TWI551695B - Steel sheet, hot dip galvanizing steel sheet, galvannealed steel sheet, and method for manufacturing thereof - Google Patents

Description

Steel plate, hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, and the like Field of invention

The present invention relates to a steel sheet, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and excellent delayed fracture resistance as a structural member of an automobile, a building, a home electric appliance, or the like. And these manufacturing methods.

Background of the invention

In recent years, steel sheets used as structural members of automobiles, buildings, home electric appliances, and the like are required to have excellent resistance to delayed fracture characteristics in addition to the required strength and moldability. Delayed damage is a phenomenon in which hydrogen is concentrated in the stress concentration and invades the steel after invading the steel.

It has been known that delayed fracture is produced in high-strength steel such as high-strength screws, PC steel wires, and pipelines. It has been proposed to propose various measures for improving the resistance to delayed fracture of these high-strength steel materials.

For example, Non-Patent Document 1 discloses that elements such as Cr, Mo, and V are effective for improving the resistance to delayed fracture. This is by making Cr, Carbides such as Mo and V are precipitated in the crystal grains, and these carbides are used as a site for trapping hydrogen (hydrogen trap position) to suppress grain boundary embrittlement.

Because high-strength materials are difficult to plastically deform and are not easily broken, most of the cases are used in environments with high stress. Moreover, as a steel plate for automobiles, a steel material as a member is used after molding, and residual stress is generated after the molding process. Since the residual stress is also increased as the strength of the steel sheet increases, the fear of delayed fracture is improved in the high-strength steel sheet.

Therefore, in order to apply a high-strength steel sheet to an automobile part, it is necessary to improve the formability of the steel sheet to form a steel sheet to obtain a part, and it is necessary to improve the delayed fracture resistance of the steel sheet to withstand the use in a high stress environment. .

Further, the function of the carbides of the elements such as Cr, Mo, and V as the hydrogen trap position comes from the integration (integrated strain) at the interface between the parent phase and the carbide, and the above functions are reduced by cold rolling and heat treatment. . Therefore, the use of a carbide of an element such as Cr, Mo, or V as a hydrogen trap position cannot be applied to a steel sheet of a type that requires cold rolling and heat treatment.

Patent Document 1 discloses that an oxide composed mainly of Ti and Mg is effective in order to suppress hydrogen defects (elevation resistance to delayed fracture characteristics). In the hydrogen embrittlement measures disclosed in Patent Document 1, in particular, in order to improve hydrogen embrittlement after large heat input welding, the object of Patent Document 1 is a thick steel plate, and high moldability required for the steel sheet. The coexistence with the delayed damage resistance characteristics is not considered.

Regarding hydrogen embrittlement of a steel sheet, for example, in Non-Patent Document 2, It is revealed that the hydrogen embrittlement of the steel sheet is promoted by the processing induced metamorphosis caused by the residual Worthite iron amount. That is, it has been revealed that in order to prevent the delayed fracture resistance from deteriorating, it is necessary to limit the amount of Worstian iron remaining in the steel sheet.

However, the countermeasure against the delayed fracture characteristic disclosed in Non-Patent Document 2 is a high-strength steel sheet having a specific structure, and it cannot be said that it is a fundamental countermeasure against the deterioration of the delayed fracture characteristics.

Patent Document 2 discloses a steel sheet for a tantalum container having excellent fish scale resistance, and is a steel sheet for the purpose of improving both the delayed fracture resistance and the moldability. This steel sheet captures hydrogen which intrudes into the steel sheet at the time of manufacture by the oxide in the steel sheet, and suppresses "scale explosion" (surface defect) which occurs after mounting the crucible.

Therefore, the steel sheet disclosed in Patent Document 2 contains a large amount of oxide therein. However, when the oxide is dispersed in the steel sheet at a high density, moldability is deteriorated. Therefore, the technique disclosed in Patent Document 2 cannot be applied to a steel sheet for automobiles which is required to have high moldability.

On the other hand, as a method for improving the moldability of the steel sheet, it is known to disperse the residual Worthite iron in the steel sheet, and to use the residual Worthite iron to become the granulated iron in the processing of the steel sheet (during molding). A method of abnormally induced plasticity (TRIP effect) (see Patent Documents 3 and 4). However, in the steel sheet which is improved in moldability and delayed fracture resistance, it is difficult to utilize the TRIP effect (see Non-Patent Document 2). Thus, it is difficult to improve both the moldability and the delayed fracture resistance of the steel sheet.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open No. Hei 11-293383

Patent Document 2: Japanese Laid-Open Patent Publication No. 11-100638

Patent Document 3: Japanese Laid-Open Patent Publication No. 01-230715

Patent Document 4: Japanese Laid-Open Patent Publication No. 02-217425

[Non-patent literature]

[Non-Patent Document 1] "New Development of Delayed Destruction" (Japan Iron and Steel Association, issued in January 1997)

[Non-Patent Document 2] CAMP-ISIJ Vol. 5 (Vol. 5) No. 6 (Phase 6), pages 1839 to 1842, Yamazaki et al., October 1992, issued by the Japan Iron and Steel Association

Summary of invention

As described above, it is difficult to improve both the moldability and the delayed fracture resistance of the steel sheet. In the steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, the moldability is improved and the delayed fracture resistance is improved. An object of the present invention is to provide a steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and a method for producing the same.

The present inventors have intensively studied in order to solve the above problems. As a result, the present inventors have found that in a steel sheet having a C content of 0.05 to 0.40% and a tensile strength of 780 MPa or more, the tempered granules of the main phase are contained in a necessary volume fraction, preferably in a necessary volume. The fraction contains one or two kinds of ferrite iron and toughened iron of the second phase, and forms a structure that limits the volume fraction of the other phases, so that the iron carbides are tempered at a necessary number density or more. When the iron-based carbide of 20% or more is made into an ε- based carbide, the moldability of the steel sheet can be ensured, and the delayed fracture resistance can be improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) A steel sheet according to an aspect of the present invention has a chemical composition of C: 0.05 to 0.40%, Si: 0.05 to 3.00%, Mn: 1.50% or more and less than 3.50%, and P: 0.04% by mass%. Hereinafter, S: 0.01% or less, N: 0.01% or less, O: 0.006% or less, Al: 0 to 2.00%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0~1.00%, Nb: 0~0.30%, Ti: 0~0.30%, V: 0~0.50%, B: 0~0.01%, Ca: 0~0.04%, Mg: 0~0.04%, and REM: 0~0.04%, the remaining part is composed of Fe and impurities, and the tissue of 1/4 part of the thickness is based on volume fraction, containing tempered Ma Tian loose iron: 70% or more, and fertilized iron and toughened iron 1 or 2 types: less than 20% in total; in the 1/4 part of the thickness of the aforementioned plate, the residual Worthite iron is less than 10% by volume fraction, and the new Ma Tian loose iron is 10% or less. The iron content is 10% or less, and the total volume fraction of the above-mentioned residual Worthite iron, the aforementioned new Ma Tian loose iron, and the aforementioned bun iron is 15% or less; in the aforementioned tempering Ma Tian loose iron of 1/4 of the aforementioned plate thickness The number density of iron-based carbides having a long diameter of 5 nm or more is 5 × 10 ⁷ /mm ² or more; relative to the thickness of the aforementioned plate The number of the iron-based carbides having a long diameter of 5 nm or more in the 1/4 portion, the ratio of the number of the ε- based carbides is 20% or more, and the tensile strength is 780 MPa or more.

(2) The steel sheet according to (1) above, wherein the chemical component is in a mass%, and may further contain Cr: 0.05 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.05 to 1.00%, and Cu: 0.05 to 1.00% of one or more.

(3) The steel sheet according to (1) or (2) above, wherein the chemical component is in a mass%, and may further contain Nb: 0.005 to 0.30%, Ti: 0.005 to 0.30%, and V: 0.005 to 0.50%. One or two or more.

The steel sheet according to any one of the above items (1) to (3), wherein the chemical component is in a mass%, and may contain B: 0.0001 to 0.01%.

(5) The steel sheet according to any one of the above items (1) to (4), wherein the chemical component is in a mass%, and may further contain Ca: 0.0005 to 0.04%, Mg: 0.0005 to 0.04%, and REM: One or two or more of 0.0005 to 0.04%.

The steel sheet according to any one of the above items (1) to (5), wherein the iron-based carbide has an average major axis of 350 nm or less.

(7) The hot-dip galvanized steel sheet according to any one of the items (1) to (6) above, wherein Fe is 15% by mass or less and the remainder is A hot-dip galvanized layer composed of Zn, Al, and impurities.

(8) The alloyed hot-dip galvanized steel sheet according to any one of the above (1) to (6), wherein Fe is 15% by mass or less and the remainder is formed. An alloyed hot-dip galvanized layer composed of Zn, Al, and impurities.

According to the present invention, it is possible to provide a structural member suitable for use as an automobile, a building, a home electric appliance, or the like and having a tensile strength of 780 MPa or more. A steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet having excellent resistance to delayed fracture, and a method for producing the same.

Fig. 1 is a graph showing the relationship between the number density (iron/mm ² ) of iron-based carbides in the tempered granulated iron and the delayed fracture resistance.

