JP5114747B2 - Manufacturing method of high-strength steel sheet with extremely good balance between hole expansibility and ductility and manufacturing method of galvanized steel sheet - Google Patents

Description

  The present invention relates to a high-strength steel sheet excellent in workability such as hole expansibility and ductility suitable for automobiles, building materials, home appliances, and the like, a manufacturing method thereof, and a manufacturing method of a galvanized steel sheet.

  In recent years, high-strength steel sheets have been applied in the automobile field in order to achieve both a function for protecting passengers in the event of a collision and weight reduction for the purpose of improving fuel efficiency. In particular, with regard to ensuring collision safety, in addition to increasing safety awareness, high-strength steel sheets are applied from strengthening laws and regulations to parts with complex shapes that have only been used with low-strength steel sheets until now. There is a need to try. However, since the formability of the material deteriorates as the strength increases, it is necessary to manufacture a steel sheet that satisfies both formability and high strength when applying a high strength steel sheet to a member having a complex shape. is there. Even if it is said that the formability is a bit, when applying to a member having a complicated shape such as an automobile member, for example, it is required to simultaneously have different formability such as ductility, stretch formability, and hole expandability. .

It is known that ductility and stretch formability, which are important as formability of a thin steel sheet, are correlated with a work hardening index (n value), and a steel sheet having a high n value is known as a steel sheet having excellent formability. For example, as steel plates excellent in ductility and stretch formability, there are DP (Dual Phase) steel plates whose steel plate structure is composed of ferrite and martensite, and TRIP (Transformation Induced Plasticity) steel plates containing residual austenite in the steel plate structure (Patent Document 1). Patent Document 2). On the other hand, as a steel sheet excellent in hole expansibility, a steel sheet having a ferrite single phase structure in which the steel sheet structure is precipitation strengthened and a steel sheet having a bainite single phase structure are known (Patent Documents 3 to 5, Non-Patent Document 1).
The DP steel sheet has excellent ductility by having ferrite having high ductility as a main phase and dispersing martensite which is a hard structure in the steel sheet structure. Further, soft ferrite is easily deformed, and a large amount of dislocations are introduced and hardened together with the deformation, so that the n value is also high. However, if the steel sheet structure is composed of soft ferrite and hard martensite, the deformability of both structures is different, so in forming with large machining such as hole expansion, there is a minute amount at the interface between both structures. There is a problem that microvoids are formed and the hole expandability is significantly deteriorated. In particular, the DP steel sheet with a maximum tensile strength of 540 MPa or higher has a relatively high martensite volume fraction, and since there are many ferrite and martensite interfaces, the microvoids formed at the interface easily connect and crack formation occurs. To break. From this, it is known that the hole expandability of DP steel sheet is inferior (for example, Non-Patent Document 2).

Similarly, in the TRIP steel plate whose steel plate structure is composed of ferrite and retained austenite, the hole expandability is low. This is because hole expansion processing and stretch flange processing, which are molding processes for automobile members, are performed after punching or mechanical cutting. The retained austenite contained in the TRIP steel sheet transforms into martensite when subjected to processing. For example, in the case of stretch-stretching or overhanging, it is possible to ensure high formability by increasing the strength of the processed portion and suppressing the concentration of deformation by transforming residual austenite into martensite. However, once punching, cutting, or the like is performed, the vicinity of the end face is subjected to processing, so that residual austenite contained in the steel sheet structure is transformed into martensite. As a result, the structure becomes similar to that of the DP steel sheet, and the hole expandability and stretch flange formability are inferior. Alternatively, since the punching process itself involves a large deformation, after punching, microvoids exist at the interface between ferrite and hard structure (here, martensite transformed with retained austenite), which deteriorates hole expandability. It has been reported that
Or the steel sheet in which a cementite and a pearlite structure exist in a grain boundary is also inferior in hole expansibility. This is because the boundary between ferrite and cementite is the starting point for microvoid formation.
As a result, as shown in Patent Documents 3 to 5 and Non-Patent Document 1, the development of a steel sheet excellent in hole expansibility has a single-phase structure of ferrite in which the main phase of the steel sheet is bainite or precipitation strengthened, and grains In order to suppress the formation of cementite phase at the boundary, by adding a large amount of alloy carbide forming elements such as Ti and making C contained in the alloy alloy carbide, a high strength hot-rolled steel sheet with excellent hole expandability can be obtained. Has been developed.

A steel sheet having a bainite single-phase structure as a steel sheet structure has a defect that the bainite structure is a structure containing many dislocations, and therefore has poor workability and is difficult to apply to members that require ductility and stretchability. It was.
A steel sheet having a precipitation-strengthened ferrite single-phase structure has a high strength of 780 MPa or more by increasing the strength of the steel sheet by using precipitation strengthening by carbides such as Ti, Nb or Mo, and suppressing the formation of cementite and the like. Although it is possible to achieve both strength and excellent hole expansibility, a cold-rolled steel sheet that has undergone cold-rolling and annealing processes has a drawback that precipitation strengthening is difficult to utilize. That is, precipitation strengthening is achieved by consistent precipitation of alloy carbides such as Nb and Ti in ferrite.
In cold-rolled steel sheets with cold rolling and annealing, since ferrite is processed and recrystallized at the time of annealing, the orientation relationship with Nb and Ti precipitates that were coherently precipitated at the hot-rolled sheet stage is lost. The strengthening ability is greatly reduced, and it is difficult to secure the strength. Nb and Ti are known to significantly delay recrystallization, and high temperature annealing is required to ensure excellent ductility, resulting in poor productivity. Moreover, even if ductility comparable to that of a hot-rolled steel sheet is obtained, the precipitation-strengthened steel is inferior in ductility and overhanging compared to DP steel sheets, and cannot be applied to parts that require a large overhang. In addition, a large amount of an expensive alloy carbide forming element such as Nb or Ti must be added, resulting in a problem of high cost.

As steel plates that have overcome these drawbacks and ensured ductility and hole expandability, steel plates described in Patent Documents 6 and 7 are known. These steel sheets have a composite structure consisting of ferrite and martensite, and then tempered and softened, thereby improving the strength-ductility balance and hole expansibility obtained by strengthening the structure. We are going to get it at the same time. However, even if the hard structure is softened by tempering the martensite, the martensite is still hard, so it is impossible to avoid deterioration of the hole expansibility. In addition, the softening of martensite causes a decrease in strength, so the martensite volume ratio must be increased to compensate for the decrease in strength, and this causes a problem that the hole expandability deteriorates as the hard tissue fraction increases. (Patent Document 7).
Further, when the cooling end point temperature fluctuates, the volume ratio of martensite varies. As a means to solve these problems, or in order to ensure a sufficient martensite volume fraction, by quenching to room temperature using a water tank or the like, a sufficient amount of martensite volume fraction may be ensured, When cooling is performed using water or the like, shape defects such as warpage of the steel sheet and camber after cutting are likely to occur. The cause of these shape defects is not simply due to deformation of the plate, but may be due to residual stress due to temperature unevenness during cooling. Even if the plate shape is good, the shape such as warp or camber after cutting May cause failure. Moreover, it has the subject that it is hard to correct in a post process. Therefore, there is a problem not only in securing the material but also in terms of ease of use.
As described above, since the steel sheet structures necessary for ensuring ductility, stretch formability, and hole expandability are very different, it is extremely difficult to simultaneously provide these characteristics.
CAMP-ISIJ vol. 13 (2000), p411 CAMP-ISIJ vol. 13 (2000), p391 JP-A-53-22812 Japanese Patent Laid-Open No. 1-2230715 JP 2003-321733 A JP 2004-256906 A Japanese Patent Application Laid-Open No. 11-296991 JP-A-63-293121 JP 57-137453 A

As described above, in order to increase the ductility, it is desirable that the steel sheet structure is a composite structure composed of a soft structure and a hard structure, and in order to increase hole expansibility, a uniform structure with a small hardness difference between the structures is used. It is desirable. That is, conventionally, since the structures required for securing the respective characteristics are different, it has been difficult to provide a steel sheet having both characteristics.
The present invention has been made in consideration of such circumstances, and its purpose is to achieve both excellent ductility comparable to that of DP steel and excellent hole expansibility equivalent to that of a single-structure steel plate. Another object of the present invention is to provide a steel plate having a high strength and a method for producing the steel plate.

