JP2009191348A - Method for manufacturing pre-coated steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a pre-coated steel sheet, which inhibits the steel sheet from lowering its gloss after having been worked. <P>SOLUTION: This manufacturing method includes the steps of: preparing a slab which has a composition including, by wt%, 0.007% or less C, 0.5% or less Si, 1.0% or less Mn, 0.02% or less S, 0.007% or less N, 0.05 to 0.2% Ti and/or 0.05 to 0.2% Nb and the balance Fe with unavoidable impurities; rolling the slab at a finishing temperature between (Ar<SB>3</SB>point-20) and 940°C to obtain a hot-rolled steel plate; winding the plate at 600 to 730°C (a1 to a4); pickling the hot-rolled steel plate; rolling it at a rolling rate of 60 to 90% to obtain a cold-rolled steel sheet (a5 and a6); and annealing the cold-rolled steel sheet at 700 to 950°C (a7). The steel sheet obtained after annealing has crystal grains with sizes of 20 μm or smaller, an r-value of 1.5 or more, and is plated and painted to produce the pre-coated steel sheet (a8 and a9). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for producing a pre-coated steel sheet that is used for exterior members of various products and is subjected to processing in a state having a coating layer on one or both surfaces of the steel sheet.

  Steel plates used as exterior members of various products are often coated from the viewpoint of corrosion resistance life and aesthetics. When the steel sheet used for the exterior member is coated after being processed into a predetermined shape, it is called post-coating. Since post-coating must be applied in the shape of the processed member, it is difficult to apply the post-coating when it is processed into a complicated shape, which is a cause of hindering productivity. Steel sheets include painted steel sheets with a coating layer pre-formed on the surface at the time of shipment from the factory, but they are especially used for exterior members such as electrical appliances and heating equipment that are subjected to complicated processing and severe processing. A material that can be applied in advance and can withstand processing in a state having a coating layer is desired. In response to this, a steel sheet that can be processed in a state having a coating film layer has been provided by improving the characteristics of the paint and the coating technique. A steel plate that can be processed with this coating layer is called a pre-coated steel plate.

  A pre-coated steel plate is a cold-rolled steel plate or a plated steel plate as a coating original plate, and a thermosetting resin or the like is used for a coating layer. The pre-coated steel sheet that has been pre-coated does not need to be coated after processing, so that productivity is improved compared to post-coating in which a member is processed into a predetermined shape and then coated. Precoated steel sheets with excellent productivity are used as materials for various applications. As the application expands, precoated steel sheets are used as processing materials even for members having complicated shapes, and severe processing such as deep drawing may be performed on the precoated steel sheets. When subjected to severe processing, there is a problem in that the gloss of the pre-coated steel sheet is partially reduced, and gloss is uneven between parts where gloss reduction does not occur or with other members and the appearance is not harmonized.

Assuming that the cause of the gloss reduction generated in the precoated steel sheet subjected to processing is local elongation of the coating film, it has been proposed to improve the gloss reduction by defining the physical properties of the coating film (see Patent Document 1). In patent document 1, while limiting the glass transition point of a coating film to a specific range and suppressing the local elongation of a coating film, a gloss fall is prevented, and the universal hardness and breaking elongation of a coating film are limited to a specific range. In addition, it is possible to prevent galling during deep drawing.
JP 2007-44922 A

  It seems that the improvement of the physical properties of the precoated steel sheet as in Patent Document 1 is effective in suppressing the reduction in gloss after processing. However, the gloss reduction is not caused only by the coating film, but may be caused by the steel plate which is the coating raw plate of the precoated steel plate. Therefore, the gloss reduction of the precoated steel sheet after processing cannot be sufficiently suppressed only by improving the coating film.

  When the precoated steel sheet is subjected to processing, the steel sheet as the coating original sheet is plastically deformed. At this time, when the surface of the steel sheet is roughened, the coating film follows the roughening of the steel sheet, and irregularities are formed on the surface of the coating film, resulting in a decrease in gloss. Therefore, in order to suppress the reduction in gloss of the precoated steel sheet after processing, it is necessary to prevent the steel sheet from being roughened due to processing.

  An object of the present invention is to provide a method for producing a pre-coated steel sheet that prevents a steel sheet as a coating original sheet from roughening even when subjected to, for example, deep drawing, and suppresses a decrease in gloss. is there.

