JPH05195149A - Ultrahigh strength cold rolled steel sheet excellent in bendability and shock resistance - Google Patents

Abstract

PURPOSE:To provide an ultrahigh strength cold rolled steel sheet for an automobile excellent in bendability and shock resistance, having >=100kgf/mm<2> tensile strength and contg. soft layers in the surface layer parts. CONSTITUTION:This cold rolled steel sheet consists of 0.12-0.30% C, <=1.2 Si, 1-3% Mn, <=0.02% P, <=0.01% S, 0.01-0.06% sol. Al and the balance Fe with inevitable impurities, contains soft layers contg. <=0.1% C in the surface layer parts by 3-15vol.% per one side and has a combined structure consisting of <10vol.% retained austenite and a phase transformed at a low temp. and/or ferrite at the inner layer part.

Description

Detailed Description of the Invention

[0001]

The present invention has a tensile strength of 100 kgf.
/ Mm ² or more, an ultra-high strength cold-rolled steel sheet having a soft layer on the surface layer and excellent in bendability is provided.

[0002]

2. Description of the Related Art In recent years, in order to improve the fuel efficiency of automobiles and the safety of passengers in the event of a collision, the tensile strength is 100 kgf.
Ultra-high-strength cold-rolled steel sheets of / mm ² or more are being actively used mainly for reinforcing members. For this reason, the requirements for workability of ultrahigh-strength cold-rolled steel sheets have become increasingly strict, and accordingly, various inventions relating to ultrahigh-strength cold-rolled steel sheets having excellent workability have been proposed.

When bending a steel sheet, a large tensile stress in the circumferential direction is applied to the outer peripheral surface layer of the bend, and a large compressive stress is applied to the inner peripheral surface layer of the bend. In addition to the tissue, the state of the superficial layer also has a great influence as a factor affecting the above.

In view of this, by forming a soft layer on the surface layer of an ultra-high strength cold rolled steel sheet, it is possible to relieve the tensile stress and compressive stress generated on the surface of the steel sheet during bending and to significantly improve the bending workability. And a high-strength steel sheet having a soft layer on the surface layer is disclosed in Japanese Patent Laid-Open No. 2-1.
75839 discloses a high-strength steel sheet having a surface layer having at least 10 vol% of a soft surface layer and a hard center layer containing 10 vol% or more of retained austenite, and a method for producing the same.

[0005]

However, as in the above-mentioned proposed technique, 10 v of residual austenite is added to the central hard layer.
If the content of ol% or more is also included, the presence of martensite formed by transformation of the retained austenite during molding has a markedly adverse effect on the product properties after molding, especially the impact properties. That is, since these high-strength materials are mainly used for the impact absorbing member, the deterioration of the impact property after molding is inevitably a serious problem.

[0006]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made extensive studies to solve the above-mentioned problems of the prior art, and have a tensile strength of a soft layer as the surface layer and a hard layer as the remaining layer.
For ultra-high strength cold-rolled steel sheets exceeding 00 kgf / mm ² , various studies were conducted to see if they have good bendability and can secure good impact properties after forming.
% Of the retained austenite and the low temperature transformation phase or further ferrite, C:
Even with a relatively small volume ratio of 0.1 wt% or less of the soft layer on one side of 3 to 15 vol%, the soft layer sufficiently absorbs the strain of the steel sheet surface layer that occurs during bending, and has excellent bendability. It was later confirmed that an ultra-high-strength cold-rolled steel sheet having good impact properties could be obtained, and is as follows.

(1) wt%, C: 0.12 to 0.30%,
Si: 1.2% or less, Mn: 1 to 3%, P: 0.020% or less,
A steel containing S: 0.010% or less and sol.Al: 0.01 to 0.06% with the balance being Fe and inevitable impurities,
C: 0.1 wt% or less soft layer on the surface layer on one side for 3 to 15 vo
An ultrahigh strength cold rolled steel sheet having excellent bending workability and impact characteristics, which is characterized by having a composite structure of 1% on both sides and the balance being less than 10 vol% of retained austenite and a low temperature transformation phase or further ferrite.

(2) In the steel described in the item (1), Nb:
0.005 to 0.05%, V: 0.005 to 0.05%, Ti: 0.0.
Any one or two or more of 005 to 0.05% in total
An ultrahigh strength cold rolled steel sheet excellent in bending workability and impact characteristics, characterized by being contained in the range of 0.005 to 0.10%.

(3) Cr: 0.1 to 1%, B: 0.0005
~ 0.005% of any one kind or two kinds is contained, The super-high-strength cold-rolled steel sheet excellent in bending workability and impact characteristics according to the above (1) or (2).

