JPH04365814A - Production of cold rolled high strength steel sheet excellent in baking hardenability - Google Patents

Abstract

PURPOSE:To produce a cold rolled high strength steel sheet excellent in baking hardenability by successively subjecting a steel, having a specific composition where respective contents of Ti, Nb, Mo, and Cr are specified, to hot rolling, coiling, and continuous annealing after cold rolling under respectively specified conditions. CONSTITUTION:The steel is prepared by adding 0.01-0.20%, in total, of Ti and/or Nb to a steel having a composition consisting of, by weight, 0.05-0.25% C, <=0.8% Si, 0.50-3.0% Mn, <=0.1% P, <=0.020% S, 0.01-0.1% Al, <=0.01% N, and the balance Fe with inevitable impurities and further adding 0.01-3.0% Mo and 0.01-3.0% Cr to the above steel. This steel is hot-rolled at a temp. not lower than the Ar3 transformation point, coiled at >=500 deg.C, cold-rolled, and then continuously annealed at a temp. not lower than the recrystallization temp. at about 5-5000 deg.C/sec heating rate. By this method, the cold rolled high strength steel sheet increased in tensile strength as well as yield strength due to baking hardening and excellent in baking hardenability can be obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high-strength cold-rolled steel sheets with excellent bake hardenability.

0002

[Prior Art] Conventionally, high-strength cold-rolled steel sheets have been produced by water cooling as described in Japanese Patent Application Laid-Open No. 58-22327, or by increasing the amount of alloying elements added when the cooling rate is slow as in box annealing. Methods are used to increase strength. In the water cooling method, the cooling rate is too fast, so a large amount of strain accumulates in the steel plate during the cooling process, making it impossible to obtain excellent bending and other processing properties.Also, in the case of box annealing, where the cooling rate is extremely slow, In order to obtain high tensile strength, it is necessary to add a large amount of alloying elements, which impairs weldability and becomes economically expensive.In addition, the steel sheet before cold rolling is hard and difficult to use in a cold rolling mill. It is considered difficult to manufacture thin steel sheets due to the manufacturing capacity, and conventional manufacturing methods have not been able to manufacture high-strength thin steel sheets with high tensile strength that satisfies bendability and other processing characteristics.

On the other hand, as a method for improving the bake hardenability of cold-rolled steel sheets, for example, as described in JP-A-55-141526 and JP-A-55-141555, in Nb-added steel, C, Nb is added according to the N and Al contents, and at. By limiting Nb/(solid solute C + solid solute N) within a certain range, the solid solute C and solid solute N in the steel sheet can be reduced.
A method for adjusting the cooling rate after annealing and further controlling the cooling rate after annealing is disclosed.

Furthermore, as described in Japanese Patent Publication No. 61-45689, it has been disclosed that a steel sheet having excellent bake hardenability can be obtained by adding Ti and Nb in combination. However, even with such a method, it is still difficult to obtain satisfactory results on an industrial scale, and the amount of bake hardening is still small. Moreover, there is no disclosure of a method for producing a cold-rolled steel sheet that has high strength and bake hardenability.

[0005] Thus, there is a strong demand for improving the bake hardenability of high-strength cold-rolled steel sheets.

[0006]

OBJECTS OF THE INVENTION The present invention has been made to advantageously satisfy these needs.

[0007]

[Means for Solving the Problems] The gist of the present invention is that when producing a cold rolled steel sheet, C:
0.05-0.25%, Si: 0.8% or less, Mn: 0
．． 50-3.0%, P: 0.1% or less, S: 0.020
% or less, Al: 0.01 to 0.1%, N: 0.01% or less, and the balance is Fe and unavoidable impurities, and the total amount of one or both of Ti and Nb is 0. .01
~0.20% added, further Mo: 0.001~3.0
%, Cr: 0.01~3.0% steel added with Ar3
Yield strength and tensile strength by bake hardening characterized by hot rolling at a temperature higher than the transformation point, then winding at a temperature higher than 500°C, followed by cold rolling and continuous annealing at a temperature higher than the recrystallization temperature. The present invention provides a method for producing a high-strength cold-rolled steel sheet with excellent bake hardenability.

