JP2631437B2 - Cold rolled steel sheet excellent in workability, bake hardenability and aging, and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet having excellent workability, bake hardenability and aging, and a method for producing the same. The cold-rolled steel sheet referred to here is a so-called cold-rolled steel sheet and a steel sheet subjected to hot-dip plating and electroplating.

[0002]

2. Description of the Related Art As a method for improving the workability and bake hardenability of a cold rolled steel sheet, for example, Japanese Patent Application Laid-Open No. Sho 55-14152.
No. 6, JP-A-55-141555, Nb is added to Nb-added steel in accordance with the content of C, N, and Al in the steel. % Nb / (Solute C +
A method is disclosed in which the solid solution C) and the solid solution N in the steel sheet are adjusted by limiting the solid solution N) to a certain range, and further, the cooling rate after annealing is controlled.

Further, Japanese Patent Publication No. Sho 61-45689 discloses that a cold rolled steel sheet having excellent bake hardenability is obtained by adding a combination of Ti and Nb.
No., JP-B-55-12167 describes Mo, C
Disclosed are a low-yield-ratio high-strength steel sheet having a low yield strength due to the addition of r, a high tensile strength, and a remarkable recovery by strain aging treatment, and a method for producing the same.

[0004] Further, as to a method for producing an ultra-deep drawing steel sheet having no bake hardenability, there are disclosed a Ti-killed steel sheet (Japanese Patent Publication No. 61-45689) and an Nb-killed steel sheet (Japanese Patent Publication No. 54-1245). Two families are disclosed. However, these steel sheets have excellent workability because C and N in the steel sheets are completely fixed as precipitates such as Ti or Nb, but the strain aging phenomenon occurs at the time of coating baking after pressing. Therefore, it has no bake hardenability.

As described above, there is a strong demand for improving both the workability and the bake hardenability of cold rolled steel sheets.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold-rolled steel sheet excellent in workability, bake hardenability and aging which advantageously satisfies the above requirements and a method for producing the same.

[0007]

The gist of the present invention is as follows. (1) In weight%, C: 0.0010% or more and 0.01
%, Si: 0.005% to 0.8%, Mn:
0.05% or more and 3% or less, P: 0.001% or more and 0.1
5% or less, S: 0.020% or less, Al: 0.01% or more and 0.1% or less, N: 0.01% or less, and Mo:
A cold-rolled steel sheet containing 0.001% or more and 3.0% or less and excellent in workability, bake hardenability, and aging, with the balance being Fe and inevitable impurities.

(2) C: 0.0010% by weight
Not less than 0.01%, Si: 0.005% to 0.8%
Hereinafter, Mn: 0.05% or more and 3% or less, P: 0.001
% To 0.15%, S: 0.020% or less, Al:
Steel containing not less than 0.01% and not more than 0.01%, N: not more than 0.01%, Mo not less than 0.001% and not more than 3.0%, and the balance consisting of Fe and inevitable impurities is 700 ° C or more. After hot rolling at 500 ° C. or higher, then cold rolling at a reduction ratio of 50% or more and 90% or less, and then performing continuous annealing at a temperature between the recrystallization temperature and the A ₃ transformation point. A method for producing cold-rolled steel sheets with excellent workability, bake hardenability, and aging characteristics.

The cold-rolled steel sheet to which the present invention is applied is a so-called cold-rolled steel sheet which is not subjected to plating or the like, a plated steel sheet which is plated with zinc or the like. Any method using an open hearth or the like may be used, and a slab cast by ingot casting using a mold and then slab-rolled, a slab cast by continuous casting, or the like may be used as a starting material, but the manufacturing method is not limited.
The present inventors have conducted various studies in order to improve the workability, bake hardenability, and aging properties of cold-rolled steel sheets. As a result, C: 0.0010% to 0.01% by weight. , S
i: 0.005% or more and 0.8% or less, Mn: 0.05%
3% or less, P: 0.001% or more and 0.15% or less,
S: 0.020% or less, Al: 0.01% or more and 0.1%
In the following, in addition to N: 0.01% or less, steel containing 0.001% or more and 3.0% or less of Mo and the balance of Fe and unavoidable impurities is hot-rolled at 700 ° C or more, and then 500%. ℃ coiling at temperatures above after cold rolling at a reduction ratio of thereafter 50% to 90%, the recrystallization temperature or more a ₃
By conducting continuous annealing at a temperature equal to or lower than the transformation point, it has been found that a cold-rolled steel sheet which is excellent in workability and aging properties and has greatly improved bake hardenability can be obtained.

