2 O 4, 1 or more kinds selected from the group of MnO, and a Mn 2 SiO 4 and Cr 2 O 3, a total average content 0.01 An ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, characterized by being contained at 30% by mass and having a maximum tensile strength of 980 MPa or more.

(3) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total content of each is 0.3% or more, and the balance of the steel sheet is made of iron and unavoidable impurities, and 7 to 15 mass% of Fe is contained, and the balance is made of Zn, Al and unavoidable impurities. Forming an alloyed hot-dip galvanized layer, and crystal grain boundaries within the steel plate within 10 μm from the interface between the steel sheet and the alloyed hot-dip galvanized layer; any one of the galvanized layer or two or more, as oxides, SiO 2, FeSiO 3 Fe 2 SiO 4, MnSiO 3, Al 2 O 3, MnAl 2 O 4, 1 or more kinds selected from the group of MnO, and a Mn 2 SiO 4 and Cr 2 O 3, in total, the average content An ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, characterized by containing at a rate of 0.01 to 30% by mass and having a maximum tensile strength of 980 MPa or more.

(4) Any one of the above (1) to (3), further comprising Ni: 0.05 to 1.0% and Cu: 0.05 to 1.0% by mass% An ultra high strength steel plate with excellent hydrogen embrittlement resistance.

(5) Further, by mass%, Nb: 0.005-0.3%, Ti: 0.005-0.3%, V: 0.005-0.3%, W: 0.005-0. The ultra-high strength steel sheet having excellent hydrogen embrittlement resistance according to any one of the above (1) to (4), comprising one or more selected from the group of 3%.

(6) Further, in the mass%, B: 0.0001 to 0.1% is contained, and the superb resistance to hydrogen embrittlement according to any one of the above (1) to (5) High strength steel plate.

(7) Further, in mass%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, La: 0.0005 to 0.01%, Ce: 0.0005 to 0.00. 01%, Y: 1 type or 2 types or more selected from the group of 0.0005-0.01%, The resistance in any one of said (1) thru | or (6) characterized by the above-mentioned Super high strength steel sheet with excellent hydrogen embrittlement.

(8) A method of heat treating a high strength steel sheet consisting of chemical components according to any one of (1) to (7), and H 2 contains 1 to 60 vol%, the remainder N 2, H 2 In an atmosphere composed of O, O 2 and inevitable impurities, the water pressure PH 2 O and the hydrogen partial pressure PH 2 in this atmosphere are expressed by the following formula { −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5 } Is controlled to satisfy any of the following, by oxidizing to one or more of grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the surface layer within 10 μm from the surface of the steel sheet. One or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, and Mn 2 SiO 4 and Cr 2 the O 3, a total average content Method for manufacturing ultra-high strength steel sheet excellent in embrittlement and controls to contain at .01~30 wt%.

( 9 ) The cast slab composed of the chemical component according to any one of (1) to (7) above is directly or once cooled and then heated to 1200 ° C or higher, and then hot-rolled at a temperature equal to or higher than the Ar3 transformation point. Next, it is wound up in a temperature range of 630 ° C. or less, then cold-rolled with a rolling reduction of 40 to 70%, and then subjected to continuous annealing, which is a continuous annealing method. At this time, 1 to 60% by volume of H 2 is contained, the balance is made of N 2 , H 2 O, O 2 and unavoidable impurities, and the water pressure PH 2 O and the hydrogen partial pressure PH 2 are expressed by the following formula { −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5 } In an atmosphere controlled to satisfy ≦ log (PH 2 O / PH 2 ) ≦ −0.5 } , heating is performed at an average heating rate of 0.7 ° C./second or more at 550 to 750 ° C. Above and at 900 ° C or lower, then up to 650 ° C Cooling is performed at a uniform cooling rate of 0.1 to 200 ° C./second, and a temperature between 650 ° C. and 500 ° C. is cooled at an average cooling rate of 4 to 200 ° C./second to obtain a temperature of 200 to 500 ° C. By holding for 2 seconds or more , any one or two or more kinds of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the surface layer within 10 μm from the surface of the steel sheet are used as oxides, SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , or one or more selected from the group of MnO, and Mn 2 SiO 4 and Cr 2 O 3 in total, The manufacturing method of the ultra high strength steel plate excellent in hydrogen embrittlement resistance characterized by controlling so that it may contain at an average content rate of 0.01-30 mass% .

( 10 ) The cast slab composed of the chemical component according to any one of (1) to (7) above is directly or once cooled and then heated to 1200 ° C. or higher, and then hot-rolled at a temperature equal to or higher than the Ar3 transformation point. Is then rolled in a temperature range of 630 ° C. or lower, then cold rolled at a rolling reduction of 40 to 70%, and then hot dip galvanized, followed by a method of manufacturing an ultra high strength hot dip galvanized steel sheet. In carrying out the hot dip galvanization, 1 to 60% by volume of H 2 is contained, the balance is made of N 2 , H 2 O, O 2 and unavoidable impurities, and water pressure PH 2 O and hydrogen partial pressure PH 2 is controlled so as to satisfy the following formula { −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5 } , an average heating rate of 0.7 ° C./second between 550 and 750 ° C. Heat above, 750 ℃ or more and 900 ℃ or less Then, it is cooled to 650 ° C. at an average cooling rate of 0.1 to 200 ° C./second, and is cooled between 650 ° C. and 500 ° C. at an average cooling rate of 3 to 200 ° C./second (zinc plating bath temperature). −40) to (temperature of galvanizing bath +50) ° C., and then immersed in a galvanizing bath to obtain crystal grains in the steel plate within 10 μm from the interface between the steel plate and the hot dip galvanized layer. Any one or more of the boundary, the crystal grain, the crystal grain boundary and the crystal grain, and the hot dip galvanized layer may be SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al as an oxide. One or two or more selected from the group of 2 O 3 , MnAl 2 O 4 and MnO, and Mn 2 SiO 4 and Cr 2 O 3 in total, the average content is 0.01 to 30% by mass To contain High-strength method for manufacturing a galvanized steel sheet having excellent resistance to hydrogen embrittlement, characterized by.

