JP6102902B2 - Cold-rolled steel sheet, manufacturing method thereof, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet - Google Patents

Description

  The present invention relates to a cold-rolled steel sheet, a method for producing the same, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet.

In recent years, with the increasing awareness of global environmental protection, there has been a strong demand for improved fuel efficiency to reduce CO ₂ emissions from automobiles. Along with this, there is an active movement to increase the strength of materials for automobile parts to reduce the thickness of automobile parts and reduce the weight of the vehicle body. However, when the strength of a steel plate used as a vehicle body material is increased, the ductility of the steel plate tends to be reduced. Therefore, development of a high strength and high ductility steel sheet is desired.

  In order to increase the strength of the steel sheet, a solid solution strengthening element such as Si, Mn, or Cr may be included in the steel sheet. In particular, Cr increases the strength of the steel sheet with a smaller content than other elements. For this reason, Cr is effective in strengthening the material of the steel plate.

  However, Cr and the like are easily oxidizable elements that are easier to oxidize than Fe. When manufacturing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that use a high-strength steel sheet containing a large amount of these solid solution strengthening elements as a base material, there are the following problems.

  When manufacturing a hot-dip galvanized steel sheet, the steel sheet is heat-annealed at a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or a reducing atmosphere, and then hot-dip galvanized. However, since easily oxidizable elements in steel are selectively oxidized in a generally used non-oxidizing atmosphere or reducing atmosphere, these elements are concentrated on the surface to form oxides during heat annealing. This oxide reduces wettability with molten zinc during the plating process and causes non-plating. That is, as the concentration of the easily oxidizable element in the steel increases, the wettability rapidly decreases and non-plating frequently occurs. Further, even when non-plating does not occur, if there is an oxide between the steel plate and the plating, the plating adhesion deteriorates.

  For this problem, Patent Document 1 proposes a method for improving the wettability with molten zinc. Specifically, in Patent Document 1, the steel sheet is heated in an oxidizing atmosphere in advance, and the oxidation of additive elements on the steel sheet surface is prevented by rapidly generating an Fe oxide film on the surface at an oxidation rate higher than a predetermined value. Then, a method of reducing annealing the Fe oxide film is described. However, in the technique described in Patent Document 1, when the amount of oxidation of the steel sheet is large, there arises a problem that iron oxide adheres to the in-furnace roll and the steel sheet is pressed.

  Further, Patent Document 2 proposes a method in which the surface oxide is removed by pickling the steel plate after annealing, and then the steel plate after pickling is annealed again and hot dip galvanized. However, the technique described in Patent Document 2 cannot remove an acid-insoluble oxide, and thus does not improve the plating appearance of a steel sheet on which an acid-insoluble oxide is formed. As a steel plate on which an acid-insoluble oxide is formed, a steel plate containing Cr is known.

  Moreover, in patent document 3, the method of improving the chemical conversion property of a cold-rolled steel sheet is proposed by removing the Cr oxide layer formed by annealing by performing hydrochloric acid pickling after annealing. However, under the condition that the average value of the Cr concentration in the surface layer portion is 1.0% by mass or less, the Cr concentration in the surface layer is not sufficiently reduced, and the hot dip galvanizing has poor adhesion.

Japanese Patent No. 2587724 Japanese Patent No. 3957550 JP 2010-138458 A

  In view of such circumstances, the present invention relates to a cold-rolled steel sheet for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet. The present invention provides a technique for improving the surface appearance and plating adhesion of a plated steel sheet.

  As a result of intensive studies to solve the above problems, the present inventors have found the following facts.

