JP2009249732A - High-strength steel sheet having extremely excellent stretch flange formability, method for producing the same, and cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet having extremely excellent stretch flange formability and suitable for automobiles, building materials, house appliances or the like. <P>SOLUTION: Disclosed is a high-strength steel sheet having a composition comprising, by mass, prescribed amounts of C, Si, Mn, P, S, Al, N and O, and the balance iron with inevitable impurities, and has a structure composed mainly of ferrite and bainite, and in which the number density of nonmetallic inclusions of &gt;5 &mu;m comprised in the steel sheet is &le;15 pieces/mm<SP>2</SP>, and the maximum tensile strength is &ge;540 MPa. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a high-strength steel sheet having extremely excellent stretch flangeability suitable for automobiles, building materials, home appliances and the like, and a method for producing the same.

  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. .

In particular, a member for an automobile needs to be spot-welded or the like for joining the members, and the member is often provided with a flange. In many cases, the end face of the flange portion is cut as it is, and it is particularly easy to break due to the damage caused by cutting. Therefore, it is required that the flange portion does not break during the processing.
Since the stretch flangeability is obtained by processing the end face as it is cut, it is required that the stretch flangeability is good as a material characteristic, and at the same time, damage to the end face due to mechanical cutting of a shear or the like is slight.
As shown in Non-Patent Document 1, the material properties necessary for improving stretch flangeability are uniform stretch and hole expandability. For this reason, it is required to have both uniform elongation and hole expansibility.
On the other hand, there are many damages that are thought to have dragged inclusions at the time of cutting on the shears and punched end faces, and this is the starting point, and it is known that cracks occur during stretch flange molding and hole expansion tests (non- Patent Document 1). For this reason, it is also extremely important to suppress damage to the end face during cutting.

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 expansibility of DP steel sheet is inferior (for example, Non-Patent Document 3).

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 bainite single phase structure. Therefore, in manufacturing a cold-rolled steel sheet, it must be heated to a high temperature at which it becomes an austenite single phase, resulting in poor productivity. . Further, since the bainite structure is a structure containing many dislocations, it has a drawback that it is difficult to apply to a member that requires poor workability and requires ductility and stretchability.
Moreover, the steel sheet having a precipitation-strengthened ferrite single-phase structure is 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 high strength and excellent hole expansibility, a cold-rolled steel sheet that has undergone a cold-rolling and annealing process has the disadvantage that its 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.
In addition, these steel sheets pay attention to improving material properties such as elongation and hole expansibility, but in research that pays attention to end-face damage due to machining such as punching with shears and punches, and characteristic deterioration due to this, Absent.
CAMP-ISIJ vol. 13 (2000), p399 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 improve stretch flangeability, in addition to improving material properties such as ductility and hole expandability, it is essential to improve the properties of the cut end face.
The present invention has been made in consideration of the improvement of material properties such as ductility and hole expansibility, as well as suppression of end face damage after cutting. An object of the present invention is to provide a high-strength steel sheet having an excellent hole expandability similar to that of a structure and at the same time having an extremely excellent stretch flangeability having a feature that damage of an end face after cutting is extremely slight, and a method for producing the same.

The gist of the present invention aimed at solving the above problems is as follows.
(1) The present invention is based on 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 bainite, and the number density of non-metallic inclusions exceeding 5 μm contained in the steel sheet is 15 pieces / mm ² or less, A high-strength steel sheet having extremely excellent stretch flangeability, characterized by having a maximum tensile strength of 540 MPa or more.
(2) The present invention further includes B: 0.0001 to less than 0.010% by mass%, and the high-strength steel sheet having extremely excellent stretch flangeability as described in (1).
(3) The present invention further includes, in mass%, Cr: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Mo: 0.00. The high-strength steel sheet having extremely excellent stretch flangeability as described in (1) or (2), comprising one or more of 01 to 0.3%.