The gist of the present invention aimed at solving the above problems is as follows.
(1) The present invention is mass%, C: 0.05% to 0.20%, Si: 0.3 to 2.0%, Mn: 1.3 to 2.6%, P: 0.001. -0.03%, S: 0.0001-0.01%, Al: less than 0.10%, N: 0.0005-0.0100%, O: 0.0005-0.007%, The balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and the crystal orientation difference between any of the ferrites adjacent to the hard structure and the hard structure is less than 9 °. In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility with a maximum tensile strength of 540 MPa or more, the cast slab having the above composition is directly or once cooled and then heated to 1050 ° C. or more, and Ar 3 Complete hot rolling above the transformation point, 400-620 ° C Wound at a temperature range, after pickling, subjected to rolling reduction of 40% to 70% cold rolled, when to Tsuban a continuous annealing line, the average cooling rate maximum heating temperature to 740 ° C. After annealing at Ac3 transformation point or more Cool at 7 to 30 ° C./s, hold at 740 to 680 ° C. for 8 seconds or more, cool at 630 to 570 ° C. at an average cooling rate of 3 ° C./second or more, and in a temperature range of 450 to 300 ° C. The present invention relates to a method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility, characterized by holding for 30 seconds or more.
(2) The present invention is a high-strength steel sheet containing, as a composition, B: 0.0001 to less than 0.010% by mass%, and the hole expansibility described in (1) Further, the present invention relates to a method for producing a high-strength steel sheet having a very good balance of ductility.
(3) In the present invention, the composition further includes, in mass%, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: A high-strength steel sheet containing one or more of 0.01 to 1.0%, characterized by a very good balance between hole expansibility and ductility as described in (1) or (2) The present invention relates to a method for producing a high strength steel sheet.
(4) The present invention is characterized in that the composition further comprises a high-strength steel plate containing 0.001 to 0.14% of Nb, Ti, or V in total by mass of 1 type or 2 types or more. The present invention relates to a method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility according to any one of (1) to (3).
(5) The present invention further provides a high-strength steel sheet containing 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM as the composition. The method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility according to any one of (1) to (4).

(6) The present invention is mass%, C: 0.05% to 0.20%, Si: 0.3 to 2.0%, Mn: 1.3 to 2.6%, P: 0.001. -0.03%, S: 0.0001-0.01%, Al: less than 0.10%, N: 0.0005-0.0100%, O: 0.0005-0.007%, The balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and the crystal orientation difference between any of the ferrites adjacent to the hard structure and the hard structure is less than 9 °. In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility having a maximum tensile strength of 540 MPa or more, the cast slab having the chemical component is directly or once cooled, and then heated to 1050 ° C. or more, Hot rolling is completed at the Ar3 transformation point or higher, 400 to 67 After winding in a temperature range of ℃, pickling, cold rolling with a rolling reduction of 40 to 70%, and passing through a continuous hot dip galvanizing line, annealing at a maximum heating temperature of Ac3 transformation point or higher, 740 ° C Until it is cooled at an average cooling rate of 7 to 30 ° C./s, held at 740 to 680 ° C. for 8 seconds or more, and between 630 to 570 ° C. at an average cooling rate of 3 ° C./second or more (zinc plating bath temperature −40) After cooling to ℃ ~ (Zinc plating bath temperature +50) ℃, either before or after immersion in the galvanization bath, or both, (Zinc plating bath temperature +50) Temperature range from ℃ to 300 ℃ It is related with the manufacturing method of the high intensity | strength hot-dip galvanized steel plate with the extremely favorable balance of hole expansibility and ductility characterized by hold | maintaining for 30 seconds or more.
(7) The present invention further provides a high-strength steel sheet containing B: 0.0001 to less than 0.010% by mass% as the composition, and the hole expansibility described in (6) And a method for producing a high-strength hot-dip galvanized steel sheet with a very good balance of ductility.
(8) In the present invention, the composition further includes, in mass%, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: High-strength steel sheet containing one or more of 0.01 to 1.0%, characterized by a very good balance between hole expandability and ductility as described in (6) or (7) The present invention relates to a method for producing a high strength hot dip galvanized steel sheet.
(9) The present invention is characterized in that the composition further comprises a high-strength steel sheet containing 0.001 to 0.14% in total of one or more of Nb, Ti, and V in mass%. The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility according to any one of (6) to (8).
(10) The present invention further provides a high-strength steel sheet containing 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM as the composition. The method for producing a high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility according to any one of (6) to (9).

(11) The present invention is mass%, C: 0.05% to 0.20%, Si: 0.3 to 2.0%, Mn: 1.3 to 2.6%, P: 0.001. -0.03%, S: 0.0001-0.01%, Al: less than 0.10%, N: 0.0005-0.0100%, O: 0.0005-0.007%, The balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and the crystal orientation difference between any of the ferrites adjacent to the hard structure and the hard structure is less than 9 °. In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility having a maximum tensile strength of 540 MPa or more, the cast slab having the chemical component is directly or once cooled, and then heated to 1050 ° C. or more, Hot rolling is completed at the Ar3 transformation point or higher, 400 to 6 After winding in a temperature range of 0 ° C., pickling, cold rolling with a rolling reduction of 40 to 70%, and annealing through a continuous hot-dip galvanizing line with a maximum heating temperature equal to or higher than the Ac3 transformation point 740 The sample is cooled to an average cooling rate of 7 to 30 ° C./s, and held at 740 to 680 ° C. for 8 seconds or more, and between 630 to 570 ° C. at an average cooling rate of 3 ° C./s or more (zinc plating bath temperature−40 ) After cooling to 0 ° C to (Zinc plating bath temperature +50) ° C, if necessary, alloying treatment is performed at a temperature of 460 to 540 ° C, and before immersion, after immersion in the zinc plating bath, or after alloying treatment High strength alloyed molten zinc with a very good balance between hole expansibility and ductility, characterized by being held in a temperature range of (Zinc plating bath temperature +50) ° C. to 300 ° C. for at least 30 seconds. Manufacturing method of plated steel sheet.
(12) A cast slab containing B: 0.0001 to less than 0.010% by mass% as the composition is used, and the balance between hole expansibility and ductility according to (11) A very good method for producing high-strength hot-dip galvanized steel sheets.
(13) As the composition, further, by mass, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.00. A high-strength melt having a very good balance between hole expansibility and ductility according to (11) or (12), characterized by being a high-strength steel sheet containing one or more of 01 to 1.0% Manufacturing method of galvanized steel sheet.
(14) The composition further comprises a high-strength steel sheet containing 0.001 to 0.14% of one or more of Nb, Ti, and V in total by mass% (11 )-(13) The manufacturing method of the high intensity | strength hot-dip galvanized steel plate with the extremely favorable balance of the hole expansibility and ductility in any one of.
(15) The composition is further characterized by being a high-strength steel sheet containing 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM by mass%. (11) A method for producing a high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility according to any one of (11) to (14).

According to the present invention, by controlling the steel plate components and annealing conditions, mainly composed of ferrite and hard structure, the crystal orientation difference between the adjacent ferrite and hard structure is less than 9 °, thereby the maximum tensile strength is 540 MPa. A high-strength steel sheet having the above excellent ductility and excellent hole expandability can be obtained stably.
That is, the content of C, Si, Mn, P, S, Al, N, and O is within a specified range, and an excellent strength of 540 MPa or more can be exhibited at the maximum tensile strength, and at the same time, it is mainly composed of ferrite and a hard structure. By making the crystal orientation difference between the ferrite and the hard structure less than 9 °, a high-strength steel sheet having excellent ductility and excellent hole expansibility can be stably provided.

