Method for manufacturing a steel plate

Abstract

To provide a production method of a steel sheet that has high Si content, suppressed unevenness in alloying, and good pickling properties without including an actual pickling evaluation process.SOLUTION: A production method of a steel sheet includes the step of annealing a steel material with a Si content of 1.0 mass% or more and a Cr content of 1 mass% or less under conditions that satisfy the relation between a soaking temperature T (°C) during annealing, which is 500°C or higher, a soaking time t (seconds) during annealing and a Cr content Cr[%] (mass%) of the steel material, depending on the Cr content of the steel material. For example, the method includes a step of annealing under conditions satisfying the following formula 1A when the Cr content of the steel material is 0.2 mass% or more and 0.6 mass% or less, and satisfying the following formula 1B when the Cr content of the steel material is less than 0.2 mass%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a steel sheet, which is suitably used as a base material for a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a high Si content, high strength, and high workability.

Background Art

[0002] In the automotive industry, from the viewpoints of improving fuel efficiency for CO2 reduction and improving collision safety performance, weight reduction and high strength of automotive members such as automobile bodies are required. Therefore, ultra-high strength steel sheets with a tensile strength of 980 MPa or more are applied to automotive members such as automobile bodies. In order to improve the workability of such high-strength steel sheets, a method of containing inexpensive Si in the chemical composition of the steel sheet is known. By containing Si in the chemical composition of the steel sheet, not only the strength of the steel sheet but also the workability can be improved.

[0003]

[0004] Generally, when applying Si-added steel to automotive members, hot-dip galvanized steel sheets (GI steel sheets) and alloyed hot-dip galvanized steel sheets (GA steel sheets) obtained by alloying the hot-dip galvanized steel sheets are used from the viewpoints of ensuring corrosion resistance and weldability. However, in the production process of hot-dip galvanized steel sheets with Si added to the steel sheet, since the Si oxide layer covers the surface of the steel sheet, finally, problems such as non-galvanization, reduction of galvanization adhesion, and uneven alloying in the alloying treatment are likely to occur. Furthermore, problems such as peeling of the plating during processing of the alloyed hot-dip galvanized steel sheet may also occur. In order to suppress such problems caused by Si addition, hot-dip galvanized steel sheets containing Si in the steel material are often manufactured using an oxidation-reduction method using an annealing furnace having an oxidation heating zone and a reduction heating zone. According to the oxidation-reduction method, since the iron oxide generated in the oxidation heating zone is formed into a reduced Fe layer during reduction annealing, the plating wettability during plating can be improved. Furthermore, a method of forming an internal oxide layer containing SiO2 or the like necessary for plating in advance on the steel sheet by increasing the coiling temperature in hot rolling is also used.In recent years, various developments have been made to further improve the strength and workability of hot-dip galvanized steel sheets, such as increasing the Si content of the steel sheet to 1% by mass or more and methods for forming a good internal oxide layer.

[0005] Specifically, for example, Patent Document 1 discloses a high-strength alloyed hot-dip galvanized steel sheet with good appearance, which contains, by mass, C: 0.05 - 0.25%, Si: 0.3 - 2.5%, Mn: 1.5 - 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 - 0.5%, N: 0.0060% or less, with the balance being Fe and inevitable impurities. On this high-strength steel sheet, there is an alloyed hot-dip galvanized layer containing Fe, with the balance being Zn and inevitable impurities. Oxides containing Si are present at an average content rate of 0.6 - 10% by mass in the grain boundaries and within the grains on the steel sheet side within 5 μm from the interface between the high-strength steel sheet and the plating layer, and oxides containing Si are present at an average content rate of 0.05 - 1.5% by mass in the plating layer.

[0006] Also, for example, Patent Document 2 discloses a method for manufacturing a high-strength hot-dip galvanized steel sheet excellent in plating adhesion, workability, and appearance. After hot-rolling a slab containing, by mass, C: 0.05 - 0.30%, Si: 0.1 - 2.0%, and Mn: 1.0 - 4.0%, it is coiled at a specific temperature T C and pickled in a hot-rolling process, cold-rolled in a cold-rolling process on the hot-rolled sheet obtained in the hot-rolling process, annealed under specific conditions in an annealing process on the cold-rolled sheet obtained in the cold-rolling process, and then subjected to a hot-dip galvanizing process in a hot-dip galvanizing bath containing 0.12 - 0.22% by mass of Al on the annealed sheet after the annealing process. A method for manufacturing a high-strength hot-dip galvanized steel sheet having these steps is described.

[0007] Furthermore, for example, Patent Document 3 describes a cold-rolled steel sheet obtained by subjecting a steel slab material to heat treatment in a substantially non-reducing atmosphere at a temperature range of 650 to 950°C with black scale attached after hot rolling, forming an internal oxidation layer in the base metal surface layer portion of the steel sheet, and then performing pickling, cold rolling, and recrystallization annealing according to a conventional method.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] However, in order to obtain a high-strength and highly workable hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, when the Si content is increased to 1 mass% or more, it is difficult to obtain an alloyed hot-dip galvanized steel sheet with uniform alloying over the entire coil by simply applying a conventional manufacturing method. In particular, compared with the vicinity of the center in the coil width direction of the steel sheet (hereinafter, also simply referred to as the "width direction center"), it is difficult to uniformly alloy the zinc plating in the vicinity of the edge in the coil width direction of the steel sheet (hereinafter, also simply referred to as the "width direction edge").

[0010] Specifically, when using high-Si added steel, when the coil is cooled after hot rolling, the cooling of the coil is steep near the edge in the width direction of the steel plate. Therefore, near the edge in the width direction of the steel plate, it is difficult for the internal oxide layer to grow, and the layer is formed thinly. On the other hand, near the center in the width direction of the steel plate, the internal oxide layer grows sufficiently, and the layer is formed thickly. Furthermore, in the subsequent pickling process, the internal oxide layer near the edge in the width direction is preferentially dissolved. Thus, due to the difference in the thickness of the internal oxide layer in the coil width direction, alloying unevenness occurs.

[0011] Such problems cannot be solved even by using the techniques described in the above-mentioned patent documents. For example, in the manufacturing method of the steel plate described in Patent Document 1, since the rapid cooling of the coil near the edge in the width direction is not considered, it is impossible to leave the internal oxide layer near the edge in the width direction. Also, regarding the manufacturing method described in Patent Document 2, since it is necessary to lower the coiling temperature as the contents of Si and Mn increase, it is difficult to generate a predetermined amount of oxide near the edge in the width direction. As a result, even by using the techniques disclosed in Patent Document 1 and Patent Document 2, it is difficult to manufacture an alloying hot-dip galvanized steel plate without alloying unevenness uniformly in the coil width direction of the steel plate.

[0012] On the one hand, as described in Patent Document 3, according to the method of heat-treating the steel plate after hot rolling again, more internal oxide layers can be formed. However, in addition to the heating during hot rolling, by subjecting the steel plate to heat treatment again, the oxide scale formed on the surface of the steel plate further increases. As a result, there may arise a problem of poor pickling property that even if pickling is performed later, the oxide scale cannot be sufficiently removed and remains. This is because the oxide scale on the surface of the steel plate is partially reduced to become reduced iron. For example, according to the manufacturing method described in Patent Document 3, since the heat treatment temperature is high, the surface of the steel plate is covered with reduced iron, and the scale cannot be removed by pickling. As a result, contamination of the steel plate and decarburization near the surface of the steel plate progress, and it becomes difficult to obtain a steel plate having a predetermined strength, for example, a tensile strength of 980 MPa. The principle of the generation of reduced iron is described in detail, for example, in Japanese Patent Application Laid-Open No. 2017-222887. Reduced iron tends to be formed more in the vicinity of the widthwise edge of the steel plate than in the vicinity of the center in the width direction of the steel plate due to the influence of the furnace atmosphere and the like. Furthermore, such reduced iron is formed more as the amount of Si added to the steel increases.

[0013] In addition, currently, regarding the pickling property evaluation of such oxide scale on the steel plate surface, after actually performing pickling, if the scale is removed, the pickling property is considered good, and if the scale cannot be removed, it is evaluated as poor pickling property. In other words, there is no quantitative pickling property evaluation index for the cases where the pickling property is good and the cases where the pickling property is poor.

[0014] Therefore, in order to efficiently manufacture a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet with high Si content, high strength, and high workability, a manufacturing method of a steel plate that simultaneously solves the problem of alloying unevenness and the problem regarding the index of pickling property evaluation due to the generation of reduced iron is required.

[0015] Therefore, an object of the present invention is to provide a manufacturing method of a steel plate that contains a high amount of Si, can suppress alloying unevenness, and has good pickling property even without actually including a pickling property evaluation step.

