JPH10130782A - Ultrahigh strength cold rolled steel sheet and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength steel sheet excellent in delayed fracture resistance after parts forming without causing deterioration in workability and weldability required of machine parts by specifying the composition of a steel sheet, applying decarburizing treatment to the steel sheet during annealing, and forming a ferrite layer in the surface layer and a bainite.martensite layer in the inner layer. SOLUTION: A steel, having a composition consisting of, by weight, 0.05-0.20% C, 0.0001-0.030% P, 0.001-0.050% S, 0.001-0.100% Al, 0.0002-0.0050% N, one or >=2 kinds among 0.10-2.50% Si, 0.5-3.50% Mn, 0.10-1.5% Cr, 0.10-1.5% Mo, and 0.001-0.005% B, and the balance Fe with inevitable impurities, is used. This steel is hot-rolled, descaled, and cold-rolled, followed by continuous annealing. At this time, the resultant steel sheet is subjected to decarburizing treatment, by which each surface layer part, in the region between the surface or the rear surface and a position at a depth of 0.010-0.20mm per side from the surface or the rear surface of the steel sheet, becomes composed essentially of ferrite and also the inner layer part becomes composed essentially of bainite.martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled thin steel sheet used for structural parts such as automobiles and machines, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, structural parts such as automobiles and mechanical structures have been required to reduce weight in terms of resource saving and energy saving. There is a demand for high strength characteristics that are excellent in properties.
Therefore, application of ultra-high strength steel sheets having a strength of 780 MPa or more to parts has been actively considered. However, ordinary ultra-high-strength steel sheets are inferior in workability at the time of forming parts and weldability at the time of assembling parts, and are prone to delayed fracture phenomena that lead to destruction due to aging after forming parts. It is rare.

Several techniques have been proposed to improve the workability of ultra-high strength steel sheets. For example, Dual, in which the structure of a steel sheet is ferrite and martensite
Phase steel (Japanese Patent Application Laid-Open No. 56-116833) and a steel sheet (Japanese Patent Application Laid-Open No. 62-18224) utilizing work-induced plasticity due to retained austenite are excellent in ductility while having high strength. However, any processing that requires local ductility of the steel sheet surface, such as bending, is insufficient. Further, as an attempt to improve bending workability and spot weldability, a steel sheet in which the surface layer is decarburized and annealed and the inner layer contains 10% by volume or more of retained austenite has been proposed (Japanese Patent Laid-Open No. 2-175839).

However, since the inner layer of the steel sheet contains high carbon, welding conditions must be selected so that only the surface of the steel sheet is melted, and the molten portion during welding is melted over the inner layer of the steel sheet. Butt welding, arc welding, and the like cannot be applied, and thus are not suitable as structural members that require the strength of the welded portion. Further, no technique has been proposed for sufficiently improving the property deterioration of the ultra-high strength steel due to the delayed fracture phenomenon.

[0005]

DISCLOSURE OF THE INVENTION In order to solve the above-mentioned problems, the present inventors have developed a steel sheet which does not impair workability or weldability as a mechanical part and has excellent delayed fracture resistance after forming the part. And intensive research on its manufacturing method.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the delayed fracture phenomenon of a part made of a steel plate, and as a result, hydrogen that has invaded the part from the external environment has segregated in the strained part during the part processing. It has been found that it accumulates and causes a destructive phenomenon over time, and the delayed fracture resistance of the entire component is improved by improving the delayed fracture resistance of the surface layer by using the surface layer of the steel sheet exposed to the external environment as a layer mainly composed of ferrite. It was found that it was effective in improving the characteristics. Further, it has been found that this effect is remarkably effective in improving delayed fracture resistance by making the inner layer a layer mainly composed of martensite and bainite. Here, the layer mainly composed of ferrite is preferably a layer in which the fraction of ferrite in the steel material is 90% or more. Further, the layer mainly composed of martensite and bainite means a layer having a combined fraction of martensite and bainite of 30% or more and less than 10% of retained austenite.

Further, as a result of intensive studies on the workability of the steel sheet, it has been found that the processing limit of a part using an ultra-high strength steel sheet is dominated by crack fracture from the steel sheet surface, and a part of the steel sheet surface is distorted. It has been found that it is important to prevent the occurrence of cracks and constrictions by concentrating and to prevent the growth of cracks and constrictions. In other words, by making the surface layer of the steel sheet a layer mainly composed of ferrite, the deformation of the surface progresses uniformly and it is hard to cause local necking, and even if necking occurs, sharp necking is hard to occur, so cracks It was found that the occurrence and development of cracks were difficult, and it was found that even if the inner layer of the steel sheet was a hard layer mainly composed of bainite and martensite, the working limit as a steel sheet was dramatically improved.

