JP2868870B2 - High tensile cold rolled steel sheet and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet having a tensile strength of 38 kgf / mm ² or more, which is suitable for use as an outer plate and an inner plate of an automobile, particularly as a strength member. It is.

In recent years, an outer plate or inner plate of the motor vehicle, for the parts relatively harsh moldability is required, but TS is 35 kgf / mm ² or more high-strength cold-rolled steel sheet is widely used, further,
In response to demands for reducing body weight, higher strength steel sheets (TS40kg
The requirement of f / mm ^{2 to} 45 kgf / mm ² ) is increasing. And even such a high-tensile steel sheet is required to have good stretch flangeability at the same time.

(Prior Art) Many techniques have been proposed for high-tensile cold-rolled steel sheets having good workability. In these methods, P with little material deterioration is added as a strengthening component. However, in a method of performing batch annealing of a steel sheet to which P is added, the strengthening element is P.
And a part of Mn, and a large amount of P is added, so there is a problem that a surface defect occurs after processing due to macrosegregation,
Further, when the surface concentration of the added component is extremely severe, there is a problem that the chemical conversion property is deteriorated. Another major problem is that the batch annealing process itself is inferior in productivity.

On the other hand, when performing the continuous annealing method which is excellent in productivity, if low C steel is used, sufficient properties such as deep drawability and aging property cannot be obtained.

Therefore, when the continuous annealing method is used, an extremely low C steel having a smaller amount of C than that of the low C steel is used. However, when an extremely low C steel is used, the strength of the steel sheet is low as it is, so that a large amount of strengthening components, P , Si, Mn, etc. need to be added.

As an example using extremely low C steel, Mn, P is added as a basic reinforcing component in JP-A-61-104031, and Si, Mn, P is added as a basic reinforcing component in JP-A-63-243226. There is disclosed a technique using a steel as described above. However, since it contains a large amount of the above-mentioned reinforcing components, there are problems that deterioration of secondary working brittleness resistance, chemical conversion treatment properties, spot weldability and the like are inevitable, and that the production cost is not necessarily inexpensive. .

(Problems to be Solved by the Invention) The present invention provides a workability, which can be manufactured by a continuous annealing process without using an extremely low C steel having many problems.
It is an object of the present invention to provide a low-yield-ratio high-tensile cold-rolled steel sheet having good stretch flangeability and less aging deterioration, and a method for producing the same.

(Means for Solving the Problems) The present invention is characterized in that steel having a high level of C content, which has not been used conventionally, is used for inner and outer panels of automobiles. By using steel, problems such as chemical conversion property and secondary working brittleness are solved.

In addition, the formation of band structure is eliminated by using low Mn steel, and aging and stretch flangeability (side bend elongation)
By increasing the hot-rolling winding temperature, the agglomeration of carbides is promoted to obtain relatively good drawability (high value). Further, by limiting the continuous annealing temperature, Proper dispersion of carbides and desirable texture are obtained.

That is, the present invention provides a high C / low Mn steel-high temperature winding-
It is based on a combination of continuous annealing in an appropriate temperature range. The gist is as follows: C: 0.10 wt% or more, 0.20 wt% or less, Si: 0.10 wt% or less, Mn: 0.30 wt% or less, Al: 0.015 wt% or more , 0.100 wt% or less, P: 0.06 wt% or less, S: 0.005 wt% or less, and N: 0.0030 wt% or less, with the balance consisting of iron and unavoidable impurities and a tensile strength of 38 kgf / mm A high-tensile cold-rolled steel sheet characterized by having a characteristic of not less than ² and a yield ratio of not more than 70%. Further, the present invention provides: C: 0.10 wt% or more, 0.20 wt% or less, Si: 0.10 wt% or less, Mn: 0.30 wt% or less, Al: 0.015 wt% or more, 0.100 wt% or less, P: 0.06 wt% or less, S: 0.005 wt% or less, and N: 0.0030 wt% or less, with the balance being iron and Using steel adjusted to the unavoidable impurity composition as a material, hot-rolling, finishing rolling at a finishing temperature of 800 ° C or more, then cooling at a rate of 10 ° C / s or more, 600 ° C or more to 750 ° C
Coiled at a temperature of less than ℃, then cold-rolled at a reduction of more than 50%, and then 700 ℃ or more by continuous annealing.
A method for producing a high-tensile cold-rolled steel sheet, wherein annealing is performed at a temperature of 50 ° C. or lower.

(Action) First, the reasons for limiting the component composition range in the present invention will be described.

C: a required component for high tension of, is less than 0.10 wt%, to obtain a TS38kgf / mm ² or more, to suppress the aging property low, the yield ratio it is difficult to 70% or less, 0.20
If it exceeds wt%, the spot weldability deteriorates significantly. Therefore, its content should be 0.10 wt% or more and 0.20 wt% or less.

