JPH0734138A - Production of high strength cold rolled steel sheet and surface treated steel sheet excellent in baking hardenability and deep drawability - Google Patents

Abstract

PURPOSE:To produce a high strength surface treated steel sheet excellent in baking hardenability and deep drawability by subjecting an ultralow carbon steel cold rolled sheet having a specified compsn. to continuous annealing in a specified temp. range, applying it with hot dip metal plating according to necessary and furthermore executing surface treatment. CONSTITUTION:A slab having a compsn. contg., by weight, 0.001 to 0.004% C, <0.4% Si, 1 to 2% Mn, <0.07% P, <0.01% S, 0.01 to 0.08% Sol.Al, <0.004% N, 0.003 to 0.02% Nb and >0.002% Ti and satisfying Ti-(48/14)N<0.004% is subjected to hot rolling into a hot rolled sheet. After the surface is subjected to pickling treatment, it is subjected to cold rolling into a cold rolled steel sheet. This cold rolled steel sheet is subjected to continuous annealing in the temp. ranges of T1 to T2 expressed by the formulae I and II, or furthermore, hot dip metal plating and an alloyed plating layer of molten metal and Fe in the steel sheet are formed, and after that, a hot dip metal plating layer of an alloy of Zn, Al, Zn-Al or the like is formed, by which the high strength surface treated steel sheet excellent in baking hardenability and deep drawability is produced.

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 steel sheet and a surface-treated steel sheet having excellent bake hardenability and aging resistance. For example, a steel sheet suitable for use as an outer panel of an automobile is provided. It is intended to provide a method that can be appropriately manufactured.

[0002]

2. Description of the Related Art In recent years, attention has been paid to global environmental problems,
Regarding automobiles, reducing exhaust gas has become an important issue. As one of measures for reducing exhaust gas, automobile outer panel and the like are thinned, and fuel consumption is being improved by reducing the weight of the vehicle body. The outer panel can be made thinner by increasing the strength of the steel plate, but the steel plate is also required to have excellent deep drawability so that complicated press working can be performed even if the strength is increased. In addition, since the outer panel is remarkably impaired in appearance due to dents, excellent dent resistance (difficulty in denting the steel sheet, proportional to the square of the thickness of the steel sheet, and proportional to the yield point of the steel sheet) is required. In order to maintain good dent resistance before and after thinning, it is particularly effective that the steel sheet has excellent bake hardenability (the property that the yield point increases in the bake coating step after pressing).

However, in order to meet the above requirements,
A bake hardening type steel sheet has been developed in which a very small amount of carbonitride forming elements such as Nb and Ti are added to ultra-low carbon steel to improve deep drawability, and strength is enhanced with Si, Mn, P and the like. Among them Mn
Strengthening due to is an advantageous strengthening method from the viewpoint of the deterioration of the surface properties of the steel sheet and the embrittlement of the steel sheet, but Mn is generally considered to deteriorate the deep drawability, and Mn is actively Examples of utilization are Japanese Patent Application Laid-Open No. 2-111841, “Materials and Processes” (CAMP-ISIJ), 5 (199).
2), p. It can only be seen in 2051 and so on. That is, the former is intended to improve the r value by optimizing the amount of Mn and hot rolling conditions to make the structure of the hot rolled sheet uniform and fine. Also,
The latter is a technique in which the Ac3 transformation point is lowered by the addition of Mn and soaking is performed in the α-γ two-phase region and the γ single-phase region during annealing.

[0004]

However, the above-mentioned conventional techniques still have their respective problems.
That is, since the technique according to Japanese Patent Laid-Open No. 2-111841 has a high Ti addition amount and produces Ti-based carbides that are difficult to dissolve during annealing,
Satisfactory bake hardenability cannot be obtained. The latter "material and process" also requires high temperature annealing in the α-γ two-phase region (or γ single-phase region) in order to obtain sufficient bake hardenability.

[0005]

The present invention has been studied to solve the problems in the prior art as described above, and succeeded in overcoming such problems. The method for producing the high-strength cold-rolled steel sheet and the surface-treated steel sheet having excellent deep drawability is as follows.

