JP3498504B2 - High ductility type high tensile cold rolled steel sheet and galvanized steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is suitable as a steel sheet for automobile bodies, which has excellent surface properties and shock absorption properties, and is pursued for "improvement in fuel consumption by weight reduction" and "improvement in shock absorption property in collision". High ductility high tension cold rolled steel sheet and its
The present invention relates to a galvanized steel sheet used as a base material .

[0002]

2. Description of the Related Art In recent years, the weight reduction of vehicle bodies has been actively promoted by reducing the thickness of steel sheets for vehicle bodies for the purpose of improving the fuel efficiency of automobiles. There is a tendency to be strengthened, and it is becoming a situation in which it is not possible to deal with these issues simply by reducing the weight by reducing the thickness.

Therefore, for steel sheets for automobile bodies, it is required to make the steel sheets thinner and at the same time further strengthen the strength to improve the impact absorption performance. In order to reduce the thickness, it is necessary to apply high strength steel sheets having higher strength levels. I was under pressure. However,
Generally, higher strength of steel plate deteriorates formability, which leads to failure (cracking) during press forming. In order to apply high strength steel plate with high strength level, "deterioration of formability due to high strength" It was necessary to overcome the problem.

However, in the past, in order to overcome the above problems by providing a high tensile steel sheet having high ductility, "C: 0.30 ~
0.55% (Hereafter,% representing the component ratio shall be% by weight), S
After heating a steel sheet containing i: 0.7 to 2.0% and Mn: 0.5 to 2.0% in the austenite single phase region, it is held at 650 to 750 ° C for 4 to 15 seconds, and then at 450 to 650 ° C during the cooling process. By holding for 10 to 50 seconds in total, "a high-tensile steel sheet showing high ductility" is realized by realizing "a mixed structure containing 10% or more by volume of ferrite and residual austenite in martensite or bainite". Has been proposed (Japanese Patent Application Laid-Open No. 60-43464).

Further, in Japanese Patent Laid-Open No. 61-157625, "C: 0.12-0.55%, Si: 0.4-1.8%, Mn: 0.2-"
2.5%, if necessary, appropriate amount of P, Ni, Cu, Cr, Ti, N
After heating a steel sheet containing at least one of b, V and Mo to the "ferrite + austenite two-phase region", 500
A method of realizing a "ferrite + bainite + residual austenite mixed structure" by maintaining the temperature range of up to 350 ° C for 30 seconds to 30 minutes to obtain a "high tensile steel sheet exhibiting high ductility" has been proposed. .

However, the high-strength steel sheets proposed by these proposals have a high ductility by utilizing the "phenomenon in which retained austenite undergoes strain-induced transformation during deformation during deformation to show a large elongation (transformation-induced plasticity)". Although it has been secured, in reality, transformation-induced plasticity due to processing is expressed at the initial stage of deformation, so local ductility (hole expandability, etc.) is inferior, and 590 N / mm ² grade strength is also achieved. It was difficult to obtain, and the performance required for steel sheets for automobiles was not sufficiently satisfied.

Therefore, Al is used as a high-strength steel plate that solves the problem of local ductility deterioration, has excellent formability including hole expandability, and can secure a strength level of 590 N / mm ² class, and suppresses the formation of carbides. Proposals have been made regarding "high-strength steel sheets containing residual austenite (residual γ steel sheets)" of Al addition type used as an element for stabilizing the retained austenite (Japanese Patent Laid-Open Nos. 5-70886 and 6-8686). -14578
No. 8).

Further, Japanese Patent Laid-Open No. 6-145808 discloses a Si-added residual gamma steel sheet using Si as an element for suppressing the formation of carbides and stabilizing the retained austenite. The process of studying "residual austenite morphology affecting high impact absorption characteristics" to improve the absorption performance and the clarified "chemical composition and manufacturing process for obtaining optimum morphology" are disclosed. There is.

As described above, a steel sheet excellent in shock absorbing property has been studied under the tightening of collision safety regulations for automobiles, and "the energy absorbing property at the time of impact is the integral value of the resistance force at the time of deformation and the deformation amount. From the viewpoint of impact absorption performance, it has been found that a material that exhibits high work hardening characteristics and high ductility due to plastic deformation is excellent in impact absorption. However, the application to actual vehicles is being considered because the residual γ steel plate showing high work hardening and high ductility is excellent.