According to the method for producing a high-strength steel sheet of the present invention, the content of C, Si, Mn, P, S, Al, N, and O as a steel sheet component is within a specified range, mainly composed of ferrite and a hard structure, and adjacent to each other. In order to produce a steel sheet having a crystal orientation difference of less than 9 ° between the ferrite and the hard structure, after passing through a hot rolling process and a cold rolling process, a continuous annealing line is mainly passed as a condition during annealing. At this time, after annealing at a maximum heating temperature of Ac 3 ° C. or higher, it is cooled to 740 ° C., held at 740 to 680 ° C. for 8 seconds or longer, a heating rate between 200 ° C. and 600 ° C., and a heating rate between 600 ° C. and the maximum temperature. The desired high-strength steel sheet can be produced by regulating the cooling rate between 570 ° C. and 630 ° C. and the holding conditions in the temperature range between 450 ° C. and 300 ° C. after annealing.
Moreover, according to the manufacturing method of the high-strength hot-dip galvanized steel sheet of the present invention, in addition to the above-described conditions, at least one before and after being immersed in the Zn plating bath, in the temperature range of the galvanizing bath temperature + 50 ° C. to 300 ° C. This can be realized by defining the holding conditions.

  Moreover, according to the manufacturing method of the high-strength galvannealed steel sheet of the present invention, in addition to the above-described conditions, the alloying treatment is performed, and at least one before and after being immersed in the Zn plating bath, the galvanizing bath temperature +50 This can be realized by defining the holding conditions in the temperature range of from ° C to 300 ° C.

The inventors of the present invention have intensively studied for the purpose of achieving both excellent ductility and excellent hole expansibility in a high-strength steel sheet having a maximum tensile strength of 540 MPa or more. As a result, while the steel sheet structure is a ferrite and a hard structure, the ratio of the hard structure in which the crystal orientation difference between the hard structure and any adjacent ferrite is within 9 ° is 50% or more of the volume ratio of the entire hard structure In other words, while maintaining the excellent ductility that is a feature of the composite structure steel sheet by mainly having a hard structure whose crystal orientation difference with any adjacent ferrite is within 9 °. Also found that excellent hole expansibility can be secured.
The present invention is described in detail below.
First, the reasons for limiting the structure of the steel sheet will be described.
As a result of diligent investigations on the structure of this type of steel sheet, the present inventors have made the steel sheet structure a composite structure of ferrite and hard structure, and the crystal orientation difference between any one of the ferrites adjacent to the hard structure is 9 °. It has been found that by making the ratio of the hard structure within 50% or more of the volume ratio of the entire hard structure, it is possible to achieve both excellent ductility and hole expansibility while maintaining high strength. In particular, it is most important in the present invention that the crystal orientation difference between adjacent ferrite and hard structure is within 9 °.

  The hard structure mentioned here indicates bainite or martensite. Bainite is a structure having a bcc structure, like ferrite. Depending on the case, it is a structure containing cementite or retained austenite inside the lath-like or massive bainitic ferrite constituting the bainite structure. In addition, bainite has a smaller particle size than ferrite or has a low transformation temperature, so it contains a large amount of dislocations and is therefore harder than ferrite. On the other hand, martensite is very hard because it has a bct structure and contains a large amount of C therein.

The reason why the steel sheet structure is a multiphase structure of ferrite and hard structure is to obtain excellent ductility. Since soft ferrite is rich in ductility, it is essential to obtain excellent ductility. In addition, it is possible to increase the strength while ensuring excellent ductility by dispersing an appropriate amount of hard structure. In order to ensure excellent ductility, it is necessary to use a ferrite main phase. The steel sheet structure of the present invention may contain retained austenite. Residual austenite is transformed into martensite at the time of deformation, thereby hardening the processed portion and hindering concentration of deformation. As a result, particularly excellent ductility can be obtained.
Next, the volume ratio of the hard structure adjacent to the ferrite with the crystal orientation difference within 9 ° is set to 50% or more of the volume ratio of the entire hard structure. This is to suppress the formation of microvoids at the ferrite and hard tissue interface.
Generally, ferrite and hard structure, which are soft tissues, have different deformability. That is, soft ferrite is easily deformed, but hard bainite and martensite are not easily deformed. As a result, after the uniform elongation of the tensile test, when large deformation such as hole expansion processing or stretch flange processing is performed, the deformation concentrates at the interface of both tissues, leading to microvoid formation, crack formation, crack propagation, and fracture. Therefore, it was considered impossible to achieve both excellent ductility and hole expandability.

However, as a result of intensive studies by the present inventors, it has been found that even a hard structure can be deformed by reducing the orientation difference between adjacent ferrites. That is, the present inventors have found that strain concentration relaxation at the interface between the soft tissue and the hard tissue and the formation of microvoids can be suppressed. This is considered to be due to the fact that the crystal structures of ferrite and hard structure are similar. That is, since both structures have similar crystal structures, it is considered that the slip system of dislocations responsible for deformation is the same. In addition, if the crystal orientation difference is small, it is considered that the same deformation as that generated in the ferrite also occurs in the hard structure. In addition, under large deformation such as hole expansion processing, ferrite is sufficiently hardened and hard, and since the difference in deformability from a hard structure is small, it is considered that even a hard structure can be deformed. On the other hand, at the initial stage of deformation, ferrite is soft and easily deformed because it has not undergone much processing. As a result, it is considered that the steel sheet of the present invention can simultaneously have ductility and hole expansibility comparable to those of a composite structure steel sheet.
Such an effect becomes remarkable when the volume ratio of the hard structure in which the crystal orientation difference between the adjacent ferrite and the hard structure, particularly bainite is within 9 °, is 50% or more of the total hard structure. If it exceeds 9 °, the deformability is poor even under large deformation, strain concentration at the interface and formation of microvoids are promoted, and the hole expandability is greatly deteriorated. For this reason, the crystal orientation difference needs to be 9 ° or less.

On the other hand, in the present invention, it is only necessary to satisfy the crystal orientation relationship in the above range with the hard structure and any ferrite adjacent thereto. Although it is desirable that the azimuth difference with all adjacent ferrites is less than 9 °, for that purpose, it is necessary to make all the ferrites have the same azimuth, which is extremely difficult technically. Even if the orientation difference between the adjacent ferrite is large, the ferrite having the same orientation is deformed, so that the strain concentration can be alleviated. Furthermore, the hard structure to be formed often has a crystal orientation similar to that of the ferrite with the most interfaces. For this reason, the present inventor believes that even if all adjacent ferrites and hard structures do not have the above orientation relationship, the hole expandability is improved by suppressing the formation of microvoids.
Moreover, in the steel plate of this invention, you may contain a retained austenite, a pearlite, or cementite other than the said bainite and a martensite. This is because most of the hard structure is the hard structure having the above-mentioned crystal orientation, so that there is a hard structure having other orientations, causing strain concentration and microvoid formation at the interface between the ferrite and the hard structure. Even if it is, the frequency is small and sufficiently dispersed, it is difficult to connect to each other, and the hole expandability and stretch flangeability of the steel sheet are not deteriorated.

It is desirable that the volume ratio of the hard structure having an adjacent crystal orientation difference of 9 ° or less is 50% or more of the volume ratio of the entire hard structure. This is because if the volume ratio is less than 50%, the effect of suppressing the hole expandability due to the suppression of microvoid formation is small. The volume ratio of the hard tissue is desirably 5% or more. This is because it is difficult to ensure the strength of 540 MPa or more when the volume fraction of the hard tissue is less than 5%. More desirably, 50% or more of the total volume ratio of bainite, martensite, retained austenite, cementite, and pearlite structure present in the steel sheet is used as the bainite structure. For this reason, the lower limit of the volume ratio is set to 5%. The upper limit is not particularly defined, and excellent ductility and hole expandability, which are the effects of the present invention, are provided. However, if the TS range is 590 to 1080 MPa, both the ductility of the steel sheet and the hole expandability or stretch flangeability can be achieved. For this purpose, it is desirable to include ferrite having a volume ratio exceeding 50%.
The steel sheet structure is basically a composite structure of ferrite and bainite, but may include residual austenite, martensite, cementite, pearlite, and the like as other hard structures.

