WO2011039885A1 - Cold-rolled steel sheet - Google Patents

Abstract

Disclosed is a cold-rolled steel sheet having improved processability and improved hydrogen embrittlement resistance. The cold-rolled steel sheet comprises not less than 0.03% by mass and less than 0.30% by mass of C, 2.0% by mass or less (including 0% by mass) of Si, 0.1 to 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 1.00% by mass of Al. In the cold-rolled steel sheet, the area ratio of tempered bainite is 50% or more (including 100%), the total area ratio of tempered martensite and remaining austenite is less than 3% (including 0%), and a structure comprising ferrite makes up the remainder. In the cold-rolled steel sheet, the distribution of precipitates in the tempered bainite and/or the distribution of cementite particles in the tempered bainite are controlled.

Description

Cold rolled steel sheet

The present invention relates to a cold-rolled steel sheet (cold-rolled thin steel sheet) suitable for automobile parts and the like, and in particular, a high-strength cold-rolled steel sheet having excellent workability, or excellent hydrogen embrittlement resistance in addition to workability. It relates to a high-strength cold-rolled steel sheet.

For example, cold-rolled steel sheets used for automobile frame parts and the like are required to have high strength for the purpose of achieving both collision safety and fuel efficiency reduction by reducing the weight of the vehicle body, and excellent for processing into complex frame parts Also, moldability is required.
For this reason, it has been desired to provide a high-strength steel sheet having both increased elongation (total elongation; El) and stretch flangeability (hole expansion ratio; λ). There is a demand for a steel sheet having a hole expansion ratio of 90% or more at 10% or more.

In response to such demands, a number of high-strength steel sheets with improved balance between elongation and stretch flangeability have been proposed based on various structural control concepts. However, a steel sheet that has both elongation and stretch flangeability so as to satisfy the above-mentioned required level has not yet been completed.
For example, Patent Document 1 discloses a high-tensile cold-rolled steel sheet containing 1.6 to 2.5% by mass in total of at least one of Mn, Cr, and Mo and substantially comprising a martensite single-phase structure. Has been. In this steel sheet, the hole expansion ratio (stretch flangeability) is 90% or more, but the elongation does not reach 10%.
Further, Patent Document 2 discloses a high-tensile steel plate having a two-phase structure of ferrite having an area ratio of 65 to 85% and the balance being tempered martensite. Patent Document 3 discloses a high-tensile steel plate having a two-phase structure in which the average crystal grain sizes of ferrite and martensite are both 2 μm or less and the volume ratio of martensite is 20% or more and less than 60%. . Although both the high-tensile steel sheets disclosed in Patent Document 2 and Patent Document 3 ensure an elongation of 10% or more, the hole expansion ratio (stretch flangeability) does not reach 90%.
In Patent Document 4, 1.0 to 2.0% by mass of Si is added, and the space factor is 5% or more of retained austenite and 60% or more of bainitic ferrite. A high-strength cold-rolled steel sheet having a phase structure is disclosed. Although this steel sheet has high elongation and relatively high stretch flangeability, it is mainly used to improve elongation. Although elongation of 10% or more is obtained, stretch flangeability is up to about 60%. It has only been obtained.
Further, in Patent Document 5, 1.0 to 2.0% by mass of Si is added, and the Baini is composed of a structure including a space factor of 5 to 20% of retained austenite and 50% or more of bainitic ferrite. A high-strength cold-rolled steel sheet composed of a multiphase structure mainly composed of tick ferrite is disclosed. Although this steel sheet exhibits very excellent elongation of 20% or more, stretch flangeability is only obtained up to about 80%.

On the other hand, in high-strength steels often used for applications such as bolts, PC steel wires and line pipes, when the tensile strength is 780 MPa or more, particularly 980 MPa or more, hydrogen embrittlement (pickling brittleness, It has been widely known that plating brittleness, delayed fracture, etc.) occur. Delayed fracture is a phenomenon in which hydrogen generated from a corrosive environment or atmosphere in high-strength steel diffuses into defects such as dislocations, vacancies, and grain boundaries, embrittles the material, and breaks when stress is applied. That is, it causes 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 fracture by precipitating alloy carbides and utilizing them as hydrogen trap sites to shift the delayed fracture mode from grain boundaries to intragranular fracture. However, these findings are for application to medium carbon steel, and cannot be used as it is for a thin steel sheet having a low carbon content that requires weldability and workability.

Therefore, the present applicants developed an ultra-high strength thin steel sheet having excellent hydrogen embrittlement resistance, satisfying the carbon content C: more than 0.25 to 0.60% and the balance being iron and inevitable impurities ( Patent Document 6). In this high-strength thin steel sheet, the metal structure after tensile processing with a processing rate of 3% satisfies the area ratio of the entire structure, and the residual austenite structure: 1% or more, bainitic ferrite and martensite: 80% or more in total In addition, the average axial ratio (major axis / minor axis) of the residual austenite crystal grains satisfies 5 or more.

The thin steel sheet described in Patent Document 6 exhibits excellent strength, elongation, and hydrogen embrittlement resistance. However, even in the thin steel sheet described in Patent Document 6, retained austenite becomes a starting point of fracture and becomes a factor of reducing stretch flangeability. As described above, the stretch flangeability, which is becoming increasingly important in recent years, is assured of a desired level (at least TS × λ: 60000 (MPa ·%) [unit of Ts: MPa], preferably λ: 100% or more). It was difficult to achieve.

Japanese Published Patent Publication: 2002-161336 Japan Published Patent Publication: 2004-256872 Japanese Published Patent Publication: 2004-232022 Japan Published Patent Publication: 2005-240178 Japan Published Patent Publication: 2005-330584 Japan Published Patent Publication: 2006-207019

In view of the above situation, an object of the present invention is to provide a cold-rolled steel sheet (cold-rolled thin steel sheet) with improved workability (specifically, stretch flangeability).
More specifically, one object of the present invention is to provide a high-strength cold-rolled steel sheet excellent in workability, in which both elongation and stretch flangeability are enhanced. Another object of the present invention is to provide a high-strength cold-rolled steel sheet with improved stretch flangeability while ensuring excellent hydrogen embrittlement resistance.