Fig. 2 is a graph showing the relationship between the ratio of the ε- based carbide in the iron-based carbide and the delayed fracture resistance.

Form for implementing the invention

The carbides such as Cr, Mo, and V function as hydrogen trap positions, and it is known that the delayed fracture resistance due to hydrogen embrittlement is improved (see Non-Patent Document 1). However, since the heat treatment requires a long time for the precipitation of carbides such as Cr, Mo, and V, the steel sheet manufactured by using a production line (continuous annealing line, continuous plating line, etc.) which must be heat-treated for a short period of time is used for the purpose of upgrading. It is difficult to use a carbide precipitation system such as Cr, Mo, or V to withstand the delayed fracture characteristics.

The function of capturing hydrogen comes from the integration (integrated strain) at the interface between the base metal and the carbide, because the trapping ability of the carbides such as Cr, Mo, and V precipitated during hot rolling is due to cold rolling and heat treatment. However, it is difficult to improve the resistance to delayed fracture characteristics by depositing carbides such as Cr, Mo, and V in a steel sheet of a type which is required to be cold-rolled and heat-treated.

The inventors of the present invention have obtained the following findings, and by appropriately controlling the structure and the iron-based carbide (especially ε- based carbide) in a steel sheet having a tensile strength of 780 MPa or more, it is possible to maintain moldability while maintaining moldability. Increased delay damage characteristics.

In the following, a steel sheet having excellent delayed fracture resistance according to an embodiment of the present invention (hereinafter referred to as "the steel sheet of the present embodiment") will be described.

The steel plate according to the present embodiment is based on a steel sheet having a tensile strength of 780 MPa or more and having excellent delayed fracture resistance, a molten plated steel sheet, and an alloyed hot-dip galvanized steel sheet, and a fine iron-based carbide is used. (Xingming carbon iron and ε- based carbides) are formed by arranging the main phase of the structure as tempered granulated iron in the tempered granulated iron, and using the iron-based carbide as a hydrogen trap position. While maintaining formability, the resistance to delayed damage is improved.

Specifically, in the case of a steel sheet having excellent delayed fracture resistance according to an embodiment of the present invention (hereinafter referred to as "the steel sheet of the present embodiment"), the chemical composition thereof is C: 0.05 in mass%. ~0.40%, Si: 0.05 to 3.00%, Mn: 1.50% or more and less than 3.50%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, O: 0.006% or less, Al: 0 to 2.00 %, Cr: 0~1.00%, Mo: 0~1.00%, Ni: 0~1.00%, Cu: 0~1.00%, Nb: 0~0.30%, Ti: 0~0.30%, V: 0~0.50% , B: 0~0.01%, Ca: 0~0.04%, Mg: 0~0.04%, and REM: 0~0.04%, the remaining part is composed of Fe and impurities, and the tissue of 1/4 part of the thickness is Volume fraction meter, containing tempered Ma Tian loose iron: 70% or more, and one or two kinds of fertilized iron and toughened iron: a total of less than 20%; in the above-mentioned plate thickness of 1/4 part of the tissue, by volume The meter has a residual Worthite iron of less than 10%, a new Maeda iron of 10% or less, and a Bora iron of 10% or less, and the above-mentioned residual Worthite iron, the aforementioned new Maeda iron and the aforementioned Bora iron. The volume fraction is 15% or less; the aforementioned tempering in the 1/4 portion of the aforementioned plate thickness The number density of bulk iron carbide long diameter of 5nm or more of iron 5 × 10 ⁷ / mm ^{2 or} more; with respect to the iron carbide in one or more of the preceding 5nm thickness of one quarter of the length of the path The ratio of the number of ε- based carbides is 20% or more, and the tensile strength is 780 MPa or more.

In the case of the hot-dip galvanized steel sheet having the excellent delayed fracture resistance (hereinafter referred to as "the hot-dip galvanized steel sheet of the present embodiment"), the steel sheet of the present embodiment is formed with Fe of 15% by mass. Hereinafter, the remaining portion is a hot-dip galvanized layer composed of Zn, Al, and impurities.

In the case of the alloyed hot-dip galvanized steel sheet (hereinafter referred to as "the alloyed hot-dip galvanized steel sheet of the present embodiment") having excellent delayed fracture resistance, the steel sheet of the present embodiment is formed with Fe. The alloyed hot-dip galvanized layer is 15 mass% or less and the remainder is composed of Zn, Al, and impurities.

First, the chemical composition of the steel sheet of the present embodiment will be described. The unit "% by mass" of the content of each element contained in the chemical component is described below as "%".

C: 0.05~0.40%

The steel sheet according to the present embodiment is a steel sheet containing 0.05 to 0.40% of C having a tensile strength of 780 MPa or more. In order to improve the strength of the steel sheet and the function of depositing iron-based carbides (snow-carbon iron, ε- based carbide, etc.) as hydrogen trap positions, C is an essential element. When the C content is less than 0.05%, it is difficult to obtain a tensile strength of 780 MPa or more. Further, when the C content is less than 0.05%, the amount of the iron-based carbide precipitated is insufficient, and the delayed fracture resistance cannot be improved.

On the other hand, when the C content is more than 0.40%, the metamorphic initial temperature of the granulated iron is lowered, and a sufficient amount of granulated iron is not secured, so that it is difficult to ensure that 70% by volume or more of the tempered granules are dispersed.

Therefore, the C content of the steel sheet of the present embodiment is 0.05 to 0.40%. A preferred lower limit of the C content is 0.10%. A preferred upper limit of the C content is 0.25%.

Si: 0.05~3.00%

Si is an effective element for enhancing strength. Further, the Si-based element has an action of suppressing the precipitation of the iron-based carbide in the Worthite iron and suppressing the coarsening of the iron-based carbide formed in the granulated iron. Since the iron-based carbide in the granulated iron is finer and the delayed fracture resistance is improved, the Si system has an effect of resisting delayed fracture characteristics.

When the Si content is less than 0.05%, the above effects cannot be sufficiently obtained, so the Si content must be 0.05% or more. Preferably, the Si content is 0.10% or more. On the other hand, when the Si content is more than 3.00%, the steel sheet strength is excessively increased and the formability of the steel sheet is lowered, so the Si content must be 3.00% or less. The Si content is preferably 2.00% or less.

Mn: 1.50 to less than 3.50%

Mn is an effective element for improving the strength of the steel sheet. Further, in the heat treatment for annealing or hot-dip galvanizing, the Mn system has an effect of suppressing the deformation of the ferrite-grain during the cooling. In order to make the amount of tempered iron in the steel sheet of the present embodiment within a predetermined range, it is considered that this action is necessary.

Since the Mn content is less than 1.50%, the above cannot be sufficiently obtained. The effect is not able to get the necessary volume fraction of the tempered 麻田散铁. Therefore, the Mn content must be set to 1.50% or more. Preferably, the Mn content is 1.70% or more. On the other hand, when the Mn content is 3.50% or more, the strength of the steel blank and the hot-rolled sheet is excessively increased, so that the manufacturability of the steel sheet is lowered, so the Mn content must be made less than 3.50%. Preferably, the Mn content is 3.00% or less.

P: 0.04% or less

The P-based impurity element segregates in the central portion of the thickness of the steel sheet to impede the toughness, and is an element that causes the welded portion to be embrittled. When the P content is more than 0.04%, the toughness is lowered and the welded portion embrittlement system becomes conspicuous. Therefore, the P content must be made 0.04% or less. It is preferred that the P content is 0.02% or less. Since the P content is preferably as small as possible, the lower limit of the P content is not particularly limited. However, since the P content is less than 0.0001%, it is economically disadvantageous, so 0.0001% is a substantially lower limit of the P content.

S: 0.01% or less

S is an element which is an element of impurities and which hinders weldability, and which hinders manufacturability at the time of casting and hot rolling. Further, S forms an element which forms coarse MnS and impedes hole expandability. When the S content is more than 0.01%, the weldability is low, the manufacturability is low, and the hole expandability is remarkable, and the S content must be 0.01% or less. Preferably, the S content is 0.005% or less. Since the S content is preferably as small as possible, the lower limit of the S content is not particularly limited. When the S content is less than 0.0001%, it is economically disadvantageous, so 0.0001% is a substantially lower limit of the S content.

N: 0.01% or less

N system hinders flexibility and hole expandability due to the formation of coarse nitride The element, in turn, becomes an element of the cause of the stomata at the time of welding. When the N content is more than 0.01%, the bendability and the hole expandability are lowered, and the generation of the pores becomes remarkable. Therefore, the N content must be 0.01% or less. Since the N content is preferably as small as possible, the lower limit of the N content is not particularly limited. When the N content is less than 0.0005%, the production cost is greatly increased. Therefore, the 0.0005% is a substantially lower limit of the N content.

O: 0.006% or less

O is an element which forms an oxide and hinders moldability. Since the moldability is significantly reduced when the O content is more than 0.006%, the O content must be made 0.006% or less. Since the O content is preferably as small as possible, the lower limit is not particularly limited, and when the O content is less than 0.001%, the cost is too high and economically unfavorable, so 0.001% is a substantially lower limit of the O content.

