JPH10130776A - High ductility type high tensile strength cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high ductility type high strength steel sheet capable of stable production even by the ordinary method using continuous casting equipment, suitable for use in an automobile body, etc., and excellent in formability, surface characteristic, and shock absorbing power. SOLUTION: The high ductility type high tensile strength cold rolled steel sheet is constituted so that it has a composition, which consists of 0.06-0.25% C, <=2.5% Si, 0.5-3.0% Mn, <=0.1% P, <=0.03% S, 0.1-2.5% Al, 0.003-0.08% Ti, <=0.01% N, and the balance Fe with inevitable impurities and also contains, if necessary, one or more kinds among proper amounts of Ni, Cr, Mo, Cu, Nb, V, Zr, and B and in which Ti content satisfies [(48/14)N<=Ti<=(48/14)N +(48/32)S+0.01], and also has a structure, after (cold rolling)-(recrystallization annealing), containing retained austenite by >=5% by volume percentage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has excellent surface properties and shock absorption, and is suitable as a steel sheet for an automobile body in which "improvement in fuel economy by weight reduction" and "improvement in shock absorption in a collision" are pursued. The present invention relates to a high-ductility type high-tensile cold-rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, the weight of a vehicle body has been actively reduced by reducing the thickness of a steel plate for a vehicle body in order to improve the fuel efficiency of the vehicle. There is a tendency to be strengthened, and the situation cannot be met simply by reducing the weight by simply reducing the thickness.

[0003] Therefore, steel sheets for automobile bodies are required to have higher strength, which leads to improvement in shock absorption performance at the same time as reduction in thickness. For thinning, it is necessary to use high strength steel sheets having higher strength levels. I was under pressure. However,
In general, increasing the strength of a steel sheet deteriorates formability, which leads to the occurrence of defects (cracks) during press forming. To apply a high-strength steel sheet with a high strength level, "deterioration of formability due to higher strength" It was necessary to overcome the problem.

However, in order to overcome the above problem by providing a high-tensile steel sheet exhibiting high ductility, "C: 0.30-
0.55% (Hereinafter,% representing the component ratio is referred to as weight%), S
After heating a steel sheet containing i: 0.7 to 2.0% and Mn: 0.5 to 2.0% to the austenite single phase region, it is kept at 650 to 750 ° C. for 4 to 15 seconds, and then cooled to 450 to 650 ° C. during the cooling process. By holding for a total of 10 to 50 seconds, a "mixed structure containing 10% or more by volume of ferrite and residual austenite in martensite or bainite" is realized, and "a high-tensile steel sheet showing high ductility". Method "(JP-A-60-43464).

Japanese Patent Application Laid-Open No. 61-157625 discloses "C: 0.12 to 0.55%, Si: 0.4 to 1.8%, Mn: 0.2 to 0.5%.
In addition to 2.5%, appropriate amounts of P, Ni, Cu, Cr, Ti, N
After heating a steel sheet containing at least one of b, V and Mo to "two phase region of ferrite + austenite", 500
A method has been proposed in which a "ferrite + bainite + retained austenite mixed structure" is realized by holding the steel sheet in a temperature range of about 350 DEG C. for 30 seconds to 30 minutes to obtain a "high-tensile steel sheet exhibiting high ductility". .

However, the high-strength steel sheets proposed in these proposals have high ductility by utilizing "a phenomenon in which retained austenite undergoes strain-induced transformation during deformation during work and exhibits large elongation (transformation-induced plasticity)". Although it is ensured, in practice, the transformation induced plasticity due to processing is manifested in the initial stage of deformation, so the local ductility (hole expanding property etc.) is inferior, and the strength is 590 N / mm class ² strength. It was difficult to obtain, and did not fully satisfy the performance required for steel sheets for automobiles.

[0007] Therefore, Al is used as a high-tensile steel sheet which solves the problem of local ductility deterioration, is excellent in formability including hole expandability and the like, and can secure a strength level of 590 N / mm ² class. A proposal has been made for an Al-added "high-strength steel sheet containing retained austenite (residual gamma steel sheet)" which is used as an element for stabilizing retained austenite (Japanese Patent Application Laid-Open Nos. 5-70886 and 6-70886). -14578
No. 8).

Japanese Unexamined Patent Publication No. 6-145808 discloses a Si-added residual γ steel sheet using Si as an element for suppressing carbide formation and stabilizing retained austenite. In order to improve the absorption performance, the process of studying the "retained austenite form on high impact absorption characteristics" was studied, and the "chemical composition and manufacturing process for obtaining the optimum form" disclosed thereby were disclosed. I have.

