WO2016111272A1 - 高強度めっき鋼板、並びにその製造方法 - Google Patents
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- 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
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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
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
Definitions
- the present invention is a high strength plated steel sheet having a tensile strength of 980 MPa or more, good plating properties, workability including elongation, bendability, and hole expansibility, and excellent delayed fracture resistance, and its production Regarding the method.
- the plated steel sheet of the present invention includes both hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets.
- Hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets which are widely used in fields such as automobiles and transportation equipment, are not only high-strength, but also workability such as elongation, bendability, and hole expandability (synonymous with stretch flangeability) Furthermore, it is required to have excellent delayed fracture resistance.
- Patent Document 1 discloses a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability and corrosion resistance of a processed part. Specifically, in Patent Document 1, in order to suppress the occurrence of bending cracks and damage to the plating film due to the internal oxide layer formed on the steel plate side from the interface between the steel plate and the plating layer, the growth of the internal oxide layer is suppressed. Therefore, the growth of the decarburized layer is remarkably accelerated. Further, a near-surface structure is disclosed in which the thickness of the internal oxide layer in the ferrite region formed by decarburization is controlled to be thin.
- Patent Document 2 discloses a hot-dip galvanized steel sheet having a fatigue strength, hydrogen embrittlement resistance (synonymous with delayed fracture resistance), and a tensile strength excellent in bendability of 770 MPa or more.
- the steel plate portion is configured to have a soft layer that is in direct contact with the interface with the plating layer, and a soft layer that has ferrite with a maximum area ratio structure.
- the thickness D of the soft layer and the depth d from the plating / base metal interface of the oxide containing one or more of Si and Mn existing in the steel sheet surface layer portion are d / 4 ⁇ D ⁇ 2d.
- a hot-dip galvanized steel sheet that satisfies the requirements is disclosed.
- JP 2011-231367 A Japanese Patent No. 4943558
- the present invention has been made in view of the above circumstances, and its purpose is that the plating strength is good, the tensile strength excellent in workability of elongation, bendability and hole expansibility, and delayed fracture resistance is 980 MPa or more.
- An object of the present invention is to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet.
- the other object of this invention is to provide the manufacturing method of the said hot dip galvanized steel plate and an alloyed hot dip galvanized steel plate.
- the high-strength galvanized steel sheet having a tensile strength of 980 MPa or more is a galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the base steel plate.
- the base steel sheet is, by mass, C: 0.10 to 0.5%, Si: 1.0 to 3%, Mn: 1.5 to 8%, Al: 0.005 to 3%, P: It contains more than 0% and 0.1% or less, S: more than 0% and 0.05% or less, and N: more than 0% and 0.01% or less, with the balance being iron and inevitable impurities.
- an internal oxide layer containing at least one oxide selected from the group consisting of Si and Mn, and a layer containing the internal oxide layer are scanned.
- the low-temperature transformation generation phase is 20 to 85 area% with respect to the entire metal structure, and the polygonal ferrite is more than 10 area% with respect to the entire metal structure.
- an average depth D of the soft layer is 20 ⁇ m or more
- an average depth d of the internal oxide layer Has a gist in that it satisfies 4 ⁇ m or more and less than D.
- the average depth d of the internal oxide layer and the average depth D of the soft layer satisfy a relationship of D> 2d.
- the low-temperature transformation generation phase includes high-temperature range bainite having an average interval of 1 ⁇ m or more between adjacent residual austenite, adjacent carbides, or adjacent residual austenite and carbide, Low-temperature region-generated bainite that is greater than 10% by area and less than or equal to 85% by area with respect to the entire structure, and whose average distance between adjacent residual austenite, adjacent carbides, or adjacent residual austenite and carbide is less than 1 ⁇ m, and tempering Martensite may be included, and the total of the low-temperature region bainite and the tempered martensite may be 0 area% or more and less than 10 area% with respect to the entire metal structure.
- the low-temperature transformation generation phase is, between adjacent residual austenite, adjacent carbides, or high temperature region bainite having an average interval between adjacent residual austenite and carbide of 1 ⁇ m or more, adjacent residual austenite, adjacent carbides, Or a low-temperature zone bainite having an average interval between adjacent retained austenite and carbide of less than 1 ⁇ m, and tempered martensite, and the high-temperature zone bainite is 10 to 75 area% with respect to the entire metal structure, The total of the low temperature region bainite and the tempered martensite may be 10 to 75 area% with respect to the entire metal structure.
- the low temperature transformation product phase includes adjacent low temperature austenite, adjacent carbides, or low temperature region bainite having an average interval between adjacent residual austenite and carbide of less than 1 ⁇ m, and tempered martensite.
- the total of bainite and the tempered martensite is more than 10 area% and 85 area% or less with respect to the entire metal structure, and the average distance between adjacent residual austenite, adjacent carbides, or adjacent residual austenite and carbide.
- the base steel plate is further in mass%, (A) at least one selected from the group consisting of Cr: more than 0% and 1% or less, Mo: more than 0% and 1% or less, and B: more than 0% and 0.01% or less, (B) at least one selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.2% or less, (C) at least one selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less, (D) at least one selected from the group consisting of Ca: more than 0% and 0.01% or less, Mg: more than 0% and 0.01% or less, and rare earth elements: more than 0% and 0.01% or less, It may contain.
- A at least one selected from the group consisting of Cr: more than 0% and 1% or less, Mo: more than 0% and 1% or less, and B: more than 0%
- the high strength plated steel sheet A hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 600 ° C. or higher; Pickling and cold rolling so that the average depth d of the internal oxide layer remains 4 ⁇ m or more; Oxidizing in an oxidation zone at an air ratio of 0.9 to 1.4; (I) In the reduction zone, soaking in a range equal to or higher than the higher of Ac 3 point or 750 ° C., After soaking, it is cooled to 600 ° C. at an average cooling rate of more than 0 ° C./second and not more than 20 ° C./second, and from 600 ° C., it is cooled to an arbitrary stop temperature Z satisfying 100 to 540 ° C.
- the range up to the higher one of the stop temperature Z or 500 ° C. is higher than the average cooling rate up to 600 ° C. after the soaking, and is cooled at an average cooling rate of 10 ° C./second or more.
- the high-strength plated steel sheet is A hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 500 ° C. or higher; Maintaining the temperature at a temperature of 500 ° C. or more for 60 minutes or more; Pickling and cold rolling so that the average depth d of the internal oxide layer remains 4 ⁇ m or more; Oxidizing in an oxidation zone at an air ratio of 0.9 to 1.4; (I) In the reduction zone, soaking in a range equal to or higher than the higher of Ac 3 point or 750 ° C., After soaking, it is cooled to 600 ° C.
- the low-temperature transformation generation phase contains the high-temperature region generation bainite more than 10% by area and not more than 85% by area with respect to the entire metal structure, and the total of the low-temperature region generation bainite and the tempered martensite is in the entire metal structure.
- the high-strength plated steel sheet that is 0 area% or more and less than 10 area% can be manufactured by the following [Ia] or [Ib] manufacturing method.
- [Ia] a hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 600 ° C.
- the low-temperature transformation generation phase contains 10 to 75 area% of the high-temperature region-generated bainite with respect to the entire metal structure, and the total of the low-temperature region-generated bainite and the tempered martensite is 10 to the total metal structure.
- the high-strength plated steel sheet containing ⁇ 75 area% can be manufactured by the following manufacturing method [IIa] or [IIb]. [IIa] a hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 600 ° C.
- the low-temperature transformation generation phase includes the low-temperature region generation bainite in an amount of more than 10 area% to 85 area% or less with respect to the entire metal structure.
- the high-strength plated steel sheet that is less than area% can be produced by the following production method [IIIa] or [IIIb]. [IIIa] a hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 600 ° C.
- the plated steel sheet of the present invention includes an internal oxide layer containing at least one oxide selected from the group consisting of Si and Mn from the interface between the plating layer and the base steel sheet to the base steel sheet side, and a region of the internal oxide layer.
- a hard layer that is a region other than the soft layer and mainly includes a low-temperature transformation generation phase, includes retained austenite, and may include polygonal ferrite.
- the average depth d of the internal oxide layer is controlled to be 4 ⁇ m or more and used as a hydrogen trap site, hydrogen embrittlement can be effectively suppressed, and processing of elongation, bendability, and hole expandability is possible.
- a high-strength plated steel sheet having a tensile strength of 980 MPa or more, which is excellent in all properties and delayed fracture resistance, can be obtained.
- the bendability is further enhanced.
- FIG. 1 is a schematic diagram for explaining a layer structure from the interface between a plating layer and a base steel plate to the base steel plate side in the plated steel plate of the present invention.
- FIG. 2 is a schematic diagram for explaining the procedure for measuring the average depth d of the internal oxide layer in the plated steel sheet of the present invention.
- FIG. 3 is a diagram for explaining the measurement position of the Vickers hardness used for determining the average depth D of the soft layer.
- FIG. 4 is a schematic diagram for explaining a procedure for measuring the distance between the center positions of retained austenite, carbides, or retained austenite and carbide.
- FIG. 5 b are diagrams schematically showing a distribution state of high-temperature region-generated bainite and low-temperature region-generated bainite and tempered martensite.
- FIG. 6 is a schematic diagram for explaining heat patterns in the T1 temperature range and the T2 temperature range.
- FIG. 7 is a schematic diagram for explaining heat patterns in the T3 temperature range and the T4 temperature range.
- the present inventors provide a high-strength plated steel sheet having a high strength of 980 MPa or more and excellent in all of plateability, workability, and delayed fracture resistance in a base steel sheet rich in Si and Mn. For this reason, in particular, studies have been made focusing on the layer structure from the interface between the plating layer and the base steel plate to the base steel plate side. As a result, as shown in the schematic diagram of FIG.
- A a soft layer including an internal oxide layer containing at least one oxide selected from the group consisting of Si and Mn, the layer structure from the interface between the plating layer and the base steel plate to the base steel plate side, and the soft layer And having a hard layer containing a low-temperature transformation generation phase, polygonal ferrite, and residual austenite
- B When the average depth d of the internal oxide layer is controlled to be 4 ⁇ m or more, the internal oxide layer functions as a hydrogen trap site, and hydrogen embrittlement can be effectively suppressed, so that the intended purpose can be achieved.
- C Preferably, if the relationship between the average depth d of the internal oxide layer and the average depth D of the soft layer including the region of the internal oxide layer is appropriately controlled, the bendability is particularly improved. The present invention has been completed.
- the plated steel sheet includes both a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet.
- the said base steel plate means the steel plate before a hot dip galvanized layer and an alloying hot dip galvanized layer are formed, and the said plated steel plate is a hot dip galvanized layer or alloy on the surface of a base steel plate. It means a steel sheet having a hot dip galvanized layer.
- high strength means a tensile strength of 980 MPa or more.
- excellent in workability means excellent in all of elongation, bendability, and hole expandability. For details, when these characteristics are measured by the method described in the examples described later, those satisfying the acceptance criteria of the examples are referred to as “excellent workability”.
- the plated steel sheet of the present invention has a hot dip galvanized layer or an alloyed hot dip galvanized layer (hereinafter, may be represented by a plated layer) on the surface of the base steel sheet.
- the characteristic part of the present invention is that it has the following layer configurations (A) to (C) in order from the interface between the base steel plate and the plating layer toward the base steel plate side.
- the average depth d of the internal oxide layer is 4 ⁇ m or more and less than the average depth D of the soft layer described in (B) described later.
- (B) Soft layer including the internal oxide layer, where the thickness of the base steel sheet is t, the Vickers hardness satisfies 90% or less of the Vickers hardness at t / 4 part of the base steel sheet.
- the average depth D of the soft layer is 20 ⁇ m or more.
- the “low temperature transformation product phase” means bainite and tempered martensite, and does not include martensite (sometimes referred to as fresh martensite) as quenched in the low temperature transformation product phase. Fresh martensite is classified here for convenience in other tissues.
- the layer structure of the plated steel sheet according to the present invention on the base steel sheet 2 side is from the interface between the plating layer 1 and the base steel sheet 2 toward the base steel sheet 2 side,
- a hard layer 5 of (C) is provided inside the base steel plate 2 from the layer 4.
- the soft layer 4 of (B) includes the internal oxide layer 3 of (A). The soft layer 4 and the hard layer 5 are continuously present.
- the portion directly in contact with the interface between the plating layer 1 and the base steel plate 2 has an internal oxide layer 3 having an average depth d of 4 ⁇ m or more.
- the average depth means an average value of the depth from the interface, and a detailed measuring method thereof will be described with reference to FIG.
- the internal oxide layer 3 is composed of an oxide containing at least one of Si and Mn, and a Si and Mn depletion layer in which Si and Mn form an oxide to form a solid solution Si or a small amount of solid solution Mn. .
- the greatest feature is that the average depth d of the internal oxide layer 3 is controlled to be 4 ⁇ m or more.
- the internal oxide layer 3 can be used as a hydrogen trap site, hydrogen embrittlement can be suppressed, and bendability, hole expansibility, and delayed fracture resistance are improved.
- a composite oxide film having Si oxide, Mn oxide, and a composite oxide of Si and Mn is formed on the base steel sheet surface during annealing. It is easy to form and the plating property is hindered.
- the time of annealing corresponds to an oxidation / reduction process in a continuous hot dip galvanizing line which will be described later.
- a method in which the surface of the base steel sheet is oxidized in an oxidizing atmosphere to form a Fe oxide film, and then annealed (that is, reduction annealing) in an atmosphere containing hydrogen. Furthermore, by controlling the atmosphere in the furnace, the oxidizable elements are fixed as oxides inside the base steel sheet surface layer, and by reducing the oxidizable elements dissolved in the base steel sheet surface layer, A method for preventing the formation of an oxide film on the surface of the base steel plate is also known.
