TWI613297B - Steel plate manufacturing method - Google Patents

Abstract

A method for producing a steel sheet, comprising the steps of: obtaining a steel blank by continuous casting of a molten steel having a Si content of 0.4% by mass to 3.0% by mass; and performing a hot rolling of the steel blank to obtain a hot rolled steel sheet; a step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; a step of annealing the cold rolled sheet of the cold rolled steel sheet; a step of pickling after the cold rolled sheet is annealed; and a step of washing with water after pickling And, after washing, the step of drying. In the cold-rolled sheet annealing, the dew point is set to -35 ° C or less, and the conductivity of the washing water used for washing is set to 5.0 mS / m or less, and the washing time is set to within 15 seconds in the water washing, and Drying begins within 60 seconds from the end of the washing.

Description

Steel plate manufacturing method

The present invention relates to a method of producing a steel sheet.

In recent years, in terms of protecting the global environment, there is a constant demand for improving the fuel consumption performance of automobiles. Moreover, from the viewpoint of ensuring the safety of the passengers at the time of collision, it is also constantly required to improve the safety of the automobile. In order to respond to these requirements, it is desirable to achieve the weight reduction and high strength of the vehicle body at the same time. Therefore, the cold rolled steel sheet, which is the material of the automobile parts, continues to be thinned while maintaining high strength.

For the above high-strength steel sheets, rust resistance is required. Therefore, the steel sheet is subjected to chemical conversion treatment and electrodeposition coating after press forming. However, in the chemical conversion treatment, if the rust preventive oil or the press-formed lubricating oil applied to ensure the rust preventive property during transportation adheres to the surface of the steel sheet, the rust preventive oil or the lubricating oil hinders the chemical conversion reaction. Therefore, the anti-rust oil and lubricating oil will be degreased before the chemical conversion treatment.

In order to improve the chemical conversion treatability of the high-strength steel sheet, there is a case where the steel sheet is subjected to Ni plating treatment. Further, in the case of a non-high-strength Si-containing steel sheet, good chemical conversion treatability may be required, and therefore, the steel sheet may be subjected to Ni plating treatment. On the other hand, when the steel sheet is subjected to Ni plating treatment, the degreasing property is deteriorated.

Up to now, although various techniques have been proposed, it is difficult to balance chemical conversion treatability and degreasing property. In recent years, the chemical conversion film which is used for the chemical conversion treatment has been improved, so that the desired chemical conversion film is easily formed. Therefore, a technique of omitting the Ni plating treatment has been proposed. However, if the Ni plating treatment is omitted, the chemical conversion treatability is insufficient. Even with the above technique, it is still difficult to balance chemical conversion treatability and degreasing property.

CITATION LIST Patent Literature Patent Literature 1: Japanese Patent Publication No. Sho 58-37391 Patent Document 2: Japanese Patent Laid-Open Publication No. Hei No. Hei No. Hei. No. Hei. Japanese Patent Laid-Open No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei.

Disclosure of the Invention Problems to be Solved by the Invention An object of the present invention is to provide a method for producing a steel sheet which can achieve both chemical conversion treatability and degreasing property.

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above problems. As a result, it is understood that when the Si content is 0.4% by mass or more, the Si oxide is formed on the surface of the steel sheet in the cold-rolled sheet annealing, and the Si oxide lowers the chemical conversion treatability. It is also known that although the Si oxide can be removed by pickling, if it is pickled, in the water washing after pickling, the Fe oxide film is formed on the surface of the steel sheet and grows and remains. Further, it is also understood that the thicker the Fe oxide film formed on the surface of the steel sheet, the more deteriorated the chemical conversion treatability. Although the chemical conversion treatability can be improved by the Ni plating treatment, as described above, when the Ni plating treatment is performed, the degreasing property is deteriorated. As a result of the study by the inventors of the present invention, it has been found that when the Si content is 0.4% by mass or more, it is difficult to achieve both chemical conversion treatability and degreasing property.

Then, the inventors of the present invention have further intensively studied in order to suppress the formation of the Fe oxide film in the water washing after pickling. As a result, it was found that the higher the conductivity of the washing water used for the water washing, the thicker the Fe oxide film grows, and the longer the water washing time, the thicker the Fe oxide film grows. Further, it has been found that the longer the time from the completion of the washing to the start of drying, the thicker the Fe oxide film grows.

The inventors of the present invention have carried out various studies based on the above-mentioned knowledge and have conducted intensive studies, and have come up with various aspects of the invention described below.

(1) A method for producing a steel sheet, comprising the steps of: obtaining a steel blank by continuous casting of a molten steel having a Si content of 0.4% by mass to 3.0% by mass; and performing hot rolling of the steel preform a step of hot rolling the steel sheet; a step of cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet; a step of annealing the cold rolled sheet of the cold rolled steel sheet; and a step of pickling after the cold rolled sheet is annealed; After the pickling, the step of washing with water; and the step of drying after the washing with water, the dew point of the cold-rolled sheet annealing is set to -35 ° C or less, and the conductivity of the washing water used in the water washing is performed. When it is 5.0 mS/m or less, the washing time is set to 15 hours or less in the water washing, and the drying is started within 60 seconds from the end of the water washing.

(2) The method for producing a steel sheet according to (1), wherein the molten steel has a Mn content of 0.5% by mass to 4.0% by mass.