(4) The present invention further comprises 0.001 to 0.14% of Nb, Ti, or V in a total of 0.001 to 0.14% by mass%. A high-strength steel sheet having extremely excellent stretch flangeability as described in any of the above.
(5) The present invention further comprises, in mass%, 0.0001 to 0.5% of one or more of Ca, Ce, Mg, and REM in total (1) to ( 4) A high-strength steel sheet having extremely excellent stretch flangeability described in any one of 4).
(6) The present invention is characterized by having zinc-based plating on the surface of the high-strength steel sheet according to any one of (1) to (5).

(7) In casting the casting slab having the chemical component according to any one of (1) to (5), the present invention includes casting a nozzle in molten steel while blowing Ar gas. A method for producing a high-strength steel sheet having extremely excellent stretch flangeability as a feature.
(8) The present invention provides a cast slab characterized in that the number density of coarse oxides exceeding 5 μm contained in the slab cast by the method described in (7) is less than 15 / mm ^2. .
(9) In the present invention, the cast slab described in (8) is directly or once cooled and then heated to 1050 ° C. or higher, and hot rolling is completed at the Ar 3 transformation point or higher, in a temperature range of 400 to 670 ° C. Winding, pickling, cold rolling with a rolling reduction of 40 to 70%, and passing through a continuous annealing line, annealing at a maximum heating temperature of 760 to 870 ° C, and then an average cooling rate between 630 and 570 ° C A method for producing a high-strength steel sheet having extremely excellent stretch flangeability, characterized by cooling at 3 ° C./second or more and holding at 450 ° C. to 300 ° C. for 30 seconds or more.

(10) In the present invention, the cast slab described in (8) is directly or once cooled, and then heated to 1050 ° C. or higher, and hot rolling is completed at the Ar3 transformation point or higher, in a temperature range of 400 to 670 ° C. Winding, pickling, cold rolling with a rolling reduction of 40-70%, passing through a continuous hot dip galvanizing line, annealing at a maximum heating temperature of 760-870 ° C, then averaging between 630 ° C-570 ° C After cooling to (zinc plating bath temperature -40) ° C to (zinc plating bath temperature +50) ° C at a cooling rate of 3 ° C / second or more, either before or after immersion in the zinc plating bath, or both The method for producing a high-strength hot-dip galvanized steel sheet having extremely excellent stretch flangeability, characterized by being held in a temperature range of (galvanizing bath temperature + 50) ° C. to 300 ° C. for 30 seconds or more.
(11) In the present invention, the cast slab described in (8) is directly or once cooled, and then heated to 1050 ° C. or higher, and hot rolling is completed at an Ar 3 transformation point or higher, in a temperature range of 400 to 670 ° C. Winding, pickling, cold rolling with a rolling reduction of 40-70%, passing through a continuous hot dip galvanizing line, annealing at a maximum heating temperature of 760-870 ° C, then averaging between 630 ° C-570 ° C After cooling to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C. at a cooling rate of 3 ° C./second or more, alloying treatment is performed at a temperature of 460 to 540 ° C. as necessary, and galvanization It is extremely characterized in that it is kept for 30 seconds or more in a temperature range of (zinc plating bath temperature +50) ° C. to 300 ° C. before or after immersion in the bath, after alloying treatment, or in total. High with excellent stretch flangeability Method of manufacturing a degree gold galvannealed steel sheet.
(12) The present invention is a high-strength electrozinc system having extremely excellent stretch flangeability as described in (9), wherein a steel sheet is produced by the method of (9) and then zinc-based electroplating is performed. Manufacturing method of plated steel sheet.

According to the present invention, by controlling the steel plate components and annealing conditions, the steel plate structure is mainly composed of ferrite and bainite, and the number density of non-metallic inclusions exceeding 5 μm contained in the steel plate is 15 pieces / mm ² or less. Therefore, it is possible to stably obtain a high-strength steel plate having excellent ductility of 540 MPa or more in maximum tensile strength and extremely excellent stretch flangeability.
That is, the content of C, Si, Mn, P, S, Al, N, and O is within a specified range, and an excellent tensile strength of 540 MPa or more can be exhibited at the same time, and at the same time, mainly composed of ferrite and bainite, By making the number density of non-metallic inclusions exceeding 5 μm contained in 15 / mm ² or less, it is possible to stably provide a high-strength steel sheet having extremely excellent stretch flangeability.