However, the newly proposed residual γ steel also
"The work hardening behavior and ductility change sensitively depending on the components and the manufacturing process, and it is very difficult to stably manufacture the" high-strength steel sheet with high ductility and excellent impact absorption "on an industrial scale. The problem of "Yes" cannot be fully overcome. That is, it is important to exhibit high work hardening characteristics in a low strain to high strain region in order to obtain a better shock absorbing property, but in the residual γ steel sheet, the C concentration in austenite (γ) is low. And even at low strain, residual austenite is excessively transformed into martensite and does not show high work hardening in the high strain region, and if the amount of retained austenite is small, excellent ductility cannot be obtained. Therefore, there has been an urgent need to develop a technique for stably mass-producing industrially-scale high-strength steel sheets with stable performance that does not cause these inconveniences.

Incidentally, "improvement of C concentration in austenite" necessary for obtaining high work hardening from a low strain region to a high strain region.
For Al, it is preferable to use Al, which has a stronger action of expanding the solid solubility limit of C in austenite than Si, and the residual austenite formed when the solid solubility limit of C in austenite is extended. Since the C concentration in the austenite is also high, it was considered that the Al-added type residual γ steel sheet is more advantageous than the Si-added type in terms of impact absorption characteristics. The behavior is still unclear compared to that of the Si-added type. For example, since Al, which has a strong action of expanding the solid solubility limit of C in austenite, is added, the Si-added type near the high strain range. Although it has a higher work hardening ability than that of the Si-added type, it has a problem that the work hardening ability is smaller in the low strain region than that of the Si-added type. However, since the Al-added residual γ steel sheet still exhibits relatively good work hardening characteristics in a wide range from low strain to high strain area, it stably improves the impact absorption characteristics of the Al-added residual γ steel sheet. I had to say that the establishment of methods was extremely important in industry. In addition to that, in the case of manufacturing a high Al-added steel sheet, "a surface flaw of the slab is likely to occur due to AlN generation during continuous casting,
There is also a problem that this is easily brought to products, and it is an essential item to solve such a problem when manufacturing Al-added residual γ steel sheet for automobile bodies.

From the above, the object of the present invention is to solve the above-mentioned problems of high-strength steel sheets used for automobile bodies and the like, and to enable stable production by a usual method using continuous casting equipment. It was put on providing "high ductility type high strength steel sheet excellent in formability, surface properties and impact absorbing ability".

[0013]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventors of the present invention have conducted researches focusing on the impact absorption characteristics of Al-added residual γ steel sheet, When the contents of C, Mn, P, S and N in the steels were regulated to the proper ranges, respectively, and an appropriate amount of Ti considering the contents of N and S was added, it was found in the Al-added residual γ steel. The above-mentioned problems are wiped out, and while exhibiting excellent moldability, it exhibits high work hardening performance from a low strain range to a high strain range.
It is possible to stably obtain good surface properties even by the manufacturing process that undergoes continuous casting in the usual manner. "

That is, in the Al-added type residual γ steel sheet, the formability could be stabilized in the favorable range by first adjusting the contents of C, Mn, P and S. Addition of an appropriate amount of Ti after regulation has an extremely effective effect on the improvement of surface properties and work hardening performance, and is greatly effective in securing the high level of shock absorption, workability and surface properties required for steel sheets for automobile bodies. I found that I would contribute.

More specifically, for example, the following actions were confirmed as the actions of Ti that can be individually captured when an appropriate amount of Ti is added to Al-added residual γ steel. a) Since N is fixed as TiN by the addition of Ti, the generation of AlN is suppressed, and brittle cracking of the slab during continuous casting is suppressed, so that a steel slab with excellent surface properties can be obtained, and therefore, after cold rolling, The steel sheet also stably has excellent surface properties. b) As mentioned above, Al-added type residual γ steel shows rather high work hardening characteristics in the high strain region, and in the low strain region, work hardening performance is smaller than that of the Si added type, but Ti
When added, the precipitation hardening strengthens the ferrite ground, and the weak point that the work hardening is small in the low strain region is compensated by the increase in the deformation resistance, and the shock absorbing performance is sufficiently excellent.

