JP2015218365A - High strength hot-dip galvannealed steel sheet excellent in yield strength and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot-dip galvannealed steel sheet having high yield strength, high elongation and high stretch-flangeability.SOLUTION: There is provided the high strength hot-dip galvannealed steel sheet that has a component composition containing, by mass%, C:0.05 to 0.30%, Si:0.1 to 3.0%, Mn:1.0 to 3.0%, P:0.1% or less, S:0.01% or less, N:0.01% or less, Al:0.001 to 0.10% and the balance Fe with inevitable impurities, and that has structure in which an average area ratio of a mixed structure MA composed of martensite and austenite to the whole structure Vis 2 to 5%; an area where an MA area ratio is locally 60% or less of the average area ratio Vis 10% or less by area ratio to the whole structure; the average area ratio of ferrite to the whole structure is 2% or less; the balance is bainite and martensite; and dislocation density is 2.0×10to 8.0×10m.

Description

  The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet used for a framework member for automobiles, and more specifically, a high-strength alloyed hot-dip galvanized steel sheet (hereinafter referred to as “high-strength alloyed hot-dip galvanized steel sheet”). Also referred to as “GA steel plate”.

  Alloyed hot-dip galvanized steel sheets are excellent in corrosion resistance, and are therefore used for automobile frame members. Steel sheets used as frame members for automobiles are required to have high strength for the purpose of improving fuel efficiency by reducing the weight of the vehicle body, and it is necessary to ensure collision safety. For this reason, a high strength galvannealed steel sheet having a high yield strength is required. On the other hand, excellent workability is also required in order to form a complex shaped part.

  For this reason, there is a strong desire to provide a high-strength galvannealed steel sheet having both high yield strength and enhanced elongation (total elongation; El) and stretch flangeability (hole expansion ratio; λ). For example, a steel sheet having a tensile strength of 1180 MPa class is required to have a yield strength of 900 MPa or more, a total elongation of 12% or more, and a stretch flangeability λ of 60% or more.

  In response to the above needs, a number of high-strength galvannealed steel sheets with improved balance of yield strength, elongation, and stretch flangeability have been proposed based on various structural control concepts. Those that have satisfied the level have not yet been completed.

  For example, Patent Document 1 proposes a high-strength steel sheet having a composite structure mainly composed of a ferrite phase and a bainite phase. By precipitation strengthening ferrite with a carbide containing fine Ti, Mo, etc., a high strength of 780 MPa or more is proposed. It is said that high yield ratio, good elongation and stretch flangeability can be obtained even in strength. However, the high-strength steel sheet described in the same document is based on the premise that it contains a considerable amount of soft ferrite (20% or more by volume) compared to martensite (see paragraph [0015] of the same document), The introduction of soft ferrite is minimized (area ratio is 2% or less), and the technical idea is completely different from the present invention. In the same document, it is said that high yield strength is realized by precipitation strengthening of Ti and Mo-based carbides. However, as shown in Table 4 of the example, in the steel sheet subjected to alloying hot dip galvanizing, The yield strength YS does not reach 800 MPa because bainite is softened by heating during the conversion.

  Patent Document 2 proposes a composite steel sheet mainly composed of ferrite and martensite. In the ferrite structure, ferrite grains surrounded by grain boundaries having a crystal orientation difference of less than 10 ° are increased. It is said that a high yield ratio high-strength hot-dip galvanized steel sheet excellent in workability having a tensile strength of 980 MPa or more can be obtained. However, since the steel sheet described in the literature requires the presence of ferrite, one example (test No. 2-3) is excluded as shown in Table 5 and Table 6 of the examples of the literature. In any case, the yield strength YP does not reach 900 MPa, and the stretch flangeability is hardly considered to be 60% or more. In addition, Test No. In 2-3, the yield strength YP is 900 MPa or more, but the tensile strength TS and the elongation EL do not reach the desired levels of the present invention.

JP 2007-146209 A JP 2010-106323 A

  Accordingly, an object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having both yield strength and elongation and stretch flangeability.

The high-strength galvannealed steel sheet excellent in yield strength and workability according to the first invention of the present invention is
% By mass
C: 0.05 to 0.30%
Si: 0.1-3.0%
Mn: 1.0 to 3.0%
P: 0.1% or less,
S: 0.01% or less,
N: 0.01% or less,
Al: 0.001 to 0.10%
And the balance has a component composition consisting of iron and inevitable impurities,
The mixed structure MA consisting martensite and austenite, the average area ratio _{V MA} with respect to the entire organization is 2-5%,
Area MA area fraction of less than or equal to 60% of the locally the average area ratio V _MA is 10% or less in area ratio to the whole organization,
The average area ratio of ferrite to the entire structure is 2% or less,
The balance is bainite and martensite,
A structure having a dislocation density of 2.0 × 10 ^{15 to} 8.0 × 10 ¹⁵ m ⁻² ;
It is characterized by that.

