WO2010109702A1 - Cold-rolled steel sheet - Google Patents

Abstract

Disclosed is a high-strength cold-rolled steel sheet which has improved stretch-flange formability while keeping excellent hydrogen embrittlement resistance. The cold-rolled steel sheet comprises 0.03 to 0.30% by mass of C, 3.0% by mass or less (including 0% by mass) of Si, more than 0.1% by mass and not more than 2.8% by mass of Mn, 0.1% by mass or less of P, 0.005% by mass or less of S, 0.01% by mass or less of N, and 0.01 to 0.50% by mass of Al. The cold-rolled steel sheet additionally comprises V in an amount of 0.001 to 1.00% by mass or one or more elements selected from Nb, Ti and Zr in the total amount of 0.01% by mass or more, with the remainder being made up by iron and unavoidable impurities, wherein the contents of one or more elements selected from Nb, Ti and Zr fulfils the requirement represented by the following formula: [%C]-[%Nb]/92.9×12-[%Ti]/47.9×12-[%Zr]/91.2×12 > 0.03. In the cold-rolled steel sheet, the area ratio of tempered martensite is 50% or more (including 100%), and ferrite makes up the remainder. In the cold-rolled steel sheet, the distribution of precipitates in the tempered martensite is as follows: the number of precipitates each having a circle-equivalent diameter of 1 to 10 nm is 20 particles or more per 1 µm2 of the tempered martensite and the number of precipitates each containing V or at least one element selected from Nb, Ti and Zr and each having a circle-equivalent diameter of 20 nm or more is 10 particles or less per 1 µm2 of the tempered martensite.

Description

Cold rolled steel sheet

The present invention relates to a cold-rolled steel sheet suitable for automobile parts and the like, and particularly to a high-strength cold-rolled steel sheet excellent in hydrogen embrittlement resistance and workability.

For example, cold rolled steel sheets used for automobile frame parts and the like are required to have a high strength of 980 MPa or more in order to achieve both collision safety and fuel efficiency reduction by reducing the weight of the vehicle body. At the same time, such cold-rolled steel sheets are also required to have excellent formability in order to process into skeleton parts having complicated shapes.

Conventionally, in a high-strength steel often used for applications such as bolts, PC steel wires, and line pipes, when the tensile strength is 980 MPa or more, hydrogen embrittlement (pickling brittleness, plating brittleness, It is well known that delayed destruction occurs. Delayed fracture is a high-strength steel in which hydrogen generated from a corrosive environment or atmosphere diffuses into defects such as dislocations, vacancies, and grain boundaries, embrittles the material, and breaks when stress is applied. It is a phenomenon that brings about adverse effects such as a decrease in the ductility and toughness of the metal material. Most of the techniques for improving the resistance to hydrogen embrittlement are for steel materials used for bolts and the like. For example, Non-Patent Document 1 describes that if a metal structure is mainly tempered martensite and an element (Cr, Mo, V, etc.) exhibiting temper softening resistance is added, it is effective in improving delayed fracture resistance. ing. This is a technology that suppresses the fracture by transferring the delayed fracture mode from the grain boundary to the intragranular fracture by precipitating the alloy carbide and utilizing it as a hydrogen trap site. However, these findings are for application to medium carbon steel, and cannot be used as it is for thin steel sheets with low carbon content that require weldability and workability.

Therefore, the present applicants have developed an ultra-high strength thin steel sheet with excellent hydrogen embrittlement resistance, which has a carbon content of C: more than 0.25 to 0.60% by mass and the balance is made of iron and inevitable impurities. (Patent Document 1). In this ultra high strength steel sheet, the metal structure after the tensile process with a processing rate of 3% is an area ratio with respect to the entire structure, the retained austenite structure: 1% or more, bainitic ferrite and martensite: a total of 80% or more, The above-mentioned residual austenite crystal grains satisfy an average axial ratio (major axis / minor axis) of 5 or more.

The above thin steel sheet exhibits excellent strength and elongation and resistance to hydrogen embrittlement. However, even in the above-mentioned thin steel sheet, retained austenite becomes a starting point of fracture and becomes a factor of lowering the stretch flangeability. Therefore, the stretch flangeability, which is becoming increasingly important in recent years, has a desired level (at least 70%, preferably 90%) was difficult to achieve reliably.

Japan Published Patent Publication: 2006-207019

Japan Iron and Steel Institute, New Development of Delayed Fracture Elucidation, January 1997, p. 111-120

Therefore, an object of the present invention is to provide a high-strength cold-rolled steel sheet that has improved stretch embrittlement properties while ensuring excellent hydrogen embrittlement resistance.

The cold-rolled steel sheet according to the present invention has C: 0.03 to 0.30 mass%, Si: 3.0 mass% or less (including 0 mass%), Mn: more than 0.1 mass%, 2.8 mass% %: P: 0.1 mass% or less, S: 0.005 mass% or less, N: 0.01 mass% or less, and Al: 0.01 to 0.50 mass%, and V: 0 0.001 to 1.00% by mass, or a combination of one or more of Nb, Ti, and Zr so as to satisfy the following formula 1 and 0.01% by mass or more, with the balance being from iron and inevitable impurities A cold-rolled steel sheet comprising tempered martensite in an area ratio of 50% or more (including 100%), the remainder having a structure made of ferrite, and a precipitate having a circle-equivalent diameter of 1 to 10 nm. and a martensite 1 [mu] m ² per 20 or more, or comprising a V, or, b, containing at least one of Ti and Zr, a circle equivalent diameter 20nm or more precipitates, wherein the at tempered martensite 1 [mu] m ² 10 per below.
[% C] − [% Nb] /92.9×12 − [% Ti] /47.9×12 − [% Zr] /91.2×12> 0.03 (Formula 1 )
(However, [% C], [% Nb], [% Ti], and [% Zr] mean the contents (mass%) of C, Nb, Ti, and Zr, respectively)

In the cold-rolled steel sheet according to the present invention, at least 1 type of Nb, Ti, and Zr is combined so as to satisfy the above-mentioned formula 1, and 0.01% by mass or more, and the crystal grain difference is 15 ° or more at a large angle grain boundary. The average particle size of the surrounded ferrite is preferably 5 μm or less. Further, it is preferable that the number of precipitates containing V: 0.001 to 0.20 mass% and having an equivalent circle diameter of 20 nm or more containing V is 10 or less per 1 μm ^{2 of the} tempered martensite.

In the cold-rolled steel sheet according to the present invention, Cr: 0.01 to 1.0 mass%, Mo: 0.01 to 1.0 mass%, Cu: 0.05 to 1.0 mass%, and Ni: 0 It is preferable to include one or more of 0.05 to 1.0% by mass.

The cold-rolled steel sheet according to the present invention preferably contains B: 0.0001 to 0.0050 mass%.

In the cold-rolled steel sheet according to the present invention, one of Ca: 0.0005 to 0.01 mass%, Mg: 0.0005 to 0.01 mass%, and REM: 0.0004 to 0.01 mass% It is preferable to include the above.

In the cold rolled steel sheet according to the present invention, the number of cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm is 10 or more per 1 μm ^{2 of the} tempered martensite, and the cementite particles having an equivalent circle diameter of 0.1 μm or more. The number is preferably 3 or less per 1 μm ^{2 of the} tempered martensite.