As described above, while the safety regulations for automobiles are being tightened, studies are being made on steel sheets having excellent shock absorption, and the energy absorption at impact is determined by the integral value of the resistance to deformation and the amount of deformation. The material that exhibits high work hardening characteristics and high ductility due to plastic deformation is excellent in shock absorption. " Also, application to an actual vehicle is being studied as a residual γ steel sheet exhibiting high work hardening and high ductility is excellent.

[0010] However, the newly proposed residual gamma steel also
"The work hardening behavior and ductility change sensitively depending on the components and the manufacturing process, and it is extremely difficult to stably produce the high-tensile steel with high ductility and excellent shock absorption intended on an industrial scale. Is not enough to overcome the problem. In other words, in order to obtain better shock absorption, it is important to exhibit high work hardening characteristics from low strain to high strain, but in the residual γ steel sheet, the C concentration in austenite (γ) is low. However, even at low strains, the residual austenite is excessively transformed into martensite, does not show high work hardening in the high strain range, and if the amount of retained austenite is small, excellent ductility cannot be obtained. Therefore, development of technology for stably mass-producing high-strength steel sheets with stable performance without causing these inconveniences on an industrial scale was urgently needed.

The "enhancement of C concentration in austenite" necessary for obtaining high work hardening from a low strain range to a high strain range.
It is preferable to use Al, which has a stronger action of expanding the solid solubility limit of C in austenite than Si, and it is preferable to use residual aluminum generated when the solid solubility limit of C in austenite is expanded. Since the C concentration in the austenite also increased, it was thought that the Al-added type residual γ steel sheet was more advantageous than the Si-added type in terms of impact absorption characteristics. The behavior is still more unclear than that of the Si-added type. For example, Al, which has a strong effect of expanding the solid solubility limit of C in austenite, is added. Although it exhibited a higher work hardening ability than that of the Si-added type, it had a problem that the work hardening ability was lower than that of the Si-added type. However, since the Al-added residual γ steel sheet still shows relatively good work hardening characteristics in a wide range from low strain to high strain, the impact-absorbing property of the Al-added residual γ steel sheet is stably improved. I had to say that establishing a method was extremely important in industry. In addition, when manufacturing a high Al-added steel sheet, "Surface scratches on the slab due to AlN generation during continuous casting are likely to occur,
There is also a problem that this is easily brought to a product, and it is indispensable to solve such a problem when manufacturing an Al-added residual γ steel sheet for an automobile body or the like.

Accordingly, an object of the present invention is to solve the above-mentioned problem of a high-strength steel sheet used for an automobile body or the like, and it is possible to stably manufacture the steel sheet by a usual method using a continuous casting facility. The aim was to provide a "high-ductility type high-strength steel sheet with excellent formability, surface properties, and shock absorption capacity".

[0013]

Means for Solving the Problems In order to achieve the above object, the present inventors have repeatedly conducted research while paying particular attention to the impact absorption characteristics of Al-added residual γ steel sheets. When the contents of C, Mn, P, S and N in the above were regulated to appropriate ranges respectively, and an appropriate amount of Ti was added in consideration of the contents of N and S, it was observed in the Al-added residual γ steel. The above-mentioned problems have been eliminated, and while exhibiting high workability from low strain range to high strain range while having excellent moldability,
Good surface properties can be stably ensured even in a manufacturing process that involves continuous casting as usual. "

That is, in the Al-added residual γ steel sheet, the formability could be stabilized in a good range by adjusting the contents of C, Mn, P and S first. The addition of an appropriate amount of Ti after regulation has an extremely effective effect on the improvement of surface texture and work hardening performance, and greatly contributes to ensuring the high shock absorption, workability, and surface texture required for steel sheets for automobile bodies. I found something to contribute.

More specifically, for example, the following were confirmed as the effects of individually separable Ti when an appropriate amount of Ti was added to the Al-added residual γ steel. a) Since N is fixed as TiN by the addition of Ti, the production of AlN is suppressed, and brittle cracking of the slab during continuous casting is suppressed, so that a steel slab having excellent surface properties can be obtained, and therefore, after cold rolling, The steel plate also has excellent surface properties stably. b) As described above, the Al-added residual γ steel shows rather high work-hardening properties in the high strain range, and has a lower work-hardening performance in the low strain range than the Si-added type.
When added, the ferrite ground is strengthened by precipitation hardening, and the weak point of low work hardening in a low strain range is compensated for by the increase in deformation resistance, and sufficiently excellent shock absorbing performance is exhibited.