Identification of each phase of the above microstructure, ferrite, pearlite, cementite, martensite, bainite, austenite and the remaining structure, observation of the existing position and measurement of the area ratio are disclosed in Nital reagent and JP-A-59-219473. It is possible to corrode the steel plate rolling direction cross section or the rolling direction perpendicular direction cross section with the above-mentioned reagent, and to quantify by 1000 times optical microscope observation and 1000 to 100000 times scanning and transmission electron microscopes. The structure can also be discriminated from crystal orientation analysis using the FESEM-EBSP method (high-resolution crystal orientation analysis method) and micro region hardness measurement such as micro Vickers hardness measurement.
Further, the crystal orientation relationship can be identified by observation of the internal structure with a transmission electron microscope (TEM) and crystal orientation mapping using the FESEM-EBSP method. In particular, crystal orientation mapping using the FESEM-EBSP method is particularly effective because a wide field of view can be easily measured. In the present invention, after taking a photograph with an SEM, a crystal orientation mapping of a field of view of 100 μm × 100 μm was performed using a FESEM-EBSP method with a step size of 0.2 μm. However, it is difficult to distinguish bainite and martensite having a similar crystal structure only by orientation analysis using the FESEM-EBSP method. However, since the martensite structure is a structure containing many dislocations, it can be easily discriminated by comparison with an image quality image. That is, since martensite is a structure containing many dislocations, the image quality is much lower than that of ferrite and bainite and can be easily discriminated. Therefore, in the present invention, when the bainite and martensite are discriminated using the FESEM-EBSP method, the discriminating is performed using the Image Quality image. It is possible to obtain an area ratio of each tissue by observing 10 fields of view or more and using a point counting method or image analysis.

In measuring the difference in crystal orientation, the crystal orientation relationship between [1-1-1], which is the main slip direction of the ferrite that is the main phase and the adjacent hard structure, was measured. However, even if the [1-1-1] direction is the same, it may rotate around this axis. Therefore, the difference in crystal orientation in the normal direction of the (110) plane, which is the slip surface of the [1-1-1] slip, was also measured, and the difference in crystal orientation between both was 9 ° or less. It was defined as a hard structure having a crystal orientation difference of 9 ° or less in the invention.
In determining the orientation difference, steel plates having various components and manufacturing conditions are prepared, and after the hole expansion test or the test piece after the tensile test is embedded and polished, the deformation behavior near the fractured part, especially the microvoid When the formation behavior was investigated, a remarkable suppression of microvoid formation was observed at the ferrite-hard structure interface where the difference in crystal orientation between the adjacent ferrite and hard structure determined as described above was 9 ° or less. Furthermore, it has been found that by controlling the ratio of the hard structure having a crystal orientation difference of 9 ° or less between the adjacent ferrite and the hard structure in the entire hard structure to 50% or more, there is a significant hole expandability improving effect. It was. For this reason, it is necessary that the ratio of the hard structure having a crystal orientation difference of 9 ° or less in the entire hard structure be 50% or more. In addition, the suppression of microvoid formation not only improves the hole expansibility, but also improves the local elongation in the tensile test. Therefore, the composite structure steel sheet in which the crystal orientation difference of the hard structure of the present invention is controlled is normal. Excellent local elongation compared to DP steel.

If TS is 540 MPa or more, if it is less than this strength, it is possible to achieve both high ductility and hole expandability with TS of less than 540 MPa by increasing the strength of the ferrite single-phase steel using solid solution strengthening. It is because it can plan. In particular, when considering securing TS of 540 MPa or more, in order to ensure excellent ductility, it is necessary to perform reinforcement using martensite or retained austenite, and deterioration of hole expansibility becomes remarkable.
In the present invention, the crystal grain size of ferrite is not particularly limited, but it is desirable that the nominal grain size is 7 μm or less from the viewpoint of balance of strength elongation.

Next, the reasons for limiting the components of the present invention will be described.
C: 0.05% to 0.20%
C is an essential element when strengthening the structure using bainite or martensite. If C is less than 0.05%, it is difficult to ensure a strength of 540 MPa or more, so the lower limit was set to 0.05%. On the other hand, the reason why the content of C is 0.20% or less is that if C exceeds 0.20%, the volume fraction of the hard structure becomes too large, and the crystal orientation difference between most hard structures and ferrite is increased. Even if it is 9 ° or less, the volume ratio of the hard structure that does not have the above crystal orientation relationship inevitably increases, and strain concentration and microvoid formation at the interface cannot be suppressed, and the hole expansion value is inferior. It is to become.
Si: 0.3-2.0%
In addition to being a strengthening element, Si does not dissolve in cementite, so it suppresses the formation of coarse cementite at grain boundaries. If less than 0.3% is added, strengthening due to solid solution strengthening cannot be expected, or formation of coarse cementite at grain boundaries cannot be suppressed, so 0.3% or more needs to be added. On the other hand, addition exceeding 2.0% excessively increases the retained austenite, and deteriorates the hole expandability and stretch flangeability after punching or cutting. Therefore, the upper limit needs to be 2.0%. In addition, since the oxide of Si has poor wettability with hot dip galvanizing, it causes non-plating. Therefore, in manufacturing a hot-dip galvanized steel sheet, it is necessary to control the oxygen potential in the furnace and suppress the formation of Si oxide on the steel sheet surface.

Mn: 1.3 to 2.6%
Since Mn is an austenite stabilizing element at the same time as a solid solution strengthening element, it suppresses the transformation of austenite to pearlite. If it is less than 1.3%, the rate of pearlite transformation is too high, and the steel sheet structure cannot be made a composite structure of ferrite and bainite, and a TS of 540 MPa or more cannot be secured. Moreover, the hole expansibility is also inferior. For this reason, the lower limit is set to 1.3% or more. On the other hand, when Mn is added in a large amount, co-segregation with P and S is promoted and workability is significantly deteriorated. Therefore, the upper limit is set to 2.6%.
P: 0.001 to 0.03%
P tends to segregate in the central part of the plate thickness of the steel sheet, causing the weld to become brittle. If it exceeds 0.03%, embrittlement of the weld becomes significant, so the appropriate range is limited to 0.03% or less. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.001%.
S: 0.0001 to 0.01%
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01% or less. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%. In addition, since S is combined with Mn to form coarse MnS, the hole expandability is lowered. For this reason, it is necessary to reduce as much as possible in order to improve hole expansibility.
Al: less than 0.10% Al promotes ferrite formation and improves ductility, so it may be added. It can also be used as a deoxidizer. However, excessive addition increases the number of Al-based coarse inclusions, causing deterioration of hole expansibility and surface scratches. From this, the upper limit of Al addition was set to 0.1%. The lower limit is not particularly limited, but it is difficult to set the lower limit to 0.0005% or less, which is a practical lower limit.

N: 0.0005 to 0.01%
N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.01% or less. In addition, it is better to use less because it causes blowholes during welding. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased, and this is a substantial lower limit.
O: 0.0005 to 0.007%
O forms an oxide and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. In particular, oxides often exist as inclusions, and when they are present on the punched end surface or cut surface, they form notched scratches and coarse dimples on the end surface, so when expanding holes or during strong processing, It causes stress concentration and becomes the starting point of crack formation, resulting in a significant deterioration of hole expansibility or bendability. This is because this tendency becomes significant when O exceeds 0.007%, so the upper limit of the O content is set to 0.007% or less. Setting it to less than 0.0005% invites excessive costs such as deoxidation at the time of steel making, and is not economically preferable. However, even if O is less than 0.0005%, TS of 540 MPa or more, which is the effect of the present invention, and excellent ductility can be secured.
B: 0.0001 to 0.010%
B is effective for strengthening grain boundaries and strengthening steel by addition of 0.0001% by mass or more. However, when the addition amount exceeds 0.010% by mass, the effect is not only saturated but also heat The upper limit is set to 0.010% because the manufactured product at the time of extension is lowered.
Cr: 0.01 to 1.0%
Cr is a strengthening element and is important for improving hardenability. However, if it is less than 0.01%, these effects cannot be obtained, so the lower limit was set to 0.01%. If the content exceeds 1%, a significant increase in cost is caused, so the upper limit was made 1%.

