JP6237657B2 - Galvanized steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a galvanized steel sheet such as an electrogalvanized steel sheet, a hot dip galvanized steel sheet, and an alloyed hot dip galvanized steel sheet for automobiles and home appliances and a method for producing the same.

  Steel plates used for automobiles and household appliances are usually subjected to press forming to become products. In recent years, in order to reduce the manufacturing cost of these products, etc., the tendency of integral molding to form using larger steel plates has progressed, and steel plates with even better deep drawability have been required. . As such a steel sheet for deep drawing, so-called IF (Interstitial Free) steel obtained by adding a carbonitride-forming element such as Ti or Nb to ultra-low carbon steel is generally used. Such an ultra-low carbon steel sheet for deep drawing has a problem that the recrystallization temperature is greatly increased as compared with a steel not added with a carbonitride-forming element, and unrecrystallized tends to remain on the steel sheet surface. Non-recrystallized grains remain on the surface of the steel sheet, causing local deep drawability to deteriorate, or generating streak patterns on the plated surface after hot dip galvanizing or electrogalvanizing treatment, thereby impairing the appearance. There is a case.

  Several proposals have been made regarding methods for suppressing streaks in Ti-added ultra-low carbon hot dip galvanized steel sheets and preventing local deep drawability. For example, in Patent Document 1, the slab heating temperature before hot rolling is reduced according to the Ti content, and the crystal grain size or texture of the surface layer of the base iron is made uniform, thereby improving the deep drawability. However, a technique for preventing streaks is disclosed. However, if the slab heating temperature is low, it is difficult to ensure the finishing temperature during hot rolling over the entire length of the coil, and the difference in plating surface properties due to the difference in texture in the longitudinal direction of the coil becomes a problem.

  Patent Document 2 discloses a method of preventing streaking by increasing the hot rolling finish finishing temperature and leaving no unrecrystallized structure after annealing. Furthermore, Patent Document 3 discloses a method of suppressing streaks by increasing the hot rolling finish finishing temperature and controlling the texture after annealing. However, the methods of increasing the hot rolling finishing temperature as in the techniques disclosed in Patent Document 2 and Patent Document 3 cause generation of scale wrinkles, and thus surface defects due to scale are likely to occur.

  As described above, in the techniques disclosed in Patent Documents 1 to 3 described above, a galvanized steel sheet having no streaks, good surface properties, and sufficient deep drawability has not been provided yet.

JP-A-7-228944 JP 2001-342522 A JP-A-10-18011

  The present invention has been made in order to solve the above-mentioned problems, and an object thereof is to provide a galvanized steel sheet having a good surface property without streaks and an excellent deep drawability. It is.

  In the conventional Ti-added IF steel sheet for automobile exterior panels, uneven appearance may occur. Therefore, the steel sheets in which such appearance irregularities were generated were investigated in detail. As a result, it was found that non-recrystallized grains having a {100} plane texture (hereinafter referred to as {100} non-recrystallized grains) exist in the plate thickness surface layer portion of the steel sheet where the appearance unevenness occurs. It was also found that when {100} unrecrystallized grains remain in the vicinity of the surface layer, unevenness of appearance occurs because the alloying speed is locally different during the alloying treatment. It was found that there are many very fine precipitates having a particle size of less than 20 nm in the portion where {100} unrecrystallized grains remain. Such fine precipitates remain undissolved under the general annealing conditions applied to steel sheets for automobile exterior plates, and recrystallization does not easily proceed near the surface layer due to the so-called pinning effect. , {100} non-recrystallized grains are considered to remain. Therefore, in the present invention, it was found that recrystallization is promoted by giving a high dislocation density to the steel sheet surface, and generation of {100} unrecrystallized grains on the steel sheet surface layer can be suppressed.

  Based on the above examination results, the present invention is an IF steel containing Ti, which has good deep drawability and a good surface appearance by suppressing surface defects that are likely to occur in IF steel. In order to obtain a galvanized steel sheet, the {100} plane X-ray intensity in the direction parallel to the plate surface is 0.8 or less in random intensity ratio on the surface of the steel sheet, and the area ratio occupied by unrecrystallized grains is 0.10. % Or less.

