EP4296385A1 - Method for manufacturing steel sheet - Google Patents

Abstract

A method for manufacturing a steel sheet according to the present invention includes a step of annealing a steel raw material having an Si content of 1.0 mass% or more and a Cr content of 1 mass% or less, under a condition satisfying: a following formula 1A if the Cr content of the steel raw material is 0.2 mass% or more and 0.6 mass% or less; a following formula 1B if the Cr content of the steel raw material is less than 0.2 mass%; or a following formula 1C if the Cr content of the steel raw material is more than 0.6 mass% and 1.0 mass% or less (in the formulas 1A, 1B, and 1C, T is 500°C or higher and represents a soaking temperature (°C) during annealing, t represents a soaking time (seconds) during annealing, and Cr [%] represents the Cr content (mass%) of the steel raw material).

Description

Technical Field
• The present invention relates to a method for manufacturing a steel sheet, which is preferably used as a substrate of a high-strength and high-formability hot-dip galvanized steel sheet and a substrate of a hot-dip galvannealed steel sheet, having a high Si content.
Background Art
• In the automobile industry, weight reduction and strength enhancement of an automobile member such as an automobile body are required from the viewpoint of improving fuel efficiency for reducing CO₂ and improving crashworthness. Thus, an ultra-high-strength steel sheet having a tensile strength of 980 MPa or more is applied to the automobile member such as the automobile body. In order to improve the formability of such a high-strength steel sheet, a method is known in which inexpensive Si is contained in a chemical composition of the steel sheet. When Si is contained in the chemical composition of the steel sheet, not only the strength but also the formability of the steel sheet can be improved.
• In general, when Si-added steel is applied to the automobile member, a hot-dip galvanized steel sheet (GI steel sheet) and a hot-dip galvannealed steel sheet (GA steel sheet) obtained by alloying the hot-dip galvanized steel sheet are used from the viewpoint of securing corrosion resistance and weldability. However, in the hot-dip galvanized steel sheet in which Si is added to the steel sheet, since an Si oxide layer covers a steel sheet surface in the manufacturing process, problems such as bare spot, reduction in plating adhesion, and an alloying unevenness in alloying treatment are likely to occur finally. In addition, problems such as peeling of the plating during processing of the hot-dip galvannealed steel sheet may also occur. In order to suppress such a problem caused by the addition of Si, the hot-dip galvanized steel sheet containing Si in a steel raw material is 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 iron oxide generated in the oxidation heating zone is generated in a reduced Fe layer during reduction annealing, plating wettability during plating can be improved. In addition, a method of forming an internal oxide layer containing SiO₂ and the like necessary for plating in advance in a steel sheet by increasing a coiling temperature in hot rolling is also used.
• In recent years, in order to further improve the strength and formability of the hot-dip galvanized steel sheet, various developments have been made on a method of favorably forming a hot-dip galvanized steel sheet or an internal oxide layer in which an Si content of the steel sheet is increased to 1 mass% or more.
• Specifically, for example, Patent Literature 1 describes a high-strength hot-dip galvannealed steel sheet with a good appearance containing Fe on a high-strength steel sheet containing C: 0.05 to 0.25%, Si: 0.3 to 2.5%, Mn: 1.5 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5%, and N: 0.0060% or less in terms of mass%, with the balance being Fe and unavoidable impurities, and having a hot-dip galvannealed layer containing Zn and unavoidable impurities as the balance, in which an oxide containing Si at a crystal grain boundary on a steel sheet side of 5 µm or less from an interface between a high-strength steel sheet and a plating layer and within the crystal grains is present at an average content of 0.6 to 10 mass%, and an oxide containing Si is present in the plating layer at an average content of 0.05 to 1.5 mass%.
• In addition, for example, Patent Literature 2 describes a method for manufacturing a high-strength hot-dip galvanized steel sheet having excellent plating adhesion, formability, and appearance, and this method includes a hot rolling step of hot rolling a slab containing C: 0.05 to 0.30%, Si: 0.1 to 2.0%, and Mn: 1.0 to 4.0% in terms of mass%, then coiling the steel sheet into a coil at a specific temperature Tc, and pickling the steel sheet, a cold rolling step of cold rolling the hot-rolled steel sheet resulting from the hot rolling step, an annealing step of annealing the cold-rolled steel sheet resulting from the cold rolling step under specific conditions, and a hot-dip galvanizing step of hot-dip galvanizing the annealed sheet resulting from the annealing step in a hot-dip galvanizing bath containing 0.12 to 0.22 mass% Al.
• In addition, for example, Patent Literature 3 describes a cold-rolled steel sheet obtained by heat-treating a raw steel piece in a temperature range of 650 to 950°C in an atmosphere in which reduction does not substantially occur after hot rolling while keeping adhesion of a black scale, thereby forming an internal oxide layer on a surface portion of a base steel sheet of the steel sheet, and then performing pickling, cold rolling, and recrystallization annealing according to a conventional method.
Citation List Patent Literature
• Patent Literature 1: JP 2006-233333 A
• Patent Literature 2: WO 2016/038801 A
• Patent Literature 3: JP 2000-309824 A
Summary of Invention
• An object of the present invention is to provide a method for manufacturing a steel sheet which has a high Si content, can suppress alloying unevenness, and has good pickling properties without actually including a step of evaluating pickling properties.
• As a result of intensive studies to solve the above problems, the present inventors have accomplished the present invention.
• That is, a method for manufacturing a steel sheet according to a first aspect of the present invention includes a step of annealing a steel raw material having a Si content of 1.0 mass% or more under a condition satisfying a following formula 1:
  [Mathematical Formula 1] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63$$

  
• and a following formula 2:
  [Mathematical Formula 2] $$\mathrm{<figure-callout\; id="2"\; label="P\; H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}\left({\mathrm{H}}_{}\right)\left({\mathrm{}}_2\right)<-\frac{1}{25}\mathit{T}+25$$
  
• (in the formulas 1 and 2, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and P (H₂) is an H₂ concentration (vol%) in a surrounding gas atmosphere during annealing).
• Alternatively, a method for manufacturing a steel sheet according to another first aspect of the present invention includes a step of annealing a steel raw material having an Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less, under a condition satisfying:
• a following formula 1A if the Cr content of the steel raw material is 0.2 mass% or more and 0.6 mass% or less:
  [Mathematical Formula 3] $$0.19\le \mathit{exp}\left(-\frac{10770}{\mathit{T}+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.75}\mathit{Cr}\left[\%\right]+0.48$$
  
• a following formula 1B if the Cr content of the steel raw material is less than 0.2 mass%:
  [Mathematical Formula 4] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.63}$$
  
• or a following formula 1C if the Cr content of the steel raw material is more than 0.6 mass% and 1.0 mass% or less:
  [Mathematical Formula 5] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.93}$$
  