( 11 ) The cast slab composed of the chemical component according to any one of (1) to (7) above is directly or once cooled and then heated to 1200 ° C or higher, and then hot-rolled at a temperature equal to or higher than the Ar3 transformation point. And then rolled up in a temperature range of 630 ° C. or lower, then cold rolled at a rolling reduction of 40 to 70%, and then subjected to hot dip galvanizing and alloying treatment. In the production method, the hot dip galvanizing is performed, the H 2 content is 1 to 60% by volume, the balance is N 2 , H 2 O, O 2 and unavoidable impurities, and the moisture pressure PH 2 An average heating rate between 550 and 750 ° C. in an atmosphere in which O and the hydrogen partial pressure PH 2 are controlled so as to satisfy the following formula { −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5 } Heat at 0.7 ° C / second or more, 750 More than this and annealing at 900 ° C. or less, and then cooling to 650 ° C. at an average cooling rate of 0.1 to 200 ° C./second, and cooling between 650 ° C. and 500 ° C. at an average cooling rate of 3 to 200 ° C./second ( Zinc plating bath temperature −40) ° C. to (Zinc plating bath temperature + 50) ° C., then immersed in a zinc plating bath, further subjected to alloying treatment at a temperature of 460 ° C. or higher, and then to room temperature By cooling , crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains within the steel sheet within 10 μm from the interface between the steel sheet and the alloyed hot dip galvanized layer, within the alloyed hot dip galvanized layer Any one or two or more of these may be selected from the group consisting of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , and MnO as an oxide. More than species, And Mn 2 SiO 4 and Cr 2 O 3 are controlled so as to contain an average content of 0.01 to 30% by mass in total, and an ultra-high strength alloy having excellent hydrogen embrittlement resistance is provided. Manufacturing method of hot dip galvanized steel sheet.

According to the ultra high strength steel sheet of the present invention, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% or more and 3.0% or less, Al: 2 in mass%. 0.0% or less, Cr: 3.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and The total content of each of Si, Mn, Al and Cr is 0.3% or more, and the surface layer within 10 μm from the surface of the steel plate, the balance being iron and inevitable impurities, or this steel plate and hot dip galvanized layer In a steel plate within 10 μm from the interface with the steel, or in a steel plate within 10 μm from the interface between the steel plate and the alloyed hot dip galvanized layer, crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains, molten zinc In the plating layer or the alloyed hot-dip galvanized layer, any one or more of the oxide and One or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, and Mn 2 SiO 4 and Cr 2 Since O 3 is contained in an average content of 0.01 to 30% by mass in total, it has moldability and corrosion resistance suitable for structural members, reinforcing members, and suspension members for automobiles. It is possible to provide an ultra-high strength steel sheet having a maximum strength of 980 MPa or more and having excellent resistance to hydrogen embrittlement at a weld.

The best mode of the ultra high strength steel sheet excellent in hydrogen embrittlement resistance of the present invention, its manufacturing method, the manufacturing method of the ultra high strength hot dip galvanized steel sheet, and the manufacturing method of the ultra high strength alloyed hot dip galvanized steel sheet will be described. Here, a high yield ratio high strength cold-rolled steel sheet having good ductility and tensile strength (TS) of 780 MPa class will be described as an example.
In addition, since this form is demonstrated in detail in order to make the meaning of invention be better understood, unless otherwise specified, this invention is not limited.

The ultra-high-strength steel sheet of the present invention is any one of the following steel sheets (1) to (3).
(1) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total of each content is 0.3% or more, and any of crystal grain boundaries, crystal grains, crystal grain boundaries, and crystal grains in the surface layer within 10 μm from the surface of the steel plate, the balance being iron and inevitable impurities Or one or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO as the oxide , And Mn 2 SiO 4 and Cr 2 O 3 in total, an average content of 0.01 to 30 It is contained in mass% and has a maximum tensile strength of 980 MPa or more.

(2) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total content of each is 0.3% or more, the balance is less than 7% by mass of Fe on the surface of the steel plate made of iron and inevitable impurities, and the remainder is made of Zn, Al and inevitable impurities. A galvanized layer is formed, and any one of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the steel sheet within 10 μm from the interface between the steel sheet and the hot dip galvanized layer, or in the hot dip galvanized layer. one kind or two or more, as oxides, SiO 2, FeSiO 3, Fe 2 SiO 4, nSiO 3, Al 2 O 3, MnAl 2 O 4, 1 or more kinds selected from the group of MnO, and a Mn 2 SiO 4 and Cr 2 O 3, a total average content 0.01 It is contained at 30% by mass, has a maximum tensile strength of 980 MPa or more, and is called an ultra-high strength hot-dip galvanized steel sheet.

(3) By mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3 0.0% or less, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less, and the Si, Mn, Al and Cr The total content of each is 0.3% or more, and the balance of the steel sheet is made of iron and unavoidable impurities, and 7 to 15 mass% of Fe is contained, and the balance is made of Zn, Al and unavoidable impurities. Forming an alloyed hot-dip galvanized layer, and crystal grain boundaries within the steel plate within 10 μm from the interface between the steel sheet and the alloyed hot-dip galvanized layer; any one of the galvanized layer or two or more, as oxides, SiO 2, FeSiO 3 Fe 2 SiO 4, MnSiO 3, Al 2 O 3, MnAl 2 O 4, 1 or more kinds selected from the group of MnO, and a Mn 2 SiO 4 and Cr 2 O 3, in total, the average content It is contained at a rate of 0.01 to 30% by mass, has a maximum tensile strength of 980 MPa or more, and is called an ultra-high-strength galvannealed steel sheet.

Here, the reason for limiting the steel composition as described above will be described. Here, “%” means “mass%”.
C is an element that can increase the strength of the steel sheet. Here, if the C content is less than 0.06%, the maximum tensile strength cannot be 980 MPa or more. Therefore, the lower limit was made 0.06%. In addition, when the content is less than 0.06%, the maximum tensile strength cannot be increased to 980 MPa or more. However, the improvement of chemical conversion by suppressing the formation of oxide on the steel sheet surface layer, the wettability of hot dipping, and the alloy Effects such as crystallization promotion can be obtained. On the other hand, if the C content exceeds 0.25%, it becomes difficult to ensure the strength of the weld. Therefore, the upper limit is limited to 0.25%. However, if the strength of the weld is not a problem, the effect of improving hydrogen embrittlement resistance, which is an effect of the present invention, can be brought out.

Si is a strengthening element and is effective in increasing the strength of the steel sheet. In addition, it is desirable to add the oxide because it contributes to the improvement of hydrogen embrittlement resistance. Since addition exceeding 2.0% lowers moldability, the upper limit was made 2.0%. The lower limit is not particularly limited, but it is difficult to set it to 0.0005% or less, so this is a substantial lower limit.
Mn is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it exceeds 3.0%, the formability of the steel sheet is lowered, so the upper limit was made 3.0%. The lower limit is not particularly limited, but it is difficult to set it to 0.0005% or less, so this is a substantial lower limit.

Al may be added because it promotes ferrite formation and improves ductility. It can also be used as a deoxidizer. In addition, the oxide may contribute to the improvement of hydrogen embrittlement resistance and may be added. Here, excessive addition deteriorates moldability, so the upper limit was made 2.0%. Moreover, although a minimum is not specifically limited, Since it is difficult to set it as 0.0005% or less, this is a substantial minimum.
Cr may be added because it contributes to improvement of hydrogen embrittlement resistance by using an oxide. Here, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%. Moreover, although a minimum is not specifically limited, In order to utilize the hydrogen embrittlement improvement effect by Cr addition, adding 0.05% or more is preferable. In addition, it may be added to increase the strength.