  The {100} plane X-ray intensity in the direction parallel to the plate surface can be measured by an inverse pole figure method. Detailed conditions of the measurement method will be described later.

This invention is made | formed based on such knowledge, The summary is as follows.
[1] By mass%, C: 0.0005% to 0.0050%, Si: 0.20% or less, Mn: 0.40% or less, P: 0.050% or less, S: 0.030% In the following, Al: 0.010% or more and 0.080% or less, N: 0.0050% or less, Ti: 0.005% or more and 0.100% or less, with the balance being Fe and inevitable impurities Have
On the surface, the {100} plane X-ray intensity in the direction parallel to the plate surface is 0.8 or less in random intensity ratio, and the area ratio occupied by non-recrystallized grains is 0.10% or less,
A galvanized steel sheet having a galvanized film formed on the surface of the original steel sheet.
[2] The galvanized steel sheet according to the above [1], further containing, by mass%, Nb: 0.001% or more and 0.010% or less as the component composition.
[3] The galvanized steel sheet according to the above [1] or [2], wherein the component composition further contains B: 0.0002% to 0.0030% by mass%.
[4] The above-mentioned component composition further contains, in mass%, Sb: 0.001% or more and 0.100% or less and / or Sn: 0.001% or more and 0.100% or less. The galvanized steel sheet according to any one of [1] to [3].
[5] A slab comprising the component composition according to any one of [1] to [4] is heated at a heating temperature of 1000 ° C. to 1300 ° C., and hot at a finishing temperature of 800 ° C. to 1000 ° C. Rolled and rolled up at a temperature of 600 ° C. or higher and 800 ° C. or lower, pickled, and cold-rolled, and then the equivalent dislocation density ρ from the surface to the depth of 200 μm is 1.0 × 10 ¹⁵ m ⁻² or higher. The manufacturing method of the galvanized steel plate which anneals 700 degreeC or more and 900 degrees C or less with respect to the said steel plate surface, and performs the galvanization process on the said steel plate surface. Here, the corresponding transition density ρ shows 14.4ε ² / ^{b 2} (ε represents the uneven distortion of the steel sheet, b is 2.5 × ^{10 -10} m.).

  According to the present invention, it is possible to provide a galvanized steel sheet having no streak pattern, good surface properties, and excellent deep drawability. Moreover, the galvanized steel sheet of the present invention can be manufactured without lowering the slab heating temperature or increasing the finish rolling temperature.

  The galvanized steel sheet according to the present invention is, in mass%, C: 0.0005% or more and 0.0050% or less, Si: 0.20% or less, Mn: 0.40% or less, P: 0.050% or less, S: 0.030% or less, Al: 0.010% or more and 0.080% or less, N: 0.0050% or less, Ti: 0.005% or more and 0.100% or less, with the balance being Fe and inevitable The surface composition has a {100} plane X-ray intensity in the direction parallel to the plate surface at a random intensity ratio of 0.8 or less, and the area ratio occupied by non-recrystallized grains is 0.8. It has a steel plate original plate that is 10% or less, and a galvanized film formed on the surface of the steel plate original plate. Hereinafter, the galvanized steel sheet according to the present invention will be described in detail according to the component composition and structure of a steel sheet original sheet to which a galvanized film is applied, the galvanized film, and the method for producing the galvanized steel sheet.

(About steel plate)
[Ingredient composition]
First, the component composition of the steel plate original plate to which the galvanizing according to the present invention is applied will be described. In the following, “%” of the component amount means “% by mass” unless otherwise specified.

<C: 0.0005% or more and 0.0050% or less>
C must be reduced as much as possible in order to obtain excellent press workability, particularly excellent deep drawability, and the C content is regulated to 0.0050% or less. Further, the C content is preferably 0.0030% or less. On the other hand, if the C content is less than 0.0005%, the crystal grain size becomes extremely coarse, the yield strength is remarkably lowered, the in-plane rigidity is lowered, and surface defects such as hip breakage tend to occur. 0005% or more.