- the use of at least one oxide selected from the group consisting of Si and Mn is effective in improving the deterioration of bendability and hole expansibility due to crystallization.
- the oxide is useful as a hydrogen trap site that can prevent hydrogen from entering the base steel sheet during reduction and improve the deterioration of bendability and hole expandability due to the deterioration of delayed fracture resistance.
- the d is preferably 6 ⁇ m or more, more preferably 8 ⁇ m or more, and still more preferably more than 10 ⁇ m.
- the upper limit of the average depth d of the internal oxide layer 3 is at least less than the average depth D of the soft layer 4 described later (B).
- the upper limit of d is preferably 30 ⁇ m or less. In order to make the internal oxide layer 3 thick, it is necessary to keep it for a long time in a high temperature range after hot rolling, but the above preferred values are generally obtained due to restrictions on productivity and equipment.
- the d is more preferably 18 ⁇ m or less, and still more preferably 16 ⁇ m or less.
- the average depth d of the internal oxide layer 3 is controlled so as to satisfy the relational expression D> 2d in relation to the average depth D of the soft layer 4 (B) described later. Is preferable, and in particular, the bendability is further improved.
- Patent Document 2 the oxide existing depth d and the soft layer thickness substantially correspond to the average depth d of the internal oxide layer and the average depth D of the soft layer described in the present invention.
- a hot-dip galvanized steel sheet satisfying d / 4 ⁇ D ⁇ 2d is disclosed for the depth D, and the directivity of control is completely different from the relational expression (D> 2d) defined in the present invention.
- Patent Document 2 describes that the range of the oxide depth d is basically controlled while satisfying the above-described relationship of d / 4 ⁇ D ⁇ 2d.
- the average depth of the internal oxide layer 3 in the cold-rolled steel sheet before passing through the continuous hot dip galvanizing line is 4 ⁇ m. It is necessary to control as described above. This is because the internal oxide layer after pickling and cold rolling is succeeded to the internal oxide layer in the finally obtained plated steel sheet after passing through the plating line, as shown in the examples described later. Details will be described together with the manufacturing method.
- the soft layer 4 is a layer including the region of the internal oxide layer 3 of (A) as shown in FIG. This soft layer 4 satisfies the Vickers hardness of 90% or less of the Vickers hardness at the t / 4 part of the base steel plate 2.
- t is the thickness (mm) of the base steel plate. A detailed method for measuring the Vickers hardness will be described in the column of Examples described later.
- the soft layer 4 is a soft structure having a Vickers hardness lower than that of the hard layer 5 described later (C), and is excellent in deformability. Therefore, when the soft layer 4 is formed, the bendability is particularly improved. That is, at the time of bending, the base steel plate surface layer portion becomes the starting point of cracking, but the bendability is particularly improved by forming the predetermined soft layer 4 on the base steel plate surface layer as in the present invention. Furthermore, the formation of the soft layer 4 can prevent the oxide in (A) from becoming a starting point of cracking during bending, and can enjoy only the above-described merit as a hydrogen trap site. As a result, not only bendability but also delayed fracture resistance is further improved.
- the average depth D of the soft layer 4 is set to 20 ⁇ m or more.
- the D is preferably 22 ⁇ m or more, and more preferably 24 ⁇ m or more.
- the upper limit is preferably 100 ⁇ m or less, and more preferably 60 ⁇ m or less.
- the hard layer 5 is formed in the base steel plate 2 side of the soft layer 4 of the said (B), as shown in FIG.
- the hard layer 5 is composed of a structure including a low-temperature transformation generation phase, polygonal ferrite, and residual ⁇ .
- the “low-temperature transformation generation phase” means bainite and tempered martensite, and bainite means bainitic ferrite.
- Bainite is a structure in which carbide is precipitated, and bainitic ferrite is a structure in which carbide is not precipitated.
- the area ratio of the low-temperature transformation generation phase is preferably 30 area% or more, more preferably 40 area% or more, and further preferably 50 area% or more.
- the upper limit of the area ratio of the low-temperature transformation product phase is preferably, for example, 85 area% or less in order to ensure the production amount of polygonal ferrite and residual ⁇ .
- the hard layer contains polygonal ferrite in a range of more than 10 area% and less than 70 area% with respect to the entire metal structure.
- the polygonal ferrite is softer than the low-temperature transformation generation phase and acts to improve the workability by increasing the elongation of the steel sheet.
- the area ratio of polygonal ferrite is more than 10%, preferably 15% or more with respect to the entire metal structure.
- the area ratio of polygonal ferrite is preferably 70% or less, more preferably 65% or less, and still more preferably 60% or less with respect to the entire metal structure.
- the residual ⁇ has the effect of accelerating hardening of the deformed portion by transforming into martensite when the steel sheet is deformed under stress, thereby preventing strain concentration. Thereby, the uniform deformability is improved and good elongation is exhibited. Such an effect is generally called a TRIP effect.
- the residual ⁇ needs to be contained in an amount of 5% by volume or more based on the entire metal structure when the metal structure is measured by the saturation magnetization method.
- the residual ⁇ is preferably 8% by volume or more, more preferably 10% by volume or more, and still more preferably 12% by volume or more.
- the upper limit of the residual ⁇ is about 30% by volume or less, preferably 25% by volume or less.
- Residual ⁇ is mainly generated between the laths of the metal structure.
- Residual ⁇ is mainly generated between the laths of the metal structure.
- the hard layer may contain other structures that may be inevitably mixed in the manufacturing process, for example, pearlite, quenched martensite, and the like, as long as the effects of the present invention are not impaired.
- MA mixed phase that is a composite phase of quenched martensite and residual ⁇ may be included.
- the other structure is preferably 15 area% or less at the maximum, and the smaller the better.
- the formation of the hard layer improves elongation, bendability, and hole expandability. That is, the bendability and hole expandability can be improved by generating a predetermined amount of a hard phase such as bainite, and the elongation can be improved by generating a predetermined amount of a soft phase such as polygonal ferrite. Therefore, in the present invention, the internal structure of the base steel sheet is 20 to 85 area% of the low-temperature transformation generation phase such as bainite that is the hard phase, and the proportion of polygonal ferrite that is the soft phase is more than 10 area% and less than 70 area%. The hard layer was suppressed.
- the low-temperature transformation generation phase mainly includes (C6-1) high-temperature range generation bainite, (C6-2) includes a high-temperature range generation bainite, a low-temperature range generation bainite, and a tempered martensite composite structure.
- C6-3 It is preferable to mainly contain low-temperature region bainite and tempered martensite.
- the high-temperature region-generated bainite means that the average distance between adjacent residual ⁇ , adjacent carbides, or adjacent residual ⁇ and carbide is 1 ⁇ m or more when a cross section of a steel plate subjected to nital corrosion is observed with a scanning electron microscope. It is an organization.
- the high-temperature region-generated bainite is a bainite structure that is generally generated in a temperature range of 400 ° C. or more and 540 ° C. or less in the cooling process after heating to a temperature of Ac 1 point or higher.
- the low-temperature region-generated bainite means that the average distance between adjacent residual ⁇ , adjacent carbides, or adjacent residual ⁇ and carbide is less than 1 ⁇ m when a cross section of a steel plate subjected to nital corrosion is observed with a scanning electron microscope. It is an organization.
- the low temperature region-generated bainite is a bainite structure that is generally generated in a temperature range of 200 ° C. or higher and lower than 400 ° C. in the cooling process after heating to the Ac 1 point or higher.
- the tempered martensite is a structure having the same action as the low temperature region bainite.
- the low temperature region bainite and the tempered martensite cannot be distinguished even when observed with a scanning electron microscope, in the present invention, the low temperature region bainite and the tempered martensite are collectively referred to as “low temperature region bainite and the like”. I will call it.
- the high-temperature region-generated bainite contributes particularly to the improvement of elongation among the mechanical properties of the steel sheet
- the low-temperature region-generated bainite and the tempered martensite contribute particularly to the improvement of hole expandability among the mechanical properties of the steel sheet.
- bainite structure and tempered martensite When these two types of bainite structure and tempered martensite are included, it is possible to increase the elongation while ensuring good hole expansibility, and to improve the workability in general. This is considered to be because work hardening ability is increased because non-uniform deformation occurs due to the composite of bainite structure and tempered martensite having different strength levels. That is, since the high temperature region generation bainite is softer than the low temperature region generation bainite and the like, it contributes to improving the workability by increasing the elongation EL of the steel sheet. On the other hand, bainite, etc. produced at low temperatures has small carbides and residual ⁇ , and stress concentration is reduced during deformation.
- the hole expandability and bendability of the steel sheet are enhanced to improve local deformability and improve workability. Contribute to. And by mixing such a high temperature range generation bainite, a low temperature range generation bainite, etc., work hardening ability improves, elongation improves and workability is improved.
- the distance between the center positions of the adjacent residual ⁇ is obtained by measuring the center position of each residual ⁇ or each carbide when measuring the most adjacent residual ⁇ , the most adjacent carbides, and the most adjacent residual ⁇ and carbide. This means the distance between the center positions.
- the center position determines the major axis and minor axis of the residual ⁇ or carbide, and is the position where the major axis and minor axis intersect.
- the distance between the lines formed by the residual ⁇ and carbide, or the residual ⁇ or carbide continuous in the major axis direction may be 12.
- the distance between lines may be referred to as the distance between laths.
- 11 indicates retained austenite or carbide.
- the reason for distinguishing bainite into “high temperature region bainite” and “low temperature region bainite” by the difference in the generation temperature region and the difference in the average interval such as residual ⁇ as described above is a general academic reason. This is because it is difficult to clearly distinguish bainite in the tissue classification. For example, lath-shaped bainite and bainitic ferrite are classified into upper bainite and lower bainite according to the transformation temperature. However, in the steel type containing a large amount of Si of 1% or more as in the present invention, since precipitation of carbides accompanying bainite transformation is suppressed, it is possible to distinguish these including the martensite structure in the scanning electron microscope observation. Have difficulty. Therefore, in the present invention, bainite is not classified based on an academic organization definition, but is distinguished based on the difference in generation temperature range and the average interval such as residual ⁇ as described above.
- the average interval is greatly influenced by the holding temperature, but the lath shape of the bainite structure is a flat plate shape, and the above-mentioned interval is observed narrowly or widely depending on the observation surface. . Therefore, the area ratio of bainite generated in each of the high temperature region and the low temperature region is defined including the variation in the interval depending on the observation direction.
- the distribution state of the high temperature zone bainite and the low temperature zone bainite is not particularly limited.For example, both the high temperature zone bainite and the low temperature zone bainite may be generated in the prior austenite grains. High temperature region bainite, low temperature region bainite, and the like may be generated for each austenite grain.
- FIG. 5 schematically shows the distribution state of the high temperature region bainite and the low temperature region bainite.
- reference numeral 21 denotes a high temperature region bainite
- 22 denotes a low temperature region bainite
- 23 denotes a prior austenite grain boundary (old ⁇ grain boundary)
- 24 denotes an MA mixed phase.
- the high temperature region generation bainite is hatched, and the low temperature region generation bainite is marked with fine dots.
- FIG. 5a shows a state in which both high-temperature region-generated bainite and low-temperature region-generated bainite are mixed and formed in the prior austenite grains.
- FIG. 5b shows a state in which high temperature region bainite and low temperature region bainite are generated for each prior austenite grain.
- the black circles shown in FIG. 5 indicate the MA mixed phase. The MA mixed phase will be described later.
- any of the following (C6-1), (C6-2), and (C6-3) may be used.
- the low temperature transformation generation phase includes the high temperature region generation bainite, and the high temperature region generation bainite is more than 10 area% and not more than 85 area% with respect to the entire metal structure.
- the tempered martensite may be included, and the total of the low temperature region bainite and the tempered martensite is 0 area% or more and less than 10 area% with respect to the entire metal structure.
- the low-temperature transformation generation phase includes the high-temperature range generation bainite, the low-temperature range generation bainite, and the tempered martensite, and the high-temperature range generation bainite is 10 to 75 area% with respect to the entire metal structure. In addition, the sum of the low temperature region bainite and the tempered martensite is 10 to 75 area% with respect to the entire metal structure.
- C6-3 When the low temperature transformation product phase contains the low temperature product bainite and tempered martensite, the total of the low temperature product bainite and the tempered martensite is 10 areas with respect to the entire metal structure. More than 85% by area and less than 85% by area, and may include the high-temperature region-generated bainite.
- the elongation of the steel sheet is improved and the workability can be improved by setting the amount of the high-temperature region-generated bainite to be more than 10 area%.
- the high temperature region bainite is preferably 15 area% or more, more preferably 20 area% or more, and further preferably 25 area% or more.
- the high temperature region-generated bainite is preferably 85 area% or less, more preferably 70 area% or less, and still more preferably 60 area% or less.
- the production amount a of the high temperature region bainite is 10 area% or more, whereby the elongation of the steel sheet is improved, and the production amount b of the low temperature region bainite or the like is 10 area% or more.
- the high temperature region bainite is preferably 10 area% or more, more preferably 15 area% or more, still more preferably 20 area% or more, and particularly preferably 25 area% or more.
- the low temperature region bainite and the like are preferably 10 area% or more, more preferably 15 area% or more, still more preferably 20 area% or more, and particularly preferably 25 area% or more.
- the high temperature region bainite is preferably 75 area% or less, more preferably 70 area% or less, and still more preferably 65 area% or less.
- the low-temperature region-generated bainite and the like are preferably 75 area% or less, more preferably 70 area% or less, and still more preferably 65 area% or less.