Means for Solving the Problem

[0016] As a result of intensive studies to solve the above problems, the present inventors have reached the present invention. That is, the present invention includes the following preferred embodiments.

[0017] The method for manufacturing a steel sheet according to the first aspect of the present invention is to use a steel material having a Si content of 1.0 mass% or more, the following formula 1,

Number

Number

[0018] Alternatively, the method for manufacturing a steel sheet according to another first aspect of the present invention is to use a steel material having a Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less, when the Cr content of the steel material is 0.2 mass% or more and 0.6 mass% or less, the following formula 1A,

Number

Number

Number

[0019] In the method for manufacturing a steel sheet according to the above-described another first aspect, a steel material having a Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less is when the Cr content of the steel material is 0.6 mass% or less, the following Formula 1A, [Number] or when the Cr content of the steel material is more than 0.6 mass% and 1.0 mass% or less, the following Formula 1C, [Number] (In Formula 1A and Formula 1C, T is the soaking temperature (°C) during annealing at 500 °C or higher, t is the soaking time (seconds) during annealing, and Cr[%] is the Cr content (mass%) of the steel material) preferably includes a step of annealing under conditions that satisfy

[0020] In the method for manufacturing a steel sheet described above, it is preferable to further include a step of hot-rolling the steel material and winding it at 500 °C to 700 °C before the annealing.

[0021] In the method for manufacturing a steel sheet described above, it is more preferable to further include a step of pickling the steel sheet after the annealing and then cold-rolling it. [Advantages of the Invention]

[0022] According to the present invention, it is possible to provide a method for manufacturing a steel sheet that has a high Si content, can suppress alloying unevenness, and has good pickling properties even without actually including a step of evaluating pickling properties. [Brief Description of the Drawings]

[0023]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0024] The inventors have conducted various studies on a method for manufacturing a steel sheet that can suppress alloying unevenness even when the Si content is high and has good pickling properties without actually including the pickling property evaluation process. Then, in the annealing process of the steel sheet manufacturing method, it has been found that by satisfying a predetermined relational expression among the soaking holding temperature T, the soaking holding time t, and the H2 concentration P(H2) in the surrounding gas atmosphere, the problems of alloying unevenness and pickling properties can be solved.

[0025] Furthermore, from another perspective, in the annealing process of the steel sheet manufacturing method, it has been found that by satisfying a predetermined relational expression among the soaking holding temperature T, the soaking holding time t, and the Cr content according to the Cr content contained in the steel material, the problems of alloying unevenness and pickling properties can be solved.

[0026] Hereinafter, embodiments of the present invention will be described in detail by taking the first embodiment and the second embodiment as examples. Note that the scope of the present invention is not limited to the embodiments described here, and various modifications can be made without departing from the spirit of the present invention.

[0027] In this specification, the "internal oxide layer" means an internal oxide layer containing SiO2 (including both the oxidized portions of grain boundary oxidation and intragranular oxidation) that can be formed inside the steel sheet during heating for hot rolling and annealing. Further, the internal oxide layer exists between the surface layer of the steel sheet and the steel substrate portion inside the steel sheet that does not contain oxides such as SiO2 in the steel sheet manufactured by the method according to the embodiment of the present invention. Also, as will be described in detail in later examples, the amount of the internal oxide layer can be measured as the dissolution amount per unit area (g / m 2 ) by immersing and dissolving it in an acidic solution such as hydrochloric acid.

[0028] In this specification, the "(coil width direction edge of the steel sheet)" basically refers to both edges in the coil width direction, that is, both ends in the sheet width direction, unless a specific position is indicated. Also, in this specification, the "(vicinity of the coil width direction edge of the steel sheet)" means the peripheral portion of the position of the coil width direction edge. When indicating a specific position from the coil width direction edge, the distance from the said width direction edge (in other words, the position of 0 mm in the width direction) is also noted.

[0029] 1. Method for manufacturing a steel sheet In the method for manufacturing a steel sheet according to the first embodiment of the present invention, there is no particular limitation as long as a steel material (steel or steel sheet) having a Si content of 1.0 mass% or more is used and an annealing step is included under conditions satisfying a predetermined relational expression including a relational expression of H2 concentration as described later.

[0030] In the method for manufacturing a steel sheet according to the second embodiment of the present invention, there is no particular limitation as long as a steel material (steel or steel sheet) having a Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less is used and an annealing step is included under conditions satisfying a predetermined relational expression according to the Cr content as described later.

[0031] In the first and second embodiments of the present invention, any of the steps described below may be included.

[0032] Hereinafter, an example of a method for manufacturing a steel sheet in the first and second embodiments will be described.

[0033] (Preparation of Steel Material for Rolling) First, a steel material such as a slab for rolling having a chemical composition with a Si content of 1.0 mass% or more is produced. In the second embodiment including an annealing process under conditions according to the Cr content, a steel material such as a slab for rolling having a chemical composition with a Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less is produced. The details of the chemical composition of the steel material will be described later. The steel material such as a slab can be prepared by any known method. Examples of the method for producing a slab include a method of melting steel having the chemical composition described later and producing a slab by ingot casting or continuous casting. If necessary, the casting obtained by ingot casting or continuous casting may be subjected to block rolling to obtain a slab.

[0034] (Hot Rolling) Next, hot rolling is performed using the obtained steel material such as a slab to obtain a hot-rolled steel sheet.

[0035] The hot rolling may be performed by a method under any known conditions. The coiling temperature is preferably set to 500°C to 700°C. By setting the coiling temperature to 500°C or higher, the internal oxidation layer can be sufficiently grown, and after passing through subsequent processes, it becomes easier to secure the internal oxidation layer in the vicinity of the width direction edge. The coiling temperature is more preferably 520°C or higher, and even more preferably 530°C or higher. By setting the coiling temperature to 700°C or lower, the amount of reduced iron generated during cooling after hot rolling can be more reliably reduced, and a steel sheet having better pickling properties can be obtained. The coiling temperature is more preferably 680°C or lower, and even more preferably 660°C or lower.

[0036] Regarding other conditions during hot rolling, there are no particular limitations. For example, in hot rolling, the slab before hot rolling is soaked and held at a temperature of 1000°C to 1300°C or lower according to a conventional method, the finish rolling temperature is set to 800°C or higher, and then it may be wound up as a coiled steel plate. Furthermore, the coiled hot-rolled steel plate after hot rolling may be naturally cooled to room temperature.

[0037] (Annealing) Furthermore, the wound-up steel plate is annealed under the conditions of the first embodiment or the second embodiment described below.

[0038] In the first embodiment, the wound-up steel plate is annealed so as to satisfy the following relational expressions. Specifically, the steel plate is annealed under the conditions satisfying the following Formula 1, [Number] and the following Formula 2, [Number] (In Formula 1 and Formula 2, T is the soaking holding temperature (°C) during annealing which is 500°C or higher, t is the soaking holding time (seconds) during annealing, and P(H2) is the H2 concentration (volume %) in the ambient gas atmosphere during annealing).

[0039] In the second embodiment, the wound-up steel plate is annealed so as to satisfy the following relational expressions according to the Cr content contained in the steel material.

[0040] When the Cr content is 0.2 mass% or more and 0.6 mass% or less, the steel plate is annealed under the conditions satisfying the following Formula 1A. [Number]

[0041] When the Cr content is less than 0.2 mass%, the steel plate is annealed under the conditions satisfying the following Formula 1B. [Number]

[0042] Or, when the Cr content is more than 0.6% by mass and 1.0% by mass or less, anneal the steel sheet under the conditions satisfying the following formula 1C. [Number]

[0043] In the above formula 1A, the above formula 1B, and the above formula 1C, T is the soaking holding temperature (°C) during annealing that is 500°C or higher, t is the soaking holding time (seconds) during annealing, and Cr[%] is the Cr content (% by mass) of the steel material.

[0044] Furthermore, in the method for manufacturing a steel sheet according to the second embodiment, it is preferable to anneal the wound steel sheet so as to satisfy the following conditions according to the Cr content contained in the steel material.

[0045] When the Cr content is 0.6% by mass or less, it is preferable to anneal the steel sheet under the conditions satisfying the above formula 1A.

[0046] Or, when the Cr content is more than 0.6% by mass and 1.0% by mass or less, it is preferable to anneal the steel sheet under the conditions satisfying the above formula 1C.

[0047] Also in this case, in the above formula 1A and the above formula 1C, T is the soaking holding temperature (°C) during annealing that is 500°C or higher, t is the soaking holding time (seconds) during annealing, and Cr[%] is the Cr content (% by mass) of the steel material.

[0048] Furthermore, in the method for manufacturing a steel sheet according to the second embodiment, the H2 concentration (volume%) in the surrounding gas atmosphere during annealing is preferably 0% by volume.