<Si: 0.20% or less>
Si is an element useful for strengthening steel by solid solution strengthening without degrading workability. However, if its content exceeds 0.20%, it concentrates on the surface during annealing and increases galvanizing properties. Inhibition and surface appearance deteriorate, so the Si content is 0.20% or less.

<Mn: 0.40% or less>
Mn has an action of strengthening steel, and a necessary amount is contained according to a desired strength. However, when the content exceeds 0.40%, fine MnS is generated and a {100} plane texture remains. Therefore, the Mn content is set to 0.40% or less. Preferably it is 0.30% or less.

<P: 0.050% or less>
P has an effect of strengthening steel, and a necessary amount is contained depending on a desired strength. However, when the content exceeds 0.050%, the plating property and appearance are deteriorated, so the P content is 0. .050% or less. Preferably it is 0.030% or less.

<S: 0.030% or less>
S is present in steel as an unavoidable impurity, and if the S content exceeds 0.030%, hot cracking during steel plate production is likely to occur, and a large amount of sulfide is generated in the steel, causing recrystallization. Therefore, surface defects are likely to occur. Therefore, the S content is 0.030% or less.

<Al: 0.010% or more and 0.080% or less>
Al is an element to be added as a deoxidizer and needs to be 0.010% or more. On the other hand, if the content exceeds 0.080%, a large number of unrecrystallized grains remain due to the pinning effect of nitride, and surface defects are likely to occur. Therefore, the Al content is set to 0.010% or more and 0.080% or less.

<N: 0.0050% or less>
N forms nitrides with Al and Ti, and non-recrystallized grains due to the pinning effect remain, so that surface defects are likely to occur. Therefore, the N content is set to 0.0050% or less. Preferably it is 0.0030% or less.

<Ti: 0.005% or more and 0.100%>
Ti is a carbonitride-forming element and is contained as necessary to improve deep drawability. If Ti is less than 0.005%, the effect is insufficient, so the Ti content is 0.005% or more. If the content exceeds 0.100%, the remaining of non-recrystallized grains and the formation of an abnormal structure are promoted due to the formation of nitride in the steel plate, and the surface appearance deteriorates. Therefore, the Ti content is 0.100% or less. In addition, Preferably 0.010% or more of Ti is contained. Further, preferably, Ti is contained by 0.060% or less.

  The balance other than the above components is Fe and inevitable impurities. In the present invention, Nb, B, Sb, and Sn are not essential components, but can be contained in the following ranges as necessary.

<Nb: 0.001% or more and 0.010% or less>
Nb, like Ti, is an element that is advantageous for forming a carbonitride to improve workability. When Nb is contained in order to obtain the workability improvement effect, the content is made 0.001% or more. However, if the content exceeds 0.010%, the recrystallization temperature is greatly increased, and unrecrystallized grains may remain. Therefore, when Nb is contained, the Nb content is set to 0.001% or more and 0.010% or less.

<B: 0.0002% or more and 0.0030% or less>
B is an element effective for strengthening the grain boundary of the soft IF steel sheet. When secondary work brittleness resistance is required, it is effective to contain 0.0002% or more. However, if B is contained in excess of 0.0030%, it may cause deterioration of the surface properties and an increase in the recrystallization temperature during the production of the steel sheet. Therefore, when it contains B, B content shall be 0.0002% or more and 0.0030% or less.

<Sb: 0.001% or more and 0.100% or less>
When Sb is contained in an amount of 0.001% or more, it concentrates in the outermost layer of the steel plate, and suppresses nitriding to suppress the remaining of non-recrystallized grains. However, when the content exceeds 0.100%, non-recrystallized grains having {111} plane texture decrease, and non-recrystallized grains having {100} plane texture tend to remain. Therefore, when it contains Sb, Sb content shall be 0.001% or more and 0.100% or less.

<Sn: 0.001% or more and 0.100% or less>
When Sn is contained in an amount of 0.001% or more, it concentrates in the outermost surface layer of the steel plate, and suppresses nitriding to suppress the remaining of non-recrystallized grains. However, when the content exceeds 0.100%, non-recrystallized grains having {111} plane texture decrease, and non-recrystallized grains having {100} plane texture tend to remain. Therefore, when it contains Sn, Sn content shall be 0.001% or more and 0.100% or less.