Ni: 0.01 to 1.0%
Ni is a strengthening element and is important for improving hardenability. However, if it is less than 0.01%, these effects cannot be obtained, so the lower limit was set to 0.01%. If the content exceeds 1%, a significant increase in cost is caused, so the upper limit was made 1%.
Cu: 0.01 to 1.0%
Cu is a strengthening element and is important for improving hardenability. However, if it is less than 0.01%, these effects cannot be obtained, so the lower limit was set to 0.01%. On the other hand, if the content exceeds 1%, the manufacturability at the time of production and hot rolling is adversely affected, so the upper limit was made 1%.
Mo: 0.01 to 1.0%
Mo is a strengthening element and is important for improving hardenability. However, if it is less than 0.01%, these effects cannot be obtained, so the lower limit was set to 0.01%. If the content exceeds 1%, the cost is significantly increased, so the upper limit is 1%, but 0.3% or less is more preferable.
Nb: Nb is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the addition amount is less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. If the content exceeds 0.14%, the precipitation of carbonitride increases and the formability deteriorates, so the upper limit was made 0.14%.
Ti: Ti is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the addition amount is less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. If the content exceeds 0.14%, the precipitation of carbonitride increases and the formability deteriorates, so the upper limit was made 0.14%.
V: V is a strengthening element. It contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. If the addition amount is less than 0.001%, these effects cannot be obtained, so the lower limit was set to 0.001%. If the content exceeds 0.14%, the precipitation of carbonitride increases and the formability deteriorates, so the upper limit was made 0.14%.
One or two or more selected from Ca, Ce, Mg, and REM can be added in a total amount of 0.0001 to 0.5%. Ca, Ce, Mg, and REM are elements used for deoxidation. By containing one or more kinds in total of 0.0001% or more, the oxide size after deoxidation can be reduced, and the hole is expanded. Contributes to improved performance.
However, when the content exceeds 0.5% in total, it causes deterioration of molding processability. Therefore, the content is made 0.0001 to 0.5% in total. Note that REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM and Ce are often added by misch metal and may contain a lanthanoid series element in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. However, the effects of the present invention are exhibited even when metal La or Ce is added.

Next, the reasons for limiting the production conditions of the steel sheet of the present invention will be described.
Since martensite and bainite are transformed from austenite, it is known to have a specific orientation relationship with austenite. On the other hand, it is known that when an austenite single-phase region annealing is performed and then held at a high temperature for a long time, ferrite having a specific orientation relationship may be formed at the austenite grain boundary. However, although the ferrite having the specific orientation relationship can be formed by holding at high temperature for a long time, the ferrite is very easy to form at a low temperature. Therefore, many ferrites having no specific orientation relationship are formed. In the cooling process, since new ferrite may be formed in the austenite grains or at the grain boundaries of austenite and ferrite, the ratio of the ferrite and austenite interface having a specific orientation relationship is reduced.
When considering a short manufacturing process such as continuous annealing equipment or hot dip galvanizing equipment, it is difficult to form a ferrite having a specific orientation because it is difficult to hold for a long time. As a result, when ferrite is produced by high-temperature and long-time heat treatment, even if there is a specific orientation relationship between ferrite and austenite, if a large amount of ferrite with a volume ratio of 50% or more is formed, the orientation relationship is It will be lost. As a result, it was impossible to control the orientation relationship between hard structures (such as bainite and martensite) formed by transformation from ferrite and austenite present in the steel sheet structure.

  As a result of earnest studies, the present inventors have been able to secure ferrite having a specific orientation relationship with austenite by controlling the cooling conditions from the austenite single phase region, and 450 ° C. to 300 ° C. Hard structure whose crystal orientation difference with ferrite as the main phase is less than 9 ° by controlling the crystal orientation relationship of the hard structure transformed from austenite by retaining for 30 seconds or more in the temperature range of ° C. It was found that can be formed. As a result, it was possible to produce a steel sheet that contributed to the increase in strength but did not deteriorate the ductility and hole expandability, that is, simultaneously provided with the maximum tensile strength, ductility, and hole expandability of 540 MPa or more.

The reason for limiting the detailed manufacturing conditions will be described below.
The slab used for hot rolling in the present invention is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.
The hot-rolled slab heating temperature needs to be 1050 ° C. or higher. If the slab heating temperature is excessively low, the finish rolling temperature falls below the Ar3 transformation point, resulting in a two-phase rolling of ferrite and austenite, and the hot rolled sheet structure becomes a heterogeneous mixed grain structure, which has undergone cold rolling and annealing processes. However, the non-uniform structure is not eliminated and the ductility and hole expansibility are poor. Moreover, since the steel plate which concerns on this invention ensures the tensile maximum strength of 540 Mpa or more after annealing, since the comparatively large amount of alloy elements are added, the intensity | strength at the time of finish rolling tends to become high. The decrease in the slab heating temperature causes a decrease in the finish rolling temperature, further increases the rolling load, and there is a concern that rolling may become difficult or the shape of the steel sheet after rolling may be poor. It is necessary to set it to 1050 ° C. or higher. The upper limit of the slab heating temperature is not particularly defined, and the effect of the present invention is exhibited. However, since it is economically undesirable to make the heating temperature too high, the upper limit of the heating temperature should be less than 1300 ° C. Is desirable.

The finish rolling temperature needs to be higher than the Ar3 transformation point. When the finish rolling temperature is in the two-phase region of austenite + ferrite, the structure non-uniformity in the steel sheet increases, and the formability after annealing deteriorates. Therefore, the Ar3 transformation temperature or higher is desirable.
The Ar3 transformation temperature can be calculated and grasped by the following equation according to the alloy composition.
Ar3 = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)

On the other hand, the upper limit of the finishing temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finishing rolling temperature is excessively high, the slab heating temperature is excessively increased in order to secure the temperature. I have to. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1000 ° C. or less.
The coiling temperature after hot rolling needs to be 620 ° C. or less. When the temperature exceeds 620 ° C., coarse ferrite and pearlite structures exist in the hot-rolled structure, so that the structure non-uniformity after annealing increases and the ductility of the final product deteriorates. It is more preferable to wind up at 600 ° C. or less from the viewpoint of improving the strength ductility balance by making the microstructure after annealing fine, and further improving the hole expandability by uniformly dispersing the second phase. In addition, winding at a temperature exceeding 620 ° C. is not preferable because the thickness of the oxide formed on the steel sheet surface is excessively increased, and the pickling property is inferior. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, since it is technically difficult to wind up at a temperature of room temperature or lower, this is the actual lower limit. Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.

  The hot-rolled steel sheet thus manufactured is pickled. Since pickling can remove oxides on the surface of steel sheets, it can improve the chemical conversion properties of cold-rolled high-strength steel sheets as final products, and improve the hot-plating properties of cold-rolled steel sheets for hot-dip galvanized or alloyed hot-dip galvanized steel sheets. It is important for that. Moreover, pickling may be performed once, or pickling may be performed in a plurality of times.

  The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70% and passed through a continuous annealing line or a continuous hot-dip galvanizing line. If the rolling reduction is less than 40%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, this is the lower limit. On the other hand, cold rolling exceeding 70% makes the cold rolling difficult because the cold rolling load becomes too large. A rolling reduction of 45 to 65% is a more preferable range. The effect of the present invention is exhibited without particularly specifying the number of rolling passes and the rolling reduction for each pass.

  The annealing temperature when passing through the continuous annealing line needs to be higher than the Ac3 transformation temperature. This is because an austenite single-phase structure is once formed, and ferrite having a specific orientation is formed at the austenite grain boundary. When the temperature is lower than the Ac3 transformation temperature, recrystallized ferrite that does not have a specific orientation relationship with austenite exists. Therefore, even when it is cooled, most of the ferrite is formed. As a result, it is difficult to control the orientation relationship between austenite and ferrite. On the other hand, excessively high-temperature annealing exceeding the Ac3 transformation temperature exhibits the strength, ductility, and hole expansibility that are effects according to the present invention, but the economical efficiency is lowered. For this reason, the upper limit of the annealing temperature is desirably 900 ° C. or less.

A ferrite having a specific orientation relationship is more likely to be produced as it is held at a higher temperature, but it takes a long time to form. On the other hand, ferrite tends to appear in a shorter time as the temperature is lower, but does not have a specific orientation relationship. As a result of intensive studies by the present inventors, it has been found that ferrite having a specific orientation relationship can be formed by setting the residence time between 680 and 740 ° C. to 8 seconds or more.
Although the cooling rate at the maximum heating temperature to 740 ° C. is not particularly limited, when a facility having a finite length is used, the decrease in the cooling rate in this temperature range is due to the residence time between 740 to 680 ° C. Means decrease. For this reason, it is desirable to increase the cooling rate between the maximum heating temperature and 740 ° C. as much as possible.