The high-strength galvannealed steel sheet excellent in yield strength and workability according to the second invention of the present invention is the above first invention,
Ingredient composition is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0002 to 0.0050%
1 type or 2 types or more are included.

The high-strength galvannealed steel sheet excellent in yield strength and workability according to the third invention of the present invention is the above first or second invention,
In addition, the component composition is
Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
1 type or 2 types or more are included.

The high-strength galvannealed steel sheet excellent in yield strength and workability according to the fourth invention of the present invention is any one of the first to third inventions,
Ingredient composition is further mass%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
1 type or 2 types are included.

  According to the present invention, as a steel sheet structure, bainite + martensite is a main structure, while introduction of ferrite is limited as much as possible, a predetermined amount of MA is uniformly dispersed, and a dislocation density is controlled within a predetermined range. As a result, it has become possible to provide a high-strength galvannealed steel sheet having both high yield strength and elongation and stretch flangeability.

It is a figure which shows typically the heat processing pattern recommended for manufacturing this invention steel plate.

  As a result of various studies to solve the above problems, the present inventors have made the steel structure of the GA steel sheet mainly composed of bainite + martensite, while limiting the introduction of ferrite as much as possible, and a predetermined amount. It was found that a GA steel sheet having both high yield strength and elongation and stretch flangeability can be obtained by uniformly dispersing MA and controlling the dislocation density within a predetermined range, thereby completing the present invention.

  Hereinafter, the structure characterizing the high-strength GA steel sheet according to the present invention (hereinafter also referred to as “the present steel sheet”) will be described first.

  Hereinafter, the structure characterizing the steel sheet of the present invention will be described first.

[Structure of the steel sheet of the present invention]
As described above, the steel sheet of the present invention has bainite + martensite as the main structure, and in particular, the ferrite content is limited as much as possible, a predetermined amount of MA is uniformly dispersed, and the dislocation density is within a predetermined range. It is characterized in that it is controlled.

<Average area ratio V _{MA of the} mixed structure MA composed of martensite and austenite with respect to the entire structure: 2 to 5%>
In order to ensure the elongation, it is necessary that the average area ratio VMA of _MA with respect to the entire structure is 2% or more, preferably 2.4% or more, more preferably 2.8% or more. However, if there is too much MA, the stretch flangeability deteriorates, so the average area ratio VMA of _{MA with} respect to the entire structure is limited to 5% or less, preferably 4.5% or less, more preferably 4% or less.

<Area where the area ratio of MA is locally 60% or less of the average area ratio _VMA : 10% or less in terms of the area ratio with respect to the entire tissue>
By appropriately controlling the local MA dispersion state, it is possible to improve the balance between elongation and stretch flangeability. That is, MA is effective in improving ductility, but if it is dispersed non-uniformly in the steel sheet structure, non-uniform deformation occurs in the steel sheet, and fracture occurs in a region with a small amount of MA, so that stretch flangeability deteriorates. Therefore, it is necessary to reduce the area with less MA and disperse the MA as uniformly as possible. Specifically, a region where the area ratio of the MA is equal to or less than 60% of the locally the average area ratio V _MA, 10% in area ratio to the whole organization less, preferably 9% or less, more preferably 8% or less And Thereby, the nonuniformity of the said deformation | transformation can be suppressed and stretch flangeability can be improved.

<Average area ratio of ferrite to the entire structure: 2% or less>
By reducing the introduction of soft ferrite as much as possible, the yield strength can be improved, and the generation of voids due to the hardness difference from martensite can be suppressed, and the stretch flangeability can be improved. In order to effectively exhibit such an action, the average area ratio of ferrite with respect to the entire structure is limited to 2% or less, preferably 1.5% or less, and more preferably 1% or less.

<Balance: Bainite and martensite>
The balance is bainite and martensite to ensure strength.

<Dislocation density: 2.0 × 10 ^{15 to} 8.0 × 10 ¹⁵ m ⁻² >
Although the as-quenched martensite is a structure having a very high dislocation density, since many of these dislocations are movable dislocations, yield strength is low and ductility is also poor. Therefore, yield strength and ductility can be increased by reducing the movable dislocations introduced during martensite generation by tempering. In order to effectively exhibit such an action, the dislocation density is limited to 8.0 × 10 ¹⁵ m ⁻² or less, preferably 7.0 × 10 ¹⁵ m ⁻² or less. However, if tempering is performed more than necessary, the dislocation density decreases excessively and the strength is insufficient, so the dislocation density is 2.0 × 10 ¹⁵ m ⁻² or more, preferably 2.5 × 10 ¹⁵ m ⁻² or more. More preferably, it is necessary to secure 3.0 × 10 ¹⁵ m ⁻² or more.

  Next, the component composition which comprises this invention steel plate is demonstrated. Hereinafter, all the units of chemical components are mass%.