In the cold rolled steel sheet according to the present invention, the dislocation density in the entire structure is 1 × 10 ¹⁵ to 1 × 10 ¹⁶ m ⁻² , and the Si equivalent defined by the following formula 2 satisfies the following formula 3. It is preferable to do.
[Si equivalent] = [% Si] +0.36 [% Mn] +7.56 [% P] +0.15 [% Mo] +0.36 [% Cr] +0.43 [% Cu] (Formula 2)
[Si equivalent] ≧ 4.0−5.3 × 10 ⁻⁸ √ [dislocation density] (Equation 3)

According to the present invention, in the tempered martensite single phase structure or the two-phase structure composed of ferrite and tempered martensite, the area ratio of the tempered martensite is appropriately controlled, V is contained, or Nb, Ti and Zr The distribution state of the precipitate precipitated in the tempered martensite containing one or more types is appropriately controlled. This makes it possible to improve stretch flangeability while ensuring hydrogen embrittlement resistance, and to provide a high-strength thin steel sheet that is excellent in both hydrogen embrittlement resistance and stretch flangeability.

The present inventors paid attention to a high-strength steel sheet having a tempered martensite (hereinafter sometimes simply referred to as “martensite”) single phase or a two-phase structure composed of ferrite and tempered martensite. The inventors then added V as an alloy element, or added one or more of Nb, Ti, and Zr to the carbides and carbonitrides of V that strongly act as hydrogen trap sites. It was thought that stretch flangeability could be improved while securing hydrogen embrittlement resistance by introducing carbides and carbonitrides of Nb, Ti, and Zr into martensite with appropriate sizes. The inventors have conducted intensive studies such as investigating the influence of various factors on hydrogen embrittlement resistance and stretch flangeability. Hereinafter, V carbides and carbonitrides, Nb, Ti, and Zr carbides and carbonitrides may be collectively referred to as “V-containing precipitates”.
As a result, the inventors have found that, in addition to reducing the proportion of ferrite, by refining precipitates containing V or the like, it is possible to improve stretch flangeability while ensuring hydrogen embrittlement resistance, The present invention has been completed based on this finding.

[Structure of the steel sheet of the present invention]
Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.
As described above, the steel sheet of the present invention is based on a tempered martensite single phase or a two-phase structure (ferrite + tempered martensite), and in particular, the distribution state of precipitates containing V and the like in the tempered martensite. Is characterized in that is controlled.

<Tempered martensite: 50% or more in area ratio (including 100%)>
By using a tempered martensite-based structure, breakage at the interface between ferrite and tempered martensite can be prevented, and stretch flangeability can be secured.

In order to effectively exhibit the above action, the tempered martensite is 50% or more in area ratio, preferably 60% or more, more preferably 70% or more (including 100%). The balance is ferrite.

<Precipitates with equivalent circle diameter of 1 to 10 nm: 20 or more per 1 μm ^{2 of} tempered martensite>
When fine V-containing precipitates that effectively act as hydrogen trap sites are appropriately dispersed in the structure, the hydrogen embrittlement resistance can be improved and delayed fracture resistance after processing can be ensured. In other words, when a large amount of fine V-containing precipitates having a large specific surface area are dispersed in a large amount, the number of hydrogen trap sites can be increased. A consistent strain field is applied around the object. Thereby, the capability as a trap site for hydrogen that tends to collect in the strain field can be enhanced, and the hydrogen embrittlement resistance is improved. In this particle size range (equivalent circle diameter of 1 to 10 nm), there are almost no precipitates that do not contain any of V, Nb, Ti, and Zr. All precipitates were targeted without limitation as in the case.

In order to effectively exert the above action, the number of fine precipitates having an equivalent circle diameter of 1 to 10 nm is 20 or more, preferably 50 or more, more preferably 100 or more per 1 μm ^{2 of} tempered martensite. A preferable range of the size (equivalent circle diameter) of the fine precipitate is 1 to 8 nm, and a more preferable range is 1 to 6 nm.

The reason why the lower limit of the equivalent circle diameter of the fine precipitate is set to 1 nm is that the fine precipitate having a smaller diameter is less effective as a hydrogen trap site.

<Precipitates containing V or one or more of Nb, Ti and Zr and having an equivalent circle diameter of 20 nm or more: 10 or less per 1 μm ^{2 of} tempered martensite>
Precipitates containing V, such as VC, or precipitates containing Nb, TiC, ZrC, or other Nb, Ti, or Zr, have very high rigidity and critical shear stress compared to the parent phase. Even so, the precipitate itself is not easily deformed. Therefore, when these precipitates have a size of 20 nm or more, a large strain occurs at the interface between the parent phase and the precipitates, and breakage occurs. For this reason, if a large amount of coarse precipitates of 20 nm or more is present, stretch flangeability deteriorates. Therefore, stretch flangeability can be improved by restricting the existence density of coarse V-containing precipitates.

In order to effectively exhibit the above-described action, coarse V-containing precipitates having an equivalent circle diameter of 20 nm or more are limited to 10 or less, preferably 5 or less, more preferably 3 or less per 1 μm ^{2 of} tempered martensite. The

It is essential that the structure of the steel sheet of the present invention satisfies the above requirements. Further, when one or more of Nb, Ti and Zr are contained, it is recommended to satisfy the following organization regulations in addition to the essential organization regulations.
<Average grain size of ferrite surrounded by large grain boundaries with a crystal orientation difference of 15 ° or more: 5 μm or less>
By making effective ferrite finer, even if fatigue cracks occur at the interface with martensite, the cracks are difficult to propagate into the ferrite grains. Thereby, stretch flangeability can be improved.
In order to effectively exhibit the above action, the average grain size of ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more is limited to 5 μm or less, preferably 10 μm or less.
Further, when it contains one or more of Nb, Ti and Zr and also contains V, it is recommended that the content of V is 0.001 to 0.20 mass%.
V, like Nb, Ti and Zr, is an element that contributes to the improvement of hydrogen embrittlement resistance because it functions as a hydrogen trap site by being present in steel as fine carbides and carbonitrides. When the amount of V is less than 0.001% by mass, the effect of improving the hydrogen embrittlement resistance cannot be sufficiently obtained. On the other hand, when Nb, Ti, or Zr is contained and the amount of V exceeds 0.20% by mass, V is present in the steel in an insoluble state during heating during annealing. Thereby, since the V carbide or V carbonitride which grows coarsely increases, stretch flangeability deteriorates. The range of V content in the case of containing one or more of Nb, Ti and Zr is more preferably 0.01% by mass or more and less than 0.15% by mass, particularly preferably 0.02% by mass or more and 0.12% by mass. %.

The structure of the steel sheet of the present invention preferably satisfies the following recommended structure provision (a) or (b) in addition to the above essential structure provision.

<(A) Cementite particles with an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm: 10 or more per 1 μm ^{2 of} tempered martensite, 3 cementite particles with an equivalent circle diameter of 0.1 μm or more: 3 or less per 1 μm ^{2 of} tempered martensite>
In addition to controlling the dispersion state of the V-containing precipitates, by controlling the size and number of cementite particles precipitated in martensite during tempering, both elongation and stretch flangeability can be improved. . In other words, a large amount of moderately fine cementite particles are dispersed in martensite to increase the work hardening index by acting as a growth source of dislocations, contributing to improvement of elongation, and at the time of deformation of the stretch flange, Stretch flangeability can be further improved by reducing the number of coarse cementite particles as starting points.

In order to effectively exhibit the above action, the moderately fine cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm are 10 or more per 1 μm ^{2 of} tempered martensite, 15 or more, especially 20 or more. Good. On the other hand, it is recommended that coarse cementite particles having an equivalent circle diameter of 0.1 μm or more be limited to 3 particles or less, further 2.5 particles or less, particularly 2 particles or less per 1 μm ^{2 of} tempered martensite.

Note that the lower limit of the equivalent circle diameter of the moderately fine cementite particles is 0.02 μm because the finer cementite particles do not give sufficient strain to the martensite crystal structure, and the growth source of dislocations. It is because it is thought that it hardly contributes.