[0049] By annealing under the conditions defined by the lower limit values of the above formula (1), formula (1A), formula (1B) and formula (1C), the internal oxide layer can be favorably grown and left up to the vicinity of the edge in the width direction of the steel sheet. As a result, a steel sheet that can be alloyed uniformly without unevenness can be obtained. Preferably, not only from the center in the width direction to the edge in the width direction of the steel sheet, but also from the front end in the direction parallel to the rolling direction of the steel sheet (hereinafter also referred to as "front end in the rolling direction") to the rear end in the direction parallel to the rolling direction of the steel sheet (hereinafter also referred to as "rear end in the rolling direction"), the internal oxide layer can be favorably grown and left. As a result, a steel sheet that can be alloyed substantially uniformly and surely without unevenness over substantially the entire surface of the steel sheet can be obtained. Note that it is difficult to sufficiently grow the internal oxide layer up to the edge in the width direction only by the heating during coiling in the above-described hot rolling.

[0050] Furthermore, by annealing under the conditions defined by the upper limit value of the above formula (1) and the formula (2), or under the conditions defined by the upper limit value of the above formula (1A), formula (1B) or formula (1C) according to the Cr content, the generation of reduced iron on the surface of the steel sheet can be sufficiently suppressed. As a result, since a steel sheet having good pickling properties can be obtained without including the actual pickling property evaluation process, scale removal in subsequent pickling does not become difficult.

[0051] Here, first, the process leading to the above formula (1) and formula (2) in the first embodiment will be described.

[0052] The amount x (g / m 2 ) of the internal oxide layer generated during annealing is proportional to the value represented by the following formula (3), where T (°C) is the soaking holding temperature during annealing and t (seconds) is the soaking holding time during annealing.

Equation

[0053] Here, in the above formula 3, R is the gas constant 8.31 [J / (K·mol)], and Q is the activation energy for oxygen diffusion in iron = 89.5 (kJ / mol). Therefore, substituting these values, the amount x (g / m 2 ) of the internal oxidation layer can be expressed by the following formula 4. In formula 4, A is a coefficient.

Number

[0054] Here, x obtained by substituting the soaking holding temperature T of 540 °C and the soaking holding time t of 30 hours (108000 seconds) into the above formula 4 2 is defined as a lower limit as represented by the following formula 5. The definition of this lower limit can be a condition for manufacturing a steel sheet capable of suppressing alloying unevenness. In this specification, such a lower limit is also simply referred to as the "lower limit regarding alloying unevenness of the internal oxidation layer" or the "lower limit". Note that if the soaking holding temperature T is too low, an internal oxidation layer cannot be formed, so T in the following formula 5 is 500 °C or higher.

[0055]

Number

[0056] Furthermore, x obtained by substituting the soaking holding temperature T of 620 °C and the soaking holding time t of 30 hours (108000 seconds) into the above formula 4 2 is defined as an upper limit as represented by the following formula 6. The definition of this upper limit can be a condition for manufacturing a steel sheet that suppresses the generation of reduced iron and has good pickling properties. In this specification, such an upper limit is also simply referred to as the "upper limit regarding pickling properties of the internal oxidation layer" or the "upper limit".

[0057]

Number

[0058] When the above formula 5 and the above formula 6 derived in this way are combined, the above formula 1 is derived. Further, the H2 concentration P(H2) (volume %) in the surrounding gas atmosphere during annealing needs to satisfy the conditions of the above formula 2 in relation to the soaking holding temperature T (°C).

[0059] Next, the process leading to the derivation of the above formula 1A, the above formula 1B, and the above formula 1C in the second embodiment will be described.

[0060] Also in the second embodiment, the method for defining the lower limit value "0.19" regarding the uneven alloying of the internal oxide layer is the same as that in the first embodiment described above. The upper limit value in the second embodiment is defined as follows.

[0061] When the Cr content is less than 0.2% by mass, the same upper limit value is defined based on the numerical value of the upper limit value when the Cr content is 0.2% by mass. Specifically, when the Cr content is less than 0.2% by mass, x obtained by substituting the conditions of a soaking holding temperature T of 620 °C and a soaking holding time t of 30 hours (108,000 seconds) into the above formula 4 2 is defined as the upper limit value regarding the pickling property of the internal oxide layer as represented by the above formula 6. This definition of the upper limit value can be a condition for suppressing the generation of reduced iron and producing a steel sheet having good pickling property when the Cr content is less than 0.2% by mass.

[0062] When the Cr content is more than 0.6% by mass and 1.0% by mass or less, the same upper limit value is defined based on the numerical value of the upper limit value when the Cr content is 0.6% by mass. Specifically, when the Cr content is more than 0.6% by mass and 1.0% by mass or less, x obtained by substituting the conditions of a soaking holding temperature T of 650 °C and a soaking holding time t of 30 hours (108,000 seconds) into the above formula 4 2 is defined as the upper limit value regarding the pickling property of the internal oxide layer as represented by the following formula 7. This definition of the upper limit value can be a condition for suppressing the generation of reduced iron and producing a steel sheet having good pickling property when the Cr content is more than 0.6% by mass and 1.0% by mass or less.

Number

[0063] When the Cr content is 0.2% by mass or more and 0.6% by mass or less, the straight line of the upper limit value with respect to the Cr content passing through two points of the upper limit value of 0.63 when the Cr content is 0.2% by mass and the upper limit value of 0.93 when the Cr content is 0.6% by mass is defined as the upper limit value regarding the pickling property of the internal oxide layer as represented by the following formula (8). This definition of the upper limit value can be a condition for suppressing the generation of reduced iron and manufacturing a steel sheet having good pickling property when the Cr content is 0.2% by mass or more and 0.6% by mass or less.

Equation

[0064] When the above formula (6), the above formula (7) and the above formula (8) derived in this way are summarized, the above formula (1A), the above formula (1B) and the above formula (1C) are derived according to the Cr content contained in the steel material.

[0065] Preferably, even when the Cr content is less than 0.2% by mass, similar to the case where the Cr content is 0.2% by mass or more and 0.6% by mass or less, the straight line of the upper limit value with respect to the Cr content passing through two points of the upper limit value of 0.63 when the Cr content is 0.2% by mass and the upper limit value of 0.93 when the Cr content is 0.6% by mass may be defined as the upper limit value regarding the pickling property of the internal oxide layer as represented by the above formula (8).

[0066] (Pickling) Next, it is preferable to pickle the annealed steel sheet. The pickling method is not particularly limited, and any known method may be applied. For example, the scale may be removed by immersing using hydrochloric acid or the like.

[0067] According to the method for manufacturing a steel sheet in the first embodiment, annealing is performed in the previous annealing step under the upper limit value of Formula 1 and the conditions defined by Formula 2. Alternatively, according to the method for manufacturing a steel sheet in the second embodiment, in the previous annealing step, depending on the Cr content contained in the steel material, annealing is performed under the conditions defined by the upper limit value of Formula 1A, Formula 1B, or Formula 1C (preferably under the conditions defined by the upper limit value of Formula 1A or Formula 1C). Therefore, the generation of reduced iron on the surface of the steel sheet is sufficiently suppressed, and the steel sheet to be pickled has good pickling properties. Therefore, according to the conventional method, by setting pickling conditions such as the concentration of the pickling solution, the temperature of the pickling solution, and the pickling time to general numerical values, the problem of residual oxide scale does not occur, and the scale adhering to the steel sheet can be easily and efficiently removed.

[0068] For example, when hydrochloric acid is used as the pickling solution, the hydrochloric acid concentration is preferably set to 3% by mass or more, more preferably 5% by mass or more. Also, when hydrochloric acid is used as the pickling solution, the hydrochloric acid concentration is preferably set to 20% by mass or less, more preferably 15% by mass or less. Further, for example, the temperature of the pickling solution is preferably set to 60°C or higher, more preferably 70°C or higher. Also, the temperature of the pickling solution is preferably set to 90°C or lower, more preferably 80°C or lower. The pickling time may be appropriately adjusted according to the concentration and temperature of the pickling solution.

[0069] (Cold rolling) Furthermore, cold rolling may be performed on the pickled steel sheet. The method of cold rolling is not particularly limited, and any known method may be applied. For example, in order to obtain a desired sheet thickness, the cold rolling reduction rate can be in the range of 10% to 70%. The sheet thickness of the steel sheet is not particularly limited.

[0070] By including the annealing step and any arbitrary steps as described above, the steel sheet in the first embodiment or the second embodiment can be manufactured.

[0071] 2. Method for manufacturing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet The steel sheet manufactured by the method according to the first or second embodiment of the present invention is suitably used as a base material for a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having a high Si content, high strength, and high workability. Hereinafter, an example of a method for manufacturing such a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet will be described.