The steel sheet of the present embodiment may contain one or more of Al, Cr, Mo, Ni, and Cu, and one or more of Nb, Ti, and V, in addition to the above-mentioned elements; One or two or more of B and/or Ca, Mg, and REM. However, since the steel sheet of the present embodiment does not necessarily contain such elements, the lower limit of the content of these elements is 0%.

Al: 0~2.00%

Al is an effective element of the deoxidizing material, and Si, in the same manner, has an action of suppressing the precipitation of iron-based carbides in the Worthite iron. Further, since the Al oxide system contributes to the improvement of the delayed fracture resistance, the steel sheet of the present embodiment may contain Al. However, when the Al content is more than 2.00%, the Al oxide is excessively formed and the manufacturability is deteriorated, so the Al content must be set to 2.00% or less. Preferably, the Al content is 1.00% or less. Since Al is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Al content is 0%. However, since it is difficult to completely remove Al contained in the material of the steel sheet as an impurity, the lower limit of the Al content may be 0.001%.

Cr: 0~1.00%

The Cr element is an element which enhances the tensile strength of the steel sheet and the like, and at the same time, when it is cooled after annealing in the annealing equipment or the hot-dip galvanizing apparatus, it has an element of suppressing the deformation of the ferrite and iron and thereby increasing the amount of the tempered iron in the squash. . Since Cr is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Cr content is 0%. However, in order to obtain the above effects, the Cr content may be set to 0.05% or more. Preferably, the Cr content is 0.10% or more. On the other hand, when the Cr content is more than 1.00%, the manufacturability at the time of production and hot rolling is hindered, so the Cr content is preferably 1.00% or less. Preferably, the Cr content is 0.70% or less.

Mo: 0~1.00%

Mo is an element which enhances the tensile strength of the steel sheet and the like, and at the same time, when it is cooled after annealing in the annealing apparatus or the hot-dip galvanizing apparatus, it has an element of suppressing the deformation of the ferrite and iron and thereby increasing the amount of the tempered iron in the hemp field. . Since Mo is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Mo content is 0%. However, in order to obtain the above effects, the Mo content may be set to 0.01% or more. The Mo content is preferably 0.05% or more. On the other hand, when the Mo content is more than 1.00%, the manufacturability at the time of production and hot rolling is hindered, so the Mo content is preferably 1.00% or less. The Mo content is preferably 0.70% or less.

Ni: 0~1.00%

Ni is an element that enhances the tensile strength of a steel sheet, etc., while annealing When the preparation or the hot-dip galvanizing equipment is cooled after annealing, it has an action of suppressing the deformation of the ferrite and iron and thereby increasing the amount of the tempered iron in the field. Since Ni is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Ni content is 0%. However, in order to obtain the above effects, the Ni content may be 0.05% or more. The Ni content is preferably 0.10% or more. On the other hand, when the Ni content is more than 1.00%, the manufacturability at the time of production and hot rolling is hindered, so the Ni content is preferably 1.00% or less. The Ni content is preferably 0.70% or less.

Cu: 0~1.00%

Cu is an element which enhances the tensile strength of the steel sheet and the like, and at the same time, when it is cooled after annealing in the annealing equipment or the hot-dip galvanizing apparatus, it has an element of suppressing the deformation of the ferrite and iron and thereby increasing the amount of the tempered iron in the hemp field. . Cu is not necessarily contained in the steel sheet of the present embodiment because the lower limit of the Cu content is 0%. However, in order to obtain the above effects, the Cu content may be 0.05% or more. The Cu content is preferably 0.10% or more. On the other hand, when the Cu content is more than 1.00%, the manufacturability at the time of production and hot rolling is hindered, so the Cu content is preferably 1.00% or less. The Cu content is preferably 0.70% or less.

Nb: 0~0.30%

Nb is an element that contributes to the strength of the steel sheet by precipitation strengthening, fine grain strengthening, and dislocation strengthening. Since Nb is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Nb content is 0%. However, in order to obtain the above effects, the Nb content may be 0.005% or more. The Nb content is preferably 0.010% or more. On the other hand, when the Nb content is more than 0.30%, the precipitation amount of the carbonitride is increased and the moldability is deteriorated, and the Nb content is preferably 0.30% or less. The Nb content is preferably 0.20% or less.

Ti: 0~0.30%

Ti is an element that contributes to the strength of the steel sheet by precipitation strengthening, fine grain strengthening, and dislocation strengthening. Since Ti is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the Ti content is 0%. However, in order to obtain the above effects, the Ti content may be 0.005% or more. The Ti content is preferably 0.010% or more. On the other hand, when the Ti content is more than 0.30%, the precipitation amount of the carbonitride is increased and the moldability is deteriorated, so the Ti content is preferably 0.30% or less. The Ti content is preferably 0.15% or less.

V: 0~0.50%

V is an element that contributes to the strength of the steel sheet by precipitation strengthening, fine grain strengthening, and dislocation strengthening. Since V is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the V content is 0%. However, in order to obtain the above effects, the V content may be made 0.005% or more. The V content is preferably 0.10% or more. On the other hand, when the V content is more than 0.50%, the precipitation amount of the carbonitride is increased and the moldability is deteriorated, so the V content is preferably 0.50% or less. The V content is preferably 0.35% or less.

B: 0~0.01%

The B-based layer strengthens the elements of the grain boundary, and further has an effect of suppressing the ferrite-grain metamorphism and thereby increasing the amount of the tempered granulated iron in the annealing apparatus or the galvanizing apparatus after cooling. Since B is not necessarily contained in the steel sheet of the present embodiment, the lower limit of the B content is 0%. However, in order to obtain the above effects, the B content may be made 0.0001% or more. The B content is preferably 0.0005% or more. On the other hand, when the B content is more than 0.01%, the manufacturability at the time of hot rolling is low, and it is preferable that the B content is 0.01% or less. B content It is preferably 0.005% or less.

Ca: 0~0.04%

Mg: 0~0.04%

REM: 0~0.04%

Ca, Mg, and REM are elements that control the form of oxides and sulfides and contribute to the improvement of the hole expandability of the steel sheet. Since Ca, Mg, and REM are not necessarily contained in the steel sheet of the present embodiment, the lower limit of each of the Ca content, the Mg content, and the REM content is 0%. However, in order to obtain the above effects, the Ca content, the Mg content, and the REM content may each be 0.0005% or more. The Ca content, the Mg content, and the REM content are each preferably 0.0010% or more. On the other hand, when the Ca content, the Mg content, and the REM content are each more than 0.04%, since the castability is deteriorated, the Ca content, the Mg content, and the REM content are each preferably 0.04% or less. The Ca content, the Mg content, and the REM content are each preferably 0.01% or less.

In addition, "REM" means a total of 17 elements composed of Sc, Y and Lanthanoid, and the above-mentioned "REM content" means the total content of these 17 elements. When a lanthanoid element is used as the REM, the REM is mostly added in the form of a Misch metal in the industry. In this case, the steel sheet of the present embodiment can also exhibit the effect of the steel sheet of the embodiment. Further, even if the metal REM such as metal La or metal Ce is contained, the steel sheet of the present embodiment can exhibit the effect of the steel sheet of the present embodiment.

The steel sheet according to the present embodiment is composed of iron and impurities in addition to the above elements. The so-called impurity means that it is working. When a steel material is produced in the industry, a raw material such as ore or scrap, or a component mixed in due to various important reasons of the production step, is allowed to be in a range that does not adversely affect the present invention.

Tensile strength: 780MPa or more

The tensile strength of the steel sheet according to the present embodiment is 780 MPa or more. The tensile strength can be obtained by controlling the chemical composition of the steel sheet within the above range and the structure of the steel sheet as described below.

Next, the structure of the 1/4 portion of the thickness of the steel sheet according to the present embodiment (hereinafter, abbreviated as "organization") will be described. The term "1/4" of the plate thickness refers to the surface between the surface of the steel sheet (the upper and lower surfaces of the steel sheet) which is 1/8 of the thickness t of the steel sheet, and the surface of the steel sheet thickness t from the surface of the steel sheet. region. The surface of the steel plate thickness t which is 1/4 depth from the surface of the steel sheet is the center surface of the 1/4 portion of the plate thickness. Since the 1/4 portion of the sheet thickness is located between the center surface of the steel sheet and the surface of the sheet, it has an average structure. Therefore, the steel sheet according to the present embodiment is defined as a structure having a thickness of 1/4.

The structure of the 1/4 portion of the thickness of the steel sheet according to the present embodiment is defined as: by volume fraction, (structure A) tempered granulated iron: 70% or more, (tissue B) ferrite iron and toughened iron One or two: a total of less than 20%, and (organization C) residual Worthite iron, new Ma Tian loose iron, and Borite: each less than 10%.

The structure A has a structure which has the most significant influence on the tensile strength and the delayed fracture resistance of the steel sheet of the present embodiment because of the ε- based carbide, that is, the main phase. The structure B has an effect of improving various characteristics of the steel sheet according to the embodiment. However, since the steel sheet of the present embodiment can solve the problem even if the structure B is not contained, the lower limit of the content of the tissue B is 0% by volume. . Since the structure C does not have the effect of improving various characteristics of the steel sheet of the present embodiment, it is not necessary to contain the structure, and the lower limit of the content is 0% by volume.

