JPH0559489A - Cold rolled steel sheet for deep drawing as well as galvanized product thereof and there manufacturing method - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in deep drawability, platability and secondary working brittleness resistance by subjecting a steel contg. specified C, Si, Mn, P, S, N, Al, Ti and Fe to heat treatment and machin ing under prescribed conditions. CONSTITUTION:A steel having a compsn. constituted of, by weight, <=0.008% C, <=0.3% Si, 0.1 to 0.5% Mn, <0.02% P,<=0.02% S, <=0.01% N,',0.l% acid soluble Al and <=0.2% Ti as well as >=1 Ti/(C+N+S) and the balance Fe is melted. This steel is hot-rolled, is thereafter coiled at 500 to 720 deg.C an is cold-rolled at 60 to 95% draft. Next, the steel is heated to the recrystallization temp. to the Ac3 transformation point and is annealed. After that, treatment in which the extent to <=50mum from the surface is reheated to >=Ac3 transformation point and is cooled to <=Ac3. Then, the average grain size of ferrite in the surface layer part of the steel sheet is regulated to <=5mum.

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 for drawing which is used after being processed into various shapes as it is, and a zinc-based plating excellent in formability and surface quality obtained by applying hot-dip galvanized plating to the cold-rolled steel sheet. TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet and a method for producing them stably, uniformly and inexpensively.

[0002]

2. Description of the Related Art A cold-rolled steel sheet for deep drawing is a cold-rolled steel sheet having a high r value and excellent formability and is widely used in industrial fields such as home appliances, automobiles and building materials.

As a cold-rolled steel sheet for deep drawing, low carbon has hitherto been used.
Al-killed steel sheets were often used, but due to the continuous movement of annealing equipment in recent years, it is possible to obtain stable and excellent deep drawability without being affected by manufacturing factors.
The use of ultra low carbon steel called (Interstitial Free) steel is increasing. In addition, as another trend in recent years, the demand for cold-rolled steel sheets plated to increase corrosion resistance is rapidly increasing. Among them, relatively hard and brittle plating such as Fe-Zn and Ni-Zn is often used as the plating having excellent anticorrosion property, and the deep drawing property is usually deteriorated by such plating treatment. Therefore, it is required to increase the deep drawability of the cold rolled steel sheet base material by that amount.

The IF steel is an extremely low carbon low alloy steel in which an inevitable interstitial solid solution element such as C and N is added as an alloy element such as Ti or Nb and fixed as carbon / nitride, and has a non-aging property. It is a material with high workability. The basic chemical composition of IF steel is C <0.003%, Si <0.04%, Mn: 0.10
~ 0.50%, P: 0.01 ~ 0.02%, S: 0.008 ~ 0.02%, N
It is known that <0.02%, acid-soluble Al <0.1%, Ti: 0.02 to 0.06%, and optionally one or more of Nb <0.015% and B <0.002% are further added.

As a problem in using the cold-rolled steel sheet of IF steel as it is, there is known a so-called secondary working brittleness which causes brittle cracking when an impact force is applied after press forming. The cause of this secondary work embrittlement is that the ferrite grain boundaries become too clean and the grain boundaries become brittle because the interstitial solid solution elements such as C and N are reduced. Therefore, the secondary work embrittlement is very sensitive to the crystal grain size because the crack originates from the grain boundary, and the secondary work embrittlement susceptibility becomes dull in proportion to the reciprocal of the crystal grain size. Is known to do. As a method of preventing the secondary working embrittlement, for example, there is a method of reducing the annealing temperature to make the crystal grains finer. However, if this is done, the growth of the {111} grains becomes insufficient and the r value decreases, resulting in a deep drawability. It will drop significantly.

A steel sheet obtained by alloying a cold-rolled steel sheet with galvanization is excellent in paintability, corrosion resistance, weldability, and economical efficiency, and in recent years, it has been used in industrial fields such as home appliances, automobiles and building materials. Widely used.

The alloyed hot-dip galvanized steel sheet is obtained by hot-dip galvanizing a steel sheet and then heating it to cause mutual diffusion between the Zn phase on the surface of the plating layer and the steel sheet as the base material. Is an Fe-Zn alloy. As a base material of this alloyed hot-dip galvanized steel sheet, low carbon Al killed steel sheet has been often used until now, but as the use of the alloyed hot-dip galvanized steel sheet has expanded in recent years, excellent deep drawability is also required. As a result, the steel sheet of the above-mentioned IF steel (hereinafter referred to as the IF steel sheet) is often used as the base material.