The cold-rolled steel sheet of the present invention that has achieved the above-mentioned object is C: 0.03 mass% or more, less than 0.30 mass%, Si: 2.0 mass% or less (including 0 mass%), Mn: 0.1 to 2.8% by mass, P: 0.1% by mass or less, S: 0.005% by mass or less, N: 0.01% by mass or less, and Al: 0.01 to 1.00% by mass, In a cold-rolled steel sheet containing tempered steel, the area ratio of tempered bainite is 50% or more (including 100%), the total area ratio of tempered martensite and retained austenite is less than 3% (including 0%), and the balance is ferrite. And a distribution state of precipitates in the tempered bainite and / or a distribution state of cementite particles in the tempered bainite are controlled.

By adopting such a configuration, it is possible to provide a high-strength cold-rolled steel sheet with excellent workability or a high-strength cold-rolled steel sheet with improved workability while ensuring excellent hydrogen embrittlement resistance. It becomes possible.

More specifically, in order to provide a high-strength cold-rolled steel sheet excellent in workability, in which both elongation and stretch flangeability are improved, the above-described cold-rolled steel sheet contains Si: 1.0% by mass or less (0 Mn: 0.5 to 2.4% by mass, and Al: 0.01% by mass or more and less than 0.10% by mass, and the area ratio of the tempered bainite is 70% or more (100%). It is recommended that the number of cementite particles having an equivalent circle diameter of 0.1 μm or more be 3 or less per 1 μm ^{2 of the} tempered bainite.

In addition, in order to provide a high-strength cold-rolled steel sheet with improved stretch embrittlement characteristics while ensuring excellent hydrogen embrittlement resistance, in the above-described cold-rolled steel sheet, V: 0.001 to 1.00 mass 20% or more of precipitates having an equivalent circle diameter of 1 to 10 nm per 1 μm ^{2 of the} tempered bainite, and 10 precipitates or less of equivalent circle diameter of 20 nm or more containing V per 1 μm ^{2 of the} tempered bainite. It is recommended that there be.

In the invention in which the precipitates are controlled in this way, it is further recommended that 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 bainite.

The above-described cold-rolled steel sheet of the present invention preferably contains Cr: 0.1 to 3.0% by mass.

The cold-rolled steel sheet of the present invention described above contains B: 0.0002 to 0.0050 mass%, and Nb and / or Ti is ((N) −0.003) / 12 ≦ [Nb] / It is preferable to include so as to satisfy the relationship of 96+ [Ti] / 48 ≦ ([N] +0.01) / 12 (however, [] means the content (% by mass) of each element).

The cold-rolled steel sheet of the present invention described above has Mo: 0.01 to 1.0% by mass, Cu: 0.05 to 1.0% by mass, and Ni: 0.05 to 1.0% by mass. It is also preferable that it contains 1 or more types.

The cold-rolled steel sheet of the present invention described above has 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. It is also preferable that it contains 1 or more types.

According to the present invention, the ratio of tempered martensite and retained austenite, which is the main cause of fracture in a tempered bainite single phase structure or a multiphase structure mainly composed of ferrite and tempered bainite, is reduced as much as possible. At the same time, by appropriately controlling the number of coarse cementite particles precipitated in tempered bainite and / or appropriately controlling the distribution of precipitates precipitated in tempered bainite, It may be simply referred to as “strength”), and the stretch flangeability can be further improved over conventional steel while ensuring hydrogen embrittlement resistance. Accordingly, the present invention can provide a high-strength cold-rolled steel sheet excellent in workability or a high-strength cold-rolled steel sheet excellent in workability and hydrogen embrittlement resistance.

It is a figure which shows the relationship between the number of cementite particles with an equivalent circle diameter of 0.1 μm or more and stretch flangeability.

The present inventors have a high structure composed mainly of tempered bainite (hereinafter simply referred to as “bainite”), which has a higher deformability than tempered martensite (hereinafter sometimes simply referred to as “martensite”). We focused on the strength steel plate. And as a result of intensive studies, the present inventors have found that by controlling the distribution of precipitates and cementite particles in bainite, it is possible to improve stretch flangeability and further improve hydrogen embrittlement resistance, The present invention has been completed.

Specifically, the present invention includes C: 0.03% by mass or more and less than 0.30% by mass, Si: 2.0% by mass or less (including 0% by mass), Mn: 0.1 to 2.8 In a cold-rolled steel sheet containing, by mass, P: 0.1% by mass or less, S: 0.005% by mass or less, N: 0.01% by mass or less, and Al: 0.01 to 1.00% by mass, The area ratio of tempered bainite is 50% or more (including 100%), the total area ratio of tempered martensite and residual austenite is less than 3% (including 0%) and the balance has a structure made of ferrite, A cold-rolled steel sheet characterized by controlling the distribution of precipitates in the tempered bainite and / or the distribution of cementite particles in the tempered bainite (hereinafter, this invention is referred to as a basic invention). is there).

The features of the basic invention will be described below.
[Organization of basic invention]
As described above, the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite. The present invention particularly reduces the tempered martensite structure and residual austenite structure as much as possible, and controls the distribution of precipitates in the tempered bainite and / or the distribution of cementite particles present in the tempered bainite. It has the characteristics.

<Tempered bainite: 50% or more in area ratio (including 100%), remaining ferrite>
Bainite is a homogeneous structure having high strength and excellent plasticity. By using a bainite-based structure having such properties, stretch flangeability can be improved while securing tensile strength and elongation.
When the ratio of tempered bainite is reduced, the structure becomes non-uniform and stretch flangeability cannot be ensured, so that the area ratio of tempered bainite is 50% or more, preferably 70% or more, in order to effectively exhibit the above-described action. Preferably, it is 90% or more (including 100%).
In addition, since ferrite has high ductility but low strength, increasing the proportion of ferrite improves the elongation but decreases the strength. In addition, when the proportion of ferrite increases, strain concentrates on the ferrite during deformation, strain at the interface between ferrite and bainite increases, and cracking at the interface is promoted, so that stretch flangeability is improved. to degrade. Therefore, from the viewpoint of ductility, the balance of the steel sheet of the present invention is ferrite, but the ferrite area ratio is smaller than that of tempered bainite.