The present invention has been completed based on the above findings and the like, and provides the following high-ductility type high-tensile cold-rolled steel sheet. C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, N: 0.01% And the balance consists of Fe and unavoidable impurities, and the Ti content satisfies the relationship of (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01, A high-ductility type high-tensile cold-rolled steel sheet excellent in surface properties and impact absorption, characterized in that the structure after cold rolling and recrystallization annealing is a structure containing residual austenite in a volume ratio of 5% or more. . C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, N: 0.01% Ni: 0.1% or less, Cr: 1.0% or less, Mo: 0.6% or less, Cu: 1.0% or less, Nb: 0.05% or less, V: 0.08% or less, Zr: 0.05% or less, B: It contains at least one of 0.003% or less, and the balance consists of Fe and inevitable impurities, and the Ti content is (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01. A high-quality steel having excellent surface properties and shock absorption, characterized in that the structure after cold rolling and recrystallization annealing is a structure containing a residual austenite of 5% or more by volume. Ductile high-tensile cold-rolled steel sheet.

It is needless to say that the high-ductility type high-tensile cold-rolled steel sheet according to the present invention can be applied as a material for an automobile body or the like after being made into a product by cold rolling and then subjected to recrystallization annealing. Of course, it can be used in the form of a hot-dip galvanized steel sheet or an electro-plated steel sheet.

The reason why the "chemical composition" and "structure after cold rolling and recrystallization annealing" of the steel sheet are limited as described above in the present invention will be described below. (A) Chemical composition of steel sheet C: C is a strengthening component of steel and has a large amount and stability of retained austenite necessary for "improvement of ductility by transformation induced plasticity", which is characteristic of the steel sheet of the present invention. It is also an influencing element. That is, C, which is an austenite-stable element, is concentrated from ferrite into austenite during the two-phase region or bainite transformation to improve the chemical stability of austenite. Acts to promote the residual. However, C
If the content is less than 0.06%, it is difficult to secure 5% or more of retained austenite as defined in the present invention, while
If the content of C exceeds 0.25%, the weldability is deteriorated and the strength is excessively increased. Therefore, the C content is determined to be 0.06 to 0.25%.

Si: Si is an element which suppresses precipitation without dissolving in cementite, and therefore has an effect of delaying transformation from austenite to promote C concentration in austenite. . This action increases the stability of the retained austenite, so that it is easy to secure "retained austenite exhibiting transformation-induced plasticity" at room temperature. However, Si is an oxide-forming element and
If the Si content exceeds 2.5%, an island scale is formed to reduce the pickling property and deteriorate the flash butt weldability. Therefore, the Si content is set to 2.5%.
The steel sheet of the present invention is premised on the addition of Al which exerts the same action as Si. However, since the addition of Si gives a favorable result of increasing the amount of retained austenite, the steel sheet is preferably 0.5% or more. It is better to add.

Mn: Mn is an austenite stabilizing element, and also has an effect of preventing austenite from being decomposed into pearlite during cooling of the steel sheet. However, if the Mn content is less than 0.5%, the desired effect cannot be obtained by the above-mentioned effect. On the other hand, if the Mn content exceeds 3.0%, the hardenability of the steel sheet becomes excessively high, causing an excessive increase in strength and ductility deterioration. Therefore, the content of Mn was determined to be 0.5 to 3.0%.

P: P is an element that affects ferrite formation like Si and Al, but in the steel sheet of the present invention, P
Since addition is essential, it is not necessary to specify the lower limit of the amount of P added. However, if the content of P exceeds 0.1%, the deterioration of the secondary work resistance becomes remarkable,
The content was determined to be 0.1% or less.

S: Although S does not affect the formation of retained austenite, the increase in the amount of S results in the formation of a large number of A-based inclusions, thereby deteriorating hole expandability. And
Since this tendency becomes remarkable when the S content exceeds 0.03%, the S content is regulated to 0.03% or less.