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, and is mainly composed of ferrite and bainite. To produce a high-strength steel sheet having extremely excellent stretch flangeability with the number density of non-metallic inclusions exceeding 5 μm included being 15 pieces / mm ² or less, after undergoing a hot rolling process and a cold rolling process The maximum heating temperature is defined as a condition during annealing, and the desired high-strength steel sheet is defined by defining the cooling rate between 630 ° C. and 570 ° C. and the holding conditions in the temperature range between 450 ° C. and 300 ° C. after annealing. Can be manufactured.
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 to develop a steel plate excellent in stretch flangeability. As a result, even if the steel sheet structure as conventionally known is controlled, the stretch flangeability is not necessarily improved, or the inclusion size that is conventionally said to cause deterioration such as elongation. It was found that much smaller inclusions promoted damage to the end face and deteriorated hole expansibility. As a result, it has been found that when the number density of inclusions of 5 μm or more is 15 pieces / mm ² or less, significant end face damage improvement and stretch flangeability improvement can be achieved.
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 investigation, the present inventors have made the steel sheet structure a structure composed of a hard structure such as ferrite and bainite and martensite, while reducing the hardness difference between the hard structure such as bainite and martensite and ferrite. The present inventors have found that by improving the end face damage caused by reducing the number of inclusions, it is possible to significantly improve stretch flangeability. In particular, it has been found that a significant improvement in stretch flangeability can be achieved by simultaneously suppressing crack formation sites by improving end face damage and suppressing crack propagation by reducing the hardness difference between hard and soft structures.

First, in the steel plate of steel number A-4 which has the composition of steel number A and was not blown with Ar, the end face after cutting was observed, and a large number of I as shown in the end face structure photograph of FIG. It was found that damage called mold dimples was observed, and that there was a clear correlation between the end face damage, inclusion number density, and hole expansion rate. In particular, the relationship between the end face damage and the number density of inclusions was investigated, and the best correlation was obtained with the inclusion number density of 5 μm or more, which is far smaller than the inclusion size, which is said to cause ductile deterioration. At the same time, by controlling the deoxidation method, a significant improvement in hole expansibility was observed by reducing the number of inclusions of 5 μm or more. For this reason, it is desirable to reduce the inclusion number density of 5 μm or more.
Here, the number density of inclusions of 5 μm or more is limited to 15 pieces / mm ² or less. By making the number density of inclusions of 5 μm or more 15 pieces / mm ² or less, there is a remarkable effect of improving hole expansibility. Because it was seen. The cause of the correlation between the inclusion number density of 5 μm or more and the hole expandability is considered to be as follows.
For example, the inclusion number density exceeding 10 μm is small, and in the process of punching a 10 mmφ hole such as a hole expansion test, there is very little possibility of inclusions on the end face. As a result, it is estimated that the variation in the hole expansion test value when a large number of hole expansion tests are performed is difficult to detect in a test performed with a small number of tests that can be measured.

In addition, it is considered that the reason why inclusions of 5 μm or less are less likely to have an adverse effect is due to the size of the structure of the composite structure steel plate. For example, in DP steel having a steel sheet structure of ferrite and martensite, the size of martensite, which is a hard structure, is as small as 5 μm or less, so it is considered that martensite itself plays the same role as inclusions. From this, it is considered that it was difficult to obtain a correlation with the inclusion number density of 5 μm or less. The reason why the number density of inclusions was set to 15 pieces / mm ² or less was that a remarkable effect of improving the hole expandability was observed, preferably 9 pieces / mm ² or less, more preferably 6 pieces / mm ² or less. It is desirable to make it.