(3) The method for producing a steel sheet according to (1) or (2), wherein the H ⁺ concentration (mol/L) contained in the washing water is [H ⁺ ], Na ⁺ concentration (mol/L) ) is [Na ⁺ ], Mg ²⁺ concentration (mol/L) is [Mg ²⁺ ], K ⁺ concentration (mol/L) is [K ⁺ ], and Ca ²⁺ concentration (mol/L) is [Ca ^{2 ] +} ], Fe ²⁺ concentration (mol/L) is [Fe ²⁺ ], Fe ³⁺ concentration (mol/L) is [Fe ³⁺ ], Cl ^- concentration (mol/L) is [Cl ^- ], NO ₃ ^- concentration (mol / L) of [NO ₃ ^-], and SO ₄ ^2- concentration (mol / L) is [SO ₄ ^2-], satisfy formula ^{1: 349.81 [H +] +50.1} [Na +] +53.05×2[Mg ²⁺ ] +73.5[K ⁺ ]+595×2[Ca ²⁺ ]+53.5×2[Fe ²⁺ ] +68.4×3[Fe ³⁺ ]+76.35[Cl ^- ]+71.46 [NO ₃ ^- ] +80.0 × 2 [SO ₄ ^2- ] ≦ 5/100 (Formula 1).

Advantageous Effects of Invention According to the present invention, since good chemical conversion treatability can be obtained without performing Ni plating treatment, both chemical conversion treatability and degreasing property can be achieved.

Embodiments for Carrying Out the Invention Hereinafter, embodiments of the present invention will be described in detail. In the method for producing a steel sheet according to the present embodiment, continuous casting of the molten steel, hot rolling, hot rolling, pickling, cold rolling, cold rolling sheet annealing, post-annealing pickling, water washing, and drying are performed. In the following description, the unit of content of each element contained in the molten steel is "%", and means "% by mass" unless otherwise specified.

First, in continuous casting and hot rolling of molten steel, continuous casting of molten steel having a Si content of 0.4% to 3.0% is carried out to produce a steel preform, and heating and hot rolling of the steel preform are performed.

Continuous casting and heating can be carried out under normal conditions. As described above, when the Si content is 0.4% or more, Si oxide which is required to be pickled is formed. When the Si content is more than 3.0%, a large amount of Si oxide is formed on the surface of the steel sheet during the cold-rolled sheet annealing, and even if the pickling is performed, the Si oxide is not sufficiently removed, so that it becomes difficult to ensure chemical conversion treatability. Therefore, the Si content is set to be 3.0% or less.

In the hot rolling, it is preferred to carry out the finishing rolling in the temperature range of 850 ° C to 1000 ° C. The coiling temperature of the obtained hot-rolled steel sheet is preferably set in the range of 550 ° C to 750 ° C.

Pickling after hot rolling can be carried out under normal conditions.

Next, cold rolling is performed on the obtained hot-rolled steel sheet to obtain a cold-rolled steel sheet. If the rolling rate of the cold rolling is to be less than 50%, the hot-rolled steel sheet must be excessively thinned in advance, so that the production efficiency is lowered. Therefore, the rolling rate of the cold rolling is preferably set to 50% or more. If the rolling rate of the cold rolling is to be more than 85%, there is a case where the load of the cold rolling delay is significantly increased. Therefore, the rolling rate of the cold rolling is preferably set to 85% or less. In addition, the rolling ratio is a value calculated by (h1-h2)/h1 when the thickness of the steel sheet before the cold rolling is h1 and the thickness of the steel sheet after the cold rolling is h2.

Next, the cold rolled sheet of the obtained cold rolled steel sheet is annealed. The cold rolled sheet annealing can be performed, for example, by using a continuous annealing furnace including a preheating chamber, a heating chamber, a soaking chamber, a cooling chamber, and an overaging chamber.

The holding temperature of the cold rolled sheet annealing is preferably set to 750 ° C or higher, and the holding time is preferably set to 1 minute or longer. When the holding temperature of the cold rolled sheet annealing is lower than 750 ° C and the holding time is less than 1 minute, there is a case where the desired ductility and other mechanical properties cannot be obtained by recrystallization annealing.

The ambient gas in the annealing furnace is mainly composed of N ₂ , and 1 vol% to 40 vol% of H ₂ may be added, and water vapor may be added as needed. The ambient gas in the annealing furnace contains H ₂ O and other impurity gases that are inevitably mixed.

When the dew point of the ambient gas in the annealing furnace exceeds -35 ° C, the surface layer of the steel sheet is unavoidably decarburized, and the mechanical properties of the steel sheet are deteriorated. Therefore, the dew point of the ambient gas in the annealing furnace should be set to -35 ° C or lower. It is also possible to add water vapor in the annealing furnace, considering that the equilibrium vapor pressure of H ₂ O at -35 ° C is 3.2 × 10 ^-4 atmosphere, and the total pressure of the ambient gas in the annealing furnace is the same as the normal atmospheric pressure. The amount of water vapor is about 0.03 vol%. In some cases, water vapor is inevitably mixed into the annealing furnace, and the amount of water vapor is about 0.02 vol%. When water vapor is inevitably mixed, the dew point of the ambient gas in the annealing furnace is about -40 °C.

After the cold rolled sheet is annealed, pickling is performed. The Si oxide or Mn oxide formed on the surface of the steel sheet during annealing of the cold rolled sheet is removed by pickling. The method of pickling is not particularly limited, but it can be carried out, for example, by transporting a steel sheet annealed in a cold-rolled sheet into an acid bath filled with an acid pickling liquid and continuously immersing it.