[0072] (Oxidation treatment and reduction treatment) First, annealing by an oxidation-reduction method is applied to the surface of the steel sheet manufactured in the first or second embodiment described above. First, an Fe oxide layer is formed on the surface of the steel sheet by subjecting the surface of the steel sheet to an oxidation treatment. Further, the Fe oxide layer is subjected to a reduction treatment (also referred to as "reduction annealing treatment" in this specification) in a reducing atmosphere to form a reduced Fe layer. At this time, oxygen supplied from the Fe oxide layer by reduction oxidizes Si and Mn in the steel sheet. That is, by applying annealing by such an oxidation-reduction method, the Fe oxide layer becomes a barrier layer, and the oxide of Si can be retained inside the steel sheet, and an increase in the amount of solid-solution Si in the vicinity of the surface layer of the steel sheet can be suppressed. As a result, the wettability with respect to hot-dip galvanizing can be improved, and finally, alloying unevenness can be more reliably reduced.

[0073] The oxidation treatment and the reduction treatment may be carried out using any known single or plural pieces of equipment. Preferably, from the viewpoints of production efficiency, cost, and quality maintenance, equipment of a continuous hot-dip galvanizing line (CGL) is used. By using a continuous hot-dip galvanizing line, the oxidation treatment and the reduction treatment by the oxidation-reduction method and the hot-dip galvanizing treatment and the alloying treatment described later can be continuously carried out in a series of production lines. More specifically, the oxidation treatment and the reduction treatment by the oxidation-reduction method are more preferably carried out using, for example, an annealing furnace in a continuous hot-dip galvanizing line of a non-oxidizing furnace (NOF) type or a direct-fired furnace (DFF) type.

[0074] The pickling treatment is preferably carried out on the surface of the steel sheet at a heating temperature of 750 °C or lower, for example, in the oxidation heating zone in a NOF type or DFF type annealing furnace. By setting the steel sheet temperature to 750 °C or lower, a hot-dip galvanized steel sheet having good plating adhesion can be obtained.

[0075] The steel sheet temperature in the pickling treatment is preferably 730 °C or lower, more preferably 720 °C or lower, and still more preferably 700 °C or lower. The lower limit of the steel sheet temperature in the pickling treatment is not particularly limited, and any temperature at which an Fe oxide layer is formed on the surface of the steel sheet under the gas atmosphere described below is acceptable. For example, the steel sheet temperature in the pickling treatment is preferably 650 °C or higher, more preferably 670 °C or higher.

[0076] The temperature rising time in the pickling treatment is preferably 10 seconds or more, more preferably 15 seconds or more. Also, for example, the temperature rising time in the pickling treatment is preferably 120 seconds or less, more preferably 90 seconds or less.

[0077] The pickling treatment is not particularly limited, but can be carried out, for example, in a gas atmosphere containing O2, CO2, N2, and H2O. More specifically, the pickling treatment can be carried out, for example, in a NOF type or DFF type annealing furnace or the like, in a combustion gas such as coke oven gas (COG) or liquefied petroleum gas (LPG), in a gas atmosphere in which the concentration of unburned O2 is controlled. The O2 concentration is preferably controlled in the range of 100 ppm to 17000 ppm. The O2 concentration is more preferably controlled to be 500 ppm or higher, and still more preferably 2000 ppm or higher. Also, the O2 concentration is more preferably controlled to be 15000 ppm or lower, and still more preferably 13000 ppm or lower.

[0078] The heating temperature ( soaking temperature ) of the steel sheet in the reduction annealing treatment is not particularly limited, and it may be carried out at a temperature at which the Fe oxide layer formed by the oxidation treatment becomes a reduced Fe layer. Specifically, it is preferably carried out at a soaking temperature of Ac3 point or higher. The Ac3 point can be calculated by the following formula (i) ( "Leslie's Steel Materials Science" (published by Maruzen Co., Ltd., written by William C. Leslie, p273)). The element symbol enclosed in [ ] in formula (i) represents the content ( mass% ) of the element. Ac3 ( °C ) = 910 - 203×[C] 1 / 2 - 15.2×[Ni] + 44.7×[Si] + 104×[V] + 31.5×[Mo] + 13.1×[W] - {30×[Mn] + 11×[Cr] + 20×[Cu] - 700×[P] - 400×[Al] - 120×[As] - 400×[Ti]} …(i)

[0079] In addition, the heating time ( soaking time ) in the reduction treatment is not particularly limited, and it may be appropriately adjusted so that the Fe oxide layer formed by the oxidation treatment becomes a reduced Fe layer. For example, the heating time in the reduction treatment is preferably 30 seconds or more, more preferably 45 seconds or more. Also, the heating time in the reduction treatment is preferably 600 seconds or less, more preferably 500 seconds or less.

[0080] The reduction annealing treatment can be carried out by any known treatment method in, for example, the reduction heating zone in a NOF type or DFF type annealing furnace. Specifically, it can be carried out by heating the surface of the steel sheet mainly in a reducing atmosphere containing H2 gas and an inert gas such as N2. When using a mixed gas containing H2 gas and an inert gas such as N2, for example, H2 gas can be contained at a ratio of 3 to 25% by volume, and the remaining part can be an inert gas such as N2.

[0081] (Hot-dip galvanizing treatment) Furthermore, a hot-dip galvanized steel sheet can be manufactured by subjecting the steel sheet after the reduction treatment to a hot-dip galvanizing treatment to form a zinc plating layer on the surface of the steel sheet.

[0082] The method of hot-dip galvanizing treatment is not particularly limited, and any known method can be applied. For example, a galvanized layer can be formed on the surface of a steel sheet by immersing the steel sheet in a zinc plating bath at a steel sheet temperature of about 400°C to 500°C. Further, the immersion time of the steel sheet in the zinc plating bath may be adjusted according to the desired amount of zinc plating adhesion.

[0083] (Alloying treatment) The method for manufacturing an alloyed hot-dip galvanized steel sheet further includes a step of alloying the galvanized layer formed on the hot-dip galvanized steel sheet obtained by the above-described method.

[0084] Specifically, by heating the hot-dip galvanized steel sheet at a predetermined alloying temperature, Fe atoms contained in the steel sheet diffuse into the galvanized layer, and the galvanized layer can be alloyed. The alloying method is not particularly limited, and any known method can be applied. The alloying temperature is not particularly limited, but for example, it can be preferably set at 480°C to 650°C. The heating time at the alloying temperature is also not particularly limited, but for example, it can be preferably set at 10 seconds to 40 seconds. Further, the heating for alloying can be, for example, in an air atmosphere.

[0085] 3. Chemical composition of steel material The chemical composition of the steel material used in the method for manufacturing a steel sheet in the first embodiment is not particularly limited except for Si. Also, the chemical composition of the steel material used in the method for manufacturing a steel sheet in the second embodiment is not particularly limited except for Si and Cr.

[0086] Hereinafter, an example of the chemical composition of the steel material in the first embodiment and the second embodiment will be described.

[0087] [Si: 1% by mass or more] Si is an inexpensive steel strengthening element and is less likely to affect the workability of steel plates. Also, Si is an element that can suppress the decomposition of retained austenite, which is useful for improving the workability of steel plates, and the formation of carbides. In order to effectively exert such an effect, the Si content is 1.0 mass% or more, preferably 1.1 mass% or more, and more preferably 1.2 mass% or more. The upper limit of the Si content is not particularly limited. However, if the Si content is too high, the solid solution strengthening effect of Si becomes significant, which may increase the rolling load, and Si scale may occur during hot rolling, resulting in surface defects of the steel plate. Therefore, for example, from the viewpoint of manufacturing stability, the Si content is preferably 3.0 mass% or less, more preferably 2.7 mass% or less, and even more preferably 2.5 mass% or less.

[0088] [Mn: Preferably 1.5 mass% or more and 3.0 mass% or less] Similar to Si, Mn is also an inexpensive steel strengthening element and is effective in improving the strength of steel plates. Mn, together with Si and, if necessary, C as well, is added to the steel, and is a particularly effective strengthening element for finally ensuring a tensile strength of 980 MPa or more for hot-dip galvanized steel plates. Furthermore, Mn stabilizes austenite and contributes to the improvement of the workability of steel plates due to the formation of retained austenite. In order to effectively exert such an effect, the Mn content is preferably 1.5 mass% or more, more preferably 1.8 mass% or more, and even more preferably 2.0 mass% or more. However, if the Mn content is too high, the ductility of the steel plate decreases, which may adversely affect the workability of the steel plate and reduce the weldability of the steel plate. From such a viewpoint, the Mn content is preferably 3.0 mass% or less, more preferably 2.8 mass% or less, and even more preferably 2.7 mass% or less.

