JP3020617B2 - Ultra-strength cold-rolled steel sheet with good bending workability and impact properties and method for producing the same - Google Patents

Abstract

PURPOSE:To improve the bendability and impact properties of a steel sheet by subjecting a steel slab in which C, Si, Mn, Cr, Nb, B, Fe or the like are specified to hot rolling at prescribed heating temp., finishing temp. and coiling temp. and furthermore executing soaking, rapid cooling and gradual cooling. CONSTITUTION:A steel having a compsn. constituted of, by weight, 0.1 to 0.2% C, <=0.2% Si, 2 to 3% Mn, 0.2 to 1% Cr, 0.005 to 0.05% Nb, 0.0003 to 0.002% B, 0.02 to 0.1% Al, etc., and the balance Fe or the like is melted. The slab of this steel is heated to >=1200 deg.C, is hot-rolled, is subjected to finish rolling at >=800 deg.C, is thereafter cooled and is coiled at 550 to 750 deg.C. Next, the steel is pickled and is cold-rolled at >=40% draft. Successively, this rolled stock is heated at >=5 deg.C/sec heating rate and is subjected to continuous annealing at 800 to 900 deg.C for 20 to 300sec. Next, the steel is rapidly cooled to 300 to 450 deg.C at >=20 deg.C/sec cooling rate and is gradually cooled to <=200 deg.C at >=5 deg.C/sec cooling rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an automobile
Relatively light working, door impact beams high strength is required, with the bumper preferred TS 100 kgf / mm ² or more, the bending of the yield ratio of 80% or less workability, good ultra high strength cold rolled impact properties The present invention relates to steel and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, as a method for producing an ultra-high strength cold-rolled steel sheet, for example, there are the following methods, each of which has a problem.

First, as a method for producing a cold-rolled steel sheet having a tensile strength exceeding 100 kgf / mm ² , a so-called water quenching method as disclosed in Japanese Patent Application Laid-Open No. 62-13533 is used. Is applied to high-strength steel. However, although this method has an advantage that it can be manufactured relatively inexpensively, it has a problem that a large strain occurs due to rapid cooling, and the uniformity of the material in the width direction is inferior. Had a problem that the embrittlement was remarkable after molding.

Japanese Patent Application Laid-Open No. 51-86015 discloses a method of batch annealing a steel sheet to which a large amount of Si is added. In this case, although the ductility is excellent, it is caused by the scale due to the inclusion of Si. The surface defects are remarkable, leading to a decrease in the reliability of strength members such as door impact beams. Also, in such high Si steels, the deterioration of the shape due to the deterioration of rolling workability is also a major problem, and the cause is not clear, but the band structure develops under certain hot rolling conditions, Bending workability was deteriorated due to development of anisotropy and the like, which had a problem in use.

Further, Japanese Patent Application Laid-Open No. 62-182224 discloses that
A method is disclosed in which a heat treatment is performed using a steel sheet containing C, Si, and Mn to form a composite structure including retained austenite, ferrite, bainite, and martensite. In this case, however, a considerably special heat treatment condition is required. Also, there were problems such as difficulty in welding.

JP-A-59-143027 discloses that C, Mn
A method is disclosed in which a steel sheet containing P and P and a steel sheet to which an additive element is added are heat-treated to obtain a composite structure mainly composed of ferrite, bainite, and the like. However,
In this composition, when TS is set to 100 kgf / mm ² or more, there is a problem that the impact characteristics are poor. Although the cause is not necessarily clear, it is presumed that one of the causes is that a large internal stress remains due to an inappropriate cooling pattern.

[0007]

SUMMARY OF THE INVENTION The present invention provides a TS 100 kg
In an ultrahigh-strength cold-rolled steel sheet having a f / mm ² or more and a yield ratio of 80% or less and a method for producing the same, the following items including the above-mentioned problems are to be improved.

[0008] Bending workability Impact properties Yield ratio (reduced) Weldability Uniformity of materials What can be manufactured in a conventional continuous furnace

Here, the characteristics from (1) to (4) are characteristics required for strength members of automobiles, and are indispensable for securing the reliability of the members. Not economically good.

[0010]

SUMMARY OF THE INVENTION The present invention provides high strength, low yield ratio, high ductility, good bending workability, and good impact characteristics.
In order to obtain a cold-rolled steel sheet with a good balance of materials without particularly large disadvantages, the composition of the steel is limited, and C, Mn, Cr, N
b, B components are added appropriately to form a fine and uniform bainite structure containing 5% to 15% of retained austenite. In the production, the hot rolling, cold rolling and annealing conditions are optimized. Things.

