JP2013163827A - High-strength cold-rolled steel sheet, high-strength galvanized steel sheet, and high-strength alloying hot-dip galvanized steel sheet, excellent in bendability, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet having excellent bendability and 900 MPa or higher tensile maximum strength, and to provide a method for manufacturing the high-strength steel sheet.SOLUTION: A steel sheet is composed of steel containing 0.075-0.300% C, 0.30-2.50% Si, 1.30-3.50% Mn, 0.001-0.050% P, 0.0001-0.0100 S, 0.001-1.500% Al, 0.0001-0.0100% N, 0.0001-0.0100% O, and having the structure containing ≥50% of bainite and 3-30% of retained austenite, and a ratio of the hardness Hvs on the steel sheet surface layer to the hardness Hvb at 1/4 thickness of the steel sheet being 0.35-0.90. The steel sheet is manufactured in such a manner that after cold rolling, an annealing is applied in the temperature range near the Ac3 transformation point under atmosphere of -3.0 to 0.0 log (water partial pressure/hydrogen partial pressure), and retention at 300-500°C for 10-1000 sec is performed on the way of cooling after annealing.

Description

  The present invention relates to a high-strength cold-rolled steel sheet, a high-strength galvanized steel sheet, and a high-strength galvannealed steel sheet, and relates to a high-strength steel sheet having excellent bendability and a method for producing the same.

  In recent years, demands for higher strength of plated steel sheets used in automobiles and the like have increased, and high-strength steel sheets having a tensile maximum stress of 900 MPa or more are also being used. Examples of a method for forming a vehicle or a member of an automobile using such a high-strength steel plate include bending work such as press work. Usually, the bendability becomes worse as the strength of the steel plate is increased. For this reason, when bending was performed on a high-strength steel plate, there was a problem that a crack (crack) occurred inside the steel plate of the deformed portion or necking occurred on the steel plate surface.

As factors governing the bendability of high-strength steel sheets, it is known that (a) the difficulty of necking and (b) the difficulty of occurrence of cracks (voids) inside the steel sheet are important ( For example, Non-Patent Document 1). It has been known that, in a steel sheet having a low elongation, necking occurs during the bending process and the deformation is localized, so that the bending processability deteriorates.
On the other hand, it has been known that a steel made of ferrite and martensite has inferior bendability due to martensite cracking and void formation at the interface. As a result, increasing the strength leads to deterioration of elongation, so the bendability is poor. In addition, since the increase in strength may be accompanied by an increase in the martensite volume fraction, the increase in strength tends to cause deterioration in bendability.

  As a technique for improving the bendability of a steel sheet, Patent Document 1 discloses that the chemical composition of the steel sheet is, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0%. Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.0004-0. 015%, Ti: 0.03 to 0.2%, the balance being Fe and impurities, the metallographic structure of the steel sheet contains 30 to 95% ferrite by area ratio, the remaining second phase is It consists of one or more of martensite, bainite, pearlite, cementite and retained austenite, and when martensite is contained, the area ratio of martensite is 0 to 50%, and the steel sheet has a particle size of 2 ~ 30nm Ti carbonitride precipitate average particle Distance contained in 30 to 300 nm, and high-tensile galvanized steel sheet containing particle diameter 3μm or more crystallization-based TiN with an average distance between particles 50~500μm have been proposed. However, since precipitation strengthening is used, there is a problem that the strength-elongation balance is inferior.

  In Patent Document 2, as a hot-dip galvanized steel sheet having excellent bendability, C: 0.03 to 0.11%, Si: 0.005 to 0.5%, Mn: 2.0 to 2.0% by mass 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0%, N: 0.01% or less, and Ti: 0.50% or less and Nb: 0.50% or less of one or two of Ti + Nb / 2 ≧ 0.03 (Ti and Nb in the formula indicate the content (unit: mass%) of each element), with the remainder having a chemical composition composed of Fe and impurities, The average interval in the plate width direction of the Mn-concentrated portion expanded in the rolling direction at the 20t depth position (t: plate thickness of the steel plate) is 300 μm or less, the area ratio of ferrite is 60% or more, and the average of ferrite A hot-dip galvanized steel sheet having a steel structure having a grain size of 1.0 to 6.0 μm, containing precipitates having a grain size of 1 to 10 nm in ferrite at 100 pieces / μm 2 or more, and having a tensile strength of 540 MPa or more. Have been described. However, since the main phase is ferrite and the retained austenite volume fraction is limited to less than 3%, there is a problem that it is difficult to apply to a high strength steel plate of 900 MPa or more.

  Moreover, in patent document 3, as a hot-dip galvanized steel plate which made ductility and bendability compatible, by mass%, C: 0.08-0.25%, Si: 0.7% or less, Mn: 1.0- 2.6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less and N: 0.01% or less, and the relationship between Si and Al is 1 0.0% ≦ Si + Al ≦ 1.8%, having a chemical composition consisting of the balance Fe and impurities, TS ≧ 590 (TS: tensile strength (MPa)), TS × El ≧ 17500 (El: total elongation ( %)), And ρ ≦ 1.5 × t (ρ: critical bending radius (mm), t: plate thickness (mm)), a cold-rolled steel sheet having a plated layer containing zinc on the surface thereof Is described. However, since the steel plate strength is less than 900 MPa, it has been difficult to apply to a further high strength steel plate.

  In Patent Document 4, as a cold-rolled steel sheet having good ductility and bendability, C: 05. 08 to 0.20%, Si: 1.0% or less, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005 to 0.5%, N: 0.01% or less, and Ti: 0.02 to 0.2%, having a chemical composition consisting of the balance Fe and impurities, and in volume%, ferrite : 10% or more, bainite: 20 to 70%, residual austenite: 3 to 20% and martensite: 0 to 20%, the ferrite has an average particle size of 10 μm or less, and the bainite has an average particle size of 10 μm or less. And having a steel structure having an average particle size of the retained austenite of 3 μm or less and an average particle size of the martensite of 3 μm or less, a tensile strength (TS) of 780 MPa or more, a tensile strength (TS) and a total elongation (El). Product (TS × El value) is 14000 MPa ·% or more, and the minimum bending radius in the bending test is 1.5 t or less (t: plate thickness), and the plate thickness is 2.0. A cold-rolled steel sheet that is at least mm is described, and it is described that the surface of the cold-rolled steel sheet is plated. However, further improvements in strength and bending balance have been required.