  A Cr oxide is formed on the surface of the cold-rolled steel sheet after annealing, and a region having a low Cr concentration is formed inside the cold-rolled steel sheet immediately below the surface. Here, if the Cr oxide on the surface of the cold-rolled steel sheet can be removed, since the Cr concentration on the surface of the cold-rolled steel sheet is low, Cr is hardly supplied to the surface of the cold-rolled steel sheet even after annealing again. Formation of Cr oxide is suppressed. However, as described above, since Cr oxide is insoluble in acid, a means for removing Cr oxide on the surface is necessary. Here, the Cr oxide itself is insoluble in acid. However, by dissolving and removing the surface layer of the steel plate with a strong acid or the like, the Cr oxide can be removed together with the surface layer portion. At this time, if the pickling reduction amount of the steel sheet can be appropriately controlled, it is possible to leave the low Cr concentration region immediately below the surface of the cold-rolled steel sheet and to remove the Cr oxide on the surface. On the other hand, when a strong acid is used to dissolve and remove the surface layer of the steel sheet, iron-based oxides are slightly formed on the steel sheet surface. Since the amount of this iron-based oxide is small, it has been found that it has little effect on the plating appearance, but affects the plating adhesion of the hot-dip galvanized steel sheet. Specifically, when hot dip galvanizing is applied to a steel sheet on which an iron-based oxide is formed, it has been found that the plating may be peeled off when the conditions for processing the steel sheet become severe. For this reason, in particular, in hot-dip galvanized steel sheets that are not alloyed, it is possible to improve the plating adhesion by removing the iron-based oxides by removing the iron oxide after removing the surface layer of the steel sheet with a strong acid. I understood that. It has also been found that it is effective to use a non-oxidizing acid that is different from the acid used for the pickling before the re-pickling for the re-pickling.

  The present invention has been made based on the above findings and has the following features.

  (1) A cold-rolled steel sheet used for production of a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet, wherein C: 0.040% or more and 0.500% or less, Si: 1.00% by mass % Or less, Mn: 2.00% or more and 3.80% or less, P: 0.010% or less, S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Cr: 0.3% or more and 1.00% or less, the balance is made of Fe and inevitable impurities, the Cr concentration within 0.5 μm in the thickness direction from the surface is 0.20% or less, and the Mn concentration is 1 Cold-rolled steel sheet characterized by being 8% or less.

  (2) Further, in terms of mass%, Mo: 0.01 to 0.50, Nb: 0.010% to 0.100%, B: 0.0001% to 0.0050% and Ti: 0.00. The cold-rolled steel sheet according to (1), containing at least one element selected from 010% to 0.100%.

  (3) Further, by mass%, Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca The cold-rolled steel sheet according to (1) or (2), which contains at least one element selected from: 0.0100% or less and REM: 0.005% or less.

(4) A method for producing the cold-rolled steel sheet according to any one of (1) to (3), wherein the hydrogen concentration is 3.0 to 25.0 vol%, and the dew point is -40 to -10 ° C. The heating process which hold | maintains the steel plate which has the component composition of Claims 1-3 in the temperature range of 700-900 degreeC for 20-600 second, the cooling process which cools the steel plate after this holding | maintenance, and the steel plate after this cooling A method for producing a cold-rolled steel sheet, comprising: a pickling step for pickling and removing the surface under conditions where the pickling loss is 4.0 to 20.0 g / m ^{2 in} terms of Fe.

  (5) The method for producing a cold-rolled steel sheet according to (4), wherein the acid used in the pickling step is at least one selected from nitric acid, hydrochloric acid, hydrofluoric acid, and sulfuric acid.

(6) The pickling solution used in the pickling step includes nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g / L and not more than 200 g / L, and the ratio of the hydrochloric acid concentration to the nitric acid concentration (HCl / HNO ₃ ) is 0. The pickling solution used in the pickling step contains nitric acid and hydrofluoric acid, the nitric acid concentration is more than 50 g / L and not more than 200 g / L, and the hydrofluoric acid concentration relative to the nitric acid concentration The ratio (HF / HNO ₃ ) is 0.01 to 1.00, the method for producing a cold-rolled steel sheet according to (4) or (5).

  (7) The method according to (6), further comprising a re-pickling step of re-pickling the steel sheet surface with a non-oxidizing acid different from the acid used in the pickling step after the pickling step. The manufacturing method of the cold-rolled steel sheet of description.