(Organization A) Tempered 1/4 field tempered 麻田散铁 (main phase): 70% or more

In the organization, in order to ensure the strength of the steel sheet and the resistance to delayed fracture, the tempered Matian iron is an important organization.

An assembly of tempered granulated iron-based lath-like crystal grains containing iron-based carbides inside. The iron-based carbides belong to a plurality of iron-based carbide groups elongated in different directions and function as hydrogen trap positions. The long-length system of the iron-based carbide is, for example, 5 nm or more. A part of the iron-based carbide in the tempered granulated iron is a ε- based carbide which will be described later by heat treatment under appropriate conditions.

The tempered granulated iron can be obtained by tempering the quenched granulated iron. When the volume fraction of the tempered granulated iron is 70% or more, the tensile strength of the steel sheet can be surely 780 MPa or more. Therefore, the volume fraction of the tempered granulated iron is 70% or more. The volume fraction of the tempered granulated iron is preferably 75% or more.

Since the upper limit of the volume fraction of the tempered granulated iron is not particularly limited, it may be 100%. However, since the volume fraction of the tempered granulated iron is more than 90%, the tensile strength of the steel sheet is excessively increased and the steel sheet is In the case where the moldability is low, the volume fraction of the tempered granulated iron is preferably 90% or less. The volume fraction of the tempered granulated iron is preferably 85% or less.

(Organization B) 1 or 2 types of fertilized iron and toughened iron (second phase): less than 20% in total

In the steel sheet according to the present embodiment, the structure other than the tempered granules is mainly composed of one or two types of ferrite iron and toughened iron.

The fat iron is a soft structure and causes the strength of the steel sheet to decrease. When the amount of ferrite is excessive, the tensile strength of the steel sheet may be less than 780 MPa. Therefore, the steel sheet of this embodiment may contain ferrite iron. However, when the ferrite iron which is softer than the tempered granulated iron is replaced by a part of the tempered granulated iron, the steel sheet of the present embodiment may contain the ferrite iron. .

Similarly, the toughened iron is an aggregate of lath-like crystal grains, and contains, for example, a structure of iron-based carbide having a long diameter of 5 nm or more. The iron-based carbide functions as a hydrogen trap position to improve the delayed fracture resistance of the steel sheet. On the other hand, the tensile strength improvement effect of the toughened iron is small compared to the tensile strength improvement effect of the granulated iron. The steel sheet according to the present embodiment has a tensile strength of 780 MPa or more by a granulated iron having a volume fraction of 70% or more. When the amount of toughened iron is excessive, the tensile strength of the steel sheet may be less than 780 MPa. Therefore, the steel sheet of the present embodiment does not need to contain toughened iron. However, since it contains a tougher iron which is softer than the tempered granulated iron, it has an effect of improving the formability of the steel sheet in place of a part of the tempered granulated iron. Therefore, as long as the tempered granulated iron is not damaged The steel sheet of this embodiment may also contain toughened iron.

Further, the toughened iron containing iron-based carbide is also a structure which contributes to the improvement of the delayed fracture resistance. However, the toughened iron system is different from the granulated iron which can control the precipitation of carbides by the heat treatment after the generation of the granulated iron, and the iron-based carbide cannot be formed because the structure is formed at a necessary temperature for a long time. Part of it stays in the state of the ε- based carbide.

The inventors of the present invention classify the structure contained in the steel sheet according to the present embodiment into an essential structure A containing ε- based carbide (that is, tempered granulated iron), and a steel sheet not containing ε- based carbide and in the present embodiment. It is not necessary, but there is a structure B (ie, ferrite iron and toughened iron) which brings about a good effect on the improvement of moldability, etc.; and the structure C which does not have to be contained in the steel plate of this embodiment; Moreover, it is judged in order to control well. It is necessary to specify the content of each group for all of the delayed fracture resistance characteristics, moldability, and tensile strength. Therefore, in the steel sheet according to the present embodiment, the volume fraction of the total of the ferrite iron and the toughened iron is specified. In the steel sheet according to the present embodiment, the total volume fraction of the ferrite iron and the toughened iron is set to be less than 20%, and the tensile strength of the steel sheet is surely 780 MPa or more. The total volume fraction of the ferrite iron and the toughened iron is preferably 10% or less.

The lower limit of the total volume fraction of fertilized iron and toughened iron is 0%. When fermented iron and toughened iron are used to improve the formability of the steel sheet, the total volume of ferrite iron and toughened iron may also be used. The lower limit of the fractional rate is set to 5% or more.

(Organization C) Residual Worth Iron: less than 10% by volume

(Organization C) New Ma Tian loose iron: 10% by volume or less

(Organization C) Borite: 10% by volume or less

(Organization C) Total amount of residual Worthite iron, new Maeda loose iron, and Borne iron: 15% by volume or less

In addition to the tempered granulated iron, the granulated iron, and the toughened iron, the steel sheet according to the present embodiment may contain a residual Worthite iron, a new Maeda iron, and a Bora iron.

The residual Worth Iron system helps to improve formability by the TRIP effect. However, when the volume fraction of the residual Worthite iron is increased, when it is molded into a member for automobiles, it is feared that the metamorphosis becomes a hard new ramification iron and the processing characteristics are low.

The inventors of the present invention have confirmed by experiments that the processing characteristics are deteriorated when the volume fraction of the remaining Worstian iron in the steel sheet structure is 10% or more. Therefore, in the steel sheet of the present embodiment, the volume fraction of the remaining Worthite iron is set to be less than 10%. The volume fraction of the residual Worthite iron is preferably 7% or less. On the other hand, even if the volume fraction of the remaining Worthite iron is 0%, the steel sheet of the present embodiment has sufficient moldability. Therefore, since the steel sheet of the present embodiment does not need to contain the residual Worthite iron, the lower limit of the volume fraction of the remaining Worth iron is 0%.

The new Ma Tian loose iron is a Ma Tian loose iron that does not contain Fe carbide. Although the steel sheet containing the new Ma Tian loose iron is high in strength, the steel sheet according to the present embodiment limits the volume fraction of the new Ma Tian loose iron to 10% or less because of poor processing characteristics. On the other hand, even if the volume fraction of the new Ma Tian loose iron is 0%, the steel sheet of the present embodiment has sufficient strength. Therefore, since the steel sheet of the present embodiment does not have to contain the new Ma Tian loose iron, the lower limit of the volume fraction of the new Ma Tian loose iron is 0%.

The Bora iron system reduces the processing characteristics of the steel sheet. Therefore, in the steel sheet according to the present embodiment, the volume fraction of the ferrite is 10% or less. On the other hand, although the ferrite is a structure of ferritic carbon iron containing Fe carbide, the ferritic carbon is not sufficiently changed to become ε- based carbide, so the ferritic iron is not sufficiently delayed. The effect of feature enhancement. Therefore, since the steel sheet of the present embodiment does not need to contain the pulverized iron, the lower limit of the volume fraction of the pulverized iron is 0%.

In addition, the total volume fraction of the remaining Worthite iron, the new Maeda iron, and the pulverized iron of the steel sheet of the present embodiment must be 15% or less, and preferably 12% or less. The residual Worthite iron, the new Maeda iron, and the Borne iron with a total volume fraction of more than 15% are likely to impair the processing characteristics of the steel sheet.

The tempering of the granulated iron, the ferrite iron, the toughening iron, and the residual Worthite iron, and the identification of the new Matian iron, the Borne iron, and other tissues, the confirmation of the existence position, and the determination of the volume fraction, By using the NITAL (Nitrate Ethanol Etching Solution) reagent and the reagent disclosed in JP-A-59-219473, the steel sheet is subjected to a cross-section in the direction of the rolling direction or a direction perpendicular to the rolling direction, and is used in a 1000~ shape. The cross section was observed by a scanning electron microscope and a transmission electron microscope at 100,000 times.

Further, from the use of FE-SEM (using an EBSD attached to an electric field emission type scanning electron microscope (FE-SEM): Electron Back-Scatter Diffraction (crystal backscattering analyzer) Analytical method) Determination of the hardness of a small region such as crystal orientation analysis or micro Vickers hardness measurement, It is also able to identify organizations.

For example, as described above, the tempered granulated iron and the toughened iron are different in the formation position and crystal orientation (elongation direction) of the carbide, and the internal iron of the lath-like crystal grain is observed by using FE-SEM. It is the direction in which the carbides are elongated, and the tempered iron and the toughened iron can be easily distinguished.

The volume fraction of tempered loose iron, ferrite iron, and toughened iron in the 1/4 portion of the thickness of the steel sheet, and/or the volume fraction of the ferrite, by the direction of the steel sheet and the rolling direction The parallel plate thickness profile was taken as the observation surface, and the observation surface was polished, and the measurement was performed by etching using a NITAL liquid and observing the plate thickness of 1/4 portion using FE-SEM (the thickness of the plate thickness was 1/4) The area fraction of each organization obtained from the range of 1/8 to 3/8 is obtained as the volume fraction. In addition, the area fraction of each tissue refers to the average value of the area fraction of each tissue in each field of view obtained by measuring 10 fields of view at a magnification of 5000 times.