In Patent Document 5, as an alloyed hot-dip galvanized steel sheet having excellent bendability, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 2.0- 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0% and N: 0.01% or less, and Ti: N0: 0.50% or less and Nb: 0.50% or less, Ti + Nb / 2 ≧ 0.03 is contained, and the balance has a chemical composition composed of Fe and impurities, the area ratio of ferrite is 60% or more, and the average grain size of ferrite is 1.0 to 6.0 μm. The alloyed hot-dip galvanized layer has a steel structure and contains, by mass%, Fe: 8 to 15% and Al: 0.08 to 0.50%, with the balance being Zn and impurities, The hot-dip galvanized steel sheet has a tensile strength of 540 MPa or more, and an alloyed hot-dip galvanized steel sheet having excellent bendability is described.
However, since the amount of addition of C is limited to a low range of 0.12% or less, although it can be applied to a steel plate of 780 MPa or less, it has a problem that it is difficult to apply to a further high strength steel plate. In addition, since the area ratio of retained austenite is less than 3%, there is a problem that ductility is low.

  In Patent Document 6, as a high-strength hot-dip galvanized steel sheet having excellent workability, C: 0.03 to 0.17%, Si: 0.01 to 0.75%, Mn: 1.5 -2.5%, P: 0.080% or less, S: 0.010% or less, sol. Al: 0.01 to 1.20%, Cr: 0.3 to 1.3%, the balance is Fe and inevitable impurities, the steel structure is 30 to 70% ferrite by volume, 3% There is a description of a steel sheet having a hot dip galvanized layer on a base steel sheet that is composed of less retained austenite and the remaining martensite, and 20% or more of the martensite is tempered martensite. However, since the volume ratio of retained austenite is limited to less than 3%, it has a problem that it has excellent bendability but low uniform elongation. As a result, also in bending, when a thick plate is bent, there is a concern that cracks due to necking may occur on the surface of the steel plate.

  In Patent Document 7, as an ultra-high-strength cold-rolled steel sheet having excellent bending workability, wt%, C: 0.12 to 0.30%, Si: 1.2% or less, Mn: 1 to 3%, P: 0.020% or less, S: 0.010% or less, sol. Al: 0.01 to 0.06% steel, the balance being Fe and inevitable impurities, C: 0.1 wt% or less soft layer on one side and 3 to 15 vol% on both sides In addition, it is described that the balance is a composite structure of residual austenite with a content of less than 10 vol% and a low-temperature transformation phase or further ferrite. However, in order to form the soft layer of the steel sheet surface layer, the decarburization annealing must be performed twice in total after hot rolling and after cold rolling, resulting in poor productivity.

JP 2007-16319 A JP 2009-215616 A JP 2009-270126 A JP 2010-59452 A JP 2010-65269 A JP 2010-70843 A JP-A-5-195149

Hasegawa et al .: CAMP-ISIJ Vol.20 (2007) p437

  As described above, according to the conventional technique, particularly when a high-strength steel sheet is bent, sufficient bendability cannot be obtained. Therefore, it is required to further improve bendability. In view of such a current situation, the present invention provides a high-strength steel sheet having excellent bendability and a method for producing the same.

The present inventors obtain a high-strength steel sheet having a tensile maximum strength of 900 MPa or more that can obtain excellent bendability by preventing cracks in the steel sheet generated in the deformed portion by bending and necking of the steel sheet surface. In order to achieve this goal, we made extensive studies.
As a result, the present inventors have a predetermined chemical composition, can control the steel sheet structure to a predetermined structure, and can soften the steel sheet surface layer by performing a decarburization process, and have a maximum tensile strength of 900 MPa. As described above, it was clarified that even a high-strength steel plate can obtain excellent bendability as if it were a low-strength steel plate. This effect is obtained when the ratio of the hardness of the steel sheet surface layer to the hardness of 1/4 thickness “(surface layer hardness) / (1/4 thickness hardness)” is 0.35 to 0.90.

In addition, when the steel sheet structure contains 3-20% retained austenite in volume fraction, cracking due to necking can be suppressed, and further improvement in bendability can be obtained. In particular, since bending becomes larger as the surface layer is bent, a large bendability improvement effect can be obtained by setting the hardness of the surface layer and the inside of the steel sheet within the notation range.
The present invention has been completed on the basis of such knowledge, and the gist thereof is as follows.

(1) In mass%,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0100%,
Al: 0.001-1.500%,
N: 0.0001 to 0.0100%,
O: 0.0001 to 0.0100%
Is a high-strength steel plate having a tensile maximum strength of 900 MPa or more consisting of iron and inevitable impurities, and the structure contains 50% or more of bainite as a main phase in a volume fraction of 3 to 30%. Residual austenite is contained, and the ratio (Hvs / Hvb) of the hardness (Hvs) of the steel sheet surface layer to the 1/4 thickness (Hvb) of the steel sheet is 0.35 to 0.90. High-strength cold-rolled steel sheet with excellent bendability.

(2) The high-strength cold-rolled steel sheet with excellent bendability according to (1), wherein the steel sheet structure is such that fresh martensite is limited to 15% or less.
(3) The base steel plate is further in mass%, Ti: 0.005 to 0.150%, Nb: 0.005 to 0.150%, V: 0.005 to 0.150%, or one type The high-strength cold-rolled steel sheet having excellent bendability as described in (1) or (2) above, comprising two or more kinds.
(4) The base material steel plate is further mass%, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0. .01-2.00%, Mo: 0.01-1.00%, W: 0.01-1.00% 1 type or 2 types or more are contained, (1)-( A high-strength cold-rolled steel sheet excellent in bendability as described in any one of 3).
(5) The base steel plate further contains 0.0001 to 0.5000 mass% of one or more of Ca, Ce, Mg, Zr, Hf, and REM in total (1 A high-strength cold-rolled steel sheet excellent in bendability according to any one of (1) to (4).

(6) A bendability characterized in that a film composed of phosphorus oxide and / or composite oxide containing phosphorus is formed on the surface of the cold-rolled steel sheet according to any one of (1) to (5). High strength cold-rolled steel sheet with excellent resistance.
(7) A high-strength galvanized steel sheet excellent in bendability characterized by having an electrogalvanized layer on the surface of the cold rolled steel sheet according to any one of (1) to (5).
(8) A high-strength galvanized steel sheet excellent in bendability characterized by having a hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of (1) to (5).
(9) A high-strength galvanized steel sheet excellent in bendability characterized by having an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of (1) to (5).
(10) The bending according to any one of (7) to (9), wherein a film made of a phosphorus oxide and / or a composite oxide containing phosphorus is formed on the surface of the galvanized layer. High strength galvanized steel sheet with excellent properties.