[Physical properties]
Next, in the galvanized steel sheet excellent in surface appearance and deep drawability of the present invention, the physical properties of the steel sheet original plate to which the galvanized film is applied will be described.

<{100} plane X-ray intensity in a direction parallel to the plate surface is 0.8 or less in terms of random intensity ratio>
If the {100} plane X-ray intensity in the direction parallel to the plate surface on the steel plate surface is 0.8 or less in terms of the random intensity ratio, the {100} plane in the direction parallel to the plate surface on the surface of the steel plate plate The area ratio of the recrystallized grains becomes sufficiently low, and a galvanized steel sheet having an excellent surface appearance after galvanizing treatment can be obtained. The {100} plane X-ray intensity in the direction parallel to the plate surface can be measured by an inverted pole figure method. At the same time, the {100} plane X-ray intensity (random intensity) of a random sample (standard sample) in which the crystal orientation without selective orientation is irregularly distributed is measured, and the real sample relative to the {100} plane X-ray intensity of the random sample is measured. The random intensity ratio can be calculated from the ratio of the {100} plane X-ray intensity of the test piece. The X-ray intensity can be measured using white X-rays as an X-ray source and a Ge semiconductor detector for detecting {100} plane X-rays. This random intensity ratio of {100} plane X-ray intensity can be adjusted to 0.8 or less by controlling the equivalent dislocation density of the steel sheet before annealing, which will be described later.

<The ratio of non-recrystallized grains on the surface of the steel sheet is 0.10% or less>
By setting the ratio of non-recrystallized grains on the surface of the steel sheet original plate to 0.10% or less, it is possible to obtain a galvanized steel sheet having an excellent surface appearance after the galvanized film treatment. In order to obtain the non-recrystallized ratio of the surface layer of the galvanized steel sheet, the surface of the steel sheet is observed after dissolving and removing the plated layer with 15% hydrochloric acid containing inhibitor, and the ratio of the unrecrystallized structure to the entire structure ( The area ratio) may be obtained and used as the ratio of unrecrystallized grains. The ratio of the non-recrystallized grains on the surface of the steel plate can be adjusted to 0.10 or less by controlling the equivalent dislocation density of the steel plate before annealing, which will be described later.

  Further, the steel sheet original plate can have a ferrite single phase structure. The structure of the steel sheet can be observed using a scanning electron microscope. Specifically, first, the steel sheet surface is polished and then corroded with nital (alcohol solution containing nitric acid). Subsequently, a tissue photograph at a magnification of 3000 times was taken with a scanning electron microscope, and a desired region in the obtained tissue photograph data was extracted by image analysis, and image analysis software (Image-Pro ver. 7 manufactured by Nippon Roper Co., Ltd.) was extracted. ), It can be determined that the region having dark contrast is ferrite.

(Zinc plating film)
In the present invention, a galvanized film is formed on the surface of the steel plate. The galvanized steel sheet of the present invention may be any of an electrogalvanized steel sheet (EG), a hot dip galvanized steel sheet (GI), and an alloyed hot dip galvanized steel sheet (GA). Although not particularly limited, the composition of the zinc coating is, for example, in the case of a hot dip galvanized steel sheet (GI) or an alloyed hot dip galvanized steel sheet (GA), and Fe: 7 to 15% by mass. , Al: 0.08 to 0.22% by mass, and a plating layer of ^{20 to} 150 g / m ² per side. Moreover, in the case of an electrogalvanized steel sheet (EG), it can be set as the plating layer with the adhesion amount of Zn of 0.5-1000 g / m < ² >. In the galvanized steel sheet of the present invention, the original steel sheet on which galvanization is performed has the above-described component composition, and the {100} plane X-ray intensity in the direction parallel to the plate surface is 0.8 or less in terms of random intensity ratio. And the ratio of non-recrystallized grains is 0.10% or less, so that {100} non-recrystallized grains can be prevented from occurring in the vicinity of the surface layer of the steel sheet, and the surface properties are improved without streaking And deep drawability can be improved.

