JP3489295B2 - Method of manufacturing cold-rolled steel strip for deep drawing by continuous annealing - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel strip for deep drawing by continuous annealing, and in particular, it is processed into various shapes by pressing or the like, and is used for automobile exterior parts, home electric appliance products,
The present invention relates to a method for producing a steel strip used in a portion of a kitchen product or the like where appearance is important by continuous annealing without causing surface flaws.

[0002]

2. Description of the Related Art When a cold-rolled steel sheet is press-formed, flaws may occur on the surface of which linear irregularities are formed. This flaw is
Since it does not disappear even after painting, it is a fatal defect in parts such as automobile exterior materials where appearance is important.

In the case of rimmed steel or core-killed steel, this defect is called a ghost band, and Al introduced for deoxidization is locally enriched at the boundary between the rim layer and the core layer and hardened with other portions. It is reported that it makes a difference (iron and steel,
Vol.54, 1968, 455 and Iron and Steel, Vol.61, 1975, S17
0).

It is considered that when the steel sheet has a non-uniform hardness portion, the soft portion is preferentially plastically deformed during press forming to cause unevenness of the sheet thickness. As another cause, P segregation and segregation have been considered, and in order to solve it, it is effective to perform slabbing at a solidification rate of 80 to 97% in the thickness direction. -75807.

Further, such a flaw occurs in aluminum-killed steel and is called a ferrite band. It is reported that the cause of this is that the hardness is nonuniform corresponding to the nonuniform thickness of the nitrification layer formed during annealing (iron and steel, Vol. 61, 1975, S168).

The present inventors have experienced the occurrence of defects similar to ghost bands and ferrite bands in deep-drawn cold-rolled steel strips produced by continuous annealing, but until now, defects due to cooling factors during continuous annealing have been experienced. Regarding the occurrence of, no solution has been proposed for its cause and solution.

Factors affecting the mechanical properties of the cold-rolled steel sheet for deep drawing produced by continuous annealing are mainly the soaking temperature that governs the grain growth degree, the dispersion of cementite, and the amount of dissolved carbon. The cooling rate after soaking, the overaging temperature, and the time have been studied in detail. However, the effect of continuous annealing conditions on the occurrence of surface defects has not been examined.

On the other hand, from the viewpoint of equipment, it is preferable to increase the cooling rate because the cooling zone in the continuous annealing line can be made compact. Therefore, various cooling techniques for rapid cooling have been proposed. For example, Japanese Examined Patent Publication No. 60-58766 discloses a technology for cooling a steel strip by using a roll in which a refrigerant is circulated. Compared with that technology, a gas jet cooler, water spray, water immersion cooling, and those The advantages and disadvantages of the cooling method by the combination of are described. However, nothing is mentioned about the effect on the material surface and surface defects of the steel strip.

[0009]

[Problems to be Solved by the Invention] As described above,
When a cold-rolled steel sheet for deep drawing is manufactured by continuous annealing, flaws similar to ghost bands and ferrite bands may occur. This flaw is a fatal defect in parts where the appearance is important, such as automobile exterior materials, because the surface becomes linear irregularity when press-forming a steel plate and it does not disappear even if painting is applied. .

This flaw not only lowers the material yield, but in a continuous press line, it requires a great deal of labor to sort defective products, resulting in a marked decrease in productivity. Further, since this kind of flaw cannot be found by visual inspection after continuous annealing, it is discovered only when a problem occurs in the customer, and therefore a fundamental solution is desired.

Therefore, an object of the present invention is to provide a method of manufacturing a cold-rolled steel strip for deep drawing which can guarantee no flaws over the entire length of the coil. More specifically, an object of the present invention is to provide a method for manufacturing a cold-rolled steel strip for deep drawing, which can improve both material yield and press productivity.

[0012]

Means for Solving the Problems As a result of an intensive investigation on the cause of defects in the cold-rolled steel sheet for deep drawing that has been continuously annealed, the present inventors have revealed the following.

That is, the substance of the flaw is a local increase in hardness, like the ghost band and the ferrite band, and a portion having a high hardness is difficult to be deformed during pressing and becomes a defect having a convex surface. The cause of hardening is that local plastic deformation occurs due to the thermal stress generated during cooling in the continuous annealing line,
This is because that portion was strain age hardened. Therefore, it has become clear that it is important to suppress local plastic deformation and subsequent strain aging in order to prevent defects, and the present invention has been completed.