(11) The slab having the chemical component according to any one of (1) and (3) to (5) is heated to 1050 ° C. or higher, and hot rolling is completed at a finish hot rolling temperature not lower than the Ar3 transformation point, After hot rolling to be wound in a temperature range of 750 ° C. or lower, after cold rolling at a reduction rate of 30 to 80%, in a temperature range of (Ac3 transformation point−20) ° C. to (Ac3 transformation point + 100) ° C., And after performing annealing for 20 seconds to 600 seconds in an atmosphere having a log (moisture pressure / hydrogen partial pressure) of −3.0 to 0.0, a cooling rate of 0.5 to 500 ° C./second is applied between 700 to 500 ° C. The bendability according to any one of (1) to (5) above, wherein the bendability is cooled to 300 to 500 ° C. and subsequently held for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. An excellent method for producing high-strength cold-rolled steel sheets.

(12) The step of applying a film made of a complex oxide containing phosphorus oxide and / or phosphorus to the surface of the cold-rolled steel sheet produced by the production method according to (11) above ( The manufacturing method of the high intensity | strength cold-rolled steel plate excellent in the bendability as described in 6).

(13) Manufacture of a high strength galvanized steel sheet having excellent bendability as described in (7) above, wherein the surface of the cold rolled steel sheet manufactured by the manufacturing method as described in (11) is subjected to electrogalvanization. Method.
(14) The slab is hot-rolled in accordance with the manufacturing method described in (11), cold-rolled, and then cooled after being annealed, and maintained for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. 9. The method according to claim 8, further comprising heating or cooling to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C. and then immersing in the zinc plating bath and cooling. The manufacturing method of the high intensity | strength galvanized steel plate excellent in the bendability of description.
(15) The slab is hot-rolled according to the production method described in (11), cold-rolled, and then cooled after annealing, and then held for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. Then, after heating or cooling to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C., it is immersed in a zinc plating bath and further alloyed at a temperature of 460 ° C. or higher. The method for producing a high-strength galvanized steel sheet having excellent bendability according to claim 9, wherein the steel sheet is cooled after being applied.

(16) A step of applying a film made of a composite oxide containing phosphorus oxide and / or phosphorus to the surface of the galvanized steel sheet manufactured by the manufacturing method according to (14) or (15) is performed. The manufacturing method of the high intensity | strength galvanized steel plate excellent in the bendability as described in said (10).

  According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet or galvanized steel sheet having excellent bendability and a tensile maximum strength of 900 MPa or more, and a method for producing them.

  In the present invention, the chemical composition of the steel sheet is specified, and the steel sheet structure is controlled to a specific structure, and then the decarburization treatment is performed to soften the steel sheet surface layer so that the maximum tensile strength is 900 MPa or more. Above, it is set as the high strength steel plate which was further excellent in bendability.

  In the present invention, the steel sheet excellent in bendability was defined as a steel sheet excellent in bendability in the 90-degree V-bending test having a minimum limit bending radius of 1.0 R or less. In addition, the steel sheet of the present invention has good elongation because the retained austenite used for the suppression of necking at the time of bending works to suppress necking at the time of tensile tests and press processing. .

(Chemical composition of steel)
First, the chemical composition (composition) of steel constituting the cold-rolled steel sheet or galvanized steel sheet of the present invention will be described. In addition,% in the following description represents the mass%.

"C: 0.075-0.300%"
C is contained in order to increase the strength of the base steel sheet. However, if the C content exceeds 0.300%, weldability becomes insufficient. From the viewpoint of weldability, the C content is preferably 0.280% or less, and more preferably 0.260% or less. On the other hand, if the C content is less than 0.075%, the strength is lowered, and the maximum tensile strength of 900 MPa or more cannot be ensured. In order to increase the strength, the C content is preferably 0.090% or more, and more preferably 0.100% or more.

“Si: 0.30 to 2.50%”
Si is the most important additive element because it promotes the decarburization reaction and softens the surface layer of the steel sheet. For this reason, it is necessary to add 0.30 or more. However, if the Si content exceeds 2.50%, the base steel sheet becomes brittle and the ductility deteriorates. From the viewpoint of ductility, the Si content is preferably 2.20% or less, and more preferably 2.00% or less. On the other hand, if the Si content is less than 0.30%, a large amount of coarse iron-based carbides are generated, the residual austenite cannot be made 3 to 30%, and the elongation is lowered. In this respect, the lower limit value of Si is preferably 0.50% or more, and more preferably 0.70% or more. In addition, Si is an element necessary for suppressing the coarsening of iron-based carbides in the base steel sheet and increasing the strength and formability. Moreover, it is necessary to add as a solid solution strengthening element in order to contribute to the strengthening of a steel plate.

“Mn: 1.30 to 3.50%”
Mn is contained to increase the strength of the base steel sheet. However, if the Mn content exceeds 3.50%, a coarse Mn-concentrated portion is generated at the center of the thickness of the base steel sheet, and brittleness is likely to occur, and troubles such as cracking of the cast slab are likely to occur. . Further, when the Mn content exceeds 3.50%, the weldability is also deteriorated. Therefore, the Mn content needs to be 3.50% or less. From the viewpoint of weldability, the Mn content is preferably 3.20% or less, and more preferably 3.00% or less. On the other hand, when the content of Mn is less than 1.30%, a large amount of soft structure is formed during cooling after annealing, so that it is difficult to secure a maximum tensile strength of 900 MPa or more. For this reason, the Mn content needs to be 1.30% or more. In order to further increase the strength, the Mn content is preferably 1.50% or more, and more preferably 1.70% or more.

“P: 0.001 to 0.050%”
P tends to segregate in the central part of the thickness of the base steel sheet, and causes the weld to become brittle. When the P content exceeds 0.050%, the welded portion is significantly embrittled, so the P content is limited to 0.050% or less. Although the lower limit of the content of P is not particularly defined, the effect of the present invention is exhibited. However, since the content of P is less than 0.001% is accompanied by a significant increase in production cost, 0.001 % Is the lower limit.

“S: 0.0001 to 0.0100%”
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit value of the S content is set to 0.0100% or less. Further, since S is combined with Mn to form coarse MnS to lower the ductility and stretch flangeability, it is preferably 0.0050% or less, and more preferably 0.0025% or less. The lower limit of the content of S is not particularly defined, and the effect of the present invention is exhibited. However, if the content of S is less than 0.0001%, a significant increase in production cost is caused, so 0.0001% Is the lower limit.