The new Ma Tian loose iron and the residual Worth iron are able to clearly distinguish the above-mentioned tissues by etching the steel sheet profile using LePera liquid and observing the 1/4 portion of the thickness using FE-SEM (returning the above-mentioned tissue) Iron, ferrite iron, toughened iron). Therefore, the volume fraction of the new Ma Tian loose iron can be set as the area fraction of the uncorroded area observed by FE-SEM, and the difference in the area fraction of the residual Worthite iron measured by X-rays. Seek.

Next, it is explained that the number density of iron-based carbides in the tempered granulated iron is specified to be 5 × 10 ⁷ (pieces / mm ² ) or more, and the number of ε- based carbides is relative to the total number of all iron-based carbides. The ratio of the number is set at 20% or more.

In the tempered granulated iron of 1/4 of the plate thickness, the number density of iron carbides having a long diameter of 5 nm or more: 5 × 10 ⁷ (pieces / mm ² ) or more

In the steel sheet according to the present embodiment, in order to improve both the delayed fracture resistance and the moldability, iron is carbonized with a long diameter of 5 nm or more in the main phase of the structure having a thickness of 1/4 of the thickness, that is, the tempered granulated iron. The number density of the objects is specified to be 5 × 10 ⁷ (pieces/mm ² ) or more. In the present embodiment, the "number of iron-based carbides in the tempered granulated iron" is obtained by dividing the number of iron-based carbides contained in the tempered granulated iron in the observation surface by observation. The value obtained from the area of the tempered granulated iron in the surface.

Although the quenched iron of the Ma Tian is high in strength, it has to be improved because of its low resistance to delayed fracture. Therefore, the granulated iron of the granules is tempered into the tempered granulated iron, and the iron-based carbide having a long diameter of 5 nm or more is analyzed to be 5 × 10 ⁷ (pieces/mm ² ) or more in the 1/4 portion of the thickness. Compared with the untempered Ma Tian loose iron, the tempering Ma Tian loose iron (main phase) has superior resistance to delayed damage.

The inventors of the present invention investigated the relationship between the delayed fracture resistance and the number density of iron-based carbides in the tempered granulated iron in the 1/4 portion of the sheet thickness. The result is shown in Fig. 1.

The number density of iron-based carbides was measured by using a plate thickness section parallel to the rolling direction of the steel sheet as an observation surface, and the observation surface was polished, and etching was performed using NITAL liquid and observed at a magnification of 5000 times using FE-SEM. The field of view is 1/4 of the plate thickness, and the number of iron-based carbides with a long diameter of 5 nm or more contained in the tempered granulated iron in each field of view is divided by the area of the tempered granulated iron in the field of view. The values obtained were averaged to determine. Further, the number of iron-based carbides having a long diameter of less than 5 nm is not measured. Because the long diameter is less than 5nm Iron-based carbides have little effect on the delayed-destructive properties of steel sheets. In the following, there is a case where iron-based carbide having a long diameter of 5 nm or more is simply referred to as "iron-based carbide".

The delayed fracture resistance of the steel sheet is obtained by bending a thin rectangular test piece having a length of 100 mm, a width of 30 mm, and a thickness of 1.3 mm or 1.6 mm at a right angle in the rolling direction of the steel sheet, and performing a three-point bending process on the thin rectangular shape. After the surface of the sheet was attached with the water resistance strain gauge, the thin rectangular test piece was immersed in an aqueous solution of ammonium thiocyanate and the aqueous ammonium thiocyanate solution was electrolyzed at a current density of 0.1 mA/cm ² to invade the thin rectangular shape. In the test piece, after 2 hours, the evaluation was performed by confirming the presence or absence of cracks.

The bending radius of the thin rectangular test piece was set to 10 mm. The load stress applied to the thin rectangular test piece having a thickness of 1.3 mm is 60% of the tensile strength (TS) of the steel sheet, and the load stress applied to the thin rectangular test piece having a thickness of 1.6 mm is set as the steel sheet. 90% of the tensile strength (TS). A thin rectangular test piece which is broken by a load stress of 60% of tensile strength (TS) is evaluated as "VERY BAD", and the load stress at 60% of the tensile strength (TS) is not broken, but A thin rectangular test piece having a load stress fracture of 90% of tensile strength (TS) was evaluated as "BAD (poor)", and a thin rectangular test piece in which both load stresses were not broken was evaluated as "GOOD (good) )".

The present inventors have obtained the following findings: as shown in FIG. 1, the number density of iron-based carbides in the tempered granulated iron in the 1/4 portion of the sheet thickness is at least 5 × 10 ⁷ (pieces/mm ² ). In the above case, the delayed damage resistance characteristics are remarkably improved.

In this case, the number density of the iron-based carbides in the tempered granulated iron in the 1/4 portion of the sheet thickness is specified to be 5 × 10 ⁷ (pieces/mm ² ) or more. The number density of iron-based carbides in the tempered granulated iron in the 1/4 portion of the sheet thickness is preferably 1 × 10 ⁸ (pieces / mm ² ) or more, preferably 3 × 10 ⁸ (pieces / mm). ² ) Above.

By the effect of the iron-based carbide in the tempered iron in the tempering iron, the effect of the delayed fracture resistance is obtained, and the smaller the iron-based carbide is, the more remarkable. Further, since most of the iron-based carbides are precipitated in the slabs of the granulated iron, the mechanical properties necessary for the steel sheets such as ductility and moldability are not hindered. Therefore, the smaller the long diameter system of the iron-based carbide in the tempered granulated iron, the better, preferably 350 nm or less. The long diameter of the iron-based carbide in the tempered granulated iron is preferably 250 nm or less, more preferably 200 nm or less. On the other hand, since the iron-based carbide having a too small long diameter does not have the effect of improving the delayed fracture resistance, the steel sheet according to the present embodiment does not consider iron-based carbide having a long diameter of less than 5 nm.

Further, as described above, since the 1/4 portion of the plate thickness is located between the center surface of the steel sheet and the surface of the plate, it has an average structure. Therefore, in the steel sheet according to the present embodiment, it is possible to obtain excellent characteristics throughout the entire steel sheet as long as the number density of the iron-based carbides in the tempered granulated iron in the 1/4 portion of the sheet thickness is within a suitable range.

The ratio of the number of ε- based carbides to the total number of iron-based carbides: 20% or more

The ratio of the total number of iron-based carbides in the tempered granulated iron in the present embodiment to the number of ε- based carbides (hereinafter, abbreviated as "the ratio of the ε- based carbides") is set. More than 20%. Thereby, the delayed fracture resistance can be improved without impairing the moldability, particularly the hole expandability.

The iron-based carbides in the tempered granulated iron are mainly ferritic carbon iron (Fe ₃ C). It is generally considered that the interface between the iron (bcc structure) of the mother phase and the ferritic carbon (Fe ₃ C) functions as a trapping position for trapping hydrogen. Therefore, it is generally believed that the presence of ferritic carbon iron contributes to the improvement of the delayed damage resistance characteristics.

However, since the Schönming carbon-iron system is the starting point for ductile damage, it is difficult to improve both the moldability and the delayed fracture resistance characteristics only by using the Schönming carbon-iron system.

As a result of intensive studies by the present inventors, it has been found that when ε- based carbides (Fe _2.4 C) among various iron-based carbides are used, both moldability and delayed fracture resistance can be improved.

The iron-based carbides composed of Fe and C are ε- based carbides, lanthanide-based carbides, and swarf-carbons (θ-based carbides) having different crystal structures. These iron-based carbides are precipitated in the granulated iron in a state in which the iron of the bcc structure of the parent phase has a specific crystal orientation relationship.

Among the above various iron-based carbides, the ε- based carbide (Fe _2.4 C) and the bcc-structured iron form a near-integrated interface (Coherent interface; at the interface of the two phases, all atoms are satisfied The interface of the interface of the relationship of the most adjacent atoms. Because the interface between ε- based carbide (Fe _2.4 C) and iron (bcc structure) is superior to that of Xueming carbon-iron and iron (bcc structure), the capture ability is better than that of Xueming carbon iron. high. Further, since the ε- based carbide is finer than the sulphur-carbon, it is not likely to be a starting point for ductile fracture.

Therefore, the present inventors focused on the ε- based carbide (Fe _2.4 C) and investigated the relationship between the ε- based carbide ratio and the delayed fracture resistance in the iron-based carbide. The result is shown in Figure 2.

Since the crystal structures of the ε- based carbide (hexagonal crystal) and the swarf carbon iron (orthogonal crystal) are different, the diffraction pattern of the X-ray diffraction or the electron ray diffraction can be easily distinguished. The inventors of the present invention identified the type of iron-based carbide by observing the film sample using an electron microscope. An ε- based carbide (Fe _2.4 C) was identified by irradiating an electron beam to an iron-based carbide and analyzing the obtained diffraction pattern.

The ratio of the ε- based carbide (Fe _2.4 C) in the iron-based carbide of each sample was 10,000 times the observation magnification, and the ε- based carbide (Fe _2.4 C) of each field of view obtained by measurement in 10 fields of view was measured. The ratio is calculated on average. Evaluation of the delayed fracture resistance was carried out using the aforementioned evaluation method.

As shown in Fig. 2, it is found that the ratio of the ε- based carbide (Fe _2.4 C) in the iron-based carbide is 20% or more, whereby excellent processing characteristics and delayed fracture resistance can be ensured. In order to further improve the processing characteristics and the delayed fracture resistance, the ratio of the ε- based carbide (Fe _2.4 C) in the iron-based carbide is preferably 30% or more, more preferably 40% or more.