(Manufacturing method of galvanized steel sheet)
Then, the manufacturing method of the galvanized steel plate of this invention is demonstrated. In the method for producing a galvanized steel sheet according to the present invention, the slab having the above-described component composition is heated at a heating temperature of 1000 ° C. to 1300 ° C., hot-rolled at a finish rolling temperature of 800 ° C. to 1000 ° C., and 600 ° C. above 800 ° C. coiling at a temperature below, pickling, after cold rolling, relative to corresponding dislocation density ρ is 1.0 × ¹⁰ 15 ^{m -2} or more steel to 200μm in the depth direction from the surface, 700 ° C. Annealing is performed at 900 ° C. or less and galvanizing treatment is performed on the surface of the original steel sheet.

<Slab heating at a temperature of 1000 ° C to 1300 ° C>
In the present invention, the method for melting the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. In addition, after melting, it is preferable to use a slab (steel material) by a continuous casting method because of problems such as segregation, but it may also be used as a slab by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. good. The obtained slab is roughly rolled, or directly inserted into a hot finish rolling mill and hot rolled. Slab heating temperature shall be 1000 degreeC or more from a viewpoint of ensuring the finishing temperature mentioned later. When the slab heating temperature is 1300 ° C. or higher, a large amount of nitride is generated and unrecrystallized grains remain after annealing.

<Hot rolling at a finish rolling temperature of 800 ° C to 1000 ° C>
In hot rolling, after rough rolling is performed as necessary, finish rolling is performed at a finish rolling temperature of 800 ° C. or higher and 1000 ° C. or lower. If the finish rolling temperature is below 800 ° C., the structure of the steel plate becomes uneven and the workability and surface appearance deteriorate, so the temperature is set to 800 ° C. or higher. Further, if the rolling is performed at a temperature exceeding 1000 ° C., it causes scale wrinkles and the surface appearance is impaired.

<Winding at a temperature of 600 ° C to 800 ° C>
When the coiling temperature is lower than 600 ° C., the growth rate of precipitates is reduced, the amount of fine precipitates is increased, the deep drawability is deteriorated, and the recrystallization temperature is increased. For this reason, the coiling temperature is 600 ° C. or higher. Further, if it exceeds 800 ° C., the scale of the surface layer grows and tends to cause surface defects.

<Controlling the equivalent dislocation density from the steel sheet surface to 200 μm in the depth direction from the steel sheet surface before annealing to 1 × 10 ¹⁵ m ⁻² or more>
After winding, pickling, cold rolling, washing, and then annealing, the equivalent dislocation density from the steel sheet surface before annealing to 200 μm in the depth direction shall be 1 × 10 ¹⁵ m ⁻² or more. Thus, the recrystallization of the steel sheet surface layer is promoted, and the remaining {100} unrecrystallized grains can be suppressed. More preferably, it is 1 × 10 ¹⁶ m ⁻² or more. Although the method of setting the equivalent dislocation density of the steel sheet surface layer to 1 × 10 ¹⁵ m ⁻² or more is not particularly specified, it is usually performed when manufacturing an automobile outer sheet or a household appliance outer sheet, which is 50% to In the range of about 95% cold rolling reduction, it is difficult to obtain an equivalent dislocation density of 1 × 10 ¹⁵ m ⁻² or more in the steel sheet surface layer. For example, shot blasting after cold rolling or straining with a high strength brush It can be realized by performing the low-pressure rolling with a high roughness roll after the cold rolling final stage or after cold rolling.

[Equivalent dislocation density]
The equivalent dislocation density can be measured by the following method. A 10 mm × 10 mm test piece is collected from each steel plate, polished from the back surface of the test piece until the plate thickness becomes 200 μm, and then the polishing strain layer on the back surface layer is removed with hydrofluoric acid. An X-ray diffraction experiment is performed using this test piece, and the half width of the peak of the (110), (211), (220) crystal plane of the steel sheet is obtained. Using this half width, the non-uniform strain ε of the test piece is obtained by the Williamson-Hall method. This inhomogeneous strain ε is expressed by the formula described in Non-Patent Document 1 (Nakashima et al. “Method of evaluating dislocation density using X-ray diffraction”, CAMP-ISIJ, Vol. 17, 2004, p. 396): ρ = Substituting for 14.4ε ² / b ² , the equivalent dislocation density ρ is obtained. Note that b is the size (m) of the Burgers vector, and the value of b is 2.5 × 10 ⁻¹⁰ m.