That is, the gist is as follows: C: 0.10 wt% to 0.20 wt%, Si: 0.20 wt% or less, Mn: 2.0 wt% to 3.5 wt%, Cr: 0.20 wt% to 1.00 wt%, Nb : 0.005 wt% to 0.050 wt%, B: 0.0003 wt% to 0.0020 wt%, Al: 0.020 wt% to 0.100 wt%, P: 0.020 wt% or less, S: 0.010 wt% or less, and N: 0.0150 wt% % contained the following, the balance being a composition of iron and unavoidable impurities, the remainder contain residual austenite with 5% to 15% or less has a structure consisting of bainite, TS 100 kgf / mm ² or more, 80% or less yield ratio It is an ultra-high tensile strength cold rolled steel sheet having good bending workability and impact properties characterized by having the following characteristics: In addition, Ti: 0.050 wt% or less is added to the above component composition.

Further, the steel slab having each of the above-mentioned component compositions is heated to 1200 ° C. or more, hot-rolled, subjected to finish rolling at a temperature of 800 ° C. or more, cooled, and then cooled to 750 ° C. or less to 550 ° C.
It is wound on a coil at the above-mentioned predetermined temperature, then pickled, cold-rolled at a rolling reduction of 40% or more, and subsequently heated at a heating rate of 5 ° C./sec or more following this cold rolling. , 800
After performing continuous annealing at a predetermined temperature of not less than ℃ 900 and not more than 900 ° C for a time of not less than 20 seconds and not more than 300 seconds, quenching to a predetermined temperature of not more than 450 ℃ and not less than 300 ℃ at a cooling rate of not less than 20 ℃ / Subsequently, it is a method for producing an ultra-high strength cold rolled steel sheet having good bending workability and impact properties, characterized by gradually cooling to a temperature of 200 ° C or less at a cooling rate of 5 ° C / second or less.

[0013]

First, the reasons for limiting the component composition of the present invention will be described.

C: Inexpensive reinforcing component. If the content is less than 0.10 wt%, the effect of suppressing the ferrite transformation and increasing the strength of bainite is reduced due to the relatively low alloy system, and high strength cannot be obtained. If it exceeds 0.20 wt%, spot weldability and impact The characteristics deteriorate. Therefore its content is
0.10 wt% or more and 0.20 wt% or less.

Si: A powerful reinforcing component, however, if it exceeds 0.20 wt%, it inhibits cold rolling and causes deterioration of surface properties due to its scale. Therefore, the upper limit of its content is 0.
20 wt%.

Mn: stabilizes the austenite phase, suppresses ferrite transformation, forms a bainite-based structure, refines the structure, and improves strength, bending workability, impact characteristics, and the like. If the content is less than 2.0 wt%, the above effect is insufficient, and 3.
When the content exceeds 5 wt%, the effect is saturated and the cost is increased. Therefore, the content is 2.0 wt% or more and 3.5 w%.
t% or less.

Cr: One of the important components in the present invention. The effect is similar to that of Mn and is added for stabilizing the austenite phase. Then, the effect of Mn is supplemented, and the effect is further improved. In particular, it is considered to contribute to the stabilization of the austenite phase in a lower temperature range than Mn. In order to secure the target austenite of 5% to 15%, which is the target of the present invention, and to obtain good workability, the content needs to be 0.20% by weight or more. However, if the content exceeds 1.00% by weight, the hot-rolled sheet hardens remarkably, hindering the cold rolling, deteriorating the chemical conversion property, saturating the stabilization tendency of the austenite phase, and increasing the cost. May be invited. Therefore its content is 0.20
The content should be not less than wt% and not more than 1.00 wt%.

Ti: Effective for avoiding cracking of the slab and effective for homogenizing the structure. However, 0.050wt%
If added in excess of, the weldability will deteriorate. Therefore, its content should be 0.050 wt% or less.

Nb: One of the important components in the present invention. It refines and homogenizes the structure and contributes to high strength by improving hardenability. If the content is less than 0.005 wt%, the effect is insufficient, and if the content exceeds 0.050 wt%, the effect is saturated and hot rolling and cold rolling become difficult. Therefore, its content should be not less than 0.005 wt% and not more than 0.050 wt%.

B: One of the important components of the present invention.
Although the reason is not necessarily clear, the combined addition with Mn and Nb provides a large increase in strength that cannot be obtained by single addition. To obtain such an effect, 0.0003 wt% or more is required. However, if it is added in excess of 0.0020 wt%, the effect is saturated, and the structure becomes coarse and the impact characteristics deteriorate. Therefore its content is 0.0003wt
% And 0.0020 wt% or less.