- the mixing ratio of the high temperature region bainite and the low temperature region bainite may be determined according to the characteristics required for the steel sheet. Specifically, in order to further improve the hole expandability of the workability of the steel sheet, the ratio of the high temperature region-generated bainite should be as small as possible and the ratio of the low temperature region-generated bainite should be as large as possible. On the other hand, in order to further improve the elongation of the workability of the steel sheet, the ratio of the high-temperature region-generated bainite should be as large as possible, and the ratio of the low-temperature region-generated bainite should be as small as possible. Further, in order to further increase the strength of the steel sheet, the ratio of the low temperature region bainite or the like may be increased as much as possible, and the ratio of the high temperature region bainite may be decreased as much as possible.
- the hole expandability of the steel sheet can be improved and the workability can be improved by setting the amount of the low temperature region bainite and the like to exceed 10% by area.
- the low-temperature region-generated bainite or the like is preferably 15 area% or more, more preferably 20 area% or more, and further preferably 25 area% or more.
- the low temperature region bainite and the like are preferably 85 area% or less, more preferably 70 area% or less, and still more preferably 60 area% or less.
- the number ratio of the MA mixed phase having an equivalent circle diameter exceeding 5 ⁇ m is 0 with respect to the total number of the MA mixed phases. % Or more and less than 15%.
- the MA mixed phase is generally known as a composite phase of quenched martensite and residual ⁇ , and a part of the structure that was present as untransformed austenite before the final cooling is martensite during the final cooling. It is a structure formed by transformation into a site and the remainder remaining as austenite.
- the MA mixed phase is a particularly hard structure because carbon is concentrated at a high concentration in the course of the austempering process and a part thereof has a martensite structure. Therefore, the hardness difference between the bainite and the MA mixed phase is large, and stress concentrates during deformation and tends to become a starting point of void formation. Therefore, if the MA mixed phase is excessively generated, the hole expandability and bendability are reduced and the local deformability is reduced. Decreases.
- the MA mixed phase when MA mixed phase produces
- the MA mixed phase is easily generated as the residual ⁇ amount is increased and the Si content is increased.
- the generated amount is preferably as small as possible.
- the number ratio of MA mixed phases having an equivalent circle diameter exceeding 5 ⁇ m is preferably 0% or more and less than 15% with respect to the total number of MA mixed phases.
- a coarse MA mixed phase having an equivalent circle diameter exceeding 5 ⁇ m adversely affects local deformability.
- the MA mixed phase is recommended to be as small as possible because experiments have shown that the MA mixed phase tends to generate voids as its particle size increases.
- the above metal structure can be measured by the following procedure.
- High temperature zone bainite, low temperature zone bainite, etc. (low temperature zone bainite + tempered martensite), polygonal ferrite, and pearlite are subjected to nital corrosion at 1/4 of the thickness of the cross section parallel to the rolling direction of the steel sheet. However, it can be identified by observing with a scanning electron microscope at a magnification of about 3000 times.
- High temperature region bainite and low temperature region bainite are mainly observed in gray, and are observed as a structure in which white or light gray residual ⁇ and the like are dispersed in crystal grains. Therefore, according to the observation with a scanning electron microscope, since the high temperature region bainite and the low temperature region bainite include residual ⁇ and carbides, the area ratio including the residual ⁇ is calculated.
- Polygonal ferrite is observed as crystal grains that do not contain the above-described white or light gray residual ⁇ or the like inside the crystal grains.
- Pearlite is observed as a structure in which carbide and ferrite are layered.
- both carbide and residual ⁇ are observed as a white or light gray structure, and it is difficult to distinguish them from each other.
- carbides such as cementite, for example, tend to precipitate in the laths rather than between the laths as they are produced in the low temperature range.
- Residual ⁇ is usually generated between the laths, but the size of the lath becomes smaller as the tissue generation temperature decreases. Therefore, when the distance between the residual ⁇ is wide, it is considered that the residual ⁇ was generated in a high temperature range.
- the interval of is narrow, it can be considered that it was generated in a low temperature region. Therefore, in the present invention, when the cross-section subjected to Nital corrosion is observed with a scanning electron microscope, focusing on residual ⁇ observed as white or light gray in the observation field, the distance between the center positions between adjacent residual ⁇ is measured. Further, a structure having an average interval of 1 ⁇ m or more is referred to as a high-temperature region-generated bainite, and a structure having an average interval of less than 1 ⁇ m is referred to as a low-temperature region-generated bainite.
- the volume ratio is measured by the saturation magnetization method. This volume ratio value can be read as the area ratio as it is.
- the detailed measurement principle by the saturation magnetization method may be referred to “R & D Kobe Steel Engineering Reports, Vol.52, No.3, 2002, p.43-46”.
- the volume fraction of residual ⁇ is measured by the saturation magnetization method
- the area ratios of high-temperature region bainite and low-temperature region bainite are measured including the residual ⁇ by observation with a scanning electron microscope. Therefore, the sum of these may exceed 100%.
- the MA mixed phase undergoes repeller corrosion at 1/4 position of the plate thickness in the cross section parallel to the rolling direction of the steel plate, and is observed as a white structure when observed with an optical microscope at a magnification of about 1000 times. Based on this, the ratio of the number of MA mixed phases having an equivalent circle diameter exceeding 5 ⁇ m may be calculated.
- the base steel plate has C: 0.10 to 0.5%, Si: 1.0 to 3%, Mn: 1.5 to 8%, Al: 0.005 to 3%, P: more than 0%, and 0.1%. 1% or less, S: more than 0% and 0.05% or less, and N: more than 0% and 0.01% or less, with the balance being iron and inevitable impurities.
- C is an element necessary for increasing the strength of the steel sheet and generating residual ⁇ .
- the amount of C is 0.10% or more, preferably 0.13% or more, more preferably 0.15% or more.
- the C content is 0.5% or less, preferably 0.4% or less, more preferably 0.3% or less.
- the Si contributes to increasing the strength of the steel sheet as a solid solution strengthening element, and also suppresses the precipitation of carbides during holding in the temperature range of 100 to 540 ° C. (during austempering), effectively reducing residual ⁇ . It is an extremely important element for the formation of selenium.
- the Si amount is 1.0% or more, preferably 1.1% or more, more preferably 1.2% or more.
- Si when Si is contained excessively, reverse transformation to the ⁇ phase does not occur during annealing and soaking, and a large amount of polygonal ferrite remains, resulting in insufficient strength.
- Si scale is generated on the surface of the steel sheet during hot rolling to deteriorate the surface properties of the steel sheet. Therefore, the amount of Si is 3% or less, preferably 2.5% or less, more preferably 2.0% or less.
- Mn is an element necessary for obtaining bainite and tempered martensite. Mn is an element that effectively acts to stabilize ⁇ and generate residual ⁇ .
- the amount of Mn is 1.5% or more, preferably 1.8% or more, more preferably 2.0% or more.
- Mn content is 8% or less, preferably 7% or less, more preferably 6% or less, still more preferably 5.0% or less, and particularly preferably 3% or less.
- Al like Si, is an element that suppresses the precipitation of carbides during austempering and contributes to the formation of residual ⁇ .
- Al is an element that acts as a deoxidizer in the steel making process.
- the Al content is 0.005% or more, preferably 0.01% or more, more preferably 0.03% or more.
- the Al content is 3% or less, preferably 1.5% or less, more preferably 1% or less, still more preferably 0.5% or less, and particularly preferably 0.2% or less.
- the amount of P is an impurity element inevitably contained in steel, and when the amount of P becomes excessive, the weldability of the steel sheet deteriorates. Therefore, the amount of P is 0.1% or less, preferably 0.08% or less, more preferably 0.05% or less. The amount of P is preferably as small as possible, but it is industrially difficult to reduce it to 0%.
- the amount of S is 0.05% or less, preferably 0.01% or less, more preferably 0.005% or less.
- the amount of S should be as small as possible, but it is industrially difficult to make it 0%.
- N is an impurity element that is inevitably contained in the steel as in the case of P described above.
- the N content is 0.01% or less, preferably 0.008% or less, more preferably 0.005% or less.
- the amount of N should be as small as possible, but it is industrially difficult to reduce it to 0%.
- the high-strength steel sheet according to the present invention satisfies the above component composition, and the remaining components are iron and unavoidable impurities other than P, S, and N.
- the inevitable impurities include, for example, O (oxygen) and, for example, trump elements such as Pb, Bi, Sb, and Sn.
- O is preferably, for example, more than 0% and 0.01% or less.
- O is an element that causes a decrease in elongation, hole expansibility, and bendability when contained in excess. Accordingly, the O content is preferably 0.01% or less, more preferably 0.008% or less, and still more preferably 0.005% or less.
- the steel sheet of the present invention is further as another element, (A) at least one element selected from the group consisting of Cr: more than 0% and 1% or less, Mo: more than 0% and 1% or less, and B: more than 0% and 0.01% or less, (B) at least one element selected from the group consisting of Ti: more than 0% and 0.2% or less, Nb: more than 0% and 0.2% or less, and V: more than 0% and 0.2% or less, (C) at least one element selected from the group consisting of Cu: more than 0% and 1% or less, and Ni: more than 0% and 1% or less, (D) at least one element selected from the group consisting of Ca: more than 0% and not more than 0.01%, Mg: more than 0% and not more than 0.01%, and rare earth elements: more than 0% and not more than 0.01%, Etc. may be contained.
- A at least one element selected from the group consisting of Cr: more than 0% and 1% or less, Mo: more than
- (A) Cr, Mo, and B are elements that effectively act to obtain bainite and tempered martensite, as in the case of Mn, and these elements may be added alone or in combination of two or more. May be used.
- Cr and Mo are each preferably contained alone in an amount of 0.1% or more, more preferably 0.2% or more.
- B is preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more.
- Cr and Mo are each preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. When Cr and Mo are used in combination, the total amount is recommended to be 1.5% or less.
- the amount of B is preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.004% or less.
- Ti, Nb, and V are elements that act to strengthen the steel sheet by forming precipitates such as carbides and nitrides in the steel sheet.
- Ti, Nb, and V are each preferably contained in an amount of 0.01% or more, more preferably 0.02% or more.
- Ti, Nb, and V are each independently preferably 0.2% or less, more preferably 0.18% or less, and still more preferably 0.15% or less.
- Ti, Nb, and V may each be contained alone, or two or more elements that are arbitrarily selected may be contained.
- Cu and Ni are elements that effectively act to stabilize ⁇ and generate residual ⁇ . These elements can be used alone or in combination. In order to exhibit such an action effectively, it is preferable to contain Cu and Ni individually by 0.05% or more, more preferably 0.1% or more. However, when Cu and Ni are contained excessively, the hot workability deteriorates. Therefore, in the present invention, Cu and Ni are each preferably 1% or less, more preferably 0.8% or less, still more preferably 0.5% or less. In addition, when Cu is contained in excess of 1%, hot workability deteriorates. However, when Ni is added, deterioration of hot workability is suppressed. However, Cu may be added in excess of 1%.
- Ca, Mg, and rare earth elements are elements that act to finely disperse inclusions in the steel sheet.
- Ca, Mg and rare earth elements are each preferably contained in an amount of 0.0005% or more, more preferably 0.001% or more.
- Ca, Mg, and rare earth elements are each independently preferably 0.01% or less, more preferably 0.005% or less, and still more preferably 0.003% or less.
- Ca, Mg, and a rare earth element may each be contained alone, or two or more elements selected arbitrarily may be contained.
- the rare earth element is meant to include lanthanoid elements which are 15 elements from La to Lu, and Sc (scandium) and Y (yttrium).
- lanthanoid elements which are 15 elements from La to Lu, and Sc (scandium) and Y (yttrium).
- the group consisting of La, Ce and Y It is preferable to contain at least one element selected from the group consisting of La and Ce, and more preferable to contain at least one element selected from the group consisting of La and Ce.
- the component composition of the base steel sheet used in the present invention has been described above.
- the manufacturing method of the present invention includes a first manufacturing method in which pickling is performed immediately after hot rolling without holding the heat, and a second manufacturing method in which pickling is performed after warming after hot rolling.
- the lower limit of the hot rolling coiling temperature is different between the first manufacturing method that does not retain heat depending on the presence or absence of heat retention and the second manufacturing method that retains heat, but the other steps are the same. Details will be described below.
- the first production method according to the present invention includes a hot rolling step, pickling, cold rolling step, oxidation step, continuous reduction Zn plating line [CGL (Continuous Galvanizing Line)], reduction step, cooling step, and plating. It is roughly divided into processes. And the characteristic part of this invention is: A hot rolling step of obtaining a hot-rolled steel sheet in which an internal oxide layer is formed by winding a steel sheet satisfying the steel components of the base steel sheet at a temperature of 600 ° C.
- it demonstrates in order of a process.
- a hot rolled steel sheet satisfying the steel components of the base steel sheet is prepared.
- Hot rolling may be performed according to a conventional method.
- the heating temperature is preferably about 1150 to 1300 ° C.
- finish rolling temperature is approximately 850 to 950 ° C.
- the coiling temperature after hot rolling it is important to control the coiling temperature after hot rolling to 600 degreeC or more.
- an internal oxide layer is formed on the surface of the base steel plate, and a soft layer is also formed by decarburization, so that a desired internal oxide layer and soft layer can be obtained on the steel plate after plating.
- the coiling temperature is preferably 620 ° C. or higher, more preferably 640 ° C. or higher.
- the upper limit is preferably 750 ° C. or lower.
- the hot-rolled steel sheet thus obtained is pickled and cold-rolled so that the average depth d of the internal oxide layer remains at 4 ⁇ m or more.
- the soft layer remains, so that a desired soft layer can be easily formed after plating.
- mineral acids such as hydrochloric acid, sulfuric acid, nitric acid can be used.