<Total area ratio of tempered martensite and retained austenite: less than 3%>
When steel containing a large amount of alloying elements such as the high-strength cold-rolled steel sheet according to the present invention is bainite transformed, the transformation speed becomes slow, and not only bainite but also martensite is easily formed. Since martensite is hard, strain concentrates on the interface between ferrite and martensite and breakage easily occurs, and stretch flangeability deteriorates. Residual austenite also transforms into hard martensite when strain is applied and becomes the starting point of fracture, so that stretch flangeability is also deteriorated. Therefore, stretch flangeability can be improved by reducing the ratio of these structures as much as possible. Therefore, it is recommended that the total area ratio of tempered martensite and retained austenite be limited to less than 3%, further 2% or less, particularly 1% or less.
By controlling the distribution state of precipitates in the tempered bainite and / or the distribution state of cementite particles present in the tempered bainite on the basis of the above-described structure morphology, elongation and elongation flange are achieved. It is possible to provide a high-strength cold-rolled steel sheet with improved properties, or a high-strength cold-rolled steel sheet with improved stretch flangeability while ensuring excellent hydrogen embrittlement resistance.

Hereinafter, each measuring method of each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite will be described.
Regarding the area ratio of ferrite, first, each test steel sheet was mirror-polished and corroded with a 3% nital solution to reveal a metal structure. Thereafter, five fields of view were observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and the equiaxed region not containing cementite was determined to be ferrite by image analysis, and the area ratio of ferrite was determined from the area ratio of the ferrite region to the entire structure. Calculated.

Regarding the total area ratio of tempered martensite and retained austenite, first, each test steel sheet was mirror-polished and corroded with a repeller corrosive solution to reveal a metal structure. Thereafter, five fields of view were observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and the area that appeared white from the image contrast by image analysis was made tempered martensite and retained austenite. The total area ratio of sites and retained austenite was calculated.

Regarding the area ratio of tempered bainite, by subtracting the area ratio of ferrite calculated above and the total area ratio of tempered martensite and retained austenite from 100%, with the area other than ferrite, martensite and retained austenite as bainite. Asked.

Next, the basic component composition constituting the steel sheet of the present invention will be described.
[Component composition of the basic invention]
(C: 0.03 mass% or more, less than 0.30 mass%)
C is an important element that affects the area ratio of bainite and affects strength and stretch flangeability. If the C content is less than 0.03% by mass, the bainite area ratio is insufficient, so that the strength cannot be ensured. On the other hand, if the C content exceeds 0.30% by mass, the hardenability becomes too high, the transformation to bainite is too suppressed, the ratio of martensite and austenite increases, and stretch flangeability cannot be ensured. The range of the C content is preferably 0.05 to 0.25% by mass, more preferably 0.07 to 0.20% by mass.

(Si: 2.0 mass% or less (including 0 mass%))
Si is a useful element that can increase strength without deteriorating elongation as a solid solution strengthening element. If the Si content exceeds 2.0% by mass, the formation of austenite during heating is inhibited, so that the area ratio of bainite cannot be ensured and stretch flangeability cannot be ensured. In this case, the formation of cementite is inhibited, and retained austenite (residual γ) and martensite are likely to remain. The range of the Si content is preferably 1.8% by mass or less, more preferably 1.5% by mass or less (including 0% by mass).

(Mn: 0.1 to 2.8% by mass)
Mn is a useful element having an effect of enhancing the strength and stretch flangeability by increasing the hardenability and securing the bainite area ratio during rapid cooling after heating during annealing. If the Mn content is less than 0.1% by mass, the bainite area ratio is insufficient, so that the strength and stretch flangeability cannot be ensured. On the other hand, if the Mn content exceeds 2.8% by mass, bainite transformation is suppressed, martensite and austenite remain, 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, but segregates at the prior austenite grain boundaries and causes the grain boundaries to become brittle, thereby deteriorating stretch flangeability. Therefore, the P content is 0.1% by mass or less. 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, forms MnS inclusions, and becomes a starting point of a crack when the hole is expanded, thereby reducing stretch flangeability. Therefore, the P content is 0.005% by mass or less, 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 N content is preferably as low as 0.01% by mass or less.

(Al: 0.01 to 1.00% by mass)
Al combines with N to form AlN and reduces 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 1.00% by mass, the formation of austenite during heating is inhibited, so that the area ratio of bainite cannot be ensured and stretch flangeability cannot be ensured.

Next, the high-strength cold-rolled steel sheet excellent in workability, in which both elongation and stretch flangeability are improved, will be described more specifically.
The inventors of the present invention ensured elongation by using a tempered bainite structure having a higher deformability than tempered martensite as a main structure and introducing a ferrite structure into the bainite structure as necessary. On the other hand, the present inventors reduced the ratio of tempered martensite and retained austenite (hereinafter, sometimes referred to as “residual γ”), which is the main cause of destruction, and other main causes of destruction. By appropriately controlling the size and the number of the cementite particles precipitated in the tempered bainite, the hole expansion ratio is 90% at a total elongation of 10% or more with respect to the steel sheet having the desired level, that is, the tensile strength of 800 MPa class or more. It has been found that stretch flangeability of at least% can be secured. That is, a high-strength cold-rolled steel sheet having both improved elongation and flangeability and excellent workability is based on the basic invention described above, Si: 1.0 mass% or less (including 0 mass%), Mn : 0.5 to 2.4% by mass and Al: 0.01% by mass or more and less than 0.10% by mass, the area ratio of the tempered bainite is 70% or more (including 100%), This is a cold-rolled steel sheet having 3 or less cementite particles having an equivalent diameter of 0.1 μm or more per 1 μm ^{2 of the} tempered bainite (hereinafter, the present invention may be referred to as the first invention).

Hereinafter, the organization characterizing the first invention will be described.
[Organization of the first invention]
The steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite. In particular, while reducing the tempered martensite structure and the retained austenite structure as much as possible, the tempered bainite It is characterized in that the number of coarse cementite particles precipitated therein is controlled.