Si: An extremely advantageous component for achieving high tension while ensuring ductility, but excessive addition causes deterioration of toughness and surface properties. Therefore, the content is 0.10 wt% or less.

Mn: The content of Mn is one of the important constituent elements in the present invention. That is, conventional high-strength cold-rolled steel sheets require addition of 0.50 wt% or more from the viewpoint of strength, and particularly when the amount of C is low, there is a tendency to increase Mn to supplement the strength. And, by increasing the amount of Mn, the band structure remarkably develops, thereby deteriorating the side bend elongation.
There is a problem that the aging property is significantly deteriorated.

However, in the present invention, it is not necessary to add a large amount of Mn in terms of strength by setting the C content to 0.10 wt% or more and 0.20 wt%, and the Mn content is 0.30 wt% or less, preferably 0.20 wt%.
By setting as below, a very uniform microstructure can be obtained, and in addition, cementite of a hot-rolled sheet can be coarsely aggregated, and stretch flangeability (side bend elongation),
Deep drawability, aging, etc. can be improved.

Therefore, the content is 0.30 wt% or less,
0.20 wt% or less is desirable.

The lower limit is not particularly limited, but may be any content that can prevent the generation of FeS and prevent hot brittleness. If a low-temperature slab reheating treatment method (low SRT process) is applied, the content is further increased. Can be reduced.

Al: Necessary as deoxidized, 0.015wt% in terms of workability
The above addition is necessary. However, if it is added in excess of 0.100 wt%, deterioration of the surface properties tends to occur due to alumina clusters. Therefore, the content is 0.015 wt% or more and 0.100 wt% or less.

P: Although effective as a strengthening component, it is regulated in terms of spot welding and yield ratio, and its content is limited to an upper limit of 0.06 wt%.

The lower limit is not particularly limited, but is 0.005w in view of cost.
A value around t% is a reasonable value.

It is necessary to keep S: Mn at a low level.
Further, it is necessary to reduce the workability and the stretch flangeability, and the upper limit of the content is 0.005 wt%.

The lower limit is not particularly limited, but is 0.001 in view of cost.
The values before and after are reasonable values.

N: From the viewpoints of workability and ductility, it is desirable to reduce as much as possible, but if it is 0.0030% by weight or less, satisfactory characteristics can be obtained. Therefore, the upper limit is made 0.0030 wt%.

 Next, the manufacturing conditions of the present invention will be described.

First, among the hot rolling conditions, the finish rolling temperature, the subsequent cooling rate, and the winding temperature are particularly important.

Since the transformation point is lowered due to the high carbon content, lower temperatures are possible, but processing in the low temperature region of austenite makes the distribution of the second phase (pearlite) non-uniform and increases in-plane anisotropy. Therefore, it is necessary to set the finish rolling temperature to 800 ° C. or more because the stretch flange characteristic is deteriorated.

The cooling rate after hot rolling must be 10 ° C./s or more so that the ferrite transformation occurs unevenly and a remarkable band structure is not formed.

If the winding temperature is less than 600 ° C, sufficient workability (especially the value) cannot be obtained, and if it exceeds 750 ° C, the carbide becomes too coarse and the stretch flangeability is deteriorated. This leads to deterioration and is not desirable. Therefore, it is necessary to perform high-temperature winding at 600 ° C. or more, preferably 640 ° C. or more and 750 ° C. or less.

Cold rolling reduction rate is 50% to obtain proper recrystallization texture.
The above is necessary.

The continuous annealing temperature needs to be 700 ° C. or higher to obtain sufficient ductility. However, when annealing at a temperature exceeding 850 ° C. is considered to be due to a marked increase in the austenite phase generated during annealing, YP increases and deterioration of aging becomes significant. Therefore, its temperature is 700 ℃
Not less than 850 ° C.

Here, the steel sheet obtained by the present invention is late-aged at room temperature, but has a bake hardenability of 3 kgf / mm ² or more in a so-called paint baking treatment after heating at 170 ° C. for 30 minutes after a pre-strain of 2%. .

This room-temperature aging characteristic is characterized in that the recovery of the yield point elongation is particularly slow, and the reason is not always clear. However, since the hard phase is finely and uniformly dispersed, a so-called dual-phase steel (dual-phase steel) is used.
phase steel) (based on non-uniform strain around the hard phase, non-uniform solute atom distribution).

(Example) Example 1 A total of 12 types of steel, 4 types of compatible steel of the present invention and 8 types of comparative steel, were melted in a converter to form a continuous cast slab, and then hot-rolled, cold-rolled, and annealed. A cold-rolled sheet having a thickness of 0.8 mm was manufactured.