- the concentration and temperature of the pickling solution are high and the pickling time is long, the internal oxide layer tends to dissolve and become thin.
- the concentration and temperature of the pickling solution are low and the pickling time is short, removal of the black skin scale layer by pickling becomes insufficient. Therefore, for example, when hydrochloric acid is used, it is recommended to control the concentration to about 3 to 20%, the temperature to 60 to 90 ° C., and the time to about 35 to 200 seconds.
- the number of the pickling tanks used at the time of pickling is not particularly limited, and a plurality of pickling tanks may be used.
- a pickling inhibitor such as an amine, that is, an inhibitor or a pickling accelerator may be added to the pickling solution.
- cold rolling is performed so that the average depth d of the internal oxide layer remains 4 ⁇ m or more.
- the cold rolling conditions are preferably controlled so that the cold rolling rate is in the range of about 20 to 70%.
- oxidation and reduction are performed. Specifically, first, oxidation is performed in an oxidation zone at an air ratio of 0.9 to 1.4.
- the air ratio means the ratio of the amount of air actually supplied to the amount of air that is theoretically required to completely burn the supplied combustion gas. In the examples described later, CO gas is used. When the air ratio is higher than 1, oxygen is in an excess state, and when the air ratio is lower than 1, oxygen is in a shortage state.
- Oxidation in an atmosphere where the air ratio falls within the above range promotes decarburization, so that a desired soft layer is formed and bendability is improved.
- an Fe oxide film can be generated on the surface, and generation of the composite oxide film and the like harmful to the plating property can be suppressed.
- the air ratio needs to be controlled to 0.9 or more, and is preferably controlled to 1.0 or more.
- the air ratio is as high as 1.4 or more, an Fe oxide film is excessively generated and cannot be sufficiently reduced in the next reduction furnace, thereby impairing the plateability.
- the air ratio needs to be controlled to 1.4 or less, and is preferably controlled to 1.2 or less.
- the preferable lower limit of the oxidation temperature is 500 ° C. or higher, more preferably 750 ° C. or higher.
- the upper limit with preferable oxidation temperature is 900 degrees C or less, More preferably, it is 850 degrees C or less.
- the Fe oxide film is reduced in a hydrogen atmosphere in the reduction zone.
- heating is performed in a range equal to or higher than the higher temperature of Ac 3 point or 750 ° C. which is an austenite single phase region, As described in the above (II), it is necessary to heat in the range of higher temperature of Ac 1 point + 20 ° C. or 750 ° C. which is a two-phase region and less than Ac 3 point, and soaking is performed in this temperature range. .
- the soaking temperature is preferably Ac 3 point + 15 ° C. or higher.
- the upper limit of soaking temperature is not specifically limited, For example, 1000 degrees C or less is preferable.
- the Ac 3 point is calculated based on the following formula (i).
- [] represents the content (% by mass) of each element. Calculation is performed by substituting 0 (zero) into the term of the element not contained. This equation is described in “Leslie Steel Material Science” (published by Maruzen Co., Ltd., William C. Leslie, p273).
- the atmosphere in the reduction zone contains, for example, hydrogen and nitrogen, and the hydrogen concentration is preferably controlled in the range of about 5 to 25% by volume.
- the dew point is preferably controlled to, for example, ⁇ 30 to ⁇ 60 ° C.
- the holding time at the time of soaking is not particularly limited, and for example, it is preferably controlled to about 10 to 100 seconds, particularly about 10 to 80 seconds.
- cooling is performed at an average cooling rate of 0 ° C./second to 20 ° C./second or less up to 600 ° C., and from 600 ° C. to an arbitrary stop temperature Z satisfying 100 to 540 ° C.
- an average cooling rate 10 ° C./second or more, which is larger than the average cooling rate up to 600 ° C. Hold at a temperature range of ⁇ 540 ° C. for 50 seconds or more.
- the average cooling rate up to 600 ° C. needs to be controlled to more than 0 ° C./second, and preferably 2 ° C./second or more.
- the upper limit of the average cooling rate up to 600 ° C. needs to be 20 ° C./second or less in order to secure the amount of polygonal ferrite produced.
- the average cooling rate up to 600 ° C. is preferably 15 ° C./second or less, more preferably 10 ° C./second or less.
- the average cooling rate from 600 ° C. needs to be controlled to 10 ° C./second or more, preferably 20 ° C./second or more.
- the upper limit of the average cooling rate from 600 ° C. is not particularly limited, but it is generally preferably 100 ° C./second or less in consideration of easy control of the base steel plate temperature and equipment costs.
- the average cooling rate from 600 ° C. is more preferably 50 ° C./second or less, and further preferably 30 ° C./second or less.
- the rate when cooling to 600 ° C. is the average slow cooling rate
- the rate when cooling from 600 ° C. to the higher one of the stop temperature Z or 500 ° C. is the average quench rate
- the average The rapid cooling rate needs to be larger than the average slow cooling rate, and the formation of polygonal ferrite can be promoted.
- Polygonal ferrite becomes excessive when the soaking temperature falls below the lower one of Ac 1 point + 20 ° C. or 750 ° C.
- the soaking temperature is preferably Ac 1 point + 25 ° C. or higher.
- the upper limit of the soaking temperature is less than the Ac 3 point in order to soak in the two-phase region.
- a preferable upper limit of the soaking temperature is Ac 3 point ⁇ 10 ° C. or less.
- the Ac 1 point is calculated based on the following formula (ii).
- [] represents the content (% by mass) of each element. Calculation is performed by substituting 0 (zero) into the term of the element not contained. This formula is expressed in “Leslie Steel Materialology” (published by Maruzen Co., Ltd., William C. By Leslie, p 273).
- Ac 1 (° C.) 723 + 29.1 ⁇ [Si] ⁇ 10.7 ⁇ [Mn] + 16.9 ⁇ [Cr] ⁇ 16.9 ⁇ [Ni] + 290 ⁇ [As] + 6.38 ⁇ [W] ⁇ (Ii)
- the average cooling rate after soaking should be controlled to 10 ° C./second or more, and preferably 20 ° C./second or more.
- the upper limit of the average cooling rate after the soaking is not particularly limited, it is generally preferably 100 ° C./second or less in consideration of easy control of the base steel sheet temperature, equipment cost, and the like.
- the average cooling rate after the soaking is more preferably 50 ° C./second or less, and further preferably 30 ° C./second or less.
- the holding time in the temperature range is preferably 60 seconds or longer, more preferably 70 seconds or longer.
- the upper limit of the holding time in the said temperature range is not specifically limited, For example, Preferably it is 1500 seconds or less, More preferably, it is 1400 seconds or less, More preferably, it is 1300 seconds or less.
- the specific conditions for cooling to an arbitrary stop temperature Z satisfying the above 100 to 540 ° C. and holding in the temperature range of 100 to 540 ° C. are not particularly limited, and may be held at the stop temperature Z at a constant temperature. In this temperature range, constant temperature holding may be performed so that the holding temperature is two or more stages.
- the cooling rate may be changed and the cooling may be performed over a predetermined time within the temperature range, or the heating may be performed over the predetermined time within the temperature range. .
- two or more stages of multi-stage cooling with different cooling rates may be performed, or two or more stages of multi-stage heating with different heating rates may be performed.
- the low temperature transformation generation phase includes the high temperature region generation bainite, and the high temperature region generation bainite is more than 10 area% and not more than 85 area% with respect to the entire metal structure,
- the low-temperature region-generated bainite and the tempered martensite may be included, and the total of the low-temperature region-generated bainite and the tempered martensite produces a base steel sheet having a total area of 0 to less than 10 area% with respect to the entire metal structure.
- cooling to 600 ° C. is performed at an average cooling rate of more than 0 ° C./second and not more than 20 ° C./second, and from 600 ° C. is greater than the above average cooling rate to 600 ° C. after the soaking.
- satisfies the following (a1) After the soaking in (II) above, it is preferable that the following (a1) is satisfied.
- high-temperature region-generated bainite can be mainly generated in the low-temperature transformation generation phase.
- the lower limit of the temperature at which the cooling is stopped is more preferably 430 ° C. or higher.
- the upper limit of the temperature at which the cooling is stopped is more preferably 480 ° C. or less, and further preferably 460 ° C. or less.
- the holding time in the above temperature range is more preferably 70 seconds or more, still more preferably 100 seconds or more, and particularly preferably 200 seconds or more.
- the upper limit of the holding time in the said temperature range is not specifically limited, For example, Preferably it is 1500 seconds or less, More preferably, it is 1400 seconds or less, More preferably, it is 1300 seconds or less.
- the average cooling rate up to 500 ° C. is preferably controlled to 10 ° C./second or more, more preferably 20 ° C./second or more.
- the upper limit of the average cooling rate up to 500 ° C. is not particularly limited, but it is preferable to control the temperature to about 100 ° C./second or less in consideration of ease of control of the base steel sheet temperature and equipment cost.
- the average cooling rate up to 500 ° C. is more preferably 50 ° C./second or less, and still more preferably 30 ° C./second or less.
- the low-temperature transformation generation phase includes the high-temperature region generation bainite, the low-temperature region generation bainite, and the tempered martensite.
- the total of the low temperature region bainite and the tempered martensite is 10 to 75 area% with respect to the entire metal structure.
- the above-mentioned cooling stop temperature Za2 is set to 380 ° C. or higher and lower than 420 ° C., and maintained in this temperature range for 50 seconds or longer, thereby generating high temperature region bainite, low temperature region bainite, and tempered martensite as a low temperature transformation generation phase.
- the average interval is higher than that of high-temperature region bainite having an average interval of 1 ⁇ m or more.
- a state in which low-temperature region bainite having an interval of less than 1 ⁇ m is mixed is obtained.
- the lower limit of the temperature at which the cooling is stopped is more preferably 390 ° C. or higher.
- the upper limit of the temperature at which the cooling is stopped is more preferably 410 ° C. or lower.
- the holding time in the above temperature range is more preferably 70 seconds or more, still more preferably 100 seconds or more, and particularly preferably 200 seconds or more.
- the upper limit of the holding time in the said temperature range is not specifically limited, For example, Preferably it is 1500 seconds or less, More preferably, it is 1400 seconds or less, More preferably, it is 1300 seconds or less.
- the amount of high-temperature region-generated bainite can be controlled by holding in the T1 temperature region for a predetermined time, and the untransformed austenite is converted into low-temperature region-generated bainite or martensite by the austempering process that is maintained in the T2 temperature region for a predetermined time. While transforming into sites, carbon can be concentrated to austenite to generate residual ⁇ , and a metal structure defined in the present invention can be generated.
- the concentration of carbon is limited to the concentration indicated by the To line where the free energy of polygonal ferrite and austenite becomes equal, and the bainite transformation also stops. Strictly speaking, the bainite transformation stops at a concentration slightly deviated from the To line. Since the To line becomes lower in carbon concentration as the temperature is higher, if the austempering process is performed at a relatively high temperature, the bainite transformation stops at a certain level even if the processing time is increased. At this time, since the stability of untransformed austenite is low, a coarse MA mixed phase is generated.
- the allowable amount of C concentration to the untransformed austenite can be increased by holding in the T2 temperature range, so the low temperature range is higher than the high temperature range.
- the bainite transformation proceeds and the MA mixed phase becomes smaller.
- the size of the lath-like structure is smaller when held at the T2 temperature range than when held at the T1 temperature range, the MA mixed phase itself is subdivided even if the MA mixed phase exists. Thus, the MA mixed phase can be reduced.
- the T1 temperature range defined by the above formula (1) is specifically 400 ° C. or more and 540 ° C. or less.
- a high temperature range bainite can be generated. That is, when the temperature is maintained at a temperature exceeding 540 ° C., the formation of high temperature bainite is suppressed.
- polygonal ferrite is excessively generated and pseudo pearlite is generated, so that desired characteristics cannot be obtained. Therefore, the upper limit of the T1 temperature range is preferably 540 ° C. or less, more preferably 520 ° C. or less, and further preferably 500 ° C. or less.
- the lower limit of the T1 temperature range is preferably 400 ° C. or higher, more preferably 420 ° C. or higher.
- the holding time in the T1 temperature range is preferably 10 to 100 seconds. If the holding time exceeds 100 seconds, the high-temperature region-generated bainite is excessively generated. Therefore, as will be described later, the amount of low-temperature region-generated bainite or the like cannot be ensured even if the predetermined time is maintained in the T2 temperature region. Accordingly, it is impossible to achieve both strength and workability. Further, if the temperature is held for a long time in the T1 temperature range, carbon is excessively concentrated in the austenite, so that a coarse MA mixed phase is generated even if austempering is performed in the T2 temperature range, and workability deteriorates. Therefore, the holding time is 100 seconds or less, preferably 90 seconds or less, more preferably 80 seconds or less.
- the holding time in the T1 temperature range is 10 seconds or longer, preferably 15 seconds or longer, more preferably 20 seconds or longer, and even more preferably 30 seconds or longer.
- the holding time in the T1 temperature range means the time from when the surface temperature of the steel sheet reaches the upper limit temperature in the T1 temperature range to the lower limit temperature in the T1 temperature range.
- FIG. 6 shows a case where one-stage constant temperature holding is performed, the present invention is not limited to this, and two or more constant temperature holdings with different holding temperatures are performed within the T1 temperature range. May be.
- FIG. 6 (Ii) in FIG. 6 is, after soaking, after rapidly cooling to an arbitrary temperature Z b satisfying the above formula (1), after changing the cooling rate and cooling over a predetermined time within the T1 temperature range, This is an example in which the cooling rate is changed again to cool to an arbitrary temperature satisfying the above expression (2).