  The ultra-high-strength steel sheet of the present invention can improve hydrogen embrittlement resistance by forming an oxide containing Si, Mn, Al and Cr alone or a composite oxide containing a plurality of these elements on the surface of the steel sheet. Therefore, the total content of Si, Mn, Al and Cr needs to be 0.3% or more. Here, when the total content is less than 0.3%, the content of oxides is too small to significantly improve the hydrogen embrittlement resistance. Therefore, the lower limit was made 0.3%. The upper limit is not particularly defined, but if it exceeds the upper limit of each content, the moldability deteriorates, so it is necessary to be within the respective upper limit range.

P tends to segregate in the central part of the plate thickness of the steel sheet, causing the weld to become brittle. When the content exceeds 0.04%, the weld becomes brittle, so the appropriate range is limited to 0.04% or less. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001%.
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01%. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%. In addition, S combines with Mn to form coarse MnS, reducing the hole expandability. Therefore, it is necessary to reduce as much as possible in order to improve hole expandability.

O forms an oxide and degrades the moldability, so the amount added must be suppressed. In particular, when O exceeds 0.01%, this tendency becomes remarkable. Therefore, the upper limit of the O content is set to 0.01%. Moreover, since making it less than 0.001% invites excessive cost and is not preferable economically, 0.001% is preferable as a lower limit.
Here, the content of O is the content of O contained in the steel sheet after removing the internal oxide contained in the steel sheet surface layer and the oxide contained in the plating layer. In particular, since the steel sheet contains an oxide in one or both of the steel sheet surface layer and the plating layer, the O content in the surface layer is higher than that in the steel sheet interior. However, since these oxides are present in the plating layer or on the surface layer of the steel sheet, the formability is hardly adversely affected.

  N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the N content range is set to 0.01% or less. In addition, it is better to use less because it causes blowholes during welding. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased, and this is a substantial lower limit. .

In addition to the above composition, the ultra high strength steel sheet of the present invention has
Furthermore, it is preferable to contain Ni: 0.05-1.0% and Cu: 0.05-1.0% by the mass%.
Ni is a strengthening element and is important for improving hardenability. However, since these effects cannot be obtained if the content is less than 0.05%, the lower limit is set to 0.05%. On the other hand, if it exceeds 1%, the manufacturability at the time of production and hot rolling will be adversely affected, so the upper limit was made 1%. In addition, it may be added because it improves wettability and promotes the alloying reaction.

  Cu is a strengthening element and is important for improving hardenability. However, since these effects cannot be obtained if the content is less than 0.05%, the lower limit is set to 0.05%. On the other hand, if it exceeds 1%, the manufacturability at the time of production and hot rolling will be adversely affected, so the upper limit was made 1%. In addition, it may be added because it improves wettability and promotes the alloying reaction.

In addition to the above composition, the ultra high strength steel sheet of the present invention has
Furthermore, by mass%, Nb: 0.005-0.3%, Ti: 0.005-0.3%, V: 0.005-0.3%, W: 0.005-0.3% It is preferable to contain 1 type (s) or 2 or more types selected from the group.
Nb is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. On the other hand, if the content exceeds 0.3%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.3%.

Ti is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. On the other hand, if the content exceeds 0.3%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.3%.
V is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. On the other hand, if the content exceeds 0.3%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.3%.

W is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the content is less than 0.005%, these effects cannot be obtained, so the lower limit was made 0.005%. On the other hand, if the content exceeds 0.3%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.3%.
Further, since carbides and nitrides containing Ti, Nb, V and W alone or in combination improve the hydrogen embrittlement resistance of the base material of the steel sheet, by containing these elements, the hydrogen embrittlement resistance is improved. Further improve.

In addition to the above composition, the ultra high strength steel sheet of the present invention has
Furthermore, it is preferable to contain B: 0.0001 to 0.1% by mass%.
B is effective for strengthening grain boundaries and strengthening steel by containing 0.0001% by mass or more, but when the content exceeds 0.0045% by mass, the effect is not only saturated but also heat The upper limit is set to 0.0045% because the production at the time of rolling is lowered.

In addition to the above composition, the ultra high strength steel sheet of the present invention has
Furthermore, in mass%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, La: 0.0005 to 0.01%, Ce: 0.0005 to 0.01%, It is preferable to contain 1 type (s) or 2 or more types selected from the group of Y: 0.0005-0.01%.
The total content of one or more selected from the group of Ca, Mg, La, Ce, and Y is preferably 0.0005 to 0.04%.
Ca, Mg, La, Ce, and Y are elements used for deoxidation, and it is preferable to contain one or more elements in total of 0.0005% or more. However, when the content exceeds 0.04% in total, it causes deterioration of molding processability. Therefore, the total content is set to 0.0005 to 0.04%.

In addition, in this invention, the effect of this invention is exhibited by containing La and Ce. La and Ce are often added by misch metal, and in addition to La and Ce, a plurality of other rare earth elements may be contained. Even if rare earth elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited.
Here, the steel sheet strength is set to 980 MPa or more because the steel having a strength of less than 980 MPa is less concerned with hydrogen embrittlement.
By including an oxide containing Si, Mn, and Al alone or in combination in the steel sheet surface layer or plating layer, the hydrogen embrittlement resistance of the welded portion can be enhanced. The steel sheet surface layer means not the steel sheet surface but the inside of the steel sheet. In the plated steel sheet, it means the steel sheet side at the interface between the plated layer and the steel sheet. Of course, a part of the oxide may be present on the surface of the steel sheet, but if the oxide is formed inside the steel sheet, it is assumed that the oxide is formed on the surface layer of the steel sheet.

The oxide containing Si, Mn, Al and Cr alone or in combination is controlled by controlling the atmosphere during heating in the annealing line or the continuous plating line, so that Si, Mn, Al and Cr contained in the steel sheet Alternatively, an oxide can be formed on either one or both of the plating layers.
Here, the oxide containing Si, Mn, Al, and Cr alone or in combination is used in the steel sheet surface layer or plating layer, without impairing formability, which is a necessary characteristic of the thin steel sheet, which is an effect of the present invention. This is to improve hydrogen embrittlement resistance.
In addition, the oxide is formed in the surface layer or plating layer of the steel sheet. When the oxide with poor formability is dispersed to the center of the plate thickness, the formability, which is a necessary characteristic for a thin steel sheet, is greatly increased. It is because it will deteriorate. If it is only in the steel sheet surface layer or plating layer, only hydrogen embrittlement resistance can be improved without impairing formability.