"Al: 0.001-1.500%"
Al is an important additive element because it accelerates the decarburization reaction and softens the steel sheet surface layer. Therefore, it may be added. However, if the Al content exceeds 1.500%, weldability deteriorates, so the upper limit of the Al content is set to 1.500%. From this viewpoint, the Al content is preferably 1.200% or less, and more preferably 0.900% or less. Further, Al is an element effective as a deoxidizing material, but if the Al content is less than 0.001%, the effect as the deoxidizing material cannot be obtained sufficiently, so the lower limit of the Al content is 0. 0.001% or more. In order to obtain a sufficient deoxidation effect, the Al content is preferably 0.003% or more.

“N: 0.0001 to 0.0100%”
N forms coarse nitrides and degrades ductility and stretch flangeability, so it is necessary to suppress the addition amount. When the N content exceeds 0.0100%, this tendency becomes remarkable, so the N content range is set to 0.0100% or less. Further, N is better because it causes blowholes during welding. The lower limit of the content of N is not particularly defined, and the effect of the present invention is exhibited. However, if the content of N is less than 0.0001%, a significant increase in manufacturing cost is caused, so 0.0001% Is the lower limit.

“O: 0.0001 to 0.0100%”
O forms an oxide and deteriorates ductility and stretch flangeability, so the content needs to be suppressed. When the O content exceeds 0.0100%, the stretch flangeability deteriorates significantly, so the upper limit of the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0060% or less. Although the lower limit of the content of O is not particularly defined, the effects of the present invention are exhibited. However, if the content of O is less than 0.0001%, a significant increase in manufacturing cost is caused, so 0.0001% Was the lower limit.

  The base steel plate of the high-strength cold-rolled steel plate or high-strength galvanized steel plate of the present invention contains the above elements, and the balance is basically composed of iron and unavoidable impurities. In addition, the following elements may be included.

“Ti: 0.005 to 0.150%”
Ti is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the Ti content exceeds 0.150%, precipitation of carbonitrides increases and the formability deteriorates, so the Ti content is preferably 0.150% or less. From the viewpoint of moldability, the Ti content is more preferably 0.120% or less, and further preferably 0.100% or less. The lower limit of the Ti content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the strength increasing effect by Ti, the Ti content is preferably 0.005% or more. In order to increase the strength of the base steel sheet, the Ti content is more preferably 0.010% or more, and further preferably 0.015% or more.

“Nb: 0.005 to 0.150%”
Nb is an element that contributes to an increase in the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the Nb content exceeds 0.150%, carbonitride precipitation increases and the formability deteriorates, so the Nb content is preferably 0.150% or less. From the viewpoint of moldability, the Nb content is more preferably 0.120% or less, and further preferably 0.100% or less. The lower limit of the Nb content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Nb, the Nb content is preferably 0.005% or more. In order to increase the strength of the base steel sheet, the Nb content is more preferably 0.010% or more, and further preferably 0.015% or more.

"V: 0.005-0.150%"
V is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. However, if the V content exceeds 0.150%, carbonitride precipitation increases and the formability deteriorates, so the V content is preferably 0.150% or less. The lower limit of the content of V is not particularly limited, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by V, the content of V is preferably 0.005% or more.

“B: 0.0001 to 0.0100%”
B is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. When the content of B exceeds 0.0100%, hot workability is impaired and productivity is lowered. Therefore, the B content is preferably 0.0100% or less. From the viewpoint of productivity, the B content is more preferably 0.0050% or less, and further preferably 0.0030% or less. The lower limit of the content of B is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by B, the content of B should be 0.0001% or more. preferable. For increasing the strength, the B content is more preferably 0.0003% or more, and more preferably 0.0005% or more.

"Cr: 0.01-2.00%"
Cr is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Cr content exceeds 2.00%, hot workability is impaired and productivity is lowered. Therefore, the Cr content is preferably 2.00% or less. Although the lower limit of the Cr content is not particularly defined, the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cr, the Cr content may be 0.01% or more. preferable.

"Ni: 0.01-2.00%"
Ni is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Ni content exceeds 2.00%, weldability is impaired, so the Ni content is preferably 2.00% or less. The lower limit of the Ni content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Ni, the Ni content should be 0.01% or more. preferable.
"Cu: 0.01-2.00%"
Cu is an element that increases the strength by being present in the steel as fine particles, and can be added instead of a part of C and / or Mn. If the Cu content exceeds 2.00%, weldability is impaired, so the Cu content is preferably 2.00% or less. The lower limit of the Cu content is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cu, the Cu content should be 0.01% or more. preferable.

"Mo: 0.01-1.00%"
Mo is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Mo content exceeds 1.00%, hot workability is impaired and productivity is lowered. For this reason, the Mo content is preferably 1.00% or less. The lower limit of the content of Mo is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Mo, the content of Mo is 0.01% or more. preferable.

"W: 0.01-1.00%"
W is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the W content exceeds 1.00%, hot workability is impaired and productivity is lowered. Therefore, the W content is preferably 1.00% or less. The lower limit of the W content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by W, the W content may be 0.01% or more. preferable.

“Totally 0.0001 to 0.5000% of one or more of Ca, Ce, Mg, Zr, Hf, and REM”
Ca, Ce, Mg, Zr, Hf, and REM are effective elements for improving formability, and one or more of them can be added. However, if the total content of one or more of Ca, Ce, Mg, Zr, Hf, and REM exceeds 0.5000%, the ductility may be impaired. For this reason, the total content of each element is preferably 0.5000% or less. The lower limit of the content of one or more of Ca, Ce, Mg, Zr, Hf, and REM is not particularly defined and the effect of the present invention is exhibited, but the effect of improving the formability of the base steel sheet In order to obtain it sufficiently, the total content of the elements is preferably 0.0001% or more. From the viewpoint of formability, the total content of one or more of Ca, Ce, Mg, Zr, Hf, and REM is more preferably 0.0005% or more, and 0.0010% or more. Is more preferable.
Note that REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM and Ce are often added by misch metal and may contain a lanthanoid series element in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. Even if the metal La or Ce is added, the effect of the present invention is exhibited.

(Steel sheet structure)
Next, the reason for defining the steel sheet structure will be described.

“Bainnight: 50% or more”
Bainite is a structure excellent in the balance between strength and bendability, and in order to ensure the maximum tensile strength of 900 MPa or more, the volume fraction of bainite as a main phase needs to be 50% or more. If the volume fraction is less than 50%, it is difficult to secure a strength of 900 MPa or more, and the bendability deteriorates, which is not preferable. Furthermore, it is more preferable that the volume fraction is 60% or more.