- FIG. 6 (ii) shows a case where the cooling is performed for a predetermined time within the range of the T1 temperature range, but the present invention is not limited to this, and if it is within the range of the T1 temperature range, it takes a predetermined time. And a step of heating may be included, and cooling and heating may be repeated as appropriate.
- multi-stage cooling of two or more stages having different cooling rates may be performed instead of single-stage cooling. Further, one-stage heating or multi-stage heating of two or more stages may be performed (not shown).
- (Iii) in FIG. 6 shows that after soaking, the cooling rate is changed after quenching to an arbitrary temperature Z b satisfying the above formula (1), and up to an arbitrary temperature satisfying the above formula (2).
- This is an example of gradual cooling at a cooling rate.
- the residence time in the T1 temperature range may be 10 to 100 seconds.
- the present invention is not intended to be limited to the heat patterns shown in FIGS. 6 (i) to (iii), and any other heat pattern can be adopted as long as the requirements of the present invention are satisfied.
- the T2 temperature range defined by the above formula (2) is specifically preferably 200 ° C. or higher and lower than 400 ° C.
- untransformed austenite that has not been transformed in the T1 temperature range can be transformed into low temperature range bainite or martensite.
- the bainite transformation proceeds, finally residual ⁇ is generated, and the MA mixed phase is subdivided.
- this martensite exists as quenching martensite immediately after transformation, it is tempered while being maintained in the T2 temperature region and remains as tempered martensite. This tempered martensite exhibits the same characteristics as low temperature region bainite generated in the temperature region where martensitic transformation occurs.
- the T2 temperature range is preferably less than 400 ° C., more preferably 390 ° C. or less, and further preferably 380 ° C. or less.
- the low temperature region bainite is not generated even if kept at a temperature lower than 200 ° C., the carbon concentration in the austenite becomes low, the amount of residual ⁇ cannot be secured, and more quenching martensite is generated, so the strength is high. It becomes high and elongation and local deformability deteriorate.
- the lower limit of the T2 temperature range is preferably 200 ° C. or higher, more preferably 250 ° C. or higher, and still more preferably 280 ° C. or higher.
- the holding time in the T2 temperature range satisfying the above formula (2) is preferably 50 seconds or more.
- the holding time is less than 50 seconds, the amount of low-temperature region bainite and the like is reduced, the carbon concentration in the austenite is lowered and the residual ⁇ amount cannot be secured, and more hardened martensite is produced, It becomes high and elongation and local deformability deteriorate. Further, since carbon concentration is not promoted, the amount of residual ⁇ is reduced, and the elongation cannot be improved.
- the holding time is preferably 50 seconds or more, more preferably 70 seconds or more, still more preferably 100 seconds or more, and particularly preferably 200 seconds or more.
- the upper limit of the holding time is not particularly limited, productivity decreases when held for a long time, and concentrated carbon cannot be precipitated as carbides to generate residual ⁇ , resulting in a decrease in elongation and workability. to degrade. Therefore, the upper limit of the holding time may be set to 1800 seconds or less, for example.
- the holding time in the T2 temperature range means the time from when the surface temperature of the steel sheet reaches the upper limit temperature in the T2 temperature range to the lower limit temperature in the T2 temperature range.
- the method of holding in the T2 temperature range is not particularly limited as long as the residence time in the T2 temperature range is 50 seconds or more. Even if the temperature is kept constant like the heat pattern in the T1 temperature range shown in FIG. It may be cooled or heated within the T2 temperature range. Further, multistage holding may be performed at different holding temperatures.
- the Ms point is calculated based on the following formula (iii).
- [] represents the content (% by mass) of each element. Calculation is performed by substituting 0 (zero) into the term of the element not contained. This equation is based on the equation described in “Leslie Steel Materialology” (published by Maruzen Co., Ltd., William C. Leslie, p231) and takes into account the polygonal ferrite fraction.
- Vf represents the polygonal ferrite fraction (area%), but since it is difficult to directly measure the polygonal ferrite fraction during production, separately from heating, soaking to cooling.
- the soaking As shown in FIG. 7, it is preferable to rapidly cool to an arbitrary temperature Z c1 or Ms point satisfying the above formula (3) at an average cooling rate of 10 ° C./second or more.
- the range up to an arbitrary temperature Z c1 or Ms point satisfying the above formula (3) is quenched to suppress the transformation of austenite to polygonal ferrite, and low-temperature range bainite and martensite are located.
- a fixed amount can be generated.
- the average cooling rate in this section is more preferably 15 ° C./second or more.
- the upper limit of the average cooling rate of the said area is not specifically limited, For example, what is necessary is just about 100 degreeC / second.
- the holding time in the T3 temperature range means that after the soaking, the surface temperature of the steel sheet is lower than 400 ° C., the heating is started after being held in the T3 temperature range, and the surface temperature of the steel sheet is It means the time to reach 400 ° C. Therefore, in this invention, since it cools to room temperature after hold
- the holding time in the T4 temperature range means heating after holding in the T3 temperature range, and starting cooling after holding in the T4 temperature range from the time when the surface temperature of the steel plate reaches 400 ° C. It means the time until the temperature reaches 400 ° C. Therefore, in the present invention, as described above, after soaking, the T4 temperature range is passed while cooling to the T3 temperature range. In the present invention, the time for passing this cooling is in the T4 temperature range. Not included in stay time. This is because, during this cooling, the residence time is too short, so that almost no transformation occurs and no high temperature region bainite is generated.
- the present invention it is possible to generate a predetermined amount of high temperature region bainite by appropriately controlling the time for holding in the T3 temperature region and the T4 temperature region. Specifically, by maintaining for a predetermined time in the T3 temperature range, the untransformed austenite is transformed into low-temperature range bainite, bainitic ferrite, or martensite, and the austempering process is performed by holding for a predetermined time in the T4 temperature range. In this way, untransformed austenite is transformed into high-temperature-range-generated bainite and bainitic ferrite, and the amount of formation is controlled, and carbon is concentrated to austenite to generate residual ⁇ .
- the metal structure to be produced can be generated.
- miniaturize MA mixed phase is also exhibited by hold
- the T3 temperature range defined by the above formula (3) is specifically preferably 100 ° C. or more and less than 400 ° C.
- the untransformed austenite can be transformed into low temperature range bainite, bainitic ferrite, or martensite. Further, by securing a sufficient holding time, the bainite transformation proceeds, finally residual ⁇ is generated, and the MA mixed phase is subdivided.
- this martensite exists as quenching martensite immediately after transformation, it is tempered while being maintained in a T4 temperature range described later, and remains as tempered martensite. This tempered martensite does not adversely affect the elongation, hole expansibility, or bendability of the steel sheet.
- the T3 temperature range is preferably less than 400 ° C. (however, when the Ms point is lower than 400 ° C., it is equal to or lower than the Ms point).
- the T3 temperature range is more preferably 390 ° C. or less (provided that the Ms point ⁇ 10 ° C.
- the Ms point is lower than 390 ° C., the Ms point ⁇ 10 ° C. or less), more preferably 380 ° C. or less (provided that the Ms point ⁇ 20 ° C. is 380 ° C.).
- the Ms point is ⁇ 20 ° C. or lower.
- the martensite fraction becomes too large, so that workability deteriorates.
- the lower limit of the T3 temperature range is preferably 100 ° C. or higher.
- the T3 temperature range is more preferably 110 ° C. or higher, and still more preferably 120 ° C. or higher.
- the holding time in the T3 temperature range satisfying the above formula (3) is preferably 5 to 180 seconds.
- the holding time is preferably 5 seconds or more, more preferably 10 seconds or more, still more preferably 20 seconds or more, and particularly preferably 40 seconds or more.
- the holding time exceeds 180 seconds, there is a tendency that the low temperature region bainite is excessively generated, and as will be described later, it is difficult to secure the amount of high temperature region bainite and the like even if it is held for a predetermined time in the T4 temperature region. Become. Accordingly, the elongation decreases.
- the holding time is preferably 180 seconds or less, more preferably 150 seconds or less, still more preferably 120 seconds or less, and particularly preferably 80 seconds or less.
- the method of holding in the T3 temperature range satisfying the above formula (3) is not particularly limited as long as the residence time in the T3 temperature range is in the above-described range.
- the heat shown in (iv) to (vi) of FIG. A pattern may be adopted.
- the present invention is not intended to be limited to this, and heat patterns other than those described above can be appropriately employed as long as the requirements of the present invention are satisfied.
- (Iv) in FIG. 7 is an example in which, after soaking, the sample is rapidly cooled to an arbitrary temperature Z c1 that satisfies the above formula (3), and then kept at this temperature Z c1 for a predetermined time. Heating to an arbitrary temperature satisfying 4).
- (iv) of FIG. 7 shows the case where one-step constant temperature holding is performed, the present invention is not limited to this, and two or more steps with different holding temperatures are provided as long as they are within the T3 temperature range. Constant temperature holding may be performed (not shown).
- FIG. 7 shows that, after soaking, after rapidly cooling to an arbitrary temperature Z c1 that satisfies the above formula (3), the cooling rate is changed, and cooling is performed for a predetermined time within the range of the T3 temperature range. This is an example of heating to an arbitrary temperature that satisfies the above expression (4).
- FIG. 7 (v) shows a case where one-stage cooling is performed, the present invention is not limited to this, and multi-stage cooling of two or more stages having different cooling rates may be performed (not shown). ).
- FIG. 7 shows that after soaking, the sample is rapidly cooled to an arbitrary temperature Z c1 that satisfies the above formula (3), and then heated for a predetermined time within the T3 temperature range, thereby satisfying the above formula (4).
- This is an example of heating to an arbitrary temperature.
- FIG. 7 shows the case where one-stage heating is performed, the present invention is not limited to this, and multi-stage heating including two or more stages with different heating rates may be performed (not shown). )
- the T4 temperature range defined by the above formula (4) is specifically preferably 400 ° C. or more and 500 ° C. or less.
- the upper limit of the T4 temperature range is preferably 500 ° C. or less, more preferably 490 ° C. or less, and further preferably 480 ° C. or less.
- the lower limit of the T4 temperature range is preferably 400 ° C. or higher, more preferably 420 ° C. or higher, and still more preferably 425 ° C. or higher.
- the time for holding in the T4 temperature range satisfying the above formula (4) is preferably 30 seconds or more. According to the present invention, even if the holding time in the T4 temperature range is about 30 seconds, the low temperature range bainite or the like is generated in the high temperature range because the low temperature range bainite or the like is generated by holding the T3 temperature range for a predetermined time in advance. Since the generation of the generated bainite is promoted, the generation amount of the high temperature region generated bainite can be ensured. However, when the holding time is shorter than 30 seconds, many untransformed portions remain and carbon concentration is insufficient, so that martensitic transformation occurs during the final cooling from the T4 temperature range.
- the holding time in the T4 temperature range is more preferably 50 seconds or more, and still more preferably 100 seconds. Above, especially preferably 200 seconds or more.
- the upper limit when holding in the T4 temperature range is not particularly limited, but even if it is held for a long time, the formation of the high temperature range bainite is saturated and the productivity is lowered, so that it is preferably 1800 seconds or less, more preferably 1500 seconds. Hereinafter, it is more preferably set to 1000 seconds or less.
- the method of holding in the T4 temperature range satisfying the above formula (4) is not particularly limited as long as the residence time in the T4 temperature range is 30 seconds or more, and in the T4 temperature range as in the heat pattern in the T3 temperature range. It may be held at a constant temperature, or may be cooled or heated within the T4 temperature range.
- the present invention after being held in the T3 temperature range on the low temperature side, it is held in the T4 temperature range on the high temperature side. However, the low temperature zone bainite generated in the T3 temperature range is heated to the T3 temperature range.
- the present inventors have confirmed that the lath interval, that is, the above average interval does not change, although the tempering causes recovery of the underlying structure.
- the average cooling rate up to 500 ° C. is preferably controlled to 10 ° C./second or more, more preferably 20 ° C./second or more.
- the upper limit of the average cooling rate up to 500 ° C. is not particularly limited, but it is preferable to control the temperature to about 100 ° C./second or less in consideration of ease of control of the base steel sheet temperature and equipment cost.
- the average cooling rate up to 500 ° C. is more preferably 50 ° C./second or less, and still more preferably 30 ° C./second or less.
- the low-temperature transformation generation phase contains the low-temperature region-generated bainite and tempered martensite, and the total of the low-temperature region-generated bainite and tempered martensite is 10 with respect to the entire metal structure. It is more than area% and not more than 85 area%, and may include the high-temperature region-generated bainite.
- the high-temperature region-generated bainite is used for producing a base steel sheet having a surface area of 0 to 10% by area with respect to the entire metal structure.
- the holding time in the above temperature range is more preferably 70 seconds or more, still more preferably 100 seconds or more, and particularly preferably 200 seconds or more.
- the upper limit of the holding time in the said temperature range is not specifically limited, For example, Preferably it is 1500 seconds or less, More preferably, it is 1400 seconds or less, More preferably, it is 1300 seconds or less.
- the condition of (c2) is the same as that of (c1) above.
- the cooling stop temperature Z c2 is set within the T3 temperature region depending on the components.
- the martensite is tempered to become tempered martensite.
- low temperature region bainite and the like are mainly used.
- high temperature region bainite is also generated, but since the amount of tempered martensite is increased, the result is mainly low temperature region bainite.
- the average cooling rate up to 500 ° C. is preferably controlled to 10 ° C./second or more, more preferably 20 ° C./second or more.
- the upper limit of the average cooling rate up to 500 ° C. is not particularly limited, but it is preferable to control the temperature to about 100 ° C./second or less in consideration of ease of control of the base steel sheet temperature and equipment cost.
- the average cooling rate up to 500 ° C. is more preferably 50 ° C./second or less, and still more preferably 30 ° C./second or less.