The pickling liquid is not particularly limited, and hydrochloric acid, sulfuric acid, or nitric acid, or a solution containing a combination of these may be used in a total amount of 1% by mass to 20% by mass. The temperature of the pickling liquid is not particularly limited, and may be 30 ° C to 90 ° C. The immersion time for immersing the steel sheet in the pickling liquid is not particularly limited, and may be 2 seconds to 20 seconds.

Next, the pickled steel sheet was washed with water. The method of washing with water is not particularly limited, but can be carried out, for example, by transporting the pickled steel sheet into a bath filled with washing water for washing with water and continuously immersing it.

When the conductivity of the washing water exceeds 5.0 mS/m, the Fe oxide film tends to grow on the surface of the steel sheet during water washing, so that excellent chemical conversion treatability cannot be obtained. Therefore, the conductivity of the washing water is 5.0 mS/m or less, preferably 1.0 mS/m or less. The lower the conductivity of the washing water, the more the growth of the Fe oxide film can be suppressed, so that it is easy to ensure chemical conversion treatability. On the other hand, even in theoretical pure water, there are 10 ^-7 mol/L of H ⁺ ions and OH ^- ions generated by self-dissociation in water. Also, according to the literature (Introduction to Electrical Chemistry, Matsuda Koki, Iwakura Chiaki, Maruzen, Tokyo, 1994, p. 15), the molar conductivity of H ⁺ ions and OH ^- ions is 349.81 S‧ cm ^{2 /} mol, 198.3, respectively. S‧cm ² /mol. From the above, it is expected that the conductivity of theoretical pure water is 5.4 μS/m. Therefore, the conductivity of the washing water cannot be set to be lower than 5.4 μS/m. In order to maintain a low electrical conductivity of, for example, less than 10 μS/m, it is necessary to use not only ultrapure water but also carbon dioxide which is dissolved in water in the atmosphere to generate carbonate ions, thereby increasing the conductivity. Therefore, it is not economical to manage ambient air. Therefore, setting the conductivity of the washing water to less than 10 μS/m makes the cost unnecessarily excessive, which is not preferable.

When the water washing time exceeds 15 seconds, the Fe oxide film tends to grow on the surface of the steel sheet during water washing, so that excellent chemical conversion treatability cannot be obtained. Therefore, the water washing time should be 15 seconds or less, preferably 5 seconds or less. When the washing time is less than 1 second, the acid cannot be removed by washing with water, and the acid remaining in the steel sheet causes Fe ²⁺ ions to be eluted from the steel sheet, and the Fe ²⁺ ions react with the surrounding oxygen to form a thick Fe oxide film. Therefore, it causes deterioration of chemical conversion treatability and causes yellowing of the appearance of the product. Therefore, it is advisable to make the washing time longer than 1 second.

Si forms Si oxide on the surface of the steel sheet during cold-rolled sheet annealing, which deteriorates chemical conversion treatability. Even if the Si oxide can be removed by pickling, Si which is solid-melted in the steel sheet deteriorates the chemical conversion treatability. The chemical conversion treatability is related to the Si content in the steel sheet. The more the Si content in the steel sheet, the more easily the chemical conversion treatability is deteriorated. Therefore, it is preferable to control the conductivity of the washing water to be low and to control the water washing time to be short in accordance with the Si content in the steel sheet.

Table 1 shows the relationship between the Si content in the steel sheet, the conductivity of the washing water, and the washing time. When the Si content in the steel sheet is 0.4% or more and less than 1.25%, the conductivity of the washing water is preferably 5.0 mS/m or less, and the washing time is preferably 15 seconds or less. When the Si content in the steel sheet is 1.25% or more and less than 2.5%, the conductivity of the washing water is preferably 3.0 mS/m or less, and the washing time is preferably 9 seconds or less. When the Si content in the steel sheet is 2.5% or more and 3.0% or less, the conductivity of the washing water is preferably 1.0 mS/m or less, and the water washing time is preferably 3 seconds or shorter. By controlling the conductivity of the washing water and the washing time as described above, the chemical conversion treatability can be sufficiently ensured.

[Table 1]