"Residual austenite: 3-30%"
The structure of the steel sheet contains 3-30% residual austenite in volume fraction in the range of 1/8 thickness to 3/8 thickness centered on 1/4 of the plate thickness. Residual austenite is effective in suppressing necking that occurs during bending by greatly improving ductility. On the other hand, retained austenite becomes a starting point of fracture and deteriorates bendability. For this reason, in the galvanized steel sheet, the retained austenite contained in the structure of the base steel sheet is limited to 3 to 30% by volume fraction.

"Fresh martensite: 15% or less"
In addition to the above-mentioned retained austenite, the structure of the steel sheet needs to limit fresh martensite to 15% or less in the range of 1/8 thickness to 3/8 thickness centered on 1/4 of the thickness. is there. When the structure of the steel sheet of the present invention (base steel sheet in the case of a galvanized steel sheet) has such a structure, it becomes a high-strength steel sheet having a strength of 900 MPa or more and excellent bendability.
Although fresh martensite greatly improves the tensile strength, on the other hand, it becomes a starting point of fracture and greatly deteriorates the bendability. Therefore, it is preferable to limit the structure of the base steel sheet to 15% or less in terms of volume fraction. In order to improve bendability, the volume fraction of fresh martensite is more preferably 10% or less, and further preferably 5% or less. Fresh martensite is martensite that does not contain iron-based carbides, and is very hard and brittle. As a result, when bending is performed, it becomes a starting point of cracking and the bendability is greatly deteriorated. For this reason, it is desirable to make the volume ratio as small as possible.

  In addition to the steel sheet structure, one or more of tempered martensite, ferrite, pearlite, and cementite may be included. If it is the range demonstrated below, the objective of this invention can be achieved.

“Tempered martensite: 47% or less”
Tempered martensite is a structure that greatly improves the tensile strength, and may be contained in the structure of the base steel sheet in a volume fraction of 47% or less. Tempered martensite is martensite in which iron-based carbides such as θ, ε, and η are precipitated by holding martensite at 200 to 500 ° C., and causes cracking compared to fresh martensite. It ’s hard to be. Therefore, tempered martensite may be contained. However, if the volume fraction exceeds 47%, the yield stress becomes excessively high and the shape freezing property deteriorates, so the volume fraction needs to be 47% or less.

"Ferrite: 30% or less"
Ferrite is an effective structure for improving ductility, and may be contained in the structure of the base steel sheet in a volume fraction of 30% or less. Moreover, since ferrite is a soft structure, if the volume fraction exceeds 30%, sufficient strength may not be obtained. For this reason, the ferrite volume fraction needs to be 30% or less.

"Perlite: 5% or less"
When the amount of pearlite increases, the ductility deteriorates. Therefore, the volume fraction of pearlite contained in the base steel sheet structure is preferably 5% or less, more preferably 3% or less.

"Other"
Other structures such as coarse cementite may be included as other structures. However, when coarse cementite increases in the structure of the base steel sheet, the bendability deteriorates. From this, the volume fraction of coarse cementite contained in the base steel sheet structure is preferably 10% or less, and more preferably 5% or less. Coarse cementite means cementite having a nominal particle size of 2 μm or more. Cementite is fragile compared to iron, and the interfacial strength between iron and cementite is also small. Therefore, it becomes a starting point of crack formation and void formation during bending, and deteriorates bendability. For this reason, it is necessary to reduce the volume ratio of coarse cementite. On the other hand, fine iron-based carbides contained in the bainite structure or tempered martensite may be contained because the bendability is not deteriorated.

The volume fraction of each tissue as described above can be measured by the following method, for example.
The volume fraction of retained austenite is X-ray diffracted using a plane parallel to the plate surface of the base steel sheet and a thickness of 1/4 as the observation surface, and the area fraction is calculated and taken as the volume fraction. Can do.
The volume fractions of ferrite, pearlite, bainite, cementite, tempered martensite and fresh martensite were collected by taking a sample with the thickness cross section parallel to the rolling direction of the base steel sheet as the observation surface, and polishing the observation surface. Etching and observing the range of 1/8 to 3/8 thickness centered on 1/4 of the plate thickness with a field emission scanning electron microscope (FE-SEM). You can measure the rate and take it as a volume fraction.
In addition, the measurement position of the volume fraction of each structure was set to 1/4 thickness, 1/8 thickness to 3/8 thickness from the surface. The steel sheet surface layer was 1/8 thickness from the thickness due to decarburization. This is because the thickness center is a structure containing a lot of martensite due to Mn segregation, and the steel sheet structure is greatly different from other positions.

(Hardness ratio of steel sheet)
Next, the reason for defining the ratio between the hardness of the steel sheet surface layer and the hardness of the quarter thickness of the base steel sheet will be described.

The present inventors have found that excellent bendability can be obtained by subjecting a steel sheet having the above chemical components and structure to decarburization to soften the steel sheet surface layer. That is, excellent bendability can be obtained by setting the ratio “Hvs / Hvb” of the hardness Hvb of the surface layer of the steel sheet and the hardness Hvb of the quarter thickness of the base steel sheet to 0.35 to 0.90.
The reason why the hardness ratio is set to 0.35 or more is that if the hardness ratio is less than 0.35, the steel sheet is too soft and it is difficult to ensure the maximum tensile strength of 900 MPa or more. On the other hand, if it exceeds 0.90, the effect of improving the bendability which is the effect of the present invention cannot be obtained.

  The “hardness” used here is a steel plate surface layer and a 1/4 position of the plate thickness in a plate thickness cross section parallel to the rolling direction of the steel plate, and 10% indentation load of 10 g using a Vickers hardness tester. The hardness was measured point by point, and the average value was defined as each hardness.

  When investigating the relationship between hardness and bendability, the present inventors investigated the relationship between bendability and steel sheet characteristics as a preliminary test. Does not depend on the position, and at the thickness center (1/2 thickness), the steel sheet structure is different due to the center segregation of Mn, and the average hardness is also different from the position of 1/8 thickness to 3/8 thickness. I found out. From this, the hardness at the 1/4 position where the hardness of the steel plate base metal can be represented is taken as the base material hardness (Hvb).

  On the other hand, when investigating the relationship between the decarburization conditions and the hardness of the steel sheet surface layer, as the decarburization progresses, the hardness of the steel sheet surface layer decreases and the softened area expands in the sheet thickness direction. It was found that the thickness and the degree of softening of the softened layer can be represented by measuring the hardness at a certain depth position. From this, the hardness at the position of 20 μm from the surface of the steel sheet was measured, and in the case of a plated steel sheet, the hardness at the position of 20 μm was measured from the plating layer / base metal interface to obtain the hardness (Hvs) of the steel sheet surface layer.

