JPH10265898A - Cold rolled high tensile strength steel sheet and hot dip galvanized high tensile strength steel sheet, excellent in external surface appearance and workability, and their production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled high tensile strength steel sheet and a hot dip galvanized high tensile strength steel sheet, capable of combining superior external surface appearance with superior mechanical properties and excellent in workability, and their production. SOLUTION: These steel sheets have a composition containing, by weight, 0.005-0.015% C, <=0.45% Si, 0.3-1.5% Mn, 0.02-0.08% P, 0.003-0.015% S, 0.02-0.06% sol.Al, <=0.004% N, and 0.03-0.15% Ti and satisfying 0.8<=Ti*/C<=1.5, where Ti*=(12/48)×[Ti-(48/14)×N-(48/32)×S] is satisfied. Further, 0.0003-0.0020% B can be incorporated into this composition.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-tensile cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet having excellent surface appearance and workability suitable for automobiles, transportation machines, building materials, and the like, and methods for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a high-tensile steel sheet used for drawing, for example, as disclosed in Japanese Patent Application Laid-Open No. Hei 7-216452, an IF made by adding Ti or the like to ultra-low carbon steel is disclosed.
A steel obtained by adding a solid solution strengthening element such as Si, Mn and P to steel has been put to practical use.

[0003] However, this steel precipitates and fixes a solid solution element and nitrogen and has advantages such as non-aging and high workability. Have. As a countermeasure for the secondary working embrittlement, a technique of intentionally adding B, which is a grain boundary strengthening element, is well known as described in the above publication. Also, since it is based on ultra-low carbon steel with low strength, a large amount of Si, M
It is necessary to add n or the like, which increases the cost.

In order to solve such a problem, CAMP
-ISIJ8 (1995) pp. As reported in U.S. Pat. No. 752, attempts have been made to reduce the above solid solution strengthening elements by increasing C and Ti.

[0005]

However, in this technique, Nb and Ti are added in a complex manner, and high-temperature annealing is required to obtain good workability. When high-temperature annealing is performed, carbides that have been precipitated once during the annealing heating re-dissolve, so that sufficient precipitation strengthening cannot be obtained.
In addition, since a large amount of elements that deteriorate the surface properties such as Si and Mn are added, it is still a problem to deteriorate the surface properties such as non-plating during hot-dip galvanizing, and tencaling in cold-rolled steel sheets. I have.

As described above, a high-strength steel sheet for processing capable of satisfying both the surface appearance and mechanical properties, which are indispensable properties of a thin plate, in order to improve workability has not yet been proposed.

The present invention has been made in view of the above circumstances, and provides a high-tensile cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet which can achieve both good surface appearance and mechanical properties and are excellent in workability. It is an object to provide a method for producing them.

[0008]

Means for Solving the Problems The present inventors have intensively studied to solve the above-mentioned problems. As a result, Si,
It has been concluded that it is effective to reduce the elements that degrade the surface properties such as Mn as much as possible and compensate for the accompanying decrease in strength by precipitation strengthening. Also, by using TiC as a precipitate used for precipitation strengthening and optimizing the amount thereof, it has been found that, in addition to high strength, grain boundary strengthening can be achieved by using C which is dissolved in a trace amount during annealing. .

[0009] As a result of examining the preferable production conditions of such a steel sheet, it has been found that the winding temperature is strictly controlled, the precipitates are controlled, the annealing conditions are specified, and the strain-induced precipitation and re-solidification of TiC are performed. It has been found that better processability can be obtained by controlling the dissolution.

[0010] The present invention has been made based on such findings, and provides the following (1) to (10). (1) By weight%, C: 0.005 to 0.015%, S
i: 0.45% or less, Mn: 0.3 to 1.5%, P:
0.02 to 0.08%, S: 0.003 to 0.015
%, Sol. Al: 0.02 to 0.06%, N: 0.004
%, Ti: 0.03 to 0.15%, and satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5, characterized by excellent surface appearance and workability, and high tension cold rolling. steel sheet. (2) In (1), B: 0.0003-0.
A high-tensile cold-rolled steel sheet excellent in surface appearance and workability, characterized by containing 0020%.