The washing water used for washing contains Na ⁺ , Mg ²⁺ , K ⁺ , and Ca ²⁺ from the rock components of the water source, and may contain H ⁺ , Fe ²⁺ , Fe mixed by pickling. ³⁺ , Cl ^- , NO ₃ ^- , SO ₄ ^2- . The conductivity of the washing water is related to the ion concentration of the plasma, and can be calculated by calculating the product of the ion concentration (mol/L) of each ion and the conductivity per 1 mole, and totaling the respective ions. That is, the H ⁺ concentration (mol/L) contained in the washing water is [H ⁺ ], the Na ⁺ concentration (mol/L) is [Na ⁺ ], and the Mg ²⁺ concentration (mol/L) is [Mg. ²⁺ ], K ⁺ concentration (mol/L) is [K ⁺ ], Ca ²⁺ concentration (mol/L) is [Ca ²⁺ ], Fe ²⁺ concentration (mol/L) is [Fe ²⁺ ], The Fe ³⁺ concentration (mol/L) is [Fe ³⁺ ], the Cl ⁻ concentration (mol/L) is [Cl ⁻ ], the NO ₃ ⁻ concentration (mol/L) is [NO ₃ ⁻ ], and SO ₄ ^{2 - When the} concentration (mol/L) is [SO ₄ ^2- ], the formula 1 is preferably satisfied. According to the literature (Introduction to Electrical Chemistry, Matsuda Koki, Iwakura Chiaki, Maruzen, Tokyo, 1994, p. 15), the conductivity of each ion species per 1 mol/L is H ⁺ : 349.81 (S ‧ cm ² / mol), Na ⁺ : 50.1 (S ‧ cm ² /mol), Mg ²⁺ : 53.05 × 2 (S ‧ cm ² /mol), K ⁺ : 73.5 (S ‧ cm ² / mol), Ca ²⁺ : 59.5 × 2 ( S‧cm ² /mol), Fe ²⁺ : 53.5×2 (S‧cm ² /mol), Fe ³⁺ : 68.4×3 (S‧cm ² /mol), Cl ⁻ : 76.35 (S‧cm ² / Mol), NO ₃ ^- : 71.46 (S‧ cm ² /mol), SO ₄ ^2- : 80.0 × 2 (S‧ cm ² /mol). Therefore, Equation 1 can be used to calculate the conductivity of the washing water. Furthermore, 1 (S‧ cm ² /mol) is converted to 100 (mS‧l/m‧mol). 349.81[H ⁺ ]+50.1[Na ⁺ ]+53.05×2[Mg ²⁺ ] +73.5[K ⁺ ]+595×2[Ca ²⁺ ]+53.5×2[Fe ²⁺ ] +68.4×3[Fe ³⁺ ]+76.35[Cl ^- ]+71.46[NO ₃ ^- ] +80.0×2[SO ₄ ^2- ] ≦ 5/100 (Formula 1)

The reason why the higher the conductivity of the washing water is, the easier it is to form an Fe oxide film on the surface of the steel sheet in the water washing is as follows. In the water washing, Fe derived from the steel sheet component is eluted into the Fe ²⁺ ion in the washing water by the following anode reaction. Fe→Fe ²⁺ +2e ^-

On the other hand, because of atmospheric oxygen dissolved in the cleaning water of the cathodic reaction is generated as follows, to generate OH ^- ions. 1/2O ₂ +H ₂ O+2e ^- →2OH ^-

Thereafter, Fe ²⁺ and 2OH ^- are combined in the washing water to precipitate as iron hydroxide (Fe(OH) ₂ ). And an oxide film of FeO is formed by the detachment of H ₂ O from the iron hydroxide. Fe ²⁺ +2OH ^- →Fe(OH) ₂ Fe(OH) ₂ →FeO+H ₂ O

In this series of reactions, when the conductivity of the washing water is low, the positive and negative charges become excessive in the vicinity of the Fe ²⁺ ions and the OH ^- ions generated in the washing water, thereby hindering the Fe ² or more of the predetermined amount or more. ⁺ ions and OH ^- ions are generated. On the other hand, when the conductivity of the washing water is high, since the washing water contains many kinds of cations/anions which become carriers, as long as Fe ²⁺ ions are generated, the surrounding anions are close, and conversely, as long as OH ^- ions are generated The surrounding cations will be close, so the neutral state will be maintained electrically, and the above-mentioned series of reactions will be promoted. From these circumstances, it is presumed that the longer the washing time is, the more the above-described series of reactions are promoted, so that the Fe oxide film is more likely to form on the surface of the steel sheet.

The water-washed steel sheet can also be rolled by, for example, a general rubber ring roll. It can sweep away the washing water attached to the surface of the washed steel sheet. By reducing the amount of washing water attached to the surface of the washed steel sheet, the energy and time required for subsequent drying can be reduced.

Next, the water-washed steel plate is dried. The drying method is not particularly limited, and it can be carried out, for example, by placing the water-washed steel sheet in the conveying direction and spraying the hot air on the conveyed steel sheet by the dryer. Further, the drying ability of the dryer (hair dryer) is not particularly limited, and the steel sheet can be sufficiently dried in consideration of the speed at which the steel sheet is conveyed.

Drying starts within 60 seconds from the end of the washing. When the time from the completion of the washing to the start of drying is more than 60 seconds, an Fe oxide film is formed on the surface of the steel sheet, the chemical conversion treatability is deteriorated, and the surface appearance of the steel sheet is deteriorated. It is assumed that even if the washing water used for washing is clean, when the washing water adheres to the surface of the steel sheet for a fixed period of time, there is still a fear that an Fe oxide film is formed on the surface of the steel sheet.

In the washing of the steel sheet, the anodic reaction produces ions Fe ²⁺ Fe eluted from the steel sheet composition in wash water, and the atmosphere of oxygen dissolved in the wash water to generate OH ^- ions in the cathode reaction. Since these reactions are also carried out during the period from the completion of the washing to the start of the drying, it is presumed that the amount of the Fe oxide film formed is increased.

In this way, the steel sheet of this embodiment can be manufactured. Further, the steel sheet may be taken up in a roll shape after drying. The rust preventive agent may also be applied to the steel sheet before being wound into a roll. The film formed on the surface of the steel sheet by the rust preventive agent protects the surface of the steel sheet by insulating the surrounding water and oxygen in the atmosphere, thereby suppressing the formation of the Fe oxide film. Therefore, the chemical conversion treatability of the steel sheet can be ensured, and the surface appearance of the steel sheet can be kept beautiful.