Here, the measurement position was set to 20 μm from the surface for the following reason.
In the softening position, the steel sheet hardness was Hv 100 to 400, the indentation size was about 8 to 13 μm, and when the measurement position was too close to the steel sheet surface, accurate hardness measurement was difficult. On the other hand, if the measurement position is too far from the steel sheet surface, the softening layer is not included, and thus the relationship between the bendability and the hardness of the steel sheet surface layer could not be obtained accurately. From this, the measured value was set at a position of 20 μm.
In measuring the hardness of the steel sheet surface layer, in order to prevent sagging of the surface of the steel sheet during polishing, it is preferable to perform polishing and hardness measurement after applying a plate to the steel sheet and embedding resin.

(Plate form)
The high-strength steel sheet of the present invention may be any of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and an electrogalvanized steel sheet as long as the hardness of the steel sheet surface layer satisfies the above range.
The galvanized layer is not particularly limited. For example, as the galvanized layer, an alloyed galvanized layer containing less than 7% by mass of Fe and the balance consisting of Zn, Al and unavoidable impurities is used. As for, what contains 7-15 mass% of Fe, and the remainder consists of Zn, Al, and an unavoidable impurity etc. can be used.
Further, the galvanized layer may be one or two of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, and REM. The above may be contained or mixed. Even if the alloyed galvanized layer contains or mixes one or more of the above-mentioned elements, the effects of the present invention are not impaired, and the corrosion resistance and workability are improved depending on the content. In some cases, it is preferable.

Furthermore, the high-strength steel sheet of the present invention may have a coating layer made of a complex oxide containing phosphorus oxide and / or phosphorus on the surface of a cold-rolled steel sheet or the surface of a galvanized steel sheet. .
A film made of a phosphorus oxide and / or a composite oxide containing phosphorus can function as a lubricant when processing the steel sheet, and can protect the surface of the steel sheet and the alloyed galvanized layer.

(Steel plate manufacturing method)
Next, the method for producing the high-strength steel plate of the present invention will be described in detail.
To manufacture a steel plate, first, a slab having the above-described chemical component (composition) is cast. As the slab used for hot rolling, a slab produced by a continuous casting slab, a thin slab caster or the like can be used. In addition, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting may be used.

  In the hot rolling of the slab, the slab heating temperature is to secure a finish rolling temperature equal to or higher than the Ar3 transformation point, and the decrease in the slab heating temperature leads to an excessive increase in rolling load, making rolling difficult. Since there is a concern of causing a shape defect of the base steel plate after rolling, it is necessary to set the temperature to 1050 ° C. or higher. The upper limit of the slab heating temperature 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 excessively high, the upper limit of the slab heating temperature is 1350 ° C. or less. It is desirable.

Hot rolling needs to be completed at a finish rolling temperature not lower than the Ar3 transformation point temperature. If the finish rolling temperature is lower than the Ar3 transformation point, it becomes a two-phase rolling of ferrite and austenite, and the hot-rolled sheet structure becomes a heterogeneous mixed grain structure, even if it undergoes a cold rolling process and a continuous annealing process. Is not eliminated, and the steel sheet is inferior in ductility and bendability.
On the other hand, the upper limit of the finish rolling temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finish rolling temperature is excessively high, the slab heating temperature must be excessively high in order to secure the temperature. I must. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1100 ° C. or lower.

The Ar3 transformation point is calculated by the following formula using the content (mass%) of each element.
Ar3 = 901-325 * C + 33 * Si-92 * (Mn + Ni / 2 + Cr / 2)
+ Cu / 2 + Mo / 2) + 52 × Al

The hot rolling coiling temperature is set to 750 ° C. or lower in order to prevent the thickness of the oxide formed on the surface of the hot-rolled steel sheet from increasing excessively and deteriorating the pickling property. In order to further improve the pickling property, the winding temperature is preferably 720 ° C. or lower, and more preferably 700 ° C. or lower.
On the other hand, when the coiling temperature is less than 400 ° C., the strength of the hot-rolled steel sheet is excessively increased and cold rolling becomes difficult, and therefore the coiling temperature is preferably 400 ° C. or higher. In order to reduce the cold rolling load, the winding temperature is preferably 420 ° C. or higher. However, even if it winds at less than 400 degreeC, since it can be cold-rolled by annealing in a box furnace and performing a softening process of a hot-rolled sheet after that, it winds at less than 400 degreeC. It doesn't matter.

  Next, it is preferable to perform pickling on the hot-rolled steel sheet thus manufactured. Pickling removes oxides on the surface of the hot-rolled steel sheet, and is therefore important for improving the plateability of the base steel sheet. Moreover, pickling may be performed once or may be performed in a plurality of times.

The hot-rolled steel sheet after pickling is cold-rolled for the purpose of plate thickness adjustment and shape correction. In cold rolling, in order to obtain a base steel plate having a high thickness accuracy and an excellent shape, it is preferable that the rolling reduction is in the range of 30 to 80%. If the rolling reduction is less than 30%, it is difficult to keep the shape flat, and the ductility of the final product may be deteriorated. The rolling reduction in cold rolling is preferably 35% or more, and more preferably 40% or more. On the other hand, when the rolling reduction ratio exceeds 80%, the cold rolling load becomes too large and cold rolling becomes difficult. Therefore, the rolling reduction is preferably 80% or less. However, even if cold rolling is performed at a cold rolling rate exceeding 80%, excellent bendability that is the effect of the present invention can be obtained.
In the cold rolling process, the effect of the present invention is exhibited without particularly defining the number of rolling passes and the rolling reduction for each rolling pass.

  Next, the obtained cold-rolled steel sheet is passed through an annealing line and annealed in a temperature range of (Ac3 transformation point−20) ° C. to (Ac3 transformation point + 100) ° C. In order to obtain a steel sheet with excellent bendability, it is necessary to decarburize the surface layer during annealing to soften the steel sheet surface layer. The decarburization treatment means that the atmosphere in the furnace during annealing is in the following range, C contained in the steel sheet surface layer is diffused into the atmosphere, the C concentration of the steel sheet surface layer is reduced, and the fraction of the hard structure is reduced. It is a process to reduce.