In the conventional cold-rolled steel sheet, the workability is excellent but no bake hardenability is obtained, or even if it has the bake hardenability, the amount thereof is small and the aging property is impaired. They were incompatible, unstable, and poor in workability. The reasons for limiting the components of the steel of the present invention are as follows. First, C: C is 0.0010% or more and 0.0
1% or less. The lower limit was set to 0.0010% because C
The lower the amount, the better the workability, but below this, the solid solution C
Is not sufficient and is not an amount necessary to enhance bake hardenability. Also, the upper limit is set to 0.01% because
If it exceeds this, C is a strengthening element of steel, so that the strength becomes too high and the workability is impaired.

Si: Si is set to 0.005% or more and 0.8% or less. Si is a strengthening element for steel and is added according to the required strength, and the lower limit is made 0.005%. The upper limit is set to 0.8% because if it exceeds this, the strength becomes too high and the workability is impaired, and zinc is hardly adhered when performing galvanizing or the like, and the adhesion is impaired.

Mn: Mn is at least 0.05% and at most 3%. Mn is a strengthening element for steel and is added according to the required strength, with the lower limit being 0.05%. The upper limit is set to 3% because if it exceeds 3%, the strength becomes too high and the workability is impaired. P: P is set to 0.001% or more and 0.15% or less. P is a strengthening element for steel even in a small amount, and is added according to the required strength, and the lower limit is 0.001%. The upper limit is 0.1
The reason for setting the content to 5% is that if it exceeds this, the strength becomes too high and the workability is impaired. In addition, P is an element which is concentrated at the crystal grain boundaries and easily causes grain boundary embrittlement. %
If added in excess of, the workability is impaired.

S: The content of S of 0.020% or less is an element that is essentially meaningless to be present in steel.
If added in excess of 0%, if the amount of sulfide forming elements such as Mn is small, red hot embrittlement occurs during hot rolling, and the surface cracks.
This is because so-called hot embrittlement may occur. Al: Al is set to 0.01% or more and 0.1% or less. The lower limit is set to 0.01% because if it is less than 0.01%, it becomes difficult to precipitate N as AlN and to improve the aging property by N. Further, the upper limit is set to 0.1% because, even if added in excess of this, the improvement of aging is saturated, and the strength becomes too high, thereby impairing workability.

N: The N content is set to 0.01% or less. If the N content exceeds this range, the aging property cannot be ensured unless the amount of Al added is too large, and the strength becomes too high, and the workability becomes too high. This is to spoil. Mo: Mo is set to 0.001% or more and 3.0% or less. The lower limit is set to 0.001% because there is no effect of increasing the bake hardenability below this value. Also, the upper limit is 3.0
The reason for setting it to% is that if it exceeds this, Mo is a strengthening element of steel, and the strength becomes too high to impair workability.
This is because the bake hardenability is also saturated, so that it is no longer expensive and economical. Mo is known to be an element that suppresses the nucleation of Fe ₃ C and suppresses the pearlite transformation, but the reason for increasing the bake hardenability is not clear, but it was added for low carbon steel. Because Mo forms a solid solution and creates many strain fields, precipitates with solute carbon and solute nitrogen that remain easily even at a low temperature of about 170 ° C when baking paint in a part with little processing strain Or a cluster is formed, and the movable dislocations are fixed and hardened.
Conversely, instead of forming precipitates or clusters of Mo and C, a compression is formed in which C gathers around Mo, and at normal temperature, the movement of C is suppressed, and the baking temperature of the paint is 170 to 200 ° C. It is also considered that Mo-C separates and C fixes dislocations and hardens. In any case, it is considered that the effect of adding Mo appears as an improvement in bake hardenability.