  (8) Hot-dip galvanizing treatment is applied to the cold-rolled steel sheet according to any one of (1) to (3) above or the cold-rolled steel sheet manufactured by the manufacturing method according to any one of (4) to (7) above. High strength hot-dip galvanized steel sheet.

  (9) A high-strength galvanized steel sheet obtained by subjecting the high-strength hot-dip galvanized steel sheet according to (8) to a hot-dip galvanizing process and then an alloying process.

  Although the cold-rolled steel sheet of the present invention is a steel sheet containing Cr, the surface appearance and plating adhesion are excellent. Applying high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets manufactured using this cold-rolled steel sheet to automotive parts such as automotive structural members will improve fuel efficiency by reducing the weight of the vehicle body. Can do.

It is a figure which shows one example of the measurement of the density | concentration of Cr and Mn.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

  First, the component composition of the cold-rolled steel sheet of the present invention will be described. In the following description of the component composition, “%” means “mass%”.

  The cold-rolled steel sheet of the present invention is C: 0.040% or more and 0.500% or less, Si: 1.00% or less, Mn: 2.00% or more and 3.80% or less, P: 0.010 by mass%. %: S: 0.010% or less, Al: 0.100% or less, N: 0.0100% or less, Cr: 0.3% or more and 1.00% or less.

C: 0.040% or more and 0.500% or less C is an austenite generating element, which is an element that combines an annealed plate structure and contributes to improvement in strength and ductility. In order to acquire the said effect, it is necessary to make content of C 0.04% or more. When the C content exceeds 0.50%, the welded part and the heat-affected zone are markedly cured, and the mechanical properties of the welded part deteriorate. That is, when the C content exceeds 0.50%, spot weldability, arc weldability, and the like deteriorate. Since automobile parts are often joined to other parts by welding, automobile parts are required to have good weldability. Therefore, the C content is 0.50% or less.

Si: 1.00% or less Si is a ferrite-forming element and contributes to solid solution strengthening and work hardening ability of the ferrite of the annealed plate. However, when the Si content is excessive, an oxide is formed on the surface of the cold-rolled steel sheet during annealing, and the plateability of the cold-rolled steel sheet is deteriorated. Therefore, the Si content is 1.00% or less.

Mn: 2.00% or more and 3.80% or less Mn is an austenite generating element and an element contributing to securing the strength of the annealed plate. In order to acquire this effect, it is necessary to make Mn content 2.00% or more. On the other hand, if the Mn content exceeds 3.80%, a large amount of oxide is formed on the steel sheet surface during annealing, and the plating appearance deteriorates. Therefore, the Mn content is 3.80% or less.

Cr: 0.3% or more and 1.00% or less Cr is an austenite generating element and an element contributing to securing the strength of the annealed plate. If the Cr content is less than 0.3%, it is difficult to ensure the strength of the cold-rolled steel sheet. On the other hand, if the Cr content exceeds 1.00%, the appearance of plating deteriorates because Cr oxide formed on the surface of the cold-rolled steel sheet cannot be removed even with the present invention. Therefore, the Cr content is 1.00% or less.

P: 0.010% or less P is an element effective for strengthening steel. In order to obtain this effect, the P content is preferably 0.001% or more. On the other hand, when the P content exceeds 0.010%, embrittlement occurs due to grain boundary segregation, and impact resistance deteriorates. Therefore, when it contains P, content of P shall be 0.010% or less.

S: 0.010% or less S becomes inclusions such as MnS, and deteriorates impact resistance or causes cracks along the metal flow of the welded portion. For this reason, the S content is preferably as low as possible. However, it is difficult to manufacture a cold-rolled steel sheet so as not to completely contain S from the viewpoint of manufacturing cost and the like. Then, when it contains S, content of S shall be 0.010% or less.

Al: 0.100% or less When Al is excessively contained, surface properties and moldability are deteriorated due to an increase in oxide inclusions. Moreover, the excessive content of Al increases the manufacturing cost. Therefore, when Al is contained, the Al content is 0.100% or less. Preferably it is 0.050% or less.