However, when hot dip galvanizing is applied to an IF steel sheet or an alloying treatment is applied to the IF steel sheet, defects in appearance and other problems often occur due to the influence of uneven grain size of the steel sheet base material. One of them is so-called spangle diameter unevenness when hot-dip galvanizing, which is caused by unevenness of the Fe-Zn alloy layer during plating due to the grain size and orientation of the base material, and unevenness of the alloy layer causes Zn. It is caused by a change in the size of the critical nucleus during solidification. The other defect is the color tone of the surface when the alloying treatment is performed after hot dip galvanizing, that is, unevenness of unevenness, and this phenomenon is also controlled by the crystal grain size and orientation of the base material. That is, the alloying reaction occurs preferentially on the surface of the base material having a specific crystallographic orientation and preferentially at the grain boundaries of the base material, so that it is greatly affected by the structure of the base material. In IF steel, since the crystal grain boundaries are too clean, the reaction between the steel sheet base material and zinc occurs selectively at the crystal grain boundaries of the steel sheet base material, so unevenness is likely to occur.

[0009] As a method of manufacturing hot dip galvanized steel sheet,
A method of plating on a base material annealed outside the line is also adopted, and the cold rolled steel sheet for plating used in that case needs to solve the above problems. Further, cold-rolled steel sheets are sometimes used after being treated with zinc phosphate, and in the case of IF steel, the distribution of zinc phosphate crystals may be non-uniform due to the non-uniform structure of the base material of the cold-rolled steel sheet. The present inventors are aware. In order to improve these points, it is important to make the texture of the substrate uniform.

Since the crystal structure of the steel sheet surface layer has an important influence on the prevention of the above-mentioned secondary working embrittlement and the prevention of the surface defects of the plated steel sheet, the above problems are solved. It is conceivable that, even if the structure is not refined over the entire thickness direction of the steel sheet, the crystals of only the surface layer of the steel sheet should be finely homogenized. However, it is extremely difficult to make the ferrite grain structure of the surface layer portion of the IF steel sheet uniform and fine in a normal manufacturing process. One of the causes is a small amount of Ti
The continuous nitriding phenomenon that occurs in the surface layer part in the steps of continuous casting, hot rolling, etc., causes unevenness in the type and dispersion morphology of precipitates containing Ti, and causes ferrite grains during annealing after cold rolling. Unevenness is caused in the growth of the plating film, which causes unevenness of the plating film. Although there is such a phenomenon other than the IF steel plate, impurities such as C segregate at the grain boundaries of the mother steel plate, so that the alloying reaction does not occur preferentially at the grain boundaries and the problem does not become apparent. .. As a means for solving such a problem in galvanized IF steel sheet, there is a method conventionally used in the process of in-line annealing of CGL (hot dip galvanizing line), for example, before reaching the annealing temperature which is the maximum heating temperature in CGL. A method is considered in which the surface layer portion is lightly oxidized and then reduced to form a reduced iron layer composed of ultrafine particles on the surface layer portion. However, it is difficult to carry out the above reaction in the CGL line at a line speed that does not impair economic efficiency, and in this method, the amount of Fe eluted into the plating bath increases and dross is generated. There is also the problem of increase. Furthermore, such a method cannot be carried out in an out-of-line annealing, that is, a process premised on pre-annealing.

[0011]

DISCLOSURE OF THE INVENTION The present invention has been made to reduce the secondary work embrittlement of an IF steel sheet, and to reduce the defects on the plated surface when hot dip galvanizing the IF steel sheet. It is a thing.

[0012]

The gist of the present invention resides in the following cold-rolled steel sheet and plated steel sheet, and their manufacturing methods.

% By weight, C: 0.0080% or less, Si: 0.
3% or less, Mn: 0.1 to 0.5%, P: less than 0.020%, S:
0.02% or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, Ti: 0.2% or less, and Ti / (C + N + S) ≧ 1, or further Nb: 0.003 to 0.03% and B: 0.0003 to 0.00
20% of 1 type or 2 types, the balance consists essentially of Fe and unavoidable impurities, and the average grain size of ferrite in the surface layer of the steel sheet is 5 μm or less. Excellent cold-rolled steel sheet for deep drawing.