[0089] [C: Preferably 0.08 mass% or more and 0.30 mass% or less] C is an element effective for improving the strength of the steel sheet. Together with Si and, if necessary, Mn as well, it is a particularly effective strengthening element for finally ensuring a tensile strength of 980 MPa or more for the hot-dip galvanized steel sheet. Further, C is an element necessary for securing retained austenite and improving workability. In order to effectively exert such an effect, the C content is preferably 0.08% by mass or more, more preferably 0.11% by mass or more, still more preferably 0.13% by mass or more. From the viewpoint of ensuring the strength of the steel sheet, a higher C content is preferable, but if the C content is too high, there is a risk of deterioration in corrosion resistance, spot weldability and workability. Therefore, the C content is preferably 0.30% by mass or less, more preferably 0.25% by mass or less, still more preferably 0.20% by mass or less.

[0090] [P: Preferably more than 0% by mass and 0.1% by mass or less] P is an element that inevitably exists as an impurity element. If the P content becomes excessive, there is a risk of deteriorating weldability. Therefore, the P content is preferably suppressed to 0.1% by mass or less, more preferably 0.08% by mass or less, still more preferably 0.05% by mass or less.

[0091] [S: Preferably more than 0% by mass and 0.05% by mass or less] S is an element that inevitably exists as an impurity element. Usually, steel inevitably contains S at about 0.0005% by mass. If the S content becomes excessive, there is a risk of forming sulfide-based inclusions, promoting hydrogen absorption in a corrosive environment, deteriorating the stress corrosion cracking resistance of the steel sheet, and deteriorating the weldability and workability of the steel sheet. Therefore, the S content is preferably suppressed to 0.05% by mass or less, more preferably 0.01% by mass or less, still more preferably 0.005% by mass or less.

[0092] [Al: Preferably more than 0% by mass and 1.0% by mass or less] Al is an element having a deoxidizing effect. To effectively exert such an effect, the Al content is preferably more than 0% by mass, more preferably 0.005% by mass or more, and still more preferably 0.02% by mass or more. If the Al content becomes excessive, inclusions such as alumina may increase, and the workability of the steel sheet may deteriorate. Therefore, the Al content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and still more preferably 0.5% by mass or less.

[0093] [Cr: Preferably more than 0% by mass and 1.0% by mass or less] Cr is an element effective for improving the strength of the steel sheet. Further, Cr is an element that improves the corrosion resistance of the steel sheet and has the effect of suppressing the generation of hydrogen due to the corrosion of the steel sheet. Specifically, Cr has the effect of promoting the formation of iron oxide (α-FeOOH). Iron oxide is said to be thermodynamically stable and protective among the rusts generated in the atmosphere. By promoting the formation of such rust, the intrusion of the generated hydrogen into the steel sheet can be suppressed, and hydrogen-assisted cracking can be sufficiently suppressed even when the steel sheet is used in a severe corrosion environment, for example, in the presence of chlorides. Also, like B and Ti, Cr is an element effective for the stress corrosion cracking resistance of the steel sheet, so it can be added in an amount that does not affect the strength and workability such as elongation of the steel sheet. To effectively exert these effects, the Cr content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and still more preferably 0.01% by mass or more. On the other hand, if the Cr content becomes excessive, the workability such as the elongation of the steel sheet may deteriorate. Therefore, the Cr content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and still more preferably 0.6% by mass or less.

[0094] In the method for manufacturing a steel sheet according to the first embodiment, in order to more reliably obtain a good effect regarding pickling properties, the Cr content is preferably more than 0% by mass and 0.4% by mass or less, more preferably 0.1% by mass or more and 0.3% by mass or less, still more preferably 0.2% by mass or more and 0.3% by mass or less, and particularly preferably 0.2% by mass.

[0095] On the other hand, in the method for manufacturing a steel sheet according to the second embodiment, the Cr content may be 1% by mass or less. Specifically, depending on the Cr content, by adjusting the soaking temperature T, the soaking time t, and the Cr content so as to satisfy a predetermined relational expression, a good effect regarding pickling property can be obtained.

[0096] [Cu: Preferably more than 0% by mass and 1.0% by mass or less] Similar to Cr, Cu is also effective in improving the strength of the steel sheet, and has the effect of suppressing the generation of hydrogen due to corrosion of the steel sheet, and is an element that improves the corrosion resistance of the steel sheet. Similar to Cr, Cu also has the effect of promoting the formation of iron oxide. In order to effectively exert these effects, the Cu content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and still more preferably 0.05% by mass or more. Also, from the viewpoint of the workability of the steel sheet, the Cu content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and still more preferably 0.5% by mass or less.

[0097] [Ni: Preferably more than 0% by mass and 1.0% by mass or less] Similar to Cr and Cu, Ni is also effective in improving the strength of the steel sheet, and has the effect of suppressing the generation of hydrogen due to corrosion of the steel sheet, and is an element that improves the corrosion resistance of the steel sheet. Similar to Cr and Cu, Ni also has the effect of promoting the formation of iron oxide. In order to effectively exert these effects, the Ni content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and still more preferably 0.05% by mass or more. Also, from the viewpoint of the workability of the steel sheet, the Ni content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and still more preferably 0.5% by mass or less.

[0098] [Ti: Preferably more than 0% by mass and 0.15% by mass or less] Like Ti, Cr, Cu, and Ni, Ti is effective in improving the strength of the steel sheet and has the effect of suppressing the generation of hydrogen due to corrosion of the steel sheet, thereby improving the corrosion resistance of the steel sheet. Like Cr, Cu, and Ni, Ti also has the effect of promoting the formation of iron oxide. Further, like B and Cr, Ti is an element effective for the hydrogen embrittlement resistance of the steel sheet, and thus can be added in an amount that does not affect the workability such as the strength and elongation of the steel sheet. To effectively exhibit these effects, the Ti content is preferably more than 0% by mass, more preferably 0.003% by mass or more, and still more preferably 0.05% by mass or more. Further, from the viewpoint of the workability of the steel sheet, the Ti content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and still more preferably 0.10% by mass or less.

[0099] [Nb: Preferably more than 0% by mass and 0.15% by mass or less] Nb is an element effective in improving the strength of the steel sheet and acts to refine austenite grains after quenching and improve the toughness of the steel sheet. To effectively exhibit such an effect, the Nb content is preferably more than 0% by mass, more preferably 0.03% by mass or more, and still more preferably 0.005% by mass or more. On the other hand, when the Nb content becomes excessive, a large amount of carbides, nitrides or carbonitrides are generated, and there is a risk that the workability or hydrogen embrittlement resistance of the steel sheet deteriorates. Therefore, the Nb content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and still more preferably 0.10% by mass or less.

[0100] [V: Preferably more than 0% by mass and 0.15% by mass or less] Like Nb, V is also an element effective in improving the strength of the steel sheet and acts to refine austenite grains after quenching to improve the toughness of the steel sheet. To effectively exert such an effect, the V content is preferably more than 0% by mass, more preferably 0.03% by mass or more, and still more preferably 0.005% by mass or more. On the other hand, when the V content becomes excessive, similar to Nb, a large amount of carbides, nitrides or carbonitrides may be formed, and the workability or hydrogen embrittlement resistance of the steel sheet may deteriorate. Therefore, the V content is preferably 0.15% by mass or less, more preferably 0.12% by mass or less, and still more preferably 0.1% by mass or less.

[0101] [B: Preferably more than 0% by mass and 0.005% by mass or less] B is an element useful for improving the hardenability and weldability of the steel sheet. Also, like Ti and Cr, B is an element effective in the hydrogen embrittlement resistance of the steel sheet, and thus can be added in an amount that does not affect the strength and workability such as elongation of the steel sheet. To effectively exert these effects, the B content is preferably more than 0% by mass, more preferably 0.0002% by mass or more, still more preferably 0.0003% by mass or more, and particularly preferably 0.0004% by mass or more. On the other hand, when the B content becomes excessive, such effects become saturated, and there is a risk that the ductility decreases and the workability deteriorates. Therefore, the B content is preferably 0.005% by mass or less, still more preferably 0.004% by mass or less, and still more preferably 0.003% by mass or less.

[0102] [N: Preferably more than 0% by mass and 0.01% by mass or less] N is an element that inevitably exists as an impurity element. When the N content becomes excessive, there is a risk that nitrides are formed and the workability of the steel sheet deteriorates. In particular, when the steel sheet contains B for improving hardenability, N combines with B to form BN precipitates, inhibiting the hardenability improvement effect of B. Therefore, the N content is preferably suppressed to 0.01% by mass or less, more preferably 0.008% by mass or less, and still more preferably 0.005% by mass or less.