Al: Necessary as a deoxidizing component of steel, the effect of cleaning steel and refining the structure together is insufficient if it is less than 0.02 wt%, and if it exceeds 0.100 wt%, alumina Surface defects due to the formation of clusters become significant. Therefore, its content is more than 0.020wt% and 0.100wt%
The following is assumed.

P: Has a strong tendency to segregate, particularly in high-Mn steel, and causes deterioration in bending workability and spot weldability. In order to suppress such material deterioration, the upper limit is made 0.020 wt%. The lower limit is not particularly limited, but a lower content is preferable in terms of material.On the other hand, reducing the content is a factor of cost increase. Good.

S: Exists in steel as MnS and is extremely harmful to ductility due to its stretched shape, the degree of which increases with increasing strength of the steel sheet. For steel with a TS of 100 kgf / mm ² or more, its content needs to be 0.010 wt% or less, and preferably 0.003 wt% or less.
In addition, like P, the lower limit is not particularly limited,
The smaller the content, the better in terms of the material. On the other hand, reducing the content is a factor in increasing the cost. Therefore, in practical use, the components should be designed in consideration of these factors.

N: Although a certain amount is unavoidable, it is an austenite phase stabilizing component and is not particularly a component to be removed. However, if the content exceeds 0.0150 wt%, the strength as a hot-rolled sheet becomes too large, which hinders cold rolling. Therefore, the upper limit of the content is set to 0.0150 wt% or less.

Next, the reasons for limiting the steel structure will be described.
In order to improve the bending workability and the impact characteristics aimed at by the present invention, it is necessary to form a fine and uniform bainite structure containing 5% or more and 15% or less of retained austenite.

If a suitable heat treatment is carried out using the steel of the present invention, a uniform bainite structure can be obtained.
Impact properties are not always sufficient. On the other hand, the impact characteristics are also improved by forming a structure containing the retained austenite in the uniformly fine bainite structure. The reason for this is not always clear, but it is presumed that the retained austenite resists the propagation of brittle cracks through its martensitic transformation. If the retained austenite is less than 5%, the effect is insufficient, and if it exceeds 15%, problems such as long-term dimensional change occur in practical use.

Next, the reasons for limiting the manufacturing conditions will be described. At the time of annealing after hot rolling and cold rolling, the slab heating temperature is set to 1200 ° C. or higher in order to secure a sufficient amount of solute Nb and obtain a desired fine bainite structure.

If the finish rolling temperature is too low, the hardenability improving components Nb and B cannot be ensured in a solid solution state, resulting in deterioration of the material. In the steel of the present invention, the lower limit is set to 800 ° C., since the deterioration of the material becomes remarkable at 800 ° C. as a boundary. The upper limit is not particularly limited, but is preferably set to 950 ° C. because the difficulty of the hot rolling step increases.

The coil winding temperature after hot rolling has a relatively small effect on the material, but when it exceeds 750 ° C., the thickness of the scale increases remarkably, and not only the pickling property is lowered, but also the surface properties are reduced. This can also lead to surface defects of the parts after press molding. On the other hand, if it is below 550 ° C,
The hot-rolled sheet hardens abnormally, and significantly impairs cold-rolling properties. Therefore, the upper limit of the temperature is 750 ° C and the lower limit is 550 ° C.

After the hot rolling, the cold rolling reduction in the cold rolling following the pickling needs to have a lower limit of 40% in order to make the structure fine and obtain a fine and uniform bainite structure. In addition, there is no demerit on the material due to the excessively low rolling reduction, but the increase in the rolling reduction is a hindrance factor of the process, and there is little merit of improving the material. Therefore, in practical use, it is preferable to set in consideration of these factors.

The desired structure can be made uniform by increasing the heating rate in the annealing after cold rolling.
This is advantageous for preventing deterioration in bending workability and impact characteristics due to the increase in strength. In order to obtain this effect, the lower limit of the heating rate is set to 5 ° C./sec. The upper limit is not particularly limited, but there is no deterioration of the material due to the speed.

The soaking temperature of this annealing is one of the important factors. If this temperature is too low, the structure becomes extremely non-uniform, and the bending workability and the impact properties are remarkably deteriorated. Also,
If this temperature is too high, the structure is locally coarsened during the soaking, resulting in a non-uniform structure, and the bending workability and impact characteristics are also deteriorated. Therefore, the lower limit of the soaking temperature is 800 ° C., preferably 850 ° C., and the upper limit is 900 ° C.