Further, when the ratio of the ε- based carbide in the iron-based carbide is less than 20%, not only the delayed fracture resistance is deteriorated, but also good processing characteristics cannot be obtained.

As described above, since the 1/4 portion of the plate thickness is located between the center surface of the steel sheet and the surface of the plate, it has an average structure. Therefore, in the steel sheet according to the present embodiment, it is possible to obtain the number of iron-based carbides in the tempered granulated iron in the 1/4 portion of the thickness of the steel sheet. Good properties of the steel plate as a whole.

In the galvanized steel sheet according to the present embodiment, a hot-dip galvanized layer in which Fe is 15% by mass or less and the remainder is composed of Zn, Al, and impurities is formed on the surface of the steel sheet of the present embodiment. Usually, the Fe concentration in the hot-dip galvanizing layer is set to be less than 7% by mass. The lower limit of the Fe concentration in the hot-dip galvanizing is not particularly limited, and is preferably 1.0% by mass.

In the galvannealed steel sheet according to the present embodiment, a hot-dip galvanized layer in which Fe is 15% by mass or less and the remainder is composed of Zn, Al, and impurities is formed on the surface of the steel sheet of the present embodiment. Alloying. The lower limit of the Fe concentration in the alloyed hot-dip galvanizing is not particularly limited, and is usually set to 7% by mass.

Next, a method of producing the steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet according to the embodiment will be described.

First, a method of producing the steel sheet of the present invention will be described.

A method for producing a steel sheet according to the present invention, comprising the steps of: (a) casting a steel preform having the same composition as that of the steel sheet of the embodiment, (a1) directly providing hot rolling, second winding, or (a2) After the temporary cooling, heating is performed to provide hot rolling, followed by coiling; (b) after pickling, cold rolling is performed, followed by annealing; then, (c) the annealed steel sheet is cooled, followed by tempering; d) The tempered steel sheet is subjected to two-stage cooling. (d) is an important step for making the ratio of the ε- based carbide in the iron-based carbide 20% or more.

Providing a hot-rolled cast steel embryo, which is a steel embryo after casting, and Not limited by a specific cast steel blank. For example, a steel embryo made of a continuously cast steel blank or a thin steel blank casting machine may be used. The cast steel germ system is hot rolled. At this time, after the cast steel blank is cast, it may be directly supplied with hot rolling, or may be temporarily cooled, and then reheated to provide hot rolling.

When casting steel blanks are directly continuous casting - direct rolling (CC-DR) or when hot rolling is provided, the cast steel blank must be preheated at the beginning of hot rolling to be able to start at the Ar ₃ metamorphic point (when the steel is cooled, start the fat The temperature range of the granular iron is changed to the temperature range above the hot rolling. Since the finishing rolling temperature is in the two-phase temperature region of (Worstian Iron + Fertilizer Iron), the unevenness of the microstructure of the hot-rolled steel sheet becomes large, or the formability of the finally obtained steel sheet deteriorates.

The steel sheet of the present embodiment having a maximum tensile strength of 780 MPa or more is in a state in which a large amount of alloying elements are contained. At this time, since the rolling load at the time of hot rolling of the cast steel blank is increased, it is preferable to carry out hot rolling at a high temperature. Because of the above, the finishing rolling temperature is set to be higher than the Ar ₃ metamorphic point.

As a result of the experiment, the inventors of the present invention confirmed that, for example, when the heating temperature before hot rolling is about 1150 ° C and the finishing rolling end temperature is 920 ° C, the steel sheet obtained finally has good moldability. .

Further, in the hot rolling, the rough-rolled sheets may be joined to each other to be continuously hot-rolled, or the rough-rolled sheets may be temporarily wound up to provide the second hot rolling.

The coiling temperature after the completion of the hot rolling must be set to a temperature at which the thickness of the oxide formed on the surface of the steel sheet is not excessively excessive and the pickling property is lowered. Further, the coiling temperature after the completion of the hot rolling must be such that the coarse ferrite iron and the ferrite are not generated in the hot-rolled structure, so that the unevenness of the structure after annealing becomes large and the moldability of the final product is deteriorated. .

As a result of the experiment, the inventors of the present invention confirmed that, for example, when the coiling temperature is set to about 590 ° C, the microstructure after annealing is made fine, the strength-ductility balance is improved, and the second phase is uniformly dispersed. The moldability of the finally obtained steel sheet can be improved.

The coiled hot-rolled steel sheet was unwound, subjected to pickling and cold-rolled, whereby a cold-rolled steel sheet was obtained. The oxide on the surface of the hot-rolled steel sheet is removed by pickling to impart chemical conversion treatability and plating property to the cold-rolled steel sheet. The pickling system can be carried out once or in multiples.

The hot-rolled steel sheet after pickling must be cold-rolled at a high reduction ratio which maintains the shape of the cold-rolled steel sheet flat and can impart sufficient ductility to the final product. On the other hand, when the rolling reduction ratio is too high, the rolling load becomes too large and rolling becomes difficult. As a result of the experiment, the inventors of the present invention confirmed that, for example, when the cumulative rolling reduction ratio (cold rolling ratio) at the time of cold rolling is 50%, good results can be obtained. On the other hand, for example, when the cumulative rolling reduction ratio at the time of cold rolling is 90%, the steel slab having the chemical composition of the steel sheet of the present embodiment is cold-rolled, and then cracks occur in the steel sheet. Further, the number of times of the rolling pass and the rolling reduction ratio per pass are not particularly limited.

Next, the cold rolled steel sheet is annealed. Annealing is used to improve manufacturability, preferably continuous annealing.

When the annealing temperature is insufficient (for example, 750 ° C), it is impossible to form a sufficient cold-rolled steel sheet after annealing in the field, and the volume fraction of the tempered iron in the last obtained steel sheet is 70% or more. difficult.

On the other hand, when the annealing temperature is excessive, it causes manufacturing cost. It is economically unsatisfactory, and the shape of the steel sheet becomes inferior, for example, causing a problem that the life of the roller for transporting the steel sheet in the continuous annealing apparatus is low.

Further, when the annealing time is insufficient (for example, about 1 second), the iron-based carbide formed in the hot rolling cannot be dissolved, and since the iron-containing iron contained in the cold-rolled steel sheet after annealing is insufficient, the last cannot be made. The volume fraction of the tempered loose iron of the obtained steel sheet was 70% or more. On the other hand, when the annealing time is excessive, the manufacturing cost is increased and it is economically unsatisfactory.

As a result of experiments conducted by the inventors of the present invention, it has been confirmed that, for example, when the annealing temperature is set to about 880 ° C and the annealing time is set to about 100 seconds, the amount of tempered granulated iron in the finally obtained steel sheet can be within an appropriate range. .

The cold rolled steel sheet after the completion of the continuous annealing is cooled. In the future, there is a case where the cooling after annealing and before tempering is referred to as primary cooling.

When the cooling stop temperature (primary cooling stop temperature) of the primary cooling is too low (for example, about 80 ° C), the new Ma Tian loose iron system containing a large amount of strain tends to remain after the tempering described later, so that tempering cannot be ensured. The ε- based carbide is 20% or more. On the other hand, when the primary cooling stop temperature is too high (for example, about 550 ° C), the amount of toughening iron is more than 20%, and the tensile strength of 780 MPa or more cannot be ensured.

Further, the cooling method may be any of roll cooling, air cooling, water cooling, and the like.

As a result of experiments conducted by the inventors of the present invention, it was confirmed that, for example, in a temperature range from an annealing temperature to 400 ° C, after cooling at a cooling rate of about 2 ° C/sec, the amount of ε- based carbide and toughened iron can be appropriately adjusted. In the range.

Following the above cooling, the cold rolled steel sheet is tempered, whereby the structure control is performed. The granulated iron contained in the cold-rolled steel sheet is tempered by the tempering, and the number density of the iron-based carbides in the tempered granulated iron is 5 × 10 ⁷ (pieces/mm ² ) or more.

In the tempering, the steel sheet temperature is maintained at a predetermined holding temperature (isothermal holding temperature) for a predetermined time (isothermal holding time). When the temperature of the tempering is too low (for example, about 150 ° C), it is difficult to obtain an iron-based carbide number density of 5 × 10 ⁷ (pieces/mm ² ) or more, and sufficient delayed fracture resistance cannot be obtained.

On the other hand, when the temperature of the tempering is excessive (for example, about 550 ° C), the granulated iron is excessively tempered, so that the tensile strength of the finally obtained steel sheet is less than 780 MPa. Further, when the temperature at which the tempering is maintained is excessive, the precipitated iron-based carbide is coarsened and the delayed fracture resistance cannot be improved.

When the tempering holding time is insufficient (for example, about 1 second), the tempering of the granulated iron is insufficient, and it is difficult to make the number density of the iron-based carbides 5 × 10 ⁷ (pieces/mm ² ) or more.

As a result of experiments conducted by the inventors of the present invention, it was confirmed that, for example, when the tempering holding temperature is set to about 400 ° C and the tempering holding time is about 280 seconds, the steel sheet obtained last can be tempered. The amount of iron in the field and the number of iron-based carbides are within an appropriate range.