The present invention has been completed based on the above findings and the like, and provides the following high-ductility-type high-strength cold-rolled steel sheet. (1) C: 0.06 to 0.25%, Si: 0.5 % or more ( excluding 0.5 % ) 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, the balance consisting of Fe and inevitable impurities, and the Ti content is (48/14) N ≤ Ti ≤ (48/14) N + (48 /
32) S + 0.01 is satisfied and the structure after cold rolling-recrystallization annealing is a structure containing 5% or more of retained austenite by volume ratio, surface properties and impact. High ductility high tensile cold rolled steel sheet with excellent absorbency. (2) C: 0.06 to 0.25%, Si: 0.5 % or more ( excluding 0.5 % ) 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 is satisfied, and the structure after cold rolling-recrystallization annealing is a structure containing 5% or more of retained austenite by volume ratio. A high-ductility-type high-tensile cold-rolled steel sheet that has excellent surface properties and shock absorption. (3) C: 0.06 to 0.25 %, Si : 0.5 % or more ( excluding 0.5 % ) 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 , and the balance consisting of Fe and inevitable impurities.
One Ti content (48/14) N ≦ Ti ≦ ( 48/14) N + (48/32) have satisfied S + 0.01 the relationship, cold - structure after recrystallization annealing is
Contains 5% or more of retained austenite by volume
Galvanized steel sheet with cold-rolled steel sheet as a base material . (4) C: 0.06 to 0.25 %, Si : 0.5 % or more ( excluding 0.5 % ) 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 ~ 0.08%, N: to 0.01% or less, further 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 is Fe and inevitable impurities.
And the Ti content is (48/14) N ≤ Ti ≤ (48/14) N + (48/32) S + 0.01 , and the structure after cold rolling-recrystallization annealing is
A structure containing 5% or more by volume of retained austenite
Galvanized steel sheet made from a certain cold-rolled steel sheet .

Needless to say, the high-ductility-type high-tensile-strength cold-rolled steel sheet according to the present invention can be applied as a material for an automobile body, etc. after being recrystallized and annealed after being made into a product by cold rolling. Of course, it can be used in the form of a hot-dip galvanized steel sheet, an electroplated steel sheet, or the like as described above .

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

Si: Si is an element that suppresses the precipitation of cementite without forming a solid solution, and thus delays the transformation from austenite and promotes the concentration of C in austenite. Demonstrate. This action increases the stability of the retained austenite, which facilitates the securing of "retained austenite exhibiting transformation-induced plasticity" at room temperature. However, Si is an oxide-forming element, and if the Si content exceeds 2.5%,
Island scale - flash butt weldability causes Le decrease the pickling property generated by the ing to degradation. It should be noted that the steel sheet of the present invention is premised on the addition of Al, which exhibits the same effect as Si, but the preferable result that the amount of retained austenite produced is increased by the addition of Si can be obtained.
Therefore, it is desirable to add 0.5% or more of Si.

Mn: Mn is an austenite stabilizing element, and also exerts an action of preventing decomposition of austenite into pearlite during cooling of the steel sheet. However, if the Mn content is less than 0.5%, the desired effect due to the above action cannot be obtained.On the other hand, if the Mn content exceeds 3.0%, the hardenability of the steel sheet is excessively increased, resulting in excessive strength increase and ductility deterioration. Therefore, the Mn content was set to 0.5 to 3.0%.

P: P is also an element that affects ferrite formation, like Si and Al, but in the steel sheet of the present invention, Al
Since the addition is essential, it is not necessary to specify the lower limit of the P addition amount. However, if P is contained in excess of 0.1%, the deterioration of the secondary workability becomes remarkable, so P
The content was set to 0.1% or less.

S: S does not affect the formation of retained austenite, but as the amount of S increases, a large number of A-type inclusions are formed, which causes deterioration of the 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 it suppresses the generation of AlN, which is a cause of surface cracking during casting, and exerts the action of precipitating and fixing N as TiN.
The precipitation hardening by this action strengthens the ferrite base and increases the deformation resistance, thereby compensating the work hardening ability in the low strain region and contributing to the improvement of the impact absorption characteristics of the Al-containing residual γ steel. However, when “Ti <(48/14) N”, N cannot be completely fixed as TiN and AlN which causes slab surface cracking is generated. Also, "Ti> (48 /
14) N + (48/32) S + 0.01 "or the Ti content is
If it exceeds 0.08%, the amount of TiC produced increases, the precipitation strengthening causes an excessive increase in strength, and the absolute amount of C enriched in austenite decreases, resulting in austenite due to C enrichment. The effect on the chemical stability of
The amount of stenite produced decreases and ductility deteriorates. by the way,
In order to satisfy the condition of “(48/14) N ≦ Ti” when the Ti content is 0.003%, the N content needs to be less than 0.0009%, and it is not possible to reduce the N content to such a range. It is extremely difficult and impractical at the current steelmaking technology level. Therefore, the Ti content should be adjusted within the range of 0.003 to 0.08% and satisfy the condition of (48/14) N ≤ Ti ≤ (48/14) N + (48/32) S + 0.01. Specified.