[0103] In addition, the chemical composition of the steel material in the first and second embodiments of the present invention may further contain other well-known optional components, in addition to the above components, within a range that does not inhibit strength and sufficient workability.

[0104] [Balance] The balance is Fe and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. Since P, S, and N as described above are preferably present in a smaller amount, they can also be regarded as unavoidable impurities. However, since the present invention can exhibit its effects by suppressing the content of these elements to a specific range, they are defined as above. Therefore, in this specification, the "unavoidable impurities" constituting the balance are a concept excluding elements whose composition ranges are defined.

[0105] According to the method for manufacturing a steel sheet in the first and second embodiments of the present invention, a steel sheet that achieves both suppression of alloying unevenness and good pickling property can be obtained even though the Si content is 1% by mass or more. In particular, in the manufacturing process of the steel sheet, it is not necessary to include a step of evaluating the pickling property of the scale on the surface of the steel sheet before and after pickling, a step of measuring the amount of reduced iron generated on the surface of the steel sheet, and the like.

[0106] Specifically, according to the method for manufacturing a steel sheet in the first embodiment, by simply setting the annealing conditions of the soaking holding temperature T, the soaking holding time t, and the H2 concentration P(H2) in the surrounding gas atmosphere so as to satisfy a predetermined relational expression defined in advance, a steel sheet having the above-described effects can be efficiently obtained.

[0107] According to the method for manufacturing a steel sheet in the second embodiment, by simply setting the annealing conditions of the soaking holding temperature T and the soaking holding time t so as to satisfy a predetermined relational expression defined in advance according to the Cr content, a steel sheet having the above-described effects can be efficiently obtained.

[0108] Furthermore, as described above, when manufacturing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet using the steel sheet manufactured by the method in the first embodiment or the second embodiment, if a continuous hot-dip galvanizing line is used, oxidation treatment, reduction treatment, hot-dip galvanizing treatment, and alloying treatment can be continuously performed in a series of manufacturing lines. According to such a manufacturing line, it is possible to manufacture an alloyed hot-dip galvanized steel sheet with high strength and high workability without alloying unevenness more inexpensively and efficiently while maintaining the product quality. Specifically, the alloyed hot-dip galvanized steel sheet manufactured in this way can have a tensile strength of 980 MPa or more.

Example

[0109] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited by the examples.

[0110] (Example 1) In Example 1, the lower limit value of the amount x (g / m 2 ) of the internal oxide layer capable of suppressing alloying unevenness was determined.

[0111] Specifically, using a steel material having a Si content of 1.0 mass% or more, a steel sheet was actually manufactured, and the amount of solid-solved Si (weight%) from the surface of the steel sheet to a depth of 1 μm (specifically, the average value (weight%) of the amount of solid-solved Si) and the amount of the internal oxide layer (g / m 2 ) were examined for their relationship with the alloying unevenness suppression effect.

[0112] First, a steel material (steel grade A) with the chemical composition shown in Table 1 below was melted in a converter and then a slab was produced by continuous casting. The obtained slab was hot-rolled until the finish rolling temperature was 900 °C and the plate thickness became 2.0 mm, coiled at 640 °C, and the obtained hot-rolled steel plate was cooled to room temperature. Then, the hot-rolled steel plate was put into an annealing furnace and annealed. The annealing conditions were as follows: in a non-reducing atmosphere of N2 - 0.5 vol% H2, the hot-rolled steel plate was heated to 580 °C in about 8.5 hours, held isothermally at 580 °C for 30 hours, and then cooled to 200 °C or less in about 5 hours. Then, the obtained annealed steel plate was pickled by immersing it in hydrochloric acid with a concentration of 8 wt% at 85 °C for 40 seconds. Finally, cold rolling was performed until the steel plate thickness decreased from 2.0 mm to 1.4 mm to obtain the target steel plate.

[0113]

Table 1

[0114] First, specimens of 20 mm × 20 mm × 1.4 mm (plate thickness) were cut out from various positions on the obtained steel plate using a shear cutting machine. Then, for each specimen, the amount of dissolved Si (wt%) from the surface of the steel plate to a depth of 1 μm, specifically the average value of the amount of dissolved Si (wt%), was measured. The amount of dissolved Si on the surface of the steel plate was measured using a fully automatic scanning X-ray photoelectron spectrometer (manufactured by ULVAC-PHI, Inc., "Quantera-SXM"). The measurement conditions were: X-ray output: 24.2 W, X-ray beam diameter: 100 μm, and analysis position: depth of 1 μm. Specifically, it was calculated using the following formula. That is, the ratio of the peak area intensity of Si(Si - Si,Fe - Si) to {Si(SiO x )+Si(Si - Si,Fe - Si)} was obtained, and the amount of dissolved Si (wt%) was calculated by multiplying the obtained ratio by the actual Si content in the steel. Amount of dissolved Si (wt%) = [Si(Si - Si,Fe - Si) / {Si(SiO x )+Si(Si - Si,Fe - Si)}] × Si content in steel

[0115] Furthermore, simultaneously, the amount of the internal oxide layer (g / m 2 ) of the test piece in which the amount of dissolved Si (wt%) was measured was measured. Specifically, the cut-out test piece was immersed under the condition of a temperature of 80°C using hydrochloric acid with a concentration of 10% by mass, and the dissolution amount per unit area (g / m 2 ) was measured.

[0116] The graph of FIG. 1 shows the correlation between the amount of dissolved Si (wt%) and the amount of the internal oxide layer (g / m 2 ) measured in this way. Here, the following formula indicated by the broken line in the graph of FIG. 1 is a formula derived by regression analysis. R is the correlation coefficient. y (amount of dissolved Si (wt%)) = -0.1169x (amount of internal oxide layer (g / m 2 )) + 1.8723 (R 2 = 0.997)

[0117] Next, in order to examine the relationship between the amount of dissolved Si (wt%) and the amount of the internal oxide layer (g / m 2 ) and the suppression effect of alloying unevenness, an alloyed hot-dip galvanized steel sheet was produced from the obtained steel sheet. First, the obtained steel sheet was subjected to an oxidation treatment, a reduction treatment, a hot-dip galvanizing treatment, and an alloying treatment by applying a continuous hot-dip galvanizing line having a NOF type annealing furnace. In the oxidation treatment, the steel sheet was heated under a combustion exhaust gas atmosphere containing less than 17000 ppm of O2 and CO2, N2, and H2O so that the steel sheet temperature became about 710°C (680°C to 730°C) with a heating-up time of 45 seconds. Here, the "steel sheet temperature" means the maximum reached sheet temperature of the steel sheet controlled by heating in the NOF which is the oxidation heating zone. The reduction treatment was carried out by heating for 50 seconds at a soaking holding temperature of about 800°C (770°C to 820°C) under a gas atmosphere of N2-H2. In the hot-dip galvanizing treatment, the reduced steel sheet was immersed in a zinc plating bath at 430°C to form a hot-dip galvanized layer. In this way, a hot-dip galvanized steel sheet was obtained, and then an alloyed hot-dip galvanized steel sheet was obtained by an alloying treatment.

[0118] Furthermore, the alloyed hot-dip galvanized steel sheet thus obtained was evaluated as to whether or not the alloying unevenness was suppressed. Specifically, the appearance of the obtained alloyed hot-dip galvanized steel sheet was visually observed, and when the Zn-Fe alloying had progressed and the metallic luster of Zn had disappeared, it was evaluated as "〇". On the other hand, when the metallic luster of Zn remained, it was evaluated as "×".

[0119] As a result of evaluating the alloying unevenness, it was found that if the amount of dissolved Si within a depth of 1 μm from the surface of the steel sheet was 1.36% by weight or less, the alloying unevenness could be suppressed at the surface portion of the steel sheet having such an amount of dissolved Si. As can be seen from the graph of Fig. 1, the fact that the amount of dissolved Si is 1.36% by weight or less corresponds to the fact that the amount of the internal oxide layer is 4.4 g / m 2 or more. That is, it was found that if the amount of the internal oxide layer was 4.4 g / m 2 or more, the alloying unevenness could be suppressed at the surface portion of the steel sheet showing such an amount of the internal oxide layer.

[0120] (Example 2) Next, an example of the method for manufacturing a steel sheet from which the lower limit value "0.19" of the formula 1 was derived will be described in detail.

[0121] In Example 2, first, the coiling temperature of hot rolling was set to 550°C, the soaking holding temperature during annealing was set to 540°C, the soaking holding time during annealing was set to 30 hours (108,000 seconds), and the steel sheet was manufactured by the same method as in Example 1 for the rest. Furthermore, in the same manner as in Example 1, the amount (g / m 2 ) of the internal oxide layer of the test piece at a predetermined position of the steel sheet was measured. In this Example 2, not only the steel material of Steel Type A but also the steel material of Steel Type B shown in Table 2 below was used to manufacture the steel sheet, and the amount (g / m 2 ) of the internal oxide layer was measured. The test pieces of the steel sheet were cut out from a position 10 m from the front end in the rolling direction of the steel sheet and at positions 0 mm to 20 mm, 20 mm to 40 mm, 40 mm to 60 mm, or 60 mm to 80 mm from the edge in the coil width direction of the steel sheet.