A steel slab having the composition of is hot-rolled, then wound at a temperature of 500 to 720 ° C., cold-rolled at a rolling reduction of 60 to 95%, and further in a temperature range not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point. heated to the annealing, plating resistance and two subsequently characterized by applying from the surface to a depth of less than 50μm reheated above Ac ₃ transformation point the process is cooled to below Ac ₃ transformation point at least once Secondary process Cold rolled steel sheet for deep drawing with excellent brittleness.

% By weight, C: 0.0080% or less, Si: 0.
3% or less, Mn: 0.1 to 0.5%, P: less than 0.020%, S:
0.02% or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, Ti: 0.2% or less and Ti / (C + N + S) ≧ 1, or further Nb: 0.003 to 0.03% and B: 0.0003 to 0.00
A cold-rolled steel sheet containing 20% of one or two kinds, the balance substantially consisting of Fe and unavoidable impurities, and having a ferrite average crystal grain size of 5 μm or less in the steel sheet surface layer was subjected to hot dip galvanizing A zinc-based plated cold-rolled steel sheet with excellent surface properties.

A steel having the composition of is formed into a billet, hot rolled, and then wound at a temperature of 500 to 720 ° C.
Cold rolled at 60-95%, further heated in the temperature range above recrystallization temperature and below Ac ₃ transformation point to anneal, then 50μ from the surface.
galvanized process, or a molten zinc-based plating and alloying treatment performed after performing at least once the process of cooling below Ac ₃ transformation point m up to less depth reheated to above Ac ₃ transformation point A method for producing a zinc-based plated cold-rolled steel sheet having excellent surface properties, which is characterized by the following.

[0017]

The present invention achieves the initial object by the overall effect of each alloy component described below and the synergistic effect of each condition of the manufacturing process. The most important feature is that the ferrite of the surface layer portion of the steel sheet is The grain structure should be an average grain structure of 5 μm or less. As a result, secondary work brittleness is prevented, and unevenness of the plating film does not occur when zinc-based plating is applied.

First, the function of alloy components in the cold rolled steel sheet of the present invention and the reason for limiting the content thereof as described above will be explained. In the present specification, “%” is “% by weight” unless otherwise specified.

C: Inevitably contained in the steel, but in the steel sheet of the present invention, the lower the more preferable.

If the C content exceeds 0.0080%, fine NbC and TiC are precipitated during low-temperature winding, and the precipitation of AlN is insufficient, so that a large amount of solute nitrogen remains in the steel, and the recrystallization temperature is increased. It remarkably enhances and hinders the development of a recrystallization texture that is favorable for deep drawability. Therefore, the allowable upper limit of C is set to 0.0080%.
It is desirable to keep this value or less, or 0.0020% or less.

Si: If Si is excessively contained, an oxide of Si is formed on the surface of the steel sheet, the wettability of zinc is impaired, and the plating film becomes uneven. Therefore, the Si content should be 0.3% or less.

Mn: S is fixed as MnS and added in order to prevent hot embrittlement of steel. To achieve this purpose, a content of 0.1% or more is required. However, if added in a large amount, the solid solution Mn strengthens the steel, which not only hardens the steel but also increases the cost of the product. Therefore, the upper limit of the content is set to 0.5%.

P: Impurities that are unavoidably contained in the steel, and the lower the more preferable. When it is 0.020% or more, recrystallization is suppressed or solid solution strengthening occurs to harden the steel and deteriorate the elongation of the steel sheet. It is also an element that induces secondary work embrittlement, and in order to prevent this, the content must be less than 0.020%.

Like S: P, it is an unavoidable impurity contained in steel, and the smaller the content, the better. If the S content is high,
Along with this, the amount of Mn added to prevent hot embrittlement must be increased, which not only increases the cost, but also causes voids starting from MnS particles to reduce the formability. The allowable upper limit is 0.02% and it is desirable to keep it below 0.005%.

N: The lower the better. If it exceeds 0.010%, the amount of solute N in the steel sheet increases after low-temperature winding in the hot rolling process, which hinders the development of a recrystallization texture preferable for deep drawability in the continuous annealing process.
It was set to 0.010%. Desirable is 0.0020% or less. Note that N, like C, is necessarily contained in steel, but even with current steelmaking technology, N can be reduced to about 0.0005%.

Acid-soluble Al: It is added as a deoxidizing agent, and if it is contained in a small amount, it indicates that deoxidizing is sufficient. However, if the acid-soluble Al exceeds 0.1%, the steel becomes hard and the ductility deteriorates.