The inclusions mentioned here indicate oxides such as Al ₂ O ₃ , SiO ₂ , manganese silicate, and sulfides such as MnS. In particular, a high-strength steel plate of 540 MPa or more often uses structural strengthening to increase ductility, and may contain a large amount of Si or Mn. As a result, these oxides and sulfides are likely to be present as compared with mild steel sheets not containing these elements. In addition, when the oxygen concentration is high at the refining stage, these oxides may be formed when the molten steel temperature decreases during casting, and even if the oxide is removed in the converter, it is produced during casting. There was a problem that it was difficult to remove the oxide. The mechanism by which these oxides are formed is considered as follows.
When the temperature of molten steel is high, the solid solubility limit of oxygen is high. On the other hand, the solid solubility limit greatly decreases at low temperatures. As a result, a large amount of oxygen that cannot be dissolved in the molten steel at the casting stage is released, and these oxygens are combined with Si, Mn, and Al, which are more easily oxidized than Fe, and form oxides in the steel. It is done. Since these oxides have a low formation temperature, they are finer than oxides formed in a converter and difficult to remove. Therefore, in the present invention, during the casting, these oxides were removed by blowing Ar gas into the molten steel.

  The number density of inclusions can be measured using an optical microscope, SEM (scanning electron microscope) or the like. In particular, when the inclusion composition and number density are simultaneously measured, it is desirable to perform observation using an SEM. In the measurement of inclusions, the steel plate rolling direction cross section or the rolling direction perpendicular direction cross section is corroded, and can be quantified by observation with a 1000 times optical microscope and 1000 to 100000 times scanning electron microscope. In addition, the inclusions present in the steel sheet that has undergone hot rolling and cold rolling are often stretched or crushed, are often stretched in the rolling direction, or are often arranged in a dotted line. When observed from a direction parallel to the direction or from a direction perpendicular to the direction, the distribution form and shape are different. In addition, since cracks during the hole expansion test tend to enter along the rolling direction, which is the direction in which the inclusions extend, it is necessary to correlate the inclusion number density on the plane perpendicular to the rolling direction. Therefore, in the present invention, the inclusion number density on the vertical surface in the rolling direction was measured. In the measurement of the number density, each 30 visual fields were measured and quantified.

Next, the reason for limiting the structure of the steel sheet will be described.
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 ferrite as the main phase. Further, residual austenite may be included. 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.

The hard structure is desirably 50% or more of the bainite structure.
The bainite structure is known to be softer than martensite. Therefore, by forming the hard structure into a soft bainite structure, it is possible to suppress the formation of microvoids at the interface between the ferrite and the hard structure during the hole expanding process. The reason why the bainite structure in the hard structure is 50% or more is that if it is less than 50% of the volume ratio of the hard structure, martensite and retained austenite are sufficiently separated and dispersed at the site of crack propagation during hole expansion processing. It is because it is thought that it must not be.
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%. 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 tensile strength (TS) range is 590 to 1080 MPa, ductility and hole expandability, or stretch flangeability is achieved. In order to achieve both, it is desirable to include ferrite having a volume ratio of more than 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 were disclosed in Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. The steel plate rolling direction cross section or the rolling direction perpendicular cross section is corroded by the reagent, and quantification is possible with 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 and micro region hardness measurement such as micro Vickers hardness measurement.

In the present invention, if the maximum tensile strength (TS) is set to 540 MPa or more, if it is less than this strength, it is possible to increase the strength of the ferrite single-phase steel by using solid solution strengthening to achieve a TS of less than 540 MPa. This is because both excellent ductility and hole expandability can be achieved. In particular, when considering securing TS of 540 MPa or more, in order to ensure excellent ductility, it is necessary to perform strengthening using martensite or retained austenite, and deterioration of hole expansibility becomes remarkable. In the present invention, it is important to improve the hole expandability characteristics in a steel plate of TS540 MPa or more.
The crystal grain size of ferrite is not particularly limited, but it is preferably 7 μm or less in terms of nominal grain size 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 interface between the ferrite and the hard structure also increases. This is to facilitate the connection of the holes and deteriorate the hole expandability. Further, the ferrite structure fraction tends to be less than 50%, and it is difficult to ensure excellent ductility.
Even if the difference in crystal orientation between most hard structures and ferrite is 9 ° or less, the volume ratio of the hard structures that are unavoidably present and do not have the above crystal orientation relationship becomes too large, and strain concentration and microvoids at the interface are increased. This is because the formation cannot be suppressed and the hole expansion value is inferior.
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. If the content is less than 0.0005%, deoxidation at the time of steelmaking takes time, and excessive costs are increased, which is economically undesirable. 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% or more, but when the addition amount exceeds 0.010%, the effect is not only saturated but also during hot rolling. The upper limit is set to 0.010% because manufacturing is reduced.
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.
In order to reduce the number density of inclusions, it is necessary to perform casting while blowing Ar into the molten steel during casting. Inclusions are effective in removing both inclusions formed during casting and those that could not be removed during refining in a converter. The amount of Ar blown at this time is preferably 2 to 30 l / min. This is because the inclusion removal effect is small when the blowing rate is less than 2 l / min, and when the blowing rate exceeds 30 l / min, the inclusion removal effect is saturated, and it is economically disadvantageous. This is because Ar blowing makes the surface of the molten steel unstable during casting and makes casting difficult. The reason why the blowing gas is Ar is that it is an inert gas that does not react with molten steel. N and O are not preferable because they react with Fe and added alloy elements to form nitrides and oxides, that is, inclusions. On the other hand, He is not economically preferable because it is expensive.