The soaking time at the above temperature requires a soaking time of 20 seconds or more in order to homogenize the structure and stabilize the material. However, if the soaking time is excessively long, bending workability will be degraded due to local coarsening of the structure, etc. In addition, there will be problems such as impeding productivity during operation, so the upper limit is 300 seconds. Good. Therefore, the soaking time is 20 seconds or more and 300 seconds or less.

After annealing, it is rapidly cooled. The cooling rate must be quenched at 20 ° C./sec or more in order to suppress the ferrite transformation during cooling to obtain a uniform bainite structure containing retained austenite and obtain good strength characteristics. Although the upper limit is not particularly limited, the material has no influence on the material by making it faster.

The quenching stop temperature must be 300 ° C. or higher in order to suppress martensitic transformation and prevent deterioration in bending workability and impact characteristics by partially utilizing the self-tempering effect of bainite. is there. On the other hand, 450
If the temperature exceeds ℃, a sufficiently high strength cannot be obtained. Therefore, the quenching stop temperature should be below 450 ° C and above 300 ° C.

After the rapid cooling is stopped, the cooling is gradually performed. However, to prevent the deterioration of bending workability and impact characteristics due to the above-mentioned effect of suppressing martensitic transformation and self-tempering, the temperature is reduced to 200 ° C. or less, 5 ° C./sec or less. Slow cooling at the speed of Slow cooling may be performed at lower temperatures, but the limit is determined by restrictions on equipment, and there is no adverse effect on the material if the temperature is 200 ° C. or less.

[0037]

EXAMPLE 1 Five types of steel (Table 1) and 12 types of comparative steels (Table 2) of the present invention having the composition shown in Tables 1 and 2 were melted to form steel slabs. A slab was used as a raw material, and a cold-rolled steel sheet having a thickness of 0.8 mm was formed under the hot rolling and cold rolling conditions shown in Table 3 and annealed under the annealing conditions shown in Table 4.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

With respect to these annealed steel sheets, tensile properties, structures, bending workability, and impact properties were examined. Tensile properties were measured by a normal procedure using a JIS No. 5 test piece, bending workability was evaluated by changing the bending radius, bending at 180 °, and evaluating the critical bending radius at which cracks occurred. .
Squeeze the conical cup with 8 and add
A weight of kg was dropped from a height of 80 cm and collided to determine whether or not cracks occurred.

The results of these investigations are shown in Table 5 (Compliant Example) and Table 6 (Comparative Example).

[0044]

[Table 5]

[0045]

[Table 6]

As is clear from Table 6, the comparative examples are inferior in tensile properties, bending workability, impact properties, etc., are difficult to perform cold rolling in the manufacturing process, and
In some cases, a post-process chemical conversion failure occurs.

On the other hand, the applicable examples of the present invention in Table 5 are as follows.
While having a sufficiently high tensile strength (TS), it has good ductility (EL), bendability, and excellent impact properties.

In particular, considering that the impact characteristics are extremely important considering that they are used for door impact beams, bumpers, and the like that require an impact absorbing ability,
The conforming examples can be said to be suitable for use in these applications.

It has been found that YR is related to the continuous annealing temperature, and that the higher the annealing temperature, the higher the YR.

Example 2 Steel having the composition shown in Table 7 conforming to the present invention was smelted into a steel slab, and the steel slab was used as a material to obtain a sheet thickness under the hot rolling, cold rolling and annealing conditions shown in Table 8. Manufacture 1.4 mm steel sheet,
Tensile properties and bending workability were examined under the same conditions as in Example 1.

[0051]

[Table 7]

[0052]

[Table 8]

The results are as shown in Table 9. As shown in Table 9, in the case of conforming to the production conditions of the present invention, the desired tensile properties and good bending workability were obtained, but the comparative examples deviated from the production conditions of the present invention. Examples each have inferior tensile properties and bendability, or have manufacturing problems.

[0054]

[Table 9]

An attempt was made to use a steel sheet according to the present invention for an actual part (bumper). However, since the YS is low, the shape accuracy after molding is good, and further, the damage to the press die is small. Was confirmed.