<Tempered bainite: 70% or more in area ratio (including 100%)>
Bainite is a homogeneous structure having high strength and excellent plasticity. The steel sheet of the present invention can improve stretch flangeability while securing tensile strength and elongation by using a bainite-based structure having such properties. When the ratio of tempered bainite decreases, the structure becomes non-uniform and stretch flangeability cannot be ensured. Therefore, in order to effectively exhibit the above action, the area ratio of tempered bainite is 70% or more, preferably 80% or more. Preferably, it is 90% or more (including 100%).

<Cementite particles with an equivalent circle diameter of 0.1 μm or more present in the tempered bainite: 3 or less per 1 μm ^{2 of} tempered bainite>
In the case of a steel having bainite as a main structure, such as conventional high-strength cold-rolled steel sheets (see Patent Documents 4 and 5), tempered martensite and retained austenite are usually easily formed, and these structures serve as starting points for fracture. Therefore, the dispersion state of the cementite particles precipitated in the bainite during tempering does not significantly affect the stretch flangeability. However, when tempered martensite and retained austenite are reduced, the starting point of fracture during deformation transitions to cementite particles precipitated in bainite, so that the presence state of cementite particles in bainite determines the stretch flangeability. . For this reason, in the case of steel that suppresses the formation of tempered martensite and retained austenite as much as possible, such as the high-strength cold-rolled steel sheet according to the present invention, stretch flangeability is achieved by controlling the precipitation state of cementite particles that are the starting point of fracture. Can be adjusted. Of the cementite particles, only large particles serve as the starting point of fracture, so that the number of coarse cementite particles can be reduced to improve the predetermined stretch flangeability.
In order to effectively exert the above action, the cementite particles having an equivalent circle diameter of 0.1 μm or more present in the tempered bainite are 3 or less, preferably 2.4 or less, more preferably 1 per 1 μm ^{2 of} the tempered bainite. 6 or less.

Hereinafter, each measuring method of the size of cementite particles and the number thereof will be described.
First, each specimen steel plate was mirror-polished and corroded with 3% nital to reveal the metal structure, and then a scanning electron microscope with a magnification of 10,000 times with respect to the field of view of 100 μm ² so that the region inside the bainite could be analyzed. (SEM) images were observed. Then, the white portion is marked and marked as cementite particles from the contrast of the image, and with the image analysis software, the equivalent circle diameter is calculated from the area of each marked cementite particle, and a predetermined size existing per unit area The number of cementite particles was determined.

Next, the differences from the basic invention among the component compositions constituting the first invention will be described.

(Si: 1.0% by mass or less (including 0% by mass))
Si, as a solid solution strengthening element, has the effect of increasing strength without deteriorating elongation and suppressing the coarsening of cementite particles present in bainite during tempering. Si also has the effect of improving stretch flangeability by preventing the formation of such coarse cementite particles. However, since Si promotes the formation of tempered martensite and retained austenite, it is necessary to limit the Si content to 1.0 mass% or less. The range of Si content is preferably 0.5% by mass or less, more preferably 0.2% by mass or less (including 0% by mass).

(Mn: 0.5 to 2.4% by mass)
Mn is a solid solution strengthening element similar to Si, increasing strength without deteriorating elongation, increasing hardenability and contributing to securing the bainite area ratio, and also having the effect of improving strength and stretch flangeability Element. If the Mn content is less than 0.5% by mass, the hardenability is insufficient, the bainite area ratio cannot be secured, and the strength cannot be secured. On the other hand, if the Mn content is more than 2.4% by mass, the hardenability becomes too high, and martensite is excessively formed, and the stretch flangeability deteriorates. The range of the Mn content is preferably 0.8 to 3.0% by mass, more preferably 1.0 to 2.2% by mass.

(Al: 0.01% by mass or more and less than 0.10% by mass)
Al combines with N to form AlN and reduces 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, when the Al content is 0.10% by mass or more, the formation of cementite is inhibited, and the total area ratio of tempered martensite and retained austenite becomes excessive, so that stretch flangeability deteriorates.

Next, the preferable manufacturing method for obtaining the cold rolled steel sheet of 1st invention is demonstrated below.
[Preferred production method of the first invention]
In order to produce the cold-rolled steel sheet as described above, first, steel having the above composition is melted and slab is formed by ingot forming or continuous casting and then hot-rolled. As the hot rolling conditions, the finish rolling finish temperature is set to Ar ₃ or higher, and after cooling appropriately, winding is performed in the range of 450 to 700 ° C. After hot rolling is completed, pickling is performed and then cold rolling is performed. The cold rolling rate is preferably about 30% or more.
And after the said cold rolling, although it anneals continuously, you may further temper as needed.

[Annealing conditions]
As annealing conditions, annealing heating temperature: Ac ₃ to 1000 ° C., annealing holding time: held for 3600 seconds or less, and then from annealing temperature to 400 to 550 ° C. (cooling end temperature) 10 to 200 ° C./second After rapidly cooling at a cooling rate of 10 ° C., the cooling end temperature is maintained for 10 to 600 seconds, and then cooled to room temperature.

<Annealing heating temperature: Ac ₃ to 1000 ° C.>
The reason why the annealing heating temperature is set to Ac ₃ or more and 1000 ° C. or less is to ensure an area ratio of bainite that is sufficiently transformed into austenite during annealing and is transformed from austenite during subsequent cooling.
It is less than the annealing heating temperature Ac _3, due to the lack of transformation of the austenite at the time of annealing heating, the amount of bainite to transformation product from austenite during subsequent cooling can not be ensured by the area rate of 70% or more reduced. On the other hand, if the annealing heating temperature exceeds 1000 ° C., the austenite structure becomes coarse and the bendability and toughness of the steel sheet deteriorate, and the annealing equipment deteriorates.
Further, if the annealing holding time exceeds 3600 seconds, productivity is extremely deteriorated, which is not preferable.