- hot dip galvanizing is performed according to a conventional method.
- the method of hot dip galvanizing is not particularly limited, and for example, the preferred lower limit of the plating bath temperature is 400 ° C. or higher, more preferably 440 ° C. or higher.
- the upper limit with preferable plating bath temperature is 500 degrees C or less, More preferably, it is 470 degrees C or less.
- the composition of the plating bath is not particularly limited, and a known hot dip galvanizing bath may be used.
- the cooling conditions after hot dip galvanizing are not particularly limited, and for example, the average cooling rate to room temperature is preferably controlled to about 1 ° C./second or more, more preferably 5 ° C./second or more.
- the upper limit of the average cooling rate to room temperature is not particularly defined, but it is preferably controlled to about 50 ° C./second or less in consideration of ease of control of the base steel sheet temperature, equipment cost, and the like.
- the average cooling rate to room temperature is preferably 40 ° C./second or less, more preferably 30 ° C./second or less.
- an alloying treatment may be performed by a conventional method as necessary.
- the conditions for the alloying treatment are also not particularly limited. For example, after performing hot dip galvanization under the above conditions, about 500 to 600 ° C., particularly about 500 to 550 ° C., about 5 to 30 seconds, especially about 10 to 25 seconds. It is preferable to carry out holding. If the temperature and time are below the above range, alloying becomes insufficient. On the other hand, if the temperature and time exceed the above range, the retained austenite is reduced due to precipitation of carbides, and desired characteristics cannot be obtained. Furthermore, polygonal ferrite is easily generated excessively.
- the alloying treatment may be performed using, for example, a heating furnace, a direct fire, or an infrared heating furnace.
- the heating means is not particularly limited, and for example, conventional means such as gas heating, induction heater heating, that is, heating by a high frequency induction heating device can be adopted.
- an alloyed hot-dip galvanized steel sheet is obtained by cooling according to a conventional method.
- the average cooling rate to room temperature is preferably controlled to about 1 ° C./second or more.
- the upper limit of the average cooling rate to room temperature is not particularly defined, but it is preferably controlled to about 50 ° C./second or less in consideration of ease of control of the base steel sheet temperature, equipment cost, and the like.
- the second production method is: A hot rolling step of winding a steel plate satisfying the steel components of the base steel plate at a temperature of 500 ° C. or higher; Maintaining the temperature at a temperature of 500 ° C. or more for 60 minutes or more; Pickling and cold rolling so that the average depth d of the internal oxide layer remains 4 ⁇ m or more; Oxidizing in an oxidation zone at an air ratio of 0.9 to 1.4; (I) In the reduction zone, soaking in a range equal to or higher than the higher of Ac 3 point or 750 ° C., After soaking, it is cooled to 600 ° C.
- the reduction zone soaking is carried out in the range between the higher temperature of Ac 1 point + 20 ° C or 750 ° C and higher than the Ac 3 point, After soaking, it is cooled to an arbitrary stop temperature Z satisfying 100 to 540 ° C., and the range to the higher one of the stop temperature Z or 500 ° C. is cooled at an average cooling rate of 10 ° C./second or more, Holding for 50 seconds or more in a temperature range of 100 to 540 ° C., Include in this order.
- the lower limit of the coiling temperature after hot rolling is set to 500 ° C. or more, and the heat retaining step is provided after the hot rolling step. Only the first manufacturing method is different. Therefore, only the difference will be described below. For the steps consistent with the first manufacturing method, the first manufacturing method may be referred to.
- the reason for providing the heat-retaining step as described above is that it is possible to maintain for a long time in a temperature range that can be oxidized by heat-retaining, and to expand the lower limit of the coiling temperature range from which a desired internal oxide layer and soft layer can be obtained. .
- the uniformity of the base steel sheet can be improved by reducing the temperature difference between the surface layer and the inside of the base steel sheet.
- the coiling temperature after hot rolling is controlled to 500 ° C. or higher.
- the temperature can be set lower than 600 ° C. which is the lower limit of the winding temperature in the first manufacturing method described above.
- the winding temperature is preferably 540 ° C. or higher, more preferably 570 ° C. or higher.
- the preferable upper limit of coiling temperature is the same as the 1st manufacturing method mentioned above, and it is preferable to set it as 750 degrees C or less.
- the hot-rolled steel sheet thus obtained is kept at a temperature of 500 ° C. or more for 60 minutes or more. Thereby, a desired internal oxide layer can be obtained. It is preferable to keep the hot-rolled steel sheet in a heat-insulating device, for example, so that the above-mentioned effect due to heat insulation is effectively exhibited.
- the apparatus used in the present invention is not particularly limited as long as it is made of a heat insulating material, and as such a material, for example, a ceramic fiber is preferably used.
- the heat retention temperature is preferably 540 ° C. or higher, more preferably 560 ° C. or higher.
- the heat retention time is preferably 100 minutes or more, more preferably 120 minutes or more.
- the plated steel sheet of the present invention obtained by the above-described production method is further subjected to various coatings and coating ground treatments, for example, chemical conversion treatment such as phosphate treatment; organic coating treatment, for example, formation of an organic coating such as a film laminate. May be.
- various coatings and coating ground treatments for example, chemical conversion treatment such as phosphate treatment; organic coating treatment, for example, formation of an organic coating such as a film laminate. May be.
- paint used for various coatings known resins such as epoxy resins, fluororesins, silicone acrylic resins, polyurethane resins, acrylic resins, polyester resins, phenol resins, alkyd resins, melamine resins and the like can be used. From the viewpoint of corrosion resistance, an epoxy resin, a fluororesin, and a silicon acrylic resin are preferable.
- a curing agent may be used together with the resin.
- the paint may also contain known additives such as coloring pigments, coupling agents, leveling agents, sensitizers, antioxidants, UV stabilizers, flame retardants and the like.
- the form of paint is not particularly limited, and any form of paint such as solvent-based paint, water-based paint, water-dispersed paint, powder paint, and electrodeposition paint can be used.
- the coating method is not particularly limited, and a dipping method, a roll coater method, a spray method, a curtain flow coater method, an electrodeposition coating method, and the like can be used. What is necessary is just to set suitably the thickness of coating layers, such as a plating layer, an organic membrane
- the high-strength plated steel sheet of the present invention has high strength, and is excellent in workability (elongation, bendability, and hole expandability) and delayed fracture resistance. For this reason, automotive strength components such as front and rear side members, crash parts such as crash boxes, pillars such as center pillar reinforcements, roof rail reinforcements, side sills, floor members, kick parts, etc. Can be used for parts.
- the obtained hot-rolled steel sheet was pickled under the following conditions, and then cold-rolled at a cold rolling rate of 50%.
- the plate thickness after cold rolling is 1.2 mm.
- Pickling solution 10% hydrochloric acid, temperature: 82 ° C., pickling time: as shown in Tables 2 to 4.
- annealing oxidation, reduction
- cooling were performed in the continuous hot-dip Zn plating line under the conditions shown in Tables 2 to 4 below.
- the temperature of the oxidation furnace installed in the continuous molten Zn plating line was set to 800 ° C.
- Tables 2 to 4 below show the air ratio in the oxidation furnace.
- the hydrogen concentration in the reduction furnace installed in the continuous hot dip Zn plating line was 20% by volume, the balance was nitrogen and inevitable impurities, and the dew point was controlled at -45 ° C.
- soaking was performed at the maximum reached temperature shown in Tables 2 to 4 below.
- the holding times at the highest temperatures shown in Tables 2 to 4 below were all 50 seconds.
- Tables 2 to 4 below the temperature of the Ac 3 point calculated based on the component composition shown in Table 1 and the above formula (i), and the temperature of the Ac 1 point calculated based on the above formula (ii) are shown. Indicates. Tables 2 to 4 below show that “single phase region” when the maximum temperature reached is Ac 3 point or higher, and the maximum temperature is higher than the higher temperature of Ac 1 point + 20 ° C. or 750 ° C. When the temperature was less than Ac 3 point, it was described as “two-phase region”, and when the maximum temperature reached was below the lower temperature of Ac 1 point + 20 ° C. or 750 ° C., it was expressed as “ ⁇ ”.
- any stop temperature that satisfies 100 to 540 ° C after soaking is cooled at the average quenching rate shown in the following Tables 2 to 4, and this temperature is shown in Tables 2 to 4 below. Held for hours.
- GI hot dip galvanized steel sheet
- GA alloyed hot dip galvanized steel sheet
- the average depth of the internal oxide layer was measured not only on the plated steel sheet but also on the base steel sheet after pickling and cold rolling for reference. This is to confirm that the desired average depth of the internal oxide layer has already been obtained in the cold-rolled steel sheet before annealing by controlling the coiling temperature and pickling conditions after hot rolling. It is.
- Each element amount profile in the depth direction of the base steel sheet was measured by continuously dispersing the emission lines in the Ar plasma of each element of O, Fe, and Zn to be sputtered.
- the sputtering conditions were as follows, and the measurement region was set to a depth of 50 ⁇ m from the plating layer surface. (Sputtering conditions) Pulse sputtering frequency: 50Hz Anode diameter (analysis area): Diameter 6 mm Discharge power: 30W Ar gas pressure: 2.5 hPa
- the position where the Zn content and the Fe content from the surface of the plating layer 1 are equal is defined as the interface between the plating layer 1 and the base steel plate 2.
- the average value of the O amount at each measurement position at a depth of 40 to 50 ⁇ m from the surface of the plating layer 1 is defined as the bulk O amount average value, and a range higher by 0.02%, that is, the O amount ⁇ (bulk O The amount average value + 0.02%) was defined as the internal oxide layer 3, and the maximum depth was defined as the internal oxide layer depth.
- the measurement was performed using a Vickers hardness tester with a load of 3 gf. Specifically, as shown in FIG. 3, from the measurement position of the plate thickness internal depth 10 ⁇ m from the interface between the plating layer 1 and the base steel plate 2, the measurement is performed every 5 ⁇ m pitch toward the plate thickness inside to the depth of 100 ⁇ m. Vickers hardness was measured.
- “x” indicates a measurement point of Vickers hardness, and the distance between the measurement points; that is, the distance between “x” and “x” in FIG. 3 is at least 15 ⁇ m or more.
- the area where the Vickers hardness was 90% or less compared with the Vickers hardness at t / 4 part of the base steel sheet was defined as the soft layer, and the depth was calculated.
- a similar test was performed at 10 locations on the same test piece, and the average was defined as the average depth D ( ⁇ m) of the soft layer.
- Tables 5 to 7 below also show the results of calculating the value of D / 2d based on the average depth d of the internal oxide layer and the average depth D of the soft layer.
- the metal structure of the base steel plate which comprises a plated steel plate was observed in the following procedure.
- the metal structure fraction was determined for the low temperature transformation phase, polygonal ferrite, and residual ⁇ .
- required the area ratio by distinguishing in high temperature range production
- the area ratio of high-temperature region-generated bainite, low-temperature region-generated bainite and the like that is, low-temperature region-generated bainite + tempered martensite
- SEM scanning electron microscope
- (4-1) Area ratio of polygonal ferrite such as high-temperature region-generated bainite, low-temperature region-generated bainite, etc. After polishing the surface of the cross-section parallel to the rolling direction of the base steel sheet, and further electrolytically polishing, The 1 ⁇ 4 position was observed with SEM at 5 magnifications at 3000 magnifications. The observation visual field was about 50 ⁇ m ⁇ about 50 ⁇ m.
- the average interval between residual ⁇ and carbides observed as white or light gray was measured based on the method described above.
- the area ratios of the high-temperature region-generated bainite and the low-temperature region-generated bainite, which are distinguished by these average intervals, were measured by a point calculation method.
- the hole expandability was evaluated by the hole expansion rate ⁇ .
- the hole expansion rate ⁇ was measured by conducting a hole expansion test based on the Japan Iron and Steel Federation standard JFS T1001. Specifically, after punching a hole with a diameter of 10 mm in a plated steel sheet, a punch having a 60 ° cone was pushed into the hole in a state where the periphery was constrained, and the hole diameter at the crack initiation limit was measured.
- the bendability was evaluated by the limit bending radius R.
- the critical bending radius R was measured by performing a V-bending test based on JIS Z2248.
- the test piece was obtained by cutting out the No. 1 test piece defined in JIS Z2204 from the plated steel sheet so that the direction perpendicular to the rolling direction of the plated steel sheet was the longitudinal direction, that is, the bending ridge line coincided with the rolling direction. Using.
- the plate thickness of the test piece is 1.4 mm.
- the V-bending test was performed after mechanical grinding was performed on the end face in the longitudinal direction of the test piece so that no crack was generated.
- the V-bending test is performed by setting the die-to-punch angle to 90 °, and changing the tip radius of the punch in units of 0.5 mm, and obtaining the punch tip radius that can be bent without cracks as the limit bending radius R. It was. The results are shown in Tables 5 to 7 below. In addition, the presence or absence of crack generation was observed using a loupe, and the determination was made based on the absence of hair crack generation.
- the mechanical properties of the plated steel plate were evaluated according to the criteria of elongation EL, hole expansion ratio ⁇ , and critical bending radius R according to the metal structure and tensile strength TS of the steel plate. That is, among the low-temperature transformation generation phases, when the amount of high-temperature region-generated bainite increases, the elongation is improved among the mechanical properties, and when the amount of low-temperature region-generated bainite increases, the hole expandability among the mechanical properties. It becomes easy to improve. Further, the mechanical properties of the steel plate are greatly affected by the tensile strength TS of the steel plate. Therefore, required EL, ⁇ , and R differ depending on the metal structure and tensile strength TS of the steel plate.