In addition, B is not limited to be added as an element to be contained, but is not limited to about 0.0010% in order to improve hardenability and strength and to improve secondary workability. It may be added. Next, the reason why the finish rolling end temperature was set to 700 ° C. or higher as a hot rolling condition is as follows.
If it is less than that, the rolled structure remains, and the amount of reduction during cold rolling becomes large, which is disadvantageous. Further, in order to improve the workability, it is said that the crystal grains at the end of hot rolling are preferably random, and the remaining rolled structure adversely affects the workability from the aspect of the crystal texture. is there.
The upper limit is not limited, but may be 960.
A temperature of less than ° C is suitable.

The reason why the winding temperature is set to 500 ° C. or higher is that crystal grains must be enlarged in order to improve the workability of the steel sheet, and crystal grains are expected to grow in a cooling process from a high temperature. It is. The rolling reduction of cold rolling is 50%.
Or a 90% or less, of the lower limit was 50% on the condition that the continuous annealing at a recrystallization temperature or higher A ₃ following transformation point temperature, the cold-rolling reduction has optimal point to improve the workability, 50% even if strain is stored before cold rolling
If it is less than 1, the cold-rolled texture cannot be adjusted to optimize the annealed texture and the drawability cannot be improved. The reason why the upper limit of the cold rolling reduction is set to 90% is that if strain is applied beyond that, the (100) plane of the cold rolled texture increases,
This is because the (100) plane having poor drawability remains in the texture after annealing, and the drawability is impaired. This is because the strain in cold rolling may be small when combined with the strain before cold rolling. . In other words, the cold rolling reduction is not only adjusted to the final thickness of the cold-rolled steel sheet, but also has an optimum point for improving the texture, and its industrial significance is significant.

Furthermore, as the following A ₃ transformation point temperature conditions recrystallization temperature or more continuous annealing, to that the recrystallization temperature of the lower limit is not removed the strain generated by cold rolling is less than it, yet again This is because crystals do not crystallize and do not become excellent in workability, and workability is inferior. Further, the upper limit was set to A ₃ transformation point exhibits a steel sheet surface after machining recrystallized grains are coarsened rough skin when annealed beyond it, in order to produce the appearance of problems. It is said that the higher the heating rate at the time of annealing is, the more the development of the (111) plane which improves workability is promoted, but is not particularly specified.

The annealing time is not particularly limited, but may be longer than the recrystallization completion time, which is shorter when the temperature is higher and longer when the temperature is lower. The cooling rate after annealing is not specified, but rapid cooling is desirable from the viewpoint of enhancing bake hardenability. The subsequent temper rolling may be performed for shape adjustment, but the product may be used without temper rolling as it is. There is no yield point elongation without temper rolling, and the workability is good because it is a method that can be manufactured with a large number of movable dislocations remaining as described above. This is also advantageous in that processing is easy and processing is low.

As shown in FIG. 1, the cold-rolled steel sheet manufactured by adjusting the components in this manner has a high r value at a hot rolling finish temperature of 700 ° C. or higher and lower than the A ₃ transformation point, and has a good workability. It is a steel plate. Further, as shown in FIG. 2, a steel sheet having excellent bake hardenability can be obtained by adding Mo.
In the region where the amount of Mo added is 0.001% or more and 3.0% or less, the amount of bake hardening increases, and furthermore, a cold-rolled steel sheet having no yield point elongation and good workability and excellent bake hardenability is obtained. it can.

Thus, the composition of the steel is adjusted, and the hot rolling conditions,
By adjusting the cold rolling conditions and the annealing conditions, a cold rolled steel sheet excellent in workability, bake hardenability, and aging can be obtained. As described above, the components for forming a cold-rolled steel sheet having excellent workability, bake hardening properties, and aging properties, and a method for producing the same, such as casting with a mold or continuous casting, and
, C: 0.0010% to 0.01%, Si:
0.005% to 0.8%, Mn: 0.05% to 3%, P: 0.001% to 0.15%, S:
0.020% or less; Al: 0.01% or more and 0.1% or less; N: 0.01% or less;
A steel containing 001% or more and 3.0% or less, the balance being Fe and unavoidable impurities. After hot rolling the steel having such a chemical composition at 700 ° C. or more, 50%
Winding at a temperature of 0 ° C. or more, and then cold rolling at a reduction of 50% or more and 90% or less, followed by continuous annealing at a temperature of a recrystallization temperature or more and an A ₃ transformation point or less, so that workability is excellent. It is possible to produce a cold-rolled steel sheet that does not generate stretcher strain, has excellent aging properties, and has greatly improved bake hardenability.