(3) C: 0.005-0.
015%, Si: 0.45% or less, Mn: 0.3-1.
5%, P: 0.02 to 0.08%, S: 0.003 to
0.015%, sol. Al: 0.02 to 0.06%, N:
High content excellent in surface appearance and workability, characterized by containing 0.004% or less, Ti: 0.03 to 0.15%, and satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5. Tension galvanized steel sheet. (4) In (3), B: 0.0003-0.
A high-strength galvanized steel sheet having excellent surface appearance and workability, characterized by containing 0020%.

(5) C: 0.005 to 0.5% by weight.
015%, Si: 0.45% or less, Mn: 0.3-1.
5%, P: 0.02 to 0.08%, S: 0.003 to
0.015%, sol. Al: 0.02 to 0.06%, N:
A cold-rolled steel sheet containing 0.004% or less, Ti: 0.03 to 0.15%, and satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5 is subjected to a hot rolling step, a cold rolling step, A method for producing a high-tensile cold-rolled steel sheet having excellent surface appearance and workability, wherein a winding temperature during hot rolling is set to 580 to 720 ° C. when producing by a process including at least an annealing step.

(6) In (5), B: 0.0
003 to 0.0020%,
A method for manufacturing a high-tensile cold-rolled steel sheet with excellent surface appearance and workability. (7) In (5) or (6), the heating rate in the annealing step is 10 ° C./sec or more, and the soaking temperature is 740 to 83.
A method for producing a high-tensile cold-rolled steel sheet having excellent surface appearance and workability, which is characterized by being performed at 0 ° C.

(8) C: 0.005 to 0.5% by weight.
015%, Si: 0.45% or less, Mn: 0.3-1.
5%, P: 0.02 to 0.08%, S: 0.003 to
0.015%, sol. Al: 0.02 to 0.06%, N:
A hot-dip galvanizing steel sheet containing 0.004% or less, Ti: 0.03 to 0.15%, and satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5 is subjected to a hot rolling step and a cold rolling step. When manufacturing by a process including at least an annealing step and a galvanizing step, the winding temperature during hot rolling is set to 580 to 720.
A method for producing a high-strength galvanized steel sheet having excellent surface appearance and workability, characterized in that the temperature is set to ° C.

(9) In (8), B: 0.0
003 to 0.0020%,
Manufacturing method of high tensile galvanized steel sheet with excellent surface appearance and workability. (10) In (8) or (9), the heating rate in the annealing step is 10 ° C./sec or more, and the soaking temperature is 740-8.
A method for producing a galvanized steel sheet excellent in surface appearance and workability, characterized in that the temperature is 30 ° C. However, the above Ti
^* Satisfies the following equation. Ti ^* = (12/48) × (Ti− (48/14) × N
− (48/32) × S)

[0016]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, C:
0.005 to 0.015%, Si: 0.45% or less, M
n: 0.3-1.5%, P: 0.02-0.08%,
S: 0.003 to 0.015%, sol. Al: 0.02 to
0.06%, N: 0.004% or less, Ti: 0.03 to
0.15% and 0.8 ≦ Ti ^{* /} C ≦ 1.5
Meet.

The reasons for limiting these chemical compositions will be described below. C: 0.005 to 0.015% C is desirably reduced in steel used for processing applications, but in the present invention, it is necessary to achieve both high strength and workability by precipitating with Ti,
The range is 0.005% to 0.015%. 0.008
The range of -0.012% is more preferable.

Si: 0.45% or less Si is added to enhance the strength of a steel sheet. But,
If it is contained in excess of 0.45%, the surface properties are extremely deteriorated, which is not desirable. Therefore, the Si content is set to 0.45% or less. S for hot dip galvanizing
The i content is preferably 0.2% or less.

Mn: 0.3-1.5% Mn is added to increase the strength of the steel sheet. However, an excessive content causes an increase in cost and extremely deteriorates the surface properties of the cold-rolled steel sheet. From such a viewpoint, Mn
Is 0.3 to 1.5%. When a cold-rolled steel sheet is used as it is without plating, from the viewpoint of surface properties:
2% or less is desirable.

P: 0.02 to 0.08% P is added to enhance the strength of the steel sheet. However, when excessively contained, not only extremely deteriorates workability,
It is not preferable because the steel sheet is embrittled. From such a viewpoint, the content of P is set to 0.02 to 0.08%.

S: 0.003 to 0.015% Since S deteriorates the workability of steel, it is desirable to reduce S. However, excessive reduction not only causes an increase in cost but also induces surface defects. From such a viewpoint, the content of S is set to 0.003 to 0.015%.