Table 1 shows the chemical composition of these steels, and Table 2 shows the production conditions such as hot rolling, cold rolling and annealing.

Tensile properties, side bend elongation, chemical conversion properties, spot weldability, etc. were investigated for steel sheets that had been subjected to accelerated aging treatment after skin pass.

For the tensile properties, the yield strength, tensile strength, elongation, etc. were measured using a JIS No. 5 test piece.

Here, the side bend elongation is the test piece size: width: plate thickness × 40 length: 170 mm, both sides of the test piece are constrained, side bend is performed at a bending radius of 10 mm, and the steel sheet when cracks occur in the test piece The elongation of the side edges was measured.

The chemical conversion treatment was performed by phosphate treatment with a usual dipping treatment, washing with water, and drying, and then judgment of usual “coating weight measurement”, “P ratio measurement”, “crystal size measurement”, and “visual”.

The term “visual” refers to a case where a chemical conversion treatment cave is not formed soundly in a visual determination. It can be said that this is typical of poor chemical conversion treatment.

Spot weldability is measured by RWMA (Resistance Welding Manufacture).
Welding conditions were evaluated in accordance with the recommended conditions of the Acture Association (shear tension, cross tension) with the joint strength: welding time: 8 cycles Pressure: 200 electrodes: 5 mmφCF.

 Table 3 shows the results of these investigations.

From Table 3, the comparative example in which the steel composition is out of the component composition range of the present invention is inferior in any of the survey items, whereas the conforming example using steel within the component composition range of the present invention is: All show satisfactory results.

Example 2 Two types of compatible steel of the present invention having the component compositions shown in Table 4 were melted in a converter to form a continuous cast slab.
Cold-rolled sheets were produced under a total of 12 kinds of manufacturing conditions, five kinds of conforming examples of the present invention and seven kinds of comparative examples.

The tensile properties, AI (age hardening coefficient), side vent elongation, values, BH (bake hardenability), etc. of these steel sheets were investigated.

Here, the tensile properties and side bend elongation were performed in the same manner as in Example 1. AI was pre-strained at 7.5%, and then aged at 100 ° C for 30 minutes.
Measure the difference between the strain and the deformation stress before and after aging. BH is 2.0% pre-strained and then aged at 170 ° C for 20 minutes.
The difference between the strain and the deformation stress before and after aging was measured.

 Table 6 summarizes the results of these investigations.

As is evident from Table 6, the comparative examples that deviate from the production conditions of the present invention are inferior in YR, side bend elongation, and values, respectively, but the conforming examples of the present invention all show good values. ing.

Incidentally, if S was sufficiently reduced as in this example, no deterioration of the surface properties was observed even at the normal slab heating temperature, but in order to obtain good surface properties, the slab heating temperature was as low as possible. Is preferably reduced.

(Effects of the Invention) According to the present invention, a high-temperature and high-temperature winding using a high-C, low-Mn steel having a high level of C content, which has not been conventionally used, for an inner plate and an outer plate of an automobile, is performed. By performing continuous annealing at low temperature, it is possible to obtain a low yield ratio, high tensile strength cold rolled steel sheet with excellent workability, stretch flangeability, aging resistance, and chemical conversion treatment properties. It is suitable for use as a strength member of an outer plate.

Claims (2)

(57) [Claims] C: 0.10 wt% or more, 0.20 wt% or less, Si: 0.10 wt% or less, Mn: 0.30 wt% or less, Al: 0.015 wt% or more, 0.100 wt% or less, P: 0.06 wt% or less, S: 0.005 wt% or less, and, N: 0.0030 wt% or less, containing, the balance consisting of iron and unavoidable impurities composition, a tensile strength of 38kgf / mm ² or more, a yield ratio having 70% or less of the characteristic A high-tensile cold-rolled steel sheet characterized by the following. 2. C: 0.10 wt% or more, 0.20 wt% or less, Si: 0.10 wt% or less, Mn: 0.30 wt% or less, Al: 0.015 wt% or more, 0.100 wt% or less, P: 0.06 wt% or less, S: 0.005wt% or less, and N: 0.0030wt% or less, with the remainder being hot-rolled using iron and steel adjusted to the unavoidable impurity composition, and finish rolling at a finishing temperature of 800 ° C or more. After performing, cool at a rate of 10 ° C / s or more, and
Wound on a coil at the following temperature, then cold rolled at a reduction of 50% or more, and then 700 ° C or more to 850 by continuous annealing.
A method for producing a high-tensile cold-rolled steel sheet, wherein annealing is performed at a temperature of not more than ° C.