• (in the formulas 1A, 1B, and 1C, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and Cr [%]is a Cr content (mass%) of the steel raw material).
Brief Description of Drawings
• FIG. 1 is a graph showing a correlation between a solid solution Si amount (wt%) and an amount (g/m²) of an internal oxide layer in Example 1.
• FIG. 2 is a graph showing a relationship between a reduced iron area ratio (%) and a grain boundary oxidation depth (µm) in a test for evaluating pickling properties in Example 3.
• FIG. 3 is a graph showing a correlation between a soaking temperature and an H₂ concentration during annealing based on an evaluation result of the pickling properties in Example 4.
Description of Embodiments
• As described above, the techniques described in Patent Literatures 1 to 3 relate to a method for manufacturing a hot-dip galvanized steel sheet or the like in which an Si content of a steel sheet is increased to 1 mass% or more, and a method for forming an internal oxide layer well.
• However, when the Si content is increased to 1 mass% or more in order to obtain a high-strength and high-formability hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, it is difficult to obtain a hot-dip galvannealed steel sheet uniformly alloyed over the entire surface of a coil only by applying the conventional manufacturing method. In particular, as compared with the vicinity of a center in a coil width direction of the steel sheet (hereinafter, also simply referred to as "width direction center"), it is difficult for the galvanizing to be uniformly alloyed in the vicinity of an edge of the steel sheet in the coil width direction (hereinafter, also simply referred to as "width direction edge").
• Specifically, in the case of using high Si-added steel, when the coil is cooled after coiling in hot rolling, the coil is steeply cooled in the vicinity of the width direction edge of the steel sheet. Thus, in the vicinity of the width direction edge of the steel sheet, the internal oxide layer hardly grows, and a layer is formed thin. On the other hand, in the vicinity of the width direction center of the steel sheet, the internal oxide layer sufficiently grows, and a layer is formed thick. In addition, in the subsequent pickling step, the internal oxide layer in the vicinity of the width direction edge is preferentially dissolved. As described above, the thickness of the internal oxide layer is different in the coil width direction, which causes alloying unevenness.
• Such a problem cannot be solved even by using the techniques described in the above-described Patent Literatures. For example, even in the method for manufacturing a steel sheet described in Patent Literature 1, the rapid cooling of the coil in the vicinity of the width direction edge is not taken into consideration, so that the internal oxide layer cannot remain in the vicinity of the width direction edge. In the manufacturing method described in Patent Literature 2, since it is necessary to lower a coiling temperature as the contents of Si and Mn increase, it is difficult to generate a predetermined amount of oxide in the vicinity of the width direction edge. As a result, it is difficult to uniformly manufacture a hot-dip galvannealed steel sheet having no alloying unevenness in the coil width direction of the steel sheet even by using the techniques disclosed in Patent Literatures 1 and 2.
• On the other hand, as described in Patent Literature 3, according to the method of heat-treating the hot-rolled steel sheet again, more internal oxide layers can be formed. However, by heat-treating the steel sheet again in addition to heating during hot rolling, an oxidation scale formed on a surface of the steel sheet is further increased. As a result, even if pickling is performed later, the oxidation scale is not sufficiently removed and remains, which may cause a problem of poor pickling properties. This is because the oxidation scale on the surface of the steel sheet is partially reduced to be reduced iron. For example, according to the manufacturing method described in Patent Literature 3, since the temperature of the heat treatment is high, the surface of the steel sheet is covered with reduced iron, and the scale cannot be removed by pickling. As a result, contamination of the steel sheet and decarburization in the vicinity of the surface of the steel sheet proceed, and thus it becomes difficult to obtain a steel sheet having a predetermined strength, for example, a tensile strength as high as 980 MPa. The principle of production of reduced iron is described in detail, for example, in JP 2017-222887 A . The reduced iron tends to be formed more in the vicinity of the width direction edge of the steel sheet than in the vicinity of the width direction center of the steel sheet due to an influence of an atmosphere in a furnace or the like. In addition, such reduced iron is formed more as an amount of Si added to steel increases.
• In addition, currently, in the evaluation of the pickling properties for the oxidation scale of the surface of the steel sheet as described above, the pickling properties are evaluated as good if the scale is removed after the pickling is actually performed, and the pickling properties are evaluated as poor if the scale is not removed. In other words, there is no index of quantitative evaluation of the pickling properties when the pickling properties are good or when the pickling properties are poor.
• Therefore, in order to efficiently manufacture a high-strength and high-formability hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet, having high Si contents, there is a need for a method for manufacturing a steel sheet that simultaneously solves the problem of the alloying unevenness and the problem regarding the index of the evaluation of the pickling properties due to the generation of reduced iron.
• Thus, the present inventors have conducted various studies on a method for manufacturing a steel sheet capable of suppressing the alloying unevenness even when the Si content is large, and having good pickling properties even without actually including a step of evaluating the pickling properties. In the annealing step of the method for manufacturing a steel sheet, it has been found that the problem of the alloying unevenness and the problem of the pickling properties can be solved by satisfying the predetermined relational expression between the soaking temperature T, the soaking time t, and an H₂ concentration P (H₂) in a surrounding gas atmosphere.
• In addition, from another point of view, it has been found that, in the annealing step of the method for manufacturing a steel sheet, the problem of the alloying unevenness and the problem of the pickling properties can be solved by satisfying a predetermined relational expression between the soaking temperature T, the soaking time t, and a Cr content depending on the Cr content in the steel raw material.
• That is, according to the present invention, it is possible to provide a method for manufacturing a steel sheet which has a high Si content, can suppress the alloying unevenness, and has good pickling properties without actually including a step of evaluating pickling properties.
• 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 embodiment described herein, and various modifications can be made without departing from the spirit of the present invention.
• In the present specification, the "internal oxide layer" means an internal oxide layer containing SiO₂ (including oxidized portions of both grain boundary oxidation and intragranular oxidation) that can be generated inside the steel sheet during heating in hot rolling and annealing. In addition, in the steel sheet manufactured by the method in the embodiment of the present invention, the internal oxide layer is present between a surface layer of the steel sheet and a steel sheet base portion which is an inner portion of the steel sheet not containing an oxide such as SiO₂. As will be described in detail in the following Examples, an amount of the internal oxide layer can be measured as a dissolved amount (g/m²) per unit area by immersing and dissolving the internal oxide layer in an acidic solution such as hydrochloric acid.
• In the present specification, unless a specific position is indicated, an "edge in the coil width direction (of the steel sheet)" basically intends both edges in the coil width direction, that is, both ends in a sheet width direction. In addition, in the present specification, "the vicinity of the edge in the coil width direction (of the steel sheet)" means a peripheral portion of the position of the edge in the coil width direction. When a specific position is indicated from the edge in the coil width direction, a distance from the width direction edge (in other words, the position of 0 mm in the width direction) is also described.
1. Method for manufacturing steel sheet
• The method for manufacturing a steel sheet according to the first embodiment of the present invention is not particularly limited as long as a steel raw material (steel or steel sheet) having a Si content of 1.0 mass% or more is used, and the method includes an annealing step under a condition satisfying a predetermined relational expression including a relational expression of the H₂ concentration as described later.
• The method for manufacturing a steel sheet according to the second embodiment of the present invention is not particularly limited as long as a steel raw 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 the method includes an annealing step under a condition satisfying a predetermined relational expression depending on the Cr content as described later.
• In the first embodiment and the second embodiment of the present invention, optional steps as described below may be included.
• Hereinafter, an example of the method for manufacturing a steel sheet according to the first embodiment and the second embodiment will be described.
(Provision of steel raw material for rolling)
• First, a steel raw material such as a slab for rolling having a chemical composition in which the Si content is 1.0 mass% or more is prepared. In the second embodiment including the annealing step under the condition depending on the Cr content, a steel raw material such as a slab for rolling having a chemical composition in which the Si content is 1.0 mass% or more and the Cr content is 1.0 mass% or less is prepared. Details of the chemical composition of the steel raw material will be described later. The steel raw material such as a slab can be provided by any known method. Examples of a method for preparing the slab include a method in which steel having a chemical composition described later is produced, and the slab is prepared by ingot-making or continuous casting. If necessary, a cast material obtained by ingot-making or continuous casting may be subjected to blooming and billet-making to obtain a slab.
(Hot rolling)
• Next, hot rolling is performed using the obtained steel raw material such as a slab to obtain a hot-rolled steel sheet.
• The hot rolling may be performed by a method according to any known conditions. The coiling temperature is preferably 500°C to 700°C. By setting the coiling temperature to 500°C or higher, the internal oxide layer can be sufficiently grown, and it becomes easy to secure the internal oxide layer in the vicinity of the width direction edge after the subsequent steps. The coiling temperature is more preferably 520°C or higher, and still more preferably 530°C or higher. By setting the coiling temperature to 700°C or lower, an amount of reduced iron generated by 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 still more preferably 660°C or lower.
• Other conditions at the time of hot rolling are not particularly limited. For example, in the hot rolling, the slab before hot rolling may be soakingly retained at a temperature of 1000°C to 1300°C or lower according to a conventional method, a finish rolling temperature may be set to 800°C or higher, and then the slab may be coiled as a coiled steel sheet. In addition, the coiled hot-rolled steel sheet after hot rolling may be naturally cooled to normal temperature.
(Annealing)
• In addition, the coiled steel sheet is annealed under the conditions of the first embodiment or the second embodiment described below.
• In the first embodiment, the coiled steel sheet is annealed so as to satisfy the following relational expression. Specifically, the steel sheet is annealed under a condition satisfying the following formula 1:
  [Mathematical Formula 6] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.63}$$

  
• and the following formula 2:
  [Mathematical Formula 7] $$\mathrm{<figure-callout\; id="2"\; label="P\; H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}\left({\mathrm{H}}_{}\right)\left({\mathrm{}}_2\right)<-\frac{1}{25}\mathit{T}+25$$
  
• (in the formulas 1 and 2, T is 500°C or higher and the soaking temperature (°C) during annealing, t is the soaking time (seconds) during annealing, and P (H₂) is an H₂ concentration (vol%) in a surrounding gas atmosphere during annealing).
• In the second embodiment, the coiled steel sheet is annealed so as to satisfy the following relational expression according to the Cr content in the steel raw material.
• When the Cr content is 0.2 mass% or more and 0.6 mass% or less, the steel sheet is annealed under conditions satisfying the following formula 1A.
  [Mathematical Formula 8] $$0.19\le \mathit{exp}\left(-\frac{10770}{\mathit{T}+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.75}\mathit{Cr}\left[\%\right]+0.48$$

  
• When the Cr content is less than 0.2 mass%, the steel sheet is annealed under conditions satisfying the following formula 1B.
  [Mathematical Formula 9] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63$$

  
• Alternatively, when the Cr content is more than 0.6 mass% and 1.0 mass% or less, the steel sheet is annealed under conditions satisfying the following formula 1C.
  [Mathematical Formula 10] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.93}$$

  
• In the formulas 1A, 1B, and 1C, T is 500°C or higher and the soaking temperature (°C) during annealing, t is the soaking time (seconds) during annealing, and Cr [%] is the Cr content (mass%) of the steel raw material.
• In addition, in the method for manufacturing a steel sheet according to the second embodiment, the coiled steel sheet is preferably annealed so as to satisfy the following conditions according to the Cr content in the steel raw material.
• When the Cr content is 0.6 mass% or less, the steel sheet is preferably annealed under conditions satisfying the following formula 1A.
• Alternatively, when the Cr content is more than 0.6 mass% and 1.0 mass% or less, the steel sheet is preferably annealed under conditions satisfying the formula 1C.
• Also in this case, in the formulas 1A and 1C, T is 500°C or higher and represents the soaking temperature (°C) during annealing, t is the soaking time (seconds) during annealing, and Cr [%] is the Cr content (mass%) of the steel raw material.
• In addition, in the method for manufacturing a steel sheet according to the second embodiment, the H₂ concentration (vol%) in the surrounding gas atmosphere during annealing is preferably 0 vol%.
• By performing annealing under the conditions defined by the lower limit values of the formula 1, the formula 1A, the formula 1B, and the formula 1C, the internal oxide layer can be grown well and remain to the vicinity of the width direction edge of the steel sheet. As a result, a steel sheet that can be alloyed without unevenness can be obtained. Preferably, the internal oxide layer can be grown well and remain not only from the width direction center to the width direction edge of the steel sheet but also from a front end (hereinafter, also referred to as "front end in the rolling direction") in a direction parallel to a rolling direction of the steel sheet to a rear end (hereinafter, also referred to as "rear end in the rolling direction") in the direction parallel to the rolling direction. As a result, it is possible to obtain a steel sheet that can be alloyed substantially uniformly and reliably without unevenness on substantially the entire surface of the steel sheet. It is difficult to sufficiently grow the internal oxide layer up to the width direction edge only by the heating at the time of coiling during the hot rolling described above.
• In addition, by annealing under conditions defined by the upper limit value of the formula 1 and the formula 2, or under conditions defined by the upper limit value of the formula 1A, the formula 1B, or the 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, a steel sheet having good pickling properties can be obtained without interposing the actual step of evaluating the pickling properties, so that it is not difficult to remove the scale in the subsequent pickling.
• Here, first, circumstances leading to the derivation of the formulas 1 and 2 in the first embodiment will be described.
• An amount x (g/m²) of the internal oxide layer produced by annealing is proportional to a value represented by the following formula 3, where the soaking temperature during annealing is T (°C), and the soaking time during annealing is t (sec).
  [Mathematical Formula 11] $${\mathrm{<figure-callout\; id="2"\; label="X"\; filenames="imgf0001.png"\; state="\{\{state\}\}">X</figure-callout>}}^2\propto \exp \left(-\mathrm{Q}/\mathrm{R}\left(\mathrm{T}+273\right)\right)\times \mathrm{t}$$