As described above, according to the method for producing a steel sheet according to the present embodiment, since good chemical conversion treatability can be obtained without performing Ni plating treatment, chemical conversion treatability and degreasing property can be achieved. Specifically, in the method for producing a steel sheet according to the present embodiment, by controlling the conductivity of the washing water, the washing time, and the time from the completion of the washing to the start of drying, the surface of the steel sheet may be prevented from being formed on the surface of the steel sheet after the washing and the completion of the washing. Formation and growth of Fe oxide film. Thereby, the chemical conversion treatability of the steel sheet can be stably ensured, and the Ni plating treatment for ensuring the chemical conversion treatability can be omitted. Further, in the method for producing a steel sheet according to the present embodiment, it is possible to suppress deterioration of mechanical properties due to unavoidable decarburization in the surface layer of the steel sheet by controlling the dew point during annealing of the cold-rolled sheet.

The steel sheets which can be produced according to the present embodiment are various, and high-strength steel sheets and non-high-strength Si-containing steel sheets can be produced, for example, according to the present embodiment.

When manufacturing a high-strength steel sheet, the molten steel has a chemical composition such as: C: 0.05% to 0.25%; Si: 0.4% to 3.0%; Mn: 0.5% to 4.0%; Al: 0.005% to 0.1%; P: 0.03% or less; S: 0.02% or less; Ni, Cu, Cr or Mo: 0.0% to 1.0%; and the total content of Ni, Cu, Cr and Mo: a total of 0.0% to 3.5%; B: 0.0000 %~0.005%; Ti, Nb or V: 0.000%~0.1%; and the total content of Ti, Nb and V: a total of 0.0%~0.20%; and the remainder: Fe and impurities. The impurities may be, for example, those contained in raw materials such as ore or scrap, and those included in the production steps.

(C: 0.05% to 0.25%) C will strengthen the strength of the steel sheet due to the formation of the iron phase of the Ma Tian during the rapid cooling. If the C content is less than 0.05%, the granitic iron phase may not be sufficiently formed under general annealing conditions, and it may be difficult to ensure strength. Therefore, the C content is preferably set to 0.05% or more. If the C content exceeds 0.25%, there is a case where sufficient spot weldability cannot be ensured. Therefore, the C content is preferably set to 0.25% or less.

(Si: 0.4% to 3.0%) Si suppresses the ductility deterioration of the steel sheet and enhances the strength. In order to sufficiently obtain this effect, the Si content is set to 0.4% or more. If the Si content exceeds 3.0%, there is a case where the workability of the cold rolling delay is lowered. Therefore, the Si content is set to be 3.0% or less.

(Mn: 0.5% to 4.0%) Mn improves the hardenability of steel and ensures strength. In order to sufficiently obtain the effect, the Mn content is preferably set to 0.5% or more. If the Mn content exceeds 4.0%, the workability of the hot rolling delay may be deteriorated, which may cause damage to the steel in continuous casting and hot rolling. Therefore, the Mn content is preferably set to 4.0% or less.

(Al: 0.005% to 0.1%) Al is a deoxidizing element of steel. Further, Al forms AlN to suppress the grain refinement of the crystal grains, and suppresses the coarsening of the crystal grains caused by the heat treatment, thereby ensuring the strength of the steel sheet. If the Al content is less than 0.005%, it is difficult to obtain this effect. Therefore, the Al content is preferably set to 0.005% or more. If the Al content exceeds 0.1%, the weldability of the steel sheet may deteriorate. Therefore, the Al content is preferably set to 0.1% or less. In order to make the surface defects of the steel sheet caused by the alumina cluster difficult to occur, it is preferable to set the Al content to 0.08% or less.

(P: 0.03% or less) P increases the strength of steel. Therefore, it is also possible to contain P. Since the refining cost becomes large, the P content is preferably 0.001% or more, and more preferably 0.005% or more. If the P content exceeds 0.03%, there is a case where the workability is lowered. Therefore, the P content is preferably set to 0.03% or less, and more preferably 0.02% or less.

(S: 0.02% or less) S is contained in steel as an impurity in a general steelmaking method. When the S content is more than 0.02%, the workability of the steel during the hot rolling delay is deteriorated, and the coarse MnS which is the starting point of the fracture at the time of bending or hole expanding is formed, and the workability may be deteriorated. Therefore, the S content is preferably set to 0.02% or less. If the S content is less than 0.0001%, the cost becomes large, so the S content should be set to 0.0001% or more. In order to make the surface defects of the steel sheet difficult to produce, the S content is preferably 0.001% or more.

Ni, Cu, Cr, Mo, B, Ti, Nb, and V are not essential elements, and any element which is appropriately contained in the steel sheet may be a predetermined amount.

(Ni, Cu, Cr or Mo: 0.0% to 1.0%, and the total content of Ni, Cu, Cr and Mo: 0.0% to 3.5% in total) Ni, Cu, Cr and Mo retard the formation of carbides, And contributed to the residue of Worth Iron. In addition, it will also reduce the metamorphic start temperature of the iron field in the Vostian Iron. Therefore, workability and fatigue strength can be improved. Therefore, Ni, Cu, Cr or Mo may also be contained. In order to sufficiently obtain this effect, the content of Ni, Cu, Cr or Mo is preferably set to 0.05% or more. If the content of Ni, Cu, Cr or Mo exceeds 1.0%, the effect of improving the strength is saturated, and the ductility is remarkably deteriorated. Therefore, the content of Ni, Cu, Cr or Mo is preferably set to 1.0% or less. Further, if the total content of Ni, Cu, Cr, and Mo exceeds 3.5%, the hardenability of the steel is increased to more than necessary, and it is difficult to manufacture a steel sheet having good workability mainly composed of ferrite iron, and the cost is increased. Therefore, the total content of Ni, Cu, Cr, and Mo is preferably 3.5% or less in total.