Further, the inner layer is a layer mainly composed of bainite and martensite to increase the strength of the steel sheet as a whole, so that the C content in the steel sheet can be reduced to 780% or less even when it is 0.20% or less.
It is possible to obtain ultra-high strength characteristics of not less than MPa and excellent welding characteristics. The present inventors have further studied and found that during the continuous annealing, the atmosphere was controlled to form a decarburized layer on the surface of the steel sheet. A method for separately forming a hard layer mainly composed of martensite and bainite was developed. This method is easy to control the thickness of the surface ferrite layer, and does not require significant improvement of conventional manufacturing equipment without increasing the number of special processes and equipment. It is possible to manufacture high-strength steel sheets excellent in quality.

The gist of the present invention is that (1) by weight% of C
: 0.05 to 0.20%, P: 0.001 to 0.0
30%, S: 0.001 to 0.050%, Al: 0.
001 to 0.100%, N: 0.0002 to 0.00
50%, Si: 0.10 to 2.50
%, Mn: 0.5 to 3.50%, Cr: 0.10 to 1.
5%, Mo: 0.10 to 1.5%, B: 0.001 to
0.005%, one or more of the following, with the balance being F
e and inevitable impurities, the surface layer of a portion of 0.010 to 0.20 mm per side from the front and back surfaces of the steel plate is mainly composed of ferrite, and the inner layer portion is mainly composed of bainite and martensite. High strength cold rolled steel sheet,

(2) The steel sheet according to (1) is further added with Nb: 0.010 to 0.20%, Ti:
0.010 to 0.20%, V: 0.010 to 0.20
%, Zr: 0.010 to 0.20%, a high-strength cold-rolled steel sheet containing (1) the above (1).
Or hot-rolling, descaling, and cold-rolling the steel having the component described in (2), and then, when performing continuous annealing on the cold-rolled steel sheet, performing decarburization processing on the steel sheet, A method for producing a high-strength cold-rolled steel sheet, characterized in that a surface layer of a portion of 0.010 to 0.20 mm per side is mainly composed of ferrite, and an inner layer is mainly composed of bainite and martensite.

First, the reasons for limiting the components will be described. C is
If it is less than 0.05%, the strength as a high-strength steel plate required for structural parts is insufficient, so the lower limit is made 0.05%. On the other hand, if it exceeds 0.20%, the weldability is extremely deteriorated, so the upper limit is 0.20%. Preferably it is 0.15% or less. Since reducing P to less than 0.001% increases the production cost and impairs the economic efficiency, 0.001%
Is set as the lower limit, and if it exceeds 0.030%, the workability deteriorates, so 0.030% is made the upper limit. Since the reduction of S to less than 0.001% increases the production cost and impairs the economic efficiency, the lower limit of S is 0.001%, and if it exceeds 0.050%, the workability deteriorates. Upper limit.

If the content of Al is less than 0.001%, deoxidation is insufficient and a pro-hole is formed in the steel, impairing the cleanliness as a steel sheet, causing cracks and surface flaws during pressing. 0.001% as the lower limit, and 0.100%
Exceeds 0.10, the workability deteriorates.
0% is the upper limit. N is preferably as small as possible,
If the content is less than 0.0002%, the production cost is significantly increased.
If it exceeds 50%, the workability deteriorates.
0% is the upper limit.

[0013] Next, Si:
0.10 to 2.50%, Mn: 0.5 to 3.50%, C
r: 0.10 to 1.5%, Mo: 0.10 to 1.5%,
B: One or two or more of 0.001 to 0.005% can be selected. Si is an element effective for strengthening the transformation of steel. If the content is less than 0.10%, the effect is small. Therefore, the lower limit is 0.10%, and if it exceeds 2.50%, the workability is deteriorated. The upper limit is 50%. Mn is an element effective for strengthening the transformation of steel, and if Mn is less than 0.50%, the effect is small.
If it exceeds 50%, the workability deteriorates, so the upper limit is 3.50%.

Cr is an element effective for strengthening the transformation of steel. When the content is less than 0.10%, the effect is small.
The lower limit is 10%, and if it exceeds 1.50%, the workability deteriorates, so the upper limit is 1.50%. Mo is an element effective for strengthening the transformation of steel. When the content is less than 0.10%, the effect is small. Therefore, the lower limit is 0.10%, and when it exceeds 1.50%, the workability is deteriorated. The upper limit is 50%. B is an element effective for strengthening the transformation of steel,
Less than 0.001% has little effect, so 0.001%
Is set as the lower limit, and if it exceeds 0.005%, the effect is saturated. Therefore, the upper limit is made 0.005%.

Further, as an element for strengthening steel,
Nb: 0.010 to 0.20%, Ti: 0.010 to
0.20%, V: 0.010 to 0.20%, Zr: 0.
One or two or more of 010 to 0.20% can be selected. This component system is added to generate fine carbonitrides in the steel and increase the strength of the steel. N
If b is less than 0.010%, the amount of carbonitrides generated is small and the effect of increasing the strength of the steel is small, so the lower limit is 0.010%, and if it exceeds 0.20%, the workability is degraded. Therefore, the upper limit is 0.20%.