In order to solve the problems of the present invention, in the method for producing a precoated steel sheet having a coating layer on one side or both sides of the steel sheet,
In order to set the crystal grain size of the steel sheet to 20 μm or less and the Rankford value to 1.5 or more,
% By weight
C: 0.007% or less,
Si: 0.5% or less,
Mn: 1.0% or less,
S: 0.02% or less,
N: 0.007% or less,
A steel ingot or slab containing Ti: 0.05 to 0.2% and / or Nb: 0.05 to 0.2%, the balance having a composition of Fe and inevitable impurities, the finishing temperature is (Ar ₃ points-20) to 940 ° C., and hot rolling at a coiling temperature of 600 to 730 ° C. to form a hot rolled steel sheet,
Cold rolling the hot rolled steel sheet at a rolling rate of 60 to 90% to obtain a cold rolled steel sheet,
The cold-rolled steel sheet is annealed at 700 to 950 ° C.

  In the present invention, the steel ingot or cast slab may further contain B: 0.00002 to 0.001% by weight.

  According to the present invention, the elements contained in the steel sheet used as the coating original sheet and the content thereof are appropriately set, and the finishing temperature and the coiling temperature of the hot rolling are regulated to suppress the crystal grain coarsening of the hot rolled steel sheet. Further, after cold rolling at an appropriate rolling rate and annealing at a temperature within an appropriate range, it is possible to achieve refinement of crystal grains of the steel sheet and excellent workability. When processing pre-coated steel sheets manufactured from such fine crystal grains and steel sheets with excellent workability, the steel sheets are not roughened, preventing the occurrence of coating irregularities due to roughening. Therefore, it is possible to suppress a decrease in gloss.

  Further, by containing an appropriate amount of B in the steel sheet, the grain boundary strength is improved, so that the roughening at the time of processing can be more reliably prevented, and the reduction in gloss after processing the precoated steel sheet can be suppressed.

  FIG. 1 shows a schematic manufacturing process for carrying out the method for manufacturing a precoated steel sheet according to the present invention. Hereinafter, with reference to FIG. 1, the manufacturing method of the precoat steel plate of this invention is demonstrated. Here, a steel plate is used in the meaning including both a single plate and a steel strip.

  First, in step a1, steel having a predetermined composition is made by steelmaking. There are no restrictions on the melting conditions. For example, the hot metal provided from a blast furnace or an electric furnace can be refined and adjusted in a converter or the like, and vacuum refined as necessary to be smelted. Composition: wt%, C: 0.007% or less, Si: 0.5% or less, Mn: 1.0% or less, S: 0.02% or less, N: 0.007% or less, Ti: 0 0.05 to 0.2% and / or Nb: 0.05 to 0.2%, with the balance being Fe and inevitable impurities. The molten steel may further contain B: 0.00002 to 0.001%.

Hereinafter, the reasons for limiting the composition range will be described.
C: 0.007% or less Workability improves as the C content decreases. However, to achieve extremely low C, it takes a long time for decarburization, which causes a decrease in productivity. Therefore, it should be reduced to such an extent that a workability improving effect capable of preventing roughening can be exhibited, and is set to 0.007% or less.

Si: 0.5% or less Si is effective in improving the strength. However, if added over 0.5%, the strength is increased, but the workability and plating properties are lowered, so the content was made 0.5% or less.

Mn: 1.0% or less Since Mn has a high affinity with S and easily forms MnS, it is effective in suppressing hot and cold embrittlement due to S. However, even if added over 1.0%, the effect of suppressing embrittlement due to bonding with S is saturated, so the content was made 1.0% or less.

S: 0.02% or less Since S causes hot and cold embrittlement, it is desirable to reduce S. However, since the adverse effect of S can be mitigated by the addition of Mn, it may be contained if it is 0.02% or less.

N: 0.007% or less Since N deteriorates workability, it is desirable that N be as small as possible. Therefore, it was made 0.007% or less.

Ti: 0.05 to 0.2%
Ti improves workability by fixing solute C, S, and N in steel as precipitates. In order to exhibit this effect, addition of 0.05% or more is necessary. However, the effect of improving workability is saturated even if added over 0.2%. Therefore, it was made 0.05 to 0.2%.

Nb: 0.05 to 0.2%
Nb improves workability by fixing solute C and N in steel as precipitates. In order to exhibit this effect, addition of 0.05% or more is necessary. However, the effect of improving workability is saturated even if added over 0.2%. Therefore, it was made 0.05 to 0.2%.