(1) C: 0.001-0.
004%, Si: 0.4% or less, Mn: 1-2%,
P: 0.07% or less, S: 0.01% or less, sol. Al
: 0.01 to 0.08%, N: 0.004% or less, Nb: 0.00
3 to 0.02%, Ti: 0.002% or more, and Ti- (48/14) N
A steel containing 0.004% or less and the balance being Fe and unavoidable impurities is cast, hot-rolled, pickled and cold-rolled at a temperature T ₁ ° C or higher represented by the following formula (1), and (2)
A method for producing a high-strength cold-rolled steel sheet excellent in bake hardenability and deep drawability, which comprises continuously annealing at a temperature of T ₂ ° C or less represented by the formula.

[0007]

[Formula 3] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ······· (1) T ₂ = 910 +110 × Si−40 × Mn + 200 × P ··· ... (2)

(2) The steel described in the above item (1) is cast, hot-rolled, pickled and cold-rolled, and the temperature is represented by the following formula (1) at a temperature T ₁ ° C or higher in a continuous hot-dip galvanizing line. And bake hardenability and depth characterized by continuous annealing and hot dip coating at a temperature of T ₂ ° C or less represented by the following formula (2), and alloying of the plating layer as needed. A method for producing a high-strength surface-treated steel sheet having excellent drawability.

[0009]

[Formula 4] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ········· (1) T ₂ = 910 +110 × Si-40 × Mn + 200 × P ··· ... (2)

(3) High strength excellent in bake hardenability and deep drawability characterized by subjecting the surface of the cold rolled steel sheet manufactured according to the above (1) or (2) to various surface treatments Method for manufacturing surface-treated steel sheet.

[0011]

First, the reason for the limitation of the steel composition in the present invention is as follows. C: 0.001 to 0.004%, C is added as an element necessary for obtaining bake hardenability, but if it is less than 0.001%, bake hardenability cannot be expected. Further, if it exceeds 0.004%, the steel plate becomes hard and the deep drawability deteriorates.
Limited to ~ 0.004%.

Si: 0.4% or less, Si is effective in increasing the strength of the steel sheet, but excessive addition causes scale defects occurring during hot rolling and impairs the surface properties of the product. Further, if Si is concentrated on the surface of the steel sheet and a Si-based oxide is generated, the adhesion of the surface treatment layer is extremely deteriorated. Therefore, the upper limit is 0.4%.

Mn: 1 to 2%, Mn is effective in increasing the strength of the steel sheet, but in the present invention, Mn is added in order to obtain excellent bake hardenability and excellent deep drawability, The inventors newly found that the addition of Mn refines NbC in the hot rolled sheet. That is, since NbC refined by Mn has a large surface area per unit volume, it is easily melted during annealing, and the bake hardenability of the cold rolled steel sheet is remarkably enhanced. Also, NbC
Melts to facilitate the movement of the crystal grain boundaries, so that the grain growth is improved and the deep drawability is improved with the improvement of the bake hardenability. Although the details of the mechanism by which Mn refines NbC are not always clear, Mn changes the solubility of NbC and changes the precipitation morphology of NbC during hot rolling and after hot rolling.
Alternatively, since Mn lowers the Ar ₂ transformation point, NbC accompanying transformation
It is estimated that NbC becomes finer as the precipitation of Ni moves to the low temperature side. In order to obtain the effect of Mn as described above, it is necessary to add 1% or more of Mn, and if the added amount of Mn exceeds 2%, the deep drawability deteriorates significantly. Therefore, Mn is limited to 1 to 2%.

P: 0.07% or less, P is very effective in increasing the strength of the steel sheet, but addition of an excessively large amount makes the steel sheet brittle. Further, as with Si, excessive addition causes P to be concentrated on the surface of the steel sheet and deteriorates the adhesion and properties of the surface treatment layer. Therefore, the upper limit is set to 0.07%.

S: 0.01% or less, S forms sulfides in the steel and reduces the ductility of the steel sheet, so the smaller the content, the more desirable. Therefore, the upper limit is made 0.01% so as not to reduce the ductility.