  Subsequently, it is necessary to keep the temperature between 740 and 680 ° C. for 8 seconds or more. The reason why the temperature range is 740 to 680 ° C. is that the formation of ferrite having a specific orientation relationship with austenite is the fastest at this temperature. At temperatures exceeding 740 ° C., ferrite having a specific orientation relationship can be formed, but it takes a long time, and a sufficient amount of ferrite cannot be formed by equipment having a finite length. On the other hand, when held at a temperature lower than 680 ° C., ferrite transformation proceeds in a short time, but many that do not have a specific orientation relationship are formed. As a result, even if holding for 30 seconds or more in the temperature range of 300 to 450 ° C., the volume ratio of the hard structure having a crystal orientation difference of 9 ° or less is 50% or more of the volume ratio of the entire hard structure. Difficult to do. Therefore, the lower limit temperature is set to 680 ° C. The reason why the holding time is set to 8 seconds or more is that it is necessary to secure a ferrite having a specific orientation at a volume ratio of 50% or more. If the holding time is less than 8 seconds, a ferrite volume ratio of 50% or more cannot be ensured and the ductility is poor.

  Moreover, holding | maintenance at 740-680 degreeC does not mean isothermal holding | maintenance, but means the residence time in this temperature range. In other words, in addition to isothermal holding, cooling between 740 and 680 ° C., or once cooling to 680 ° C. and heating to 740 ° C., the residence time between 740 and 680 ° C. is 8 seconds or more. It only has to be secured.

The Ac3 transformation point is determined by the following formula.
Ac3 = 910−203 × (C) ^1/2 + 44.7 × Si-30 × Mn + 700 × P + 400 × Al-11 × Cr-20 × Cu-15.2 × Ni + 31.5 × Mo + 400 × Ti

In the present invention, it is necessary to cool between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more. If the cooling rate is too low, austenite transforms into a pearlite structure during the cooling process, and thus a hard structure of an amount necessary for a strength of 540 MPa or more cannot be secured. Even if the cooling rate is increased, there is no problem in terms of the material. However, excessively increasing the cooling rate leads to an increase in manufacturing cost, so the upper limit is preferably set to 200 ° C./second. The cooling method may be roll cooling, air cooling, water cooling, or any combination of these methods.
In the present invention, it is necessary to keep the temperature in the temperature range of 450 ° C. to 300 ° C. for 30 seconds or longer. This is because austenite is transformed into bainite having a crystal orientation difference of less than 9 ° with respect to ferrite as the main phase. When holding in a temperature range higher than 450 ° C., coarse cementite precipitates at the grain boundaries, so that the hole expandability is greatly deteriorated. Therefore, the upper limit temperature is set to 450 ° C. On the other hand, when the holding temperature is less than 300 ° C., bainite transformation hardly occurs, and austenite is transformed into martensite in the subsequent cooling process. As a result, the crystal orientation difference between the main phase ferrite and the hard structure cannot be made less than 9 °, and the hole expansibility is greatly deteriorated. From this, 300 ° C. for holding for 30 seconds or more is the lower limit temperature.
Even if a bainite structure is formed in the temperature range of 450 ° C. to 300 ° C. for less than 30 seconds, the volume ratio is not sufficient, and the remaining austenite is transformed into martensite during the subsequent cooling process. Therefore, most of the hard structure has a crystal orientation difference of 9 ° or more and is inferior in hole expansibility. For this reason, the lower limit of the residence time is 30 seconds or more. The upper limit of the residence time is not particularly defined, and the effect of the present invention can be obtained. However, the increase in residence time can be achieved by lowering the plate passing speed when considering heat treatment in a facility having a finite length. This means that it is not preferable because it is not economical.

As described above, as an annealing condition with the Ac3 transformation temperature as a boundary, the conventional steel sheet as shown in FIG. 1 is heated to a temperature lower than the Ac3 transformation temperature and maintained at that temperature, and then cooled to below the Ac1 transformation temperature and then cooled. However, this method generates ferrite that cannot control the orientation relationship as described above. On the other hand, in this invention, as shown in FIG. 2, it heats to the temperature range more than Ac3 transformation temperature, hold | maintains it for a predetermined time, and starts cooling after that, and between average heating rate 26 degrees C / sec or less between maximum heating temperature -630 degreeC. Cooling, and then cooling between 630 ° C and 570 ° C at an average cooling rate of 3 ° C / second or more, and adding heat history shown in the polygonal line in Fig. 2 to hold at least 30 lines in the temperature range of 450 ° C to 300 ° C By doing so, the ferrite in which the orientation is controlled can be generated, and finally the target structure can be obtained.
In addition, it is necessary to perform the process to make it retain in this 450-300 degreeC temperature range continuously to the process of cooling above 630 degreeC-570 degreeC with an average cooling rate of 3 degrees C / second or more, 630 degreeC-570 degreeC. The crystal orientation difference can be controlled even if the sample is once cooled to a temperature lower than 300 ° C in the process of cooling at an average cooling rate of 3 ° C / second or more and then subjected to a heat treatment in the temperature range of 450 to 300 ° C again and retained. You can't do that.

In the present invention, holding means not only isothermal holding but also retention in a temperature range of 450 to 300 ° C. That is, after cooling to 300 ° C., it may be heated to 450 ° C., or may be cooled to 450 ° C. and then cooled to 300 ° C.
After the heat treatment, it is desirable to perform skin pass rolling in order to control surface roughness, plate shape control, or suppression of yield point elongation. The reduction ratio of the skin pass rolling at that time is preferably in the range of 0.1 to 1.5%. If the skin pass rolling rate is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, the productivity is remarkably lowered, so this is the upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.

The annealing temperature when passing through the hot dip galvanizing line after cold rolling is also set to the Ac3 transformation temperature or higher for the same reason as when passing through the continuous annealing line. The residence time at 680 to 740 ° C. needs to be 8 seconds or longer for the same reason as described above.
Regarding the cooling after annealing, it is necessary to cool between 630 ° C. and 570 ° C. at 3 ° C./second or more for the same reason as when the continuous annealing line is passed through.
The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature. If the bath immersion plate temperature is lower than the hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and part of the molten zinc may solidify and deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C. However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in. On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature + 50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.

Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. The upper limit is not particularly limited, but if it exceeds 600 ° C., carbide is formed and the volume ratio of hard structure (martensite, bainite, retained austenite) is reduced, and it becomes difficult to ensure the strength of 540 MPa or more. is there.
It is necessary to perform an additional heat treatment for 30 seconds or more in the temperature range of (zinc plating bath temperature + 50) ° C. to 300 ° C. either before or after immersion in the plating bath, or after immersion. The upper limit of the heat treatment temperature is set to (zinc plating bath temperature +50) ° C. Above this temperature, the formation of cementite and pearlite becomes remarkable, and the volume fraction of the hard tissue is reduced, so it is difficult to ensure the strength of 540 MPa or more. It is to become. On the other hand, if it is less than 300 ° C., the progress of the bainite transformation is too slow, and it is not possible to sufficiently secure the volume fraction of the hard structure that makes the crystal orientation difference between the main phase ferrite and the hard structure less than 9 °. Therefore, the lower limit of the heat treatment temperature is set to 300 ° C. or higher.
The holding time needs to be 30 seconds or more. When the holding time is less than 30 seconds, even if a bainite structure is formed, the volume ratio is not sufficient, and the remaining austenite is transformed into martensite in the subsequent cooling process, so that most of the hard structure is formed. The crystal orientation difference is 9 ° or more, and the hole expandability is inferior. For this reason, the lower limit of the residence time is 30 seconds or more. The upper limit of the residence time is not particularly defined, and the effect of the present invention can be obtained. However, the increase in residence time can be achieved by lowering the plate passing speed when considering heat treatment in a facility having a finite length. This means that it is not preferable because it is not economical. The holding time does not simply mean isothermal holding, but means residence in this temperature range, and includes cooling and heating in this temperature range.
Further, the additional heat treatment for 30 seconds or more in the temperature range of (zinc plating bath temperature + 50) ° C. to 300 ° C. may be performed either before or after immersion in the plating bath, or both. Absent. As long as a hard structure having a crystal orientation difference of less than 9 ° with respect to ferrite as the main phase can be secured, the strength of 540 MPa or more, which is the effect of the present invention, can be achieved regardless of the additional heat treatment. This is because excellent ductility and hole expansibility can be obtained.