(B: 0.0000%~0.005%) B will improve the hardenability of steel. Further, when reheating for the alloying treatment, the ferrite is deformed and the toughened iron is delayed. Therefore, it is also possible to contain B. In order to sufficiently obtain this effect, the B content is preferably set to 0.0001% or more. If the B content exceeds 0.005%, when it is cooled from the temperature zone in which the two phases of the ferrite iron and the Vostian iron coexist, it will not have a sufficient area ratio of the ferrite iron growth, and it is difficult to manufacture the ferrite iron. A steel sheet with good workability of the main body. Therefore, the B content is preferably set to 0.005% or less, and more preferably 0.002% or less.

(Ti, Nb or V: 0.000%~0.1%, and the total content of Ti, Nb and V: a total of 0.0% to 0.20%) Ti, Nb and V will form carbides, nitrides (or carbonitrides) And strengthen the ferrite grain iron phase, so the steel plate can be strengthened. Therefore, Ti, Nb or V may also be contained. In order to sufficiently obtain this effect, the Ti, Nb or V content is preferably made 0.001% or more. If the Ti, Nb or V content exceeds 0.1%, not only will the cost increase, but the effect of increasing the strength will also be saturated, and even worse, C will be wasted unnecessarily. Therefore, the Ti, Nb or V content is preferably set to 0.1% or less. Further, if the total content of Ti, Nb, and V exceeds 0.20%, not only the cost will increase, but also the effect of increasing the strength will be saturated, and C will be wasted unnecessarily. Therefore, the total content of Ti, Nb and V should be set to 0.20% or less.

When manufacturing a non-high-strength Si-containing steel sheet, the molten steel has a chemical composition such as C: 0.15% or less, Si: 0.4% to 1.0%, Mn: 0.6% or less, and Al: 1.0% or less, P: 0.100% or less, S: 0.035% or less, and the remainder: Fe and impurities. The impurities may be, for example, those contained in raw materials such as ore or scrap, and those included in the production steps.

(C: 0.15% or less) C is a residue which is contained in steel by reduction of iron ore by coke in steel making, and cannot be removed by primary refining of steel, but the strength of the steel sheet can be ensured. The J content should be set to 0.15% or less with reference to JIS G 3141.

(Si: 0.4% to 1.0%) Si suppresses the ductility deterioration of the steel sheet and increases the strength. Further, Si combines with oxygen in the steel during refining of steel, and also suppresses generation of bubbles when the steel block is solidified. In order to sufficiently obtain this effect, the Si content is set to 0.4% or more. The upper limit of the Si content should be set to 1.0% or less.

(Mn: 0.6% or less) Although Mn is contained in the refining of steel in order to remove S, Mn also ensures the strength of the steel sheet. Referring to JIS G 3141, the Mn content is preferably set to 0.6% or less.

(Al: 1.0% or less) Al is a deoxidizing element of steel. Further, Al forms AlN to suppress the grain refinement of the crystal grains, and suppresses the coarsening of the crystal grains caused by the heat treatment, thereby ensuring the strength of the steel sheet. The upper limit of the Al content is preferably set to 1.0% or less.

(P: 0.100% or less) P is derived from iron ore and is a residue which cannot be removed by one-time refining of steel, but the strength of the steel sheet can be improved. Referring to JIS G 3141, the P content is preferably set to 0.100% or less.

(S: 0.035% or less) S is contained in steel as an impurity in the general steel making method. Referring to JIS G 3141, the S content is preferably set to 0.035% or less.

Further, the non-high-strength Si-containing steel sheet may contain alloying elements other than the above elements, as needed.

The above is a detailed description of the preferred embodiments of the invention, but the invention is not limited by the examples. It is obvious that any person skilled in the art to which the present invention pertains can recognize various modifications or alterations within the scope of the technical idea described in the claims, and know that such techniques are also vested in the present invention. range.

[Embodiment] Next, an embodiment of the present invention will be described. The conditions in the examples are a conditional example employed to confirm the workability and effects of the present invention, and the present invention is not limited to this condition example. The present invention can adopt various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

(Example 1) Steel grades A to steel types E shown in Table 2 were cast to prepare steel slabs, and each steel slab was hot rolled by a conventional method to obtain a hot rolled steel sheet. The obtained hot-rolled steel sheet was pickled, and then cold rolled and rolled to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheet was cut into 100 mm × 50 mm. The bottom line in Table 2 indicates that the value is outside the scope of the present invention.

[Table 2]

Next, cold-rolled sheet annealing, pickling, water washing, and drying were sequentially performed on the obtained cold-rolled steel sheets in the conditions shown in Tables 3 to 11. Cold rolled sheet annealing was performed using a continuous annealing simulation apparatus and the annealing temperature was set to 800 °C. The bottom line in Tables 3 to 11 indicates that the value is outside the scope of the present invention.