Ti: Ti is an extremely important element in the steel sheet of the present invention, and has an effect of suppressing the generation of AlN, which is a cause of surface cracking during casting, and of precipitating and fixing N as TiN.
The precipitation hardening by this action strengthens the ferrite ground and increases the deformation resistance, thereby compensating the work hardening ability in a low strain region and contributing to the improvement of the shock absorption characteristics of the Al-added residual γ steel. However, when “Ti <(48/14) N”, N cannot be completely fixed as TiN, and AlN that is a factor of slab surface cracking is generated. In addition, "Ti> (48 /
14) N + (48/32) S + 0.01 ”or Ti content
If it exceeds 0.08%, the amount of TiC generated increases, and an excessive increase in strength occurs due to precipitation strengthening, or the absolute amount of C enriched in austenite decreases, and austenite due to C enrichment decreases. Has less effect on the chemical stability of
The amount of generated stainite decreases, and the ductility deteriorates. by the way,
When the Ti content is 0.003%, in order to satisfy the condition of "(48/14) N≤Ti", the N content must be less than 0.0009%, and it is not possible to reduce the N content to such a region. At the current steelmaking technology level, it is extremely difficult and impractical. Therefore, the Ti content is adjusted to be in the range of 0.003 to 0.08% and to satisfy the condition of (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01. I decided.

Al: Like Si, Al is an essential component for securing stable austenite at room temperature.
In addition, Al does not form a solid solution in cementite,
During the isothermal holding (at the time of bainite transformation), precipitation of cementite is suppressed, and the effect of delaying transformation is exhibited.
However, since Al has a stronger ferrite-forming ability than Si, the onset of transformation is faster in the case of addition of Al than in the case of addition of Si. -C becomes enriched in the stainite. Therefore, when Al is added, the chemical stability of austenite can be further improved, and as a result, the C concentration of the resulting austenite increases, and the generated residual austenite increases.
Since the amount of stain is increased, high work hardening characteristics are exhibited even in a high strain range. However, the Al content is
If the content is less than 0.1%, the above effect cannot be sufficiently ensured. On the other hand, if the content of Al exceeds 2.5%, the flash butt welding applied at the time of continuous line passing of pickling and cold rolling is used. Since the weldability is deteriorated and the cost increases, the Al content is set to 0.1 to 2.5%.

N: The content of N has a great effect on the amount of AlN that degrades the surface properties of the steel sheet. If the N content is excessive, the fixation of N by Ti cannot be achieved and AlN is formed. And the surface properties of the steel sheet deteriorate. If the N content exceeds 0.01%, the amount of Ti necessary to fix N as TiN increases, and it becomes impossible to fix N at the Ti amount specified in the present invention.
%.

Ni, Cr, Mo, Cu, Nb, V, Zr and B: These components are used as necessary to secure further strength to the steel sheet, to refine the structure of the steel sheet, and to improve corrosion resistance. One or two or more are added, and the actions of these individual components are as follows.

A) Ni Ni, like Mn, has an effect of stabilizing austenite, so that Ni can be included as necessary. However, if added in a large amount, cost and excessive strength increase are caused. The upper limit was set at 0.1%.

B) Cr Like Cr, Cr also has an action to stabilize austenite, so that it can be added as necessary. However, if added in a large amount, cost increases and unnecessary hardenability is required. To an excessively high strength, so the upper limit is 1.0
%.

C) Mo Mo, like Si and Al, is an element which has an effect on austenite retention through the action of suppressing the formation of carbides. The addition of Mo suppresses the formation of carbides and the martensitic transformation temperature. Has an effect of lowering the temperature to stabilize the retained austenite. Therefore, although it is a component that can be included as needed, adding a large amount of it causes an increase in cost and an unnecessary increase in hardenability, resulting in an excessive increase in strength. Therefore, the upper limit is set to 0.6%.

D) Cu Since Cu has an effect of improving the corrosion resistance of a steel sheet in a non-plated state, it may be added to improve the corrosion resistance of a non-plated steel sheet. However, excessive addition causes deterioration of hot workability due to Cu checking in the slab, and in order to suppress this, complex addition with Ni is indispensable. %.

E) Nb Nb has the effect of appropriately suppressing the pearlite transformation of austenite, thereby exhibiting the effect of relaxing the cooling rate constraint for obtaining retained austenite. . Therefore, it can be included as needed, but 0.05%
Even if added in excess of the above, the above effect is saturated and becomes economically disadvantageous. Therefore, the upper limit of the Nb content is set to 0.05%.

F) V V also has the effect of stabilizing retained austenite by lowering the martensitic transformation temperature, so that V can be included as necessary. However, the addition of a large amount increases the cost and hardens. Unnecessarily increase the strength, resulting in excessive strength increase. Therefore, the upper limit of the V content is set to 0.08%.

