JP3234644B2 - High r value high tensile strength cold rolled steel sheet excellent in secondary work brittleness resistance and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention proposes a high-strength cold-rolled steel sheet having high strength and excellent press formability and secondary work brittleness resistance, and a method for producing the same. The cold-rolled steel sheet according to the present invention is used, for example, for automobiles and home electric appliances after being subjected to surface treatment and press working as appropriate, and in some cases, spot-welded. And strength at the same time, it is possible to achieve a reduction in the thickness of the steel sheet, and thus a reduction in weight, and specifically, a tensile strength of 38 kgf / mm, which is excellent in economy and secondary work brittleness resistance A cold-rolled steel sheet having a Rank Ford value of ² or more and an average r value of 1.8 or more is required.

[0002]

2. Description of the Related Art Conventionally, decarburization has been sufficiently performed at the steel making stage to obtain extremely low carbon, and solid solution C and solid solution N in steel have been reduced to Ti.
High tensile strength cold-rolled steel sheets with increased strength based on steel containing C and N fixed by precipitating them as carbonitrides and containing Si, Mn and P in solid solution A proposal has been made.

For example, in a high-tensile cold-rolled steel sheet having good formability disclosed in Japanese Patent Application Laid-Open No. 63-190141 and a method for producing the same, a cold-rolled steel sheet obtained by adding a large amount of Mn and P to the ultra-low carbon Ti-added steel is used. It has been disclosed. In this case, by adding appropriate amounts of Mn and P, a small amount of solid solution C after annealing is obtained.
Remain, which significantly improves the r-value, and furthermore, the solid solution C present at the grain boundaries effectively prevents secondary working brittleness. Further, B is added as necessary to strengthen the grain boundaries. They say they can. However, when P is added without adding Si, there is a problem that spot weldability deteriorates.

[0004]

SUMMARY OF THE INVENTION The present invention is based on the object of the present invention, in addition to the addition of Ti for fixing solid solution C and solid solution N, in addition to the addition of Nb for more securely fixing solid solution C, Based on ultra-low carbon Ti, Nb, B composite-added steel with B added to suppress brittleness, this is a low-cost alloy containing Si and the target tensile strength in ordinary continuous annealing 38 kg
It is an object of the present invention to propose a high-tensile cold-rolled steel sheet having f / mm ² or more and an average r value of 1.8 or more and having excellent secondary work brittleness resistance, and a method for producing the same.

[0005]

Means for Solving the Problems In order to achieve the above object, the inventors have paid close attention to the ultra-low carbon Ti, Nb, and B composite added steels and have conducted intensive research. As a result, extremely low carbon Ti,
When Si, Mn, and P are added to Nb and B composite added steel, B steel suitable for secondary work embrittlement resistance is used according to the amount of these additions.
It was newly found that there was an added amount.

[0006] Here, in the above research, ultra-low carbon steel
It is known that when P is added, P segregates at the grain boundary and embrittles the grain boundary, but when Si and Mn are added to P-added steel in combination, the detailed reason is unknown, but the secondary work brittleness resistance is unknown. Was found to deteriorate. In order to improve the secondary work brittleness resistance, it is effective to add B, which strengthens the grain boundary. However, the addition of B particularly elongates the tensile properties and deteriorates the average r value. Increases hardenability and increases strength of hot rolled sheet. During hot rolling, delay recrystallization of austenite grains. The addition of a large amount thereof has a problem. Therefore, the present invention, based on the above results, Si, P and Mn
The optimum amount of B added in accordance with the amount of the solid solution strengthening component was found, and the deterioration of the secondary work brittle resistance was suppressed without deteriorating the average r value.

That is, the gist of the present invention is as follows: C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.31 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt% or less, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% In the following, solAl: 0.1 wt% or less and N: 0.005 wt% or less, and the content of B is determined based on the content of Si, Mn and P.