[0008] The cold-rolled steel sheets to which the present invention is applied include so-called cold-rolled steel sheets that are not subjected to plating, etc., and galvanized steel sheets that are coated with zinc or the like, and methods for producing steel include converter furnaces, electric furnaces, and open hearth furnaces. Any method such as the above may be used, and the method of manufacturing the slab does not matter, such as a slab formed by casting with a mold and then bloomed, or a slab formed by continuous casting. The present inventors have conducted various studies in order to improve the bendability and workability of high-strength cold-rolled steel sheets with excellent bake hardenability.
In, C: 0.05 to 0.25%, Si: 0.8% or less, Mn: 0.50 to 3.0%, P: 0.1% or less, S:
0.020% or less, Al: 0.01-0.1%, N: 0
．． 0.01% or less, with the remainder consisting of Fe and unavoidable impurities, one or both of Ti and Nb are added in a total amount of 0.01 to 0.20%, and further Mo: 0.00
Cr: 0.01-3.0% and Cr: 0.01-3.0% are hot-rolled at a temperature higher than the Ar3 transformation point, then coiled at a temperature of 500°C or higher, and then cold-rolled and re-rolled. We have discovered that continuous annealing at temperatures above the crystallization temperature can produce high-strength cold-rolled steel sheets with excellent bendability and workability, as well as excellent bake hardenability, which increases yield strength and tensile strength through bake hardening. Ta.

[0009] The conventional high-strength cold-rolled steel sheets described above have poor bendability and workability, have no bake hardenability, or even if they do have bake hardenability, the amount is low. Moreover, it was unstable because it impairs the statute of limitations. The reason for limiting the components of the steel of the present invention is as follows. First, C: 0.05 to 0.25%, and the lower limit was set to 0.05% because if it is less than that, C is a strengthening element for steel, and the required strength cannot be achieved. This is also because bake hardenability, which increases yield strength and tensile strength due to bake hardening, does not improve. The reason why we set the upper limit to 0.25% is because if it exceeds it, the strength will increase and the workability will be impaired, and moreover, the amount of addition of one or two elements of Ti and Nb, Cr, and Mo will increase. This is because an increase in strength due to precipitates is unavoidable, resulting in poor workability and being economically disadvantageous. In addition, when the amount of C added increases, from the viewpoint of weldability such as spot welding,
This is because the strength of the formed nugget becomes weaker, and the high-strength cold-rolled steel sheet becomes meaningless when used as a reinforcing member.

[0010]Si: The reason for setting it to 0.8% or less is because Si is a strengthening element for steel, increasing its strength and impairing workability.When performing zinc plating, etc., zinc This is because it is difficult to adhere and impairs adhesion. Mn: 0.50-3.0%, the lower limit is 0.50%
This is because if it is less than that, Mn is a strengthening element for steel and the required strength cannot be secured, and Mn is an element that fixes S and prevents hot embrittlement. This is because it is necessary to enable hot working. The reason why the upper limit is set to 3.0% is that if it exceeds the upper limit, the strength will increase and workability will be impaired.

[0011] P: The reason for setting it below 0.1% is that even a small amount of P is an element that strengthens the steel.
This is because the strength increases and the workability is impaired, and moreover, P
is an element that tends to concentrate at grain boundaries and cause grain boundary embrittlement, and adding more than this will impair workability. S: 0.020% or less is an element that has no meaning in originally existing in steel, and if added in excess of this,
This is because if the content of sulfide-forming elements such as Mn is low, red brittleness may occur during hot rolling, which may cause cracking on the surface, so-called hot brittleness.

[0012] Al: 0.01 to 0.1%, and the lower limit was set to 0.01% because if it is less than that, N will precipitate as AlN, making it difficult to improve the aging property due to N. It is. Moreover, the reason why the upper limit is set to 0.1% is that even if it is added in excess of this, the improvement in aging properties will be saturated, and the strength will also increase, impairing workability. The reason why N: is set to 0.01% or less is that if it is added in excess of this amount, aging properties cannot be ensured unless the amount of Al added is increased, and furthermore, the strength increases and workability is impaired.

[0013] The total amount of one or both of Ti and Nb is 0.01 to 0.20%, and the processability is improved by adding only Ti. This is because adding only Nb gives good bake hardenability, and adding the two in combination compensates for the drawbacks of both, making it more effective. The lower limit is 0.0
The reason why it is set at 1% is that if it is less than that, it becomes impossible to fix solid solution elements such as C and N and ensure aging properties. Moreover, the reason why the upper limit is set to 0.20% is that even if added in excess of this, the aging property will be saturated, and furthermore, the strength will increase due to precipitates, leading to deterioration of workability.