N: 0.0100% or less N is an element that greatly deteriorates the aging resistance of steel. The smaller the N content, the better. When the content of N exceeds 0.0100%, deterioration of aging resistance becomes remarkable. Therefore, the N content is 0.0100% or less.

  Further, the cold-rolled steel sheet of the present invention is further mass%, Mo: 0.01 to 0.50, Nb: 0.010% to 0.100%, B: 0.0003% to 0.0050. % Or less and Ti: at least one element selected from 0.010% or more and 0.100% or less may be contained.

Mo: 0.01% or more and 0.50% or less Mo is an austenite-generating element and contributes to securing the strength of the annealed plate. If the Mo content is less than 0.01%, it is difficult to obtain the effect of securing the strength. Moreover, since Mo is expensive, the increase in the content leads to an increase in cost. Therefore, when it contains Mo, content of Mo shall be 0.01% or more and 0.50% or less.

Nb: 0.010% or more and 0.100% or less Nb is an element that contributes to strength improvement by solid solution strengthening or precipitation strengthening. This effect can be obtained by setting the Nb content to 0.010% or more. On the other hand, when the Nb content exceeds 0.100%, the ductility of the steel sheet is lowered. That is, the workability of the steel sheet is deteriorated. Therefore, when Nb is contained, Nb is made 0.010% or more and 0.100% or less.

B: 0.0001% or more and 0.0050% or less B is an element that enhances hardenability and contributes to improving the strength of the steel sheet. This effect can be obtained by setting the B content to 0.0001% or more. On the other hand, when B is contained excessively, the ductility of the steel sheet is lowered, and as a result, the workability of the steel sheet is deteriorated. Further, excessive inclusion of B causes an increase in cost. Therefore, when it contains B, content of B shall be 0.0001% or more and 0.0050% or less.

Ti: 0.010% or more and 0.100% or less Ti is an element that contributes to improving the strength of the steel sheet by forming fine carbide or fine nitride with C or N in the steel sheet. In order to obtain this effect, the Ti content needs to be 0.010% or more. On the other hand, this effect is saturated when the Ti content exceeds 0.100%. Therefore, when Ti is contained, the content of Ti is set to 0.010% or more and 0.100% or less.

  Further, the cold-rolled steel sheet of the present invention is further, in mass%, Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, Sb: 0.10% or less, Sn: It may contain at least one element selected from 0.01% or less, Ca: 0.0100% or less, and REM: 0.005% or less.

Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less Cu, V, and Ni are elements effective for strengthening steel. If it is in the range prescribed | regulated by this invention, the cold-rolled steel plate of this invention can contain these elements for reinforcement | strengthening of steel. However, if the Cu content is 1.00%, the V content is 0.500%, and the Ni content exceeds 0.50%, the ductility of the steel sheet may decrease due to a significant increase in strength. Further, excessive inclusion of these elements also causes an increase in cost. Therefore, when these elements are contained, the amounts of Cu are 1.00% or less, V is 0.500% or less, and Ni is 0.50% or less, respectively.

Sb: 0.10% or less, Sn: 0.10% or less Sb and Sn have the effect of suppressing nitriding in the vicinity of the steel sheet surface layer. This effect is saturated when the content of these elements exceeds 0.10%. Therefore, when Sb is contained, the Sb content is 0.10% or less, and when Sn is contained, the Sn content is 0.10% or less.

Ca: 0.0100% or less, REM: 0.005% or less Ca improves the ductility of the steel sheet by shape control of sulfides such as MnS. The effect is saturated when the Ca content exceeds 0.0100%. Moreover, REM controls the form of sulfide inclusions and contributes to the improvement of the workability of the steel sheet. However, when the content of REM exceeds 0.005%, the amount of inclusions increases and the workability deteriorates. Therefore, when Ca is contained, the Ca content is 0.0100% or less, and when REM is contained, the REM content is 0.005% or less.

  In the cold-rolled steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities.

  In the cold-rolled steel sheet of the present invention having the above component composition, the Cr concentration within 0.5 μm in the thickness direction from the steel sheet surface is 0.20% or less, and the Mn concentration is 1.8% or less. Here, “%” means “mass%”.