<After quenching to 400 to 550 ° C. (cooling end temperature) at a cooling rate of 10 to 200 ° C./second, hold for 10 to 600 seconds in the cooling end temperature range>
This is because the formation of ferrite and martensite structure from austenite during cooling is suppressed as much as possible, and a bainite structure is sufficiently obtained.
When rapidly cooled to a temperature lower than 400 ° C., martensite is excessively formed and bainite cannot be obtained sufficiently. On the other hand, if quenching is terminated at a temperature higher than 550 ° C. or the cooling rate is less than 10 ° C./second, ferrite is excessively formed, bainite is insufficient, and strength cannot be secured. On the other hand, if the cooling rate exceeds 200 ° C./second, control becomes difficult, which is not realistic. Moreover, if the holding time in the cooling end temperature range is less than 10 seconds, the transformation to bainite does not proceed sufficiently, and the elongation and stretch flangeability cannot be ensured. On the other hand, when the holding time exceeds 600 seconds, the cementite particles precipitated in the bainite structure become too coarse and the stretch flangeability deteriorates.
The heat history during holding in the cooling end temperature range (400 to 550 ° C.) is not particularly limited, and may be any heat history in an isothermal holding state, a cooling state, or a re-temperature raising state.

Next, the high-strength cold-rolled steel sheet that has enhanced stretch flangeability while ensuring excellent hydrogen embrittlement resistance will be described more specifically.
The inventors of the present invention are based on the basic invention, and by adding V as an alloy element thereto, V carbides and carbonitrides (hereinafter referred to as “V-containing precipitates”) that strongly act as hydrogen trap sites. Is introduced into the bainite at an appropriate size, while maintaining the hydrogen embrittlement resistance and improving the stretch flangeability, and the influence of various factors on the hydrogen embrittlement resistance and stretch flangeability. We conducted intensive studies such as investigating.
As a result of the above studies, the main structure is a tempered bainite structure having a higher deformability than tempered martensite, and if necessary, a certain degree of elongation is ensured by introducing a ferrite structure into the bainite structure. By reducing the proportion of certain tempered martensite and retained austenite (hereinafter, sometimes referred to as “residual γ”) as much as possible and miniaturizing the V-containing precipitates, elongation while securing hydrogen embrittlement resistance is ensured. It was found that the flangeability can be improved. That is, based on the basic invention described above, V: 0.001 to 1.00% by mass, and there are 20 or more precipitates with an equivalent circle diameter of 1 to 10 nm per 1 μm ^{2 of the} tempered bainite and V is included. This is a cold-rolled steel sheet in which the number of precipitates having an equivalent circle diameter of 20 nm or more is 10 or less per 1 μm ^{2 of the} tempered bainite (hereinafter, the present invention may be referred to as a second invention).

Hereinafter, the organization characterizing the second invention will be described first.
[Organization of the second invention]
As described above, the steel sheet of the present invention is based on a tempered bainite single phase or a multiphase structure mainly composed of ferrite and tempered bainite, and particularly reduces the tempered martensite structure and residual austenite structure as much as possible. At the same time, it is characterized in that the distribution state of the V-containing precipitates in the tempered bainite is controlled.

<Precipitates with equivalent circle diameter of 1 to 10 nm: 20 or more per 1 μm ^{2 of} tempered bainite>
By appropriately dispersing fine V carbides and carbonitrides that effectively act as hydrogen trap sites in the structure, hydrogen embrittlement resistance can be improved and delayed fracture resistance after processing can be ensured. it can. That is, hydrogen trap sites are increased by dispersing a large amount of fine V-containing precipitates having a large specific surface area. In addition to this, by making the V-containing precipitates fine, it is possible to impart a matching strain field around the V-containing precipitates to the parent phase and enhance the ability as a trap site for hydrogen that tends to collect in the strain field. it can. These improve the hydrogen embrittlement resistance. In this particle size range (equivalent circle diameter of 1 to 10 nm), there are almost no precipitates that do not contain V. Therefore, in this rule, those containing V as in the case of precipitates with an equivalent circle diameter of 20 nm or more as described below. All precipitates were targeted without being limited to.

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 bainite. 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 precipitates is set to 1 nm is that finer precipitates are less effective as hydrogen trap sites.

<Precipitates containing V having an equivalent circle diameter of 20 nm or more: 10 or less per 1 μm ^{2 of} tempered bainite>
Precipitates containing V such as VC have extremely high rigidity and critical shear stress compared to the parent phase, and the precipitates themselves are not easily deformed even if the periphery of the precipitates is deformed. Therefore, when the precipitate containing V has a size of 20 nm or more, a large strain is generated at the interface between the parent phase and the precipitate, and breakage occurs. For this reason, if there are a large amount of coarse precipitates containing V of 20 nm or more, stretch flangeability deteriorates. Therefore, stretch flangeability can be improved by restricting the density of coarse V-containing precipitates.

In order to effectively exhibit the above action, coarse precipitates containing V 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 bainite.

Although the structure of the steel sheet of the present invention is required to satisfy the above-mentioned rules, it is recommended that the following structure rules be further satisfied in addition to the required structure rules.

<Cementite particles having an equivalent circle diameter of 0.1 μm or more: 3 or less per 1 μm ^{2 of} tempered bainite>
As described in the first invention, in the case of a steel having bainite as a main structure such as a conventional high-strength cold-rolled steel sheet, usually, tempered martensite and retained austenite are easily formed, and these structures are the starting points of fracture. Therefore, the dispersion state of the cementite particles precipitated in the bainite during tempering does not significantly affect the stretch flangeability. However, when tempered martensite and retained austenite are reduced, the starting point of fracture during deformation transitions to cementite particles precipitated in bainite, so that the presence state of cementite particles in bainite determines the stretch flangeability. . For this reason, in the case of steel that suppresses the formation of tempered martensite and retained austenite as much as possible, such as the high-strength cold-rolled steel sheet according to the present invention, stretch flangeability is achieved by controlling the precipitation state of cementite particles that are the starting point of fracture. Can be adjusted. Of the cementite particles, only large particles serve as the starting point of fracture, so that the number of coarse cementite particles can be reduced to improve the predetermined stretch flangeability.

In order to effectively exhibit the above action, the number of cementite particles having an equivalent circle diameter of 0.1 μm or more present in the tempered bainite is limited to 3 or less, further 2.5 or less, especially 2 or less per 1 μm ^{2 of} the tempered bainite. It is recommended that you do this.

Hereinafter, a method for measuring the size and the number of precipitates will be described.