<Annealing at a temperature of 700 ° C to 900 ° C>
Annealing may be either a continuous annealing furnace or a continuous hot dip galvanizing line. If the annealing temperature is less than 700 ° C., there is a possibility that unrecrystallized grains remain. On the other hand, when annealing is performed in a high temperature range exceeding 900 ° C., abnormal coarse particles are generated, and the surface appearance is impaired. Therefore, annealing temperature shall be 700 degreeC or more and 900 degrees C or less.

<Zinc plating treatment>
Although the galvanizing treatment is performed after annealing, the method is not particularly limited. For example, the galvanizing treatment may be performed by a method such as electrogalvanizing, hot dip galvanizing, alloying hot dip galvanizing. After the plating treatment, it is preferable to perform temper rolling for adjusting the surface roughness. At this time, the rolling rate (elongation rate) of the temper rolling is preferably about 0.5% to 1.5%.

  Examples of the present invention will be described below.

  First, the molten steel having the composition shown in Table 1 was made into a slab by continuous casting after vacuum degassing treatment. Next, the slab was heated at 1200 ° C., scale-removed, and then roughly rolled to a plate thickness of 40 mm. Next, after the steel sheet surface layer was cooled with a scale removing device, it was finish-rolled to a thickness of 3.5 mm and wound around a coil at a predetermined temperature.

Next, the steel sheet after winding was pickled and cold-rolled. Thereafter, in order to adjust the corresponding transition density, some samples were subjected to additional rolling using a roll having a roll roughness Ra = 0.2 to 7.4 μm, and the entire steel sheet was 0.7 mm thick (cold rolling) Rate: 80%). In addition, the rolling rate of additional rolling was 1%. Cold-rolled sheet is degreased and pickled as pretreatment, then annealed in hot-dip galvanizing line, hot-dip galvanized, some samples are alloyed, temper rolled with an elongation of 1.0%, and alloyed A hot dip galvanized steel sheet (GA) was obtained. Moreover, some samples were made into the electrogalvanized steel plate (EG) in the electrogalvanization line after annealing in a continuous annealing line. Table 2 shows the finish rolling temperature, the coiling temperature, and the roll roughness Ra during the additional rolling. The atmosphere during the annealing was a non-oxidizing gas containing hydrogen. As the hot dip galvanizing treatment, a 460 ° C. zinc plating bath containing 0.12% Al was used, and the immersion plate temperature was 460 ° C. and the immersion time was 3 seconds. The alloying treatment was performed at 510 ° C. for 20 seconds after the plating by adjusting the zinc adhesion amount to 60 g / m ² per side using an N ₂ gas wiper. The electrogalvanization treatment was performed in a sulfuric acid bath containing a bath temperature of 50 ° C., pH 1.5, and Zn ²⁺ at 1.5 mol / l at a flow rate of 1.5 mm / s and a current density of 100 A / dm ² . The obtained galvanized steel sheet was subjected to mechanical property evaluation, {100} plane random strength ratio measurement, surface layer non-recrystallization rate measurement, and appearance evaluation. The evaluation results are shown in Table 2.

  The equivalent transition density was measured according to the method described above.

  In the mechanical property evaluation, tensile strength (TS) and elongation (EL) were evaluated by a tensile test. Tensile properties were measured according to the test method described in JIS Z2241, after being processed into a No. 5 test piece described in JIS Z2201.

  The average r value, which is an index of deep drawability, is measured by a three-point method after giving a tensile pre-strain of 15%, and is 90 ° direction, 45 ° direction, 0 ° with respect to the rolling direction of the steel sheet. The average r value in the direction = (r (0 °) + 2 × r (45 °) + r (90 °)) / 4. Those having an average r value of 1.5 or more were considered to have excellent deep drawability.