If Ti is less than 0.010%, the amount of carbonitride formed is small and the effect of increasing the strength of steel is small, so that
The lower limit is 0.010%, and if it exceeds 0.20%, the workability deteriorates. Therefore, the upper limit is 0.20%. V is
If the amount is less than 0.010%, the amount of carbonitride is small and the effect of increasing the strength of the steel is small, so the lower limit is 0.010%, and if it exceeds 0.20%, the workability deteriorates.
The upper limit is 0.20%. If Zr is less than 0.010%, the amount of carbonitrides generated is small and the effect of increasing the strength of the steel is small, so the lower limit is 0.010%, and if it exceeds 0.20%, the workability is degraded. Therefore, the upper limit is 0.20%.

From the front and back surfaces of the steel sheet manufactured using the steel having the above composition, the surface layer of 0.010 to 0.20 mm per side is formed as a layer mainly composed of ferrite, and the inner layer is formed of bainite and martensite. Layer. Here, the layer mainly composed of ferrite means a layer having a ferrite fraction of 90% or more in the steel sheet. 10 in total
% Or less may contain pearlite, bainite, retained austenite, martensite, but if it exceeds 10%, the workability of the entire steel sheet and the delayed fracture resistance after forming the parts are reduced, so that the ferrite fraction is 90%. It is preferable that it is above. When the thickness of the surface layer is less than 0.010 mm, the effect of improving delayed fracture resistance and processability is small.
The lower limit is set to 0.010 mm, and if it exceeds 0.20 mm, the strength of the entire steel sheet decreases. Therefore, the upper limit is set to 0.20 mm.

The layer mainly composed of martensite and bainite has a fraction containing martensite and bainite of 3%.
0% or more and a layer in which retained austenite is less than 10%. In order to obtain the strength characteristics as an ultra-high strength steel sheet, the combined fraction of martensite and bainite needs to be 30% or more. In addition, the amount of retained austenite is 1
If the content is 0% or more, the delayed fracture resistance is not sufficient even if a surface layer mainly composed of ferrite is present.

Next, the manufacturing method will be described. In the present invention, a surface layer mainly composed of ferrite and an inner layer mainly composed of bainite martensite are separately formed in the continuous annealing step. A steel sheet having the above-mentioned chemical composition is melted and formed into a slab, which is formed into a steel sheet according to a conventional method of manufacturing a steel sheet. First, after continuous casting, the slab is cooled directly or once to an appropriate temperature, and then heated in a heating furnace. Heating can be performed by hot rolling 100
The temperature is preferably set to about 0 ° C. to 1300 ° C. Depending on the use of the steel sheet, the heating can be omitted. Thereafter, the hot-rolled at A ₃ transformation point or above the temperature. It may be cast below the A ₃ transformation point heat, but preferably a sufficient workability can not be obtained and A ₃ transformation point or more. After hot rolling, it is wound at an appropriate temperature to obtain a hot rolled steel sheet. Thereafter, after descaling such as pickling, cold rolling is performed.

Then, when performing annealing, decarburizing annealing is performed, and the surface layer of a portion of 0.010 to 0.20 mm per side from the front and back surfaces of the steel sheet becomes a layer mainly composed of ferrite after cooling after annealing. Decarburization as described above. Although the amount of decarburization is not particularly limited, it is difficult to make the surface layer mainly composed of ferrite unless the C content of the surface layer is less than 0.10%. It is desirable that the C content of the surface layer be less than 0.05% because the surface layer can be mainly composed of ferrite. The method of decarburization annealing is not particularly limited, but, for example, the carbon concentration in the steel sheet can be reduced by annealing in an oxygen-containing atmosphere or a high dew point atmosphere.

The annealing temperature is preferably at least 700 ° C. in order to promote the decarburization reaction. Further, when annealing is performed at a temperature of Ac ₃ or more, a uniform structure can be obtained and workability can be improved. After the annealing, cooling is performed. The cooling rate is faster than the inner layer starts ferrite-pearlite transformation, and it is necessary to cool the surface layer sufficiently lower than the ferrite transformation to start below the transformation point. Cooling rate is 5
When the temperature is as fast as 0 ° C./sec or more, the amount of the alloy element added for the purpose of strengthening the transformation can be reduced.