Ti: One of the important alloying elements of the steel sheet of the present invention. In order to sufficiently fix C, N and S as nitrides, Ti / (C + N + S) must be contained in the range of ≧ 1. .. In this formula, each element symbol represents the content (% by weight) of that element. However, addition of a large amount of Ti causes an increase in cost and promotes the formation of harmful Fe-Ti-based intermetallic compounds.
The upper limit of the content is preferably 0.2%.

One of the cold-rolled steel sheets of the present invention is one in which, in addition to the above components, the balance consists of Fe and inevitable impurities. The other one contains one or both of Nb and B in addition to the above components.

Nb: an element having the same function as Ti,
It is added as necessary to complement Ti. But,
When the content is less than 0.003%, there is no effect of promoting the development of recrystallization texture which is preferable for deep drawability, and when it exceeds 0.03%, the effect of suppressing recrystallization becomes too large.

B: When B is contained in an amount of 0.0003% or more, it has the effect of strengthening the ferrite grain boundaries and suppressing secondary work brittleness. However, the presence of more than 0.0020% hinders the development of recrystallized texture which is preferable for drawability.

The base material of the cold-rolled steel sheet and the galvanized steel sheet of the present invention has the above chemical composition and the average grain size of ferrite in the surface layer of the steel sheet is 5 μm or less.

The surface layer portion means a depth of 50 μm or less from the surface of the steel sheet, and the ferrite of this layer has an average grain size of 5 μm.
It is a uniform crystal of m or less. If the entire plate is made of such fine ferrite crystals up to a depth of more than 50 μm, the formability of the steel plate deteriorates. If the average grain size of the ferrite in the surface layer portion is 5 μm or less, if it is larger than this, the reaction between Zn and Fe preferentially proceeds at grain boundaries, resulting in uneven thickness of the Fe-Zn alloy layer and surface defects. It is easy to become. If the average grain size is 5 μm or less, the maximum grain size of ferrite is 10 μm.
It is suppressed to about m, resulting in a uniform and fine crystal structure as a whole.

Next, a desirable method for producing the cold-rolled steel sheet and the galvanized steel sheet of the present invention will be described.

(A) Production of cold-rolled steel sheet for deep drawing Steel having the above-mentioned chemical composition is formed into a steel slab by a continuous casting method or an ingot-agglomeration method, and ordinary hot rolling is performed. However, it is desirable to heat the billet at a temperature of 1000 ° C or higher. Heating below 1000 ° C tends to cause temperature unevenness in the slab, and hot rolling occurs in a temperature range including the Ar ₃ transformation point, causing local γ → α transformation and warping or bending of the steel sheet. In some cases, it becomes impossible to pass the thread. The hot rolling is preferably finished at the Ar ₃ transformation point or higher. Of course, a method of directly rolling the continuously cast slab, which has been actively used in recent years, without cooling to a low temperature may be adopted. Alternatively, so-called ferrite zone hot rolling, in which rolling is performed positively at a temperature not higher than the Ar ₃ transformation point, may be adopted.

Winding after hot rolling must be performed at a temperature of 500 ° C. or higher and 720 ° C. or lower.

When the coiling temperature is lower than 500 ° C., solute nitrogen remains in the hot-rolled sheet, which hinders the development of recrystallization texture which is favorable for drawability. On the other hand, if the winding temperature is higher than 720 ° C, the scale produced during winding becomes thicker and the descaling property deteriorates, and abnormal grain growth occurs to generate coarse grains. The preferred winding temperature is 500-600 ° C.

In order to reduce the crystal grain size of the hot-rolled sheet, it is desirable that the hot-rolled sheet be rapidly cooled to the coiling temperature immediately after the completion of the hot rolling.

The hot-rolled steel sheet thus obtained is subjected to descaling treatment such as pickling, cold-rolled at a rolling reduction of 60 to 95%, and further heated to a temperature range not lower than the recrystallization temperature and not higher than the Ar ₃ transformation point. And annealed.

When the rolling reduction of cold rolling is less than 60%, a recrystallization texture suitable for deep drawability does not develop, so the rolling reduction is preferably high. However, if the rolling reduction exceeds 95%, the drawability deteriorates.