In addition, since these oxides exist in the surface layer of the steel sheet, they function more efficiently as hydrogen trap sites for suppressing hydrogen embrittlement. This is presumed to be due to the fact that the starting point of hydrogen embrittlement cracking is the stress concentration part of the steel sheet surface layer. That is, by dispersing the oxide in the steel sheet surface layer, the effect can be exhibited even if the mass% of the oxide contained in the whole steel sheet is small. In addition, even if hydrogen enters the steel sheet, it is easily released because hydrogen is trapped in the steel sheet surface layer.
Specifically, in an actual environment where corrosion (hydrogen generation and entry) -drying (hydrogen release) occurs continuously, even if hydrogen enters, it is trapped on the steel sheet surface layer, so the diffusion distance to the steel sheet surface is It is short and the release of hydrogen takes place in a short time, which is more effective.

The required properties of thin steel sheets include corrosion resistance, but in cold-rolled steel sheets, the corrosion resistance is improved by subjecting the steel sheet surface to chemical conversion treatment and coating treatment. It is required not to deteriorate.
Since oxides containing Si, Mn, Al and Cr present on the surface of a cold-rolled steel sheet alone or in combination are known to degrade chemical conversion properties, these oxides must be formed inside the steel sheet. There is. In addition, since oxides containing Si, Mn, Al and Cr alone or in combination are difficult to conduct electricity, if these oxides cover the steel sheet surface, resistance welding such as spot or mash seam welding is difficult. Have. Therefore, an oxide containing Si, Mn, and Al alone or in combination needs to be formed as an internal oxide on the steel sheet surface layer.

  Moreover, if it is a plated steel plate, the corrosion resistance is improved by performing hot dip galvanizing treatment, or hot dip galvanizing treatment and alloying treatment on the surface of the steel plate, ensuring the wettability of these hot dip platings, or If it is an alloyed hot-dip galvanized steel sheet, it is necessary to promote alloying. Here, if there is an oxide containing Si, Mn, Al, and Cr alone or in combination before immersion in the plating bath, the wettability with hot-dip plating is reduced or alloying is delayed. Furthermore, a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet cannot be obtained. For this reason, it is necessary to suppress oxide formation on the steel sheet surface before immersion in the plating bath.

In the present invention, by controlling the atmosphere during heating, oxide formation on the surface of the steel sheet during heating is suppressed, and oxide formation inside the steel sheet becomes possible.
As long as the oxide is inside the steel plate, plating properties can be secured even if it is formed in the plating layer, in the crystal grain of the steel plate surface layer, or in the crystal grain boundary, or in the crystal grain and crystal grain boundary. In particular, within 10 μm from the surface layer of the steel plate, or within 10 μm from the interface between the steel plate and the hot dip galvanized layer (or alloyed hot dip galvanized layer), By forming it in either the crystal grains or the plating layer, it is possible to sufficiently ensure the plating property.
As a result, it became possible to manufacture a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having hydrogen embrittlement resistance at the weld.

  Here, in the plated steel sheet, the oxide may be contained in either the plating layer or the surface layer of the steel sheet even if the oxide is present inside the steel sheet before immersion in the plating bath. After or after alloying treatment, interdiffusion occurs between the plating layer and the steel plate, and the interface between the plating layer and the steel plate may move to the steel plate side, so it was formed inside the steel plate before immersion in the plating bath. Even if it is an oxide, it may exist in both a plating layer and the inside of a steel plate. Therefore, the oxide may be contained in any of the plating layer and the steel sheet surface layer. For example, in a hot dip galvanized steel sheet, since alloying has hardly progressed, an oxide containing Si is often present on the steel sheet side. On the other hand, in the alloyed hot-dip galvanized steel sheet, since alloying has progressed, oxides containing Si, Mn, Al and Cr alone or in combination exist in the plated layer compared to hot-dip galvanized steel sheet. There are many ratios to do.

The oxide containing Si, Mn, Al, and Cr alone or in combination can be controlled by controlling the atmosphere during heating in the annealing line or continuous plating line, in the plating layer or in the crystal grains within 10 μm from the steel sheet surface layer, or It can be easily generated at the crystal grain boundary, or within the crystal grain and at the crystal grain boundary.
In the case of a cold-rolled steel sheet, the effect is that the average content of oxides of Si, Mn, Al, and Cr is either in the grain boundary within 10 μm from the steel sheet surface layer, or in the crystal grain, or both. It is exhibited by containing 0.01 to 30% by mass.
Further, in the case of a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet, the grain boundary on the steel sheet side within 10 μm from the interface between the steel sheet and the plating layer, or in the crystal grain, or in the crystal grain boundary and the crystal grain In addition, the oxide is exerted by adding an oxide at an average content of 0.01 to 30% by mass in either one or both of the plating layers.

Here, the reason why it is within 10 μm from the surface layer of the steel sheet or in the plating layer is that excessively forming the oxide to the inside of the steel sheet requires excessive heat treatment and is not economically preferable.
By setting the content of the oxide to 0.01 to 30% by mass, the hydrogen embrittlement resistance is greatly improved. Here, the reason why the content is set to 0.01% by mass or more is that when the content is less than 0.01% by mass, it is difficult to obtain an effect of improving hydrogen embrittlement resistance. On the other hand, if the content exceeds 30% by mass, there is a possibility that the wettability with hot-dip plating is lowered, the alloying is delayed, etc., and it takes a long time to form an oxide, resulting in productivity. Because it will be inferior.

Oxides containing Si, Mn, Al and Cr alone or in combination exist in the steel sheet in a spherical, thread-like or string-like state, and therefore can be clearly distinguished by microscopic observation.
In the present invention, an oxide-containing layer is a layer in which the above oxide is observed by microscopic observation. The average oxide content is the average oxide content of Si, Mn, Al and Cr contained in the steel layer alone or in combination. In addition, the thickness of the steel layer containing the internal oxide is observed from the steel sheet surface in the case of cold-rolled steel sheet, and in the plated steel sheet, the oxide containing Si from the interface between the steel sheet and the plating layer is observed from the plating layer to the steel sheet interface. The distance to the part to be done is shown.
The oxide content may be measured by any method as long as the mass% of the oxide containing Si, Mn, Al, and Cr alone or in combination can be measured, but the oxide-containing layer is dissolved with an acid. After separating an oxide containing Si, Mn, Al and Cr alone or in combination, a method of measuring the mass is reliable. Moreover, although the measurement of the thickness of the steel layer containing the internal oxide of an oxide is not prescribed | regulated, the method of observing a steel plate cross section using an optical microscope or a scanning electron microscope (SEM) is reliable.