<(B) Dislocation density in all structures: 1 × 10 ¹⁵ to 1 × 10 ¹⁶ m ⁻² , [Si equivalent] ≧ 4.0−5.3 × 10 ⁻⁸ √ [dislocation density]>
In addition to controlling the dispersion state of the V-containing precipitate, the elongation can be ensured by controlling the dislocation density introduced into the entire structure. At the same time, it is possible to ensure the yield strength, which is important in evaluating the collision safety that has been regarded as important in recent years. That is, in the C—Si—Mn based low alloy steel having the above composition, the yield strength of the martensite-based structure whose tempering temperature exceeds 400 ° C. has four strengthening mechanisms (solid solution strengthening, precipitation strengthening, refinement). (Strengthening, dislocation strengthening) is particularly strongly dependent on dislocation strengthening. For this reason, in order to ensure the yield strength of 900 MPa or more, which is the desired level, it has been found that the dislocation density in the entire structure needs to be ensured to be 1 × 10 ¹⁵ m ⁻² or more.

On the other hand, the elongation has a strong negative correlation with the dislocation density at the initial stage of deformation. For this reason, it was found that the dislocation density must be limited to 1 × 10 ¹⁶ m ⁻² or less in order to ensure an elongation of 10% or more.

Therefore, it is recommended that the dislocation density in the entire structure is 1 × 10 ¹⁵ to 1 × 10 ¹⁶ m ⁻² .

And as mentioned above, in order to ensure an elongation of 10% or more, there is an upper limit to the dislocation density that can be introduced into the entire structure. As a result of further studies, it has been found that in order to reliably obtain a yield strength of 900 MPa or more, it is necessary to utilize solid solution strengthening that contributes to yield strength next to dislocation strengthening.

First, Si equivalent shown in Formula 2 was introduced as an index representing the amount of solid solution strengthening necessary to surely obtain the yield strength of 900 MPa or more. This Si equivalent is based on Si, which is a representative element that exhibits a solid solution strengthening action. The solid solution strengthening action of each element other than Si (Toshio Fujita et al .: Design and theory of steel materials, Maruzen, ( 1981), p. 8) is converted into Si concentration and formulated.

[Si equivalent] = [% Si] +0.36 [% Mn] +7.56 [% P] +0.15 [% Mo] +0.36 [% Cr] +0.43 [% Cu] (Formula 2)

Next, the yield strength increase Δσ due to dislocation strengthening is expressed as Δσ∝ √ρ as a function of the dislocation density ρ from the Bailey-Hirsh equation (Koichi Nakajima et al .: “Materials and Processes”, Vol. 17, 2004, pages 396-399). And, as a result of experimentally verifying the quantitative relationship between the yield strength increasing effect due to the solid solution strengthening and the yield strength increasing effect due to the dislocation strengthening, the yield strength of 900 MPa or more is satisfied by satisfying the following formula 3. Was found to be obtained reliably.

[Si equivalent] ≧ 4.0-5.3 × 10 ⁻⁸ √ [dislocation density] (Equation 3)

Hereinafter, each measurement method of the area ratio of tempered martensite, the size and number of precipitates, the size of effective ferrite, the size and number of cementite particles, and the dislocation density will be described.

[Measurement method of martensite area ratio]
First, each test steel plate was mirror-polished and corroded with a 3% nital solution to reveal the metal structure, and then a scanning electron microscope (SEM) image at a magnification of 2000 times with respect to five fields of view of approximately 40 μm × 30 μm region. Was observed. And the area ratio of martensite was computed from the area ratio of each area | region by making the area | region which does not contain cementite the ferrite by image analysis, and making the remaining area | region the martensite.

[Measurement method of the size and number of precipitates]
In order to measure the size and the number of precipitates, a thin film sample was first prepared by the thin film method or the extraction replica method. Then, using a field emission type transmission electron microscope (FE-TEM), an area of 2 μm ² or more of this sample was observed at a magnification of 100,000 to 300,000, and the dark portion was marked as a precipitate from the contrast of the image. Then, the equivalent circle diameter was calculated from the area of each marked precipitate by image analysis software, and the number of precipitates having a predetermined size per unit area was obtained.

However, for precipitates of 20 nm or more, only those that confirmed the presence of V, Nb, Ti, Zr in the precipitates using EDX or EELS attached to FE-TEM were counted.

[Method of measuring the size of cementite particles and their number]
In order to measure the size of cementite particles and the number of the cementite particles, first, each test steel plate was mirror-polished and corroded with picral to reveal a metal structure. Thereafter, a scanning electron microscope (SEM) image with a magnification of 10,000 times was observed for a field of view of 100 μm ² so that the region inside the martensite could be analyzed, and the white portion was discriminated as cementite particles from the image contrast. Then, by using image analysis software, the equivalent circle diameter was calculated from the area of each marked cementite particle, and the number of cementite particles having a predetermined size per unit area was obtained.

[Measurement method of dislocation density]
Further, in order to measure the dislocation density, first, the sample was adjusted so that the 1/4 depth position of the plate thickness could be measured, and then Si powder was applied to the sample surface as a standard sample. This was applied to an X-ray diffractometer (RAD-RU300, manufactured by Rigaku Corporation), and an X-ray diffraction profile was collected. Based on this X-ray diffraction profile, the dislocation density was calculated according to the analysis method proposed by Koichi Nakajima et al. (See Koichi Nakajima et al .: “Materials and Processes”, Vol. 17, 2004, p. 396-399. ).

[Measurement method of effective ferrite size]
The orientation of a large-angle grain boundary having a crystal orientation difference of 15 ° or more was measured by an electron beam backscatter diffraction (EBSD) method using a 10,000-mm TEM (transmission electron microscope) with several fields of 10,000 μm ² . And the ferrite surrounded by the large angle grain boundary whose crystal orientation difference (orientation difference angle of the ferrite grain boundary) is 15 ° or more was defined as an effective ferrite. The average grain diameter of the effective ferrite is 5000 times larger than that of the adjacent crystal grains by using an SEM (scanning electron microscope: JSM-5410 made by JEOL) and OIM (trademark) made by TSL. A certain grain boundary was measured and measured using the intercept method (see Japanese Patent Publication No. 2005-133155, paragraphs [0021]-[0022]).

Next, the component composition constituting the steel sheet of the present invention will be described.

[C: 0.03 to 0.30 mass%]
C is an important element that affects the area ratio of martensite and affects strength and stretch flangeability. Further, C combines with V, Nb, Ti, Zr to form precipitates containing V and the like. Therefore, when the V content or the balance between the N content, the content of Nb, Ti, and Zr and the C content change, the behavior of precipitation, disappearance, and coarsening of precipitates such as V during the heat treatment is affected. Affects stretch flangeability. If the C content is less than 0.03% by mass, the area ratio of martensite is insufficient, so that the strength cannot be ensured. On the other hand, if the C content exceeds 0.30% by mass, precipitates containing V and the like are too stable during heating during annealing, so that fine precipitates cannot be obtained, and hydrogen embrittlement characteristics cannot be ensured. The lower limit of the C content is preferably 0.05% by mass, more preferably 0.07% by mass, and still more preferably 0.08% by mass. The upper limit of the C content is preferably 0.25% by mass, more preferably 0.20% by mass.

[Si: 3.0% by mass or less (including 0% by mass)]
Si is a useful element that can increase strength without deteriorating elongation as a solid solution strengthening element. If the Si content exceeds 3.0% by mass, the formation of austenite during heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured. The range of the Si content is preferably 2.5% by mass or less, more preferably 2.0% by mass or less, further preferably 1.8% by mass or less, and particularly preferably 1.5% by mass or less (0% by mass). Included).