Cr concentration within 0.5 μm in the thickness direction from the steel sheet surface is 0.20% or less. When there is a region with low Cr concentration directly under the steel sheet surface of the cold rolled steel sheet, formation of Cr oxide on the steel sheet surface during annealing is suppressed. Is done. As a result, a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet with excellent surface appearance can be obtained. On the other hand, when the Cr concentration directly under the steel sheet surface is high, Cr oxide is formed on the steel sheet surface during annealing, resulting in non-plating.

  When the Cr concentration within 0.5 μm in the thickness direction from the steel sheet surface exceeds 0.20%, the Cr oxide is likely to be formed on the steel sheet surface, the plating appearance deteriorates, and the plating adhesion also decreases. For this reason, the Cr concentration within 0.5 μm in the thickness direction from the steel sheet surface is set to 0.20% or less.

  Here, glow discharge emission analysis (GDS) is used for the Cr concentration measurement immediately below the steel sheet surface. The Cr strength in the depth direction (thickness direction of the steel plate) is measured from the steel plate surface by GDS, and the average value of the Cr strength within 0.5 μm from the steel plate surface is defined as the Cr concentration. Further, the Cr intensity is measured at a depth exceeding 0.5 μm, and the average value of the intensity from the shallowest position in the region where the Cr intensity is constant to the depth of 0.5 μm is defined as the bulk Cr concentration. Since the element strength measured by GDS is proportional to the element concentration, the actual Cr concentration on the steel sheet surface is calculated from the ratio between the bulk Cr concentration calculated from GDS and the Cr concentration from the steel sheet surface to a depth of 0.5 μm. Can be calculated.

The Mn concentration within 0.5 μm from the steel sheet surface is 1.8% or less. Mn also forms an oxide on the steel sheet surface in the same manner as Cr and deteriorates the appearance of the plating surface. Similar to Cr, if there is a region with a low Mn concentration directly under the steel sheet surface, the formation of Mn oxide on the steel sheet surface during annealing is suppressed.

  When the Mn concentration within 0.5 μm in the thickness direction from the steel sheet surface exceeds 1.8%, Mn oxide is easily formed on the steel sheet surface when hot-dip galvanizing is performed on the cold-rolled steel sheet. The Mn oxide on the surface of the steel sheet deteriorates the plating appearance and lowers the plating adhesion. For this reason, the Mn concentration within 0.5 μm from the steel sheet surface is set to 1.8% or less. The concentration of Mn is also measured using GDS in the same manner as Cr.

  Then, the manufacturing method of the cold rolled sheet steel of this invention is demonstrated.

  In manufacturing the cold-rolled steel sheet of the present invention, a steel sheet having the above component composition is used. The manufacturing process of this steel plate is not specified. After steel having the above composition is melted by a known method, it is made into a slab through slabs or continuous casting, hot rolled into a hot-rolled sheet, and then pickled to remove the oxidized scale on the surface. To do. Further, cold rolling is performed as necessary.

  The cold-rolled steel sheet of the present invention is manufactured through a heating process for heating the steel sheet, a cooling process for cooling the steel sheet after the heating process, and a pickling process for pickling the steel sheet after the cooling process. Hereinafter, each step will be described.

Heating step The heating step is a temperature range of 700 to 900 ° C. in an atmosphere having a hydrogen concentration of 3.0 to 25.0 vol% and a dew point of −40 to −10 ° C., and a steel plate having the above component composition for 20 to 600 seconds. It is the process of holding.

  In this heating step, the steel sheet is heated under heating conditions that do not oxidize Fe. By this heating, oxides of Cr and Mn are formed on the surface of the steel sheet, and a region having a low concentration of Cr and Mn is formed immediately below the surface of the steel sheet.

The H ₂ concentration in the atmosphere during heating needs to be sufficient to suppress the oxidation of Fe. Therefore, in the present invention, the hydrogen concentration is set to 3.0 vol% or more. In addition, if the H ₂ concentration exceeds 25.0 vol%, the cost increases. Therefore, the H ₂ concentration is set to 25.0 vol% or less. Components other than H ₂ in the atmosphere are N ₂ and unavoidable impurities.