Sol. Al: 0.01 to 0.08%, Al is added to deoxidize molten steel, and N that cannot be fixed with Ti is AlN
It is necessary to add at least 0.01% to fix it. However, when a large amount of Al is added, the inclusions in the steel increase and the ductility of the steel sheet deteriorates. Therefore, the acid-soluble Al level (sol. Al) is limited to 0.01 to 0.08%.

N: 0.004% or less, N has a diffusion rate in steel faster than C at room temperature, deteriorates the aging resistance of the material at room temperature and deteriorates the deep drawability, so it is fixed as a precipitate. There is a need. If the amount of N is large and the precipitates in the steel increase, the ductility decreases, so the smaller the amount of N, the more desirable. Therefore, it is limited to 0.004% or less.

Nb: 0.003 to 0.02%, Nb is a part of C
It is properly fixed as C and has excellent bake hardenability,
It is added in order to refine the crystal grains of the hot-rolled sheet to obtain an excellent deep drawability, but if it is less than 0.003%, its effect cannot be expected, so the lower limit is made 0.003%. However, if more than 0.02% is added, bake hardenability cannot be expected, and
Since the amount of NbC increases and the dissolution of NbC is hindered and the ductility decreases, the upper limit is 0.02%.

Ti: 0.002% or more and Ti- (48/14) N
Is 0.004% or less, Ti is a refined grain of the hot rolled sheet,
Moreover, by fixing N as TiN, excellent deep drawability can be obtained, so it is added. At least 0.002% is required for refining the crystal grains of the hot-rolled sheet, so the lower limit is 0.002.
%. However, in Figure 1, C: 0.002%, Si: 0.1%,
Mn: 1.4 to 1.6%, P: 0.05 to 0.07%, S: 0.01
%, Nb: 0.01%, N: 0.002% and 0.003%,
Steel sheet containing Ti: 0.005 to 0.020% (annealing temperature:
2% BH at 860 ° C is shown, but from this figure (48/1
4) It is clear that when a large amount of Ti is added in excess of N + 0.004, the bake hardenability decreases. This is considered to be due to the formation of Ti-based carbides (Ti ₄ C ₂ S _{2 ,} TiC). Therefore, Ti- (48/14) N is set to 0.004% or less.

Next, the reason for limiting the production conditions in the present invention will be explained. Although hot rolling may be carried out by an ordinary method, the coiling temperature is 500 ° C. or more in order to reduce the solid solution C in the hot rolled sheet. Is desirable. Further, cold rolling is preferably performed at a rolling reduction of 70% or more in order to improve deep drawability.

The annealing method is continuous annealing. Since box annealing involves a thermal cycle of gradual heating and gradual cooling, it is difficult to control the dissolution and reprecipitation of NbC, which causes fluctuation and deterioration of the bake hardenability. Therefore, continuous annealing is performed in a continuous annealing line capable of rapid heating and rapid cooling, or a continuous hot dip galvanizing line such as zinc plating or aluminum plating. In order to obtain excellent bake hardenability and deep drawability in the present invention,
It is necessary to anneal at an appropriate temperature according to the steel composition. That is, as the amount of Mn added decreases, NbC coarsens, and therefore it is necessary to raise the annealing temperature in order to dissolve NbC. In addition, the NbC generated by the change of the Nb amount and the C amount
Since the amount and size of NbC are different, it is necessary to raise the annealing temperature as NbC increases. Therefore, as a result of repeated studies by the present inventors, the lower limit of the annealing temperature was set to 20 ° C. as the Mn was reduced by 1%.
It became clear that it had to be raised. Also, Nb
The amount of C is a parameter (NbC
The absolute value decreases with the increase of), and log (Nb ×
As the value of C) increases by 1, the lower limit of the annealing temperature becomes 15
It became clear that it was necessary to raise the temperature.

That is, in order to obtain excellent bake hardenability and deep drawability, it is necessary to anneal at a temperature T ₁ or higher represented by the following formula (1).

[0023]

Equation 5] _{T 1 = 930-20 × Mn + 15} × log (Nb × C) ············ (1) also exceeds the temperature T ₂ represented by the following formula (2) When annealed, the low temperature transformation forming phase in the structure becomes excessively large,
This T ₂ is set as the upper limit of the annealing temperature because it causes remarkable deterioration of deep drawability.