Therefore, in the present invention, in% by weight, C: 0.01 to 0.035%, Si: 1.5% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.005 ~ 0.
1%, N: 0.005% or less, optionally B: 0.0003 to 0.003%, steel having a steel composition consisting of balance Fe and inevitable impurities is hot-rolled, pickled, cold-rolled, continuously annealed, temper-rolled. In producing a cold-rolled steel strip through each step of, the recrystallization temperature or higher in the continuous annealing step,
After soaking and heating at a temperature below the Ac ₃ transformation point, ΔT (° C):
As the cooling start temperature-cooling end temperature, ΔT (° C) <350
When water spray cooling is performed in the cooling region of ℃, the cooling start temperature is set to the second temperature or lower determined by the following formula (2), and the cooling rate is 100 ℃ / s or lower (however, 30 ℃ / s or lower.
(Except in the case of 1 )) . However, the case where reheating is performed immediately before overaging treatment is excluded. Second temperature: Temp * 2 (℃) = 200 + YS x YR (Equation (2) YS: Yield point of cold-rolled steel strip at room temperature (N / mm ² ) YR: Yield ratio at room temperature <Yield point / Tensile Strength>

[0015]

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reason why the steel composition and the conditions for continuous annealing treatment are limited as described above in the present invention will be explained in detail together with its action.

C: The amount of C is an element that determines the amount of cementite. If it exceeds 0.035%, the amount of coarsely precipitated cementite increases, so that the r value decreases and the deep drawability decreases, so its upper limit is made 0.035%. . On the other hand, if it is less than 0.01%, the supersaturation degree of C before the start of cooling is low, so that stable intragranular cementite cannot be obtained and the aging property is lowered, so the lower limit is set to 0.
01% Preferably, it is 0.015 to 0.025%.

Si: Si may be added to increase the strength of the steel sheet. However, if the content exceeds 1.5%, the formability deteriorates, so the upper limit is limited to 1.5%. It is preferably limited to 1.0% or less.

Mn: Mn may also be added as appropriate to increase the strength of the steel sheet. However, if the content exceeds 2%, the formability deteriorates, so the upper limit is limited to 2%. It is preferably limited to 1.0% or less.

P: P is an impurity which is unavoidably contained in steel. You may add suitably in order to raise the strength of a steel plate. However, if added in excess of 0.1%, the formability will decrease and secondary work embrittlement will easily occur.
Limit to 0.1%. It is preferably limited to 0.05% or less.

S: S is precipitated and fixed as MnS, but if the content increases, it causes an increase in the amount of Mn that is unnecessary for fixing S, so the upper limit of S is made 0.03%. It is preferably limited to 0.02% or less.

Sol.Al: In order to sufficiently deoxidize molten steel,
It is necessary to add 0.005% or more of sol.Al. If 0.1% or more is added, the steel hardens and at the same time the elongation decreases, so the upper limit is made. Preferably, 0.03 to 0.07% is preferable.

N: N is an element contained as an unavoidable impurity, but the upper limit of N is set to 0.005% in order to increase the amount of nitrides that precipitate and to lower the elongation. Preferably 0.00
30% or less.

B: B precipitates and fixes N as a nitride (BN), suppresses aging due to N, and promotes coarse agglomeration of cementite, but when the content is less than 0.0003%, elimination of N aging and coarse cementite are observed. There is no effect due to aggregation,
The lower limit is 0.0003%.

If it exceeds 0.003%, the same effect as that of the B-added Al-killed steel is obtained, and coarse agglomeration of cementite does not occur, so the upper limit is made 0.003%. Preferably 0.0006
~ 0.0020%.

Hot rolling: Hot rolling is performed according to a conventional method. From the viewpoint of obtaining a steel having high formability, the finishing temperature is preferably set to ₃ Ar or higher.

Cold rolling: Cold rolling and pickling performed prior to it are carried out according to a conventional method. The higher the cold rolling rate, the higher the r value, which is preferable. When the rolling reduction is 60% or less, the r value is low. On the other hand, when the rolling reduction exceeds 95%, the thickness of the hot-rolled sheet becomes thick and it becomes difficult to manufacture the hot-rolled sheet. Therefore, the cold-rolling reduction rate is 60 to 95. % Is preferable.

Continuous Annealing: The present invention is applied when annealing is performed using a continuous annealing line or a continuous hot-dip galvanizing line such as a continuous hot-dip galvanizing line or a continuous galvannealing line. It goes without saying that production of a cold rolled steel strip that is subjected to surface treatment such as electroplating and painting after annealing in a continuous annealing line is also included.

Annealing temperature: In annealing below the recrystallization temperature, the steel remains hard and the elongation is low. When annealing is performed at a temperature of Ac ₃ or higher, the texture that exhibits a high r value is destroyed by the α → γ transformation, so the r value becomes low. Therefore, the annealing temperature is limited to the recrystallization temperature or higher and the Ac ₃ transformation temperature or lower.