B: 0.00002-0.001%
B segregates at the grain boundary and improves the grain boundary strength. Addition of 0.00002% or more is necessary to exhibit the effect of improving the grain boundary strength. However, even if added over 0.001%, the effect is saturated and ductility is lowered. Therefore, it was made 0.00002-0.001%.

In step a2, molten steel is cast into a steel ingot or slab having the above composition. There are no special restrictions on casting conditions. In step a3, the steel ingot or slab is hot-rolled at a finishing temperature of (Ar ₃ point−20) to 940 ° C. to obtain a hot-rolled steel plate. Ar ₃ point refers to the temperature at which the structure of steel, which is an Fe—C alloy, transforms from an austenite phase to a ferrite phase during the cooling process. The Ar ₃ point that is the phase transformation temperature can be measured by, for example, a thermal analyzer. The finishing temperature of hot rolling is the temperature at the end of the final rolling pass, and is an important factor that determines the ferrite structure of the steel sheet. When the finishing temperature of hot rolling is less than (Ar ₃ points−20) ° C., the structure of the hot rolled steel sheet becomes mixed, affecting the structure after cold rolling / annealing and the r value is lowered. If the finishing temperature exceeds 940 ° C., the crystal grains become coarse and the r value decreases. Therefore, the hot rolling finishing temperature was set to (Ar ₃ points−20) to 940 ° C.

  In step a4, the hot-rolled steel sheet is wound at a temperature of 600 to 730 ° C. on the exit side of the rolling stand that performs the final pass of hot rolling. If the coiling temperature is less than 600 ° C., the carbide in the ferrite becomes fine and the texture after cold rolling / annealing is not formed in a good state, so the r value is low. When the winding temperature exceeds 730 ° C., the winding coil is heated by recuperation, and the crystal grains of the hot rolled steel sheet become coarse. Therefore, the coiling temperature was 600 to 730 ° C.

  In step a5, the hot-rolled steel sheet is pickled and the oxide scale generated on the surface is removed. In step a6, the hot-rolled steel sheet is cold-rolled at a rolling rate of 60 to 90% to obtain a cold-rolled steel sheet. When the rolling rate is less than 60%, a texture preferable for improving workability cannot be developed, and a high r value cannot be obtained. If the rolling rate exceeds 90%, the load on the cold rolling mill becomes excessive and rolling becomes difficult. Therefore, the cold rolling rate is set to 60 to 90%. In step a7, the cold-rolled steel sheet is annealed at 700 to 950 ° C. Below 700 ° C., it becomes non-recrystallized and has poor ductility. Above 950 ° C., the crystal grains become coarser than 20 μm. Therefore, the annealing temperature was set to 700 to 950 ° C.

  In this way, the composition is adjusted, the rolling temperature and the coiling temperature are regulated and hot rolling is performed. The crystal grain size can be 20 μm or less, and the Rankford value can be 1.5 or more. Hereinafter, the Rankford value is expressed as r value.

Hereinafter, the reasons for limiting the range of the crystal grain size and r value of the steel sheet will be described.
Crystal grain size: 20 μm or less By making the crystal grain size 20 μm or less, it is possible to prevent the surface of the steel sheet from becoming rough even when subjected to, for example, deep drawing. When the surface of the steel sheet is not roughened, the coating film is not uneven, so that it is possible to prevent the precoated steel sheet from being deteriorated in gloss. However, when the crystal grain size exceeds 20 μm, for example, when subjected to deep drawing, roughening occurs on the surface of the steel sheet. The coating film follows this roughening to produce irregularities on the surface of the coating film, resulting in a decrease in gloss in the precoated steel sheet. The crystal grain size can be measured, for example, according to the cutting method defined in Japanese Industrial Standard (JIS) G0552.

r value: 1.5 or more By setting the r value to 1.5 or more, good deep drawability can be secured. The r value is a value measured in a state in which a No. 5 tensile test piece defined in JIS Z2201 is taken from the cold-rolled steel sheet after annealing and 15% elongation is given by a tensile tester. In addition, since r value has directionality, it calculates | requires as an average value of the value by the test piece extract | collected from each direction given by following formula (1).
r = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (1)
Here, r ₀ : r value obtained from a test piece taken in a direction parallel to the rolling direction r ₄₅ : r value obtained from a test piece taken in a 45 degree direction with respect to the rolling direction r ₉₀ : in the rolling direction R value obtained from a specimen taken in a direction perpendicular to the surface