[Mn: more than 0.1% by mass, 2.8% by mass or less]
Mn is a useful element having an effect of enhancing the strength and stretch flangeability by increasing the hardenability and ensuring the martensite area ratio during rapid cooling after heating during annealing. When the Mn content is 0.1% by mass or less, bainite is formed at the time of rapid cooling for quenching and the martensite area ratio is insufficient, so that the strength and stretch flangeability cannot be ensured. On the other hand, if the Mn content is more than 2.8% by mass, austenite remains at the time of quenching (during cooling after annealing), and stretch flangeability is deteriorated. The range of the Mn content is preferably 0.30 to 2.5% by mass, more preferably 0.50 to 2.2% by mass.

[P: 0.1% by mass or less]
P is unavoidably present as an impurity element and contributes to an increase in strength by solid solution strengthening. However, P segregates at the prior austenite grain boundaries and causes the grain boundaries to become brittle, thereby deteriorating stretch flangeability. P content becomes like this. Preferably it is 0.05 mass% or less, More preferably, it is 0.03 mass% or less.

[S: 0.005 mass% or less]
S is also unavoidably present as an impurity element, and by forming MnS inclusions, it becomes a starting point of cracks when the holes are expanded, thereby reducing the stretch flangeability. The S content is more preferably 0.003% by mass or less.

[N: 0.01% by mass or less]
N is inevitably present as an impurity element, and the elongation and stretch flangeability are lowered by strain aging. For this reason, the one where N content is low is preferable and it shall be 0.01 mass% or less.

[Al: 0.01 to 0.50 mass%]
Al combines with N to form AlN, thereby reducing the solid solution N that contributes to the occurrence of strain aging, thereby preventing the stretch flangeability from deteriorating and contributing to the strength improvement by solid solution strengthening. If the Al content is less than 0.01% by mass, solid solution N remains in the steel, strain aging occurs, and elongation and stretch flangeability cannot be ensured. On the other hand, if the Al content exceeds 0.50% by mass, the formation of austenite during heating is inhibited, so the area ratio of martensite cannot be ensured and stretch flangeability cannot be ensured.

[V: 0.001 to 1.00% by mass, or one or more of Nb, Ti, and Zr combined to be 0.01% by mass or more, [% C] − [% Nb] /92.9× 12-[% Ti] /47.9×12-[% Zr] /91.2×12> 0.03 is included to satisfy the condition]
(V: 0.001 to 1.00% by mass)
V promotes the production of iron oxide α-FeOOH, which is said to be thermodynamically stable and protective among rust produced in the atmosphere, and is present in steel as fine carbides and carbonitrides. Works as a hydrogen trap site. From these facts, V is an important element for improving hydrogen embrittlement resistance. When the V content is less than 0.001% by mass, the effect of improving the hydrogen embrittlement resistance cannot be sufficiently obtained. On the other hand, if the V content exceeds 1.00% by mass, the stretch flangeability deteriorates because V carbide or V carbonitride that grows coarsely and exists in the steel in an insoluble state during heating during annealing. To do. The range of V content is preferably 0.01% by mass or more and less than 0.50% by mass, and more preferably 0.02% by mass or more and less than 0.30% by mass.
As described above, when one or more of Nb, Ti, and Zr are contained and V is also contained, the content of V is recommended to be 0.001 to 0.20 mass%. .

(One or more of Nb, Ti and Zr: 0.01% by mass or more in total, and [% C]-[% Nb] /92.9×12-[% Ti] /47.9×12- [ % Zr] /91.2×12> 0.03)
Nb, Ti, and Zr are important elements for improving the hydrogen embrittlement resistance because they exist as fine carbides and carbonitrides in the steel and serve as hydrogen trap sites. Further, these elements contribute to the refinement of effective ferrite by acting as particles that pin the growth of austenite as fine carbides / carbonitrides during heating during annealing. When the total content of Nb, Ti and Zr is less than 0.01% by mass, the effect of improving the hydrogen embrittlement resistance cannot be sufficiently obtained. On the other hand, when [% C] − [% Nb] /92.9×12 − [% Ti] /47.9×12 − [% Zr] /91.2×12≦0.03, heating during annealing Sometimes the amount of carbon that dissolves in austenite is insufficient, and sufficient martensite hardness cannot be obtained. The range of the total content of Nb, Ti and Zr is preferably 0.02% by mass or more and less than 0.10% by mass, more preferably 0.03% by mass or more and less than 0.10% by mass.

The steel of the present invention basically contains the above components and the balance is substantially iron and impurities, but the following allowable components can be added as long as the effects of the present invention are not impaired.

[Cr: 0.01 to 1.0% by mass,
Mo: 0.01 to 1.0% by mass,
Cu: 0.05 to 1.0 mass%, and Ni: 0.05 to 1.0 mass%
One or more of
These elements are useful for enhancing the strength and stretch flangeability by increasing the hardenability and contributing to securing the martensite area ratio. Of these elements, Cr and Mo form alloy carbides and carbonitrides that can serve as hydrogen trap sites during tempering. Cu and Ni, like V, promote the production of α-FeOOH. All of these have the effect of improving the hydrogen embrittlement resistance. If the addition amount of each element is less than each of the above lower limit values, the above effects cannot be exhibited effectively. On the other hand, if the added amounts of Cr, Mo, and Cu exceed 1.0% by mass, the hardness of martensite becomes too high. On the other hand, if the amount of Ni exceeds 1.0% by mass, austenite remains during quenching. Thereby, stretch flangeability will fall.

[B: 0.0001 to 0.0050 mass%]
B is an element useful for enhancing the hardenability and increasing the martensite area ratio by being present in the austenite grain boundary in a solid solution state in the steel. When the addition amount of B is less than 0.0001% by mass, the above-described effects cannot be exhibited effectively. On the other hand, when the addition amount of B exceeds 0.0050 mass%, Fe ₂₃ (CB) ₆ is formed, and solid solution B does not exist, and the effect of improving hardenability cannot be obtained.

[Ca: 0.0005 to 0.01% by mass,
Mg: 0.0005 to 0.01% by mass, and REM: 0.0004 to 0.01% by mass
One or more of
These elements are useful for improving stretch flangeability by miniaturizing inclusions and reducing the starting point of fracture. When the addition amount of Ca and Mg is less than 0.0005 mass%, or the addition amount of REM is less than 0.0004%, the above-described effects cannot be exhibited effectively. On the other hand, when the addition amount of each element exceeds 0.01% by mass, the inclusions are coarsened and stretch flangeability is deteriorated.

REM refers to a rare earth element, that is, a group 3A element in the periodic table.

Next, a preferable manufacturing method for obtaining the steel sheet of the present invention will be described below.

To produce the cold-rolled steel sheet of the present invention, first, steel having the above composition is melted and formed into a slab by ingot forming or continuous casting and then hot-rolled (hot-rolled).

[Hot rolling conditions]
As hot rolling conditions, the hot rolling heating temperature is set to 900 ° C. or higher when V is contained, and 1200 ° C. or higher when Nb, Ti, or Zr is contained. In addition, the hot rolling finish rolling temperature is set to 800 ° C. or higher when V is contained, and is set to 850 ° C. or higher when Nb, Ti, or Zr is contained. It is recommended to wind up with.

By performing hot rolling under such temperature conditions, V or Nb, Ti, or Zr is completely dissolved in the heating stage, and precipitation such as precipitation during hot rolling and precipitation such as V during winding is performed. It is possible to prevent coarse precipitates containing V and the like from remaining during heating during annealing.