It is to be noted that, for example, Zn
The electroplated be a rustproof steel sheet, can be an excellent anti-corrosion steel sheet bake hardenability and the hot-rolled steel of such components, after cold rolling, the recrystallization temperature or more A ₃ After continuous annealing at a temperature below the transformation point, hot-dip galvanizing is immediately performed, and by performing alloying treatment, an alloyed hot-dip galvanized steel sheet that can contribute to high strength and high rust prevention It is possible.

[0022]

EXAMPLES Next, Examples of the present invention are shown together with Comparative Examples in Tables 1 to 3.
Listed in 3. Table 1 shows the components of the steel, and Tables 2 and 3 (continued from Table 2) show the steel production conditions and characteristic values.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

Industrial Applicability According to the present invention, a cold rolled steel sheet having excellent workability, bake hardening property, and aging property, for example, which can be used for an outer panel of an automobile, has excellent dent resistance, and is soft during processing. It is possible to provide a cold-rolled steel sheet having the property of sometimes becoming hard.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a hot rolling finish temperature of a cold rolled steel sheet and an r value. (Composition and manufacturing conditions of the cold-rolled steel sheet in FIG. 1) C: 0.0030% Si: 0.013% Mn: 0.140% P: 0.010% S: 0.004% Al: 0.040% N : 0.0025% Mo, W, Cr total amount: 0.13% Hot-rolling finishing temperature: 650 to 930 ° C Hot-rolling winding temperature: 600 ° C Cold rolling reduction: 80% Cold-rolled final sheet thickness: 0. 8mm Annealing temperature: 800 ° C × 40sec Cooling rate: 100 ° C / sec Temper rolling reduction: 0.8%

FIG. 2 is a graph showing the relationship between the amount of Mo added to a cold-rolled steel sheet and bake hardenability. (Composition of cold rolled steel sheet in FIG. 2, manufacturing conditions) C: 0.0030% Si: 0.013% Mn: 0.140% P: 0.010% S: 0.004% Al: 0.040% N : 0.0025% Mo: 0.13% Hot-rolling finishing temperature: 650 to 930 ° C Hot-rolling winding temperature: 600 ° C Cold rolling reduction: 80% Cold-rolled final sheet thickness: 0.8 mm Annealing temperature: 800 ° C × 40 sec Cooling rate: 100 ° C / sec Temper rolling reduction: 0.8%

Claims (2)

(57) [Claims] 1. In weight%, C: 0.0010% to 0.01%, Si: 0.005% to 0.8%, Mn: 0.05% to 3%, P: 0 0.001%
0.15% or less, S: 0.020% or less, Al:
0.01% or more and 0.1% or less; N: 0.01% or less; Mo: 0.001% or more and 3.0% or less; workability and bake hardening consisting of balance of Fe and unavoidable impurities Cold rolled steel sheet with excellent properties and aging. 2. In weight%, C: 0.0010% to 0.01%, Si: 0.005% to 0.8%, Mn: 0.05% to 3%, P: 0 0.001%
0.15% or less, S: 0.020% or less, Al:
Steel containing not less than 0.01% and not more than 0.01%, N: not more than 0.01%, Mo not less than 0.001% and not more than 3.0%, and the balance consisting of Fe and inevitable impurities is 700 ° C or more. After hot rolling at 500 ° C. or higher, then cold rolling at a reduction ratio of 50% or more and 90% or less, and then performing continuous annealing at a temperature between the recrystallization temperature and the A ₃ transformation point. A method for producing cold-rolled steel sheets with excellent workability, bake hardenability, and aging characteristics.