Sol. Al: 0.02 to 0.06% sol. Al is used as a steel deoxidizer. However, an excessive content causes an increase in cost. From such a viewpoint, the content of sol.Al is set to 0.02 to 0.06%.

N: 0.004% or less N deteriorates the workability of steel, so it is desirable to reduce it. Therefore, the N content is set to 0.004% or less, which has little effect on workability.

Ti: 0.03-0.15%, Ti ^* /
C: 0.8 to 1.5 Ti is an element effective for precipitating and fixing C, S, and N to improve the workability of the steel sheet. In the present invention, fine TiC
Also plays a very important role in precipitating and securing reinforcement. However, excessive addition precipitates coarsely in the winding stage, not only does not provide effective strength, but also does not provide an effect of strengthening grain boundaries by re-dissolving a small amount during annealing. Therefore, from such a viewpoint, the Ti content is set to 0.03 to 0.1.
5%, and the value of Ti ^* / C is set to 0.8 to 1.5. Here, Ti ^* / C is an important parameter for sufficiently exhibiting the effect of precipitation strengthening by TiC intended in the present invention and ensuring excellent workability. Here, Ti ^* is represented by the following equation. Ti ^* = (12/48) × ｛Ti− (48/14) × N
− (48/32) × S｝ TiC that contributes to precipitation strengthening in the steel of the present invention
Has a size of 100 nm or less.

B: 0.0003% to 0.002% It is well known that B is an element that improves the resistance to secondary working embrittlement. The effect of B is greater than that of C, so that more severe processing is required. It is added as necessary when performing the method. In order to obtain the effect, it is necessary to add 0.0003% or more. However, the addition of 0.0020% or more is not preferable because it deteriorates workability. Therefore, when adding B, the content is 0.0003 to 0.0
020%.

Next, a manufacturing method for obtaining the steel sheet of the present invention will be described. In the present invention, the steel sheet having the above composition is manufactured by a process including at least a hot rolling step, a cold rolling step, and an annealing step. In the case of producing a hot-dip galvanized steel sheet, a hot-dip galvanizing step is further added. At this time, the winding temperature during hot rolling was adjusted to 580 to 720.
° C. Further, the heating rate in the annealing step is set to 10 ° C. /
It is preferable that the soaking temperature be 740 to 830 ° C. for at least sec.

The temperature conditions during hot rolling are not particularly limited.
In order to obtain good workability, it is desirable to end rolling in the γ region. For this purpose, the heating temperature needs to be 1050 ° C. or higher. The heating temperature is desirably 1200 ° C. or less from the viewpoint of preventing the γ particle from becoming coarse and controlling the precipitates.

Further, the slab continuously cast is not directly charged into the heating furnace and directly rolled, or the slab is not completely cooled, but is charged into the heating furnace as a hot piece and then rolled. be able to. These methods can be manufactured at lower cost than the usual method of charging a cooled slab into a heating furnace. Also in these cases, it is desirable to perform rolling in the γ region.

Winding temperature: 580 to 720 ° C. The winding temperature during hot rolling is very important for controlling precipitates. Since the steel of the present invention has P added, when high-temperature winding is performed, fine (Fe, Ti) P precipitates finely and deteriorates workability. Further, carbides are precipitated in the hot-rolled sheet stage, and the amount of residual Ti and C necessary for securing the strength cannot be obtained. On the other hand, when low-temperature winding is performed, (Fe, T
i) P does not precipitate, but a large amount of TiC precipitates during annealing, and although sufficient steel sheet strength is obtained, good workability cannot be obtained. As shown in FIG. 1, controlling the amount of precipitation is effective in achieving both improvement in workability and strength.
From such a viewpoint, the winding temperature is specified to be 580 to 720 ° C.

Usually, a cold-rolled steel sheet is manufactured by pickling and cold rolling after hot rolling. In the present invention, the cold rolling conditions are not particularly defined, but a cold pressure ratio of 60 to 85% is desirable in order to obtain good workability.

Heating rate during annealing: 10 ° C./sec or more
Soaking temperature: 740 to 830 ° C. The heating rate during annealing is preferably 10 ° C./sec or more in order to precipitate TiC in a fine state of 100 μm or less. On the other hand, the soaking temperature during annealing is 740 ° C.
If the annealing temperature is less than 830 ° C., the TiC once precipitated during heating will recrystallize, causing a decrease in strength and a deterioration in toughness. Therefore, the soaking temperature during annealing is preferably in the range of 740 to 830 ° C.