  When the dew point of the atmosphere during heating is less than −40 ° C., formation of Cr oxide and Mn oxide on the steel sheet surface is suppressed. On the other hand, if the dew point exceeds −10 ° C., damage to the heating furnace due to moisture becomes remarkable, and repair costs are increased. Therefore, the dew point is set to -40 ° C or higher and -10 ° C or lower.

  When the steel plate temperature during heating is less than 700 ° C., Cr oxide and Mn oxide are not formed on the steel plate surface. Moreover, when the said steel plate temperature exceeds 900 degreeC, heating cost will start. Therefore, in the heating step, the steel sheet is heated at a predetermined temperature or temperature range of 700 ° C. or higher and 900 ° C. or lower.

  When the holding time, which is the time for holding the steel sheet at the above temperature or temperature range, is less than 20 seconds, sufficient Cr oxide and Mn oxide are not formed on the surface. When the holding time exceeds 600 seconds, the pickling efficiency decreases due to excessive Cr oxide formation, and the manufacturing efficiency decreases. Therefore, the holding time is 20 seconds or more and 600 seconds or less.

Cooling process A cooling process is a process of cooling the steel plate after this holding | maintenance. The cooling stop temperature is not particularly limited, but is generally 600 to 300 ° C. Cooling may be either water cooling or air cooling. Moreover, what is necessary is just to set cooling time, a cooling rate, etc. suitably.

Pickling Step The pickling step is a step of pickling and removing the steel sheet surface after the cooling step under the condition that the pickling loss is 4.0 to 20.0 g / m ^{2 in} terms of Fe.

As described above, in order to remove the Cr oxide formed on the steel plate surface, it is necessary to remove the oxide in the vicinity of the steel plate surface by strong pickling. At this time, if the pickling weight loss is less than 4.0 g / m ^{2 in} terms of Fe, Cr oxide remains on the steel sheet surface. On the other hand, if the pickling weight loss exceeds 20.0 g / m ² , the region where the Cr and Mn concentrations just below the surface are low is removed. Accordingly, the pickling weight loss is 4.0 g / m ² or more and 20.0 g / m ² or less in terms of Fe.

  As an acid that can be used for strong pickling, nitric acid, which is a strong oxidizing acid, can be used. Further, hydrochloric acid, hydrofluoric acid, sulfuric acid, or the like may be used as long as the above-described pickling weight loss can be satisfied, and the type of acid is not particularly limited. It is also effective to promote the dissolution of the steel sheet surface by adding a pickling accelerator to the acid or using an electrolytic treatment in combination.

When the steel plate is washed with a strong acid, Fe-based oxides are formed on the surface of the steel plate. This oxide causes non-plating in the hot dip galvanizing process. In the present invention, it is preferable to suppress the formation of Fe-based oxide as much as possible. For this purpose, the pickling solution contains nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g / L and not more than 200 g / L, and the ratio of the hydrochloric acid concentration to the nitric acid concentration (HCl / HNO ₃ ) is 0.01 to 1.00. Or the pickling solution contains nitric acid and hydrofluoric acid, the nitric acid concentration exceeds 50 g / L and is 200 g / L or less, and the ratio of the hydrofluoric acid concentration to the nitric acid concentration (HF / HNO ₃ ) is 0.01 to It is preferably any one of 1.00.

Re-pickling step The re-pickling step is a step of re-pickling the steel sheet surface with a non-oxidizing acid different from the acid used in the pickling step after the pickling step. The re-pickling step is an optional step, and performing this step has an effect of improving plating adhesion.

