EP4296385A1 - Verfahren zur herstellung von stahlblech - Google Patents

Verfahren zur herstellung von stahlblech Download PDF

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Publication number
EP4296385A1
EP4296385A1 EP22766965.2A EP22766965A EP4296385A1 EP 4296385 A1 EP4296385 A1 EP 4296385A1 EP 22766965 A EP22766965 A EP 22766965A EP 4296385 A1 EP4296385 A1 EP 4296385A1
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EP
European Patent Office
Prior art keywords
steel sheet
mass
content
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steel
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Pending
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EP22766965.2A
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English (en)
French (fr)
Inventor
Shuya MAEKAWA
Shohei Nakakubo
Ryosuke OTOMO
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority claimed from JP2021204254A external-priority patent/JP2022136964A/ja
Application filed by Kobe Steel Ltd filed Critical Kobe Steel Ltd
Publication of EP4296385A1 publication Critical patent/EP4296385A1/de
Pending legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere

Definitions

  • 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.
  • 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.
  • 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.
  • 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.
  • GI steel sheet a hot-dip galvanized steel sheet
  • GA steel sheet hot-dip galvannealed steel sheet obtained by alloying the hot-dip galvanized steel sheet
  • problems such as bare spot, reduction in plating adhesion, and an alloying unevenness in alloying treatment are likely to occur finally.
  • problems such as peeling of the plating during processing of the hot-dip galvannealed steel sheet may also occur.
  • 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.
  • an 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.
  • a method of forming an internal oxide layer containing SiO 2 and the like necessary for plating in advance in a steel sheet by increasing a coiling temperature in hot rolling is also used.
  • 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 ⁇ m 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%.
  • 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.
  • 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.
  • 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.
  • 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 ⁇ exp ⁇ 10770 T + 273 ⁇ t ⁇ 0.63
  • 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:
  • 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.
  • the coil 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.
  • the internal oxide layer hardly grows, and a layer is formed thin.
  • the internal oxide layer in the vicinity of the width direction center of the steel sheet, the internal oxide layer sufficiently grows, and a layer is formed thick.
  • 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.
  • 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.
  • 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.
  • 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.
  • the "internal oxide layer” means an internal oxide layer containing SiO 2 (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.
  • 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 SiO 2 .
  • an amount of the internal oxide layer can be measured as a dissolved amount (g/m 2 ) per unit area by immersing and dissolving the internal oxide layer in an acidic solution such as hydrochloric acid.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • 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.
  • the coiled hot-rolled steel sheet after hot rolling may be naturally cooled to normal temperature.
  • the coiled steel sheet is annealed so as to satisfy the following relational expression according to the Cr content in the steel raw material.
  • the steel sheet is annealed under conditions satisfying the following formula 1C.
  • T is 500°C or higher and the soaking temperature (°C) during annealing
  • t is the soaking time (seconds) during annealing
  • Cr [%] is the Cr content (mass%) of the steel raw material.
  • the coiled steel sheet is preferably annealed so as to satisfy the following conditions according to the Cr content in the steel raw material.
  • the steel sheet is preferably annealed under conditions satisfying the following formula 1A.
  • the steel sheet is preferably annealed under conditions satisfying the formula 1C.
  • T is 500°C or higher and represents the soaking temperature (°C) during annealing
  • t is the soaking time (seconds) during annealing
  • Cr [%] is the Cr content (mass%) of the steel raw material.
  • the H 2 concentration (vol%) in the surrounding gas atmosphere during annealing is preferably 0 vol%.
  • the internal oxide layer can be grown well and remain to the vicinity of the width direction edge of the steel sheet.
  • 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.
  • An amount x (g/m 2 ) 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] X 2 ⁇ exp ⁇ Q / R T + 273 ⁇ t
  • x 2 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.
  • 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”.
  • T in the following formula 5 is 500°C or higher.
  • x 2 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.
  • 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”.
  • the formula 1 is derived.
  • the H 2 concentration P (H 2 ) (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).
  • 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.
  • 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%, x 2 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%.
  • 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%.
  • x 2 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.
  • 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.
  • 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.
  • the annealed steel sheet is then preferably pickled.
  • the pickling method is not particularly limited, and any known method may be applied.
  • the scale may be removed by immersion using hydrochloric acid or the like.
  • 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.
  • 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.
  • the steel sheet after pickling may be cold-rolled.
  • the cold rolling method is not particularly limited, and any known method may be applied.
  • 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.
  • the steel sheet in the first embodiment or the second embodiment can be manufactured.
  • 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.
  • a method for manufacturing such hot-dip galvanized steel sheet and hot-dip galvannealed steel sheet will be described.
  • 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.
  • 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.
  • equipment of a continuous galvanizing line CGL
  • CGL 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.
  • 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.
  • NOF non-oxygen furnace
  • DFF direct fired furnace
  • 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.
  • 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.
  • 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.
  • 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 O 2 , CO 2 , N 2 , and H 2 O. 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 O 2 is controlled.
  • COG cokes oven gas
  • LPG liquefied petroleum gas
  • the O 2 concentration is preferably controlled in a range of 100 ppm to 17,000 ppm.
  • the O 2 concentration is more preferably controlled at 500 ppm or more, and still more preferably 2,000 ppm or more.
  • the O 2 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.
  • reduction annealing is preferably performed at a soaking temperature of preferably an Ac 3 point or higher.
  • the Ac 3 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.
  • 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.
  • the reduction annealing treatment can be performed by heating the surface of the steel sheet under a reducing atmosphere mainly containing H 2 gas and an inert gas such as N 2 .
  • a mixed gas containing H 2 gas and an inert gas such as N 2 is used, for example, the H 2 gas can be contained in a proportion of 3 vol% to 25 vol%, and an inert gas such as N 2 can be contained as the balance.
  • the method of the hot dip galvanizing treatment is not particularly limited, and any known method may be applied.
  • 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.
  • the immersion time of the steel sheet in the Zn-plating bath may be adjusted according to a desired Zn-plating adhesion amount.
  • 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.
  • 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.
  • the heating of the alloying can be, for example, under an air atmosphere.
  • 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.
  • Si is an inexpensive steel reinforcing element, and hardly affects the formability of the steel sheet.