A (wt%) = 0.8 Si (wt%) + 0.3 Mn (wt%) + P (wt
%)-0.2 (wt%)

## EQU6 ## It is contained in such a manner that 2A (wt%) × 10 ⁻³ ≦ B (wt%) ≦ 5A (wt%) × 10 ⁻³ , and the balance is composed of iron and unavoidable impurities. A high r-value, high tensile strength cold-rolled steel sheet with excellent secondary work brittleness resistance,

As a steel slab material having the above composition,
Finish hot rolling at a rolling finish temperature range of _Ar3 point (° C) or higher and _Ar3 point (° C) + 100 ° C or lower, wind up at a temperature of 650 ° C or lower, cold-roll, and then 750 ° C or higher A high r-value and high tensile strength cold-rolled steel sheet having excellent secondary work brittleness resistance, wherein the steel sheet is subjected to continuous annealing at a temperature of 0.5 ° C.

[0009]

The reasons for limiting the chemical composition of steel in the present invention will be described. C: 0.0001 to 0.005 wt% C is an average r when a large amount of solid solution C remains during recrystallization.
The value greatly deteriorates. Also, Ti, Nb for fixing solid solution C
Should be added according to the C content, so the C content should be as low as possible, and the allowable upper limit is 0.005 wt%.
And On the other hand, the lower limit is better as low as possible, but the lower limit is 0.001 wt% based on the current steelmaking technology.

Si: 0.31 to 1.0 wt% Si has a large solid solution strengthening ability and does not deteriorate the average r value, so it is optimal as a solid solution strengthening component and suppresses deterioration of spot weldability. At least 0.31 wt% must be contained for the manifestation of this effect. However, the content is 1.0 wt%
If it exceeds 300, the surface treatment properties will deteriorate. Therefore, its content should be 0.31 wt% or more and 1.0 wt% or less.

Mn: 0.5 to 2.0 wt% Mn is an important component in the present invention. Mn is Si or P
Unlike this, since it is a component that lowers the transformation point, by effectively utilizing it, the particle size of the hot-rolled sheet can be made extremely fine. This grain refinement of the hot-rolled sheet is very effective in improving the average r value because the (111) texture develops from the crystal grain boundaries. The effect is obtained when the content is 0.5 wt% or more. On the other hand, since Mn itself is a component that deteriorates the average r value, it is harmful to contain it in a large amount. If its content exceeds 2.0 wt%, a low-temperature transformation phase is likely to appear and the ferrite structure is not obtained. Greatly deteriorates. Therefore, its content is 0.5 wt% or more, 2.0 wt%
The following is assumed.

Also, the balance between the above-mentioned Si, the following P content and the Mn content is described.

It is desirable that 0.2 ≦ (Si (wt%) + P (wt%) / Mn (wt%) ≦ 1.0, which is Mn of the sum of the contents of Si and P.
If the ratio with respect to the content is less than 0.2, the average r-value is degraded, and if it is more than 1.0, the transformation point temperature is increased, and it is not possible to expect the hot-rolled sheet to become finer.

Ti: 0.01 to 0.2 wt% Ti is contained for fixing solid solution C, S and N as TiC, TiS and TiN. If the content is less than 0.01 wt%, the effect is not sufficient. If the content exceeds 0.2 wt%, phosphide is generated, and the elongation and the average r value are deteriorated. Therefore, its content should be 0.01 wt% or more and 0.2 wt% or less.

Nb: 0.001 to 0.02 wt% Nb is an effective component for fixing solid solution C as NbC in the same manner as Ti. Although solid solution C can be fixed with Ti alone, C can be fixed more reliably by adding Nb in combination, and the average r value can be improved. The effect is 0.001 content
If the content is less than wt%, it is not sufficient. If the content exceeds 0.02 wt%, hot rolling is performed in an austenite unrecrystallized state, which adversely affects the formability of the annealed material. Therefore, its content should be between 0.001 wt% and 0.02 wt%.

B: 0.0002 to 0.005 wt% B is contained in order to prevent secondary working embrittlement. In particular, when a solid solution strengthening component is contained in an ultra-low carbon steel sheet to which Ti and Nb are added, secondary work embrittlement resistance is deteriorated, so that B must be contained. The effect is manifested when the content is 0.0002 wt% or more.However, when the content exceeds 0.005 wt%, the recrystallization of austenite is delayed, the load at the time of hot rolling becomes large, and the material of the annealing material is changed. Deteriorate. Therefore, its content should be 0.0002 wt% or more and 0.005 wt% or less.