Examples of the non-oxidizing acid include hydrochloric acid, dilute sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and acids obtained by mixing two or more of these. When hydrochloric acid is used, the hydrochloric acid concentration is preferably 0.1 to 50 g / L, and when sulfuric acid is used, the sulfuric acid concentration is preferably 0.1 to 150 g / L. When the mixed acid is used for re-acid pickling, it is preferable to use a mixed acid with a hydrochloric acid concentration of 0.1 to 20 g / L and a sulfuric acid concentration of 0.1 to 60 g / L. Moreover, even when using any re-pickling liquid, it is preferable that the temperature of a re-pickling liquid shall be 20-70 degreeC. Moreover, it is preferable to perform the processing time of re-acid picking as 1 to 30 seconds. Incidentally, the acid used in the re-pickling, since the purpose of dissolving the iron oxides of the steel sheet surface, the acid used for the purpose of pickling step of dissolving each SiO ₂ the steel base of sparingly soluble (the base steel) to the acid Different acid. The “different acid” does not mean only acids that cannot be used in the pickling process, but also includes acids that are not used in the actual pickling process among the acids that can be used in the pickling process.

  The cold-rolled steel sheet of the present invention obtained as described above is subjected to a treatment such as degreasing as necessary, followed by an annealing treatment and a hot dip galvanizing treatment. The annealing process and the hot dip galvanizing process are not particularly defined and may be a generally known process. In addition, when the alloying treatment is performed after the hot dip galvanizing, temper rolling may be performed for shape correction after the alloying treatment.

  Steel having the component composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter and made into a slab by a continuous casting method. After the obtained slab was heated to 1200 ° C., it was hot-rolled to a thickness of 1.6 to 4.5 mm and wound up. Next, the obtained hot-rolled sheet was pickled and cold-rolled. Thereafter, heat treatment (heating process) is performed in a furnace capable of adjusting the atmosphere under the heat treatment conditions shown in Table 2 or Table 3. After the heat treatment, the steel sheet is cooled to 450 ° C., and subsequently pickling treatment shown in Tables 2 and 3 Pickling treatment was performed under the conditions, or pickling treatment and re- pickling treatment were performed under the conditions shown in Table 4 to obtain a cold-rolled steel sheet. Subsequently, annealing and hot dip galvanizing treatment were performed on a continuous hot dip galvanizing line to obtain a hot dip galvanized steel sheet. Moreover, after this plating process, a part was alloyed and the alloyed hot-dip galvanized steel plate was obtained.

  The sample collected from the cold-rolled steel sheet was subjected to GDS analysis, and the Cr and Mn concentrations from the surface to 0.5 μm were measured. FIG. 1 shows an example of measurement.

  In addition, the surface appearance and plating adhesion of the hot dip galvanized steel sheet (GI) and the alloyed hot dip galvanized steel sheet (GA) were investigated.

<Surface appearance>
Judging by visual inspection for appearance defects such as non-plating and pinholes, if there is no appearance defect, it is “Good (○ in the table)”. When there was an appearance defect in general “good (Δ)”, it was judged as “bad (× in the table)”. A surface appearance of good and almost good was regarded as acceptable.

<Plating adhesion>
The plating adhesion of the galvannealed steel sheet was evaluated by evaluating the powdering resistance. Specifically, cellophane tape is applied to the alloyed hot-dip galvanized steel sheet, the tape surface is bent 90 degrees, bent back, and the cellophane with a width of 24 mm is parallel to the bent portion on the inner side (compressed side) of the processed portion. The tape was pressed and pulled away, and the amount of zinc adhering to the 40 mm long portion of the cellophane tape was measured as the Zn count by fluorescent X-rays. Based on the criteria below, those with ranks 1 and 2 are “particularly good (◯ in the table)”, those with ranks 3 and 4 are “good (Δ in the table)”, and those with 5 or more are “bad” (in the table) X) ”. Ranks 1-4 were accepted.
X-ray fluorescence count Rank 0 or more and less than 2000: 1 (good)
2000 or more and less than 5000: 2
5000 or more and less than 8000: 3
8000 or more and less than 10,000: 4
10,000 or more: 5 (poor)
About the hot-dip galvanized steel sheet which is not alloyed, the ball impact test was performed, the processed part was peeled off with cellophane tape, and the plating adhesion was evaluated by visually judging the presence or absence of peeling of the plating layer. In the ball impact test, the ball mass was 1.8 kg, the drop height was 100 cm, and the diameter of the impact part was 3/4 inch and 3/8 inch.
A: No peeling of the plating layer at 3/4 inch and 3/8 inch. ○: No peeling of the plating layer at 3/4 inch. Slight peeling of the plating layer at 3/8 inch. X: 3/4 inch, 3 / 8 inches, both plating layers are peeled off