The annealing method may be either continuous annealing (when zinc plating is applied, continuous annealing in that line) or box annealing. Needless to say, it is necessary to heat above the recrystallization temperature because it is a recrystallization annealing, but if it is heated to a temperature higher than the Ac ₃ transformation point, it will transform into α → γ → α, and it will be formed in the recrystallization process. The annealing temperature is limited to the Ac ₃ transformation point or lower because the recrystallized texture favorable for the deep drawability is erased. In the case of continuous annealing, it is preferable that after heating to a recrystallization temperature or more and Ac ₃ transformation point or less, holding at that temperature for 5 seconds or more, and then cooling to a temperature of Ar ₁ transformation point or less at a cooling rate of 5 ° C / sec or less. ..

[0041] Then, in order to refine the surface of the tissue, from the outermost surface to a depth of less than 50μm reheated to above Ac ₃ transformation point, the process of cooling to a temperature lower than the Ac ₃ transformation point at least 1 Apply once. If the thickness of this treated layer is too thick, the proportion of the portion of the mother steel sheet that is not favorable for formability increases, and formability deteriorates. Therefore, this treatment is limited to the surface layer that does not exceed 50 μm from the outermost surface.

As a method for heating only the surface layer portion, it is recommended to apply a high frequency current to the surface layer portion to heat it by its electric resistance, but heating by a laser beam and other methods can also be adopted. An inert or reducing atmosphere is desirable as the atmosphere for this heat treatment.

As described above, a cold-rolled steel sheet of IF steel in which the surface layer structure is uniformly refined can be obtained. This cold rolled steel sheet
As described below, it is also suitable for use as a base material for hot-dip galvanized steel sheets, but it is also suitable for use as it is without plating or after being coated with zinc phosphate and then painted. .. In any case, it exhibits excellent deep drawability and exhibits excellent properties as a steel sheet with little secondary work embrittlement.

(B) Manufacture of galvannealed galvanized steel sheet A typical method for manufacturing galvannealed galvanized steel sheet is a method of hot dip galvanizing a cold rolled steel sheet in an in-line annealing type hot dip galvanizing facility. In the case of this method, the steel sheet which is the base material is a steel sheet which has been subjected to the cold rolling step of the above method (a). This steel plate is usually washed with an alkaline degreasing liquid on the side of the plating process, but this is not an essential process. After this, the steel sheet is preheated to 600-700 ° C. The means may be a conventional non-oxidizing furnace, a direct-fired heating furnace which has been widely adopted in recent years, or heating with a radiant tube in a reducing atmosphere. In principle, the means does not matter. That is, Fe oxide may or may not be formed on the surface of the steel plate in this step. Then, the steel sheet is annealed by the above-mentioned means, and is further subjected to a heat treatment for refining the surface layer portion. Steel sheet during annealing recrystallization temperature or higher, Ac ₃ to a temperature below the transformation point, usually up to seven hundred and eighty to eight hundred seventy ° C., is heated. As the annealing atmosphere, H ₂ is 5% or more,
It is desirable that the dew point is -30 ° C or lower.

After this annealing, a heat treatment for refining the ferrite in the surface layer of the steel sheet (depth of 50 μm or less from the outermost layer) is applied until the next plating step. The method and conditions are the same as those described in (a). This heat treatment of the surface layer portion can also be performed online.

The steel sheet treated as above is then
It is plated in a molten zinc bath at ± 20 ° C. 0.07-0 in Zn.
It is desirable to contain 2% of Al, which is effective in improving the powdering resistance of the plating layer. The hot-dip galvanized steel sheet is squeezed to the desired plating thickness and then
The alloying treatment is carried out at ˜600 ° C. for 3 to 60 seconds. The lower the alloying temperature is, the lower the alloying temperature may be, if the alloying treatment can be performed in a batch furnace.

The above method is a method in which the steps up to cold rolling are carried out in the step (a), and the steps following annealing are carried out in the plating line. When the steel plate which has been subjected to the end of the step (a), that is, where the surface layer portion has been refined is used as the base material, the above-mentioned plating step in the molten zinc bath may be performed.

The alloyed hot-dip galvanized steel sheet obtained as described above retains the excellent formability of the base material itself, and at the same time, the plated surface is uniform and beautiful, and the peeling resistance of the coating is also excellent.

Hereinafter, the present invention will be described more specifically with reference to Examples to clarify the effects thereof.

This is one of the embodiments of the present invention, and the present invention is not unduly limited by this.