[table 3]

[Table 4]

[table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

Further, after the end of the cold-rolled sheet annealing, the presence or absence of the decarburization layer in the surface layer of the steel sheet was evaluated. For the obtained sample, a small piece was taken from the center portion in the longitudinal direction and the vicinity of the center portion in the width direction, and the resin was embedded in the cross section, and then mechanical polishing and polished mirror polishing were performed. Then, the hardness of the outermost layer of the sample was measured at a thickness of 10 μm in the thickness direction by a micro Vickers hardness tester at a load of 0.01 kgf to obtain a hardness distribution. Further, the hardness of the central portion of the small piece taken in the thickness direction was measured and compared with the hardness distribution of the outermost layer. When the dimension in the thickness direction in the region which is softer than 90% of the hardness of the central portion is 20 μm or less, the thickness of the decarburized layer is within the allowable range and is rated as "Excellent (E)", and if it is 30 μm or more, it is evaluated. It is "Worse(W)". The results are shown in Tables 3 to 11.

The washing water used for washing is a pure water producing device for producing pure water, and a predetermined amount of potassium chloride is added to pure water as needed to adjust the conductivity. At this time, the conductivity was measured by a hand-held conductivity meter ES-51 manufactured by Horiba. The K ⁺ ion concentration and the Cl ⁻ ion concentration in the washing water are rated as “Excellent (E)” if the formula 1 is satisfied, and “Worse (W)” if the formula 1 is not satisfied. Further, when the dissolved oxygen amount of pure water was measured by the diaphragm electrode method, it was 2.4 mg/L. Table 12 shows the composition of the washing water, the measured value of the conductivity, and the calculated value of the conductivity determined by the formula (1).

[Table 12]

The water washing is performed by immediately pulling the predetermined washing water at a predetermined flow rate for a predetermined period of time at the center of each sample after the respective samples are pulled up by the pickling bath. At this time, the supply amount of the washing water was fixed to 7 L/min using Toyo Pump TP-G2 manufactured by MIYAKE KAGAKU Co., LTD. Further, since the test piece was 100 mm × 50 mm and the pumping water amount was 7 L/min, the water amount density was calculated to be 23 L/(second ‧ m ² ). Drying is performed by blowing hot air to each sample by a hair dryer.

The thickness of the oxide film was measured by a glow discharge spectrometer (GDS) with respect to the obtained sample. GDS is a GDA750 manufactured by Rigaku Corporation. The measurement of the thickness of the oxide film was carried out by confirming the concentration distribution of each element in the depth direction by the GDS on the surface layer of the sample, and confirming the depth at which the oxygen concentration became one-half of the maximum value. The dimension from the depth position to the surface layer is the thickness of the oxide film. The results are shown in Tables 3 to 11.

The chemical conversion treatability was evaluated for the obtained sample. A phosphate chemical conversion treatment film is formed on the surface of the obtained sample. Phosphate chemical conversion treatment is followed by degreasing, water washing, surface conditioning, chemical conversion treatment, re-washing, and drying. The degreasing was carried out by spraying the obtained sample with a degreaser FC-E2001 manufactured by Paka Kasuga Co., Ltd. for 2 minutes at a temperature of 40 °C. Water washing was performed by spraying room temperature tap water on the obtained sample for 30 seconds. The surface adjustment was carried out by immersing the obtained sample in a bath of the surface conditioner PL-X manufactured by Paccarat Co., Ltd., Japan for 30 seconds at room temperature. The chemical conversion treatment was carried out by immersing the obtained sample in a 35° C. bath in a chemical conversion treatment agent PB-SX manufactured by Paccarat Co., Ltd., Japan for 2 minutes. The re-washing was carried out by spraying tap water on the obtained sample for 30 seconds, followed by spraying 30 seconds of pure water. Drying is carried out by drying the obtained sample in a hot air oven. With respect to the sample in which the phosphate chemical conversion treatment film was formed as described above, the chemical conversion treatability was evaluated by the following procedure. Chemical conversion crystallization of the surface of each sample was taken by a scanning electron microscope (SEM). As long as the chemical conversion crystal is finely formed and the long side of the crystal is 2 μm or more and 4 μm or less, it is evaluated as "Excellent (E)". When the chemical conversion crystal is finely formed and the long side of the crystal is more than 4 μm and is 8 μm or less, it is evaluated as "Medium (M)". On the other hand, if the chemical conversion crystal is not finely formed, and the sample itself is observed to be exposed, or even if the chemical conversion crystal is finely formed, if the long side of the crystal exceeds 8 μm, it is evaluated as "Worse (W)". The results are shown in Tables 3 to 11.

The degreasing property was evaluated for the obtained sample. After the above degreasing, water was allowed to adhere to the sample and visually observed. If the sample will dial water, it will be "Worse(W)", and if it will not dial water, it will be rated as "Excellent(E)". The results are shown in Tables 3 to 11.