[0010]

The present invention as described above will be explained. First, the reasons for limiting the components of the steel used in the present invention are as follows. C: 0.12 to 0.3% C is an effective component for obtaining the strength of bainite and martensite, which are the quenched structures from austenite, and the lower limit of 0.12% is the minimum to obtain the desired strength. Determined as a limit. In addition, the upper limit of 0.3% was determined because the weldability and impact properties are remarkably deteriorated if more is added.

Si: 1.2% or less. Si contributes most to the ductility among the mechanical properties of the steel sheet, and an increase in the addition amount of Si improves the ductility. However, since the addition of more than 1.2% remarkably deteriorates the impact properties, this is the upper limit.

Mn: 1-3%. Mn is an element that lowers the transformation point and improves the hardenability of austenite, and plays an important role in controlling the volume ratio of the hard low-temperature transformation structure and obtaining a predetermined strength. That is, the lower limit is a limit for stably obtaining a hard low-temperature transformation structure, and the upper limit is that the effect is saturated even if it is added more, and the band structure is significantly developed due to segregation during casting. However, it is limited because it adversely affects the workability.

P: 0.020% or less, S: 0.010% or less. Considering the workability of the steel sheet, it is preferable that P and S are low, so that P is 0.020% or less, and especially S
If the content is high, inclusions (MnS) increase, which has a significant adverse effect not only on the workability of the steel sheet but also on the impact properties, so the content was made 0.010% or less.

Sol.Al: 0.01 to 0.06%. Al is used for deoxidizing steel, but sol.Al is 0.01%
If the amount is less than 0.25%, the silicate inclusions remain and the workability of the steel deteriorates. Also,
The residual amount of sol.Al exceeding 0.06% causes an increase in surface defects and is not preferable, so the upper limit was made 0.06%.

In the present invention, in addition to the above, Nb: 0.
005-0.05%, V: 0.005-0.05%, Ti: 0.0
05-0.05% 1 type or 2 or more types in total 0.005
Add in the range of up to 0.10%. These elements are carbide forming elements and increase the yield point as the structure becomes finer. Further, these elements are added when a high yield point is required, the lower limit is the minimum amount for obtaining the desired effect, and the upper limit not only saturates the effect even if added more, but also improves workability. It was decided because it will deteriorate.

Cr: 0.1 to 1%, B: 0.0005 to 0.00
Contains 1 or 2 of 5%. In the present invention, the above elements are further added if necessary. All of these elements improve the hardenability of steel and effectively act to increase the strength. The lower limit of each is defined as the minimum amount for obtaining the desired effect, and the upper limit is defined because the effect is saturated even if more is added.

Next, the reason for limiting the structure of the present invention will be described. In the present invention, it is important to define the volume ratio of the soft layer of the surface layer and the volume ratio of the retained austenite, and C: 0.1 wt% or less in the surface layer. 3 to 15 vol% on one side, and the balance is defined as a composite structure of less than 10 vol% retained austenite and a low temperature transformation phase or ferrite.
The low temperature transformation phase referred to here is a hard structure such as martensite or bainite.

When the carbon concentration of the soft layer of the surface layer is set to 0.1 wt% or less, it is difficult to obtain the desired effect because it is hardened and the tensile stress and compression stress generated in the surface layer of the steel sheet during bending are not sufficiently absorbed. Decided because. Further, if the volume ratio of the soft layer is 3 to 15% on one side, the desired effect cannot be obtained at 3% or less, while the effect is saturated at 15% or more, and the deformation during bending is not sufficient. This is because it is not preferable because it concentrates on the surface soft layer, that is, the unevenness of deformation in the plate thickness direction increases and the surface becomes rough.

The surface soft layer is required on both sides, but if the volume ratio on one side is 3 to 15%, the desired effect is not adversely affected even if there is a difference in volume ratio on both sides.

The portion of the surface layer excluding the soft layer, that is, the hard layer has a composite structure of less than 10 vol% residual austenite and a low temperature transformation layer or further ferrite. When the content of retained austenite exceeds 10 vol%, the volume ratio of the surface soft layer required for improving bending workability increases, and further, the retained austenite is formed by martensite formed by the work-induced transformation during molding. The volume ratio is limited to less than 10 vol% because it increases and significantly deteriorates impact properties. The low temperature transformation layer is martensite, bainite,
Alternatively, they may be mixed. Further, even if ferrite is further mixed, the desired effect is not adversely affected, but an increase in the volume ratio leads to a decrease in strength, so it is desirable to be within 30%.