The reason for limiting the detailed manufacturing conditions will be described below.
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 be 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 must be excessively high in order to secure the temperature. . For this reason, the upper limit temperature of the finish rolling temperature is desirably 1000 ° C. or less.
The winding temperature after hot rolling is preferably 670 ° C. or lower. When the temperature exceeds 670 ° 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 670 ° C. is not preferable because the thickness of the oxide formed on the steel sheet surface is excessively increased, and the pickling property is poor. 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 galvannealed 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.
After cold rolling, annealing is performed at a maximum heating temperature of 760 to 870 ° C. This is because if the temperature is lower than 760 ° C., an excessive time is required for reverse transformation from cementite or pearlite to austenite. In addition, when the maximum attainable temperature is less than 760 ° C., part of cementite and pearlite cannot be transformed into austenite and remain in the steel sheet structure even after annealing. Since this cementite and pearlite are coarse, it is not preferable because it causes deterioration of hole expansibility. Alternatively, bainite and martensite formed by transformation of austenite, or austenite itself can be transformed into martensite during processing, so that a strength of 540 MPa or more can be achieved. If it does not transform into, the hard structure becomes too small and it is not possible to secure a strength of 540 MPa or more. Therefore, the lower limit of the maximum heating temperature needs to be 760 ° C. On the other hand, it is economically undesirable to raise the heating temperature excessively. Therefore, it is desirable that the upper limit of the heating temperature is 870 ° C.

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. 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 structure cannot be made of ferrite and bainite, and the hole expandability is greatly deteriorated. Therefore, 300 ° C. is the lower limit temperature.

When the holding time is 450 ° C. to 300 ° C. for 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. Therefore, it is inferior to 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.
In addition, holding does not only mean isothermal holding, but means retaining 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 maximum heating temperature when passing through the hot dip galvanizing line after cold rolling is set to 760 to 870 ° C. for the same reason as when passing through the continuous annealing line. 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 bainite transformation is too slow, and the structure cannot be made of ferrite and bainite. Therefore, the lower limit 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, and therefore the hole expandability is poor. 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 volume ratio of 5% or more can be secured, even if an additional heat treatment is performed under any conditions, the strength of 540 MPa or more, which is the effect of the present invention, and excellent ductility and hole expansibility. Is 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 annealing method prior to plating, the dew point during heating is set to −20 ° C. or higher, which advantageously works on the wettability of the plating and the alloying reaction at the time of plating alloying.
In addition, even if this cold-rolled steel sheet is electroplated, the tensile strength, ductility, and hole expandability of the steel sheet are 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.
When each slab having various components shown in Table 1 is manufactured, Ar gas is blown into the molten steel at the time of casting in the blowing amounts shown in Tables 2 and 3 and cast, and each obtained slab is heated to 1200 ° C. Heating was performed, hot rolling was performed at a finish hot rolling temperature of 900 ° C., and water-cooling was performed in a water-cooling zone, followed by winding processing at temperatures shown in Tables 2 and 3. 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. N ₂ gas containing 10 vol% was introduced, and annealing was performed under the conditions shown in Tables 2 and 3. In particular, in steel numbers B, E, and F containing a large amount of Si, if the above 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 480-540 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 sheet having a good strength-hole expansibility balance.
A steel sheet having this good strength-ductility balance and a good strength-hole expansibility balance at the same time was used as a high-strength steel sheet having an excellent balance of hole expansibility and ductility.