[Method of measuring the size of precipitates and their number]
First, a thin film sample is prepared by a thin film method or an extraction replica method, and an area of 2 μm ² or more is observed with this sample using a field emission transmission electron microscope (FE-TEM) at a magnification of 100,000 to 300,000 times. . Then, mark the dark part from the contrast of the image as a precipitate, and with the image analysis software, calculate the equivalent circle diameter from the area of each marked precipitate, and the precipitate of a predetermined size present per unit area The number was determined.

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

Next, of the component compositions constituting the cold rolled steel sheet according to the second invention, differences from the basic invention will be described.

[Component composition of the second invention]
(V: 0.001 to 1.00% by mass)
V promotes the production of α-FeOOH, which is iron oxide, which is said to be thermodynamically stable and protective among rust produced in the atmosphere. V is an important element for improving hydrogen embrittlement resistance because it functions as a hydrogen trap site by being present in steel as fine carbides and carbonitrides. When the V content is less than 0.01% 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.

Next, the preferable manufacturing method for obtaining the cold rolled steel sheet of 2nd invention is demonstrated below.
[Preferred production method of the second invention]
In order to manufacture the cold-rolled steel sheet as described above, first, steel having the above-described composition is melted and formed into a slab by ingot forming or continuous casting, followed by hot rolling (hot rolling).

[Hot rolling conditions]
As hot rolling conditions, it is recommended that the hot rolling heating temperature is set to 900 ° C. or higher, the hot rolling finish rolling temperature is set to 800 ° C. or higher, and after appropriate cooling, winding is performed at a temperature of 450 ° C. or lower.

By performing hot rolling under such temperature conditions, V is completely dissolved in the heating stage, suppressing precipitation during hot rolling and precipitation of V carbide and carbonitride during winding, and annealing. During heating, coarse V carbides and carbonitrides can be prevented from remaining.

After completion of hot rolling, pickling is performed and then cold rolling is performed, but the cold rolling rate is preferably about 30% or more.

Then, after the cold rolling, annealing and further tempering are performed.

[Annealing conditions]
As annealing conditions, first, annealing heating temperature Ta (° C.): [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or higher, and Ac ₃ or higher and 1000 ° C. or lower And hold annealing time: 20 to 3600 seconds. Then, after quenching from the annealing heating temperature to the cooling end temperature: 300 ° C. to [Bs-100] ° C. at a cooling rate of 10 to 200 ° C./second, the cooling end temperature is held for 10 to 600 seconds. Good.

<Annealing heating temperature Ta (° C.): [−9500 / {log ([% C] · [% V]) − 6.72} −273] ° C. or higher and Ac ₃ or higher and 1000 ° C. or lower, annealing holding time: 20 ~ 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. This is to reduce the density of coarse V-containing precipitates of 20 nm or more. Moreover, this is because the area ratio of bainite that is transformed from austenite during subsequent cooling is secured by 50% or more by sufficiently transforming into austenite during annealing and heating.

Annealing heating temperature 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 carbide and the like remain during annealing and become coarse, and the starting point of fracture increases 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 Basics, 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.

In addition, when the annealing heating temperature Ta (° C.) <Ac ₃ , the amount of transformation to austenite is insufficient during annealing heating, so that the amount of bainite transformed from austenite during subsequent cooling is reduced, and the area ratio is 50% or more. This is not preferable because it cannot be secured. On the other hand, an annealing heating temperature 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.

Also, if the annealing holding time is less than 20 seconds, it is not preferable because V carbides cannot be completely dissolved. On the other hand, if the annealing holding time exceeds 3600 seconds, productivity is extremely deteriorated.

<Cooling end temperature: 300 ° C. or more and [Bs-100] ° C. or less, cooling rate: 10 to 200 ° C./second, holding time at cooling end temperature: 10 to 600 seconds>
This is because the bainite transformation from the austenite transformed during the annealing heating is sufficiently advanced to secure a bainite area ratio of 50% or more.

When the cooling end temperature is less than 300 ° C., martensite is formed and stretch flangeability is deteriorated. On the other hand, if the cooling end temperature exceeds [Bs-100] ° C., it takes too much time to complete the bainite transformation, martensite and retained austenite are formed, and stretch flangeability cannot be secured.

If the holding time at the cooling end temperature is less than 10 seconds, the bainite transformation does not proceed sufficiently and the elongation and stretch flangeability cannot be secured. On the other hand, if it exceeds 600 seconds, the productivity is extremely deteriorated.

[Tempering conditions]
As the tempering conditions, the temperature after the annealing cooling to the tempering heating temperature Tt (° C.): 480 ° C. or higher and Ac ₁ ° C. or lower and the tempering holding time t (second) is Pg = exp [−13123 / (Tt + 273)] After holding under the condition of xt <1.21 × 10 ⁻⁵ , it may be cooled to room temperature.

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 the precipitate described in Koichi Sugimoto et al., Material Histology, Asakura Shoten Publishing, p106 formula (4.18). It is a parameter that defines the size of the precipitate, with variables set and simplified.

Under the condition of Pg = exp [−13123 / (Tt + 273)] × t ≧ 1.21 × 10 ⁻⁵ , the coarsening of the precipitate proceeds, and the number of coarse precipitates of 20 nm or more increases too much. The stretch flangeability cannot be secured.

It is recommended that the tempering holding time t (seconds) be Pg <0.20 × 10 ⁻⁵ , whereby the stretch flangeability can be further improved by preventing the growth of cementite.

The cold-rolled steel sheet of the present invention (basic invention, first invention, second invention) basically contains the aforementioned components, with the balance being substantially iron and impurities. However, in addition, the following allowable components can be added as long as the effects of the present invention are not impaired.