[0122] As a result, the test pieces at any position exceeded the lower limit value of the amount of the internal oxide layer (i.e., 4.4 g / m 2 ) that can suppress the alloying unevenness calculated in Example 1 above. This means that the steel sheet manufactured in Example 2 can suppress alloying unevenness, particularly in the coil width direction. Furthermore, x 2 obtained by substituting the annealing conditions of Example 2 into the above formula 4 is the square of the amount of the internal oxide layer. Therefore, x 2 obtained by substituting into the formula 4 can be defined as the lower limit value regarding the alloying unevenness of the internal oxide layer, as represented by the above formula 5. This is based on the finding that when the amount of the internal oxide layer is too small, the amount of solid-solution Si near the surface of the steel sheet increases, resulting in alloying unevenness. The results of Example 2 are summarized in Table 3 below.

[0123] [Table 2]

[0124] [Table 3]

[0125] (Example 3) In Example 3, the upper limit value of the reduction iron area ratio (%) (hereinafter, also simply referred to as "reduction iron area ratio (%)") with respect to the oxide scale area of the test piece near the edge in the width direction of the annealed steel sheet for having the effect of good pickling property was determined.

[0126] Specifically, various steel sheets before pickling were manufactured in the same manner as in Example 1, except that the soaking holding temperature and soaking holding time during annealing were changed. Then, the reduction iron area ratio (%) with respect to the oxide scale area of the test piece near the edge in the width direction of each of the obtained annealed steel sheets was measured. Specifically, the test piece near the edge in the width direction of the steel sheet was cut out from the portion at 0 mm to 100 mm in the width direction where the positions in the direction parallel to the rolling direction of the steel sheet were random.

[0127] Specifically, the scale image observed in the cross-sectional SEM image of the test piece was binarized by Otsu's method, and the reduction iron area ratio was measured by calculating the area ratio occupied by the group with high luminance in the entire scale. As a reference, the grain boundary oxidation depth (μm) in the internal oxidation layer was also measured simultaneously. Specifically, similarly, using the surface image of the test piece observed in the cross-sectional SEM image, the grain boundary oxidation depth was measured from five random points in the direction horizontal to the surface of the test piece, and the average value was calculated for measurement. Generally, when the grain boundary oxidation depth (μm) in the internal oxidation layer becomes deeper, that is, when the amount of the internal oxidation layer (g / m 2 ) increases, the relationship that the reduction iron area ratio (%) increases holds.

[0128] After that, each of the obtained annealed steel plates was pickled by immersing them in hydrochloric acid with a concentration of 10% by weight at 80°C for 40 seconds. After pickling, the remaining state of the reduced iron in each test piece near the edge in the width direction of the steel plate was visually observed. Then, the case where no reduced iron remained was designated as "〇", the case where the reduced iron was peeled off by shaking in the pickling solution was designated as "△", and the case where the reduced iron remained was designated as "×" to evaluate the pickling property. Furthermore, for such evaluation, the steel plates showing the evaluation result of "〇" were taken as the examples of the present invention. These results are shown in Table 4 below together with the results of the reduction iron area ratio (%) and the grain boundary oxidation depth (μm) measured before pickling. Also, in FIG. 2, the pickling property evaluation test in Table 4 was graphed.

[0129]

Table 4

[0130] From these pickling property evaluation results, it was found that if the reduction iron area ratio (%) with respect to the oxidation scale area near the edge in the width direction of the annealed steel plate is less than 45%, it has good pickling property. As described above, since reduced iron is more likely to occur near the edge in the width direction of the steel plate than near the center in the width direction, having good pickling property near the edge in the width direction means that the steel plate as a whole also has good pickling property.

[0131] (Example 4) Next, regarding the upper limit value of "0.63" in Formula 1 and the example of the manufacturing method of the steel sheet from which Formula 2 was derived in the first embodiment, a detailed explanation will be given.

[0132] In Example 4, regarding the annealing conditions, the soaking holding time was 30 hours (108,000 seconds), and the soaking holding temperature and the H2 concentration in the ambient gas atmosphere during annealing were changed. For the rest, various steel sheets before pickling were manufactured in the same manner as in Example 1. Further, the reduced iron area ratio (%) was measured in the same manner as in Example 3. In addition, from the above-described Example 3, it was found that if the reduced iron area ratio (%) was less than 45%, the steel sheet had good pickling properties. Based on this result, the pickling properties of each steel sheet were also evaluated. These results are shown in Table 5 below together with the annealing conditions.

[0133]

Table 5

[0134] As shown in Table 5 above, in Test No. 54 with a soaking holding temperature of 620°C and a soaking holding time of 30 hours (108,000 seconds), the measured reduced iron area ratio was lower than the upper limit value of the reduced iron area ratio that can have good pickling properties calculated in Example 3 above (that is, less than 45%). Therefore, it means that the steel sheet manufactured in Test No. 54 has good pickling properties. In addition, x obtained by substituting the annealing conditions of such Test No. 54 into Formula 4 2 is the square of the amount of the internal oxide layer. Therefore, x obtained by the substitution 2 can be defined as the upper limit value regarding the pickling properties of the internal oxide layer as represented by Formula 6. This is based on the finding that if the amount of the internal oxide layer increases by increasing the soaking holding temperature, more reduced iron is generated and good pickling properties cannot be obtained.

[0135] Furthermore, FIG. 3 is a graph plotting the soaking holding temperature during annealing in Table 5 above and the H2 concentration in the surrounding gas atmosphere. Here, in FIG. 3, the annealing conditions in Test Nos. 50 to 55 are plotted together with the evaluation results of pickling properties based on the results of Example 3 described above. Specifically, when the reduced iron area ratio indicating good pickling properties is less than 45%, it is plotted as "〇", and when the reduced iron area ratio not indicating good pickling properties is 45% or more, it is plotted as "×".

[0136] The relational expression 2 between the H2 concentration P(H2) (volume%) in the surrounding gas atmosphere during annealing and the soaking holding temperature T (°C), which is the boundary line of these evaluation results and is shown in the graph of FIG. 3, was derived from a straight line connecting a point where the H2 concentration P is 0 volume% and the soaking holding temperature T is 625 °C, and a point where the H2 concentration P is 1 volume% and the soaking holding temperature T is 600 °C in the graph. The reasons for selecting these points are as follows. Under the condition of a soaking holding temperature T of 625 °C, which is the intermediate value between Test No. 54 and Test No. 55 where the pickling property evaluation is divided when the H2 concentration P is 0%, it is assumed that the reduced iron area ratio is also about 42, which is the average of the two. Under the condition of a soaking holding temperature T of 600 °C, which is the intermediate value between Test No. 50 and Test No. 53 where the pickling property evaluation is divided when the H2 concentration P is 1%, it is assumed that the reduced iron area ratio is also about 31, which is the average of the two. These all correspond to values less than 45% indicating good pickling properties. Therefore, by using these conditions, the relational expression between the H2 concentration P(H2) and the soaking holding temperature T can be derived.

[0137] Alternatively, as an alternative to the above relational expression 2, it can also be defined by the following relational expression 2' using the results of Test No. 50 (H2 concentration P is 1 volume% and soaking holding temperature T is 590 °C) and Test No. 54 (H2 concentration P is 0 volume% and soaking holding temperature T is 620 °C).

[0138]

Equation

[0139] As can be seen from the above-described Examples 1 to 4, in the method for manufacturing a steel sheet, by setting the soaking holding temperature T, the soaking holding time t, and the H2 concentration P(H2) in the surrounding gas atmosphere so as to satisfy the above Formula 1 and the above Formula 2 (or the above Formula 2'), it is possible to efficiently obtain a steel sheet that achieves both suppression of alloying unevenness and good pickling properties.

[0140] (Example 5) In Example 5, an example of the method for manufacturing a steel sheet in which "0.19", which is the lower limit value of the above Formula 1A, the above Formula 1B, and the above Formula 1C in the second embodiment, was derived will be described in detail. Further, an example of the method for manufacturing a steel sheet in which "0.75Cr[%]+0.48", which is the upper limit value of the above Formula 1A, "0.63", which is the upper limit value of the above Formula 1B, and "0.93", which is the upper limit value of the above Formula 1C in the second embodiment, were derived will also be described in detail.