In producing the hot-dip galvanized steel sheet of the present invention, the cold-rolled steel sheet obtained in the above steps is subjected to hot-dip galvanizing according to a conventional method. The conditions of the hot-dip galvanizing at that time are not particularly limited.

Means for obtaining the material steel having the above-described composition is not particularly limited, and various methods such as a blast furnace method, an electric furnace method, and a smelting reduction method can be adopted. Further, the means for obtaining the slab is not particularly limited, and any method such as a continuous casting method and a slab method may be used.

The cold-rolled steel sheet and the hot-dip galvanized steel sheet according to the present invention may be subjected to a chemical conversion treatment, an electroplating treatment or the like for the purpose of improving the surface lubricating properties and corrosion resistance, thereby impairing the effects of the present invention. It will not be.

[0035]

【Example】

Embodiment 1 Hereinafter, an embodiment of the present invention will be described.
The steel having the chemical composition shown in Table 1 was subjected to hot rolling,
The hot rolling was completed at 0 ° C. and wound at 620 ° C. Thereafter, it was pickled and cold rolled at 75% to obtain a cold-rolled steel sheet of 1.0 mm. This cold-rolled steel sheet was continuously annealed at 780 ° C., subjected to a temper rolling of 1.0%, and then subjected to a mechanical test and an evaluation of surface properties.

Further, the steel sheet after the cold rolling is subjected to annealing at 780 ° C., hot-dip galvanizing of 45 g / m ² per one side in a continuous hot-dip galvanizing line, alloying treatment, 1.0%
A hot-dip galvanized steel sheet was manufactured by continuously performing temper rolling of the steel sheet, and its surface properties were evaluated to investigate the suitability of the hot-dip galvanized steel sheet. Evaluation items in the mechanical test include tensile strength (TS), average Rankford value (r
Value), aging index (AI) and ductile brittle transition temperature after secondary processing.

The average Rankford value is, as usual, when the Rankford values in the rolling direction, 45 ° direction, and 90 ° direction are r ₀ , r ₄₅ , and r ₉₀ , respectively.
(R ₀ + 2r ₄₅ + r ₉₀ ) / 4.

The brittle transition temperature was measured by drawing a sample punched to 105φ into a cup shape at a drawing ratio of 2.1.
Thereafter, the cup sample trimmed to 35 mm was pressed in a coolant cooled to a predetermined temperature. The lowest temperature at which no brittle fracture occurred was taken as the brittle transition temperature.

The evaluation of the surface appearance of the steel sheet was carried out as follows: A having an extremely good appearance which can be used for an automobile outer panel, etc .; Those that do not withstand the use of the board and the inner board were designated as C.

Table 2 shows the evaluation results. No. 1-7, 1
Nos. 1 to 16 are steel plates equivalent to 440 MPa class,
3a, 11a, and 12a are steel plates equivalent to a 390 MPa class. Nos. 1 to 7 and 1a to 3a, which are examples of the present invention, have a predetermined strength (440 MPa, 390 MPa), an r value of 1.4 or more, an aging index (AI) of 10 MPa or less, and a Tc of -60 ° C. or less. It was confirmed that the steel sheet had not only good mechanical properties but also good surface properties.

In comparison with the example of the present invention, No. 1 having a low C was used. 11 and No. In the case of 11a steel, not only the predetermined strength is not obtained, but also the brittle transition temperature is high. In addition, when C is high,
12 and No. The r value is bad for 12a steel. Also, S
i and Mn are high. In the case of steels 13 and 14, a predetermined strength is sufficiently obtained, but the surface appearance is poor. Also, N
o. In the case of No. 15 steel, Ti ^* / C is less than 0.8, so that not only the workability is poor, but also the aging index (AI) is high. No. Steel No. 16 is a steel sheet to which Nb is added. However, since the recrystallization temperature is high, recrystallization is not completely completed at the annealing temperature in the present test, resulting in extremely poor workability.

(Example 2) For the purpose of studying a preferred method of manufacturing a steel sheet according to the present invention, No. 1 in Table 1 was used. A steel plate was manufactured using the five steels under the conditions shown in Table 3, and the same mechanical test as in Example 1 was performed. Manufacturing methods other than those described in Table 3 were as follows: hot-rolling heating temperature: 1200 ° C., hot-rolling finishing temperature: 900 ° C., cooling pressure ratio:
75%, pressure regulation rate: 1.0%.