If the oxide contains Si, Mn, Al, and Cr alone or in combination, the hydrogen embrittlement resistance is improved regardless of the type of oxide. As the oxide, it is not particularly limited, SiO 2, FeSiO 3, Fe 2 SiO 4, MnSiO 3, Mn 2 SiO 4, Al 2 O 3, MnAl 2 O 4, MnO, Cr 2 O 3 It is preferable that it is 1 type, or 2 or more types selected from the group of these.
However, when an oxide containing a composite of Si and Mn or Al and Mn is formed, the effect of improving hydrogen embrittlement resistance is remarkable, and therefore MnSiO 3 , Mn 2 SiO 4 , and MnAl 2 O 4 may be contained. desirable.
In addition, identification of the oxide which exists in a steel plate can be performed using TEM, CMA, EPMA, FE-SEM, etc. In this embodiment, an extracted replica sample was prepared, and the oxide in the sample was identified using TEM and EPMA. In addition, this oxide containing Si, Mn, Al and Cr alone or in combination may be a complex oxide containing other atoms or may contain many defects, but it is closest from elemental analysis and structural identification. I found and determined.

The steel sheet structure is not particularly limited and exhibits excellent hydrogen embrittlement resistance, which is an effect of the present invention. However, in order to obtain a maximum tensile strength of 980 MPa or more, bainite, martensite, or residual austenite is used alone. Alternatively, it is desirable to include a composite. If necessary, precipitation strengthening by Ti, Nb, V or the like may be used in combination.
Incidentally, identification of each phase of the above microstructure, ferrite, martensite, bainite, austenite and the remaining structure, observation of the existing position and measurement of the area ratio were disclosed in Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. Using a reagent or the like, the steel sheet rolling direction cross section or the rolling direction perpendicular cross section is corroded, and this cross section is subjected to 1000 times optical microscope, 1000 to 100000 times scanning electron microscope (SEM), or transmission electron microscope (TEM). ) Can be used for quantification. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis.

Next, the reasons for limiting the manufacturing conditions will be described.
In the steel sheet, Si, Mn, Al and Cr are added alone or in combination, and the atmosphere in the furnace of the continuous annealing line or continuous plating line contains 1 to 60% by volume of H 2 , and the balance is N 2 , H The atmosphere is made of 2 O, O 2 and inevitable impurities, and the water pressure PH 2 O and the hydrogen partial pressure PH 2 in the atmosphere are expressed by the following formula: −3 ≦ log (PH 2 O / PH 2 ) ≦ −0. 5
It is possible to produce a steel sheet having excellent hydrogen embrittlement resistance by controlling so as to satisfy the above.
Here, the H 2 concentration in the atmosphere was set to 1 to 60% by volume because the increase in the H 2 concentration exceeding 60% by volume resulted in excessive reduction of the oxide generated in the steel sheet. In addition, there is a possibility that the cost is increased due to an increase in the amount of H 2 used, and less than 1% by volume, Fe contained in the steel sheet may be oxidized on the steel sheet surface, and there is a possibility of chemical conversion and coating. This is because deterioration of wettability and wettability of hot dip plating cannot be avoided. Thus, the concentration of H 2 in the range of atmosphere, and 1 to 60 vol%.

Further, the moisture pressure PH 2 O and the hydrogen partial pressure PH 2 in the atmosphere are expressed by the following formula: −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5
Thus, an oxide containing Si, Mn, Al and Cr alone or in combination can be contained in the steel sheet surface layer or the plating layer. Here, the lower limit of the logarithm of the moisture pressure PH 2 O to the hydrogen partial pressure PH 2 (PH 2 O / PH 2 ) is set to −3 or more. The ratio of oxides containing Si, Mn, Al, and Cr increases and wettability and plating adhesion are reduced. Therefore, the lower limit was set to -3. On the other hand, the upper limit is set to -0.5 because the effect is saturated.

In this way, by controlling the atmosphere in the furnace as described above, the conditions under which Si, Mn, Al, and Cr are internally oxidized are obtained. Internal oxidation is a phenomenon in which an oxide is formed inside a steel sheet, and O diffused inside the steel sheet reacts with Si, Mn, Al and Cr contained in the steel sheet to form an oxide inside the steel sheet. As a result, hydrogen embrittlement resistance can be improved without reducing chemical conversion, paintability, wettability, and plating adhesion.
As a means for controlling the atmosphere in the furnace, in the present invention, the ratio (PH 2 O / PH 2 ) between the moisture pressure PH 2 O and the hydrogen partial pressure PH 2 was controlled. For example, carbon dioxide and carbon monoxide, Alternatively, the same effect can be obtained by controlling the partial pressure of nitrogen dioxide and nitric oxide, or by blowing oxygen directly into the furnace. Moreover, the method of adjusting the ratio of the moisture pressure in the furnace and the hydrogen partial pressure by blowing water vapor into the furnace is simple.

Moreover, in order to improve the chemical conversion property and paintability of a cold-rolled steel sheet, it is not deviated from the present invention even if the steel sheet is plated with Ni, Cu, Co, or Fe alone or after annealing.
Alternatively, in order to further improve the plating adhesion of the plated steel sheet, the present invention does not depart from the present invention even if the steel sheet is plated with Ni, Cu, Co, or Fe alone or before the annealing.
Furthermore, regarding the annealing of the plated steel sheet, “after degreasing pickling, heating in a non-oxidizing atmosphere, annealing in a reducing atmosphere containing H 2 and N 2 , cooling to near the plating bath temperature, Senzimer method called “immersion”, “First, the atmosphere during annealing is adjusted to an oxidizing atmosphere to oxidize the steel sheet surface, and then reduced to a reducing atmosphere to clean before plating. There is an all-reduction furnace method called "Immersion" or a flux method such as "Immerse the plating bath after degreasing and pickling the steel plate and then using ammonium chloride etc. Even if it processes by, the effect of this invention can be exhibited.

The slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
The heating temperature of the hot-rolled slab is not particularly defined, and the effect of the present invention is exhibited. However, since it is not economically preferable to make the heating temperature too high, the upper limit of the heating temperature is less than 1300 ° C. It is desirable to do. Moreover, since it will become difficult to make finishing rolling temperature more than Ar3 temperature if it heats too low temperature, it is desirable to make minimum temperature into 1100 degreeC.

When the finish rolling temperature is in the two-phase region of austenite + ferrite, the structure non-uniformity in the steel sheet increases and the formability after annealing deteriorates. Therefore, the Ar3 temperature or higher is desirable.
In addition, Ar3 temperature can be calculated | required by following Formula.
Ar3 (° C.) = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)

The cast slab finished and rolled at an Ar3 temperature or higher is wound up in a temperature range of 630 ° C or lower.
The winding temperature is preferably in the temperature range of 630 ° C. or lower and room temperature (25 ° C.) or higher.
Here, winding at a temperature exceeding 630 ° C. is not preferable because the thickness of the oxide formed on the surface of the steel sheet is excessively increased and the pickling property is poor. On the other hand, when the temperature exceeds 630 ° C., coarse ferrite or pearlite structure exists in the hot-rolled structure, so that the structure non-uniformity after annealing increases and the hole expandability of the final product deteriorates. From the viewpoint of making the microstructure after annealing fine and improving the strength ductility balance, and further from the viewpoint of uniformly dispersing the second phase and improving the hole expansibility, it is more preferable to wind up at 600 ° C. or lower. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, since it is technically difficult to wind up at a temperature of room temperature or lower, this is the actual lower limit.

Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, after winding a rough rolled plate once, it is good also as performing finish rolling.
The hot-rolled steel sheet thus manufactured is pickled. Pickling is important for improving plating properties because it can remove oxides on the surface of the steel sheet. Moreover, pickling may be performed once, or pickling may be performed in a plurality of times.
The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70%, more preferably a rolling reduction of 45 to 65%, and passed through a continuous annealing line or a continuous hot dip galvanizing line. Here, if the rolling reduction is less than 40%, it is difficult to keep the shape flat, and the ductility of the final product becomes poor, so this is the lower limit. On the other hand, cold rolling exceeding 70% makes the cold rolling difficult because the cold rolling load becomes too large.
The effect of the present invention is exhibited without particularly specifying the number of rolling passes and the rolling reduction for each pass.

In addition, when directly plating a hot-rolled sheet, it is not necessary to perform cold rolling.
The average heating rate when passing through the continuous annealing line needs to be 0.7 ° C./second or more. Although the detailed reason is unknown, when heating is performed at a rate of less than 0.7 ° C./second, SiO 2 is formed on the surface of the steel plate during heating even if the ratio of the moisture pressure and the hydrogen partial pressure during heating is within the above range. In addition, since the formation of internal oxides is suppressed, the hydrogen embrittlement resistance of the welded portion, which is the effect of the present invention, cannot be obtained. The wettability and the plating adhesion are deteriorated, so that it is desirable that the temperature be 0.7 ° C./second or more. On the other hand, setting the heating rate to more than 100 ° C. results in excessive capital investment and is not economically preferable, so this is a practical upper limit.

The maximum heating temperature is preferably in the range of 750 to 900 ° C. The reason is that when the maximum heating temperature is less than 750 ° C., it takes a long time to form an oxide containing a single amount or a combination of Si, Mn, Al, and Cr in an amount capable of suppressing hydrogen embrittlement on the steel sheet surface layer. is there. On the other hand, excessively high temperature heating not only is economically undesirable because it leads to an increase in manufacturing cost, but also induces troubles such as deterioration of the plate shape at the time of hot plate passing and reduction in roll life. Therefore, the upper limit of the maximum heating temperature is set to 900 ° C.
The heat treatment time in this temperature range is not particularly limited, but a heat treatment of 10 seconds or longer is desirable in order to form a sufficient amount of oxide. On the other hand, if the heat treatment time exceeds 600 seconds, the cost increases, which is not economically preferable. Regarding the heat treatment, the isothermal holding may be performed at the maximum heating temperature, or even if cooling is started immediately after the gradient heating is performed and the maximum heating temperature is reached, the effect of the present invention is exhibited.

The steel plate excellent in hydrogen embrittlement resistance of the welded portion of the present invention can be produced without particularly setting the heat treatment conditions after annealing. In addition, by controlling the heat treatment conditions after annealing, it becomes possible to produce a steel sheet having both hydrogen embrittlement resistance and formability.
After completion of the annealing, it is desirable to cool at 0.1 to 200 ° C./second. Cooling below 0.1 ° C. is not preferable because productivity is greatly impaired. Increasing the cooling rate exceeding 200 ° C./second leads to an increase in production cost, and therefore the upper limit is preferably set to 200 ° C./second.

  The cooling method may be roll cooling, air cooling, water cooling, or any combination of these methods. The cooling lower limit for limiting the cooling rate is not particularly limited, and the effect of the present invention can be exhibited. However, since it is technically difficult to set the cooling temperature to be room temperature or lower, this is a practical lower limit. Moreover, in the case of a continuous annealing line, you may heat-process for 30 seconds or more in the temperature range of room temperature-450 degreeC using an overaging zone. It is not preferable that the average plate temperature in the overaging zone be higher than 450 ° C. because this leads to high production costs. Since it is difficult to set the temperature of the overaging zone to room temperature or lower, this is a practical lower limit. Here, the holding time means not only mere isothermal holding but also a residence time in a temperature range of room temperature to 450 ° C., and includes cooling and heating in this temperature range.

  The reduction ratio of the skin pass rolling after the heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. Moreover, since productivity will fall remarkably when it exceeds 1.5%, this is made an upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.

Further, in order to further improve the chemical conversion property and the paintability, it is not deviated from the present invention even if the steel sheet is plated with Ni, Cu, Co, or Fe alone or after annealing.
Moreover, even if box annealing is performed, it is possible to produce a high-strength steel sheet that is excellent in hydrogen embrittlement resistance of a welded portion, which is an effect of the present invention.
The heating rate when passing through the continuous hot dip galvanizing line is desirably 0.7 to 100 ° C./second for the same reason as when passing through the continuous annealing line. Moreover, the annealing temperature needs to be 750-900 degreeC for the same reason as the case where a continuous annealing line is passed. The cooling rate until immersion in the plating bath after annealing is preferably 0.1 to 200 ° C./second.

This plating bath immersion plate temperature is 40 ° C. lower than the hot dip galvanizing bath temperature “(galvanizing bath temperature−40) ° C.”, 50 ° C. higher than the hot dip galvanizing bath temperature “(galvanizing bath temperature + 50) ° C.” It is desirable that the temperature range be up to.
When this immersion plate temperature is lower than (zinc plating bath temperature −40) ° C., the heat removal at the time of entering the plating bath is large, and a part of the molten zinc is solidified, which may deteriorate the appearance of plating. Therefore, the lower limit is set to (hot dip galvanizing bath temperature −40) ° C.
Moreover, when this immersion plate temperature exceeds (hot dip galvanizing bath temperature + 50) ° C., an operational problem accompanying an increase in the plating bath temperature is induced, which is not preferable.

Here, when the plate temperature before immersion is lower than (zinc plating bath temperature −40) ° C., reheating is performed before immersion in the plating bath to set the plate temperature to (zinc plating bath temperature −40) ° C. or higher. Then, it may be immersed in a plating bath.
Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.
Moreover, when alloying a plating layer, it is preferable to perform an alloying process at 460 degreeC or more. Here, when the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. The upper limit is not particularly limited, but alloying at a high temperature exceeding 600 ° C. is not economically preferable. Moreover, you may perform additional heat processing in the temperature range of 500-200 degreeC before plating bath immersion. Moreover, you may give a skin pass rolling to a hot-dip galvanized steel plate.