[0141] First, regarding the lower limit values related to the alloying unevenness of the internal oxide layer of the above Formula 1A, the above Formula 1B, and the above Formula 1C, regardless of the Cr content, similar to the first embodiment, based on the results of Example 2 described above, it can be defined as "0.19".

[0142] In Example 5, not only the steel material of Steel Type A shown in the above Table 1 used in the above-described Examples 1 to 4, but also the steel material of Steel Type C having different Cr contents shown in the following Table 6 was used. Regarding the annealing conditions, the soaking holding time was 30 hours (108,000 seconds), the H2 concentration in the surrounding gas atmosphere during annealing was 0% by volume, and the soaking holding temperature was changed for each test to manufacture various steel sheets before pickling. Other detailed methods are the same as those in Example 4 described above. Also, as can be seen from Table 7 shown later, the tests using the steel material of Steel Type A having a Cr content of 0.2% by mass are Test No. 54 and Test No. 55 shown in the above Table 5 of Example 4.

[0143]

Table 6

[0144] Next, the decarburization amount (mg / cm 2 ) of the produced annealed steel sheet was measured. The decarburization amount was confirmed from the carbon concentration profile in the depth direction of the surface of the test piece of each steel sheet using a glow discharge optical emission spectrometer. Specifically, first, the carbon amount was confirmed for the portion where the carbon amount is 90% or less of the steel sheet base material at a position deeper than the interface between the oxide film and the steel material. Then, the difference between the carbon amount in this portion and the carbon amount of the steel sheet base material was obtained, and from this result, the carbon amount lost per unit area of each steel sheet was calculated as the decarburization amount (mg / cm 2 ).

[0145] The reduced iron area ratios (%) of Test No. 54 and Test No. 55 are 26% and 57% respectively, as shown in Table 5 above. Therefore, based on these values, by using the following formula, the estimated values of the reduced iron area ratio (%) were calculated from the decarburization amounts (mg / cm 2 ) measured in Test No. 56 to Test No. 58. As can be seen from the following formula, by suppressing decarburization, the reduced iron area ratio can also be decreased. A detailed explanation thereof will be described later. Reduced iron area ratio (%) (estimated value) = (57 - 26) / (13.72 - 4.84) × (decarburization amount (mg / cm 2 ) - 4.84) + 26

[0146] Furthermore, from Example 3 described above, it was found that if the reduced iron area ratio (%) is less than 45%, the steel sheet has good pickling properties. Therefore, based on this result, the pickling properties of various steel sheets were also evaluated. These results are shown in Table 7 below together with the annealing conditions and the like. The numerical values marked with (※) in Table 7 below indicate that, as described above, they are not measured values but estimated values.

[0147]

Table 7

[0148] As can be seen from the results shown in Table 7 above, generally, as the Cr content in the steel material increases, decarburization is suppressed. Since reduced iron is generated by the combination of carbon in the steel during decarburization and oxygen in the scale, when the decarburization amount decreases due to an increase in the Cr content, the amount of reduced iron generated also decreases, and good pickling properties can be obtained. In other words, when the Cr content in the steel material is higher, the soaking holding temperature during annealing can be made higher without generating a large amount of reduced iron, and the amount of the internal oxidation layer can be increased. Therefore, the higher the Cr content, the value of x obtained by substituting into the above formula 4 2 The upper limit value regarding the pickling properties of the internal oxidation layer based on can be increased.

[0149] When the Cr content is 0.2% by mass, as described above in Example 4, the value of x obtained by substituting the annealing conditions of Test No. 54 into the above formula 4 2 As represented by the above formula 6, can be defined as the upper limit value "0.63" regarding the pickling properties of the internal oxidation layer.

[0150] When the Cr content is 0.6% by mass, as shown in Table 7 above, in Test No. 58 with a soaking holding temperature of 650°C and a soaking holding time of 30 hours (108,000 seconds), the estimated reduced iron area ratio was lower than the upper limit value of the reduced iron area ratio (i.e., less than 45%) that can have good pickling properties calculated in Example 3 above. Therefore, the steel sheet manufactured in Test No. 58 means that it has good pickling properties. Therefore, when the Cr content is 0.6% by mass, the value of x obtained by substituting the annealing conditions of Test No. 58 into the above formula 4 2 As represented by the above formula 7, can be defined as the upper limit value "0.93" regarding the pickling properties of the internal oxidation layer.

[0151] Considering that the upper limit value increases in accordance with the increase in the Cr content, when the Cr content is 0.2% by mass or more and 0.6% by mass or less, the upper limit value regarding the pickling property of the internal oxide layer can be derived from the results of Test No. 54 and Test No. 58. Specifically, as shown by the formula 8, a straight line of the upper limit value with respect to the Cr content passing through two points, i.e., the upper limit value "0.63" when the Cr content is 0.2% by mass and the upper limit value "0.93" when the Cr content is 0.6% by mass, can be defined as the upper limit value "0.75Cr[%]+0.48" regarding the pickling property of the internal oxide layer according to the Cr content.

[0152] Furthermore, as can be seen from Table 7 above, in Test No. 54 when the Cr content is 0.2% by mass, the measured reduced iron area ratio is 26%, which is far lower than the upper limit value of the reduced iron area ratio (i.e., less than 45%) that can have good pickling property. Therefore, even when the Cr content is less than 0.2% by mass (preferably more than 0% by mass and less than 0.2% by mass), it is considered that the upper limit value regarding the pickling property of the internal oxide layer can be defined as "0.63" in the same way as when the Cr content is 0.2% by mass. Alternatively, even when the Cr content is less than 0.2% by mass (preferably more than 0% by mass and less than 0.2% by mass), the upper limit value regarding the pickling property of the internal oxide layer may be defined as "0.75Cr[%]+0.48" from the straight line of the upper limit value with respect to the Cr content passing through two points, i.e., the upper limit value "0.63" when the Cr content is 0.2% by mass and the upper limit value "0.93" when the Cr content is 0.6% by mass, in the same way as when the Cr content is 0.2% by mass or more and 0.6% by mass or less.

[0153] When the Cr content is more than 0.6% by mass and 1% by mass or less, compared with Test No. 58 when the Cr content is 0.6% by mass, decarburization during annealing is more suppressed. Therefore, since the amount of reduced iron generated also decreases, it is assumed that the upper limit value regarding the pickling property of the internal oxide layer can be set to a larger value. Therefore, even when the Cr content is more than 0.6% by mass and 1% by mass or less, the upper limit value regarding the pickling property of the internal oxide layer can be defined as "0.93" in the same way as when the Cr content is 0.6% by mass.

[0154] As can be seen from the above-described Example 1 to Example 3 and Example 5, in the method for manufacturing a steel sheet, by setting the soaking holding temperature T, the soaking holding time t, and the Cr content in the annealing step so as to satisfy the formula 1A, the formula 1B, or the formula 1C according to the Cr content contained in the steel material, it is possible to efficiently obtain a steel sheet that achieves both suppression of alloying unevenness and good pickling properties.

[0155] The embodiments and examples disclosed this time should be understood to be illustrative in all respects and not restrictive. The scope of the present invention is indicated by the scope of claims rather than the above description, and it is intended that all modifications within the meaning and scope equivalent to the scope of claims are included.

Claims

1. Hot-rolling a steel material with a Si content of 1.0% by mass or more and 3.0% by mass or less, a Cr content of 0.2% by mass or more and 1.0% by mass or less, a Mn content of 1.5% by mass or more and 3.0% by mass or less, a C content of 0.08% by mass or more and 0.30% by mass or less, a P content of 0.1% by mass or less, an S content of 0.05% by mass or less, and an Al content of more than 0% by mass and 1.0% by mass or less, with the balance being Fe and unavoidable impurities, and then winding up the obtained hot-rolled steel sheet at 500°C to 700°C; Immediately after winding up, the hot-rolled steel sheet is placed in a gas atmosphere with an H₂ concentration of 0% by volume. When the Cr content of the steel material is 0.2% by mass or more and 0.6% by mass or less, the following formula 1A: [Equation 1] Or, when the Cr content of the steel material is more than 0.6% by mass and 1.0% by mass or less, the following formula 1C: [Equation 2] (In formula 1A and formula 1C, T is the soaking holding temperature (°C) during annealing at 500°C or higher, t is the soaking holding time (seconds) during annealing, and Cr [%] is the Cr content (% by mass) of the steel material.) And annealing the steel sheet under conditions that satisfy the above, a method for manufacturing a steel sheet.

2. In the winding-up step, the winding-up is performed at 530°C to 660°C, the method for manufacturing a steel sheet according to Claim 1.

3. After the annealing, the method for manufacturing a steel sheet according to Claim 1 or 2, further including a step of pickling the steel sheet and then cold-rolling it.

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