By limiting the components and structure as described above, the surface soft layer relaxes the stress generated in the surface layer of the steel sheet during bending and deforms in a well-balanced manner with the inner layer, and has excellent bendability. It is possible to obtain an ultra-high-strength steel sheet that also has excellent impact properties after processing.

To produce the steel of the present invention, hot rolling, pickling and decarburization annealing followed by cold rolling or hot rolling, pickling and cold rolling followed by decarburization annealing followed by continuous annealing and A _c1 point. Heating above
After soaking, it is forcibly cooled. The forced cooling may be gas jet cooling, steam cooling, roll cooling, jet water cooling or a combination thereof, and if the retained austenite volume ratio is less than 10% even if the temperature is kept at 500 ° C or lower during the forced cooling. It does not adversely affect the effects of the present invention.

[0022]

EXAMPLES The inventors of the present invention have shown that steel types 1 to 1 of the present invention shown in Table 1 below.
14, 20, 21, 25-27 and comparative steel types 15-19,
22-24 were prepared.

[0023]

[Table 1]

Each of the above-mentioned steels produced by a converter was slab-cast by continuous casting, hot-rolled, pickled and decarburized and annealed, and then cooled to a plate thickness of 1.6 mm. Hot rolling or cold rolling to a plate thickness of 1.6 mm was followed by decarburization annealing, followed by water quenching and a roll cooling type continuous annealing line. The conditions for decarburization annealing and continuous annealing are as shown in Table 2 below.

[0025]

[Table 2]

Tables 3 and 4 below show the results of an investigation of the surface soft layer volume ratio, mechanical properties, retained austenite volume ratio and impact properties of the cold rolled steel sheet after continuous annealing. Mechanical test values and impact properties are values in the direction perpendicular to the rolling direction.For impact properties, in order to investigate the deterioration behavior due to forming, cold rolling at various reduction ratios was performed, and then the transition temperature was measured by the Charpy impact test. And evaluated.

[0027]

[Table 3]

[0028]

[Table 4]

That is, as can be seen from Tables 3 and 4,
Even in the steel types 1 to 14, 20, 21, 21, 25 to 27 according to the present invention, in the steel numbers 8 and 15 in which the surface layer soft layer volume ratio is out of the range of the present invention, the bending workability is poor, and the surface layer soft layer volume is also low. Even if the ratio is within the range of the present invention, Steel Nos. 9 and 16 having a residual austenite volume ratio of 10 vol% or more have good bending workability but poor impact properties. On the other hand, in the comparative steel types 15 to 19 and 22 to 24, even if the surface soft layer volume ratio and the retained austenite volume ratio are within the scope of the present invention, the steel numbers 20 to 24 and 27 to 29 are inferior in bending workability. Or, the bending property is good, but the impact property is poor. On the other hand, when the steel composition and the volume ratio of the surface soft layer are within the range of the present invention, a material having excellent bending workability and impact characteristics together with ultra-high strength is obtained.

[0030]

[Effect of the Invention] The above tensile strength when according to the invention as described is 100 kgf / mm ² or more, a ultra-high strength, as well reach 160 kgf / mm ^2, yet in any of the bending workability and impact properties Also provides excellent cold rolled steel sheet,
This is an invention having a great industrial effect because it can advantageously obtain a product suitable for automobiles.

[Brief description of drawings]

FIG. 1 is a table showing the effects of surface soft layer volume fraction and retained austenite volume fraction on the critical bending radius and impact properties.

Claims (3)

[Claims] 1. W: wt%, C: 0.12 to 0.30%, Si: 1.
2% or less, Mn: 1 to 3%, P: 0.020% or less, S: 0.0.
010% or less, sol.Al: 0.01 to 0.06%, the balance being Fe and unavoidable impurities, and a soft layer of C: 0.1 wt% or less on one surface of the surface layer 3 An ultra-high strength cold rolled steel sheet having excellent bending workability and impact characteristics, characterized by having a composite structure of up to 15 vol% on both sides and the balance being less than 10 vol% of retained austenite and a low temperature transformation phase or further ferrite. 2. The steel according to claim 1, wherein Nb: 0.005
0.05%, V: 0.005 to 0.05%, Ti: 0.005 to 0.
Any one of 05% or two or more in total of 0.005
An ultrahigh strength cold rolled steel sheet excellent in bending workability and impact characteristics, characterized by being contained in a range of 0.10%. 3. Cr: 0.1 to 1%, B: 0.0005 to 0.0
The ultrahigh-strength cold-rolled steel sheet having excellent bending workability and impact characteristics according to claim 1 or 2, containing any one or two of 05%.