[Cold rolling conditions]
After hot rolling is completed, pickling is performed and then cold rolling is performed. The cold rolling rate is preferably about 30% or more.
After the cold rolling, annealing and further tempering are subsequently performed.

[Annealing conditions]
1) When V is contained As annealing conditions when V is contained, annealing heating temperature: [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or more, In addition, it is heated to [(Ac ₁ + Ac ₃ ) / 2] or more and 1000 ° C. or less, and the annealing holding time is held for 20 to 3600 seconds. Then, it is good to quench rapidly with the cooling rate of 50 degree-C / sec or more from annealing heating temperature to the temperature below Ms point. Or, after annealing at a cooling rate (first cooling rate) of 1 ° C./second or more and less than 50 ° C./second from an annealing heating temperature to a temperature of 600 ° C. or more (first cooling end temperature) below the annealing heating temperature, It is preferable to rapidly cool to a temperature below the Ms point (second cooling end temperature) at a cooling rate (second cooling rate) of 50 ° C./second or more. Here, [% C] and [% V] are the C content and V content (both mass%) in the steel material.

[Annealing heating temperature Ta (° C.): [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or higher and [(Ac ₁ + Ac ₃ ) / 2] or higher 1000 Or less, annealing holding time: 20 to 3600 seconds]
Ta (° C.) ≧ [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. is achieved by completely dissolving V carbides and the like during annealing heating. Further, by reducing the abundance of coarse V-containing precipitates of 20 nm or more and sufficiently transforming to austenite at the time of annealing and heating, the area ratio of martensite transformed from austenite at the time of subsequent cooling is ensured by 50% or more. Because.
Ta (° C.) <[− 9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. That is, log [% V] <[− 9500 / (Ta + 273) ] -Log [% C] is not preferable because undissolved V carbides and the like remain during annealing and heat treatment, which coarsens and increases the starting point of fracture when the stretch flange is deformed. The relational expression Ta (° C.) ≧ [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. is the edition of the Japan Iron and Steel Institute, 3rd edition Steel Handbook, Volume I Fundamentals, p. The linear plot showing the temperature dependence of the solubility product of [V] · [C] shown in FIG. 743 of 412 is read and transformed so that the temperature at which V is completely dissolved can be calculated. Asked.

When Ta (° C.) <[(Ac ₁ + Ac ₃ ) / 2] ° C., the amount of transformation to austenite is insufficient during annealing, so the amount of martensite that forms transformation from austenite during subsequent cooling is reduced and the area is reduced. Since it becomes impossible to ensure the rate of 50% or more, it is not preferable. On the other hand, Ta (° C.)> 1000 ° C. is not preferable because the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate and the annealing equipment deteriorates.

If the annealing holding time is less than 20 seconds, V carbide and the like cannot be completely dissolved, and if it exceeds 3600 seconds, the productivity is extremely deteriorated.

2) When Nb, Ti, or Zr is contained As annealing conditions when Nb, Ti, or Zr is contained, annealing heating temperature: satisfies the following formula 4, and [(Ac ₁ + Ac ₃ ) / 2] or more and 1000 ° C. The following is heated and the annealing holding time is held for 20 to 3600 seconds. Then, it is good to quench rapidly with the cooling rate of 50 degree-C / sec or more from annealing heating temperature to the temperature below Ms point. Or, after annealing at a cooling rate (first cooling rate) of 1 ° C./second or more and less than 50 ° C./second from an annealing heating temperature to a temperature of 600 ° C. or more (first cooling end temperature) below the annealing heating temperature, It is preferable to rapidly cool to a temperature below the Ms point (second cooling end temperature) at a cooling rate (second cooling rate) of 50 ° C./second or more.

[Annealing heating temperature: satisfying Pf> 0.0010 and [(Ac ₁ + Ac ₃ ) / 2] to 1000 ° C., annealing holding time: 20 to 3600 seconds]
Annealing heating temperature: Pf> 0.0010 reduces the abundance of coarse V-containing precipitates of 20 nm or more by completely dissolving Nb, Ti, Zr carbides, etc. during annealing heating. This is because by sufficiently transforming to austenite at the time of annealing and heating, the area ratio of martensite that is transformed from austenite at the time of subsequent cooling is secured by 50% or more.
The left side Pf of the above equation 4 is obtained from an equation that thermodynamically expresses the precipitation and dissolution behavior of Nb, Ti and Zr as a parameter representing the solid solution amount of Nb, Ti and Zr during annealing heating. (See Japan Iron and Steel Institute, 3rd edition Handbook of Steel, Volume I Basics, p. 412). If the annealing heating temperature is set so as to satisfy Pf> 0.0010, a sufficient amount of solute Nb and Ti can be secured.
At the annealing heating temperature Ta (° C.) <[(Ac ₁ + Ac ₃ ) / 2] ° C., the amount of transformation to austenite is insufficient during annealing heating, so the amount of martensite produced from austenite during subsequent cooling is reduced. As a result, an area ratio of 50% or more cannot be secured. On the other hand, Ta (° C.)> 1000 ° C. is not preferable because the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate and the annealing equipment deteriorates.
If the annealing holding time is less than 20 seconds, carbides such as Nb, Ti, and Zr cannot be completely dissolved, and if it exceeds 3600 seconds, productivity is extremely deteriorated, which is not preferable.

The subsequent annealing conditions are common when V is contained and when Nb, Ti, or Zr is contained.
[Rapid cooling at a cooling rate of 50 ° C / second or more to a temperature below the Ms point]
Thereby, it can suppress that a ferrite and a bainite structure are formed from austenite during cooling, and can obtain a martensitic structure.

When the rapid cooling is finished at a temperature higher than the Ms point or the cooling rate is less than 50 ° C./second, bainite is formed, and the strength of the steel sheet cannot be secured.

[Slow cooling at a cooling rate of 1 ° C./second or more and less than 50 ° C./second to a temperature below 600 ° C. below the heating temperature]
Thereby, by forming a ferrite structure having an area ratio of less than 50%, the elongation can be improved while the stretch flangeability is secured.

When the temperature is less than 600 ° C. or the cooling rate is less than 1 ° C./second, ferrite is not formed, and the strength and stretch flangeability cannot be secured.

The recommended conditions for the hot rolling conditions and annealing conditions described above are common to all steel sheets regardless of the structure rules.
However, the recommended conditions for the tempering conditions differ between the steel sheet that satisfies only the essential structure rule and a steel sheet that also satisfies the recommended structure rule (a) or (b) in addition to the essential structure rule. The explanation will be given separately.

[Tempering conditions for steel sheets that satisfy only the essential organization rules]
1) When containing V As a tempering condition of the steel sheet containing V and satisfying only the essential structure provision, the tempering heating temperature Tt (° C.): 480 ° C. or higher from the temperature after the annealing cooling and holding tempering Time t (seconds): Pg = exp [−13123 / (Tt + 273)] × t <1.8 × 10 ⁻⁵ may be maintained and then cooled.

In order to precipitate V carbide or the like during tempering, it is necessary to heat to 480 ° C. or higher, and to control the size of the precipitate, it is necessary to appropriately control the relationship between the heating temperature and the holding time.

Here, Pg = exp [−13123 / (Tt + 273)] × t is based on the grain growth model of precipitates described in the formula (4.18) of Koichi Sugimoto et al., Material Histology, Asakura Shoten Publishing, p106. , A parameter that defines the size of the precipitate that has been set and simplified.

Under the condition of Pg = exp [−13123 / (Tt + 273)] × t ≧ 1.8 × 10 ⁻⁵ , the coarsening of the precipitate proceeds and the number of coarse precipitates of 20 nm or more becomes too large. The stretch flangeability cannot be secured.