After the temperature is maintained, it is cooled so that 20% or more of the iron-based carbide contained in the tempered granulated iron is an ε- based carbide.

As described above, the iron-based carbides are ε- based carbides, lanthanide-based carbides, and swarf-carbons (theta-based carbides) having different crystal structures. Among these various iron-based carbides, since the ε- based carbide (Fe _2.4 C) is formed at the interface between the iron close to the bcc structure and the integrated interface, the capturing ability is high. Moreover, since the ε- based carbide is finer than the swarovski carbon, it is not easy to be the starting point of ductile damage.

The inventors of the present invention presume that the amount of ε- based inclusions is affected not only by the tempering cooling conditions but also by the C content, the annealing cooling temperature, the tempering holding temperature, and the tempering retention time. influences. In order to obtain an ε- based inclusion that is considered to be necessary, it is necessary to determine the manufacturing conditions in consideration of the interaction of the control factors of the amount of the ε- based inclusions.

As a result of various experiments, the inventors of the present invention have obtained the following findings: in order to increase the resistance to delayed fracture in order to generate a large amount of ε- based carbide, it is necessary to maintain the temperature in a temperature range of about 360 ° C and at about 360 ° C to 100 ° C. The temperature range is two-stage cooling with different cooling rates. Hereinafter, cooling in a temperature range of maintaining the temperature to about 360 ° C is referred to as secondary cooling, and cooling in a temperature range of approximately 360 ° C to 100 ° C is referred to as tertiary cooling.

According to experiments by the inventors of the present invention, the cooling rate in the secondary cooling range is too low (for example, about 1 ° C / sec) or too high (for example, about 75 ° C / sec), or the cooling rate in the third cooling range is When the amount is too low (for example, about 1 ° C / sec) or too high (for example, about 65 ° C / sec), the amount of ε- based carbide is insufficient.

According to experiments by the inventors of the present invention, it has been found that when the temperature at which the cooling rate is changed is set to 360 ° C ± 10 ° C, the necessary effects can be obtained. On the other hand, when the temperature at which the cooling rate is changed is too low (for example, about 200 ° C), the amount of ε- based carbide is insufficient. Moreover, when the end temperature of the two-stage cooling is too high (for example, about 200 ° C), the amount of ε- based carbide is insufficient.

The inventors of the present invention have obtained the following findings: for example, by setting the C content to 0.06%, the annealing conditions, the tempering holding time and the holding temperature are set to the above-exemplified values, and the holding temperature is maintained at 360 ° C. The cooling rate in the temperature range is about 11 ° C / sec, the cooling rate in the temperature range of 360 to 100 ° C is about 15 ° C / sec, and the end temperature of the two-stage cooling is set to 100 ° C or less. The number of iron-based carbides is 5 × 10 ⁷ /mm ² or more, and the ratio of the ε- based carbides in the iron-based carbide of 1/4 of the thickness is 20% or more.

Further, as described above, since the ε- based carbide (Fe _2.4 C) is finer than the sulphur carbon, it is not easy to be a starting point of ductile fracture, so that the moldability can be remarkably improved while the delayed fracture resistance can be remarkably improved.

The mechanism for significantly improving the delayed fracture resistance is not clear. It is presumed that during the above temperature maintenance period, a fine ε- based carbide core is formed in the tempered granulated iron, and then finely formed by the above-described two-stage cooling. Ε- based carbide.

Next, a method for producing a galvanized steel sheet according to the present embodiment and a method for producing the alloyed galvanized steel sheet according to the embodiment will be described.

The method for producing a galvanized steel sheet according to the present embodiment includes the steps of: (a) casting a steel slab having the same composition as that of the steel sheet of the embodiment, (a1) directly supplying hot rolling, second winding, or After (a2) temporary cooling, heating is performed to provide hot rolling, followed by coiling; (b) pickling is followed by cold rolling, followed by annealing; (c1) cooling the annealed steel sheet to make the steel sheet temperature become Melt galvanizing bath temperature After the vicinity, the hot-dip galvanizing is performed; or (c2) the annealed steel sheet is cooled, and then cooled to room temperature, and then heated to a temperature near the temperature of the hot-dip galvanizing bath to perform hot-dip galvanizing; (d) The steel sheet after the hot-dip galvanizing is subjected to two-stage cooling.

The hot-dip galvanizing is a hot-dip galvanization in which Fe is 15% by mass or less and the remainder is composed of Zn, Al, and impurities.

When a plating layer having Fe of less than 7% by mass is formed on the steel sheet, usually, the plating layer is not subjected to alloying treatment, and is used as a hot-dip galvanized steel sheet. On the other hand, when a plating layer having Fe of 7 mass% or more is formed on the steel sheet, the plating layer is usually alloyed and used as an alloyed hot-dip galvanized steel sheet.

(a) and (b) of the method for producing a galvanized steel sheet according to the present embodiment are the same as (a) and (b) of the method for producing a steel sheet according to the present embodiment. Further, (d) of the method for producing a galvanized steel sheet according to the present embodiment is similar to the method for producing a steel sheet according to the embodiment, and it is necessary to perform two-stage cooling.

In the method for producing a galvanized steel sheet according to the present embodiment, after annealing, the steel sheet is cooled to a temperature near the galvanization bath temperature, and then hot-dip galvanizing or annealing is performed, and then the steel sheet is cooled and then cooled to room temperature. Then, after heating to the vicinity of the galvanizing bath temperature, hot-dip galvanizing is performed. The cooling between the annealing and the hot-dip galvanizing is carried out in the same manner as the cooling contained in (c) of the method for producing a steel sheet according to the above-described embodiment.

The hot-dip galvanizing is carried out by immersing in a plating bath after the temperature of the steel sheet is near the temperature of the galvanizing bath. By making the steel plate temperature a galvanizing bath temperature After being in the vicinity of the degree, it is immersed in the plating bath, and the hot-dip galvanizing layer can be formed on the surface of the steel sheet with good adhesion and uniformity.

When the temperature at which the steel sheet is immersed in the hot-dip galvanizing bath is too low, when the steel sheet is immersed in the plating bath, heat is discharged and a part of the molten zinc is solidified to deteriorate the plating. On the other hand, when the temperature at which the steel sheet is immersed in the hot-dip galvanizing bath is too high, there is a case where the temperature of the plating bath rises to cause an operation problem. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, or the like in addition to pure zinc.

In the method for producing a hot-dip galvanized steel sheet according to the present embodiment, the steel sheet is immersed in a hot-dip galvanizing bath to perform the same structure control as the steel sheet tempering of the present embodiment. When the thermal history of the steel sheet at the time of immersion is the same as the tempering heat history of the steel sheet according to the above-described embodiment, the immersion of the steel sheet in the hot-dip galvanizing bath system does not impair the characteristics of the steel sheet.

After forming a hot-dip galvanized layer on the surface of the steel sheet, (d) of the method for producing a galvanized steel sheet according to the present embodiment, it is necessary to perform two-stage cooling in the same manner as the method (d) for producing a steel sheet according to the present embodiment.

When the plating bath is held, by the combination of the above-described two-stage cooling after plating, the fine iron-based carbide can be made to have a number density of 5 × 10 ⁷ in the tempered granulated iron of the main phase of the necessary structure ( number / mm ²⁾ or more precipitation, and that the ratio ε of the iron-based carbides in the carbide becomes 20% or more, and while maintaining the moldability, it is possible to significantly improve the delayed fracture properties.

The method for producing a galvannealed steel sheet according to the present embodiment includes the steps of: (a) casting a steel slab having the same composition as that of the steel sheet of the embodiment, (a1) Directly providing hot rolling, followed by coiling, or (a2) temporary cooling, heating to provide hot rolling, followed by coiling; (b) after pickling, providing cold rolling, followed by annealing; subsequently, (c-1 After cooling the steel sheet after annealing to bring the steel sheet temperature to the vicinity of the galvanizing bath temperature, hot-dip galvanizing is performed, followed by alloying treatment, or (c-2) cooling of the annealed steel sheet, and further cooling to room temperature Then, hot galvanization is performed until the temperature of the hot-dip galvanizing bath is heated, and then alloying treatment is performed. (d) The steel sheet subjected to the alloying treatment is subjected to two-stage cooling.

Further, the method for producing a galvannealed steel sheet according to the present embodiment may include: (e) immediately after (d), performing heat treatment after reheating, and then cooling to room temperature.

The alloyed hot-dip galvanizing is an alloyed hot-dip galvanizing in which Fe is 15% by mass or less and the remainder is composed of Zn, Al, and impurities.

The method for producing a galvannealed steel sheet according to the present embodiment is a step of alloying a hot-dip galvanized layer in the method for producing a galvanized steel sheet according to the present invention. When the alloying temperature is insufficient, the alloying layer having good adhesion cannot be formed. On the other hand, when it is excessive, the alloying layer becomes too thick, and the moldability of the plating layer is lowered.

As a result of the experiment, the inventors of the present invention confirmed that, for example, when the alloying temperature is about 480 ° C, an alloyed hot-dip galvanized steel sheet having a good alloying layer can be obtained.