Cooling conditions: The cooling process in the continuous annealing treatment is the cooling between the soaking zone and the overaging zone (referred to as primary cooling),
The over-aging zone is followed by air cooling zone (secondary cooling) and water spray cooling (third cooling).

FIG. 1 is a graph showing an example of the temperature change in the continuous cooling process according to the present invention, in which ΔT ₁ and ΔT are shown.
₂ indicates the temperature difference between the cooling start temperature and the cooling end temperature in the primary cooling and the tertiary cooling, respectively. Usually ΔT ₁ ≧ 350 ℃
> ΔT ₂ . The temperature difference in the air-cooled zone is not a problem because the temperature gradient is usually small and the thermal stress generated is also small, and is not particularly considered in the examples of the present invention described later.

The yield point and tensile strength of the steel strip at high temperature tend to decrease with increasing temperature. The lower the yield point and tensile strength at room temperature, the higher the yield point and tensile strength at high temperature. Is reduced. Further, the steel strip in which solute C is present is hardened by strain aging simultaneously with plastic deformation or after plastic deformation due to the interaction between dislocations such as interstitial atoms such as C and dislocations.

Strain aging which occurs simultaneously with plastic deformation is greatly affected by the diffusion rate and strain rate of interstitial atoms.
In the case of ε = 2.0 × 10 ⁻⁴ · S ^−1, it occurs in the range of about 100 to 350 ° C., which causes local increase in hardness. For the same yield point,
A steel strip with a higher yield ratio has a larger plastic strain, which causes an increase in hardness difference between the undeformed portion and the deformed portion in the dynamic strain aging temperature range.

Therefore, in order to prevent a local increase in hardness in the steel strip in which solid solution C is present, the thermal stress generated during cooling of the continuous annealing line is set below the yield point of the steel strip in that temperature range to prevent plastic deformation. It is necessary to prevent it. Also, the behavior of dynamic strain aging (easiness of occurrence) varies depending on the temperature range.

In the high temperature region, the diffusion of C is fast, so that a very fast strain rate is required when dynamic strain aging occurs. Further, in the low temperature region, since the diffusion rate of C is slow, dynamic strain aging occurs even at a low strain rate. Therefore, in the present invention, it is divided into a high temperature region and a low temperature region.

That is, the cooling temperature difference without overaging,
In the region of the primary cooling zone (ΔT ≥ 350 ℃) where the cooling rate is high, the temperature range above the first temperature (hereinafter referred to as Temp ^* 1) shown in equation (1) is below 150 ℃ / s. By cooling at the cooling rate or performing overaging, the cooling temperature and the cooling rate are lower than those in the primary cooling zone.
At 350 ℃), the second temperature shown in equation (2) (hereinafter Temp ^*
(2) below the temperature range of 100 ° C / s, the thermal stress caused by rapid cooling in the temperature range during cooling in the continuous annealing line falls below the yield point of the steel strip, and Deformation can be prevented, and if each is cooled more rapidly than this, or if the temperature region below the first temperature and above the second temperature is rapidly cooled, the thermal stress caused by the quenching may exceed the yield point of the steel strip and cause compositional deformation. It becomes a factor to generate.

First temperature: Temp ^* 1 (° C) = 350 −YS × YR ··· (1) Second temperature: Temp ^* 2 (° C) = 200 + YS × YR · · · (2) YS: Yield point of cold-rolled steel strip at room temperature (N / mm ² ) YR: Yield ratio at room temperature <Yield point / Tensile strength> (Note) ΔT (℃): Cooling start temperature (℃) -Cooling end temperature
(° C) A suitable cooling rate is 150 to 60 ° C / s at the first temperature or higher, and 100 to 3 ° C / s at the second temperature or lower. Therefore, at least Δ
When T ≧ 350 ° C., if cooling is completed at Temp ^* 1 (° C.) or higher, or if ΔT <350 ° C., if cooling is performed from Temp ^* 2 (° C.) or lower, uneven defects after pressing do not occur.
As the yield point and the tensile strength described above, the values obtained in advance for each lot may be used.

Temper rolling: The steel of the present invention has elongation at yield point in the as-annealed state and causes stretcher strain during press forming as it is, so temper rolling is performed. In addition, temper rolling is also performed for the purpose of flattening the steel sheet and dulling. If the temper rolling elongation increases, the yield strength increases, the elongation and the n value decrease, so 2.0% or less is preferable. Next, the working effects of the present invention will be described more specifically by way of examples.

[0039]

Example A 0.8 mm thick full-hard cold-rolled steel strip having the components shown in Table 1 was annealed at a soaking temperature of 820 ° C. in a continuous annealing line equipped with an overaged zone, and an elongation of 1.2% was applied. It was temper-rolled.