In addition, even if this cold-rolled steel sheet is electroplated, the tensile strength, formability, and hydrogen embrittlement resistance of the steel sheet are not impaired at all. That is, the steel sheet of the present invention is also suitable as a material for electroplating.
In addition, the material of the high-strength steel plate excellent in hydrogen embrittlement resistance of the welded portion of the present invention is generally manufactured through refining, steelmaking, casting, hot rolling, and cold rolling processes, which are ordinary steelmaking processes, Even if manufactured by omitting some or all of them, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied.

Next, the steel plate of this invention is demonstrated in detail in an Example and a comparative example.
A slab having the components shown in Table 1 was heated to 1220 ° C., hot-rolled at a finish hot rolling temperature of 900 ° C., water-cooled in a water-cooled zone, and then wound up at a temperature shown in Table 2. . Next, after pickling these hot-rolled sheets, a hot-rolled sheet having a thickness of 3 mm was cold-rolled to 1.2 mm to obtain cold-rolled sheets. Thereafter, these cold-rolled plates were heat-treated under the conditions shown in Table 2. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.3%.

About some steel plates, it carried out to cold rolling by the method similar to the above, and performed the heat processing and the hot dip galvanization process in the continuous alloying hot dip galvanization equipment. Further, some steel sheets were subjected to an alloying treatment subsequent to the plating treatment. The basis weight at that time was about 50 g / m 2 on both sides. The plated steel sheet was subjected to 0.3% skin pass rolling.

The obtained cold-rolled steel sheet or plated steel sheet was subjected to a tensile test, and YS, TS, and El, which are tensile properties of the base material, were measured. The yield stress was measured by the 0.2% offset method.
In the tensile test, a JIS No. 5 test piece was taken from a 1.2 mm thick steel plate in a direction perpendicular to the rolling direction, and the tensile properties were evaluated.
Tables 3 to 6 show the evaluation results of the hydrogen embrittlement characteristics of the steel sheets.
The evaluation method is that a 100 mm × 30 mm strip test piece cut out in a direction perpendicular to the rolling direction is bent, and after mounting a water-resistant strain gauge on the surface, it is immersed in 0.5 mol / L sulfuric acid. Electrolysis was performed by energizing the piece, hydrogen was intruded into the test piece, and the occurrence of cracks after 2 hours of energization was evaluated. Here, the bending radius of the machining and 10 mm, and the stress applied to each 60 kgf / mm 2 and 90 kgf / mm 2.
Table 3 shows the measured tensile properties and hydrogen embrittlement resistance.
It can be seen that the steel sheet of the present invention can suppress hydrogen embrittlement by controlling the atmosphere during annealing even if the maximum tensile strength is high. At the same time, even if this steel plate was a steel plate containing Si, good plating properties were obtained.

  The ultra high strength steel sheet of the present invention is a steel sheet having both high strength with a tensile maximum strength (TS) of 980 MPa or more and excellent hydrogen embrittlement resistance, and is inexpensive, so structural members for automobiles and reinforcing members In addition, it can be expected to make a significant contribution to the weight reduction of automobiles by applying it to suspension members. For fields other than automobiles where high strength of 980 MPa or more and excellent hydrogen embrittlement resistance are required However, it is applicable, and its industrial utility value is extremely large.

Claims (11)