In the present invention, decarburization is performed by setting the atmosphere in the furnace at the time of annealing to a log (moisture pressure / hydrogen partial pressure) in the range of −3.0 to 0.0. By setting the logarithm of the ratio between the moisture pressure of the atmospheric gas and the hydrogen partial pressure to −3.0 to 0.0, decarburization from the surface layer of the cold-rolled steel sheet by performing annealing can be appropriately promoted.
When the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is less than −3.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is insufficient. In order to promote decarburization, the logarithm of the ratio between the water pressure and the hydrogen partial pressure is preferably −2.5 or more. On the other hand, if the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is more than 0.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is excessively promoted, and the strength of the steel sheet may be insufficient. There is. In order to ensure the strength of the steel sheet, the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is preferably −0.3 or less.
Moreover, the atmosphere at the time of annealing contains nitrogen, water vapor | steam, and hydrogen, and it is preferable that it is mainly nitrogen, and oxygen may be contained in addition to nitrogen, water vapor | steam, and hydrogen.

Moreover, the temperature range in the case of annealing shall be (Ac3 transformation point-20) degreeC-(Ac3 transformation point +100) degreeC to raise the austenite volume rate at the time of annealing, and to make a steel plate structure into a bainite main phase. This is because.
When the annealing temperature is less than (Ac3 transformation point−20) ° C., the volume fraction of austenite formed during annealing is small, the volume fraction of bainite cannot be made 50% or more, and the balance between strength and bendability is poor. From this, the lower limit of the annealing temperature was set to (Ac3 transformation point−20) ° C. On the other hand, if the annealing temperature is excessively high, not only is it not economically preferable, but also the deterioration of the roll and the production equipment becomes remarkable, so the annealing temperature is set to (Ac3 transformation point +100) ° C. or lower. It is desirable. However, excellent bendability, which is an effect excluding economic efficiency, can be obtained.

The Ac3 transformation point is calculated by the following formula.
Ac3 = 910−203 × (C) ^0.5 −15.2 × Ni + 44.7 × Si + 104 × V
+ 31.5 × Mo-30 × Mn-11 × Cr-20 × Cu + 700 × P
+ 400 × Al + 400 × Ti

  In the present invention, the residence time in the annealing temperature and atmosphere is set to 20 seconds to 600 seconds. If the residence time is less than 20 seconds, the hard tissue fraction becomes too small and it is difficult to ensure a high strength of 900 MPa or more. That is, austenite is formed by dissolution of carbides, but it takes some time for dissolution. When annealing is performed for less than 20 seconds, a sufficient amount of austenite cannot be secured due to insufficient time for the carbide to dissolve. As a result, it is difficult to ensure a strength of 900 MPa or more. Therefore, the lower limit of the annealing temperature time was set to 20 seconds. On the other hand, staying longer than 600 seconds is not preferable because not only the effect is saturated but also productivity is deteriorated. For this reason, the upper limit of the annealing temperature was set to 600 seconds.

  As for cooling after annealing, it is desirable that an average cooling rate in a temperature range of 700 ° C. to 500 ° C. is 0.5 ° C./second or more. When the average cooling rate in this temperature range is less than 0.5 ° C./second, the residence time in this temperature range is long and a large amount of ferrite and pearlite is generated. For this reason, it becomes difficult to ensure the strength of 900 MPa or more. On the other hand, at a cooling rate exceeding 500 ° C./second, not only an excessive facility investment is required, but there is a concern that the temperature variation in the plate increases.

In the subsequent cooling process, holding is performed for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. The reason why the holding temperature range is set to 300 to 500 ° C. is that the bainite transformation proceeds most rapidly in this temperature range.
When the holding temperature is less than 300 ° C., the progress of bainite transformation is slow, and C does not sufficiently concentrate in the austenite, so that austenite is transformed into martensite in the subsequent cooling process. As a result, the retained austenite volume fraction cannot be increased to 3% or more, and the fresh martensite volume fraction exceeds 15%. As a result, the bendability is inferior.
On the other hand, if the holding temperature exceeds 500 ° C., the progress of the bainite transformation is slow, and it is difficult to make the volume ratio more than 50%, and a pearlite structure is formed and the strength is greatly reduced. For this reason, the cooling stop temperature and the holding temperature are set to 300 to 500 ° C.

  The holding time is set to 10 to 1000 seconds in order to cause a sufficient amount of bainite transformation. If the holding time is less than 10 seconds, a bainite structure having a volume ratio of 50% or more cannot be obtained. On the other hand, the reason for setting it to 1000 seconds or less is not preferable because excessive retention reduces productivity.

  In addition, the holding said by this invention means that a steel plate stays for the said time in said temperature range in the middle of cooling. Therefore, it does not mean only the case where the cooling is temporarily stopped and kept at an isothermal temperature, but includes heating and cooling in this temperature range.

In the production of the hot dip galvanized steel sheet, after decarburization treatment and cooling and holding to 300 to 500 ° C., heating to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C., or Cool and dip in a hot dip galvanizing bath for plating.
The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature. If the bath immersion plate temperature is lower than the hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and part of the molten zinc may solidify and deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C.

  However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in. On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature + 50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. 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 carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. If it exceeds 600 ° C., carbide precipitates in the austenite and austenite is decomposed, so that it is difficult to ensure a strength of 700 MPa or more and good bendability, so this is the upper limit.

  The galvanization of the surface of the cold-rolled steel sheet is not limited to that performed by hot dip galvanization described above, and may be performed by electroplating. In that case, it may be carried out according to a conventional method.

  Further, for the purpose of surface lubrication, a film made of a complex oxide containing phosphorus oxide and / or phosphorus may be applied to the surface of the cold rolled steel sheet of the present invention or the surface of the plated layer of the galvanized steel sheet. Absent.

  In addition, skin pass rolling can also be performed after the above-mentioned annealing. The rolling reduction at that time 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. 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.

The present invention will be described in more detail with reference to examples.
After casting a slab having chemical components (compositions) A to V and a to d shown in Table 1 and heating to a slab heating temperature of 1230 ° C., hot rolling was performed under the conditions of Tables 2-1 and 2. Then, after pickling the hot-rolled sheet, it was cold-rolled to 1.2 mm at the cold-rolling rate shown in Tables 2-1 to 3 to obtain a cold-rolled steel sheet. Then, after annealing on the conditions of Tables 2-1 to 3 and performing a decarburization treatment, the steel was cooled to various temperatures to produce high-strength cold-rolled steel sheets. In Tables 2 and 3, sample numbers are given to the steels in Table 1 to indicate steel types.
In the production of the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet, the cooled and retained steel sheet was immersed in a galvanizing bath and then cooled to room temperature. The effective Al concentration in the plating bath in the plating bath was in the range of 0.07 to 0.17 mass%. Some steel sheets were immersed in a galvanizing bath, then alloyed under various conditions, and cooled to room temperature. The weight per unit area was about 35 g / m ^{2 on} both sides. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.4%.