Al: Al, like Si, is a component essential for securing stable retained austenite at room temperature.
Also, Al does not form a solid solution in cementite, and is 350 to 600 ° C.
It also suppresses the precipitation of cementite during isothermal holding at the time of bainite transformation and exerts the effect of delaying transformation.
However, since Al has a stronger ferrite forming ability than Si, the transformation start is faster in the case of adding Al than in the case of adding Si, and even if it is maintained for a very short time, it will not be stable during annealing in the two-phase coexisting temperature range. -The C becomes enriched in the austenite. Therefore, when Al is added, the chemical stability of austenite can be further enhanced, the C concentration of austenite produced as a result is increased, and the residual austenite produced is increased.
As the amount of stenite increases, high work hardening characteristics are exhibited even in a high strain region. However, if the Al content is
If it is less than 0.1%, the above effect cannot be sufficiently secured, while if more than 2.5% is contained in Al, it can be used in flash butt welding during continuous pickling / cold rolling continuous line threading. Since the weldability deteriorates and the cost increases, the Al content was set to 0.1 to 2.5%.

N: The content of N has a great influence on the production amount of AlN which deteriorates the surface properties of the steel sheet, and when the N content becomes excessive, the fixation of N by Ti is not achieved and AlN is formed. And the surface quality of the steel sheet deteriorates. When the N content exceeds 0.01%, the amount of Ti required to fix N as TiN increases, and the Ti amount specified in the present invention cannot fix N so that the N content is 0.01.
It is stipulated that the amount should be regulated to below%.

Ni, Cr, Mo, Cu, Nb, V, Zr and B: These components are added as necessary for the purpose of ensuring further strength in the steel sheet, refining the steel sheet structure and improving the corrosion resistance. One kind or two or more kinds are added, and the effects of these individual components are as follows.

A) Ni Like Ni, Ni has a stabilizing effect on austenite, so it can be contained if necessary. However, addition of a large amount causes cost increase and excessive strength increase. , The upper limit was set at 0.1%.

B) Cr Cr, like Mn, also has the effect of stabilizing austenite, so it can be added if necessary. However, addition of a large amount causes an increase in cost and does not require hardenability. However, the upper limit is 1.0.
Defined as%.

C) Mo Mo is an element that, like Si and Al, has an effect on the retained austenite through the function of suppressing the formation of carbides, and the addition thereof suppresses the formation of carbides and the martensite transformation temperature. Has a function of stabilizing the retained austenite by lowering the temperature. Therefore, although it is a component that can be contained as needed, the addition of a large amount causes a cost increase and unnecessarily enhances the hardenability to cause an excessive strength increase. Therefore, the upper limit was set to 0.6%.

D) Cu Since Cu has an action of improving the corrosion resistance of the steel sheet in the non-plated state, it may be added to improve the corrosion resistance of the non-plated steel sheet. However, excessive addition causes deterioration of hot workability due to Cu checking in the slab, and composite addition with Ni is essential to suppress this, so its upper limit is 1.0 Defined as%.

E) Nb Nb has an effect of appropriately suppressing the pearlite transformation of austenite, and thereby exerts the effect of relaxing the cooling rate constraint condition for obtaining retained austenite. . Therefore, it can be included if necessary, but 0.05%
If it is added in excess, the above effect will be saturated, which is economically disadvantageous. Therefore, the upper limit of the Nb content is set to 0.05%.

F) VV also has a function of stabilizing the retained austenite by lowering the martensite transformation temperature, and therefore it can be contained as necessary, but addition of a large amount causes an increase in cost and quenching. It unnecessarily enhances sex and causes 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 inclusions and improving the cold workability of the hot-rolled steel sheet, so it can be contained if necessary. Therefore, the upper limit of Zr is set to 0.05%.

H) BB B has the effect of strengthening the grain boundaries and increasing the secondary work embrittlement resistance, so it can be contained if necessary, but 0.0
If added over 03%, the effect will be saturated. Therefore, B
The upper limit of the content was set to 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. If the volume percentage is less than 5%, the ductility of the austenite due to strain-induced transformation is reduced. No improvement can be expected. Therefore, the structure after cold rolling-recrystallization annealing should be described as "a residual amount of 5% or more by volume.
The structure includes "stenite".