- the main component of high-temperature region-generated bainite means the metal structure described in (C6-1) above, and the high-temperature region-generated bainite is more than 10 area% and not more than 85 area% with respect to the entire metal structure.
- low temperature region bainite and tempered martensite may be included, and the total of the low temperature region bainite and the tempered martensite is 0 area% or more and less than 10 area% with respect to the entire metal structure.
- the composite structure such as high temperature region bainite and low temperature region bainite means the metal structure described in the above (C6-2), and the high temperature region bainite is compared to the whole metal structure. 10 to 75 area%, and the total of the low temperature region bainite and tempered martensite is 10 to 75 area% with respect to the entire metal structure.
- the main component such as low temperature region bainite means the metal structure described in the above (C6-3), and the low temperature region bainite is more than 10 area% and less than 85 area% with respect to the whole metal structure, Formation bainite may be included, and the high temperature region generation bainite is 0 area% or more and less than 10 area% with respect to the entire metal structure.
- TS is 980 MPa or more and less than 1370 MPa.
- TS is less than 980 MPa or 1370 MPa or more, even if EL, ⁇ , and R are good, they are treated as exempt.
- 0.01M-KSCN 0.01M-KSCN
- No. Examples 20 to 24, 31 to 41, 44, and 45 are examples that do not satisfy the requirements defined in the present invention.
- No. No. 20 is an example in which the amount of C is small, the amount of residual ⁇ produced is small, and the strength is insufficient.
- No. No. 21 is an example in which the amount of Si is small, the internal oxide layer is not sufficiently formed, and the bendability and delayed fracture resistance are deteriorated.
- No. No. 22 is an example in which the amount of Mn is small, and the low-temperature transformation generation phase was not sufficiently generated. Further, the amount of residual ⁇ produced was small. As a result, TS decreased.
- No. 23 and 31 are examples in which the coiling temperature at the time of hot rolling is low, and the average depth of the internal oxide layer after pickling and cold rolling is shallow, so the average depth d of the internal oxide layer after plating, The average depth D has also become shallower. As a result, bendability, delayed fracture resistance, and plating properties were reduced.
- No. 24 is an example in which the heat retention time at the time of hot rolling is insufficient, and since the average depth d of the internal oxide layer after pickling and cold rolling is shallow, the average depth d of the internal oxide layer after plating and the soft layer The average depth D has also become shallower. As a result, bendability, delayed fracture resistance, and plating properties were reduced.
- No. Nos. 32 and 44 are examples in which the pickling time is long, and the internal oxide layer is dissolved, and the average depth d of the desired internal oxide layer and the average depth D of the soft layer are not obtained and become shallow. It was. As a result, bendability, delayed fracture resistance, and plating properties were reduced.
- No. No. 34 is an example in which the soaking temperature at the time of annealing is low, polygonal ferrite is excessively generated, and the low-temperature transformation generation phase is not generated. As a result, the desired hard layer was not obtained, and TS decreased.
- No. No. 35 is an example in which the average slow cooling rate after soaking during annealing was large, and polygonal ferrite was not sufficiently generated. As a result, EL became low.
- No. 36 and 37 are examples in which the average quenching rate from 600 ° C. is small, and polygonal ferrite is excessively generated during cooling, and the low-temperature transformation generation phase and residual ⁇ are not generated. As a result, TS became low.
- No. No. 38 is an example in which the austempering time is too short, and a structure such as massive MA is excessively generated, and the low-temperature transformation generation phase is not sufficiently generated. As a result, ⁇ was low and the bendability was also lowered.
- No. No. 39 is an example in which the holding temperature is too low, and the low-temperature transformation generation phase was not sufficiently generated. As a result, ⁇ was low and the bendability was also lowered.
- No. No. 40 is an example in which the cooling stop temperature after soaking is too low, and the residual ⁇ was not sufficiently generated. As a result, EL became low.
- No. No. 41 was an example in which the cooling stop temperature after soaking was too high, and the low-temperature transformation generation phase and residual ⁇ were not sufficiently generated. As a result, TS decreased.
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Abstract
Description
(a)Cr:0%超1%以下、Mo:0%超1%以下、およびB:0%超0.01%以下よりなる群から選択される少なくとも一種、
(b)Ti:0%超0.2%以下、Nb:0%超0.2%以下、およびV:0%超0.2%以下よりなる群から選択される少なくとも一種、
(c)Cu:0%超1%以下、およびNi:0%超1%以下よりなる群から選択される少なくとも一種、
(d)Ca:0%超0.01%以下、Mg:0%超0.01%以下、および希土類元素:0%超0.01%以下よりなる群から選択される少なくとも一種、
を含有してもよい。
前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は前記均熱後、600℃までの前記平均冷却速度より大きく、且つ平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
をこの順序で含む製造方法によって製造できる。
前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は前記均熱後、600℃までの前記平均冷却速度より大きく、且つ平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
をこの順序で含む製造方法によっても製造できる。
[Ia]前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ia)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a1)を満足する工程、または
(IIa)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a1)を満足する工程、
をこの順序で含む製造方法。
[Ib]前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ia)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a1)を満足する工程、または
(IIa)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a1)を満足する工程、
をこの順序で含む製造方法。
(a1)420℃以上500℃以下を満たす任意の停止温度Za1まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記420~500℃の温度域で50秒以上保持する。
[IIa]前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ib)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a2)、(b)、(c1)のいずれかを満足する工程、または
(IIb)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a2)、(b)、(c1)のいずれかを満足する工程、
をこの順序で含む製造方法。
[IIb]前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ib)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a2)、(b)、(c1)のいずれかを満足する工程、または
(IIb)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a2)、(b)、(c1)のいずれかを満足する工程、
をこの順序で含む製造方法。
(a2)380℃以上420℃未満を満たす任意の停止温度Za2まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記380℃以上420℃未満の温度域で50秒以上保持する。
(b)下記式(1)を満たす任意の停止温度Zbまで冷却すると共に、
前記停止温度Zbまたは500℃のうち高い方の温度までの範囲は平均冷却速度を10℃/秒以上で冷却し、
下記式(1)を満たす温度域T1で10~100秒間保持し、
次いで、下記式(2)を満たす温度域T2に冷却し、
この温度域T2で50秒以上保持する。
400≦T1(℃)≦540 ・・・(1)
200≦T2(℃)<400 ・・・(2)
(c1)下記式(3)を満たす任意の停止温度Zc1またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4)
[IIIa]前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ic)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a3)または(c2)のいずれかを満足する工程、または
(IIc)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a3)または(c2)のいずれかを満足する工程、
をこの順序で含む製造方法。
[IIIb]前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ic)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a3)または(c2)のいずれかを満足する工程、または
(IIc)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a3)または(c2)のいずれかを満足する工程、
をこの順序で含む製造方法。
(a3)150℃以上380℃未満を満たす任意の停止温度Za3まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記150℃以上380℃未満の温度域で50秒以上保持する。
(c2)下記式(3)を満たす任意の停止温度Zc2またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4)
(a)めっき層と素地鋼板との界面から素地鋼板側にかけての層構成を、SiおよびMnよりなる群から選択される少なくとも一種の酸化物を含む内部酸化層を含む軟質層と、当該軟質層以外の領域であって、低温変態生成相、ポリゴナルフェライト、および残留オーステナイトを含む硬質層を有するように構成すると共に、
(b)上記内部酸化層の平均深さdを4μm以上に厚く制御すると、当該内部酸化層が水素トラップサイトとして機能し、水素脆化を有効に抑制できるため、所期の目的を達成できること、
(c)好ましくは、上記内部酸化層の平均深さdと、上記内部酸化層の領域を含む軟質層の平均深さDとの関係を適切に制御すれば、特に曲げ性が一層高められることを見出し、本発明を完成した。
(A)内部酸化層:SiおよびMnよりなる群から選択される少なくとも一種の酸化物を含む層である。内部酸化層の平均深さdは、4μm以上、後記する(B)に記載の軟質層の平均深さD未満である。
(B)軟質層:上記内部酸化層を含み、上記素地鋼板の板厚をtとしたとき、ビッカース硬さが、上記素地鋼板のt/4部におけるビッカース硬さの90%以下を満足する。軟質層の平均深さDは、20μm以上である。
(C)硬質層:低温変態生成相、ポリゴナルフェライト、および残留γを含む組織で構成される。「低温変態生成相」とは、ベイナイトおよび焼戻しマルテンサイトを意味し、本明細書では、低温変態生成相に焼入ままのマルテンサイト(フレッシュマルテンサイトと呼ばれることもある。)は含まない。フレッシュマルテンサイトは、本明細書では、その他の組織に便宜上分類する。
まず、めっき層1と素地鋼板2の界面に直接接する部分は、平均深さdが4μm以上の内部酸化層3を有する。ここで、平均深さとは、上記界面からの深さの平均値を意味し、その詳細な測定方法は、後記する実施例の欄において図2を用いて説明する。
本発明において軟質層4は、図1に示すように、上記(A)の内部酸化層3の領域を含む層である。この軟質層4は、ビッカース硬さが、素地鋼板2のt/4部におけるビッカース硬さの90%以下を満足するものである。ここで、tは素地鋼板の板厚(mm)である。上記ビッカース硬さの詳細な測定方法は、後記する実施例の欄で説明する。
本発明において硬質層5は、図1に示すように、上記(B)の軟質層4の素地鋼板2側に形成される。この硬質層5は、低温変態生成相、ポリゴナルフェライト、および残留γを含む組織で構成される。
(C6-1)上記低温変態生成相は、上記高温域生成ベイナイトを含み、該高温域生成ベイナイトは、上記金属組織全体に対して10面積%超85面積%以下であり、上記低温域生成ベイナイトおよび上記焼戻しマルテンサイトを含んでもよく、上記低温域生成ベイナイトおよび上記焼戻しマルテンサイトの合計は、上記金属組織全体に対して0面積%以上10面積%未満である。
(C6-2)上記低温変態生成相は、上記高温域生成ベイナイト、低温域生成ベイナイト、および焼戻しマルテンサイトを含み、上記高温域生成ベイナイトは、上記金属組織全体に対して10~75面積%であり、上記低温域生成ベイナイトおよび上記焼戻しマルテンサイトの合計は、上記金属組織全体に対して10~75面積%である。
(C6-3)上記低温変態生成相は、上記低温域生成ベイナイト、および焼戻しマルテンサイトを含む場合は、上記低温域生成ベイナイトおよび上記焼戻しマルテンサイトの合計は、上記金属組織全体に対して10面積%超85面積%以下であり、上記高温域生成ベイナイトを含んでもよく、上記高温域生成ベイナイトは、上記金属組織全体に対して0面積%以上10面積%未満である。
(a)Cr:0%超1%以下、Mo:0%超1%以下、およびB:0%超0.01%以下よりなる群から選択される少なくとも一種の元素、
(b)Ti:0%超0.2%以下、Nb:0%超0.2%以下、およびV:0%超0.2%以下よりなる群から選択される少なくとも一種の元素、
(c)Cu:0%超1%以下、およびNi:0%超1%以下よりなる群から選択される少なくとも一種の元素、
(d)Ca:0%超0.01%以下、Mg:0%超0.01%以下、および希土類元素:0%超0.01%以下よりなる群から選択される少なくとも一種の元素、
等を含有しても良い。
本発明に係る第一の製造方法は、熱間圧延工程、酸洗、冷間圧延工程、連続溶融Znめっきライン[CGL(Continuous Galvanizing Line)]での酸化工程、還元工程、冷却工程、およびめっき工程とに大別される。そして本発明の特徴部分は、