2) When Nb, Ti, or Zr is contained When Nb, Ti, or Zr is contained, as a tempering condition of the steel sheet that satisfies only the essential structure regulations, the tempering heating temperature Tt (° C.) from the temperature after the annealing cooling. : 480 ° C. or higher and lower than 600 ° C. and tempering holding time t (seconds): Pg = exp [−13520 / (Tt + 273)] × t <1.00 × 10 ⁻⁵ .
In order to precipitate Nb, Ti, Zr carbides, etc. during tempering, it is necessary to heat to 480 ° C. or higher. To control the size of the precipitate, it is necessary to appropriately control the relationship between the heating temperature and the holding time. is there.
Here, Pg = exp [-13520 / (Tt + 273)] × t is based on the grain growth model of precipitates described in the equation (4.18) of Koichi Sugimoto et al., Material Histology, Asakura Shoten Publishing, p106. It is a parameter that defines the size of the precipitate, with variables set and simplified.
Under the condition of Pg = exp [-13520 / (Tt + 273)] × t ≧ 1.00 × 10 ⁻⁵ , the coarsening of the precipitate proceeds and the number of coarse precipitates of 20 nm or more becomes too large. The stretch flangeability cannot be secured.

[Tempering conditions for steel sheet satisfying the above recommended structure rule (a) in addition to the required structure rule]
As a tempering condition for a steel sheet that satisfies the above recommended structure rule (a) in addition to the essential structure rule, both the case of containing V and the case of containing Nb, Ti, and Zr [satisfying only the above essential structure rule] It is recommended to satisfy the following conditions while satisfying the tempering condition of the steel sheet to be performed].

That is, heating is performed at an average heating rate of 5 ° C./second or more from 100 to 325 ° C. from the temperature after the annealing cooling to the first tempering heating temperature: 325 to 375 ° C. Then, after holding the first stage tempering holding time: 50 seconds or more, the second stage tempering heating temperature T: further heating to 400 ° C. or more. The second tempering holding time t (second) was held under the condition of 3.2 × 10 ⁻⁴ <P = exp [−9649 / (T + 273)] × t <1.2 × 10 ⁻³ . Then cool down. In the case where the temperature T is changed during the second stage holding, the following equation 5 may be used.

By holding at around 350 ° C., which is the temperature range where the precipitation of cementite from martensite is the fastest, after depositing cementite particles uniformly in the martensite structure, heating and holding at a higher temperature range, The cementite particles can be grown to an appropriate size.

[First-stage tempering heating temperature: Heating from 325 to 375 ° C, between 100 and 325 ° C at an average heating rate of 5 ° C / second or more]
When the first-stage tempering heating temperature is less than 325 ° C. or more than 375 ° C., or when the average heating rate between 100 to 325 ° C. is less than 5 ° C./second, precipitation of cementite particles in martensite It occurs unevenly. Therefore, the ratio of coarse cementite particles increases due to subsequent growth during heating and holding in the second stage, and stretch flangeability cannot be obtained.

[Second-stage tempering heating temperature T: Heated to 400 ° C. or higher, second-stage tempering holding time t (second) is 3.2 × 10 ⁻⁴ <P = exp [−9649 / (T + 273)] × held under conditions of t <1.2 × 10 ⁻³ ]
Here, P = exp [−9649 / (T + 273)] × t is based on the grain growth model of precipitates described in Koichi Sugimoto et al., Material Histology, Asakura Shoten Publishing, p106 formula (4.18). It is a parameter that defines the size of cementite particles as precipitates, with variables set and simplified.

When the second stage tempering heating temperature T is less than 400 ° C., the holding time t required for growing the cementite particles to a sufficient size becomes too long.

In P = exp [−9649 / (T + 273)] × t ≦ 3.2 × 10 ⁻⁴ , the cementite particles do not grow sufficiently, and the number of appropriately fine cementite particles cannot be secured, so that the elongation cannot be secured. .

When P = exp [−9649 / (T + 273)] × t ≧ 1.2 × 10 ⁻³ , the cementite particles are coarsened, and the number of cementite particles of 0.1 μm or more becomes too large, so that stretch flangeability can be secured. Disappear.

[Tempering conditions for steel sheet satisfying the above recommended structure rule (b) in addition to the required structure rule]
As a tempering condition for a steel sheet that satisfies the above recommended structure rule (b) in addition to the essential structure rule, both [V] and Nb, Ti, and Zr are included. It is recommended to satisfy the following conditions while satisfying the tempering condition of the steel sheet to be performed].

That is, heating is performed from the temperature after the annealing cooling to the tempering heating temperature: 550 to 650 ° C., the tempering holding time is maintained for 3 to 30 seconds in the same temperature range, and then the cooling is performed.

During tempering, the higher the heating temperature and the longer the holding time, the lower the dislocation density. Further, the density of fine precipitates of 10 nm or less increases as the holding time increases.

However, the temperature dependence and time dependence of the rate of decrease of the dislocation density and the rate of increase of the density of fine precipitates are greatly different. The rate of decrease in dislocation density is more time-dependent, whereas the rate of increase in the density of fine precipitates is more temperature-dependent.

For this reason, in order to make the values of the two parameters, the dislocation density and the density of fine precipitates both within the appropriate range, in order to make the dislocation density higher than the conventional steel, it is shorter than the tempering holding time for the conventional steel. It is effective to hold for the holding time. In order to increase the existence density of fine precipitates to 20 pieces / μm ² or more even in such a short holding time, it is effective to perform tempering at a heating temperature higher than the tempering heating temperature for conventional steel. is there.

However, when tempering is performed at a temperature exceeding 650 ° C., the dislocation density rapidly decreases and becomes insufficient even in a short time treatment. Further, if the holding time is longer than 30 seconds, the dislocation density decreases too much and becomes insufficient, and the yield strength cannot be obtained. On the other hand, when tempering is performed at a temperature lower than 550 ° C. or a holding time of less than 3 seconds, fine precipitates do not increase sufficiently, resulting in insufficient hydrogen embrittlement resistance.

1) Example 1 (when V is contained)
Steels having the components shown in Table 1 below were melted to produce 120 mm thick ingots.
After this was hot rolled to a thickness of 25 mm, it was again hot rolled to a thickness of 3 mm. After pickling, this was cold-rolled to a thickness of 1.2 mm to obtain a test material, which was heat-treated under the conditions shown in Tables 2-4.

For each steel plate after the heat treatment, the structure was quantified by the measurement method described above. Specifically, the martensite area ratio, the size of precipitates, and the number (existence density) of precipitates were measured for all steel sheets heat-treated under the heat treatment conditions shown in Tables 2 to 4. And heat processing No. shown in Table 3 is shown. Only the steel sheets heat-treated under the conditions of a-1 to e-1 were measured for the size of cementite particles and the number (existence density) of the cementite particles. In addition, the heat treatment No. 1 shown in Table 4 was used. The dislocation density was measured only for the steel sheets heat-treated under the conditions a-2 to e-2.

Moreover, in order to evaluate the mechanical properties of each of the above steel plates, tensile strength TS, elongation El, and stretch flangeability λ were measured. Furthermore, in order to evaluate the hydrogen embrittlement resistance, a hydrogen embrittlement risk index was measured.

The tensile strength TS and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction.

In addition, for stretch flangeability λ, a hole expansion rate was measured in accordance with the iron standard JFST1001, and the hole expansion rate was measured.