  
• Here, in the formula 3, R is a gas constant of 8.31 [J/(K·mol)], and Q is an activation energy of oxygen diffusion in iron = 89.5 (kJ/mol). Therefore, when these numerical values are substituted, a formula regarding the amount x (g/m²) of the internal oxide layer can be expressed as the following formula 4. In the formula 4, A is a coefficient.
  [Mathematical Formula 12] $$x^2=A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(T+273\right)}\right)\times \mathrm{t}$$

  
• Here, x² obtained by substituting conditions of the soaking temperature T of 540°C and the soaking time t of 30 hours (108,000 seconds) into the formula 4 is defined as a lower limit value as represented by the following formula 5. The definition of the lower limit value can be a condition for manufacturing a steel sheet capable of suppressing alloying unevenness. In the present specification, such a lower limit value is also simply referred to as a "lower limit value related to the alloying unevenness of the internal oxide layer" or a "lower limit value". When the soaking temperature T is too low, the internal oxide layer cannot be formed, and therefore T in the following formula 5 is 500°C or higher.
  [Mathematical Formula 13] $$\begin{array}{c}A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(540+273\right)}\right)\times 108000\le A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(T+273\right)}\right)\times \mathrm{t}\\ \mathrm{that}\mathrm{is},0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\end{array}$$

  
• In addition, x² obtained by substituting conditions of the soaking temperature T of 620°C and the soaking time t of 30 hours (108,000 seconds) into the formula 4 is defined as an upper limit value as represented by the following formula 6. The definition of the upper limit value can be defined as a condition for suppressing the generation of reduced iron and manufacturing a steel sheet having good pickling properties. In the present specification, such an upper limit value is also simply referred to as an "upper limit value related to the pickling properties of the internal oxide layer" or an "upper limit value".
  [Mathematical Formula 14] $$\begin{array}{c}A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(T+273\right)}\right)\times t\le A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(620+273\right)}\right)\times 108000\\ \mathrm{that}\mathrm{is},\mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63\end{array}$$

  
• When the formulas 5 and 6 derived in this way are put together, the formula 1 is derived. In addition, the H₂ concentration P (H₂) (vol%) in the surrounding gas atmosphere during annealing also needs to satisfy the condition of the formula 2 in relation to the soaking temperature T (°C).
• Next, circumstances leading to the derivation of the formulas 1A, 1B, and 1C in the second embodiment will be described.
• Also in the second embodiment, the method for defining the lower limit value "0.19" related to the alloying unevenness 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.
• When the Cr content is less than 0.2 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 mass%. Specifically, when the Cr content is less than 0.2 mass%, x² obtained by substituting the conditions of the soaking temperature T of 620°C and the soaking time t of 30 hours (108,000 seconds) into the formula 4 is defined as the upper limit value related to the pickling properties of the internal oxide layer as represented by the formula 6. The definition of the upper limit value can be defined as a condition for suppressing the generation of reduced iron and manufacturing a steel sheet having good pickling properties when the Cr content is less than 0.2 mass%.
• When the Cr content is more than 0.6 mass% and 1.0 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 mass%. Specifically, when the Cr content is more than 0.6 mass% and 1.0 mass% or less, x² obtained by substituting conditions of the soaking temperature T of 650°C and the soaking time t of 30 hours (108,000 seconds) into the formula 4 is defined as the upper limit value related to the pickling properties of the internal oxide layer as represented by the following formula 7. The definition of the upper limit value can be defined as a condition for suppressing the generation of reduced iron and manufacturing a steel sheet having good pickling properties when the Cr content is more than 0.6 mass% and 1.0 mass% or less.
  [Mathematical Formula 15] $$\begin{array}{c}A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(T+273\right)}\right)\times t\le A\times \mathit{exp}\left(-\frac{89.5\times {10}^3}{8.31\times \left(650+273\right)}\right)\times 108000\\ \mathrm{that}\mathrm{is},\mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.93\end{array}$$

  
• When the Cr content is 0.2 mass% or more and 0.6 mass% or less, a straight line of the upper limit value with respect to the Cr content passing through two points of an upper limit value of 0.63 when the Cr content is 0.2 mass% and an upper limit value of 0.93 when the Cr content is 0.6 mass% is defined as the upper limit value related to the pickling properties of the internal oxide layer as represented by the following formula 8. The definition of the upper limit value can be defined as a condition for suppressing the generation of reduced iron and manufacturing a steel sheet having good pickling properties when the Cr content is 0.2 mass% or more and 0.6 mass% or less.
  [Mathematical Formula 16] $$\begin{array}{l}\mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \frac{0.93-0.63}{0.6-0.2}\left(\mathit{Cr}\left[\%\right]-0.2\right)+0.63\\ \mathrm{that}\mathrm{is},\mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.75\mathit{Cr}\left[\%\right]+0.48\end{array}$$

  
• When the formulas 6, 7, and 8 derived in this way are put together, the formulas 1A, 1B, and 1C according to the Cr content in the steel raw material are derived.
• Preferably, even when the Cr content is less than 0.2 mass%, as in the case where the Cr content is 0.2 mass% or more and 0.6 mass% or less, the straight line of the upper limit value with respect to the Cr content passing through the two points of an upper limit value of 0.63 when the Cr content is 0.2 mass% and an upper limit value of 0.93 when the Cr content is 0.6 mass% may be defined as the upper limit value related to the pickling properties of the internal oxide layer as represented by the formula 8.
(Pickling)
• The annealed steel sheet is then preferably pickled. The pickling method is not particularly limited, and any known method may be applied. For example, the scale may be removed by immersion using hydrochloric acid or the like.
• According to the method for manufacturing a steel sheet according to the first embodiment, the steel sheet is annealed under the conditions defined by the upper limit value of the formula 1 and the formula 2 in the previous annealing step. Alternatively, also by the method for manufacturing a steel sheet according to the second embodiment, the steel sheet is annealed under the condition defined by the upper limit value of the formula 1A, 1B, or 1C (preferably, under the condition defined by the upper limit value of the formula 1A or 1C) according to the Cr content in the steel raw material in the previous annealing step. 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. Thus, by setting pickling conditions such as a concentration of a pickling liquid, a temperature of the pickling liquid, and a pickling time to general numerical values according to a conventional method, it is possible to easily and efficiently remove the scale adhering to the steel sheet without causing a problem that the oxidation scale remains.
• For example, when hydrochloric acid is used as the pickling liquid, a concentration of hydrochloric acid may be set to preferably 3 mass% or more, and more preferably 5 mass% or more. Furthermore, when hydrochloric acid is used as the pickling liquid, the concentration of hydrochloric acid may be set to preferably 20 mass% or less, and more preferably 15 mass% or less. In addition, for example, the temperature of the pickling liquid may be set to preferably 60°C or higher, and more preferably 70°C or higher. Furthermore, the temperature of the pickling liquid may be set to preferably 90°C or lower, and more preferably 80°C or lower. The pickling time may be appropriately adjusted according to the concentration and temperature of the pickling liquid.
(Cold rolling)
• In addition, the steel sheet after pickling may be cold-rolled. The cold rolling method is not particularly limited, and any known method may be applied. For example, in order to obtain a desired sheet thickness, a cold rolling ratio in the cold rolling can be set in a range of 10% to 70%. The sheet thickness of the steel sheet is not particularly limited.
• By including the annealing step and the optional step as described above, the steel sheet in the first embodiment or the second embodiment can be manufactured.
2. Method for manufacturing hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet
• The steel sheet manufactured by the method according to the first embodiment or the second embodiment of the present invention is suitably used as a substrate of a high-strength and high-formability hot-dip galvanized steel sheet and a substrate of a hot-dip galvannealed steel sheet, having a high Si content. Hereinafter, an example of a method for manufacturing such hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet will be described.
(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 embodiment or the second embodiment described above. First, the surface of the steel sheet is subjected to an oxidation treatment to form an Fe oxide layer on the surface of the steel sheet. In addition, the Fe oxide layer is subjected to a reduction treatment (also referred to as "reduction annealing treatment" in the present specification) under 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 inside the steel sheet. That is, by applying annealing by such an oxidation-reduction method, the Fe oxide layer becomes a barrier layer, an oxide of Si can be kept inside the steel sheet, and an increase in a solid solution Si amount in the vicinity of the surface layer of the steel sheet can be suppressed. As a result, wettability to hot-dip galvanization can be improved, and finally the alloying unevenness can be more reliably reduced.
• The oxidation treatment and the reduction treatment may be performed using any known single facility or a plurality of any known facilities. Preferably, equipment of a continuous galvanizing line (CGL) is used from the viewpoint of manufacturing efficiency, cost, and quality retention. By using the continuous galvanizing line, an oxidation treatment and a reduction treatment by the oxidation-reduction method, and a hot-dip galvanizing treatment and an alloying treatment described later can be continuously performed in a series of manufacturing lines. More specifically, the oxidation treatment and the reduction treatment by the oxidation-reduction method are more preferably performed using, for example, an annealing furnace in the continuous galvanizing line of a non-oxygen furnace (NOF) type or a direct fired furnace (DFF) type.
• The oxidation treatment is preferably applied to the surface of the steel sheet at a heating temperature as the steel sheet temperature of 750°C or lower, for example, in the oxidation heating zone in a NOF-type or DFF-type annealing furnace. When the steel sheet temperature is 750°C or lower, a hot-dip galvanized steel sheet having good plating adhesion can be obtained.
• The steel sheet temperature in the oxidation 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 oxidation treatment is not particularly limited, and may be a temperature at which the Fe oxide layer is formed on the surface of the steel sheet under a gas atmosphere described later. For example, the steel sheet temperature in the oxidation treatment is preferably 650°C or higher, and more preferably 670°C or higher.
• A temperature rise time in the oxidation treatment is preferably 10 seconds or more, and more preferably 15 seconds or more. Furthermore, for example, the temperature rise time in the oxidation treatment is preferably 120 seconds or less, and more preferably 90 seconds or less.
• The oxidation treatment is not particularly limited, and can be performed, for example, under a gas atmosphere containing O₂, CO₂, N₂, and H₂O. More specifically, the oxidation treatment can be performed in a combustion gas such as cokes oven gas (COG) or liquefied petroleum gas (LPG) in, for example, the NOF-type or DFF-type annealing furnace or the like under a gas atmosphere in which a concentration of unburned O₂ is controlled. The O₂ concentration is preferably controlled in a range of 100 ppm to 17,000 ppm. The O₂ concentration is more preferably controlled at 500 ppm or more, and still more preferably 2,000 ppm or more. Furthermore, the O₂ concentration is more preferably controlled at 15,000 ppm or less, and still more preferably 13,000 ppm or less.
• The heating temperature (soaking temperature) of the steel sheet in the reduction annealing treatment is not particularly limited, and may be performed at a temperature at which the Fe oxide layer formed by the oxidation treatment becomes the reduced Fe layer. Specifically, reduction annealing is preferably performed at a soaking temperature of preferably an Ac₃ point or higher. The Ac₃ point can be calculated by the following formula (i) ("The Physical Metallurgy of Steels, Leslie", (published by Maruzen Co., Ltd., written by William C. Leslie, p. 273)). A symbol of each element enclosed by [ ] in the formula (i) denotes the content (mass%) of the element. $$\begin{array}{l}{\mathrm{Ac}}_3\left(°\mathrm{C}\right)=910-203\times {\left[\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="imgf0003.png"\; state="\{\{state\}\}">C</figure-callout>}\right]}^{1/2}-15.2\times \left[\mathrm{Ni}\right]+44.7\times \left[\mathrm{Si}\right]+104\times \left[\mathrm{V}\right]+31.5\times \left[\mathrm{Mo}\right]+\\ 13.1\times \left[\mathrm{W}\right]-\left\{\begin{array}{l}30\times \left[\mathrm{Mn}\right]+11\times \left[\mathrm{Cr}\right]+20\times \left[\mathrm{Cu}\right]-700\times \left[\mathrm{P}\right]-400\times \left[\mathrm{Al}\right]-120\times \left[\mathrm{As}\right]-400\times \\ \left[\mathrm{Ti}\right]\end{array}\right\}\end{array}$$