[0056]

According to the present invention, by optimizing the component composition and optimizing the hot rolling, cold rolling and annealing conditions,
It is intended to obtain an ultra-high tensile cold rolled steel sheet with good bending workability and impact characteristics of TS 100 kgf / mm ² or more and a yield ratio of 80% or less. The steel sheet obtained by this invention needs high strength and shock absorption capacity. It is suitable for use in automobile impact members such as door impact beams and bumpers.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02 C21D 9/46

Claims (4)

(57) [Claims] 1. C: 0.10% to 0.20% by weight, Si: 0.20% by weight or less, Mn: 2.0% to 3.5% by weight, Cr: 0.20% to 1.00% by weight, Nb: 0.005% by weight Not less than 0.050 wt%, B: 0.0003 wt% to 0.0020 wt%, Al: 0.020 wt% to 0.100 wt%, P: 0.020 wt% or less, S: 0.010 wt% or less, and N: 0.0150 wt% or less and, the balance being the composition of iron and inevitable impurities, unrealized balance of residual austenite of 15% or less than 5% has a structure consisting bay <br/> night, TS 100 kgf / mm ² or more, a yield ratio of 80 % High-strength cold-rolled steel sheet with good bending workability and impact properties, characterized by having properties of not more than 10%. 2. C: 0.10% to 0.20% by weight, Si: 0.20% by weight or less, Mn: 2.0% to 3.5% by weight, Cr: 0.20% to 1.00% by weight, Ti: 0.050% by weight Nb: 0.005 wt% to 0.050 wt%, B: 0.0003 wt% to 0.0020 wt%, Al: 0.020 wt% to 0.100 wt%, P: 0.020 wt% or less, S: 0.010 wt% or less, and N : containing less 0.0150Wt%, the balance being the composition of iron and inevitable impurities, unrealized balance of residual austenite of 15% or less than 5% has a structure consisting bay <br/> night, TS 100 kgf / mm ² or more, satisfactory ultra high strength cold-rolled steel sheet formability, impact bending properties and having the following characteristics 80% yield ratio. 3. C: 0.10% to 0.20% by weight, Si: 0.20% by weight or less, Mn: 2.0% to 3.5% by weight, Cr: 0.20% to 1.00% by weight, Nb: 0.005% by weight Not less than 0.050 wt%, B: not less than 0.0003 wt% and not more than 0.0020 wt%, Al: not less than 0.020 wt% and not more than 0.100 wt%, P: not more than 0.020 wt%, S: not more than 0.010 wt% and N: not more than 0.0150 wt% The remainder is made of steel and steel slabs adjusted to the composition of unavoidable impurities, heated to 1200 ° C or higher, hot-rolled, subjected to finish rolling at a temperature of 800 ° C or higher, cooled, and cooled to 750 ° C or lower. It is wound around a coil at a predetermined temperature of 550 ° C. or higher, then pickled, cold-rolled at a rolling reduction of 40% or more, and, following this cold rolling, at a heating rate of 5 ° C./second or more. Heat at a specified temperature between 800 ℃ and 900 ℃
After continuous annealing with soaking for 20 seconds or more and 300 seconds or less, it is rapidly cooled to a predetermined temperature of 450 ° C or less and 300 ° C or more at a cooling rate of 20 ° C / second or more, and then cooled to 5 ° C / second or less. A method for producing an ultra-high-strength cold-rolled steel sheet having good bending workability and impact properties, characterized by gradually cooling to a temperature of 200 ° C. or less at a speed. 4. C: 0.10% to 0.20% by weight, Si: 0.20% by weight or less, Mn: 2.0% to 3.5% by weight, Cr: 0.20% to 1.00% by weight, Ti: 0.050% by weight Nb: 0.005 wt% or more and 0.050 wt% or less, B: 0.0003 wt% or more and 0.0020 wt% or less, Al: 0.020 wt% or more and 0.100 wt% or less, P: 0.020 wt% or less, S: 0.010 wt% or less, and N: 0.0150wt% or less, the remainder is made of steel and slab prepared to the composition of iron and unavoidable impurities, hot-rolled by heating to 1200 ° C or more, and finish rolling at a temperature of 800 ° C or more. After cooling, it is wound around a coil at a predetermined temperature of 750 ° C or lower and 550 ° C or higher, then pickled, cold-rolled at a rolling reduction of 40% or more, and, following this cold rolling, a heating rate of 5 ° C / Heat at a heating rate of at least 800 seconds and at a specified temperature of 800 ° C to 900 ° C.
After performing continuous annealing for soaking for 20 seconds or more and 300 seconds or less, it is rapidly cooled to a predetermined temperature of 450 ° C or less and 300 ° C or more at a cooling rate of 20 ° C / second or more, and then cooled to 5 ° C / second or less. A method for producing an ultra-high-strength cold-rolled steel sheet having good bending workability and impact properties, characterized by gradually cooling to a temperature of 200 ° C. or less at a speed.