Regarding the hydrogen embrittlement risk index, a low strain rate technique (SSRT) with a strain rate of 1 × 10 ⁻⁴ / s was performed using a flat plate test piece having a thickness of 1.2 mm. The hydrogen embrittlement risk index was calculated using the following definition formula.

Hydrogen embrittlement risk index (%) = 100 × (1−E ₁ / E ₀ )

Here, E ₀ indicates the elongation at break of a test piece substantially free of hydrogen in steel, and E ₁ indicates a steel material (test piece) electrochemically charged with hydrogen in sulfuric acid. Elongation at break is shown. The hydrogen charge is performed by immersing a steel material (test piece) in a mixed solution of H ₂ SO ₄ (0.5 mol / L) and KSCN (0.01 mol / L) at room temperature and a constant current (100 A / m ^2). ).

If the hydrogen embrittlement risk index exceeds 15%, there is a risk of causing hydrogen embrittlement during use. Therefore, in the present invention, a steel material having a hydrogen embrittlement risk index of 15% or less is excellent in hydrogen embrittlement resistance. It was evaluated.

Measured results of the mechanical properties and hydrogen embrittlement resistance are shown in Tables 5-7.

First, as shown in Table 5, invention steels (steel Nos. 2 to 4, 6, 7, 10, 11, 14, 16 to 16) satisfying the essential constituent requirements of the present invention (the above-mentioned component composition rules and the above-mentioned essential structure rules). 21 to 25, 30) all satisfy a tensile strength TS of 980 MPa or more, a stretch flangeability (hole expansion ratio) λ of 70% or more, and a hydrogen embrittlement risk index of 15% or less. Therefore, it was found that a high-strength cold-rolled steel sheet having both workability and hydrogen embrittlement resistance can be obtained.

On the other hand, comparative steels (steel Nos. 1, 5, 8, 9, 12, 13, 17, 20, 26 to 29, 31, 32) lacking at least one of the essential constituent elements of the present invention are: Any one of the mechanical characteristics and the hydrogen embrittlement resistance is inferior. Steel No. Nos. 18 and 19 satisfy both properties, but the component compositions [P] or [S] are out of the specified range of the present invention, and are therefore comparative steels.

For example, steel No. No. 1 lacks the number (existence density) of fine precipitates having a circle-equivalent diameter of 1 to 10 nm, and thus has excellent tensile strength and stretch flangeability, but is inferior in hydrogenation embrittlement resistance.

Steel No. In No. 5, since the V content is too high, the number of coarse precipitates having an equivalent circle diameter of 20 nm or more becomes excessive. For this reason, although tensile strength and hydrogenation embrittlement resistance are excellent, stretch flangeability is inferior.

Steel No. No. 8 has an insufficient martensite area ratio because the Si content is too high. For this reason, the hydrogenation embrittlement resistance is excellent, but the tensile strength and stretch flangeability are inferior.

Steel No. No. 9 has an insufficient martensite area ratio because the C content is too low. For this reason, the hydrogenation embrittlement resistance is excellent, but the tensile strength and stretch flangeability are inferior.

Steel No. In No. 12, the number of coarse precipitates of 20 nm or more becomes excessive because the C content is too high. For this reason, although tensile strength and hydrogenation embrittlement resistance are excellent, stretch flangeability is inferior.

Steel No. No. 13 has an insufficient martensite area ratio because the Mn content is too low. For this reason, the hydrogenation embrittlement resistance is excellent, but the tensile strength and stretch flangeability are inferior.

Steel No. In No. 17, residual austenite remains because the Mn content is too high. For this reason, although tensile strength is excellent, stretch flangeability and hydrogenation embrittlement resistance are inferior.

Steel No. No. 20 is excellent in hydrogenation embrittlement resistance because the Al content is too high, but is inferior in tensile strength and stretch flangeability.

Steel No. Nos. 26 to 29, 31, and 32 do not satisfy at least one of the requirements for defining the structure of the present invention because the annealing condition or the tempering condition is out of the recommended range, and any of the characteristics is inferior.

Next, as shown in Table 6, all of the recommended steels (steel Nos. 34, 40, 42, 44, and 46) that satisfy the above-mentioned recommended structure provision (a) in addition to the essential constituent requirements of the present invention. The tensile strength TS is 980 MPa or more, the elongation El is 10% or more, the stretch flangeability (hole expansion ratio) λ is 100% or more, and the hydrogen embrittlement risk index is 15% or less. Therefore, it was found that a high-strength cold-rolled steel sheet that is further excellent in workability than the above-described invention steel can be obtained.

Moreover, as shown in Table 7, in addition to the above-mentioned essential constituent requirements of the present invention, any of the recommended steels (steel Nos. 48, 53, 55, 57, 58) that also satisfy the above recommended organization rules (b) The yield strength is 900 MPa or more, the tensile strength TS is 980 MPa or more, the elongation El is 10% or more, the stretch flangeability (hole expansion ratio) λ is 90% or more, and the hydrogen embrittlement risk index is 15% or less. To do. Therefore, it was found that a high-strength cold-rolled steel sheet having excellent workability and superior collision safety as compared with the above-described invention steel can be obtained.

2) Example 2 (when Nb, Ti, Zr is contained)
Steels having the components shown in Table 8 were melted to produce 120 mm thick ingots. After this was hot rolled to a thickness of 25 mm, it was again hot rolled to a thickness of 3 mm. After pickling this, it was cold-rolled to a thickness of 1.2 mm to obtain a test material, which was heat-treated under the conditions shown in Tables 9-11.

For each steel plate after the heat treatment, the structure was quantified by the measurement method described above. Specifically, for all steel sheets heat-treated under the respective heat treatment conditions shown in Tables 9 to 11, the martensite area ratio and hardness, the size and number of precipitates (existence density), and the average effective ferrite The particle size was measured. And the heat processing No. shown in Table 10 is shown. Only for the steel plates heat-treated under the conditions of a′-1 to e′-1, the size of cementite particles and the number (existence density) of the cementite particles were measured. The dislocation density was measured only for the steel sheets heat-treated under the conditions a′-2 to f′-2.

Moreover, in order to evaluate the mechanical properties of each of the above steel plates, tensile strength TS, yield strength YP, elongation El, and stretch flangeability λ were measured. Furthermore, the hydrogen embrittlement risk index was measured for each steel sheet in order to evaluate the hydrogen embrittlement resistance.

The tensile strength TS, yield strength YP, and elongation El were measured in accordance with JIS Z 2241 by preparing a No. 5 test piece described in JIS Z 2201 with the long axis perpendicular to the rolling direction.

In addition, for stretch flangeability λ, a hole expansion rate was measured in accordance with the iron standard JFST1001, and the hole expansion rate was measured.

Regarding the hydrogen embrittlement risk index, a low strain rate technique (SSRT) with a strain rate of 1 × 10 ⁻⁴ / s was performed using a flat plate test piece having a thickness of 1.2 mm. The hydrogen embrittlement risk index was calculated using the following definition formula.

Hydrogen embrittlement risk index (%) = 100 × (1−E ₁ / E ₀ )

Here, E ₀ indicates the elongation at break of a test piece substantially free of hydrogen in steel, and E ₁ indicates a steel material (test piece) electrochemically charged with hydrogen in sulfuric acid. Elongation at break is shown. The hydrogen charge is performed by immersing a steel material (test piece) in a mixed solution of H ₂ SO ₄ (0.5 mol / L) and KSCN (0.01 mol / L) at room temperature and a constant current (100 A / m ^2). ).

If the hydrogen embrittlement risk index exceeds 15%, there is a risk of causing hydrogen embrittlement during use. Therefore, in the present invention, a steel sheet having a hydrogen embrittlement risk index of 15% or less has excellent hydrogen embrittlement resistance. evaluated.