Some high-strength cold-rolled steel sheets were electrogalvanized after degreasing. The plating conditions were as follows: the current density was 15 A / dm ^{2 in} an sulfuric acid solution at 50 ° C. and 8 wt%, and after the electrolytic pickling for about 12 seconds, the following three plating baths (Zn plating, Zn—Ni plating and Zn -Co plating), plating was performed so that the adhesion amounts were 30 g / m ² and 60 g / m ² . Plating bath temperature: 50 ± 2 ° C., current density: 60 A / dm ² , plating solution flow rate: 1 m / sec.

The evaluation of bendability was based on JIS Z 2248. The obtained steel plate was cut out in a direction perpendicular to the rolling direction, the end face was mechanically ground to produce a 35 mm × 100 mm test piece, and the tip R was 0.00. The 90-degree V bending test was performed using a 5-6 mm 90 ° die and punch. The surface of the sample after the bending test was observed with a magnifying glass, and the minimum bending radius without cracking or necking was defined as the critical bending radius. A steel plate having a limit bending radius of 1 mm or less was defined as a steel plate having excellent bendability.
Further, a sample was prepared from the obtained steel plate, and the steel plate structure and hardness were measured by the method described above. Moreover, the tension test was implemented based on JISZ2241.

The results are shown in Tables 3-1 to 3-1. In Table 3, the column of the steel sheet indicates the form of the steel sheet, CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvanized steel sheet, and EG: electrogalvanized steel sheet.
Those satisfying the conditions of the present invention have both a maximum tensile strength of 900 MPa or more and good bendability. The balance of strength (TS) -total elongation (El) (TS × El) was also as good as 18000 (MPa ·%) or more.

  The present invention provides a high-strength steel sheet excellent in bendability having a maximum tensile strength of 900 MPa or more suitable for structural members, reinforcing members, and suspension members for automobiles at low cost. It can be expected to contribute greatly to industrialization, and the industrial effect is extremely high.

Claims (16)

% By mass
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0100%,
Al: 0.001-1.500%,
N: 0.0001 to 0.0100%,
O: 0.0001 to 0.0100%
Is a high-strength steel plate having a tensile maximum strength of 900 MPa or more consisting of iron and inevitable impurities, and the structure contains 50% or more of bainite as a main phase in a volume fraction of 3 to 30%. Residual austenite is contained, and the ratio (Hvs / Hvb) of the hardness (Hvs) of the steel sheet surface layer to the 1/4 thickness (Hvb) of the steel sheet is 0.35 to 0.90. High-strength cold-rolled steel sheet with excellent bendability.   The high-strength cold-rolled steel sheet having excellent bendability according to claim 1, wherein the steel sheet structure is such that fresh martensite is limited to 15% or less. The steel sheet is further in mass%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005-0.150%
The high-strength cold-rolled steel sheet with excellent bendability according to claim 1 or 2, characterized by containing one or more of the following. The steel sheet is further in mass%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%
Ni: 0.01 to 2.00%,
Cu: 0.01-2.00%,
Mo: 0.01 to 1.00%,
W: 0.01-1.00%
The high-strength cold-rolled steel sheet excellent in bendability according to any one of claims 1 to 3, characterized by containing one or more of the following.   The steel sheet further contains 0.0001 to 0.5000% by mass in total of one or more of Ca, Ce, Mg, Zr, Hf, and REM. A high-strength cold-rolled steel sheet excellent in bendability according to item 1.   A bend that is excellent in bendability, characterized in that a film made of a composite oxide containing phosphorus oxide and / or phosphorus is formed on the surface of the cold-rolled steel sheet according to any one of claims 1 to 5. Strength cold-rolled steel sheet.   A high-strength galvanized steel sheet excellent in bendability, comprising an electrogalvanized layer on the surface of the cold-rolled steel sheet according to any one of claims 1 to 5.   A high-strength galvanized steel sheet excellent in bendability, comprising a hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of claims 1 to 5.   A high-strength galvanized steel sheet excellent in bendability, comprising an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of claims 1 to 5.   The bendability according to any one of claims 7 to 9, wherein a film made of a phosphorus oxide and / or a composite oxide containing phosphorus is formed on a surface of the galvanized layer. High strength galvanized steel sheet.   A slab having the chemical component according to any one of claims 1 and 3 to 5 is heated to 1050 ° C or higher, and hot rolling is completed at a finish hot rolling temperature not lower than the Ar3 transformation point, and a temperature not higher than 750 ° C. After hot rolling to be wound in a region, after cold rolling at a reduction rate of 30 to 80%, in a temperature range of (Ac3 transformation point−20) ° C. to (Ac3 transformation point + 100) ° C. and log (moisture pressure) / Hydrogen partial pressure) after annealing for 20 seconds to 600 seconds in an atmosphere of -3.0 to 0.0, between 700 and 500 ° C. at a cooling rate of 0.5 to 500 ° C./second, The high-strength cooling excellent in bendability according to any one of claims 1 to 5, which is cooled to 500 ° C and subsequently held for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. A method for producing rolled steel sheets.   The process of providing the film | membrane which consists of a complex oxide containing phosphorus oxide and / or phosphorus on the surface of the cold-rolled steel plate manufactured with the manufacturing method of Claim 11 is performed. A method for producing a high-strength cold-rolled steel sheet with excellent bendability.   The method for producing a high-strength galvanized steel sheet having excellent bendability according to claim 7, wherein the surface of the cold-rolled steel sheet produced by the production method according to claim 11 is electrogalvanized.   The slab is hot-rolled according to the production method according to claim 11, cold-rolled, and after annealing, after holding at a temperature range of 300 to 500 ° C. for 10 to 1000 seconds, The bendability according to claim 8, further comprising heating or cooling to (zinc plating bath temperature -40) ° C to (zinc plating bath temperature +50) ° C, then immersing in the galvanizing bath and cooling. For producing high-strength galvanized steel sheets with excellent resistance.   The slab is hot-rolled according to the production method according to claim 11, cold-rolled, and after annealing, after holding at a temperature range of 300 to 500 ° C. for 10 to 1000 seconds, Furthermore, after heating or cooling to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C., after being immersed in a zinc plating bath and further subjected to an alloying treatment at a temperature of 460 ° C. or higher, The method for producing a high-strength galvanized steel sheet having excellent bendability according to claim 9, wherein cooling is performed.   The process of providing the film | membrane which consists of a complex oxide containing a phosphorus oxide and / or phosphorus on the surface of the galvanized steel plate manufactured with the manufacturing method of Claim 14 or 15 is performed. The manufacturing method of the high intensity | strength galvanized steel plate excellent in the bendability of description.