(Cr: 0.1-3.0% by mass)
Among the bainite, the upper bainite, which is mainly targeted by the steel of the present invention, is (1) formation of bainitic ferrite → (2) spout of carbon from bainitic ferrite to austenite → (3) cementite from austenite It is formed by a transformation phenomenon that proceeds in the flow of formation. In this flow, the formation of cementite from austenite is delayed by the addition of an alloy element such as Si, so that residual austenite and tempered martensite are easily formed. On the other hand, Cr is an element that increases the driving force for nucleation of cementite, and promotes the formation of cementite, thereby suppressing the formation of retained austenite and tempered martensite. In addition, the cementite formed is usually coarsened due to the diffusion rate-determining of the carbon having a high diffusion rate, and thus is coarsened. However, when Cr is added, the coarsening proceeds due to the diffusion-limited rate of Cr having a low diffusion rate. Therefore, cementite coarsening can be suppressed. As a result, it is possible to finely disperse cementite that is the starting point of fracture, and to improve stretch flangeability. If the Cr content is less than 0.1% by mass, the above effect cannot be exhibited effectively, the ratio of tempered martensite and retained austenite increases, and cementite coarsens, so that the stretch flangeability deteriorates. On the other hand, if the Cr content exceeds 3.0% by mass, retained austenite is formed during quenching, the bainite area ratio cannot be ensured, and the strength and stretch flangeability deteriorate. The range of the Cr content is preferably 0.3 to 2.5% by mass, more preferably 0.6 to 2.0% by mass.

(B: 0.0002 to 0.0050 mass%)
B is an element useful for improving the hardenability and increasing the area ratio of bainite by being present in the austenite grain boundary in a solid solution state in the steel. If the addition amount of B is less than 0.0002% by mass, the above-described effects cannot be exhibited effectively. On the other hand, if the addition amount of B exceeds 0.0050% by mass, Fe ₂₃ (CB) ₆ is formed and the solid solution B is reduced, so that the effect of improving hardenability is diminished.

(Nb and / or Ti: ([N] −0.003) / 12 ≦ [Nb] / 96 + [Ti] / 48 ≦ ([N] +0.01) / 12 ([] is the content of each element ( Mass%))))
Since N forms BN and consumes B, the hardenability improving effect by solid solution B is reduced. On the other hand, Ti and Nb are elements useful for exerting a hardenability improving effect by B because N is strongly fixed as TiN or Nb (CN) and the formation of BN is suppressed. If the addition amount of these elements is insufficient, the above BN formation inhibiting action is not effectively exhibited. On the other hand, when the addition amount of these elements becomes excessive, the formation of cementite is inhibited, the ratio of tempered martensite and retained austenite increases, and stretch flangeability deteriorates.

(Mo: 0.01 to 1.0 mass%, Cu: 0.05 to 1.0 mass%, Ni: 0.05 to 1.0 mass%)
These elements are useful elements for enhancing the strength and stretch flangeability by increasing the hardenability and contributing to securing the bainite area ratio. Mo forms alloy carbides and carbonitrides that can become hydrogen trap sites during tempering, and Cu and Ni, like V, promote the formation of α-FeOOH, thereby improving hydrogen embrittlement resistance. Also has the effect of improving. 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, when the addition amount of each element exceeds 1.0 mass%, austenite remains at the time of quenching, and stretch flangeability is deteriorated.

(Ca: 0.0005 to 0.01% by mass, Mg: 0.0005 to 0.01% by mass, REM: 0.0004 to 0.01% by mass or more)
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 the stretch flangeability is deteriorated.

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

(Example 1: Example according to the first invention)
Steels having the components shown in Table 1 were melted to produce 120 mm thick ingots.
This was hot rolled to a thickness of 25 mm, and then hot rolled again to a thickness of 3.2 mm. After pickling this, it cold-rolled to 1.6 mm in thickness to make a test material, and heat-treated on the conditions shown in Table 2.

For each steel plate after the heat treatment, the area ratios of tempered bainite, ferrite, tempered martensite and retained austenite, and the size and number of cementite particles were measured by the measurement method described above.

Further, the tensile strength TS, the elongation El, and the stretch flangeability λ were measured for each of the above steel plates. 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 in the direction perpendicular to the rolling direction. Moreover, stretch flangeability (lambda) performed the hole expansion test according to the iron continuous standard JFST1001, and measured the hole expansion rate, and made this the stretch flangeability. These measurement results are shown in Table 3.

As shown in Table 3, steel no. 1 to 6, 9 to 11, 14 to 16, 18, and 21 to 25 all have a tensile strength TS of 800 MPa or more, an elongation El of 10% or more, and a stretch flangeability (hole expansion ratio) λ of 90. %, And a high-strength cold-rolled steel sheet having both elongation and stretch flangeability that satisfies the above-mentioned required level was obtained.

On the other hand, steel No. which is a comparative example. 7, 8, 12, 13, 17, 19, 20, 26 to 28 are inferior in any of the characteristics.

For example, steel No. No. 7 is inferior in stretch flangeability because the total area ratio of martensite and retained austenite becomes excessive because the Si content is too high.

Steel No. No. 8 is inferior in tensile strength because the amount of cementite in bainite is insufficient due to the C content being too low.

Steel No. In No. 12, since the total area ratio of martensite and retained austenite becomes excessive because the C content is too high, the tensile strength is excellent, but the stretch flangeability is inferior.

Steel No. In No. 13, since the Mn content is too low, the bainite area ratio is insufficient, so that the tensile strength is ensured, but the stretch flangeability is inferior.

Steel No. No. 17, because the Mn content is too high, martensite is excessively formed, and the total area ratio of martensite and retained austenite becomes excessive, so the tensile strength is excellent, but the stretch flangeability is inferior. Yes.

Steel No. Nos. 26 to 28 do not satisfy at least one of the requirements for defining the structure of the present invention because the annealing conditions are out of the recommended range, and the stretch flangeability is inferior.

Here, among the data shown in Table 3, steel No. 1 in which the composition of the steel and the structure of the matrix structure satisfy the specified range of the present invention. The following analysis was attempted using the above data.

That is, as a result of arranging the degree of influence of the number of cementite particles on the stretch flangeability (hole expansion ratio) λ, FIG. 1 was obtained.

As shown in FIG. 1, the stretch flangeability (hole expansion ratio) λ decreases almost linearly as the number of coarse cementite particles having an equivalent circle diameter of 0.1 μm or more increases. FIG. 1 shows that the number of coarse cementite particles needs to be 3 / μm ² or less in order to ensure λ ≧ 90% above the desired level.

(Example 2: Example according to the second invention)
Steels having the components shown in Table 4 were melted to prepare an ingot having a thickness of 120 mm.
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 cold-rolled to thickness 1.2mm to make a test material, and heat-treated on the conditions shown in Table 5.