[0051]

Example 1 A 250 mm-thick slab obtained by continuous casting of IF steel having the components shown in Table 1 was heated to 1200 ° C., and then heated from 1150 ° C.
It was hot-rolled to 3.2 mm at a temperature up to 930 ° C and wound at 600 ° C to obtain a hot-rolled steel strip. This hot-rolled steel sheet was pickled, cold-rolled at a rolling reduction of 75% to 0.8 mm, and annealed at 830 ° C for 30 seconds. Thereafter, the surface layer portion was miniaturized by each method shown in Note 1 of Table 2.

Tensile test pieces were sampled from the cold-rolled steel sheet thus prepared, the r value was measured, and a disc-shaped sample having a diameter of 66 mm was sampled to perform deep-cylindrical forming with a punch diameter of 33 mm.
At various temperatures from 6 ° C. to room temperature, a cone weight-shaped steel weight weighing 30 kg was dropped from a height of 1 m to perform a drop weight test to evaluate the secondary work embrittlement characteristics.

Separately, in order to examine the plating characteristics of this steel sheet, plating was performed by a vertical hot dip galvanizing apparatus using a material that had been subjected to the above-mentioned surface layer miniaturization treatment. Material 25% H ₂
It was heated at 650 ° C. for 60 seconds in an atmosphere of + N ₂ (open air: −45 ° C.) and then plated with a hot dip galvanizing bath containing Al: 0.13% and Pb: 0.02%. The temperature of the plating bath is 460 ° C, and the coating weight is 45 kg / mm ² per side. After that, the appearance is evaluated in the state of hot-dip galvanizing, and separately 500 ° C ×
The alloying treatment was performed for 20 seconds to evaluate the uniformity of appearance.

Further, the heat-treated steel sheet was treated with zinc phosphate. This treatment was performed for 120 seconds at a liquid temperature of 43 ° C. using a treatment agent (PBL 3080) manufactured by Nippon Parkerizing Co., Ltd.

The above results are shown in Table 2. In the examples of the present invention, the crystal of the surface layer of the steel sheet is refined, and therefore the secondary work embrittlement resistance is extremely good. In addition, the r value is large and sufficient deep drawability is exhibited. Even when hot-dip galvanized, spangles are uniform and beautiful. On the other hand, in Comparative Example 1 in which the surface layer portion was not subjected to the refining treatment, secondary processing brittleness appeared due to a large crystal flow, and in the plated material,
Many irregularities were present in the plating film, and a streak-like pattern was observed in some places. In Comparative Example 2 in which the surface layer portion is made extremely fine, the surface is beautiful and the secondary work embrittlement resistance is excellent, but the r value is low and it is clearly unsuitable as a deep drawing steel sheet.

The appearance after the zinc phosphate treatment was uniform in Comparative Example 1 in contrast to the streak-like uneven density, but in the present invention example, it was clear that the effect of refining the crystal grains in the surface layer portion was clear. ..

[0057]

[Table 1]

[0058]

[Table 2]

[0059]

Example 2 A slab having a thickness of 250 mm obtained by continuously casting IF steel having the components shown in Table 3 was heated to 1200 ° C. and then heated from 1150 ° C. to 930
It was hot-rolled to 3.2 mm by ℃ and wound at 600 ℃ to obtain hot-rolled steel strip. After pickling, it was cold-rolled to 0.8 mm, preheated at 600 ° C, and annealed at 830 ° C for 30 seconds. The annealing atmosphere is the same as in Example 1. Thereafter, the surface layer portion was miniaturized by each method shown in the note of Table 4.

This steel sheet was plated and alloyed in a molten zinc bath containing 0.13% Al. Plating bath temperature is 460
℃, the steel plate temperature before immersion in the plating bath was 450 ℃, the coating weight was 45 g / mm ² per side, and the alloying treatment was performed at 500 ℃ for 20 seconds.

With respect to the alloyed hot-dip galvanized steel sheet produced in this manner, the surface properties were visually evaluated, the structure of the cross section was observed, and the r value was measured by a tensile test. The results are shown in Table 4.

In the examples of the present invention, the surface was very uniform and beautiful, and the mechanical properties of the steel sheet itself showed sufficient deep drawability. On the other hand, in Comparative Example 1, since the crystal of the surface layer portion was coarse, many irregularities were present in the plating film, and a streak-like pattern was observed at some places. On the other hand, in Comparative Example 2 in which the surface layer portion is made extremely fine, the r value is very low even though the surface is beautiful, and it is unsuitable as a galvanized steel sheet for deep drawing.