In order to investigate the relationship between the inclusion number density and end face damage after punching, the end face after punching was observed and the end face was scored. When the inclusion number density is high, there are many scratches (refer to the metal structure photograph in FIG. 1) extending in the thickness direction called I-type dimples, which are thought to have dragged inclusions during punching. Therefore, the number density of I-type dimples existing in the plate thickness direction was investigated. However, since the inclusions extend in the rolling direction or are arranged in a dot sequence, the shape of the fracture surface varies depending on the observation surface. Therefore, during the hole expansion test, a surface perpendicular to the rolling direction in which cracks due to punching cracks were most easily formed was observed.
The damage on the punched end face was investigated as described above, and the following rating was made.
○: Number density 15 pieces / mm ² or less Δ: Number density 15 to 20 pieces / mm ² or more ×: Number density 20 pieces / mm ² or more In addition, as shown in Tables 4 and 5, the inclusion number density, vertical in the rolling direction There was a correlation between the cross-sectional damage and the hole expansion value.

Steel numbers A-1 to 3, 5, 7, 9, 11, 12B-1, 4, 5, C-1, 3, D-1, 3, 5, E-1, 2 shown in Table 4 or Table 5. , F-1, G-1, H-1, I-1, 4, and 5 are within the range defined by the present invention for the chemical components of the steel sheet, and the production conditions are also defined by the present invention. Is in range.
As a result, the steel sheet structure is mainly composed of ferrite and bainite, the number density of non-metallic inclusions exceeding 5 μm contained in the steel sheet is 15 pieces / mm ² or less, and the tensile elongation is 540 MPa or more. A high-strength steel plate having flangeability can be provided.
On the other hand, steel numbers A-4, 10, 13, B-2, C-2, D-2, 4, 6, E-3, F-2, G-2, H-2 shown in Table 4 or Table 5 , I-2, 6, and J-1 were samples in which Ar gas was not blown during casting, but each sample had a large inclusion number density of more than 15 pieces / mm ² , As a result of the damage, X or Δ.
The sample of steel number A-8 shown in Table 4 or Table 5 has a hot rolling coiling temperature of 720 ° C. and is an example that is too high and an annealing temperature that is too low, but the value of TS · El was low.
The sample of steel number A-15 shown in Table 4 or Table 5 was a sample having a low average cooling rate between 630 and 570 ° C. of 0.4 ° C./s, but the value of TS · EI was small.
The samples of steel numbers A-6, B-3, E-4, G-3, H-3, and I-3 shown in Table 4 or Table 5 are all samples having a low heat treatment temperature, but TS · λ The value of became low.

The steel number K-1 sample shown in Table 4 or Table 5 is a sample having too much C, TS is lowered, and steel numbers L-1 to L-3 are samples having a lot of Mn. Decreased.
From the above, in the sample according to the present invention, the steel sheet structure is mainly composed of ferrite and bainite, the number density of non-metallic inclusions exceeding 5 μm contained in the steel sheet is 15 pieces / mm ² or less, and the maximum tensile strength is It was found that a high-strength steel sheet having an excellent EI value, an excellent TS / EI value, and an excellent TS / λ value and excellent stretch flangeability can be provided.

  The present invention provides an inexpensive 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 stretch flangeability 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, so it can be expected to make a significant contribution to the weight reduction of automobiles. Is extremely expensive.

FIG. 1 is a metallographic photograph showing a structure in which a number of damaged portions called I-type dimples are formed on a fracture surface of a steel sheet.