  In step a8, the steel sheet is plated. This plating process is not essential in the production of the precoated steel sheet. However, from the viewpoint of improving the corrosion resistance life of the precoated steel sheet and improving the adhesion of the coating film, it is desirable to carry out a plating treatment. The plating type is not particularly limited. Zinc-based, zinc alloy-based, aluminum-based, and aluminum alloy-based plating types can be used. There are no particular restrictions on the plating method. Any of hot dipping, electroplating, and chemical plating may be used. From the viewpoint of productivity, hot dipping is preferable. In particular, as the plating used for the pre-coated steel sheet, hot dip galvanizing and hot dip zinc-55% aluminum plating are suitable.

  In step a9, coating is performed on one side or both sides of the steel plate. The coating layer may be a single layer or a multilayer of two or more layers. Preferably, it is a multilayer structure including an undercoat layer called a primer layer excellent in adhesion to a steel plate and an overcoat layer. For example, a polyester-based resin or an epoxy-based resin can be used for the paint for the undercoat layer, and a high-molecular polyester-based resin can be used for the paint for the topcoat layer. There are no particular restrictions on the painting method. Any of roll coater coating, spray coating, and immersion coating may be used. There is no restriction | limiting also in the conditions of drying or baking of a coating film, The conditions according to the coating film to form can be used.

The precoated steel sheet produced by this production method has fine crystal grains of the steel sheet that is the coating original sheet and is excellent in workability. Therefore, for example, even when subjected to deep drawing, the steel sheet is not roughened, and the coating film is not uneven, so that the gloss is not lowered. Whether or not the pre-coated steel sheet after processing has deteriorated in gloss is evaluated by the gloss retention given by the following formula (2).
Gloss retention rate = Glossiness after tensile test / Glossiness before tensile test × 100 (2)

  The glossiness is measured as follows. A No. 5 tensile test piece defined in JIS Z2201 is taken from the precoated steel plate in a direction parallel to the rolling direction. The glossiness of the test piece before the tensile test is measured based on JIS Z8741. Next, the glossiness after the tensile test giving 15% elongation is measured in the same manner. The gloss retention is obtained by substituting the glossiness before and after the tensile test into Equation (2). The gloss retention rate is an index that expresses in percentage how much the glossiness before processing is retained after processing. The gloss retention rate at which gloss reduction is recognized to be suppressed is determined to be a desirable value depending on the application of the pre-coated steel sheet. For example, in the case of an exterior member for home appliances, it is approximately 70% or more. According to the precoated steel sheet manufactured by the method shown in steps a1 to a9, a gloss retention of 70% or more can be achieved, and a reduction in gloss after processing can be suppressed.

(Example)
Examples of the present invention will be described below.
Example 1
Table 1 shows the test materials No. Each of 1 to 21 was melted and cast into a mold to produce a 30 kg steel ingot. The balance other than the composition shown in Table 1 is Fe and inevitable impurities. The steel ingot was hot forged into a slab with a thickness of 30 mm. The slab was heated to 1050 to 1180 ° C., and the heated slab was hot-rolled to a thickness of 4.0 mm to obtain a hot-rolled steel sheet. Specimen No. In Nos. 1 to 21, since the Ar ₃ points were all less than 890 ° C., the hot rolling finishing temperature was set to 920 ° C. of (Ar ₃ points−20) ° C. or higher.

  A treatment equivalent to coil winding was performed in which the steel sheet wound after hot rolling was immersed in a salt bath furnace at 680 ° C. for 2 hours. The coil of the hot-rolled steel sheet as the test material has a lower cooling rate than that of the large-scale production facility because the weight of the coil is smaller than the coil weight of the hot-rolled steel plate when the large-scale production facility is used. Get faster. A process equivalent to coil winding was performed and the cooling rate was corrected so that the conditions were the same as when winding after hot rolling in a large-scale production facility. After the treatment corresponding to coil winding, the hot rolled steel sheet was cooled to room temperature.