After annealing, skin pass rolling is performed to produce a product, or the cold-rolled steel sheet obtained by the method of the present invention is
For the purpose of improving corrosion resistance, paintability, and weldability, hot-dip plating can be performed on one or both surfaces without departing from the present invention. Further, in the case of the steel sheet of the present invention, since it has excellent delayed fracture resistance, electroplating may be performed without any problem. Further, it is of course possible to add various treatments to the steel sheet of the present invention, for example, chromate treatment, phosphate treatment, treatment for improving phosphatability, lubricity improving treatment, weldability improving treatment. Even if a resin film treatment or the like is performed, the present invention does not depart from the scope of the present invention, and various treatments can be additionally performed according to necessary characteristics.

[0023]

【Example】

(Embodiment 1) Hereinafter, the present invention will be specifically described based on embodiments. Steels having the components shown in Tables 1 to 3 were melted and continuously cast into slabs according to a conventional method. And 1 in a heating furnace
The sheet was heated to 200 ° C., hot-rolled at a finishing temperature of 880 ° C. or higher, wound up at 600 ° C., and then pickled to obtain a hot-rolled steel sheet. After cold rolling at a rolling reduction of 60%, when performing continuous annealing, CO, H ₂ , C
Controls O ₂ partial pressure, subjected to decarburization annealing at 870 ° C. under high dew point atmosphere and cooled to 700 ° C. after annealing is cooled at a cooling rate shown in Table 1 to Table 3 it ultrahigh strength cold rolled steel sheet did. In addition, some (No. 43, No. 51, No. 61, N
o. 62, No. 63, no. 64, no. 65, N
o. 66), after performing cold rolling at a rolling reduction of 60% and then performing continuous annealing, controlling the partial pressures of CO, H ₂ and CO _{2 in the} atmosphere, and decarburizing annealing at 870 ° C. in a high dew point atmosphere. After annealing, the sample was gradually cooled to 700 ° C., cooled at the cooling rates shown in Tables 1 to 3, and further placed in a molten zinc bath to obtain a hot-dip galvanized ultra-high strength steel sheet. Furthermore, No. 62,
No. 63, no. 64, no. 65, no. No. 66 was then subjected to electrolytic chromate treatment to form a chromate-treated steel sheet.

A JIS No. 5 piece was prepared from the obtained steel sheet,
The mechanical property values were investigated. Further, a 90 ° V bending test was separately performed to evaluate bending workability. Further, a disc of 80φ was punched out of the obtained cold-rolled steel sheet, a cylindrical cup was formed at a drawing ratio of 2.0, and the cup was immersed in hydrochloric acid to conduct an accelerated test of delayed fracture. After the immersion, the time until the fracture occurred was evaluated for the delayed fracture resistance. The above results are also shown in Tables 1 to 3. As is clear from Tables 1 to 3, when the comparative examples are compared with the examples of the present invention, it is understood that the examples of the present invention are more excellent in workability and delayed fracture resistance.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

According to the present invention, a steel sheet excellent in workability and delayed fracture resistance can be obtained. According to the present invention, it is possible to provide a steel sheet having excellent workability and delayed fracture resistance because the steel sheet has a layer mainly composed of ferrite in a surface layer and a layer mainly composed of martensite and bainite as an inner layer. it can. According to the method of the present invention, compared to the conventional method, the surface layer is mainly composed of ferrite and the inner layer is mainly composed of martensite / bainite at a low cost without increasing the number of special steps and equipment. Can be. In addition, since the thickness of the layer mainly composed of ferrite on the surface layer can be precisely formed and the inner layer of ultra-high strength can be secured, a high-strength cold-rolled steel sheet with excellent workability and delayed fracture resistance can be manufactured. be able to.

Claims (3)

[Claims] 1. C .: 0.05 to 0.20%, P: 0.001 to 0.030%, S: 0.001 to 0.050%, Al: 0.001 to 0. 100%, N: 0.0002 to 0.0050%, Si: 0.10 to 2.50%, Mn: 0.5 to 3.50%, Cr: 0.10 to 1.0. 5%, Mo: 0.10 to 1.5%, B: 0.001 to 0.005%, one or more of the following, and the balance: Fe and unavoidable impurities, 0.010 to 0.2 per side from the front and back of the steel sheet
The surface layer of the 0 mm portion is mainly made of ferrite,
High-strength cold-rolled steel sheet whose inner layer mainly consists of bainite and martensite. 2. The steel sheet according to claim 1, further comprising: Nb: 0.010 to 0.20%, Ti: 0.010 to 0.20%, V: 0.010 to 0. 20%, Zr: 0.010 to 0.20%, A high-strength cold-rolled steel sheet containing one or more of Zr. 3. A steel having the composition of claim 1 or 2 is subjected to hot rolling, descaling treatment, and cold rolling, and then, when performing continuous annealing to a cold rolled steel sheet, performing a decarburizing treatment on the steel sheet.
0.010-0.20mm per side from front and back of steel plate
The surface layer part is mainly composed of ferrite, and the inner layer part is
A method for producing a high-strength cold-rolled steel sheet, comprising mainly bainite and martensite.