  
• The heating time (soaking time) in the reduction treatment is not particularly limited, and may be appropriately adjusted so that the Fe oxide layer formed by the oxidation treatment becomes the reduced Fe layer. For example, the heating time in the reduction treatment is preferably 30 seconds or more, and more preferably 45 seconds or more. Furthermore, the heating time in the reduction treatment is preferably 600 seconds or less, and more preferably 500 seconds or less.
• The reduction annealing treatment can be performed by any known treatment method, for example, in the reduction heating zone in the NOF-type or DFF-type annealing furnace. Specifically, the reduction annealing treatment can be performed by heating the surface of the steel sheet under a reducing atmosphere mainly containing H₂ gas and an inert gas such as N₂. When a mixed gas containing H₂ gas and an inert gas such as N₂ is used, for example, the H₂ gas can be contained in a proportion of 3 vol% to 25 vol%, and an inert gas such as N₂ can be contained as the balance.
(Hot dip galvanizing treatment)
• In addition, 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 Zn-plated layer on the surface of the steel sheet.
• The method of the hot dip galvanizing treatment is not particularly limited, and any known method may be applied. For example, the Zn-plated layer can be formed on the surface of the steel sheet by immersing the steel sheet in a Zn-plating bath at a steel sheet temperature of about 400°C to 500°C. In addition, the immersion time of the steel sheet in the Zn-plating bath may be adjusted according to a desired Zn-plating adhesion amount.
(Alloying treatment)
• The method for manufacturing a hot-dip galvannealed steel sheet further includes a step of alloying the Zn-plated layer formed on the hot-dip galvanized steel sheet obtained by the above-described method.
• Specifically, by heating the hot-dip galvanized steel sheet at a predetermined alloying temperature, Fe atoms contained in the steel sheet diffuse into the Zn-plated layer, and the Zn-plated layer can be alloyed. The alloying method is not particularly limited, and any known method may be applied. The alloying temperature is not particularly limited, and can be set to, for example, preferably 480°C to 650°C. The heating time at the alloying temperature is also not particularly limited, and can be set to, for example, preferably 10 seconds to 40 seconds. In addition, the heating of the alloying can be, for example, under an air atmosphere.
3. Chemical composition of steel raw material
• The chemical composition of the steel raw material used in the method for manufacturing a steel sheet according to the first embodiment is not particularly limited except for Si. The chemical composition of the steel raw material used in the method for manufacturing a steel sheet according to the second embodiment is not particularly limited except for Si and Cr.
• Hereinafter, an example of the chemical composition of the steel raw material in the first embodiment and the second embodiment will be described.
[Si: 1 mass% or more]
• Si is an inexpensive steel reinforcing element, and hardly affects the formability of the steel sheet. In addition, Si is an element capable of suppressing generation of carbide due to decomposition of retained austenite useful for improving the formability of the steel sheet. In order to allow such an effect to be effectively exhibited, 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, and when the Si content is too large, there is a possibility that solid-solution strengthening action by Si becomes remarkable and a rolling load increases, and there is a possibility that Si scale is generated during hot rolling to cause surface defects of the steel sheet. Thus, for example, the Si content is preferably 3.0 mass% or less, more preferably 2.7 mass% or less, and still more preferably 2.5 mass% or less from the viewpoint of manufacturing stability.
[Mn: preferably 1.5 mass% or more and 3.0 mass% or less]
• Mn is also an inexpensive steel reinforcing element, similarly to Si, and is effective for improving the strength of the steel sheet. Mn is a particularly effective reinforcing element in order to ensure the tensile strength of the hot-dip galvanized steel sheet of finally 980 MPa or more by adding Si, and optionally also C, to the steel. In addition, Mn is an element that stabilizes austenite and contributes to improvement of the formability of the steel sheet by generation of retained austenite. In order to allow such an effect to be effectively exhibited, the Mn content is preferably 1.5 mass% or more, more preferably 1.8 mass% or more, and still more preferably 2.0 mass% or more. However, when the Mn content is too large, ductility of the steel sheet is reduced, which adversely affects the formability of the steel sheet, and the weldability of the steel sheet may be reduced. From such a viewpoint, the Mn content is preferably 3.0 mass% or less, more preferably 2.8 mass% or less, and still more preferably 2.7 mass% or less.
[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, and is a particularly effective reinforcing element in order to ensure the tensile strength of the hot-dip galvanized steel sheet of finally 980 MPa or more by adding Si, and optionally also Mn, to the steel. Furthermore, C is an element necessary for securing retained austenite and improving the formability. In order to allow such an effect to be effectively exhibited, the C content is preferably 0.08 mass% or more, more preferably 0.11 mass% or more, and still more preferably 0.13 mass% or more. From the viewpoint of ensuring the strength of the steel sheet, it is preferable that the C content is large; however, when the C content is too large, corrosion resistance, spot weldability, and formability may deteriorate. Thus, the C content is preferably 0.30 mass% or less, more preferably 0.25 mass% or less, and still more preferably 0.20 mass% or less.
[P: preferably more than 0 mass% and 0.1 mass% or less]
• P is an element inevitably present as an impurity element. When the P content is excessive, the weldability may be deteriorated. Thus, the P content is preferably 0.1 mass% or less, more preferably 0.08 mass% or less, and still more preferably 0.05 mass% or less.
[S: preferably more than 0 mass% and 0.05 mass% or less]
• S is an element inevitably present as an impurity element. Usually, steel inevitably contains S in an amount of about 0.0005 mass%. When the S content is excessive, a sulfide-based inclusion is formed, hydrogen absorption is promoted under a corrosive environment, delayed fracture resistance of the steel sheet is deteriorated, and the weldability and formability of the steel sheet may be deteriorated. Thus, the S content is preferably 0.05 mass% or less, more preferably 0.01 mass% or less, and still more preferably 0.005 mass% or less.
[Al: preferably more than 0 mass% and 1.0 mass% or less]
• Al is an element having a deoxidizing action. In order to allow such an effect to be effectively exhibited, the Al content is preferably more than 0 mass%, more preferably 0.005 mass% or more, and still more preferably 0.02 mass% or more. When the Al content is excessive, inclusions such as alumina increase, and the formability of the steel sheet may deteriorate. Thus, the Al content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and still more preferably 0.5 mass% or less.
[Cr: preferably more than 0 mass% and 1.0 mass% or less]
• Cr is an element effective for improving the strength of the steel sheet. Furthermore, Cr is an element that improves the corrosion resistance of the steel sheet, and has an action of suppressing generation of hydrogen due to corrosion of the steel sheet. Specifically, Cr has an action of promoting the production of iron oxide (α-FeOOH). Iron oxide is said to be thermodynamically stable and protective among rusts produced in the atmosphere. By promoting production of such rust, it is possible to suppress intrusion of generated hydrogen into the steel sheet, and it is possible to sufficiently suppress assisted cracking due to hydrogen even when the steel sheet is used under a severe corrosive environment, for example, in the presence of chloride. Since Cr is an element effective for the delayed fracture resistance of the steel sheet similarly to B and Ti, Cr can be added in an amount that does not affect formability such as the strength and elongation of the steel sheet. In order to allow these effects to be effectively exhibited, the Cr content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and still more preferably 0.01 mass% or more. On the other hand, when the Cr content is excessive, formability such as the elongation of the steel sheet may deteriorate. Thus, the Cr content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and still more preferably 0.6 mass% or less.
• In the method for manufacturing a steel sheet according to the first embodiment, the Cr content is preferably more than 0 mass% and 0.4 mass% or less, more preferably 0.1 mass% or more and 0.3 mass% or less, still more preferably 0.2 mass% or more and 0.3 mass% or less, and particularly preferably 0.2 mass% in order to more reliably obtain a good effect on the pickling properties.
• On the other hand, in the method for manufacturing a steel sheet according to the second embodiment, the Cr content may be 1 mass% or less. Specifically, by adjusting the soaking temperature T, the soaking time t, and the Cr content so as to satisfy a predetermined relational expression according to the Cr content, a good effect on the pickling properties can be obtained.
[Cu: preferably more than 0 mass% and 1.0 mass% or less]
• Similarly to Cr, Cu is an element that is effective for improving the strength of the steel sheet, has the action of suppressing generation of hydrogen due to corrosion of the steel sheet, and improves the corrosion resistance of the steel sheet. Cu also has an action of promoting the production of iron oxide, similarly to Cr. In order to allow these effects to be effectively exhibited, the Cu content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and still more preferably 0.05 mass% or more. From the viewpoint of the formability of the steel sheet, the Cu content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and still more preferably 0.5 mass% or less.
[Ni: preferably more than 0 mass% and 1.0 mass% or less]
• Similarly to Cr and Cu, Ni is an element that is effective for improving the strength of the steel sheet, has the action of suppressing generation of hydrogen due to corrosion of the steel sheet, and improves the corrosion resistance of the steel sheet. Ni also has the action of promoting the production of iron oxide, similarly to Cr and Cu. In order to allow these effects to be effectively exhibited, the Ni content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and still more preferably 0.05 mass% or more. From the viewpoint of the formability of the steel sheet, the Ni content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less, and still more preferably 0.5 mass% or less.
[Ti: preferably more than 0 mass% and 0.15 mass% or less]
• Similarly to Cr, Cu, and Ni, Ti is an element that is effective for improving the strength of the steel sheet, has the action of suppressing generation of hydrogen due to corrosion of the steel sheet, and improves the corrosion resistance of the steel sheet. Ti also has the action of promoting the production of iron oxide, similarly to Cr, Cu, and Ni. Since Ti is an element effective for the delayed fracture resistance of the steel sheet similarly to B and Cr, Ti can be added in an amount that does not affect formability such as the strength and elongation of the steel sheet. In order to allow these effects to be effectively exhibited, the Ti content is preferably more than 0 mass%, more preferably 0.003 mass% or more, and still more preferably 0.05 mass% or more. From the viewpoint of the formability of the steel sheet, the Ti content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and still more preferably 0.10 mass% or less.
[Nb: preferably more than 0 mass% and 0.15 mass% or less]
• Nb is an element that is effective for improving the strength of the steel sheet, and acts on improving the toughness of the steel sheet by miniaturizing austenite grains after quenching. In order to allow such an effect to be effectively exhibited, the Nb content is preferably more than 0 mass%, more preferably 0.03 mass% or more, and still more preferably 0.005 mass% or more. On the other hand, when the Nb content is excessive, a large amount of carbide, nitride, or carbonitride is generated, and the formability or delayed fracture resistance of the steel sheet may deteriorate. Thus, the Nb content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and still more preferably 0.10 mass% or less.
[V: preferably more than 0 mass% and 0.15 mass% or less]
• Similarly to Nb, V is an element that is effective for improving the strength of the steel sheet, and acts on improving the toughness of the steel sheet by miniaturizing austenite grains after quenching. In order to allow such an effect to be effectively exhibited, the V content is preferably more than 0 mass%, more preferably 0.03 mass% or more, and still more preferably 0.005 mass% or more. On the other hand, when the V content is excessive, similarly to Nb, a large amount of carbide, nitride, or carbonitride is generated, and the formability or delayed fracture resistance of the steel sheet may deteriorate. Thus, the V content is preferably 0.15 mass% or less, more preferably 0.12 mass% or less, and still more preferably 0.1 mass% or less.
[B: preferably more than 0 mass% and 0.005 mass% or less]
• B is an element useful for improving the hardenability and weldability of the steel sheet. Since B is an element effective for the delayed fracture resistance of the steel sheet similarly to Ti and Cr, B can be added in an amount that does not affect formability such as the strength and elongation of the steel sheet. In order to allow these effects to be effectively exhibited, the B content is preferably more than 0 mass%, more preferably 0.0002 mass% or more, still more preferably 0.0003 mass% or more, and particularly preferably 0.0004 mass% or more. On the other hand, when the B content is excessive, such an effect may be saturated, and the ductility may be reduced to deteriorate the formability. Thus, the B content is preferably 0.005 mass% or less, more preferably 0.004 mass% or less, and still more preferably 0.003 mass% or less.
[N: preferably more than 0 mass% and 0.01 mass% or less]
• N is an element inevitably present as an impurity element. When the N content is excessive, a nitride may be formed to deteriorate the formability of the steel sheet. In particular, when the steel sheet contains B for improving the hardenability, N bonds with B to form a BN precipitate, and inhibits the hardenability improving action of B. Thus, the N content is preferably 0.01 mass% or less, more preferably 0.008 mass% or less, and still more preferably 0.005 mass% or less.
• In the chemical composition of the steel raw material in the first embodiment and the second embodiment of the present invention, in addition to the above components, other known optional components may be further contained as long as the strength and sufficient formability are not impaired.
[Balance]
• The balance is Fe and inevitable impurities. It is permitted to mix, as inevitable impurities, trace elements (e.g., As, Sb, Sn, etc.) incorporated according to the conditions of raw materials, materials, manufacturing facilities and the like. P, S, and N as described above are usually preferred as the content is smaller, and thus can be said to be inevitable impurities. However, these elements are defined as described above since the present invention can exert its effect by suppressing the content thereof to a specific range. Thus, in the present specification, "inevitable impurities" constituting the balance mean the concept excluding elements whose composition range is defined.
• According to the method for manufacturing a steel sheet according to the first embodiment and the second embodiment of the present invention, it is possible to obtain a steel sheet that achieves both suppression of the alloying unevenness and good pickling properties despite the Si content being 1 mass% or more. In particular, in the step of manufacturing the steel sheet, there is no need to include a step of evaluating the pickling properties for the scale of 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 other steps.
• Specifically, according to the method for manufacturing a steel sheet according to the first embodiment, it is possible to efficiently obtain a steel sheet having the above-described effect only by setting the conditions of the soaking temperature T, the soaking time t, and the H₂ concentration P (H₂) in the surrounding gas atmosphere during annealing so as to satisfy a predetermined relational expression defined in advance.
• According to the method for manufacturing a steel sheet according to the second embodiment, it is possible to efficiently obtain a steel sheet having the above-described effect only by setting the conditions of the soaking temperature T and the soaking time t during annealing so as to satisfy a predetermined relational expression defined in advance according to the Cr content.
• In addition, as described above, when a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet are manufactured using a steel sheet manufactured by the method according to the first embodiment or the second embodiment, using a continuous galvanizing line, 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 more inexpensively and efficiently manufacture a high-strength and high-formability hot-dip galvannealed steel sheet having no alloying unevenness while maintaining the quality of a product. Specifically, the hot-dip galvannealed steel sheet thus manufactured may have a tensile strength of 980 MPa or more.
• Although the overview of the present invention has been described above, the method for manufacturing a steel sheet according to the embodiment of the present invention is summarized as follows.
• A method for manufacturing a steel sheet according to a first aspect of the present invention includes a step of annealing a steel raw material having a Si content of 1.0 mass% or more under a condition satisfying the following formula 1:
  [Mathematical Formula 17] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63$$