上記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取ることにより内部酸化層を形成した熱延鋼板を得る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は前記均熱後、600℃までの前記平均冷却速度より大きく、且つ平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
をこの順序で含むところにある。以下、工程順に説明する。
上記(I)に記載のように、オーステナイト単相域であるAc3点または750℃のうち高い方の温度以上の範囲で加熱するか、
上記(II)に記載のように、二相域であるAc1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で加熱する必要があり、この温度範囲で均熱処理する。
均熱温度がAc3点または750℃のうち低い方の温度を下回ると、ポリゴナルフェライトが過剰になる。均熱温度は、好ましくはAc3点+15℃以上である。均熱温度の上限は特に限定されないが、例えば、1000℃以下が好ましい。
Ac3(℃)=910-203×[C]1/2-15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]-{30×[Mn]+11×[Cr]+20×[Cu]-700×[P]-400×[Al]-120×[As]-400×[Ti]} ・・・(i)
均熱温度がAc1点+20℃または750℃のうち低い方の温度を下回ると、ポリゴナルフェライトが過剰になる。均熱温度は、好ましくはAc1点+25℃以上である。均熱温度の上限は、二相域で均熱するために、Ac3点未満とする。均熱温度の好ましい上限は、Ac3点-10℃以下である。
Leslie著、p273)に記載されている。
Ac1(℃)=723+29.1×[Si]-10.7×[Mn]+16.9×[Cr]-16.9×[Ni]+290×[As]+6.38×[W] ・・・(ii)
上記(I)における上記均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a1)を満足するか、
上記(II)における上記均熱後、下記(a1)を満足することが好ましい。
上記(I)における上記均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a2)、(b)、(c1)のいずれかを満足するか、
上記(II)における上記均熱後、下記(a2)、(b)、(c1)のいずれかを満足することが好ましい。
400≦T1(℃)≦540 ・・・(1)
200≦T2(℃)<400 ・・・(2)
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4)
Ms(℃)=561-474×[C]/(1-Vf/100)-33×[Mn]-17×[Ni]-17×[Cr]-21×[Mo] ・・・(iii)
上記(I)における上記均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a3)または(c2)のいずれかを満足するか、
上記(II)における上記均熱後、下記(a3)または(c2)のいずれかを満足することが好ましい。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4)
本発明に係る第二の製造方法は、
上記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
この順序で含む。前述した第一の製造方法と対比すると、上記第二の製造方法では、熱延後巻取温度の下限を500℃以上にしたこと、熱延工程の後に保温工程を設けたことの二点でのみ上記第一の製造方法と相違する。よって、以下では、当該相違点のみ説明する。上記第一の製造方法と一致する工程は、上記第一の製造方法を参照すればよい。
酸洗液:10%塩酸、温度:82℃、酸洗時間:表2~表4のとおり。
めっき鋼板の板幅をWとしたとき、W/4部からサイズ50mm×50mmの試験片を採取した後、グロー放電発光分析法(GD-OES;Glow Discharge-Optical Emission Spectroscopy)にて、めっき層表面からのO量、Fe量、およびZn量をそれぞれ分析し、定量した。詳細には、堀場製作所製GD-PROFILER2型GDA750のGD-OES装置を用いて、上記試験片の表面を、Arグロー放電領域内で高周波スパッタリングした。スパッタされるO、Fe、Znの各元素のArプラズマ内における発光線を連続的に分光することによって、素地鋼板の深さ方向における各元素量プロファイル測定した。スパッタ条件は以下のとおりであり、測定領域は、めっき層表面から深さ50μmまでとした。
(スパッタリング条件)
パルススパッタ周波数 :50Hz
アノード径(分析面積):直径6mm
放電電力 :30W
Arガス圧 :2.5hPa
酸洗・冷間圧延後の素地鋼板を用いたこと以外は上記(1)と同様にして、内部酸化層の平均深さを算出した。算出結果を下記表2~表4に示す。
めっき鋼板の板幅W方向に対して垂直な断面であるW/4部を露出させ、サイズ20mm×20mmの試験片を採取した後、樹脂に埋め込み、めっき層と素地鋼板の界面から素地鋼板の板厚t内部に向かってビッカース硬さを測定した。
めっき鋼板を構成する素地鋼板の金属組織を次の手順で観察した。金属組織分率は、低温変態生成相、ポリゴナルフェライト、および残留γについて求めた。なお、低温変態生成相は、高温域生成ベイナイトと低温域生成ベイナイト等に区別して面積率を求めた。具体的には、金属組織のうち、高温域生成ベイナイト、低温域生成ベイナイト等(即ち、低温域生成ベイナイト+焼戻しマルテンサイト)、およびポリゴナルフェライトの面積率は走査型電子顕微鏡(SEM)観察した結果に基づいて算出し、残留γの体積率は飽和磁化法で測定した。
素地鋼板の圧延方向に平行な断面の表面を研磨し、更に電解研磨した後、ナイタール腐食させて板厚の1/4位置をSEMで、倍率3000倍で5視野観察した。観察視野は約50μm×約50μmとした。
金属組織のうち、残留γの体積率は、飽和磁化法で測定した。具体的には、素地鋼板の飽和磁化Iと、400℃で15時間熱処理した標準試料の飽和磁化Isを測定し、下記式から残留γの体積率Vγrを求めた。飽和磁化の測定は、理研電子製の直流磁化B-H特性自動記録装置「modelBHS-40」を用い、最大印加磁化を5000(Oe)として室温で測定した。結果を下記表5~表7に示す。
Vγr=(1-I/Is)×100
素地鋼板の圧延方向に平行な断面の表面を研磨し、光学顕微鏡で、倍率1000倍で5視野観察し、残留γと焼入れマルテンサイトとが複合したMA混合相の円相当直径を測定した。MA混合相の全個数に対して、観察断面での円相当直径が5μmを超えるMA混合相の個数割合を算出した。MA混合相が観察されないか、個数割合が15%未満の場合を「A」、15%以上の場合を「B」とし、評価結果を下記表5~表7に示す。なお、本発明では、評価Aであることが好ましい。
めっき鋼板の機械的特性は、引張強度TS、伸びEL、穴拡げ率λ、限界曲げ半径Rに基づいて評価した。
穴拡げ率λ(%)={(Df-D0)/D0}×100
めっき鋼板の板幅W方向に対して垂直な断面であるW/4部を露出させ、150mm(W)×30mm(L)の試験片を切り出し、最小曲げ半径にてU曲げ加工を行った後、ボルトで締め付け、U曲げ加工試験片の外側表面に1000MPaの引張応力を負荷した。引張応力の測定は、U曲げ加工試験片の外側に歪ゲージを貼り付け、歪を引張応力に換算して行った。その後、U曲げ加工試験片のエッジ部をマスキングし、電気化学的に水素をチャージさせた。水素チャージは、試験片を、0.1M-H2SO4(pH=3)と0.01M-KSCNの混合溶液中に浸漬し、室温且つ100μA/mm2の定電流の条件で行なった。上記水素チャージ試験の結果、24時間割れない場合を合格、すなわち耐遅れ破壊特性に優れると評価した。評価結果を下記表5~表7に示す。
めっき鋼板の外観を目視で観察し、不めっきの発生の有無に基づいてめっき性を評価した。不めっき発生の有無を下記表5~表7に示す。
2 素地鋼板
3 内部酸化層
4 軟質層
5 硬質層
Claims (14)
- 素地鋼板の表面に、溶融亜鉛めっき層または合金化溶融亜鉛めっき層を有するめっき鋼板であって、
前記素地鋼板は、質量%で、
C :0.10~0.5%、
Si:1.0~3%、
Mn:1.5~8%、
Al:0.005~3%、
P :0%超0.1%以下、
S :0%超0.05%以下、および
N :0%超0.01%以下を含有し、
残部が鉄および不可避不純物からなり、
前記素地鋼板と前記めっき層との界面から素地鋼板側に向って順に、
SiおよびMnよりなる群から選択される少なくとも一種の酸化物を含む内部酸化層と、
前記内部酸化層を含む層であって、且つ、前記素地鋼板の板厚をtとしたとき、ビッカース硬さが、前記素地鋼板のt/4部におけるビッカース硬さの90%以下を満足する軟質層と、
金属組織を走査型電子顕微鏡で観察したときに、
前記金属組織全体に対して低温変態生成相を20~85面積%、および
前記金属組織全体に対してポリゴナルフェライトを10面積%超70面積%以下含み、
前記金属組織を飽和磁化法で測定したときに、前記金属組織全体に対して残留オーステナイトを5体積%以上含む組織で構成される硬質層とを有し、且つ、
前記軟質層の平均深さDが20μm以上、および
前記内部酸化層の平均深さdが4μm以上、前記D未満
を満足し、引張強度が980MPa以上である高強度めっき鋼板。 - 前記内部酸化層の平均深さdと前記軟質層の平均深さDは、
D>2dの関係を満足する請求項1に記載の高強度めっき鋼板。 - 前記低温変態生成相は、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm以上である高温域生成ベイナイトを含み、
前記高温域生成ベイナイトは、前記金属組織全体に対して10面積%超85面積%以下であり、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm未満である低温域生成ベイナイト、および焼戻しマルテンサイトを含んでもよく、
前記低温域生成ベイナイトおよび前記焼戻しマルテンサイトの合計は、前記金属組織全体に対して0面積%以上10面積%未満である請求項1または2に記載の高強度めっき鋼板。 - 前記低温変態生成相は、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm以上である高温域生成ベイナイト、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm未満である低温域生成ベイナイト、および
焼戻しマルテンサイトを含み、
前記高温域生成ベイナイトは、前記金属組織全体に対して10~75面積%であり、
前記低温域生成ベイナイトおよび前記焼戻しマルテンサイトの合計は、前記金属組織全体に対して10~75面積%である請求項1または2に記載の高強度めっき鋼板。 - 前記低温変態生成相は、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm未満である低温域生成ベイナイト、および焼戻しマルテンサイトを含み、
前記低温域生成ベイナイトおよび前記焼戻しマルテンサイトの合計は、前記金属組織全体に対して10面積%超85面積%以下であり、
隣接する残留オーステナイト同士、隣接する炭化物同士、または隣接する残留オーステナイトと炭化物との平均間隔が1μm以上である高温域生成ベイナイトを含んでもよく、
前記高温域生成ベイナイトは、前記金属組織全体に対して0面積%以上10面積%未満である請求項1または2に記載の高強度めっき鋼板。 - 前記素地鋼板が、更に、質量%で、以下の(a)~(d)のいずれかに属する1種以上を含有する請求項1に記載の高強度めっき鋼板。
(a)Cr:0%超1%以下、Mo:0%超1%以下、およびB:0%超0.01%以下よりなる群から選択される少なくとも一種。
(b)Ti:0%超0.2%以下、Nb:0%超0.2%以下、およびV:0%超0.2%以下よりなる群から選択される少なくとも一種。
(c)Cu:0%超1%以下、およびNi:0%超1%以下よりなる群から選択される少なくとも一種。
(d)Ca:0%超0.01%以下、Mg:0%超0.01%以下、および希土類元素:0%超0.01%以下よりなる群から選択される少なくとも一種。 - 請求項1に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は前記均熱後、600℃までの前記平均冷却速度より大きく、且つ平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
をこの順序で含む高強度めっき鋼板の製造方法。 - 請求項1に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(I)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃までは、平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、100~540℃を満たす任意の停止温度Zまで冷却すると共に、600℃から、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は前記均熱後、600℃までの前記平均冷却速度より大きく、且つ平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、または
(II)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、100~540℃を満たす任意の停止温度Zまで冷却すると共に、前記停止温度Zまたは500℃のうち高い方の温度までの範囲は平均冷却速度10℃/秒以上で冷却し、前記100~540℃の温度域で50秒以上保持する工程、
をこの順序で含む高強度めっき鋼板の製造方法。 - 請求項3に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ia)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a1)を満足する工程、または
(IIa)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a1)を満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a1)420℃以上500℃以下を満たす任意の停止温度Za1まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記420~500℃の温度域で50秒以上保持する。 - 請求項4に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ib)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a2)、(b)、(c1)のいずれかを満足する工程、または
(IIb)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a2)、(b)、(c1)のいずれかを満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a2)380℃以上420℃未満を満たす任意の停止温度Za2まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記380℃以上420℃未満の温度域で50秒以上保持する。
(b)下記式(1)を満たす任意の停止温度Zbまで冷却すると共に、
前記停止温度Zbまたは500℃のうち高い方の温度までの範囲は平均冷却速度を10℃/秒以上で冷却し、
下記式(1)を満たす温度域T1で10~100秒間保持し、
次いで、下記式(2)を満たす温度域T2に冷却し、
この温度域T2で50秒以上保持する。
400≦T1(℃)≦540 ・・・(1)
200≦T2(℃)<400 ・・・(2)
(c1)下記式(3)を満たす任意の停止温度Zc1またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4) - 請求項5に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、600℃以上の温度で巻取る熱間圧延工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ic)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a3)または(c2)のいずれかを満足する工程、または
(IIc)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a3)または(c2)のいずれかを満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a3)150℃以上380℃未満を満たす任意の停止温度Za3まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記150℃以上380℃未満の温度域で50秒以上保持する。
(c2)下記式(3)を満たす任意の停止温度Zc2またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4) - 請求項3に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ia)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a1)を満足する工程、または
(IIa)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a1)を満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a1)420℃以上500℃以下を満たす任意の停止温度Za1まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記420~500℃の温度域で50秒以上保持する。 - 請求項4に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ib)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a2)、(b)、(c1)のいずれかを満足する工程、または
(IIb)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a2)、(b)、(c1)のいずれかを満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a2)380℃以上420℃未満を満たす任意の停止温度Za2まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記380℃以上420℃未満の温度域で50秒以上保持する。
(b)下記式(1)を満たす任意の停止温度Zbまで冷却すると共に、
前記停止温度Zbまたは500℃のうち高い方の温度までの範囲は平均冷却速度を10℃/秒以上で冷却し、
下記式(1)を満たす温度域T1で10~100秒間保持し、
次いで、下記式(2)を満たす温度域T2に冷却し、
この温度域T2で50秒以上保持する。
400≦T1(℃)≦540 ・・・(1)
200≦T2(℃)<400 ・・・(2)
(c1)下記式(3)を満たす任意の停止温度Zc1またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4) - 請求項5に記載の高強度めっき鋼板を製造する方法であって、
前記素地鋼板の鋼中成分を満足する鋼板を、500℃以上の温度で巻取る熱間圧延工程と、
500℃以上の温度で60分以上保温する工程と、
内部酸化層の平均深さdが4μm以上残るように酸洗、冷間圧延する工程と、
酸化帯にて、0.9~1.4の空気比で酸化する工程と、
(Ic)還元帯にて、Ac3点または750℃のうち高い方の温度以上の範囲で均熱し、
均熱後、600℃まで平均冷却速度0℃/秒超20℃/秒以下で冷却し、600℃からは、前記均熱後、600℃までの前記平均冷却速度より大きく、且つ下記(a3)または(c2)のいずれかを満足する工程、または
(IIc)還元帯にて、Ac1点+20℃または750℃のうち高い方の温度以上Ac3点未満の範囲で均熱し、
均熱後、下記(a3)または(c2)のいずれかを満足する工程、
をこの順序で含む高強度めっき鋼板の製造方法。
(a3)150℃以上380℃未満を満たす任意の停止温度Za3まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
前記150℃以上380℃未満の温度域で50秒以上保持する。
(c2)下記式(3)を満たす任意の停止温度Zc2またはMs点まで冷却すると共に、
500℃までの範囲は平均冷却速度10℃/秒以上で冷却し、
下記式(3)を満たす温度域T3で5~180秒間保持し、
次いで、下記式(4)を満たす温度域T4に加熱し、
この温度域T4で30秒以上保持する。
100≦T3(℃)<400 ・・・(3)
400≦T4(℃)≦500 ・・・(4)
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