After applying a tensile strain of 5% to the annealed cold-rolled steel sheet, an oil grindstone was applied to the surface of the steel sheet to visually inspect for irregularities and defects. Moreover, the tensile test of the annealed plate was conducted, and the results are shown in Table 1.
It is also shown.

[0041] (Reference Example) This example shows an example to apply the primary cooling shown in FIG. That is, ΔT = ΔT ₁ ≧ 350 ° C., and the influence of the primary cooling using steam cooling (between the soaking zone and the overaging zone) on the occurrence of irregularities was investigated by changing the end temperature. The results are shown collectively in FIG.

The average cooling rate of steam cooling is 150 ° C./s, and the cooling rate is 0.3 ° C./s since it enters the overaging zone after the completion of steam cooling.
Met. As is clear from FIG. 2, according to this example , Temp
^* To stop the air-water cooling in one of a temperature equal to or higher than the temperature area, Temp ^*
No defects occur when the temperature range below the temperature of 1 is slowly cooled in the overaging zone.

[0043] (Embodiment 1) This embodiment shows an example of applying the present invention method to the tertiary cooling shown in FIG. The steel strip that has gone out of the overaging treatment zone is gas cooled with an air cooling zone.
(Secondary cooling), and then water spray cooling (third cooling) in the water spray zone. That is, in FIG. 1, ΔT = ΔT
_{When 2} <350 ° C, the temperature of the steel strip at which this third cooling is started was changed to examine the effect on the occurrence of irregularities. The results are shown in Figure 3.

The average cooling rate for gas cooling was 10 ° C./s, and the average cooling rate for water spray cooling was 80 ° C./s. As is clear from FIG. 3, no flaws are generated when the gas cooling is performed to a temperature range of the Temp ^* 2 (° C.) temperature or lower according to the present invention.

Then, the relationship between the degree of flaws generated in the steels B to E in Table 1 and the primary cooling end point temperature and the relation between the degree of flaws similarly generated in the steels B to E steel and the third cooling end point temperature were examined. The results are collectively shown in FIGS. 4 and 5.

[0046]

[Table 1]

[0047]

According to the manufacturing method of the present invention, it is possible to completely prevent the irregularities on the surface after press forming that occur when the cold-rolled steel sheet for deep drawing is manufactured by continuous annealing. INDUSTRIAL APPLICABILITY The present invention is a technology that is very effective industrially because it can prevent defects only by lowering the cooling rate in a specific temperature range and does not require changes in the component system of the steel sheet or equipment modification.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature change in a continuous cooling process according to the present invention.

FIG. 2 is a graph showing the relationship between the degree of defects generated in Steel A and the primary cooling end point temperature.

FIG. 3 is a graph showing the relationship between the degree of flaws generated in steel A and the temperature of the tertiary cooling end point.

FIG. 4 is a graph showing the relationship between the degree of defects generated in steels B to E and the primary cooling end point temperature.

FIG. 5 is a glarler showing the relationship between the degree of flaws generated in steels B to E and the temperature of the tertiary cooling end point.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-7-3332 (JP, A) JP-A-7-216459 (JP, A) JP-A-7-228943 (JP, A) JP-B-2- 44890 (JP, B2) Japanese Patent Publication 7-56050 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/52-9/66 C21D 9/46-9/48 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. In weight%, C: 0.01 to 0.035%, Si: 1.5% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.005 to 0.
1%, N: 0.005% or less, steel having a steel composition consisting of balance Fe and unavoidable impurities is subjected to hot rolling, pickling, cold rolling, continuous annealing, temper rolling, and cold rolled steel strip. In manufacturing, in the continuous annealing step, the recrystallization temperature or higher,
After soaking and heating at a temperature below the Ac ₃ transformation point, ΔT (° C):
As the cooling start temperature-cooling end temperature, ΔT (° C) <350
In the case of water spray cooling at ℃, the cooling start temperature shall be below the second temperature determined by the following equation (2), and the cooling rate shall be 100 ℃ / s or less (excluding the case of 30 ℃ / s or less.
Manufacturing method of a cold rolled steel strip for deep drawing, characterized by cooling as Ku). However, the case where reheating is performed immediately before overaging treatment is excluded. Second temperature: Temp * 2 (℃) = 200 + YS x YR ...
Formula (2) YS: Yield point of cold-rolled steel strip at room temperature (N / mm ² ) YR: Yield ratio at room temperature <Yield point / Tensile strength> 2. The steel composition further comprises B: 0.0003 to 0.00.
The method for producing a cold-rolled steel strip for deep drawing according to claim 1, containing 3%.