In the method for producing a galvannealed steel sheet according to the present embodiment, after alloying and two-stage cooling, heat treatment may be performed again to increase the ratio of ε- based carbide in the iron-based carbide forming an interface having a high capturing ability. .

Example

Next, the embodiment of the present invention will be described, but the conditions of the embodiment are one of the conditional examples for confirming the implementation possibility and effect of the present invention, and the present invention is not limited by the one condition example. The present invention can achieve various objects without departing from the gist of the present invention.

The manufacturing method of the embodiment of the steel sheet has the following steps: (a) casting the steel preform having the composition disclosed in the table, (a1) directly providing hot rolling, second winding, or (a2) temporary cooling Thereafter, heating is performed to provide hot rolling, followed by coiling; (b) after pickling, cold rolling is provided, followed by annealing; then, (c) the annealed steel sheet is cooled, followed by tempering; and subsequently, (d) The tempered steel sheet is subjected to two-stage cooling.

The manufacturing method of the embodiment of the hot-dip galvanized steel sheet has the following steps: (a) casting the steel preform having the composition disclosed in the table, (a1) directly providing hot rolling, second winding, or (a2) After the temporary cooling, heating is performed to provide hot rolling, followed by coiling; (b) after pickling, cold rolling is performed, followed by annealing; (c1) the annealed steel sheet is cooled to make the steel sheet temperature into hot-dip galvanizing. After the bath temperature is near, hot-dip galvanizing is performed; or (c2) the annealed steel sheet is cooled, and then cooled to room temperature, and then heated to a temperature near the hot-dip galvanizing bath to perform hot-dip galvanizing; (d) The steel sheet after the hot-dip galvanizing is subjected to two-stage cooling.

The manufacturing method of the embodiment of the alloyed hot-dip galvanized steel sheet is provided The following steps: (a) casting the steel preform having the composition disclosed in the table, (a1) directly providing hot rolling, second winding, or (a2) temporary cooling, heating to provide hot rolling, and secondly (b) after pickling, cold rolling is provided, followed by annealing; (c-1) after annealing the steel sheet to cool the steel sheet to a temperature near the galvanizing bath temperature, then performing hot-dip galvanizing, followed by execution Or alloying treatment; or (c-2) cooling the annealed steel sheet and further cooling to room temperature, followed by heating to a temperature near the galvanizing bath temperature to perform hot-dip galvanization, followed by alloying treatment; The steel sheet after the alloying treatment is subjected to two-stage cooling.

All of the hot rolled steel sheets were pickled in accordance with a usual method. All of the examples and comparative examples (except for cracks generated during hot rolling or cold rolling) had a sheet thickness of 3.2 mm after hot rolling and a primary cooling rate of 2 ° C/sec. Other manufacturing conditions are as shown in the table. Since the symbol "*1" in the table indicates that cracks appear during hot rolling, the production is stopped, and the symbol "*2" in the table indicates that cracks appear in cold rolling, so that the production is stopped. For the example of the additional mark "*1" or "*2", the feature evaluation is not performed. Regarding the example in which the plating is described as "NO", plating is not performed. In the case where the plating is described as "YES" and the alloying is described as "No", the hot-dip galvanizing is performed, and the alloying and melting are described as "Yes" in both plating and alloying. Galvanized.

In the obtained steel sheet, the volume fraction of the tempered granulated iron (structure A volume fraction), the total volume fraction of one or two types of ferrite iron and toughened iron (tissue B volume fraction) were obtained. The total volume fraction (structure C volume fraction) of residual Worthite iron, new Matian loose iron, and Borne iron, and the number density of iron carbides in the tempered granulated iron (carbide number density) And the ratio of the number of ε- based carbides relative to the number of iron-based carbides (the ratio of ε- based carbides). Further, the tensile strength (TS), the total elongation (EL), and the hole expansibility (λ) of the obtained steel sheet were measured, and the delayed fracture resistance of the obtained steel sheet was evaluated.

For the tensile strength and the elongation, a JIS No. 5 test piece was taken at a right angle in the rolling direction of the steel sheet, and a tensile test was carried out in accordance with JIS Z 2242 to measure the tensile strength (TS) and the total elongation (El). For the hole expandability, the hole expansion ratio (λ (%)) was measured in accordance with the Japan Steel Association Standard JFS T1001.

The delayed fracture resistance of the steel sheet is obtained by bending a thin rectangular test piece having a length of 100 mm, a width of 30 mm, and a thickness of 1.3 mm or 1.6 mm at a right angle in the rolling direction of the steel sheet, and performing a three-point bending process on the thin rectangular shape. After the surface of the sheet was attached with the water resistance strain gauge, the thin rectangular test piece was immersed in an aqueous solution of ammonium thiocyanate and the aqueous ammonium thiocyanate solution was electrolyzed at a current density of 0.1 mA/cm ² to invade the thin rectangular shape. In the test piece, after 2 hours, the evaluation was performed by confirming the presence or absence of cracks.

The bending radius of the thin rectangular test piece was set to 10 mm. The load stress applied to the thin rectangular test piece having a thickness of 1.3 mm is 60% of the tensile strength (TS) of the steel sheet, and the load stress applied to the thin rectangular test piece having a thickness of 1.6 mm is set as the steel sheet. 90% of the tensile strength (TS). Thin rectangular test for breaking stress at 60% of tensile strength (TS) The sheet was rated as "very poor", and the load stress at 60% of the tensile strength (TS) was not broken, but the thin rectangular test piece with a tensile stress crack of 90% of the tensile strength (TS) was rated as "poor". A thin rectangular test piece in which both load stresses are not broken is evaluated as "good". Steel sheets rated as "good" are steel sheets having excellent resistance to delayed fracture.

As shown in the table, it is understood that in the steel sheet of the embodiment of the present invention, a large amount of iron-based carbides are precipitated as a hydrogen trap position function, and remarkably have excellent resistance to delayed fracture, and the phase composition in the structure. It also has excellent formability. Moreover, it was found that at least one of the strength, the delayed fracture resistance, and the moldability of the steel sheet of the comparative example was inferior.

Industrial availability

As described above, according to the present invention, it is possible to provide a steel sheet, a hot-dip galvanized steel sheet, and an alloyed melting alloy which are suitable as structural members for automobiles, buildings, home electric appliances, and the like and have excellent tensile strength at break resistance of 780 MPa or more. Galvanized steel sheets, and the methods of manufacture thereof. Therefore, in the construction and utilization of structural members, the use of the present invention is highly likely.

Claims (8)

A steel sheet characterized by having a chemical composition of C: 0.05 to 0.40%, Si: 0.05 to 3.00%, Mn: 1.50% or more and less than 3.50%, P: 0.04% or less, and S: 0.01% or less in mass%. , N: 0.01% or less, O: 0.006% or less, Al: 0 to 2.00%, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Nb: 0~0.30%, Ti: 0~0.30%, V: 0~0.50%, B: 0~0.01%, Ca: 0~0.04%, Mg: 0~0.04%, and REM: 0~0.04%, the rest It consists of Fe and impurities. The tissue of 1/4 part of the thickness is in volume fraction, containing tempered granulated iron: 70% or more, and one or two kinds of fertilized iron and toughened iron: a total of less than 20%; in the 1/4 part of the thickness of the structure, in terms of volume fraction, the residual Worth iron is less than 10%, the new Ma Tian loose iron is less than 10%, the Borne iron is less than 10%, and The total volume fraction of the above-mentioned residual Worthite iron, the aforementioned new Maeda iron and the aforementioned bun iron is 15% or less; and the iron having a long diameter of 5 nm or more in the tempered granulated iron of the 1/4 portion of the plate thickness carbide is the number density of 5 × 10 ⁷ / mm ^{2 or} more; with respect to the quarter-thickness portion 5nm or more of the number of iron-based carbide long diameter ratio of the number of ε carbide is 20% or more, a tensile strength of 780MPa or more. The steel sheet according to claim 1, wherein the chemical composition of the steel sheet contains, by mass%, Cr: 0.05 to 1.00%, Mo: 0.01 to 1.00%, Ni: 0.05 to 1.00%, and Cu: 0.05 to 1.00%. One or two or more. The steel sheet according to claim 1 or 2, wherein the chemical component of the steel sheet contains one or two kinds of Nb: 0.005 to 0.30%, Ti: 0.005 to 0.30%, and V: 0.005 to 0.50% by mass%. the above. The steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition of the steel sheet is in mass%, and B: 0.0001 to 0.01%. The steel sheet according to claim 1 or 2, wherein the chemical component of the steel sheet contains one or two kinds of Ca: 0.0005 to 0.04%, Mg: 0.0005 to 0.04%, and REM: 0.0005 to 0.04% by mass%. the above. The steel sheet according to claim 1 or 2, wherein the iron-based carbide has an average major axis of 350 nm or less. A hot-dip galvanized steel sheet characterized in that, on the surface of the steel sheet according to any one of claims 1 to 6, a melting of 15% by mass or less and a balance of Zn, Al, and impurities is formed. Galvanized layer. An alloyed hot-dip galvanized steel sheet characterized in that the surface of the steel sheet according to any one of claims 1 to 6 is formed with Fe of 15% by mass or less and the remainder consisting of Zn, Al, and impurities. The alloyed hot-dip galvanized layer.