  • 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.
  • 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.
  • 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 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.
  • Mn is an element that stabilizes austenite and contributes to improvement of the formability of the steel sheet by generation of retained austenite.
  • 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.
  • 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 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.
  • 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.
  • 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.
  • the P is an element inevitably present as an impurity element.
  • 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 is an element inevitably present as an impurity element.
  • steel inevitably contains S in an amount of about 0.0005 mass%.
  • 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.
  • the Al content is an element having a deoxidizing action.
  • 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.
  • 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 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 ( ⁇ -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.
  • 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.
  • 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.
  • 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.
  • 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 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.
  • 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
  • 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.
  • 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 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.
  • 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.
  • 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.
  • the 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.
  • 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.
  • 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 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.
  • 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.
  • 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 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 is an element inevitably present as an impurity element.
  • a nitride may be formed to deteriorate the formability of the steel sheet.
  • 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.
  • 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.
  • 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.
  • these elements are defined as described above since the present invention can exert its effect by suppressing the content thereof to a specific range.
  • "inevitable impurities" constituting the balance mean the concept excluding elements whose composition range is defined.
  • 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.
  • 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.
  • 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 H 2 concentration P (H 2 ) in the surrounding gas atmosphere during annealing so as to satisfy a predetermined relational expression defined in advance.
  • 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.
  • 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.
  • the hot-dip galvannealed steel sheet thus manufactured may have a tensile strength of 980 MPa or more.
  • 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 ⁇ exp ⁇ 10770 T + 273 ⁇ t ⁇ 0.63
  • 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 ⁇ exp ⁇ 10770 T + 273 ⁇ t ⁇ 0.75 Cr % + 0.48
  • 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 ⁇ exp ⁇ 10770 T + 273 ⁇ t ⁇ 0.75 Cr % + 0.48
  • 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.
  • Example 1 the lower limit value of the amount x (g/m 2 ) of the internal oxide layer capable of suppressing the alloying unevenness was determined.
  • 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 ⁇ m from the surface of the steel sheet, the amount (g/m 2 ) of the internal oxide layer, and an effect of suppressing the alloying unevenness was examined.
  • 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.
  • 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 ⁇ m, 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 ⁇ m, and an analysis position of 1 ⁇ m in depth.
  • the amount (g/m 2 ) of the internal oxide layer of the test piece for which the solid solution Si amount (wt%) was measured was measured.
  • 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/m 2 ) 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/m 2 ) measured in this manner.
  • 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.
  • y solid solution Si amount wt % ⁇ 0.1169 ⁇ amount g / m 2 of internal oxide layer + 1.8723
  • R 2 0.997
  • a hot-dip galvannealed steel sheet was produced from the obtained steel sheet.
  • 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.
  • 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 O 2 and CO 2 , N 2 , and H 2 O.
  • 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 N 2 -H 2 .
  • 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.
  • the alloying unevenness 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 ⁇ m 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/m 2 or more. That is, it was found that when the amount of the internal oxide layer was 4.4 g/m 2 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 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/m 2 ) 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/m 2 ) 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.
  • the test piece at any position exceeded the lower limit value (that is, 4.4 g/m 2 ) of the amount of the internal oxide layer capable of suppressing the alloying unevenness calculated in Example 1 described above.
  • the steel sheet manufactured in Example 2 can suppress the alloying unevenness particularly in the coil width direction.
  • x 2 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 x 2 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.
  • Example 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/m 2 ) (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
  • 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.
  • Example 2 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.
  • 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.
  • a grain boundary oxidation depth ( ⁇ m) in the internal oxide layer was also measured at the same time.
  • 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.
  • the relationship is established where when the grain boundary oxidation depth ( ⁇ m) in the internal oxide layer increases, that is, the amount (g/m 2 ) of the internal oxide layer increases, the reduced iron area ratio (%) increases.
  • 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.
  • 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 H 2 concentration in the surrounding gas atmosphere during annealing were changed.
  • the reduced iron area ratio (%) was measured by the same method as in Example 3.
  • 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.
  • FIG. 3 is a graph obtained by plotting the soaking temperature during annealing and the H 2 concentration in the surrounding gas atmosphere in Table 5 above.
  • 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 H 2 concentration P (H 2 ) (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 H 2 concentration P is 0 vol% and the soaking temperature T is 625°C and a point where the H 2 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.
  • the reduced iron area ratio is also assumed to be about 42 which is the average of both tests.
  • 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 H 2 concentration P (H 2 ) and the soaking temperature T can be derived.
  • 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.
  • 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.
  • 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.
  • 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.
  • the soaking time was set to 30 hours (108,000 seconds)
  • the H 2 concentration in the surrounding gas atmosphere during annealing was set to 0 vol%
  • 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.
  • 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.
  • a decarburization amount (mg/cm 2 ) 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/cm 2 ).
  • 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.
  • 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 x 2 obtained by substituting conditions into the formula 4 can be increased.
  • x 2 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.
  • 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.
  • 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.
  • 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%.
  • 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%.
  • 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%.
  • 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.
  • 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.