After the heat treatment, it is desirable to perform skin pass rolling in order to control surface roughness, plate shape control, or suppression of yield point elongation. The reduction ratio of the skin pass rolling at that time is preferably in the range of 0.1 to 1.5%. If the skin pass rolling rate is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, the productivity is remarkably lowered, so this is the upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel plate is plated with Ni, Cu, Co, or Fe alone or before the annealing.
Further, regarding annealing before plating, “after degreasing pickling, heating in a non-oxidizing atmosphere, annealing in a reducing atmosphere containing H ₂ and N ₂ , cooling to near the plating bath temperature, and invading the plating bath. Zenjimer method called “Kizuke”, an all-reduction furnace method called “immersion in the plating bath after adjusting the atmosphere during annealing, first oxidizing the steel plate surface, and then reducing it before cleaning by plating” Alternatively, there is a flux method such as “after degreasing and pickling a steel plate, and then fluxing it with ammonium chloride and soaking it in a plating bath”, etc. The effect of can be demonstrated. Regardless of the method of annealing before plating, the dew point during heating is set to −20 ° C. or more, which advantageously works on the wettability of plating and the alloying reaction in the alloying of plating.

In addition, even if it electroplates this invention steel plate, the tensile strength, ductility, and hole expansibility which a steel plate has is not impaired at all. That is, the steel sheet of the present invention is also suitable as a material for electroplating. Even if an organic film or upper layer plating is performed, the effect of the present invention can be obtained.
In addition, the material of the high strength and high ductility hot dip galvanized steel sheet excellent in formability and hole expansibility of the present invention is manufactured through refining, steel making, casting, hot rolling, and cold rolling processes that are normal iron making processes. However, even if it is manufactured by omitting part or all of it, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied.

"Example 1"
Next, the present invention will be described in detail with reference to examples.
A slab having the components shown in Table 1 is heated to 1200 ° C, hot-rolled at a final hot rolling temperature of 900 ° C, and after water cooling in a water-cooled zone, a winding process is performed at the temperatures shown in Tables 2 and 3. went. After pickling the hot-rolled sheet, the hot-rolled sheet having a thickness of 3 mm was cold-rolled to 1.2 mm to obtain a cold-rolled sheet.

"Example 2"
These cold-rolled sheets were subjected to annealing heat treatment under the conditions shown in Tables 2 and 3, and were annealed by annealing equipment. The furnace atmosphere is equipped with a device that introduces H ₂ O and CO ₂ generated by burning a gas that is a composite of CO and H _2, and N ₂ gas containing 10% by volume of H ₂ with a dew point of −40 ° C. is introduced. Then, annealing was performed under the conditions shown in Tables 2 and 3.

Moreover, about the plated steel plate, it annealed and plated with the continuous hot dip galvanization equipment. Annealing conditions and the furnace atmosphere in order to secure the plating properties, CO and H ₂ of H ₂ O generated by burning gas in complex, a device for introducing CO ₂ mounting, H ₂ that the dew point and -10 ° C. the introducing N ₂ gas containing 10 vol% were subjected to annealing under the conditions shown in tables 2 and 3. In particular, in steel numbers C, F, and H containing a large amount of Si, if the above-mentioned furnace atmosphere control is not performed, non-plating and alloying are likely to be delayed. When the alloying process is performed, it is necessary to control the atmosphere (oxygen potential). Then, about some steel plates, the alloying process was performed in the temperature range of 480-600 degreeC. The basis weight of the hot dip galvanizing of the plated steel sheet was about 50 g / m ^{2 on} both sides. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.4%.

"Example 3"
The obtained cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized steel sheet were subjected to a tensile test, and yield stress (YS), maximum tensile stress (TS), and total elongation (El) were measured. The results are shown in Tables 4 and 5.

In addition, this steel plate is a composite structure steel plate which consists of a ferrite and a hard structure, and yield point elongation does not appear in many cases. From this, the yield stress was measured by the 0.2% offset method. A steel sheet having a TS × El of 16000 (MPa ×%) or more was defined as a high-strength steel sheet having a good strength-ductility balance.
The hole expansion rate (λ) was evaluated by punching a circular hole having a diameter of 10 mm under the condition that the clearance was 12.5%, forming the burr on the die side, and molding with a 60 ° conical punch. Under each condition, five hole expansion tests were performed, and the average value was defined as the hole expansion ratio. A steel sheet having a TS × λ of 40000 (MPa ×%) or more was defined as a high-strength steel plate having a good strength-hole expansibility balance.
This high strength steel sheet having an excellent balance of hole expansibility and ductility was provided with the good strength-ductility balance and the good strength-hole expansibility balance at the same time.
The microstructure was identified using the above method to identify each tissue. However, when the retained austenite is low in chemical stability, it becomes martensite due to polishing during the preparation of the microstructure observation specimen and the disappearance of grain boundary restraints from the surrounding crystal grains due to the release of the free surface. May be transformed. As a result, as measured by X-ray, when directly measuring the volume fraction of residual austenite contained in the steel sheet, and once measuring the residual austenite existing on the surface by polishing the free surface, etc. The volume ratio may be different. In the present invention, since it is necessary to measure the crystal orientation relationship between ferrite as a main phase and a hard structure by the FESEM-EBSP method, the microstructure was identified after polishing the surface.

In addition, the orientation difference between adjacent ferrite and hard structure was measured by the above-described method, and the following rating was performed.
○: The ratio of the hard structure having a crystal orientation difference of 9 ° or less in the entire hard structure is 50% or more. Δ: The ratio of the hard structure having a crystal orientation difference of 9 ° or less in the entire hard structure is 30% or more. The ratio of the hard structure having a crystal orientation difference of 9 ° or less in the whole is less than 30%. Particularly, when the ratio of the hard structure having a crystal orientation difference of 9 ° or less in the entire hard structure is 50% or more, particularly a remarkable hole This range was made the scope of the present invention because the expansion rate was improved.

Steel numbers A-1, 2, 5-9, 11, 12, B-1 to 3, C-1, 4, 7, 8, D-1, E-1, F-1 shown in Table 4 or Table 5. -3, G-1, 2, 5, 6, H-1, 2, I-1, 2, 6, 7, 8, J-1, K-1 to 3 are chemical components of steel sheets according to the present invention. And the manufacturing conditions are also within the range defined by the present invention. As a result, the ratio of the hard structure in which the crystal orientation difference between the ferrite as the main phase and the hard structure is less than 9 ° increases, and even if the structure is strengthened by the hard structure, the hole expandability does not deteriorate.
That is, it is possible to ensure a high level of hole expansibility while taking advantage of the improvement of the strength-ductility balance by strengthening the structure. As a result, it is possible to produce a steel sheet having a very high balance of tensile maximum strength of 540 MPa or more, ductility and hole expandability.
On the other hand, steel Nos. A-3 to 5, A-10, 13 to 16, C-2, 3, 5, 6, 9, G-3, 4, and 7 shown in Table 4 or Table 5 have the same manufacturing conditions. Since it does not satisfy the scope of the invention, there are many cases where the crystal orientation difference between ferrite and hard structure exceeds 9 °, and the TS × λ value, which is an index of hole expansibility, is less than 40000 (MPa ×%), and the hole expansion is low. Inferior to sex. In particular, steel numbers A3, 4, and C-6 are samples having a short holding time between 740 ° C. and 680 ° C., and a sample of A-16 is a sample having a slow average cooling rate between 630 ° C. and 570 ° C.
If steel numbers A-5, 10, 14, 15, C-3, G-3, 7, I-5, and 10 shown in Table 4 or Table 5 are cold-rolled steel plates, the temperature range is 300 to 450 ° C. In the case of hot dip galvanized steel sheet and alloyed hot dip galvanized steel sheet, the residence time in the temperature range of (galvanizing bath temperature +50) ° C. to 300 ° C. is 30 seconds. In many cases, the crystal orientation difference between the main phase ferrite and the hard structure is more than 9 °, and the TS × λ value, which is an index of hole expansibility, is as low as less than 40000 (MPa ×%). Inferior to sex.