As shown in Tables 3 to 11, sample No. 4, sample No. 5, sample No. 7 to sample No. 9, sample No. 17, sample No. 23, and sample No. 25. Sample No. 26, sample No. 29, sample No. 31, sample No. 32, sample No. 36 to sample No. 39, sample No. 42 to sample No. 44, Sample No. 48 to sample No. 52, sample No. 57 to sample No. 60, sample No. 63 to sample No. 65, sample No. 69 to sample No. 73, and sample No.78~sample No.81, sample No.84~sample No.86, sample No.90~sample No.94, sample No.99~sample No.102, sample No. 105~sample No.107, sample No.111~sample No.115, sample No.120~sample No.123, sample No.126~sample No.128, sample No.132~ Sample No. 136, Sample No. 141, Sample No. 142, Sample No. 144 to Sample No. 147, Sample No. 150 to Sample No. 152, Sample No. 156 to Sample No. 160, sample No. 165, sample No. 166, sample No. 168 to sample No. 171, sample No. 174 to sample No. 176, sample No. 180 to sample No. 184, sample No. 189, sample No. 190, sample No. 192 to sample No. 195, sample No. 198 to sample No. 200, sample No. 204 to sample No. 208, Sample No. 213, sample No. 214, sample No. 216 to sample No. 219, sample No. 222 to sample No. 224 Sample No. 228 to Sample No. 232, Sample No. 237, Sample No. 238, Sample No. 240 to Sample No. 243, Sample No. 246 to Sample No. 248, and sample No. 252 to sample No. 256, sample No. 261, sample No. 262, sample No. 264 to sample No. 267, sample No. 270 to sample No. 272, sample No. 276~ sample No. 280, sample No. 285, sample No. 286, sample No. 288 to sample No. 291, sample No. 294 to sample No. 296, sample No. 300~ Sample No. 304, sample No. 309, sample No. 310, sample No. 312 to sample No. 315, sample No. 318 to sample No. 320, sample No. 324 to sample No. 328, sample No. 333, sample No. 334, sample No. 336 to sample No. 339, sample No. 342 to sample No. 344, sample No. 348 to sample No. 352, sample No. 357, sample No. 358, sample No. 360 to sample No. 363, sample No. 366 to sample No. 368, sample No. 372 to sample No. 376, Sample No. 381, sample No. 382, sample No. 384 to sample No. 387, sample No. 390 to sample No. 392, sample No. 396 to sample No. 400, and sample No. 405, sample No. 406, sample No. 408 to sample No. 411, sample No. 414 to sample No. 416, and sample No. 420 to sample No. 424, due to dew point , the conductivity of the washing water, the washing time Washed with water until the start to the end of the drying time and the chemical composition within the scope of the present invention, it is obtained a good chemical conversion treatability and degreasing properties. Sample No. 35, sample No. 56, sample No. 77, sample No. 98, sample No. 119, sample No. 140, sample No. 164, sample No. 188, test Sample No. 212, sample No. 236, sample No. 260, sample No. 284, sample No. 308, sample No. 332, sample No. 356, sample No. 380, and sample In No. 404, since it was dried without washing with water after pickling, thick rust was formed on the surface, and the thickness of the oxide film could not be measured.

(Test Example 1) The conductivity of the washing water disclosed in Patent Document 4 was determined, and the conductivity of the washing water used in the present invention was compared. The cleanest washing water disclosed in Patent Document 4, that is, the washing water of Experiment No. 1 was reproduced. The ion concentration was Fe ²⁺ : 3.2 g / L, NO ₃ ^- : 1.1 g / L, and Cl ^- : 2.3 g / L. First, a liquid in which 0.032 mol/L of FeCl ₂ and 0.009 mol/L of Fe(NO ₃ ) ₂ were dissolved in pure water was prepared. The conductivity of the obtained washing water was measured using a hand-held conductivity meter ES-51 manufactured by Horiba. And the results are shown in Table 13. Further, the ion concentration and conductivity of the washing water used in the above Example 1 were recorded together in Table 13.

[Table 13]

As shown in Table 13, the conductivity of the cleanest washing water disclosed in Patent Document 4 has been confirmed to be outside the scope of the present invention.

no

Claims (3)

A method for producing a steel sheet, comprising the steps of: obtaining a steel blank by continuous casting of a molten steel having a Si content of 0.4% by mass to 3.0% by mass; performing hot rolling of the steel preform to obtain a hot rolled steel sheet a step of performing cold rolling of the hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of performing cold-rolled sheet annealing of the cold-rolled steel sheet; a step of pickling after the cold-rolled sheet is annealed; After the washing, the step of washing with water; and the step of drying after the washing with water, the dew point is set to -35 ° C or less in the cold-rolled sheet annealing, and the conductivity of the washing water used for the washing is set to 5.0 mS. In the above-described water washing, the water washing time is set to be within 15 seconds, and the drying is started within 60 seconds from the end of the water washing. The method for producing a steel sheet according to claim 1, wherein the molten steel has a Mn content of 0.5% by mass to 4.0% by mass. The method for producing a steel sheet according to claim 1 or 2, wherein the H ⁺ concentration (mol/L) contained in the washing water is [H ⁺ ], and the Na ⁺ concentration (mol/L) is [Na ⁺ ], Mg ^{2 +} concentration (mol/L) is [Mg ²⁺ ], K ⁺ concentration (mol/L) is [K ⁺ ], Ca ²⁺ concentration (mol/L) is [Ca ²⁺ ], Fe ²⁺ concentration (mol /L) is [Fe ²⁺ ], Fe ³⁺ concentration (mol/L) is [Fe ³⁺ ], Cl ^- concentration (mol/L) is [Cl ^- ], and NO ₃ ^- concentration (mol/L) is [NO ₃ ^- ], and when the SO ₄ ^2- concentration (mol/L) is [SO ₄ ^2- ], the formula 1: 349.81 [H ⁺ ] ⁺ 50.1 [Na ⁺ ] + 53.05 × 2 [Mg ²⁺ ] is satisfied. +73.5[K ⁺ ]+595×2[Ca ²⁺ ]+53.5×2[Fe ²⁺ ] +68.4×3[Fe ³⁺ ]+76.35[Cl ^- ]+71.46[NO ₃ ^- ] +80.0×2 [SO ₄ ^2- ]≦5/100 (Formula 1).