  1. In mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3.0% Hereinafter, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less,
    And the total content of each of the Si, Mn, Al and Cr is 0.3% or more,
    Balance grain boundaries in the surface layer within 10μm from the surface of the steel sheet consisting of iron and unavoidable impurities, the crystal grains, any one of the crystal grain boundaries and in crystal grains or more, as oxides, SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, one or more selected from the group, and Mn 2 SiO 4 and Cr 2 O 3 in total An ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, characterized by being contained at an average content of 0.01 to 30% by mass and having a maximum tensile strength of 980 MPa or more.
  2. In mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3.0% Hereinafter, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less,
    And the total content of each of the Si, Mn, Al and Cr is 0.3% or more,
    On the surface of the steel plate, the balance of which is iron and inevitable impurities,
    Forming a hot-dip galvanized layer containing less than 7% by mass of Fe and the balance of Zn, Al and inevitable impurities;
    Any one or more of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the steel plate within 10 μm from the interface between the steel plate and the hot dip galvanized layer, or in the hot dip galvanized layer In addition, as the oxide, one or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, and Mn 2 SiO 4 In addition, a super high strength steel sheet excellent in hydrogen embrittlement resistance, containing Cr 2 O 3 in an average content of 0.01 to 30% by mass and having a maximum tensile strength of 980 MPa or more.
  3. In mass%, C: 0.06 to 0.25%, Si: 2.0% or less, Mn: 2.02% to 3.0%, Al: 2.0% or less, Cr: 3.0% Hereinafter, P: 0.04% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less,
    And the total content of each of the Si, Mn, Al and Cr is 0.3% or more,
    On the surface of the steel plate, the balance of which is iron and inevitable impurities,
    Forming an alloyed hot-dip galvanized layer containing 7 to 15% by mass of Fe and the balance of Zn, Al and inevitable impurities;
    Any one of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the steel sheet within 10 μm from the interface between the steel sheet and the alloyed hot-dip galvanized layer, and the alloyed hot-dip galvanized layer. Or, as the oxide, as the oxide, one or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, and Mn 2 SiO 4 and Cr 2 O 3 are contained in total at an average content of 0.01 to 30% by mass, and the tensile maximum strength is 980 MPa or more, which is excellent in hydrogen brittleness resistance. Strength steel plate.
  4.   Furthermore, Ni: 0.05-1.0% and Cu: 0.05-1.0% are contained by the mass%, Resistance resistance of any one of Claim 1 thru | or 3 characterized by the above-mentioned. Super high strength steel sheet with excellent hydrogen embrittlement.
  5.   Furthermore, by mass%, Nb: 0.005-0.3%, Ti: 0.005-0.3%, V: 0.005-0.3%, W: 0.005-0.3% The ultra-high strength steel sheet excellent in hydrogen embrittlement resistance according to any one of claims 1 to 4, comprising one or more selected from the group.
  6.   Furthermore, B: 0.0001-0.1% is contained by mass%, The ultra high strength steel plate excellent in hydrogen embrittlement resistance of any one of Claim 1 thru | or 5 characterized by the above-mentioned.
  7.   Furthermore, in mass%, Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.01%, La: 0.0005 to 0.01%, Ce: 0.0005 to 0.01%, It is excellent in hydrogen embrittlement resistance of any one of Claim 1 thru | or 6 characterized by including 1 type, or 2 or more types selected from the group of Y: 0.0005-0.01% Super high strength steel plate.
  8. A method for heat-treating a high-strength steel plate comprising the chemical component according to any one of claims 1 to 7,
    In the atmosphere containing 1 to 60% by volume of H 2 and the balance being N 2 , H 2 O, O 2 and unavoidable impurities, the water pressure PH 2 O and the hydrogen partial pressure PH 2 in this atmosphere are expressed by the following formula: −3 ≦ log (PH 2 O / PH 2 ) ≦ −0.5
    By controlling the heat treatment so as to satisfy the requirements, the surface grain within 10 μm from the surface of the steel sheet, within the crystal grain boundary, within the crystal grain, the crystal grain boundary, and within the crystal grain, an oxide As one or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO, and Mn 2 SiO 4 and Cr 2 O 3 is controlled so as to contain a total content of 0.01 to 30% by mass in total, and a method for producing an ultra-high strength steel sheet having excellent hydrogen embrittlement resistance.
  9. A cast slab composed of the chemical component according to any one of claims 1 to 7 is directly or once cooled and then heated to 1200 ° C or higher, then hot-rolled at a temperature not lower than the Ar3 transformation point, and then Winding in a temperature range of 630 ° C. or lower, then cold rolling with a rolling reduction of 40 to 70%, and thereafter producing a super high strength steel sheet that is continuously annealed,
    During the continuous annealing,
    H 2 is contained in an amount of 1 to 60% by volume, the balance is composed of N 2 , H 2 O, O 2 and unavoidable impurities, and the water pressure PH 2 O and the hydrogen partial pressure PH 2 are expressed by the following formula −3 ≦ log ( PH 2 O / PH 2 ) ≦ −0.5
    In an atmosphere controlled to satisfy
    Heat between 550 and 750 ° C. at an average heating rate of 0.7 ° C./second or more, anneal at 750 ° C. or more and 900 ° C. or less, and then cool to 650 ° C. at an average cooling rate of 0.1 to 200 ° C./second. The temperature between 650 ° C. and 500 ° C. is cooled at an average cooling rate of 4 to 200 ° C./second to a temperature between 200 and 500 ° C., and is kept within this temperature range for 30 seconds or more, and within 10 μm from the surface of the steel sheet. Any one or more of crystal grain boundaries, crystal grains, crystal grain boundaries, and crystal grains in the surface layer may be used as oxides such as SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3. , MnAl 2 O 4 , one or more selected from the group of MnO, and Mn 2 SiO 4 and Cr 2 O 3 in total, with an average content of 0.01 to 30% by mass To control Method for manufacturing ultra-high strength steel sheet excellent in resistance to hydrogen embrittlement to symptoms.
  10. A cast slab composed of the chemical component according to any one of claims 1 to 7 is directly or once cooled and then heated to 1200 ° C or higher, then hot-rolled at a temperature not lower than the Ar3 transformation point, and then A method for producing an ultra-high-strength hot-dip galvanized steel sheet that is wound in a temperature range of 630 ° C. or lower, then cold-rolled at a rolling reduction of 40 to 70%, and then hot-dip galvanized, When applying
    H 2 is contained in an amount of 1 to 60% by volume, the balance is composed of N 2 , H 2 O, O 2 and unavoidable impurities, and the water pressure PH 2 O and the hydrogen partial pressure PH 2 are expressed by the following formula −3 ≦ log ( PH 2 O / PH 2 ) ≦ −0.5
    In an atmosphere controlled to satisfy
    Heat between 550 and 750 ° C. at an average heating rate of 0.7 ° C./second or more, anneal at 750 ° C. or more and 900 ° C. or less, and then cool to 650 ° C. at an average cooling rate of 0.1 to 200 ° C./second. Then, the temperature between 650 ° C. and 500 ° C. is cooled at an average cooling rate of 3 to 200 ° C./second to obtain a temperature between (zinc plating bath temperature−40) ° C. and (zinc plating bath temperature + 50) ° C. By immersing, any one of crystal grain boundaries, crystal grains, crystal grain boundaries and crystal grains in the steel sheet within 10 μm from the interface between the steel sheet and the hot dip galvanized layer, or the hot dip galvanized layer. One or more selected from the group of SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 , Al 2 O 3 , MnAl 2 O 4 , MnO as the oxide or two or more species, and , Mn 2 SiO 4 and Cr 2 O 3 are controlled so as to contain a total content of 0.01 to 30% by mass in total, and a method for producing an ultrahigh strength hot-dip galvanized steel sheet excellent in hydrogen embrittlement resistance .
  11. A cast slab composed of the chemical component according to any one of claims 1 to 7 is directly or once cooled and then heated to 1200 ° C or higher, then hot-rolled at a temperature not lower than the Ar3 transformation point, and then It is a method for producing an ultra-high-strength galvannealed steel sheet that is wound in a temperature range of 630 ° C. or lower, then cold-rolled at a rolling reduction of 40 to 70%, and then subjected to hot dip galvanizing and alloying treatment. And
    When performing the hot dip galvanizing,
    H 2 is contained in an amount of 1 to 60% by volume, the balance is composed of N 2 , H 2 O, O 2 and unavoidable impurities, and the water pressure PH 2 O and the hydrogen partial pressure PH 2 are expressed by the following formula −3 ≦ log ( PH 2 O / PH 2 ) ≦ −0.5
    In an atmosphere controlled to satisfy
    Heat between 550 and 750 ° C. at an average heating rate of 0.7 ° C./second or more, anneal at 750 ° C. or more and 900 ° C. or less, and then cool to 650 ° C. at an average cooling rate of 0.1 to 200 ° C./second. , The temperature between 650 ° C. and 500 ° C. is cooled at an average cooling rate of 3 to 200 ° C./second to obtain a temperature between (zinc plating bath temperature−40) ° C. and (zinc plating bath temperature + 50) ° C. Immersion is further performed at a temperature of 460 ° C. or higher, and then cooled to room temperature, whereby crystal grains within the steel sheet within 10 μm from the interface between the steel sheet and the alloyed hot-dip galvanized layer. SiO 2 , FeSiO 3 , Fe 2 SiO 4 , MnSiO 3 as oxides in any one or more of the boundaries, crystal grains, crystal grain boundaries and crystal grains, and the alloyed hot-dip galvanized layer , Al 2 O 3 , MnAl 2 O 4 , one or more selected from the group of MnO, and Mn 2 SiO 4 and Cr 2 O 3 in total, with an average content of 0.01 to 30% by mass A method for producing an ultra-high-strength galvannealed steel sheet excellent in hydrogen embrittlement resistance, characterized by being controlled as described above.
JP2006031131A 2006-02-08 2006-02-08 Ultra-high strength steel sheet excellent in hydrogen embrittlement resistance, its manufacturing method, manufacturing method of ultra-high strength hot-dip galvanized steel sheet, and manufacturing method of ultra-high-strength galvannealed steel sheet Active JP4781836B2 (en)

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