G) Zr Zr has the effect of adjusting the shape of the inclusions and improving the cold workability of the hot-rolled steel sheet, so that Zr can be contained as necessary. Therefore, the upper limit of Zr was determined to be 0.05%.

H) BB B has the effect of strengthening the grain boundaries and increasing the resistance to secondary working brittleness, so that B can be contained as necessary.
If the addition exceeds 03%, the effect saturates. Therefore, B
The upper limit of the content was set at 0.003%.

(B) Structure after Cold Rolling-Recrystallization Annealing The ductility of the steel sheet of the present invention depends on the volume fraction of retained austenite, and when this volume fraction is less than 5%, ductility due to strain-induced transformation of austenite is caused. No improvement can be expected. Therefore, the microstructure after cold rolling and recrystallization annealing is referred to as “residual alloy having a volume percentage of 5% or more
Structure containing stainite ”.

The steel sheet according to the present invention can be manufactured as follows. First, a steel of the intended chemical composition is melted in a converter or the like as usual, and then turned into a slab by continuous casting, and then hot-rolled while hot, or once cooled to room temperature. Is heated and hot-rolled to obtain a hot-rolled steel sheet. The hot rolling may be performed under the usual conditions, but the winding after the hot rolling is 5 times from the viewpoint of the load during the cold rolling and the pickling property.
It is recommended to be 00-700 ° C.

The rolled hot-rolled coil is then pickled as usual and then subjected to cold rolling. The cold rolling conditions need not be particularly limited, but the cold rolling rate is preferably 45% or more in view of the sheet passing property during cold rolling.

The cold rolled steel sheet obtained by performing the cold rolling is subjected to recrystallization annealing before use. In this recrystallization annealing, it is sufficient to adopt the usual conditions applied to obtain the residual γ steel sheet. That is, in the recrystallization annealing, it is necessary to heat to the [two-phase region of ferrite and austenite] to concentrate C in γ, so that the heating temperature is set to the two-phase region temperature. Further, in order to re-dissolve the carbide formed by hot rolling and to enrich C in austenite, the heating and holding time in the recrystallization annealing needs to be 20 to 50 seconds. Just 150
Long-time annealing for more than one second is not preferable because it causes a decrease in productivity. Next, in order to concentrate C from ferrite to austenite, the temperature range of 650 to 750 ° C. is gradually cooled at 2 ° C./sec or more. Then, the bainite transformation proceeds by overaging, and the C concentration in austenite is further promoted. Here, if the overaging temperature is 480 ° C. or more, carbides are generated, so that C cannot be concentrated in austenite, and 35%.
At 0 ° C. or lower, care must be taken because ductility is degraded due to an increase in strength. Furthermore, it should be noted that if the overaging time is 120 seconds or less, bainite transformation does not proceed sufficiently, so that C cannot be enriched in austenite, and good characteristics cannot be obtained.

The high-tensile cold-rolled steel sheet according to the present invention manufactured as described above can be used as it is as a material for an automobile body after recrystallization annealing, but is not subjected to hot-dip galvanizing or electroplating. It can also be used later. When hot-dip galvanizing is performed, a heat treatment corresponding to recrystallization annealing may be performed in the plating step.

Next, the present invention will be described with reference to examples.

EXAMPLE Steels of various chemical compositions were melted and converted into slabs by continuous casting in a converter, cooled once to room temperature, soaked again at 1250 ° C, and hot-rolled at 1150-930 ° C. A 3.5 mm thick hot rolled steel sheet was obtained. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled to obtain a 1.6-mm-thick cold-rolled steel sheet. Tables 1 and 2 show the chemical compositions of the obtained cold rolled steel sheets.
Shown in

[0041]

[Table 1]

[0042]

[Table 2]

Then, the cold-rolled steel sheet was subjected to recrystallization annealing using a continuous annealing facility. In continuous annealing, the cold-rolled steel sheet is continuously heated to 820 ° C., kept at that temperature for 40 seconds, and then gradually cooled to 700 ° C. at a cooling rate of 3 ° C./sec. The test was performed under the condition that the temperature was maintained for 3 minutes.

Some steel types were electrogalvanized under normal conditions after continuous annealing. Further, for some of the different steel types, the cold-rolled steel sheet after cold-rolling was passed through a hot-dip galvanizing facility, and recrystallization annealing and hot-dip galvanizing were performed in one step.