  
• and the following formula 2:
  [Mathematical Formula 18] $$\mathrm{<figure-callout\; id="2"\; label="P\; H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}\left({\mathrm{H}}_{}\right)\left({\mathrm{}}_2\right)<-\frac{1}{25}\mathit{T}+25$$
  
• (in the formulas 1 and 2, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and P (H₂) is an H₂ concentration (vol%) in a surrounding gas atmosphere during annealing).
• Alternatively, a method for manufacturing a steel sheet according to another first aspect of the present invention includes a step of annealing a steel raw material having an Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less, under a condition satisfying: the following formula 1A if the Cr content of the steel raw material is 0.2 mass% or more and 0.6 mass% or less:
  [Mathematical Formula 19] $$0.19\le \mathit{exp}\left(-\frac{10770}{\mathit{T}+273}\right)\ast \mathrm{t}\le 0.75\mathit{Cr}\left[\%\right]+0.48$$

  
• the following formula 1B if the Cr content of the steel raw material is less than 0.2 mass%:
  [Mathematical Formula 20] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63$$
  
• or the following formula 1C if the Cr content of the steel raw material is more than 0.6 mass% and 1.0 mass% or less:
  [Mathematical Formula 21] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}\mathrm{.93}$$
  
• (in the formulas 1A, 1B, and 1C, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and Cr [%]is a Cr content (mass%) of the steel raw material).
• The method for manufacturing a steel sheet according to another first aspect described above preferably includes a step of annealing a steel raw material having an Si content of 1.0 mass% or more and a Cr content of 1.0 mass% or less, under a condition satisfying: the following formula 1A if the Cr content of the steel raw material is 0.6 mass% or less:
  [Mathematical Formula 22] $$0.19\le \mathit{exp}\left(-\frac{10770}{\mathit{T}+273}\right)\ast \mathrm{t}\le 0.75\mathit{Cr}\left[\%\right]+0.48$$

  
• or the following formula 1C if the Cr content of the steel raw material is more than 0.6 mass% and 1.0 mass% or less:
  [Mathematical Formula 23] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le \mathrm{0}.93$$
  