About each steel plate after the heat treatment, by the measurement method described above, each area ratio of tempered bainite, ferrite, tempered martensite and retained austenite, the size and number of precipitates (existence density), and the size of cementite particles and The existence number (existence density) was measured.

For each steel sheet, tensile strength TS, elongation El, stretch flangeability λ are measured to evaluate mechanical properties, and hydrogen embrittlement risk index is measured to evaluate hydrogen embrittlement resistance. did.

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.

Also, the stretch flangeability λ was measured according to the iron standard JFST1001, the hole expansion rate was measured, and the hole expansion rate was measured.

The hydrogen embrittlement risk index was determined by performing a low strain rate technique (SSRT) with a strain rate of 1 × 10 ⁻⁴ / s using a flat plate test piece having a thickness of 1.2 mm. The hydrogen embrittlement risk index was calculated from the 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). ).

When the hydrogen embrittlement risk index exceeds 15%, there is a risk of causing hydrogen embrittlement during use. Therefore, in the present invention, those with 15% or less were evaluated as having excellent hydrogen embrittlement resistance.

Table 6 shows the measurement results of the mechanical properties and hydrogen embrittlement resistance.

As shown in Table 6, invention steels (steel Nos. 30, 31, 38, 39, 42, 44, 45, 48, 49) satisfying the essential constituent requirements of the present invention (the above-mentioned component composition rules and the above-mentioned essential structure rules). , 54, 56, 60 to 65; all of the circles) have an tensile strength TS of 780 MPa or more, and an index TS × λ for evaluating the balance between the tensile strength TS and stretch flangeability (hole expansion ratio) λ. Is 60000 MPa ·% or more, and the hydrogen embrittlement risk index is 15% or less. Therefore, a high-strength cold-rolled steel sheet having both workability and hydrogen embrittlement resistance was obtained.

In contrast, comparative steel lacking at least one of the essential constituent elements of the present invention (steel Nos. 29, 32 to 37, 40, 41, 43, 46, 47, 50 to 53, 55; ) Is inferior in any of the mechanical properties and hydrogen embrittlement resistance properties.

For example, steel No. In No. 29, since the V content is too low, the number (existence density) of fine precipitates having an equivalent circle diameter of 1 to 6 nm is insufficient. Therefore, the tensile strength and stretch flangeability are excellent, but the hydrogenation embrittlement resistance is inferior.

Steel No. In No. 43, since the V content is too high, the number (existence density) of coarse precipitates having an equivalent circle diameter of 20 nm or more becomes excessive. Therefore, although tensile strength and hydrogenation embrittlement resistance are excellent, stretch flangeability is inferior.

Steel No. No. 46 has an excessively high Si content, so that the total area ratio of martensite and residual austenite (residual γ) becomes excessive. Therefore, although tensile strength and hydrogenation embrittlement resistance are excellent, stretch flangeability is inferior.

Steel No. 47 is insufficient in the bainite area ratio because the C content is too low. Therefore, stretch flangeability and hydrogenation embrittlement resistance are excellent, but tensile strength is inferior.

Steel No. In No. 51, when the C content is too high, the residual amount of martensite and residual austenite (residual γ) increases, and the number of coarse precipitates of 20 nm or more becomes excessive. Therefore, although tensile strength is excellent, stretch flangeability and hydrogenation embrittlement resistance are inferior.

Steel No. In No. 52, since the Mn content is too low, the bainite area ratio is insufficient, so that the stretch flangeability and hydrogenation embrittlement resistance are excellent, but the tensile strength is inferior.

Steel No. In No. 55, when the Mn content is too high, the residual amount of martensite and retained austenite increases. Therefore, although tensile strength is excellent, stretch flangeability and hydrogenation embrittlement resistance are inferior.

Steel No. Nos. 32 to 37, 40, 41 and 50 do not satisfy at least one of the requirements for 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. Yes.

Further, as shown in Table 6, in addition to the essential constituent requirements of the present invention, the recommended steels (steel Nos. 57 to 59; marked with ◎) that satisfy the above recommended structure provision (a) are all tensile. The strength TS is 980 MPa or more, the stretch flangeability (hole expansion ratio) λ is 100% or more, and the hydrogen embrittlement risk index is 15% or less, which is superior in strength and workability to the steel of the invention. It was found that a high-strength cold-rolled steel sheet can be obtained.

Claims (8)

1. C: 0.03 mass% or more, less than 0.30 mass%,
   Si: 2.0% by mass or less (including 0% by mass),
   Mn: 0.1 to 2.8% by mass,
   P: 0.1% by mass or less,
   S: 0.005 mass% or less,
   In a cold-rolled steel sheet containing N: 0.01% by mass or less, and Al: 0.01-1.00% by mass,
   The area ratio of tempered bainite is 50% or more (including 100%), the total area ratio of tempered martensite and residual austenite is less than 3% (including 0%) and the balance has a structure made of ferrite,
   A cold-rolled steel sheet characterized by controlling a distribution state of precipitates in the tempered bainite and / or a distribution state of cementite particles in the tempered bainite.
2. Si: 1.0% by mass or less (including 0% by mass),
   Mn: 0.5 to 2.4 mass%, and Al: 0.01 mass% or more, less than 0.10 mass%,
   The area ratio of the tempered bainite is 70% or more (including 100%),
   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 bainite.
3. V: 0.001 to 1.00% by mass,
   There are 20 or more precipitates having an equivalent circle diameter of 1 to 10 nm per 1 μm ^{2 of the} tempered bainite,
   The cold-rolled steel sheet according to claim 1, 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 bainite.
4. The cold-rolled steel sheet according to claim 3, 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 bainite.
5. The cold-rolled steel sheet according to claim 1, comprising Cr: 0.1 to 3.0% by mass.
6. B: Including 0.0002 to 0.0050 mass%,
   2. Nb and / or Ti are included so as to satisfy a relationship of ([N] −0.003) / 12 ≦ [Nb] / 96 + [Ti] / 48 ≦ ([N] +0.01) / 12. Cold-rolled steel sheet as described in 1. (However, [] means the content (% by mass) of each element)
7. 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%.
8. 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%.

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