The measurement results of the mechanical properties and hydrogen embrittlement resistance are shown in Tables 12-14.

First, as shown in Table 12, invention steels (steel Nos. 61 to 63, 65, 66, 69, 70, 73 to 75) that satisfy the essential constituent requirements of the present invention (the above-mentioned component composition rules and the above-mentioned essential structure rules). , 80-84, 89, 92, 93, 119, 120), the tensile strength TS is 980 MPa or more, the stretch flangeability (hole expansion ratio) λ is 70% or more, and the hydrogen embrittlement risk index Is 15% or less, and has both workability and hydrogen embrittlement resistance.

In contrast, comparative steel lacking at least one of the essential constituent elements of the present invention (steel Nos. 60, 64, 67, 68, 71, 72, 76, 79, 85 to 88, 90, 91, 121) Is inferior in any one of the above mechanical properties and hydrogen embrittlement resistance (note that steel Nos. 77 and 78 satisfy both properties, but the component composition [P] or [S] However, since it deviates from the specified range of the present invention, it was set as a comparative steel.)

For example, steel No. No. 60 does not contain Nb, Ti, or Zr, and there is no fine precipitate having an equivalent circle diameter of 1 to 10 nm. Therefore, the tensile strength and stretch flangeability are excellent, but the resistance to hydrogenation embrittlement is high. Inferior.

Steel No. 64, 121, because the content of Nb, Ti, or Zr is too high, the number of coarse precipitates having an equivalent circle diameter of 20 nm or more becomes excessive, so the hydrogenation embrittlement resistance is excellent, but the tensile strength and Stretch flangeability is inferior.

Steel No. No. 67 is excellent in hydrogenation embrittlement resistance due to insufficient martensite area ratio due to excessively high Si content, but inferior in tensile strength and stretch flangeability.

Steel No. No. 68 has a martensite area ratio that is insufficient due to the C content being too low, so that the stretch flangeability and hydrogenation embrittlement resistance are excellent, but the tensile strength is inferior.

Steel No. No. 71 has an excessively high C content, so that the number of coarse precipitates having a size of 20 nm or more becomes excessive. Therefore, although tensile strength and hydrogenation embrittlement resistance are excellent, stretch flangeability is inferior.

Steel No. No. 72 has an excessively low Mn content, resulting in an insufficient martensite area ratio, so that the hydrogenation embrittlement resistance is excellent, but the tensile strength and stretch flangeability are inferior.

Steel No. In No. 76, since the retained austenite remains because the Mn content is too high, tensile strength is excellent, but stretch flangeability and hydrogenation embrittlement resistance are inferior.

Steel No. No. 79 has a martensite area ratio that is too low due to the Al content being too high, so that the hydrogenation embrittlement resistance is excellent, but the tensile strength and stretch flangeability are inferior.

Steel No. Nos. 85 to 88, 90 and 91 do not satisfy at least one of the requirements defining the structure of the present invention because the annealing condition or tempering condition is out of the recommended range, and any of the characteristics is inferior.

Next, as shown in Table 13, the recommended steel (steel No. 93 ′, 99, 101, 103, 105, 123) satisfying the above recommended structure rule (a) in addition to the essential constituent requirements of the present invention is as follows. In any case, the tensile strength TS is 980 MPa or more, the elongation El is 10% or more, the stretch flangeability (hole expansion ratio) λ is 100% or more, and the hydrogen embrittlement risk index is 15% or less. . Thereby, it turned out that the high intensity | strength cold-rolled steel plate which was further excellent in workability than the said invention steel is obtained.

Moreover, as shown in Table 14, in addition to the above-mentioned essential constituent requirements of the present invention, all of the recommended steels (steel Nos. 107, 112, 114, 116, 125) that also satisfy the above recommended organization rules (b) The yield strength is 900 MPa or more, the tensile strength TS is 980 MPa or more, the elongation El is 10% or more, the stretch flangeability (hole expansion ratio) λ is 90% or more, and the hydrogen embrittlement risk index is 15% or less. is doing. Thus, it was found that a high-strength cold-rolled steel sheet that is further excellent in workability than the above-described invention steel and excellent in collision safety can be obtained.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. is there. This application is based on a Japanese patent application (Japanese Patent Application No. 2009-079775) filed on Mar. 27, 2009, the contents of which are incorporated herein by reference.

Claims (8)

C: 0.03 to 0.30% by mass,
Si: 3.0% by mass or less (including 0% by mass),
Mn: more than 0.1% by mass, 2.8% by mass or less,
P: 0.1% by mass or less,
S: 0.005 mass% or less,
N: 0.01 mass% or less, and Al: 0.01 to 0.50 mass%,
V: 0.001 to 1.00% by mass or 0.01% by mass or more of Nb, Ti and Zr in combination so that the following formula 1 is satisfied, with the balance being iron and inevitable Cold-rolled steel plate made of mechanical impurities,
The tempered martensite contains 50% or more (including 100%) in area ratio, and the balance has a structure made of ferrite,
There are 20 or more precipitates having an equivalent circle diameter of 1 to 10 nm per 1 μm ^{2 of the} tempered martensite,
A cold-rolled steel sheet comprising 10 or less precipitates having a circle equivalent diameter of 20 nm or more, containing V or containing one or more of Nb, Ti, and Zr, per 1 μm ^{2 of the} tempered martensite.
[% C] − [% Nb] /92.9×12 − [% Ti] /47.9×12 − [% Zr] /91.2×12> 0.03 (Formula 1 )
(However, [% C], [% Nb], [% Ti], and [% Zr] mean the contents (mass%) of C, Nb, Ti, and Zr, respectively) One or more of Nb, Ti, and Zr are combined so as to satisfy the formula 1, and include 0.01% by mass or more,
The cold-rolled steel sheet according to claim 1, wherein the average grain size of ferrite surrounded by large-angle grain boundaries having a crystal orientation difference of 15 ° or more is 5 µm or less. V: including 0.001 to 0.20 mass%,
The cold-rolled steel sheet according to claim ² , wherein the number of precipitates having an equivalent circle diameter of 20 nm or more including V is 10 or less per 1 μm ^{2 of the} tempered martensite. Cr: 0.01 to 1.0% by mass,
Mo: 0.01 to 1.0% by mass,
The cold rolled steel sheet according to claim 1, comprising at least one of Cu: 0.05 to 1.0 mass% and Ni: 0.05 to 1.0 mass%. B: The cold-rolled steel sheet according to claim 1, containing 0.0001 to 0.0050 mass%. Ca: 0.0005 to 0.01% by mass,
The cold-rolled steel sheet according to claim 1, comprising at least one of Mg: 0.0005 to 0.01 mass% and REM: 0.0004 to 0.01 mass%. There are 10 or more cementite particles having an equivalent circle diameter of 0.02 μm or more and less than 0.1 μm per 1 μm ^{2 of the} tempered martensite,
The cold-rolled steel sheet according to claim 1, wherein the number of cementite particles having an equivalent circle diameter of 0.1 µm or more is 3 or less per 1 µm ^{2 of the} tempered martensite. The dislocation density in the whole structure is 1 × 10 ¹⁵ to 1 × 10 ¹⁶ m ⁻² ,
The cold-rolled steel sheet according to claim 1, wherein the Si equivalent defined by the following formula 2 satisfies the following formula 3.
[Si equivalent] = [% Si] +0.36 [% Mn] +7.56 [% P] +0.15 [% Mo] +0.36 [% Cr] +0.43 [% Cu] (Formula 2)
[Si equivalent] ≧ 4.0−5.3 × 10 ⁻⁸ √ [dislocation density] (Equation 3)