For each of the thus obtained cold-rolled steel sheets, the amount of retained austenite (γ), the surface properties of the steel sheets (in the case of plated steel sheets, the surface properties before plating), the tensile properties,
The hole expandability and the shock absorption characteristics were investigated, and the results are shown in Tables 1 and 2 and Table 3 below.

[0046]

[Table 3]

"Amount of retained austenite (γ)"
Is, X after chemical polishing the surface of the bare steel plate up to _^1/4 of the thickness
It was measured by line diffraction. The "surface properties" were evaluated based on the presence or absence of surface flaws per coil. Those with no surface flaws were indicated as "good", and those with surface flaws as "unfavorable". Regarding "tensile properties", JIS No. 5 tensile test pieces were taken from each cold-rolled steel sheet and subjected to a tensile test, and the yield strength (YS), tensile strength (TS) and elongation (El) were measured. The “hole expanding characteristic” indicating local ductility is obtained by forming a circular punched hole having a diameter of 10 mm into 60 holes.
° It was molded with a conical punch and evaluated by the hole expansion rate.
The "shock absorption characteristics" were such that a hat-type closed-section structural member was formed from each cold-rolled steel sheet by the press-break method, and this was crushed at a speed of 4 m / sec using a gas / hydraulic high-speed compression tester to 150 mm. Evaluation was made based on the absorbed energy at the time of displacement.

From the results shown in Tables 1, 2 and 3, the following can be confirmed. That is, the cold-rolled steel sheets 1 to 20 of the present invention all have a residual γ content of 5% or more, and exhibit good tensile properties and surface properties.

The hole expandability showing local ductility generally decreases with increasing strength. However, when compared at the same strength level, the cold-rolled steel sheet 23 as a comparative material has an Al content out of the range specified in the present invention. The cold-rolled steel sheet 4 which is the material of the present invention because it is off
It can be seen that the hole expandability is inferior to that of. this is,
In the comparative material, the residual austenite is transformed into martensite at the time of punching, the difference in hardness between the phases of the structure is enlarged, and many starting points of microcracks are generated. It is thought to be due to

Although the shock absorbing property generally improves with the increase in strength, the cold-rolled steel sheets 1 to 20 of the present invention are superior to the cold-rolled steel sheets 21 to 28 of the comparative materials. The cold-rolled steel sheet 26, which is a comparative material, has a small amount of retained austenite (γ) and poor properties especially because the Ti content is out of the range specified in the present invention. Further, with respect to the cold-rolled steel sheet 24 as a comparative material, since no Ti was added, the yield stress due to precipitation strengthening could not be increased, so that the initial impact load at the time of the crush test was reduced, and the same strength and residual strength were obtained. -The impact absorption energy is inferior to that of the cold rolled steel sheet 4 of the present invention, which shows the amount of stainite (γ) and ductility. Regarding the surface properties, the cold-rolled steel sheets 24 and 28, which are comparative materials to which the Al content is within the specified range of the present invention but to which Ti is not added, are unsatisfactory.

[0051]

As described above, according to the present invention, it is possible to stably provide a high-ductility type high-tensile cold-rolled steel sheet which is excellent not only in formability but also in surface properties and shock absorption. Thus, industrially useful effects can be brought about, such as being able to sufficiently meet the demands for reducing the weight of automobile bodies and the like and improving shock absorption.

Claims (2)

[Claims] [Claim 1] C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, N: 0.01% or less, with the balance being Fe and unavoidable impurities, and having a Ti content of (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0. 01, satisfying the following relationship, characterized in that the structure after cold rolling and recrystallization annealing is a structure containing at least 5% by volume of retained austenite, and is excellent in surface properties and impact absorption. High ductility type high tensile cold rolled steel sheet. 2. C: 0.06 to 0.25%, Si: 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1 to 2.5%, Ti: 0.003 to 0.08%, N: 0.01% or less, Ni: 0.1% or less, Cr: 1.0% or less, Mo: 0.6% or less, Cu: 1.0% or less, Nb: 0.05% or less, V: 0.08% or less , Zr: 0.05% or less, B: 0.003% or less, and the balance consists of Fe and inevitable impurities, and the Ti content is (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01, wherein the structure after cold-rolling and recrystallization annealing is a structure containing at least 5% by volume of retained austenite in the surface. High ductility type high tensile cold rolled steel sheet with excellent properties and shock absorption.