[0024]

[Equation 6] T ₂ = 910 + 110 × Si−40 × Mn + 200 × P ・ ・ ・ ・ ・ ・ (2)

It is desirable that the temper rolling is performed in order to suppress the occurrence of stretcher strain during pressing and prevent the aging deterioration of the material. In order to obtain the effect of temper rolling, it is desirable to carry out at an elongation rate of 0.5% or more, but on the other hand, if the elongation rate is too high, the yield point rises and the elongation decreases, so it should be done at 3% or less. Is desirable.

The cold-rolled steel sheet produced according to the present invention has an excellent surface treatment property and is suitable for use after being subjected to various surface treatments. That is, after being annealed in the continuous hot-dip galvanizing line described above, hot-dip metal plating (zinc, aluminum, zinc-aluminum alloy, etc.) or alloying hot-dip galvanizing is performed, or electrogalvanizing, electrogalvanizing alloy plating, organic composite plating, Even if the surface treatments such as tin plating are carried out individually or appropriately in combination, there is no problem in the material and the excellent surface treatment property is exhibited.

[0027]

EXAMPLES Specific examples of the present invention will be described below.

Example 1. Si = 0.04%, P = 0.05%,
With log (Nb × C) = − 4.6, steels having various Mn contents in the range of 0.7 to 2.3% were melted, and the finishing temperature was 880 to 910.
The hot rolling was completed at ℃, and wound at 660 ℃ to obtain a hot rolled steel strip. These steel strips are pickled, and the plate thickness is 0.8 at a reduction of 75%.
After cold rolling to mm, recrystallization annealing was performed in a continuous annealing line, and temper rolling was performed at an elongation rate of 1.3%. FIG. 2 is a result showing the 2% BH and r _m value of this cold rolled sheet on a matrix of the amount of Mn and the annealing temperature.

2% BH in FIG. 2 was measured using a JIS No. 5 test piece in accordance with the procedure described in JIS G 3135 Annex. In addition, 0 ° to the rolling direction of the steel plate,
R values in the 45 ° and 90 ° directions (respectively r ₀ , r ₄₅ ,
r ₉₀ ) is measured (JIS No. 5 test piece is used), and r
_It was calculated by _m value = (r ₀ + 2 × r ₄₅ + r ₉₀ ) / 4. According to FIG. 2, when Mn is less than 1%, 2% BH is low because NbC is difficult to dissolve. Further, when Mn exceeds 2%, a good r value cannot be obtained.

The following equation (1), (2) the temperature T ₁ and temperature T ₂ is calculated as shown in FIG. 2 in formula, is less than T ₁
Due to insufficient dissolution of NbC, both 2% BH and r value are low. Further, as Mn increases, NbC becomes finer and easily dissolved, so that T ₁ decreases as the amount of Mn increases. On the other hand, when T ₂ is exceeded, 2% BH shows a high value, but the crystal orientation is significantly randomized by the transformation, and the r value is low. Therefore, 1.8% with 2% BH of 40 N / mm ² or more
In order to obtain the above r _m value, Mn is 1 to 2% and T
It is clear that there needs to be annealed in the range of ₁ through T _2.

[0031]

[Formula 7] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ········· (1) T ₂ = 910 +110 × Si−40 × Mn + 200 × P ··· ... (2)

Example 2. Si = 0.11%, Mn = 1.4%, P
= 0.04%, the value of log (Nb x C) is -4.3 and -5.2.
The steel having the composition of is hot-rolled at a finishing temperature of 870 to 910 ° C., wound at 560 to 680 ° C. to form a hot rolled steel strip, and the steel strip is pickled, with a reduction rate of 81% and a plate thickness of 0.7 mm. Cold-rolled, continuously annealed in a continuous annealed galvanizing line, hot-dip galvanized, alloyed the plated layer, and temper-rolled at an elongation of 1.1%. 2% BH of this galvannealed steel sheet, r
The relationship between the _m value and the annealing temperature is as shown in FIG.