  The hot rolled steel sheet was pickled and scaled off. After pickling, it was cold-rolled at a rolling rate of 80% to obtain a cold-rolled steel sheet having a thickness of 0.8 mm. The cold rolled steel sheet was annealed at 850 ° C. for 3 minutes. When the cold rolling rate is 60%, the test material No. In Nos. 1 to 21, since the recrystallization temperature was less than 700 ° C., the annealing temperature was set to 850 ° C. that is equal to or higher than the recrystallization temperature and exceeded 700 ° C.

  From each cold-rolled steel sheet after annealing, a No. 5 tensile test piece defined in JIS Z2201 and a structure observation test piece were collected. The r value, yield stress, tensile strength, and elongation were measured using a tensile specimen. The r value was determined based on the above formula (1). Yield stress, tensile strength and elongation were measured only for tensile specimens taken in a direction parallel to the rolling direction. Hereinafter, the yield stress, tensile strength, and elongation are expressed as YS, TS, and El. Moreover, the crystal grain size was measured according to the cutting method prescribed | regulated to JISG0552 using the structure | tissue observation test piece.

The cold-rolled steel sheet after annealing was immersed in a Zn-0.18% Al plating bath for hot dip galvanization. The bath temperature and inlet temperature of the plating bath were both 430 ° C. The plating adhesion amount was 90 g / m ² per side and the thickness was about 13 μm. Double-sided coating was performed on the steel sheet after the plating treatment. The coating conditions are shown in Table 2. A No. 5 tensile test piece defined in JIS Z2201 was taken from the coated steel plate in parallel with the rolling direction, and the glossiness before the tensile test and after the tensile test giving 15% elongation was measured based on JIS Z8741. . The apparatus used for the measurement of glossiness is a handy gloss meter PG-1M manufactured by Nippon Denshoku Industries Co., Ltd. Based on the above formula (2), the gloss retention was obtained from the gloss before and after the tensile test. The presence or absence of gloss reduction after processing was evaluated by gloss retention. When the gloss retention was 70% or more, the evaluation was good as no gloss reduction, and when the gloss retention was less than 70%, the evaluation was poor because there was a gloss reduction.

  The test results are shown in Table 3. Specimen No. In Nos. 1 to 13, the crystal grain size was 20 μm or less and the r value was 1.5 or more. In these test materials, the gloss retention was 70% or more, and a reduction in gloss after processing could be suppressed. On the other hand, specimen No. Nos. 14 to 21 were coarsened with a crystal grain size exceeding 20 μm. In 14 to 16, 18 and 21, the r value was also less than 1.5. In these test materials, the gloss retention was less than 70%, and the gloss reduction after processing could not be suppressed.

(Example 2)
In Example 2, a test result using a test material manufactured by a large-scale production facility will be described. In Table 4, the test material No. having the composition shown by weight%. 22 to 24 were melted. The balance other than the composition shown in Table 4 is Fe and inevitable impurities. The smelting was performed by using a blast furnace hot metal and adjusting the components with an approximate converter and degassing furnace. The obtained molten steel was continuously cast to produce a slab having a thickness of 200 mm. Table 5 shows the Ar ₃ points and the recrystallization temperature for each test material.

The slab was heated to 1150 ° C. and hot-rolled to obtain a hot-rolled steel sheet. Change the finishing temperature of hot rolling, ₍₃ points -20 Ar) and 850 ° C. of less than ° C., the four levels of 900 ° C., 910 ° C. and 930 ° C. in the range of (Ar ₃ point -20) to 940 ° C. I let you. Moreover, the winding temperature of the hot-rolled steel sheet was changed to three levels of 500 ° C., 660 ° C. and 750 ° C.

  The hot rolled steel sheet was pickled and scaled off. Thereafter, the rolling rate was changed to three levels of 45%, 76% and 86%, and cold rolled to a thickness of 0.8 mm to obtain a cold rolled steel sheet. By adjusting the finishing thickness of the hot-rolled steel sheet, the rolling rate when cold-rolling to a thickness of 0.8 mm was set to the above three levels.

  Two levels of 650 ° C. that are 700 ° C. or lower and lower than the recrystallization temperature of each specimen, and 850 ° C. that is within the range of 700 to 950 ° C. and higher than the recrystallization temperature after cold rolling. Annealed. From each specimen after annealing, the r value, YS, TS, El, and crystal grain size were measured in the same manner as in Example 1.