Further, from the above contents of Si, Mn and P,

[Equation 8] A (wt%) = 0.8Si (wt%) + 0.3Mn (wt%) + P (wt%)
-0.2 (wt%)

## EQU9 ## The relationship of 2A (wt%) × 10 ⁻³ ≦ B (wt%) ≦ 5A (wt%) × 10 ⁻³ is satisfied. Thereby, B suitable for secondary processing embrittlement resistance can be contained according to the content of the solid solution strengthening component.

Here, the coefficients of Si and Mn are used to convert the degree of embrittlement caused by the inclusion of Si and Mn into the amount of embrittlement caused by the inclusion of P.
And Mn do not affect the brittleness to a certain extent, and are therefore correction terms. In this way, the amount of embrittlement due to Si and Mn is added as P equivalent, and the steel is considered to be P alone added steel.
The amount of B was determined so that the transition temperature would be −45 ° C. or less.

P: 0.05 to 0.15 wt% P is an important component as a solid solution strengthening component, and its solid solution strengthening ability is higher than Si and Mn, and is also a component for improving the average r value. To achieve this effect, the content must be 0.05 wt% or more, but if it exceeds 0.15 wt%, it segregates at the grain boundaries, embrittles the grain boundaries, and further causes central segregation during solidification. . Therefore its content is 0.05wt%
Above, 0.15wt% or less.

S: not more than 0.02 wt% S does not affect the average r value, but when its content increases, sulfides such as MnS increase, and the local ductility represented by stretch flangeability decreases. Therefore, the upper limit of its content is set to 0.02wt%.

Sol Al: 0.1 wt% or less Al is a component necessary for deoxidation.
If it exceeds 1 wt%, not only the deoxidizing effect is saturated but also the number of inclusions increases, which adversely affects the formability. Therefore, its content should be 0.1 wt% or less.

N: 0.005 wt% or less N is an impurity component which is inevitably mixed into steel.
By adding Ti, it is fixed as TiN to improve formability. However, when a large amount of TiN is formed, workability is deteriorated. Therefore, the upper limit of the allowable content is 0.005 wt%
And

Next, the manufacturing conditions will be described. Steelmaking and casting conditions may be performed according to a conventional method.

Hot rolling finishing temperature: _Ar3 point to _Ar3 point + 10
0 ° C. The hot rolling finishing temperature needs to be changed according to the _Ar3 transformation point. _{Below the Ar3} transformation point, two-phase zone rolling occurs,
A texture that adversely affects the average r value of the annealed material develops, which is not preferable. On the other hand, when the temperature is relatively higher than the _Ar3 transformation point, that is, higher than 100 ° C., the grain size of the hot-rolled sheet becomes coarse, and the development of a texture effective for deep drawing during annealing becomes insufficient. Unsuitable to become. Therefore, the hot rolling finish temperature is changed from _Ar3 point (° C) to _Ar3 point (° C).
Up to + 100 ° C.

Winding temperature: 650 ° C. or less The limitation of the winding temperature is based on the finding that when the temperature exceeds 650 ° C., the average r value is greatly deteriorated. The reason for this is that at a winding temperature exceeding 650 ° C., a large amount of phosphide of Ti and Fe precipitates on the hot-rolled sheet and details thereof are unknown, but it is presumed that this adversely affects the average r value of the annealed material. Therefore, the winding temperature should be 650 ° C or less.

The cold rolling may be performed according to a conventional method.
The rolling reduction may be in the range of common sense. Annealing is preferably continuous annealing. In this case, the annealing temperature is set to 750 ° C. or higher so that recrystallization is completed, and the upper limit is preferably set to A _c1 point (° C.) + 50 ° C.

[0026]

EXAMPLES Steel slabs adjusted to the chemical composition shown in Table 1 were used as raw materials, and these were hot-rolled and wound into coils.
After pickling, the steel sheet was cold-rolled at a rolling reduction of 80% to a sheet thickness of 0.8 mm and annealed by an alundum bath treatment for 40 seconds to obtain tensile properties and secondary work brittleness (brittle transition temperature). ) investigated.