• (in the formulas 1A and 1C, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and Cr [%] is the Cr content (mass%) of the steel raw material).
• The method for manufacturing a steel sheet described above preferably further includes a step of hot rolling the steel raw material before the annealing and coiling a steel raw sheet at 500°C to 700°C.
• The method for manufacturing a steel sheet described above more preferably further includes a step of pickling the steel sheet after the annealing and then cold rolling the steel sheet.
Examples
• Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited by the examples at all.
(Example 1)
• In Example 1, the lower limit value of the amount x (g/m²) of the internal oxide layer capable of suppressing the alloying unevenness was determined.
• Specifically, a steel sheet was actually manufactured using a steel raw material having a Si content of 1.0 mass% or more, and a relevance between the solid solution Si amount (wt%) (specifically, an average value (wt%) of the solid solution Si amount) up to a depth of 1 µm from the surface of the steel sheet, the amount (g/m²) of the internal oxide layer, and an effect of suppressing the alloying unevenness was examined.
• First, a steel material (steel grade A) having a chemical composition shown in Table 1 below was produced by a converter, and then a slab was manufactured by continuous casting. The obtained slab was hot-rolled to a sheet thickness of 2.0 mm at a finish rolling end temperature of 900°C, and coiled at 640°C, and the obtained hot-rolled steel sheet was cooled to normal temperature. Thereafter, the hot-rolled steel sheet was charged into an annealing furnace and annealed. As annealing conditions, in a non-reducing atmosphere of N₂-0.5 vol% H₂, the temperature of the hot-rolled steel sheet was increased to 580°C for about 8.5 hours, soakingly retained at 580°C for 30 hours, and then cooled to 200°C or less for about 5 hours. Thereafter, the annealed steel sheet obtained was pickled by immersing the steel sheet at 85°C for 40 seconds using hydrochloric acid having a concentration of 8 wt%. Finally, cold rolling was performed until the steel sheet thickness was changed from 2.0 mm to 1.4 mm to obtain a target steel sheet. [Table 1]

  	Chemical composition (wt%)
  Steel grade	Si	Mn	C	P	S	Al	Cr
  A	1.83	2.00	0.22	0.009	0.0005	0.05	0.2

• First, a test piece of 20 mm × 20 mm × 1.4 mm (sheet thickness) was cut out from various positions in the obtained steel sheet by a shear cutting machine. Thereafter, for each test piece, the solid solution Si amount (wt%) from the surface of the steel sheet to a depth of 1 µm, specifically, the average value (wt%) of the solid solution Si amount was measured. The solid solution Si amount on the surface of the steel sheet was measured using a fully automatic scanning X-ray photoelectron spectrometer ("Quantera-SXM" manufactured by ULVAC-PHI, Incorporated.). The measurement conditions were an X-ray output of 24.2 W, an X-ray beam diameter of 100 µm, and an analysis position of 1 µm in depth. Specifically, the calculation was performed using the following formula. That is, a ratio of peak area intensity to {Si(SiO_x)+Si(Si-Si,Fe-Si)} of Si(Si-Si,Fe-Si) was determined, and the solid solution Si amount (wt%) was calculated by multiplying the determined ratio by the actual Si content in steel. $$\begin{array}{l}\mathrm{Solid}\mathrm{solution}\mathrm{Si}\mathrm{amount}\left(\mathrm{wt}\%\right)=\left[\mathrm{Si}\left(\mathrm{Si}-\mathrm{Si},\mathrm{Fe}-\mathrm{Si}\right)/\left\{\mathrm{Si}\left({\mathrm{SiO}}_{\mathrm{x}}\right)+\mathrm{Si}\left(\mathrm{Si}-\mathrm{Si},\mathrm{Fe}-\mathrm{Si}\right)\right\}\right]\times \mathrm{Si}\mathrm{content}\\ \mathrm{in}\mathrm{steel}\end{array}$$

  
• In addition, at the same time, the amount (g/m²) of the internal oxide layer of the test piece for which the solid solution Si amount (wt%) was measured was measured. Specifically, the cut test piece was immersed under a condition of a temperature of 80°C using hydrochloric acid having a concentration of 10 mass%, and the dissolved amount (g/m²) per unit area was measured.
• The graph of FIG. 1 shows a correlation between the solid solution Si amount (wt%) and the amount of the internal oxide layer (g/m²) measured in this manner. Here, the following formula indicated by a broken line in the graph of FIG. 1 is a formula derived by regression analysis. R is a correlation coefficient. $$\begin{array}{l}\mathrm{y}\left(\mathrm{solid}\mathrm{solution}\mathrm{Si}\mathrm{amount}\left(\mathrm{wt}\%\right)\right)=-0.1169\times \left(\mathrm{amount}\left(\mathrm{g}/{\mathrm{<figure-callout\; id="2"\; label="m"\; filenames="imgf0001.png"\; state="\{\{state\}\}">m</figure-callout>}}^2\right)\mathrm{of}\mathrm{internal}\mathrm{oxide}\mathrm{layer}\right)+\\ 1.8723\left({\mathrm{<figure-callout\; id="2"\; label="R"\; filenames="imgf0001.png"\; state="\{\{state\}\}">R</figure-callout>}}^2=0.997\right)\end{array}$$

  
• Next, in order to examine a relationship between the solid solution Si amount (wt%) and the amount (g/m²) of the internal oxide layer and the effect of suppressing the alloying unevenness, a hot-dip galvannealed steel sheet was produced from the obtained steel sheet. First, the obtained steel sheet was subjected to oxidation treatment, reduction treatment, hot-dip galvanizing treatment, and alloying treatment by applying the continuous galvanizing line having the NOF-type annealing furnace. In the oxidation treatment, the steel sheet was heated to a steel sheet temperature of about 710°C (680°C to 730°C) in a temperature rise time of 45 seconds under a combustion exhaust gas atmosphere containing less than 17,000 ppm of O₂ and CO₂, N₂, and H₂O. Here, the "steel sheet temperature" means a maximum reached sheet temperature of the steel sheet whose heating is controlled in the NOF which is an oxidation heating zone. The reduction treatment was performed by heating at a soaking temperature of about 800°C (770°C to 820°C) for 50 seconds under a gas atmosphere of N₂-H₂. In the hot-dip galvanizing treatment, the steel sheet after reduction was immersed in the Zn-plating bath at 430°C to form a hot-dip galvanizing layer. The hot-dip galvanized steel sheet was thus obtained, and then a hot-dip galvannealed steel sheet was obtained by alloying treatment.
• In addition, whether or not the alloying unevenness was suppressed was evaluated for the hot-dip galvannealed steel sheet thus obtained. Specifically, an appearance of the obtained hot-dip galvannealed steel sheet was visually observed, and a case where Zn-Fe alloying proceeded and metallic luster of Zn disappeared was evaluated as "O". On the other hand, a case where the metallic luster of Zn remained was evaluated as "×".
• As a result of evaluating the alloying unevenness, it was found that when the solid solution Si amount from the surface of the steel sheet to a depth of 1 µm was 1.36 wt% or less, the alloying unevenness could be suppressed at a portion of the surface of the steel sheet, which was the solid solution Si amount. As can be seen from the graph of FIG. 1, the fact that the solid solution Si amount is 1.36 wt% or less corresponds to the fact that the amount of the internal oxide layer is 4.4 g/m² or more. That is, it was found that when the amount of the internal oxide layer was 4.4 g/m² or more, the alloying unevenness could be suppressed at a surface portion of the steel sheet indicating the amount of the internal oxide layer.
(Example 2)
• Next, an example of the method for manufacturing a steel sheet from which "0.19", which is the lower limit value of the formula 1, is derived will be described in detail.
• In Example 2, first, a steel sheet was manufactured in the same manner as in Example 1 except that the coiling temperature in hot rolling was 550°C, the soaking temperature during annealing was 540°C, and the soaking time during annealing was 30 hours (108,000 seconds). In addition, the amount (g/m²) of the internal oxide layer of the test piece at a predetermined position of the steel sheet was measured in the same manner as in Example 1. In Example 2, a steel sheet was manufactured using not only the steel material of the steel grade A but also a steel material of a steel grade B shown in Table 2 below, and the amount (g/m²) of the internal oxide layer was measured. The test piece of the steel sheet was cut out at a position of 10 m from a front end of the steel sheet in the rolling direction and at a position of 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.
• As a result, the test piece at any position exceeded the lower limit value (that is, 4.4 g/m²) of the amount of the internal oxide layer capable of suppressing the alloying unevenness calculated in Example 1 described above. This means that the steel sheet manufactured in Example 2 can suppress the alloying unevenness particularly in the coil width direction. In addition, x² obtained by substituting such conditions during annealing in Example 2 into the formula 4 is the square of the amount of the internal oxide layer. Therefore, it means that x² obtained by substituting the conditions into the formula 4 can be defined as a lower limit value related to the alloying unevenness of the internal oxide layer as represented by the formula 5. This is based on the finding that when the amount of the internal oxide layer is too small, the solid solution Si amount in the vicinity of the surface of the steel sheet increases, and the alloying unevenness occurs. In the following Table 3, the results of Example 2 are collectively shown. [Table 2]

  	Chemical composition (wt%)
  Steel grade	Si	Mn	C	P	S	Al	Cr
  B	1.85	2.10	0.20	0.005	0.0005	0.04	Less than 0.01

  [Table 3]

  	Steel grade	Amount of internal oxide layer (g/m2) (after pickling) (Coiling temperature in hot rolling: 550°C, annealing conditions: soaking temperature 540°C × soaking time 30 hours)	Evaluation of alloying unevenness based on results of Example 1
  0 mm to 20 mm from edge in coil width direction	A	4.5	○
  20 mm to 40 mm from edge in coil width direction	A	5.3	○
  40 mm to 60 mm from edge in coil width direction	A	6.6	○
  60 mm to 80 mm from edge in coil width direction	A	6.3	○
  0 mm to 20 mm from edge in coil width direction	B	5.8	○
  20 mm to 40 mm from edge in coil width direction	B	9.7	○
  40 mm to 60 mm from edge in coil width direction	B	9.4	○
  60 mm to 80 mm from edge in coil width direction	B	9.9	○

