JP3106782B2 - Method for producing high-strength cold-rolled steel sheet and surface-treated steel sheet excellent in bake hardenability and deep drawability - Google Patents

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 baking hardenability and aging resistance. For example, the present invention relates to a steel sheet suitable for use as an outer panel of an automobile. It is intended to provide a method that can be appropriately manufactured.

[0002]

2. Description of the Related Art In recent years, global environmental problems have attracted attention,
For automobiles, reducing emissions is an important issue. As one of measures to reduce exhaust gas, outer panels of automobiles and the like have been made thinner, and improvement in fuel efficiency has been promoted by reducing body weight. The outer panel can be made thinner by increasing the strength of the steel sheet, but the steel sheet is also required to have excellent deep drawability so that complicated press working can be performed even when the strength is increased. In addition, since the appearance of the outer panel is significantly impaired by the dent, excellent dent resistance (the property of difficulty in dent of 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 of increasing the yield point in a bake coating process after pressing).

However, in order to satisfy the above-mentioned requirements,
A bake hardening steel sheet has been developed in which a very small amount of Nb or Ti, which is a carbonitride forming element, is added to extremely low carbon steel to improve the deep drawability, and is strengthened with Si, Mn, P or the like. Mn among them
Although high strength is an advantageous strengthening method from the viewpoint of deterioration of the surface properties of the steel sheet and embrittlement of the steel sheet, Mn is generally considered to deteriorate the deep drawability, and Mn is actively reduced. Examples of utilization are described in JP-A-2-111841 and “Materials and Processes” (CAMP-ISIJ), 5 (199)
2), p. 2051 etc. That is, in the former, the Mn content and the hot rolling conditions are optimized to make the structure of the hot-rolled sheet uniform and fine, thereby improving the r-value. Also,
The latter is a technique in which the Ac3 transformation point is lowered by adding Mn, and the temperature is soaked in an α-γ two-phase region and a γ single-phase region during annealing.

[0004]

However, each of the above-mentioned prior arts still has its own problems.
That is, the technology according to Japanese Patent Application Laid-Open No. 2-111841 has a high Ti addition amount and generates a Ti-based carbide that is difficult to dissolve during annealing.
Satisfactory bake hardenability has not been obtained. For the latter “material and process”, high-temperature annealing in an α-γ two-phase region (or a γ single-phase region) is indispensable in order to obtain sufficient bake hardenability.

[0005]

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

(1) By weight%, 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, N
b: 0.003-0.02%, Ti: 0.004% or more, and Ti- (48/14) N 0.004%
Cast steel, hot rolling, pickling, and cold rolling are performed on steel consisting of Fe and unavoidable impurities, and the temperature is at least T ₁ ° C expressed by the following equation (1) and the following equation (2) A method for producing a high-strength cold-rolled steel sheet excellent in bake hardenability and deep drawability, characterized by continuously annealing at a temperature of T ₂ ° C or lower represented by:

[0007]

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

(2) Casting, hot rolling, pickling, and cold rolling of the steel described in (1) above, and a temperature of at least T ₁ ° C represented by the following formula (1) in a continuous hot-dip galvanizing line. And baking and hot-dipping by continuous annealing at a temperature of T ₂ ° C or lower represented by the following formula (2) and alloying the plating layer if necessary. Manufacturing method of high strength surface treated steel sheet with excellent drawability.

[0009]

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 in the above item (1) or (2) to various surface treatments. Manufacturing method of surface treated steel sheet.

[0011]

First, the reasons for limiting the steel composition in the present invention are 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. On the other hand, if the content exceeds 0.004%, the steel sheet becomes hardened and the deep drawability deteriorates. Therefore, the content is limited to 0.001 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 that occur during hot rolling.
Impairs the surface properties of the product. Further, when Si is concentrated on the surface of the steel sheet to generate a Si-based oxide, the adhesion of the surface treatment layer is significantly deteriorated. Therefore, the upper limit is set to 0.4%.

Mn: 1-2%, Mn is effective for increasing the strength of the steel sheet, but in the present invention, Mn is added to obtain excellent bake hardenability and excellent deep drawability. The inventors have newly found that the addition of Mn makes NbC in a hot-rolled sheet finer. That is, NbC refined by Mn has a large surface area per unit volume and is easily dissolved at the time of annealing, so that the bake hardenability of the cold rolled steel sheet is remarkably enhanced. In addition, since NbC dissolves to facilitate the movement of the crystal grain boundaries, the grain growth is improved and the deep drawability is improved with the improvement in bake hardenability. The details of the mechanism by which Mn refines NbC are not always clear, but Mn changes the solubility of NbC to change the precipitation morphology of NbC during and after hot rolling, or Mn changes the Ar ₂ transformation. In order to lower the point, it is presumed that the precipitation of NbC due to the transformation shifts to the lower temperature side, and the NbC becomes finer. In order to obtain the effect of Mn as described above, it is necessary to add 1% or more of Mn. However, if the addition amount of Mn exceeds 2%, deep drawability is significantly deteriorated. Therefore, Mn is limited to 1-2%.