[0014] Mo: 0.001 to 3.0%, and the lower limit is set to 0.001% because if it is less than that, there is no effect of increasing bake hardenability. Also, set the upper limit to 3.0
% because Mo is a reinforcing element for steel, and the strength becomes too high, impairing workability, and the bake hardenability becomes saturated, making it expensive and economical. This is so that it will disappear. Mo is known to be an element that suppresses the nucleation of Fe3C and suppresses pearlite transformation, but the reason why it increases bake hardenability is not clear, but the added Mo dissolves in solid solution, causing a large amount of In order to create the strain field of Therefore, it is thought that movable dislocations are fixed and hardened, and the effect of adding Mo appears.

[0015] Cr: 0.01 to 3.0%, and the lower limit was set to 0.01% because if it is less than that, Cr is not effective in increasing the strength and is not effective in preventing brittle fracture of steel. The reason why the upper limit was set at 3.0% is that beyond this, the contribution to preventing brittle fracture of steel becomes saturated, and Cr combines with C and contributes to increasing strength as carbide. However, even if added in excess of this amount, the strength increasing effect will be saturated.

Next, under the hot rolling conditions, the finish rolling end temperature is set to Ar.
3 The reason for setting the transformation point or higher is that if it is lower than that, the rolling structure will remain and the reduction amount will become large during cold rolling, which is disadvantageous.In addition, in order to improve workability, it is necessary to It is said that the more random the grains are, the better, and the residual rolling structure will have a negative effect on workability from the perspective of crystal texture.

[0017] Furthermore, the reason why the coiling temperature is set to 500°C or higher is that it is necessary to increase the size of crystal grains in order to improve the workability of the steel sheet, and growth of crystal grains is expected during the cooling process from high temperature. This is because the steel is soft and requires less reduction during cold rolling. Furthermore, as a condition for continuous annealing at a temperature higher than the recrystallization temperature after cold rolling, the cold rolling rate has an optimum point for improving workability, and by adjusting it to the final thickness of the cold rolled steel sheet, it is especially important to Although not necessarily, 30 to 80% is desirable. The reason why the temperature condition for continuous annealing is set to be higher than the recrystallization temperature is because if it is lower than that, the strain generated by cold rolling will not be removed and recrystallization will not occur, resulting in poor workability. It's for a reason. Note that the higher the annealing temperature, the more the single phase in the γ region becomes, the more uniform the structure after cooling, and the better the workability, especially the bendability, that is, the local deformability. The temperature for grain growth may be 700° C. or higher. Furthermore, it is said that the faster the heating rate during annealing, the more the development of (111) planes, which improve workability, is promoted, but in the case of high-strength cold-rolled steel sheets,
There are no particular regulations. The range is about 5 to 5000°C/sec in annealing called continuous annealing, and the heating method is not specified.

[0018] The annealing time is not particularly specified, but it may be set to be equal to or longer than the recrystallization completion time, which is related to the temperature: the higher the temperature, the shorter the annealing time, and the lower the temperature, the longer the annealing time. Although the cooling rate after annealing is not specified, rapid cooling is desirable from the viewpoint of increasing bake hardenability and increasing mobile dislocations. This is due to the principle that the solute carbon that imparts bake hardenability to low carbon steel remains, and during bake hardening, the movable dislocations of the solute carbon are fixed, and the combination of carbide-forming elements such as Mo and Cr with C. It is thought that the strength increases due to the fixation of mobile dislocations due to the formation of fine carbides, which is thought to contribute to the increase in tensile strength as well as the increase in yield strength due to bake hardening.

[0019] The subsequent skin pass rolling may be carried out to adjust the shape, but it is also possible to produce a product as is without skin pass rolling. There is no yield elongation even without temper rolling,
The reason why the workability is good is because, as mentioned above, this method can be manufactured by leaving many movable dislocations, and it is also advantageous in that it has a low yield point and is easy to work by not performing skin pass rolling.

As shown in FIG. 1, the high-strength cold-rolled steel sheet produced in this manner can be made into a steel sheet with excellent bake hardenability by adding Mo. The amount of Mo added is 0.001~
The amount of bake hardening increases in the 3.0% range, and it is possible to obtain a high-strength cold-rolled steel sheet that has excellent bendability and workability, and has excellent bake hardenability that increases yield strength and tensile strength due to bake hardening. can.