The cold-rolled steel sheet after annealing was plated. There are two types of plating: molten zinc and molten zinc-55% aluminum. Hereinafter, the hot dip galvanizing is referred to as hot dip Zn plating, and the hot dip zinc-55% aluminum plating is referred to as hot dip Zn—Al plating. In the hot dip Zn plating, the bath temperature of the plating bath was 470 ° C., and the inlet temperature was 460 ° C. In the molten Zn—Al plating, the bath temperature of the plating bath was 600 ° C., and the inlet temperature was 600 ° C. The plating adhesion amount per one side is two levels of 45 g / m ² and 90 g / m ² for molten Zn plating and one level of 45 g / m ² for molten Zn-Al plating. Each test material No. Tables 6 to 8 show manufacturing conditions from hot rolling to plating treatment for 22, 23, and 24. One specimen, for example specimen no. No. 22 with different production conditions is No. 22 A subnumber was added to the end of 22 to distinguish them.

  Double-sided coating was performed on the steel sheet after the plating treatment. The coating conditions are shown in Table 9. From the steel sheet after the coating treatment, the gloss retention was obtained in the same manner as in Example 1, and the presence or absence of gloss reduction suppression was evaluated.

  Each test material No. The test results for 22, 23, and 24 are shown in Table 10, Table 11, and Table 12, respectively. The hot rolling finishing temperature is either 900 ° C., 910 ° C., or 930 ° C., the coiling temperature of the hot rolled steel sheet is 660 ° C., and the rolling rate of cold rolling is either 76% or 86%. In the test material having an annealing temperature of 850 ° C., the crystal grain size was 20 μm or less and the r value was 1.5 or more. Such a test material had a gloss retention of 70% or more, and could suppress a reduction in gloss after processing.

In the test material having a finishing temperature of hot rolling of less than (Ar ₃ points−20) ° C., the r value was less than 1.5, and the crystal grain size also exceeded 20 μm. When the finishing temperature of hot rolling is less than (Ar ₃ point−20) ° C., the hot rolled steel sheet has a mixed grain structure, and this mixed grain structure affects the workability after cold rolling and annealing. The r value decreased.

  In the test material at 750 ° C where the coiling temperature of the hot-rolled steel sheet is high, the crystal grains of the hot-rolled steel sheet are coarsened, and the influence extends after the annealing of the cold-rolled steel sheet. It became coarse. In the specimen at 500 ° C. where the coiling temperature of the hot-rolled steel sheet is low, carbides in the ferrite of the hot-rolled steel sheet become fine, and a favorable texture is not formed for improving the r value after annealing of the cold-rolled steel sheet. The r value decreased.

  With the 45% test material having a low cold rolling rate, it was not possible to develop a texture preferable for improving workability, so an r value of 1.5 or more could not be obtained. In the test material having an annealing temperature of 650 ° C., the structure of the cold-rolled steel sheet was not recrystallized, and the r value was significantly reduced.

In the test material in which the finishing temperature of hot rolling is less than (Ar ₃ point-20) ° C., the winding temperature of the hot rolled steel sheet is 500 ° C. or 750 ° C., and the cold rolling rate is 45%, One or both of the crystal grain size of 20 μm or less and the r value of 1.5 or more were not satisfied. Such a test material had a gloss retention of less than 70%, and could not suppress a reduction in gloss after processing.

It is a flowchart which shows the schematic manufacturing process for enforcing the manufacturing method of the precoat steel plate of this invention.

Claims (2)

In the method for producing a precoated steel sheet having a coating layer on one side or both sides of the steel sheet,
In order to set the crystal grain size of the steel sheet to 20 μm or less and the Rankford value to 1.5 or more,
% By weight
C: 0.007% or less,
Si: 0.5% or less,
Mn: 1.0% or less,
S: 0.02% or less,
N: 0.007% or less,
A steel ingot or slab containing Ti: 0.05 to 0.2% and / or Nb: 0.05 to 0.2%, the balance having a composition of Fe and inevitable impurities, the finishing temperature is (Ar ₃ points-20) to 940 ° C., and hot rolling at a coiling temperature of 600 to 730 ° C. to form a hot rolled steel sheet,
Cold rolling the hot rolled steel sheet at a rolling rate of 60 to 90% to obtain a cold rolled steel sheet,
A method for producing a pre-coated steel sheet, comprising annealing the cold-rolled steel sheet at 700 to 950 ° C.   The method for producing a precoated steel sheet according to claim 1, wherein the steel ingot or slab further contains B: 0.00002 to 0.001% by weight.

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