  The random intensity ratio was measured by the method described above. The {100} plane X-ray intensity on the surface was measured after the test piece was washed and dried. White X-rays were used as the X-ray source, and a Ge semiconductor detector was used to detect {100} plane X-rays. The planes measured in this example are five planes {100}, {111}, {110}, {211}, and {310}.

  The surface layer non-recrystallized rate is obtained by dissolving and removing the plating layer with 15% hydrochloric acid containing an inhibitor, and then observing the surface of the steel sheet with an optical microscope at a magnification of 400 times. The ratio was determined as the ratio of unrecrystallized grains.

  Appearance evaluation was made by visually observing the presence or absence of a streak pattern, x where the streak pattern was generated, and ◯ where the streak pattern had a uniform appearance.

  Moreover, although not shown in the table | surface, the structure | tissue observation of the steel plate was performed. The structure of the steel sheet was observed using a scanning electron microscope. Specifically, first, the steel plate surface was polished and then corroded with nital (alcohol solution containing nitric acid). Subsequently, a tissue photograph at a magnification of 3000 times was taken with a scanning electron microscope, and a desired region in the obtained tissue photograph data was extracted by image analysis, and image analysis software (Image-Pro ver. 7 manufactured by Nippon Roper Co., Ltd.) was extracted. ), The region having a dark color contrast was determined to be ferrite, and the other regions were determined to be pearlite, bainite, martensite, and retained austenite. It was confirmed that the steel sheet of the example of the present invention has a ferrite single phase structure.

  The examples of the present invention had an excellent appearance without streak patterns, were excellent in deep drawability, and had performances suitable for automobile and home appliance exterior board applications. In the examples of the present invention, local deep drawability deterioration (fine cracks in the surface layer portion) was not observed.

  On the other hand, in the comparative example, the external appearance was inferior, and the performance suitable for the exterior board use for automobiles and home appliances was not satisfied. Further, in the comparative example, local deep drawability deterioration (fine cracks in the surface layer portion) was observed.

  The steel sheet of the present invention can be suitably used for various electrical equipment and automobile parts that require deep drawability and excellent surface quality after forming.

Claims (5)

In mass%, C: 0.0005% to 0.0050%, Si: 0.20% or less, Mn: 0.40% or less, P: 0.050% or less, S: 0.030% or less, Al : 0.010% or more and 0.080% or less, N: 0.0050% or less, Ti: 0.005% or more and 0.100% or less, with the balance being composed of Fe and inevitable impurities ,
On the surface, the {100} plane X-ray intensity in the direction parallel to the plate surface is 0.8 or less in random intensity ratio, and the area ratio occupied by non-recrystallized grains is 0.10% or less,
A galvanized steel sheet having a galvanized film formed on the surface of the original steel sheet.   The galvanized steel sheet according to claim 1, further comprising Nb: 0.001% or more and 0.010% or less in terms of mass% as the component composition.   The galvanized steel sheet according to claim 1 or 2, further comprising B: 0.0002% to 0.0030% by mass% as the component composition.   The component composition further comprises, in mass%, Sb: 0.001% or more and 0.100% or less and / or Sn: 0.001% or more and 0.100% or less. The galvanized steel sheet according to any one of 3. A slab comprising the component composition according to any one of claims 1 to 4 is heated at a heating temperature of 1000 ° C to 1300 ° C, hot-rolled at a finish rolling temperature of 800 ° C to 1000 ° C, and 600 ° C. 700 ° C. with respect to a steel sheet having an equivalent dislocation density ρ of 1.0 × 10 ¹⁵ m ⁻² or more from the surface to 200 μm in the depth direction after winding at a temperature of 800 ° C. or less, pickling, and cold rolling. The method for producing a galvanized steel sheet according to any one of claims 1 to 4 , wherein the annealing is performed at 900 ° C or lower and the steel sheet surface is galvanized.
Here, the corresponding transition density ρ shows 14.4ε ² / ^{b 2} (ε represents the uneven distortion of the steel sheet, b is 2.5 × ^{10 -10} m.).