(Example 3)
• In Example 3, an upper limit value of a reduced iron area ratio (%) with respect to an oxidation scale area of the test piece in the vicinity of the width direction edge of the annealed steel sheet (hereinafter, the reduced iron area ratio (%) is also simply referred to as "reduced iron area ratio (%)") was determined in order to have a good pickling property effect.
• Specifically, various steel sheets before pickling were manufactured in the same manner as in Example 1 except that the soaking temperature and the soaking time during annealing were changed. Thereafter, the reduced iron area ratio (%) with respect to the oxidation scale area of the test piece in the vicinity of the width direction edge of each annealed steel sheet obtained was measured. Specifically, the test piece in the vicinity of the width direction edge of the steel sheet was cut out from a portion of 0 mm to 100 mm from the width direction edge where a position in the direction parallel to the rolling direction of the steel sheet was random.
• Specifically, the reduced iron area ratio was measured by binarizing a scale image observed in a cross-sectional SEM image of the test piece by Otsu's method and calculating an area ratio of a group having a large luminance to the entire scale. As a reference, a grain boundary oxidation depth (µm) in the internal oxide layer was also measured at the same time. Specifically, similarly, using a surface image of the test piece observed in a cross-sectional SEM image, the grain boundary oxidation depth from five random points in a direction horizontal to a surface of the test piece was measured, and the average value thereof was calculated. In general, the relationship is established where when the grain boundary oxidation depth (µm) in the internal oxide layer increases, that is, the amount (g/m²) of the internal oxide layer increases, the reduced iron area ratio (%) increases.
• Thereafter, each annealed steel sheet obtained was pickled by immersing the steel sheet at 80°C for 40 seconds using hydrochloric acid having a concentration of 10 wt%. After pickling, a state of reduced iron remaining in each test piece in the vicinity of the width direction edge of the steel sheet was visually observed. Then, a case where the reduced iron did not remain was evaluated as"○", a case where the reduced iron was peeled off by shaking in a pickling liquid was evaluated as "△", and a case where the reduced iron remained was evaluated as " × " to evaluate the pickling properties. In addition, regarding such evaluation, the steel sheet having an evaluation result of "○" was used as an example of the present invention. These results are shown in Table 4 below together with the results of the reduced iron area ratio (%) and the grain boundary oxidation depth (µm) measured before pickling. In FIG. 2, the evaluation test of the pickling properties in Table 4 was graphed. [Table 4]

  	Category	Reduced iron area ratio (%)	Grain boundary oxidation depth (µm)	Evaluation of pickling properties		Category	Reduced iron area ratio (%)	Grain boundary oxidation depth (µm)	Evaluation of pickling properties
  Test No. 1	Present invention example	21	9.8	○	Test No. 26	Present invention example	14	4	○
  Test No. 2	Comparative Example	53	10	×	Test No. 27	Present invention example	16	6.9	○
  Test No. 3	Comparative Example	49	9.7	Δ	Test No. 28	Present invention example	8	6.5	○
  Test No. 4	Comparative Example	47	7.9	Δ	Test No. 29	Present invention example	42	7.6	○
  Test No. 5	Comparative Example	49	12.8	Δ	Test No. 30	Present invention example	37	6.7	○
  Test No. 6	Comparative Example	47	10.7	Δ	Test No. 31	Present invention example	14	6.8	○
  Test No. 7	Present invention example	43	10.9	○	Test No. 32	Present invention example	43	5.7	○
  Test No. 8	Present invention example	17	12.7	○	Test No. 33	Present invention example	15	4.7	○
  Test No. 9	Present invention example	29	15.2	○	Test No. 34	Present invention example	17	9.9	○
  Test No. 10	Present invention example	24	13.6	○	Test No. 35	Present invention example	18	10.7	○
  Test No. 11	Present invention example	31	14.8	○	Test No. 36	Present invention example	29	13.2	○
  Test No. 12	Present invention example	25	13.7	○	Test No. 37	Present invention example	22	12.2	○
  Test No. 13	Present invention example	21	13.6	○	Test No. 38	Present invention example	18	9	○
  Test No. 14	Present invention example	19	13.3	○	Test No. 39	Present invention example	13	8.6	○
  Test No. 15	Present invention example	26	11.6	○	Test No. 40	Present invention example	12	10.6	○
  Test No. 16	Present invention example	24	11.7	○	Test No. 41	Present invention example	13	7.9	○
  Test No. 17	Comparative Example	60	18.3	×	Test No. 42	Present invention example	14	9.5	○
  Test No. 18	Comparative Example	62	12.9	×	Test No. 43	Present invention example	16	11.2	○
  Test No. 19	Comparative Example	48	14.7	×	Test No. 44	Present invention example	40	9.2	○
  Test No. 20	Comparative Example	60	14.2	×	Test No. 45	Present invention example	45	11.1	○
  Test No. 21	Comparative Example	47	16.4	×	Test No. 46	Present invention example	37	7.4	○
  Test No. 22	Comparative Example	52	14.5	×	Test No. 47	Present invention example	44	8.7	○
  Test No. 23	Comparative Example	67	11.9	×	Test No. 48	Present invention example	27	10.1	○
  Test No. 24	Comparative Example	57	11.4	×	Test No. 49	Present invention example	35	9.4	○
  Test No. 25	Present invention example	7	6.7	○

• From these evaluation results of the pickling properties, it was found that when the reduced iron area ratio (%) with respect to the oxide scale area in the vicinity of the width direction edge of the annealed steel sheet was less than 45%, the steel sheet had good pickling properties. As described above, since reduced iron is more likely to be generated in the vicinity of the width direction edge of the steel sheet than in the vicinity of the width direction center, good pickling properties are provided in the vicinity of the width direction edge, so that good pickling properties are provided in the entire steel sheet.
(Example 4)
• Next, an example of the method for manufacturing a steel sheet that derives "0.63" which is the upper limit value of the formula 1 and the formula 2 in the first embodiment will be described in detail.
• In Example 4, various steel sheets before pickling were manufactured in the same manner as in Example 1 except that with respect to the annealing conditions, the soaking time was set to 30 hours (108,000 seconds), the soaking temperature, and the H₂ concentration in the surrounding gas atmosphere during annealing were changed. In addition, the reduced iron area ratio (%) was measured by the same method as in Example 3. In addition, from Example 3 described above, it was found that the steel sheet had good pickling properties when the reduced iron area ratio (%) was less than 45%, and therefore, the pickling properties of each steel sheet were evaluated based on this result. These results, together with annealing conditions, are shown in Table 5 below. [Table 5]

  	Category	Steel grade	Cr content (mass%)	Annealing condition Soaking temperature (°C) × soaking time (hours)	H2concentration (vol%)	Reduced iron area ratio (%)	Evaluation of pickling properties based on results of Example 3
  Test No. 50	Present invention example	A	0.2	590 × 30	1	15	○
  Test No. 51	Present invention example	A	0.2	610 × 30	0	26	○
  Test No. 52	Present invention example	A	0.2	610 × 30	0.3	33	○
  Test No. 53	Comparative Example	A	0.2	610 × 30	1	47	×
  Test No. 54	Present invention example	A	0.2	620 × 30	0	26	○
  Test No. 55	Comparative Example	A	0.2	630 × 30	0	57	×

• As shown in Table 5 above, in Test No. 54 having a soaking temperature of 620°C and a soaking time of 30 hours (108,000 seconds), the measured reduced iron area ratio was below the upper limit value (that is, less than 45%) of the reduced iron area ratio that can have good pickling properties, calculated in Example 3 described above. Therefore, it means that the steel sheet manufactured in Test No. 54 has good pickling properties. In addition, x² obtained by substituting such conditions during annealing in Test No. 54 into the formula 4 is the square of the amount of the internal oxide layer. Thus, x² obtained by the substitution can be defined as the upper limit value related to the pickling properties of the internal oxide layer as represented by the formula 6. This is based on the finding that when the amount of the internal oxide layer is increased by further increasing the soaking temperature, more reduced iron is generated, and good pickling properties cannot be obtained.
• In addition, FIG. 3 is a graph obtained by plotting the soaking temperature during annealing and the H₂ concentration in the surrounding gas atmosphere in Table 5 above. Here, in FIG. 3, the annealing conditions in Test Nos. 50 to 55 are plotted together with the evaluation results of the pickling properties based on the results of Example 3 described above. Specifically, when the reduced iron area ratio exhibiting good pickling properties is less than 45%, it is plotted as "O", and when the reduced iron area ratio not exhibiting good pickling properties is 45% or more, it is plotted as "×".
• The formula 2, which is a relational expression between the H₂ concentration P (H₂) (vol%) in the surrounding gas atmosphere during annealing and the soaking temperature T (°C) shown in the graph of FIG. 3, which is a boundary line of these evaluation results, is derived from a straight line connecting a point where the H₂ concentration P is 0 vol% and the soaking temperature T is 625°C and a point where the H₂ concentration P is 1 vol% and the soaking temperature T is 600°C in the graph. The reason for selecting these points is as follows. Under a condition of the soaking temperature T of 625°C between Test Nos. 54 and 55 in which the pickling property evaluation is divided when the H₂ concentration P is 0%, the reduced iron area ratio is also assumed to be about 42 which is the average of both tests. Under a condition of the soaking temperature T of 600°C between Test Nos. 50 and 53 in which the pickling property evaluation is divided when the H₂ concentration P is 1%, the reduced iron area ratio is also assumed to be about 31 which is the average of both tests. All of these values correspond to the numerical value of less than 45% indicating good pickling properties. Therefore, by using these conditions, the relational expression between the H₂ concentration P (H₂) and the soaking temperature T can be derived.
• Alternatively, as an alternative to the formula 2, it can also be defined by the following formula 2' using the results of Test No. 50 (H₂ concentration P is 1 vol%, and soaking temperature T is 590°C) and Test No. 54 (H₂ concentration P is 0 vol%, and soaking temperature T is 620°C).
  [Mathematical Formula 24] $$\mathrm{<figure-callout\; id="2"\; label="P\; H"\; filenames="imgf0001.png"\; state="\{\{state\}\}">P</figure-callout>}\left({\mathrm{H}}_{}\right)\left({\mathrm{}}_2\right)<-\frac{1}{30}T+\frac{62}{3}$$