[0021] Thus, by adjusting the composition of the steel, the hot rolling conditions,
By adjusting the cold rolling conditions and annealing conditions, it is possible to obtain a high-strength cold rolled steel sheet with excellent bake hardenability, which increases the yield strength and tensile strength due to bake hardening. Therefore, as a method for manufacturing a high-strength cold-rolled steel sheet with excellent bake hardenability that increases the yield strength and tensile strength due to bake hardening, continuous casting is used, and C: 0.05 to 0.05 by weight. 0.2
5%, Si: 0.8% or less, Mn: 0.50-3.0%
, P: 0.1% or less, S: 0.020% or less, Al: 0
．． 0.01 to 0.1%, N: 0.01% or less, and the balance is Fe and unavoidable impurities, and one or both of Ti and Nb are added in a total amount of 0.01 to 0.20%. Additionally, Mo: 0.001 to 3.0%, Cr: 0.0
After hot-rolling steel containing 1 to 3.0% of Ar3 at a temperature higher than the transformation point, it is wound up at a temperature of 500°C or higher, and then, after cold rolling, continuous annealing is performed at a temperature higher than the recrystallization temperature. As a result, a high-strength cold-rolled steel sheet can be obtained which has excellent bendability and workability, and also has excellent bake hardenability in which the yield strength and tensile strength increase due to bake hardening.

[0022] The steel plate thus produced may be used, for example, in Z
A rust-proof steel plate can be obtained by electroplating n, and a rust-proof steel plate with excellent bake hardenability can be obtained.Furthermore, such a steel plate can be subjected to continuous annealing at a temperature higher than the recrystallization temperature after cold rolling. After that, hot-dip galvanizing is immediately applied, and alloying treatment is also performed to obtain an alloyed hot-dip galvanized steel sheet, which can contribute to high strength and high rust prevention.

[0023]

[Examples] Next, Examples of the present invention are shown in Tables 1 to 1 along with comparative examples.
Listed in Table 4. Tables 1 and 2 show the composition of the steel, and Tables 3 and 4 show the manufacturing conditions and characteristic values of the steel plate.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

[Effects of the Invention] By doing so, the workability of the high-strength cold-rolled steel sheet is improved, and a high-strength cold-rolled steel sheet with excellent bake hardenability that increases yield strength and tensile strength due to bake hardening can be obtained. For example, it is used for strength members such as automobile bumpers and door impact beams, and has excellent bendability and workability, and is soft during processing, and has the characteristics of increasing both yield point and tensile strength and becoming hard during use. Excellent effects such as being able to do this can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the amount of Mo added and bake hardenability, which is an increase in yield strength and tensile strength due to bake hardening, of a high-strength cold-rolled steel sheet manufactured under the following conditions. C: 0.15% Si: 0.05% Mn: 2.52% P: 0.012% S: 0.007% Al: 0.036% N: 0.0023% Ti: 0.048% Mo: 0 to 3.8% Cr: 0.20% Hot rolling finishing temperature: 860°C Hot rolling winding temperature: 600°C Cold rolling rate: 50% Cold rolling final plate thickness: 1.4mm Annealing temperature: 850°C x 120sec cooling rate
:150℃/sec Temper rolling rate :0.4%

Claims (1)

[Claims] [Claim 1] When manufacturing a cold rolled steel sheet, weight %
In, C: 0.05 to 0.25%, Si: 0.8% or less, Mn: 0.50 to 3.0%, P: 0.1% or less, S:
0.020% or less, Al: 0.01-0.1%, N: 0
．． 0.01% or less, with the remainder consisting of Fe and unavoidable impurities, one or both of Ti and Nb are added in a total amount of 0.01 to 0.20%, and further Mo: 0.00
Cr: 0.01-3.0% and Cr: 0.01-3.0% are hot-rolled at a temperature higher than the Ar3 transformation point, then coiled at a temperature of 500°C or higher, and then cold-rolled and re-rolled. A method for producing a high-strength cold-rolled steel sheet with excellent bake hardenability, which increases yield strength and tensile strength through bake hardening, the method comprising continuous annealing at a temperature higher than the crystallization temperature.