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JP3835083B2 (ja) 1999-02-25 2006-10-18 Ｊｆｅスチール株式会社 冷延鋼板および溶融めっき鋼板ならびにそれらの製造方法
JP4741376B2 (ja) 2005-01-31 2011-08-03 新日本製鐵株式会社 外観が良好な高強度合金化溶融亜鉛めっき鋼板及びその製造方法と製造設備
CN106715726B (zh) 2014-09-08 2018-11-06 杰富意钢铁株式会社 高强度熔融镀锌钢板的制造方法和制造设备
WO2017109539A1 (en) * 2015-12-21 2017-06-29 Arcelormittal Method for producing a high strength steel sheet having improved strength and formability, and obtained high strength steel sheet
JP2017222887A (ja) 2016-06-13 2017-12-21 株式会社神戸製鋼所 鋼板の製造方法
MX2019005637A (es) * 2016-11-16 2019-07-04 Jfe Steel Corp Lamina de acero de alta resistencia y metodo para la produccion de la misma.
WO2018220430A1 (en) * 2017-06-02 2018-12-06 Arcelormittal Steel sheet for manufacturing press hardened parts, press hardened part having a combination of high strength and crash ductility, and manufacturing methods thereof
WO2019122963A1 (en) * 2017-12-19 2019-06-27 Arcelormittal Cold rolled and heat treated steel sheet and a method of manufacturing thereof
WO2020128574A1 (en) * 2018-12-18 2020-06-25 Arcelormittal Cold rolled and heat-treated steel sheet and method of manufacturing the same
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