  
• As can be seen from Examples 1 to 4 described above, in the method for manufacturing a steel sheet, by setting the soaking temperature T, the soaking time t, and the H₂ concentration P (H₂) in the surrounding gas atmosphere in the annealing step so as to satisfy the formula 1 and the formula 2 (or the formula 2'), it is possible to efficiently obtain a steel sheet that achieves both suppression of the alloying unevenness and good pickling properties.
(Example 5)
• In Example 5, an example of the method for manufacturing a steel sheet that derives "0.19" which is the lower limit value of the formulas 1A, 1B, and 1C in the second embodiment will be described in detail. In addition, an example of the method for manufacturing a steel sheet that derives "0.75 Cr [%]+0.48" which is the upper limit value of the formula 1A, "0.63" which is the upper limit value of the formula 1B, and "0.93" which is the upper limit value of the formula 1C in the second embodiment will be described in detail.
• First, the lower limit value related to the alloying unevenness of the internal oxide layer in the formulas 1A, 1B, and 1C can be defined as "0.19" based on the result of Example 2 described above as in the first embodiment regardless of the Cr content.
• In Example 5, not only the steel material of the steel grade A shown in Table 1 above used in Examples 1 to 4 described above, but also a steel material of a steel grade C having a different Cr content shown in Table 6 below were used. With respect to the annealing conditions, the soaking time was set to 30 hours (108,000 seconds), the H₂ concentration in the surrounding gas atmosphere during annealing was set to 0 vol%, and the soaking temperature was changed in each test to manufacture various steel sheets before pickling. Other detailed methods are similar to those in Example 4 described above. As can be seen from Table 7 shown later, the tests using the steel material of the steel grade A having a Cr content of 0.2 mass% are Test Nos. 54 and 55 shown in Table 5 above of Example 4 described above. [Table 6]

  	Chemical composition (wt%)
  Steel grade	Si	Mn	C	P	S	Al	Cr
  C	1.18	2.21	0.22	0.009	0.001	0.046	0.6

• Next, a decarburization amount (mg/cm²) in the annealed steel sheet prepared was measured. The decarburization amount was confirmed from a carbon concentration profile in a depth direction of the surface of the test piece of each steel sheet using a glow discharge optical emission spectrometer. Specifically, first, a carbon amount was confirmed for a portion where a carbon amount was 90% or less of a steel sheet base material at a position deeper than an interface between an oxide film and the steel material. Then, a difference between the carbon amount in the portion and the carbon amount of the steel sheet base material was obtained, and from this result, the amount of carbon lost per unit area by each steel sheet was calculated as the decarburization amount (mg/cm²).
• The reduced iron area ratios (%) of Test Nos. 54 and 55 are 26% and 57%, respectively, as shown in Table 5 above. Therefore, based on these values, an estimated value of the reduced iron area ratio (%) was calculated from the values of the decarburization amount (mg/cm²) measured in Test Nos. 56 to 58 by using the following formula. As can be seen from the following formula, by suppressing decarburization, the reduced iron area ratio can also be reduced. A detailed description thereof will be given later. $$\mathrm{Reduced}\mathrm{iron}\mathrm{area}\mathrm{ratio}\left(\%\right)\left(\mathrm{estimated}\mathrm{value}\right)=\left(57-26\right)/\left(13.72-4.84\right)\times \left(\begin{array}{l}\mathrm{decarburization}\\ \mathrm{amount}\left(\mathrm{mg}/{\mathrm{<figure-callout\; id="2"\; label="cm"\; filenames="imgf0001.png"\; state="\{\{state\}\}">cm</figure-callout>}}^2\right)-4.84\end{array}\right)+26$$

  
• In addition, from Example 3 described above, it was found that when the reduced iron area ratio (%) was less than 45%, the steel sheet had good pickling properties. Thus, based on this result, the pickling properties of various steel sheets were also evaluated. These results, together with annealing conditions and the like, are shown in Table 7 below. As described above, the numerical value shown in (*) in Table 7 below indicates an estimated value, not an actual measured value. [Table 7]

  	Category	Steel grade	Cr content (mass%)	Annealing condition Soaking temperature (°C) × soaking time (hours)	H2concentration (vol%)	Decarburization amount (mg/cm2)	Reduced iron area ratio (%)	Evaluation of pickling properties based on results of Example 3
  Test No. 54	Present invention example	A	0.2	620 × 30	0	4.84	26	○
  Test No. 55	Comparative Example	A	0.2	630 × 30	0	13.72	57	×
  Test No. 56	Present invention example	C	0.6	540 × 30	0	1.30	14 (*)	○
  Test No. 57	Present invention example	C	0.6	630 × 30	0	3.03	20 (*)	○
  Test No. 58	Present invention example	C	0.6	650 × 30	0	2.66	18 (*)	○

• As can be seen from the results shown in Table 7 above, decarburization is generally suppressed when the Cr content in the steel raw material increases. Since reduced iron is generated by connection between carbon in steel and oxygen in the scale during decarburization, when the decarburization amount decreases due to an increase in the Cr content, reduced iron to be generated also decreases, and good pickling properties can be obtained. In other words, when the Cr content in the steel raw material is larger, the soaking temperature during annealing can be further increased without generating a large amount of reduced iron, and the amount of the internal oxide layer can be increased. Therefore, as the Cr content is larger, the upper limit value related to the pickling properties of the internal oxide layer based on x² obtained by substituting conditions into the formula 4 can be increased.
• When the Cr content is 0.2 mass%, as described above also in Example 4, x² obtained by substituting the annealing condition of Test No. 54 into the formula 4 can be defined as the upper limit value "0.63" related to the pickling properties of the internal oxide layer as represented by the formula 6.
• When the Cr content was 0.6 mass%, as shown in Table 7 above, in Test No. 58 having a soaking temperature of 650°C and a soaking time of 30 hours (108,000 seconds), the estimated reduced iron area ratio was below the upper limit value (that is, less than 45%) of the reduced iron area ratio that can have good pickling properties, calculated in Example 3 described above. Therefore, it means that the steel sheet manufactured in Test No. 58 has good pickling properties. Thus, when the Cr content is 0.6 mass%, x² obtained by substituting the annealing condition of Test No. 58 into the formula 4 can be defined as the upper limit value "0.93" related to the pickling properties of the internal oxide layer as represented by the formula 7.
• Considering that the upper limit value increases as the Cr content increases, when the Cr content is 0.2 mass% or more and 0.6 mass% or less, the upper limit value related to the pickling properties of the internal oxide layer can be derived from the results of Test Nos. 54 and 58. Specifically, 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 mass% and the upper limit value of "0.93" when the Cr content is 0.6 mass% can be defined as the upper limit value "0.75 Cr [%]+0.48" related to the pickling properties of the internal oxide layer according to the Cr content as represented by the formula 8.
• In addition, as can be seen from Table 7 above, in Test No. 54 in the case where the Cr content is 0.2 mass%, the measured reduced iron area ratio is 26%, which is significantly lower than the upper limit value (that is, less than 45%) of the reduced iron area ratio at which good pickling properties can be obtained. Therefore, even when the Cr content is less than 0.2 mass% (preferably, the Cr content is more than 0 mass% and less than 0.2 mass%), it is considered that the upper limit value related to the pickling properties of the internal oxide layer can be defined as "0.63" as in the case where the Cr content is 0.2 mass%. Alternatively, even when the Cr content is less than 0.2 mass% (preferably, the Cr content is more than 0 mass% and less than 0.2 mass%), as in the case where the Cr content is 0.2 mass% or more and 0.6 mass% or less, the upper limit value related to the pickling properties of the internal oxide layer may be defined as "0.75 Cr [%]+0.48"from the straight line of the upper limit value with respect to the Cr content passing through the two points of the upper limit value of "0.63" when the Cr content is 0.2 mass% and the upper limit value of "0.93" when the Cr content is 0.6 mass%.
• When the Cr content is more than 0.6 mass% and 1 mass% or less, decarburization during annealing is further suppressed as compared with Test No. 58 in the case where the Cr content is 0.6 mass%. Therefore, since the amount of reduced iron to be generated is also reduced, it is assumed that the upper limit value related to the pickling properties of the internal oxide layer can be set to a larger value. Therefore, even when the Cr content is more than 0.6 mass% and 1 mass% or less, the upper limit value related to the pickling properties of the internal oxide layer can be defined as "0.93" as in the case where the Cr content is 0.6 mass%.
• As can be seen from Examples 1 to 3 and 5 described above, in the method for manufacturing a steel sheet, by setting the soaking temperature T, the soaking time t, and the Cr content in the annealing step so as to satisfy the formula 1A, 1B, or 1C according to the Cr content in the steel raw material, it is possible to efficiently obtain a steel sheet that achieves both suppression of the alloying unevenness and good pickling properties.
• This application is based on Japanese Patent Application No. 2021-036228 filed on March 8, 2021 and Japanese Patent Application No. 2021-204254 filed on December 16, 2021 , the contents of which are incorporated herein.
• It should be understood that the embodiments and examples disclosed herein are exemplary in all respects and do not pose any limitation. The scope of the present invention is indicated by the scope of claims instead of the above description, and is intended to include meanings equivalent to the scope of claims and all modifications within the scope.
Industrial Applicability
• According to the present invention, it is possible to produce a steel sheet capable of suppressing the alloying unevenness and having good pickling properties even when the Si content is large. Thus, for example, it is possible to efficiently manufacture a high-strength and high-formability hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more, which are suitably applied to an automobile member such as an automobile body.

Claims (3)

1. A method for manufacturing a steel sheet comprising a step of annealing a steel raw material having an Si content of 1.0 mass% or more and a Cr content of 1 mass% or less, under a condition satisfying:

   a following formula 1A if the Cr content of the steel raw material is 0.2 mass% or more and 0.6 mass% or less:
   [Mathematical Formula 1] $$0.19\le \mathit{exp}\left(-\frac{10770}{\mathit{T}+273}\right)\ast \mathrm{t}\le 0.75\mathit{Cr}\left[\%\right]+0.48$$

   

   a following formula 1B if the Cr content of the steel raw material is less than 0.2 mass%:
   [Mathematical Formula 2] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.63$$

   

   or a following formula 1C if the Cr content of the steel raw material is more than 0.6 mass% and 1.0 mass% or less:
   [Mathematical Formula 3] $$0.19\le \mathit{exp}\left(-\frac{10770}{T+273}\right)\ast \mathrm{t}\le 0.93$$

   

   (in the formulas 1A, 1B, and 1C, T is 500°C or higher and a soaking temperature (°C) during annealing, t is a soaking time (seconds) during annealing, and Cr [%] is the Cr content (mass%) of the steel raw material).
2. The method for manufacturing a steel sheet according to claim 1, further comprising a step of hot rolling the steel raw material before the annealing and coiling a steel sheet at 500°C to 700°C.
3. The method for manufacturing a steel sheet according to claim 1 or 2, further comprising a step of pickling the steel sheet after the annealing and then cold rolling the steel sheet.

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