  The results obtained for the above evaluations are shown in Tables 2 to 4. According to Tables 2 and 3, both the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet produced using the cold-rolled steel sheet of the present invention are excellent in surface appearance and plating adhesion. On the other hand, either the surface appearance or the plating adhesion is inferior in the comparative example. Moreover, according to Table 4, the invention example which performed re-pickling is more excellent in plating adhesiveness than the invention example which does not perform re-pickling.

Claims (9)

A cold-rolled steel sheet used for manufacturing high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets,
C: 0.040% to 0.500%, Si: 1.00% or less, Mn: 2.00% to 3.80%, P: 0.010% or less, S: 0.010 by mass% %: Al: 0.100% or less, N: 0.0100% or less, Cr: 0.3% or more and 1.00% or less, Nb: 0.010% or more and 0.100% or less , the balance being Consisting of Fe and inevitable impurities,
A cold-rolled steel sheet, characterized in that the Cr concentration within 0.5 μm in the thickness direction from the surface is 0.20% or less and the Mn concentration is 1.8% or less. Further, at least selected from Mo: 0.01% or more and 0.50% or less , B : 0.0001% or more and 0.0050% or less, and Ti: 0.010% or more and 0.100% or less. The cold-rolled steel sheet according to claim 1, comprising one element.   Further, in terms of mass%, Cu: 1.00% or less, V: 0.500% or less, Ni: 0.50% or less, Sb: 0.10% or less, Sn: 0.10% or less, Ca: 0.0. The cold-rolled steel sheet according to claim 1 or 2, comprising at least one element selected from 0100% or less and REM: 0.005% or less. A method for producing the cold-rolled steel sheet according to any one of claims 1 to 3,
A steel sheet having the component composition according to claim 1 in a temperature range of 700 to 900 ° C in an atmosphere having a hydrogen concentration of 3.0 to 25.0 vol% and a dew point of -40 to -10 ° C for 20 to 600 seconds. Holding heating process;
A cooling step for cooling the steel plate after the holding;
A method for producing a cold-rolled steel sheet, comprising: pickling and removing the surface of the steel sheet after cooling under the condition that the pickling loss is 4.0 to 20.0 g / m ^{2 in} terms of Fe. .   The method for producing a cold-rolled steel sheet according to claim 4, wherein the acid used in the pickling step is at least one selected from nitric acid, hydrochloric acid, hydrofluoric acid, and sulfuric acid. The pickling solution used in the pickling step contains nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g / L and not more than 200 g / L, and the ratio of the hydrochloric acid concentration to the nitric acid concentration (HCl / HNO ₃ ) is 0.01 to 1.00 or
Alternatively, the pickling solution used in the pickling step contains nitric acid and hydrofluoric acid, the nitric acid concentration is more than 50 g / L and not more than 200 g / L, and the ratio of the hydrofluoric acid concentration to the nitric acid concentration (HF / HNO ₃ ) is 0. The method for producing a cold-rolled steel sheet according to claim 4 or 5, wherein the method is any one of .01 to 1.00.   7. The cooling according to claim 6, further comprising a re-pickling step of re-pickling the steel sheet surface with a non-oxidizing acid different from the acid used in the pickling step after the pickling step. A method for producing rolled steel sheets.   A high-strength hot-dip galvanized steel sheet obtained by subjecting the cold-rolled steel sheet according to any one of claims 1 to 3 or the cold-rolled steel sheet produced by the manufacturing method according to any one of claims 4 to 7 to a hot dip galvanizing treatment. .   A high-strength galvannealed steel sheet obtained by subjecting the high-strength galvanized steel sheet according to claim 8 to an alloying treatment.

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