Table 4 shows the evaluation results. In the conditions C and D obtained by optimizing the hot rolling conditions with respect to the conditions A and B, the r value is 1.5.
It turns out that it is favorable above. In addition to the hot rolling conditions, under the conditions E and F where the annealing conditions were optimized, the r value was at the same level despite the higher strength.
It was confirmed that they showed better mechanical test values.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

As described above, according to the present invention,
It is possible to provide a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized steel sheet having excellent surface appearance and workability suitable for automobiles, transportation equipment, building materials, and the like, and a method for producing the same, which is industrially useful. Is brought.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of a winding temperature on a mechanical test value of the present invention.

Claims (10)

[Claims] 1. C: 0.005 to 0.01% by weight
5%, Si: 0.45% or less, Mn: 0.3 to 1.5
%, P: 0.02 to 0.08%, S: 0.003 to 0.
015%, sol.Al: 0.02-0.06%, N: 0.
004% or less, Ti: 0.03 to 0.15%,
A high-tensile cold-rolled steel sheet excellent in surface appearance and workability, characterized by satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5.
However, Ti ^* = (12/48) × ｛Ti− (48/1
4) × N− (48/32) × S}. 2. B: 0.0003 to 0.0020
The high-tensile cold-rolled steel sheet having excellent surface appearance and workability according to claim 1, characterized by containing 3. C: 0.005 to 0.01% by weight
5%, Si: 0.45% or less, Mn: 0.3 to 1.5
%, P: 0.02 to 0.08%, S: 0.003 to 0.
015%, sol.Al: 0.02-0.06%, N: 0.
004% or less, Ti: 0.03 to 0.15%,
A high tensile galvanized steel sheet excellent in surface appearance and workability, characterized by satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5. Where Ti ^* = (12/48) × ｛Ti−
(48/14) × N− (48/32) × S}. 4. B: 0.0003 to 0.0020
The high tensile galvanized steel sheet having excellent surface appearance and workability according to claim 3, characterized in that: 5. C: 0.005 to 0.01 by weight%
5%, Si: 0.45% or less, Mn: 0.3 to 1.5
%, P: 0.02 to 0.08%, S: 0.003 to 0.
015%, sol.Al: 0.02-0.06%, N: 0.
004% or less, Ti: 0.03 to 0.15%,
And a cold-rolled steel sheet satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5,
When manufactured by a process including at least a hot rolling step, a cold rolling step, and an annealing step, the winding temperature during hot rolling is set to 580 to 720 ° C, and the surface appearance and workability are excellent. Manufacturing method of high tension cold rolled steel sheet. However, Ti ^* = (12/48) × ｛Ti− (48/1
4) × N− (48/32) × S}. 6. B: 0.0003 to 0.0020
The method for producing a high-tensile cold-rolled steel sheet having excellent surface appearance and workability according to claim 5, wherein 7. The heating rate in the annealing step is 10 ° C./sec.
The method for producing a high-tensile cold-rolled steel sheet excellent in surface appearance and workability according to claim 5 or 6, wherein the soaking temperature is 740 to 830 ° C or more. 8. C: 0.005 to 0.01% by weight
5%, Si: 0.45% or less, Mn: 0.3 to 1.5
%, P: 0.02 to 0.08%, S: 0.003 to 0.
015%, sol.Al: 0.02-0.06%, N: 0.
004% or less, Ti: 0.03 to 0.15%,
In producing a hot-dip galvanized steel sheet satisfying 0.8 ≦ Ti ^{* /} C ≦ 1.5 by a process including at least a hot rolling step, a cold rolling step, an annealing step, and a galvanizing step, hot rolling is performed. A method for producing a high-strength galvanized steel sheet having excellent surface appearance and workability, wherein the winding temperature at the time is 580 to 720 ° C. However, Ti ^* = (1
2/48) × ｛Ti- (48/14) × N- (48/3
2) xS}. 9. B: 0.0003 to 0.0020
The method for producing a high-strength galvanized steel sheet having excellent surface appearance and workability according to claim 8, characterized in that the steel sheet comprises: 10. The heating rate in the annealing step is 10 ° C./s.
The method for producing a galvanized steel sheet having excellent surface appearance and workability according to claim 8 or 9, wherein the soaking temperature is 740 to 830 ° C or higher.