P: not more than 0.07% P has an extremely large effect of increasing the strength of the steel sheet, but an excessively large amount makes the steel sheet brittle. Also, like Si,
Excessive addition causes P to be concentrated on the surface of the steel sheet, thereby deteriorating the adhesion and properties of the surface treatment layer. Therefore, the upper limit is set to 0.0
7%.

S: 0.01% or less Since S forms sulfide in steel and lowers the ductility of the steel sheet, the smaller the amount, the better. Therefore, the upper limit is made 0.01% so as not to lower the ductility.

Sol. Al: 0.01-0.08%, Al is added for deoxidizing molten steel, and at least 0.01% for fixing N, which cannot be fixed by Ti, as AlN.
It needs to be added. However, when a large amount of Al is added, 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 diffuses faster in steel at room temperature than C and degrades the aging resistance of the material at room temperature and also deteriorates the deep drawability. Therefore, N 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. Therefore, the smaller the N, the better. Therefore, 0.004
% Or less.

Nb: 0.003% to 0.02%, Nb is a material having a moderately fixed part of C as NbC, has excellent bake hardenability, and has excellent deep drawability by refining the crystal grains of a hot-rolled sheet. The effect is not expected at less than 0.003%, so the lower limit is made 0.003%.
However, if more than 0.02% is added, baking hardenability cannot be expected, and the amount of NbC increases so that dissolution of NbC is hindered and ductility is reduced.
The upper limit is 02%.

Ti: not less than 0.004% and Ti- (48/14) N
Is not more than 0.004%. Ti is added because the crystal grains of the hot-rolled sheet are refined and N is fixed as TiN, thereby obtaining excellent deep drawability. At least 0.0 for grain refinement of hot rolled sheet
Since it requires 04%, the lower limit is 0.004%. However, FIG. 1 shows 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%, Ti: 0.005 to 0.0
2% B of steel sheet containing 20% (annealing temperature: 860 ° C)
Although H is shown, it is clear from this figure that baking hardenability decreases when a large amount of Ti exceeds (48/14) N + 0.004. This is Ti-based carbide (Ti ₄ C ₂ S _{2 ,} TiC)
Is probably generated. Therefore, Ti- (48
/ 14) N is set to 0.004% or less.

Next, the reasons for the limitation on the production conditions in the present invention will be explained. Hot rolling may be carried out by an ordinary method, but the winding temperature is set to 500 ° C. or more in order to reduce solid solution C in the hot rolled sheet. Is desirable. In order to improve the deep drawability, the cold rolling is desirably performed at a rolling reduction of 70% or more.

The annealing method is continuous annealing. Since box annealing involves a heat cycle of slow heating and slow cooling, it is difficult to control the dissolution and re-precipitation of NbC, leading to fluctuations and deterioration in bake hardenability. Therefore, continuous annealing is performed in a continuous annealing line capable of rapid heating and rapid cooling, or a continuous hot-dip plating 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 perform annealing at an appropriate temperature according to the steel composition. That is, as the amount of Mn added decreases, NbC becomes coarser, so that it is necessary to raise the annealing temperature in order to dissolve NbC. The NbC generated by the change of the Nb amount and the C amount
Therefore, it is necessary to raise the annealing temperature with increasing NbC. Therefore, as a result of repeated studies by the present inventors, the lower limit of the annealing temperature was set to 20 ° C. with the decrease of Mn: 1%.
It became clear that we had to raise it. Also, Nb
The amount of C is a negative parameter called log (Nb × C) (NbC
The absolute value decreases as the value increases), and log (Nb ×
As the value of C) increases by 1, the lower limit of the annealing temperature is 15
It became clear that it was necessary to raise the temperature by ° C.

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

[0023]

T ₁ = 930−20 × Mn + 15 × log (Nb × C) (1) Also, when the temperature exceeds the temperature T _{2 represented} by the following equation (2): Upon annealing, the low-temperature transformation generation phase in the structure becomes excessively large,
This T ₂ is set as the upper limit of the annealing temperature, because remarkable deterioration in deep drawability is caused.

[0024]

T ₂ = 910 + 110 × Si−40 × Mn + 200 × P (2)

The temper rolling is desirably performed in order to suppress the occurrence of stretch strain during pressing and to prevent aging deterioration of the material. In order to obtain the effect of temper rolling, it is desirable to carry out the elongation at a rate of 1.1% or more. On the other hand, if the elongation is too high, the yield point increases and the elongation decreases. Is desirable.