According to FIG. 3, log (Nb × C) = − indicated by ○.
The steel of 5.2 has less NbC than the steel of -4.3 shown by △, and because NbC is easily melted, it is 2% at a lower annealing temperature.
BH is improved. Further, NbC is dissolved and 2% BH is improved, and at the same time, grain growth is improved and the r _m value is also high. In the figure, log (Nb × C) = − 5.2, −4.3 in the following (1)
Temperature T _a (log (Nb × C) = − 5.
2) and T _b (log (Nb × C) = − 4.3). Temperature to increase the 2% BH and r _m values, T _a, consistent with T _b,
It is clear that it is necessary to carry out at a temperature of T ₁ or higher represented by the following formula (1).

[0034]

[Equation 8] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (1)

Example 3. Steel having the chemical composition shown in the following Table 1 is melted, hot rolled at a finishing temperature of 870 to 900 ° C., and wound at 680 ° C. to form a hot rolled steel strip, which is pickled and rolled. It was cold-rolled at 75% to a plate thickness of 0.7 mm, continuously annealed, and then temper-rolled at an elongation of 1.2%. Annealing temperature and the tensile properties of these steel sheets, 2% BH, r _m values are as shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

Steel Nos. 4, 6 and 7 were also subjected to continuous annealing, hot dip galvanizing, alloying of the plated layer and temper rolling in a continuous hot dip galvanizing line. The results are also shown in Table 2. The plating adhesion in Table 2 was evaluated by measuring the amount of plating peeling by a draw bead test, and was evaluated on a scale of 5 from 1 to 5 (1: extremely good to 5: bad), but according to the invention method. Was extremely good.

As is clear from Table 2 as described above,
By continuously annealing a steel having a chemical composition within the scope of the present invention at the annealing temperature according to the present invention, an r _m value of 1.8 or more, 4
It is possible to obtain 2% BH of 0 N / mm ² or more. Also,
The adhesion of the galvannealed layer is also very good.

[0040]

According to the present invention as described above, a cold rolled steel sheet and a surface-treated steel sheet having high strength and excellent bake hardenability and deep drawability can be appropriately produced, It is an invention that is industrially highly effective.

[Brief description of drawings]

FIG. 1 is a chart summarizing the results of a specific study of the relationship between Ti and 2% BH.

FIG. 2 is a chart showing 2% BH and r _m value of the cold rolled steel sheet in Example 1 on a matrix of Mn amount and annealing temperature.

FIG. 3 is a graph of 2 of the galvannealed steel sheet according to Example 2.
% BH, is a table showing the relationship of r _m values and the annealing temperature.

Claims (3)

[Claims] 1. C: 0.001 to 0.004% by weight,
Si: 0.4% or less, Mn: 1-2%, P: 0.07
% Or less, S: 0.01% or less, sol. Al: 0.01 to 0.0
8%, N: 0.004% or less, Nb: 0.003 to 0.02%, T
i: 0.002% or more and Ti- (48/14) N is 0.004%
The following is obtained by casting, hot rolling, pickling, and cold rolling steel with the balance being Fe and inevitable impurities, and at a temperature T ₁ ° C or higher represented by the following equation (1) and the following equation (2). The method for producing a high-strength cold-rolled steel sheet excellent in bake hardenability and deep drawability, which comprises continuous annealing at a temperature of T ₂ ° C or less represented by. [Equation 1] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ······· (1) T ₂ = 910 +110 × Si-40 × Mn + 200 × P ··· ... (2) 2. The steel according to claim 1 is cast, hot rolled,
Pickled, cold-rolled, and at a continuous hot dip coating line at a temperature T ₁ ° C or higher represented by the following formula (1) and below (2)
High strength with excellent bake hardenability and deep drawability, characterized by continuous annealing at the temperature of T ₂ ° C or lower represented by the formula and hot dip plating, and alloying of the plating layer as needed Method for manufacturing surface-treated steel sheet. [Formula 2] T ₁ = 930−20 × Mn + 15 × log (Nb × C) ········ (1) T ₂ = 910 +110 × Si−40 × Mn + 200 × P ··· ... (2) 3. Production of a high-strength surface-treated steel sheet excellent in bake hardenability and deep drawability, characterized in that the surface of the cold-rolled steel sheet produced according to claim 1 or 2 is subjected to various surface treatments. Method.