The steel sheet according to the present invention can be manufactured as follows. First, a steel having a desired chemical composition is melted in a converter or the like as in a conventional method, and is continuously cast into a slab, and then hot-rolled while hot, or once cooled to room temperature. Is heated and then hot rolled to obtain a hot rolled steel sheet. The hot rolling may be carried out under the usual conditions, but the winding after the hot rolling is 5 from the viewpoint of the load during cold rolling and pickling property.
It is recommended to set the temperature to 00 to 700 ° C.

The wound hot rolled coil is then pickled in a conventional manner and then subjected to cold rolling. The cold rolling conditions also need not be particularly limited, but the cold rolling rate is preferably 45% or more in view of the stripability during cold rolling.

The cold-rolled steel sheet obtained by cold rolling is subjected to recrystallization annealing before use. In this recrystallization annealing, it is only necessary to adopt the conditions as in the ordinary method applied to obtain the residual γ steel sheet. That is, in the recrystallization annealing, it is necessary to heat to [ferrite + austenite two-phase region] to concentrate C in γ, so the heating temperature is set to the two-phase region temperature. Further, in order to re-dissolve the carbide produced by hot rolling and to enrich the C in austenite, the heating and holding time in the recrystallization annealing is required to be 20 to 50 seconds. Just 150
Annealing for a long time of more than one second is not preferable because it causes deterioration of productivity. Next, in order to increase the concentration of C from ferrite to austenite, the temperature range of 650 to 750 ° C. is gradually cooled at 2 ° C./sec or more. Then, overaging causes the bainite transformation to proceed to further promote the C concentration in the austenite. Here, when the overaging temperature is 480 ° C. or higher, since carbide is formed, C concentration in austenite cannot be achieved, and 35
At 0 ° C or lower, caution is required because ductility is deteriorated due to increased strength. Further, it should be noted that if the overaging time is 120 seconds or less, the bainite transformation does not proceed sufficiently, so that C concentration in austenite cannot be achieved and good characteristics cannot be obtained.

By the way, the high-strength cold-rolled steel sheet according to the present invention produced as described above can be used as it is after recrystallization annealing as a material for automobile bodies or the like, but it is subjected to hot dip galvanization or electroplating. It can also be used after being used. When performing hot dip galvanizing, a heat treatment corresponding to recrystallization annealing may be performed in the plating process.

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

EXAMPLE Steels having various chemical compositions were melted in a converter to form a slab by continuous casting, once cooled to room temperature, soaked again to 1250 ° C., and hot rolled at 1150 to 930 ° C. A 3.5 mm thick hot rolled steel sheet was obtained. Next, the obtained hot-rolled steel sheet was pickled and cold-rolled to obtain a 1.6 mm-thick cold-rolled steel sheet. The chemical composition of the obtained cold rolled steel sheet is shown in Table 1 and Table 2.
Shown in.

[0041]

[Table 1]

[0042]

[Table 2]

Then, this cold rolled steel sheet was subjected to recrystallization annealing using a continuous annealing facility. In the continuous annealing, the cold rolled steel sheet is continuously heated to 820 ° C. and kept at that temperature for 40 seconds, then gradually cooled to 700 ° C. at a cooling rate of 3 ° C./sec, and then 410 ° C. It carried out on the condition hold | maintained at that temperature for 3 minutes.

Some steel grades were galvanized 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 cold-rolled steel sheet thus obtained, the amount of retained austenite (γ), the surface texture of the steel sheet (in the case of a plated steel sheet, the surface texture before plating), the tensile properties,
The hole expandability and impact 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 property" was evaluated by the presence or absence of surface scratches per coil. When no surface scratch was observed, "good" was shown, and when surface scratch was observed, "poor" was shown. Regarding "tensile properties", JIS No. 5 tensile test pieces were taken from each cold rolled steel sheet and subjected to a tensile test to measure the yield strength (YS), tensile strength (TS) and elongation (El). The "hole-expansion property", which indicates the local ductility, is 60 mm for a circular punched hole with a diameter of 10 mm.
° Molded with a conical punch, and evaluated by the hole expansion rate.
"Shock absorption characteristics" is 150mm when a hat-type closed cross-section structural member is formed from each cold-rolled steel sheet by the press brake method and crushed at a speed of 4m / sec using a gas / hydraulic high-speed compression tester. The energy absorbed during displacement was evaluated.