[0063]

[Table 3]

[0064]

[Table 4]

[0065]

EFFECTS OF THE INVENTION The present invention provides a cold-rolled steel sheet which is excellent in deep drawability as well as in plating property and resistance to secondary work brittleness, and a method for stably producing the cold-rolled steel sheet at low cost. Further, using this steel sheet as a base material, a zinc-based plated steel sheet which has the above-mentioned basic characteristics and has an extremely uniform and beautiful plating surface and excellent peeling resistance of the plating film, and a method for producing the same are also provided. ..

[0066]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication C22C 38/14 C23C 2/06 9270-4K 2/28 9270-4K 2/40 9270-4K

Claims (6)

[Claims] 1. By weight%, C: 0.0080% or less, Si: 0.3%
Below, Mn: 0.1-0.5%, P: less than 0.020%, S: 0.02
% Or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, T
i: 0.2% or less, Ti / (C + N + S) ≧ 1, the balance substantially consisting of Fe and unavoidable impurities, and the average grain size of ferrite in the surface layer of the steel sheet is 5 μm or less. Cold rolled steel sheet for deep drawing with excellent secondary working brittleness. 2. By weight%, C: 0.0080% or less, Si: 0.3%
Below, Mn: 0.1-0.5%, P: less than 0.020%, S: 0.02
% Or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, T
i: 0.2% or less and Ti / (C + N + S) ≧ 1, further N
b: 0.003 to 0.03% and B: 0.0003 to 0.0020% of 1 type or 2 types, the balance substantially consists of Fe and unavoidable impurities, and the average grain size of ferrite in the surface layer of the steel sheet is 5 μm or less. A cold-rolled steel sheet for deep drawing which is excellent in secondary processing brittleness resistance. 3. A steel slab having the composition of claim 1 or 2 is hot-rolled and then wound at a temperature of 500 to 720 ° C., with a rolling reduction of 60 to
It is cold-rolled at 95%, further heated to a temperature range below the Ac ₃ transformation point above the recrystallization temperature and annealed, and then reheated to a depth of 50 μm or below from the surface to the Ac ₃ transformation point or above to obtain the Ac ₃ transformation point. A method for producing a cold-rolled steel sheet for deep drawing, which is excellent in secondary work embrittlement resistance, characterized by performing at least one treatment for cooling to a temperature lower than a transformation point. 4. In weight%, C: 0.0080% or less, Si: 0.3%
Below, Mn: 0.1-0.5%, P: less than 0.020%, S: 0.02
% Or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, T
i: 0.2% or less, Ti / (C + N + S) ≧ 1, the balance substantially consisting of Fe and unavoidable impurities, and the hot-dip zinc in a cold-rolled steel sheet having a ferrite average crystal grain size of 5 μm or less in the surface layer of the steel sheet. A zinc-based plated cold-rolled steel sheet having excellent surface properties, characterized by being subjected to a system-based plating. 5. By weight%, C: 0.0080% or less, Si: 0.3%
Below, Mn: 0.1-0.5%, P: less than 0.020%, S: 0.02
% Or less, N: 0.010% or less, acid-soluble Al: 0.1% or less, T
i: 0.2% or less, Ti / (C + N + S) ≧ 1, further Nb: 0.
Cold rolling containing 003 to 0.03% and B: 0.0003 to 0.0020% of 1 type or 2 types, the balance substantially consisting of Fe and unavoidable impurities, and having a ferrite average crystal grain size of 5 μm or less in the surface layer of the steel sheet. A zinc-based plated cold-rolled steel sheet having excellent surface properties, characterized in that the steel sheet is hot-dip galvanized. 6. A steel slab having the composition according to claim 4 or 5 is hot-rolled, wound at a temperature of 500 to 720 ° C., cold-rolled at a rolling reduction of 60 to 95%, and further recrystallized. Ac ₃
Heat to a temperature range below the transformation point and anneal, then from the surface
Up to a depth of less than 50μm reheated above Ac ₃ transformation point A
c ₃ galvanized processing a process of cooling below the transformation temperature after performing at least once, or galvanized cold having excellent surface properties which is characterized in that the molten zinc plating treatment and alloying treatment Manufacturing method of rolled steel sheet.