[0027]

[Table 1]

The results of the hot rolling finishing temperature, annealing temperature, tensile properties and secondary work brittleness are shown in Table 2. The winding temperatures were all set at 600 ° C.

[0029]

[Table 2]

Here, the tensile properties were measured using a JIS No. 5 test piece, and the average r value was measured by a three-point method after applying a tensile prestrain of 15%, and the L direction (rolling direction) and the D direction ( The average value in the direction of 45 ° with respect to the rolling direction and the direction of C (in the direction of 90 ° with respect to the rolling direction) was determined from the average r value = (r _L + 2r _D + r _C ) / 4. The brittleness of secondary processing is as follows: After blanking to 50 mmφ, squeezing out with a 24.4 mmφ punch, and applying a shock of 5 kg-80 cm at various temperatures to a cup cut off the ear at a height of 21 mm. Was evaluated.

As is clear from Table 2, the B content was changed to S
from the contents of i, Mn and P

[Equation 10] A (wt%) = 0.8Si (wt%) + 0.3Mn (wt%) + P (wt%)
−0.2 (wt%)

[Equation 11] The conforming example of the present invention satisfying the relationship of 2A (wt%) × 10 ⁻³ ≦ B (wt%) ≦ 5A (wt%) × 10 ⁻³ has a high average r value, It turns out that it is also excellent in processing brittleness.

[0032]

According to the present invention, an ultra-low carbon Ti, Nb, B composite added steel solid solution strengthened by Si, Mn and P
By specifying the content of B according to the content of Si, Mn and P, a high-tensile cold-rolled steel sheet having excellent secondary work brittleness and a high average r value can be obtained. The rolled steel sheet is suitable for use in automobiles, home appliances, and the like.
Further, it can be used not only as a cold-rolled steel sheet but also as an original sheet of a surface-treated steel sheet.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. (56) References JP-A-63-105931 (JP, A) JP-A-2-163318 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48 C21D 8/00-8/10

Claims (2)

(57) [Claims] 1. C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.31 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt %, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% or less, solAl: 0.1 wt % And N: 0.005 wt% or less, and the content of B is calculated from the content of Si, Mn and P by the following formula: A (wt%) = 0.8 Si (wt%) + 0.3 Mn (wt%) + P (wt
%)-0.2 (wt%) using the following parameter: 2A (wt%) × 10 ^-3 ≤ B (wt%) ≤ 5A (wt%) × 10 ^-3 A high r-value and high tensile strength cold-rolled steel sheet having excellent secondary work brittleness resistance, the balance being composed of iron and unavoidable impurities. 2. C: 0.001 wt% or more, 0.005 wt% or less, Si: 0.31 wt% or more, 1.0 wt% or less, Mn: 0.5 wt% or more, 2.0 wt% or less, Ti: 0.01 wt% or more, 0.2 wt %, Nb: 0.001 wt% or more, 0.02 wt% or less, B: 0.0002 wt% or more, 0.005 wt% or less, P: 0.05 wt% or more, 0.15 wt% or less, S: 0.02 wt% or less, solAl: 0.1 wt % And N: 0.005 wt% or less, and the content of B is calculated from the content of Si, Mn and P by the following formula: A (wt%) = 0.8 Si (wt%) + 0.3 Mn (wt%) + P (wt
%)-0.2 (wt%) using the following parameter: 2A (wt%) × 10 ^-3 ≤ B (wt%) ≤ 5A (wt%) × ^10-3 The remainder is made of steel and a steel slab composed of unavoidable impurities. Hot rolling is completed at a rolling finish temperature range of _Ar3 point (° C) or higher and _Ar3 point (° C) + 100 ° C or lower, and 650 ° C or lower. After winding at the temperature of
A method for producing a high r-value and high tensile strength cold-rolled steel sheet having excellent secondary work embrittlement resistance, comprising cold rolling and then continuously annealing at a temperature of 750 ° C or higher.