Claims (12)

% 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%,
In which the balance is composed of iron and inevitable impurities, the steel sheet structure is mainly composed of ferrite and bainite, and the number density of non-metallic inclusions exceeding 5 μm contained in the steel sheet is 15 pieces / mm ² or less. A high-strength steel sheet having extremely excellent stretch flangeability, characterized in that the maximum tensile strength is 540 MPa or more. Furthermore, in mass%,
B: 0.0001 to less than 0.010%,
The high-strength steel sheet having extremely excellent stretch flangeability according to claim 1, comprising: Furthermore, in mass%,
Cr: 0.01 to 1.0%,
Ni: 0.01 to 1.0%,
Cu: 0.01 to 1.0%,
Mo: 0.01 to 1.0%
The high-strength steel sheet having extremely excellent stretch flangeability according to claim 1 or 2, characterized by containing one or more of the following. Furthermore, in mass%,
The extremely excellent stretch flangeability according to any one of claims 1 to 3, characterized by containing 0.001 to 0.14% of one or more of Nb, Ti, and V in total. Has high strength steel plate.   Furthermore, 0.0001-0.5% of 1 type or 2 types in total of Ca, Ce, Mg, and REM is contained by mass%, The any one of Claims 1-4 characterized by the above-mentioned. High strength steel plate with extremely excellent stretch flangeability.   A high-strength steel sheet having extremely excellent stretch flangeability, characterized by having zinc-based plating on the surface of the high-strength steel sheet according to any one of claims 1 to 5.   In casting a cast slab having the chemical component according to any one of claims 1 to 5, an extremely excellent stretch flangeability characterized by casting a nozzle in molten steel and blowing Ar gas. A method for producing a high-strength steel sheet having A cast slab characterized in that the number density of coarse oxides exceeding 5 μm contained in the cast slab subjected to casting according to claim 7 is less than 15 pieces / mm ² .   The cast slab according to claim 8 is directly or once cooled and then heated to 1050 ° C or higher, completes the hot rolling at the Ar3 transformation point or higher, wound in a temperature range of 400 to 670 ° C, and pickled. When cold rolling with a rolling reduction of 40 to 70% and passing through a continuous annealing line, after annealing at a maximum heating temperature of 760 to 870 ° C, cooling between 630 ° C and 570 ° C at an average cooling rate of 3 ° C / second or more And a method for producing a high-strength steel sheet having extremely excellent stretch flangeability, characterized by holding in a temperature range of 450 ° C. to 300 ° C. for 30 seconds or longer.   The cast slab according to claim 8 is directly or once cooled and then heated to 1050 ° C or higher, completes the hot rolling at the Ar3 transformation point or higher, wound in a temperature range of 400 to 670 ° C, and pickled. When cold rolling with a rolling reduction of 40 to 70% and passing through a continuous hot dip galvanizing line, after annealing at a maximum heating temperature of 760 to 870 ° C, an average cooling rate of 630 ° C to 570 ° C is 3 ° C / second or more. (Zinc plating bath temperature−40) ° C. to (Zinc plating bath temperature + 50) ° C., and before or after immersion in the zinc plating bath, or both, +50) A method for producing a high-strength hot-dip galvanized steel sheet having extremely excellent stretch flangeability, characterized by holding for 30 seconds or more in a temperature range of from ° C to 300 ° C.   The cast slab according to claim 8 is directly or once cooled and then heated to 1050 ° C or higher, completes the hot rolling at the Ar3 transformation point or higher, wound in a temperature range of 400 to 670 ° C, and pickled. When cold rolling with a rolling reduction of 40 to 70% and passing through a continuous hot dip galvanizing line, after annealing at a maximum heating temperature of 760 to 870 ° C, an average cooling rate of 630 ° C to 570 ° C is 3 ° C / second or more. After cooling to (Zinc plating bath temperature -40) ° C to (Zinc plating bath temperature +50) ° C, alloying treatment is performed at a temperature of 460 to 540 ° C as necessary, before and after immersion in the galvanizing bath. Or, after alloying treatment, or in total (zinc plating bath temperature +50) high temperature having excellent stretch flangeability characterized by holding in a temperature range of 30 ° C. to 300 ° C. for 30 seconds or more Alloyed hot dip zinc Method of manufacturing a can steel sheet.   The method for producing a high strength electrogalvanized steel sheet having extremely excellent stretch flangeability according to claim 9, wherein the steel sheet is produced by the method according to claim 9 and then zinc-based electroplating is performed. .

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