The cold rolled steel sheet produced according to the present invention has excellent surface treatment properties and is suitable for use after being subjected to various surface treatments. That is, after annealing in the above-mentioned continuous hot-dip plating line, hot-dip metal plating (zinc, aluminum, zinc-aluminum alloy, etc.) or alloyed hot-dip galvanizing, or electro-galvanizing, electro-zinc alloy plating, organic composite plating, Even if the surface treatment is carried out solely or appropriately in combination with a treatment such as tin plating, the material is not affected at all, and excellent surface treatment properties are exhibited.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described as follows.

Embodiment 1 FIG. Si = 0.04%, P = 0.05%, log (Nb × C) = − 4.6,
A steel having various Mn contents in the range of 0.7 to 2.3% is melted, and hot rolling is completed at a finishing temperature of 880 to 910 ° C.
It was wound at 660 ° C. to form a hot-rolled steel strip. These steel strips were pickled, cold-rolled to a sheet thickness of 0.8 mm at a reduction of 75%, recrystallized and annealed at a continuous annealing line at an elongation of 1.3%. 2, the 2% BH and r _m value of the cold-rolled sheet, Mn
It is the result shown on the matrix of the amount and the annealing temperature.

The 2% BH in FIG. 2 was measured using a JIS No. 5 test piece according to the procedure described in the appendix to JIS G 3135. In addition, 0 ° with respect to the rolling direction of the steel sheet,
The r values in the 45 ° and 90 ° directions (r ₀ , r ₄₅ ,
r ₉₀ ) (using a JIS No. 5 test piece), and r
_{The m} value was calculated as (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. If 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
Both 2% BH and r value are low due to insufficient dissolution of NbC. Further, since NbC when Mn is increased is easily dissolved by miniaturization, T ₁ with an increase of Mn quantity is reduced. On the other hand, while indicating 2% BH higher value exceeds T _2, significantly randomization crystal orientation by transformation, r value is low. Therefore, 2% BH of more than 40 N / mm ² and 1.8
More in order to obtain r _m values, and a Mn of 1 to 2%, T
It is clear that there needs to be annealed in the range of ₁ through T _2.

[0031]

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

Embodiment 2 FIG. A steel having the chemical components shown in the following Table 1 was melted, hot-rolled at a finishing temperature of 870 to 900 ° C, wound up at 680 ° C to form a hot-rolled steel strip, and this steel strip was pickled and reduced. 75% thickness
Cold rolling to 0.7mm, continuous annealing, then elongation 1.2%
Temper rolling. Annealing temperature and the tensile properties of these steel sheets, 2% BH, r _m values are as shown in Table 2.

[0033]

[Table 1]

[0034]

[Table 2]

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

As is apparent from Table 2 as described above,
The steel having the chemical components of the present invention range, by continuous annealing at annealing temperatures according to the present invention, 1.8 or more r _m values, 4
2% BH of 0 N / mm ² or more can be obtained. Also,
The adhesion of the galvannealed layer is also very good.

[0037]

According to the present invention as described above, it is possible to appropriately produce a cold-rolled steel sheet and a surface-treated steel sheet having high strength and excellent bake hardenability and deep drawability, This invention is industrially effective.

[Brief description of the drawings]

FIG. 1 is a table summarizing the results of specifically examining the relationship between Ti and 2% BH.

[2] The 2% BH and r _m values of cold-rolled steel sheet in Example 1 is a table showing on the matrix of the Mn content and annealing temperature.

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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-263186 (JP, A) JP-A-6-116651 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C22C 38/00-38/60

Claims (3)

(57) [Claims] 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 0.08%, N: 0.004% or less, N
b: 0.003-0.02%, Ti: 0.004% or more, and Ti- (48/14) N 0.004%
Cast steel, hot rolling, pickling, and cold rolling are performed on steel consisting of Fe and unavoidable impurities, and the temperature is at least T ₁ ° C expressed by the following equation (1) and the following equation (2) A method for producing a high-strength cold-rolled steel sheet excellent in bake hardenability and deep drawability, characterized by continuously annealing at a temperature of T ₂ ° C or lower represented by: 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, which is cast, hot-rolled,
Pickling, cold rolling, and a continuous hot-dip plating line at a temperature T ₁ ° C or more represented by the following formula (1) and the following (2)
High strength with excellent bake hardenability and deep drawability, characterized by continuous annealing at a temperature of T ₂ ° C or less expressed by the formula and hot-dip plating, and alloying of the plating layer as necessary. Manufacturing method of surface treated steel sheet. T ₁ = 930−20 × Mn + 15 × log (Nb × C) (1) T ₂ = 910 + 110 × Si−40 × Mn + 200 × P .... (2) 3. A high-strength surface-treated steel sheet excellent in bake hardenability and deep drawability, characterized by subjecting the surface of the cold-rolled steel sheet manufactured according to claim 1 or 2 to various surface treatments. Method.