In Steel No. A-16 shown in Table 4, since the cooling rate in the temperature range of 630 to 570 ° C. is too slow, austenite is transformed into pearlite, and high strength of 540 MPa or more cannot be secured. Also, the strength-ductility balance is inferior.
Steel No. C-5 shown in Table 4 has an annealing temperature as low as 740 ° C., and a pearlite structure formed during hot rolling and cementite spheroidized in the steel sheet structure remain, so that bainite and martens which are hard structures remain. Since the site cannot secure a sufficient volume ratio, a high strength of 540 MPa or more cannot be ensured. Also, the strength-ductility balance is inferior.
In steel numbers L-1 to L-3 shown in Table 5, Si and Mn are as low as 0.01% and 1.12%, respectively, and pearlite transformation is suppressed in the cooling process after annealing, and bainite, martensite, residual Since a hard structure such as austenite cannot be secured, a high strength of 540 MPa or more cannot be secured.
Steel numbers M-1 to M-3 shown in Table 5 have a low C content of 0.034%, and cannot secure a high strength of 540 MPa or more because a sufficient amount of hard structure cannot be ensured.
Steel numbers N-1 to N-3 shown in Table 5 have a high Mn content of 3.2%, and once the ferrite volume fraction is reduced during annealing, a sufficient amount of ferrite cannot be produced in the cooling process. From this, the strength-ductility balance is remarkably inferior.

  The present invention is a steel sheet having a maximum tensile strength of 540 MPa or more suitable for structural members, reinforcing members, and suspension members for automobiles, and having excellent ductility and hole expansibility at the same time. This steel sheet is suitable for use in, for example, structural members for automobiles, reinforcing members, suspension members, and the like. The effect is extremely high.

FIG. 1 is an explanatory diagram showing an example of heat history when annealing is performed at a temperature below the Ac3 transformation temperature. FIG. 2 is an explanatory diagram showing an example of heat history when annealing is performed by heating above the Ac3 transformation temperature.

Claims (15)

% By mass
C: 0.05% to 0.20%,
Si: 0.3 to 2.0%,
Mn: 1.3-2.6%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%
Al: less than 0.10%,
N: 0.0005 to 0.0100%,
O: 0.0005 to 0.007% is contained, the balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and any ferrite adjacent to the hard structure; In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility in which the crystal orientation difference with the hard structure is less than 9 ° and the maximum tensile strength is 540 MPa or more,
The cast slab having the above composition is directly or once cooled and then heated to 1050 ° C. or higher, completes the hot rolling at the Ar 3 transformation point or higher, wound up in the temperature range of 400 to 620 ° C., pickled, and then reduced. When 40% to 70% cold rolling is applied and the continuous annealing line is passed through, the maximum heating temperature is annealed at the Ac3 transformation point or higher, and then cooled to 740 ° C at an average cooling rate of 7-30 ° C / s , 740-680. Hole expansibility characterized by holding for 8 seconds or more at ℃, cooling between 630 ℃ and 570 ℃ at an average cooling rate of 3 ℃ / second or more, and holding for 30 seconds or more in a temperature range of 450 ℃ to 300 ℃ A method for producing high-strength steel sheets with a very good balance of ductility.   The balance of hole expansibility and ductility according to claim 1, characterized in that the composition further comprises a high-strength steel sheet containing B: 0.0001 to less than 0.010% by mass%. Method for manufacturing high strength steel sheets.   As the composition, further, by mass, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1 The method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility according to claim 1 or 2, characterized in that the steel sheet is a high-strength steel sheet containing 0.0% or one or more kinds.   The high-strength steel sheet containing 0.001 to 0.14% in total of one or more of Nb, Ti, and V in terms of mass% as the composition. A method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility.   2. The high-strength steel sheet containing 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM in mass% as the composition. A method for producing a high-strength steel sheet having a very good balance between hole expansibility and ductility according to any one of. % By mass
C: 0.05% to 0.20%,
Si: 0.3 to 2.0%,
Mn: 1.3-2.6%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%
Al: less than 0.10%,
N: 0.0005 to 0.0100%,
O: 0.0005 to 0.007% is contained, the balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and any ferrite adjacent to the hard structure; In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility in which the crystal orientation difference with the hard structure is less than 9 ° and the maximum tensile strength is 540 MPa or more,
The cast slab having the chemical component is directly or once cooled and then heated to 1050 ° C. or higher, and the hot rolling is completed at the Ar 3 transformation point or higher, wound in a temperature range of 400 to 670 ° C., pickled, and then reduced. When performing cold rolling at a rate of 40 to 70% and passing through a continuous hot dip galvanizing line, after annealing at a maximum heating temperature of the Ac3 transformation point or higher , cooling to 740 ° C at an average cooling rate of 7 to 30 ° C / s , After holding at 740-680 ° C. for 8 seconds or more, and cooling between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C. The hole expandability is characterized by holding for at least 30 seconds in a temperature range of (zinc plating bath temperature +50) ° C. to 300 ° C. either before or after immersion in the zinc plating bath, or both. And the balance of ductility Method for producing a good high-strength galvanized steel sheet Te fit.   The balance of hole expansibility and ductility according to claim 6, wherein the composition further comprises a high-strength steel sheet containing B: 0.0001 to less than 0.010% by mass%. Manufacturing method for high strength hot-dip galvanized steel sheet.   As the composition, further, by mass, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1 A high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility according to claim 6 or 7, characterized in that it is a high-strength steel sheet containing 0.0% of one or more kinds. Method.   The composition further comprises a high-strength steel sheet containing 0.001 to 0.14% of Nb, Ti, or V in a total of 0.001 to 0.14% by mass. A method for producing a high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility.   7. The high-strength steel sheet containing, as a composition, 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM in mass%. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate with the extremely favorable balance of the hole expansibility and ductility in any one of -9. % By mass
C: 0.05% to 0.20%,
Si: 0.3 to 2.0%,
Mn: 1.3-2.6%,
P: 0.001 to 0.03%,
S: 0.0001 to 0.01%
Al: less than 0.10%,
N: 0.0005 to 0.0100%,
O: 0.0005 to 0.007% is contained, the balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and a hard structure, and any ferrite adjacent to the hard structure; In producing a high-strength steel sheet having a very good balance between hole expansibility and ductility in which the crystal orientation difference with the hard structure is less than 9 ° and the maximum tensile strength is 540 MPa or more,
The cast slab having the chemical component is directly or once cooled and then heated to 1050 ° C. or higher, and the hot rolling is completed at the Ar 3 transformation point or higher, wound in a temperature range of 400 to 670 ° C., pickled, and then reduced. When performing cold rolling at a rate of 40 to 70% and passing through a continuous hot dip galvanizing line, after annealing at a maximum heating temperature of the Ac3 transformation point or higher , cooling to 740 ° C at an average cooling rate of 7 to 30 ° C / s , After holding at 740-680 ° C. for 8 seconds or more, and cooling between 630 ° C. and 570 ° C. at an average cooling rate of 3 ° C./second or more to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C. If necessary, alloying treatment is performed at a temperature of 460 to 540 ° C., and before or after immersion in the galvanizing bath, or after alloying treatment, or in total (zinc plating bath temperature +50) ℃ ～ 300 ℃ の 温度Extremely good high intensity method of manufacturing a galvannealed steel sheet balance of hole expandability and ductility, characterized by holding at frequency 30 seconds or more.   The balance of hole expansibility and ductility according to claim 11, wherein the composition further includes a high-strength steel sheet containing B: 0.0001 to less than 0.010% by mass%. Manufacturing method for high strength hot-dip galvanized steel sheet.   As the composition, further, by mass, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.01 to 1 A high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility according to claim 11 or 12, characterized in that it is a high-strength steel sheet containing 0.0% of one or more kinds. Method.   The high-strength steel sheet containing 0.001 to 0.14% in total of one or more of Nb, Ti, and V in terms of mass% as the composition. A method for producing a high-strength hot-dip galvanized steel sheet having a very good balance between hole expansibility and ductility.   The high-strength steel sheet containing 0.0001 to 0.5% in total of one or more of Ca, Ce, Mg, and REM in terms of mass% as the composition. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate with the extremely favorable balance of the hole expansibility and ductility in any one of -14.

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