From the results shown in Table 1, Table 2 and Table 3, the following can be confirmed. That is, the cold-rolled steel sheets 1 to 14 which are materials of the present invention all have a residual γ amount of 5% or more, and show good tensile properties and surface properties.

The hole expandability showing local ductility generally decreases with increasing strength, but when comparing the same strength levels, the cold rolled steel sheet 17 as a comparative material has an Al content within the range defined by the present invention. The hole expandability is inferior to that of the cold-rolled steel sheet 4 which is the material of the present invention because it is outside
I understand. This is because in the comparative material, the retained austenite transforms to martensite during punching, increasing the hardness difference between each phase of the structure and generating many microcrack starting points. , It is considered to be due to the propagation.

Further, the shock absorbing property generally improves as the strength increases, but the cold-rolled steel sheets 1 to 14 which are the materials of the present invention are superior to the cold-rolled steel sheets 15 to 22 which are the comparative materials. Regarding the cold rolled steel sheet 20 which is a comparative material,
Since the Ti content is out of the specified range of the present invention, the amount of retained austenite (γ) is small and the characteristics are poor. Furthermore, for the cold-rolled steel sheet 18 , which is a comparative material, since Ti is not added, it is not possible to increase the yield stress due to precipitation strengthening, so the initial impact load during the crushing test is reduced and the strength and residual strength are the same. Cold rolling which is the material of the present invention showing austenite (γ) content and ductility
The shock absorption energy is inferior to the steel plate 4. Regarding the surface texture, cold-rolled steel sheets 18 and 22 which are comparative materials in which the Al content is within the specified range of the present invention but to which Ti is not added are unsatisfactory.

[0051]

[Summary of Effects] As described above, according to the present invention, it is possible to stably provide a high-ductility-type high-tensile cold-rolled steel sheet and a galvanized steel sheet which are excellent not only in formability but also in surface properties and shock absorption. Therefore, industrially useful effects are brought about, such as being able to fully meet the demand for weight reduction of automobile bodies and the like and improvement of shock absorption.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-5-247586 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. A weight ratio of C: 0.06 to 0.25%, Si: 0.5 % or more ( excluding 0.5 % ) 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1-2.5%, Ti: 0.003-0.08%, N: 0.01% or less and the balance Fe and unavoidable impurities, and the Ti content is (48/14) N ≤ Ti ≤ (48/14 ) N + (48/32) S + 0.01 is satisfied, and the structure after cold rolling-recrystallization annealing is a structure containing 5% or more of retained austenite by volume ratio. , High ductility high tensile cold rolled steel sheet with excellent surface properties and shock absorption. 2. C: 0.06 to 0.25%, Si: 0.5 % or more ( excluding 0.5 % ) 2.5% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.03% or less, by weight ratio. Al: 0.1-2.5%, Ti: 0.003-0.08%, 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 is Fe and inevitable impurities, and the Ti content is (48/14) N. ≤ Ti ≤ (48/14) N + (48/32) S + 0.01, the structure after cold rolling-recrystallization annealing contains 5% or more of retained austenite by volume. Is a high ductility high tensile cold rolled steel sheet having excellent surface properties and shock absorption. 3. C: 0.06 to 0.25 % by weight , Si : 0.5 % or more ( excluding 0.5 % ) 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 , and the balance is Fe and inevitable impurities.
One Ti content (48/14) N ≦ Ti ≦ ( 48/14) N + (48/32) have satisfied S + 0.01 the relationship, cold - structure after recrystallization annealing is
A structure containing 5% or more by volume of retained austenite
Galvanized steel sheet made from a certain cold-rolled steel sheet . 4. By weight ratio, C: 0.06 to 0.25 %, Si : 0.5 % or more ( excluding 0.5 % ) 2.5 % or less, Mn : 0.5 to 3.0 %, P: 0.1 % or less, S: 0.03 % or less, al: 0.1 ~ 2.5%, Ti : 0.003 ~ 0.08%, N: to 0.01% or less, further 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 is Fe and inevitable impurities.
And the Ti content is (48/14) N ≤ Ti ≤ (48/14) N + (48/32) S + 0.01 , and the structure after cold rolling-recrystallization annealing is
A structure containing 5% or more by volume of retained austenite
Galvanized steel sheet made from a certain cold-rolled steel sheet.