[Component composition of the steel sheet of the present invention]
C: 0.05-0.30%
C is an important element that greatly affects the strength of the steel sheet. If the amount of C is too small, the strength is insufficient and a tensile strength of 1180 MPa cannot be ensured, so the C content needs to be 0.05% or more, preferably 0.07% or more, and more preferably 0.1% or more. On the other hand, when the amount of C is excessive, coarse carbides are likely to precipitate during tempering, and in addition to deteriorating stretch flangeability, it is desirable that the amount of C is low from the viewpoint of securing weldability. 30% or less, preferably 0.25% or less, more preferably 0.20% or less.

Si: 0.1-3.0%
Si is a useful element that has the effect of suppressing the coarsening of carbide particles during tempering, contributes to improvement in stretch flangeability, and also contributes to an increase in yield strength of the steel sheet as a solid solution strengthening element. In order to effectively exhibit such an action, the amount of Si needs to be 0.1% or more, preferably 0.3% or more, and more preferably 0.5% or more. However, if the amount of Si is excessive, the weldability is remarkably lowered, so that it is 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less.

Mn: 1.0-3.0%
Mn, like Si, has an effect of suppressing the coarsening of cementite during tempering, contributes to the improvement of stretch flangeability, and is a useful element that also contributes to an increase in the yield strength of the steel sheet as a solid solution strengthening element. is there. Moreover, it has the effect of suppressing the ferrite transformation at the time of cooling by improving hardenability. In order to effectively exhibit such an action, the amount of Mn needs to be 1.0% or more, preferably 1.3% or more, more preferably 1.6% or more. However, if the amount of Mn becomes excessive, the amount of MA in the final structure becomes excessive and conversely deteriorates the stretch flangeability. Therefore, it is 3.0% or less, preferably 2.7% or less, more preferably 2.4%. The following.

P: 0.1% or less P is unavoidably present as an impurity element, and contributes to an increase in strength by solid solution strengthening, but segregates at the prior austenite grain boundaries and deteriorates bendability by embrittlement of the grain boundaries. Therefore, the P content is limited to 0.1% or less, preferably 0.05%, and more preferably 0.03% or less.

S: 0.01% or less S is also unavoidably present as an impurity element, forms MnS inclusions, and deteriorates bendability by becoming a starting point of cracks during bending deformation, so the amount of S is 0.01% or less. , Preferably 0.005% or less, more preferably 0.003% or less.

N: 0.01% or less N is also unavoidably present as an impurity, and deteriorates the workability of the steel sheet by strain aging. Therefore, a lower value is preferable, 0.01% or less, preferably 0.007% or less, and more preferably. 0.005% or less.

Al: 0.001 to 0.10%
Al is a useful element added as a deoxidizing material, and 0.001% or more, preferably 0.01% or more, more preferably 0.03% or more is necessary to obtain such an action. However, if the amount of Al becomes excessive, the cleanliness of the steel is deteriorated, so that it is 0.10% or less, preferably 0.08% or less, more preferably 0.06% or less.

  The steel of the present invention basically contains the above components, and the balance is iron and inevitable impurities. In addition, the following allowable components can be contained as long as the effects of the present invention are not impaired.

Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0002 to 0.0050%
One or more of these elements are useful elements having the effect of enhancing the hardenability and suppressing the ferrite transformation from the structure austenitic single-phased by annealing. In order to obtain such an action, it is preferable that each element is contained in the above lower limit value or more. The above elements may be contained alone or in combination of two or more. However, even if these elements are contained excessively, the effect is saturated and it is economically wasteful, so that each element is set to the above upper limit value or less.

Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
These elements are useful elements for improving the strength without degrading the moldability. In order to obtain such an action, it is preferable that each element is contained in the above lower limit value or more. The above elements may be contained alone or in combination of two or more. However, if these elements are contained excessively, coarse carbides are formed and formability deteriorates, so that each element is set to the upper limit value or less.

Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
These elements are useful elements for improving the stretch flangeability by refining inclusions and reducing the starting point of fracture. In order to obtain such an action, it is preferable to contain 0.0005% or more of any element. The above elements may be used alone or in combination of two. However, if it is contained excessively, inclusions are coarsened and stretch flangeability deteriorates. Therefore, both elements are made 0.01% or less.

[Preferred production method of the steel sheet of the present invention]
In order to manufacture the steel sheet of the present invention that satisfies the above requirements, it is preferable to manufacture the steel sheet so as to satisfy the following manufacturing requirements.

  The feature in producing the steel sheet of the present invention is the heat treatment conditions after the slab is hot-rolled and cold-rolled. Therefore, a conventionally well-known manufacturing method can be employ | adopted regarding the manufacturing method to hot rolling and cold rolling. That is, a steel having the above component composition is melted to form a slab by ingot forming or continuous casting, and then hot rolling and further cold rolling may be performed. Hereinafter, the heat treatment conditions will be described with reference to the heat treatment heat pattern schematically shown in FIG.

  As shown in FIG. 1, first, a steel sheet (cold rolled material) after cold rolling is heated to an annealing heating temperature of [Ac3 point + 50 ° C.] to 930 ° C., and then 30 to 1200 s (annealing) at the annealing heating temperature. Hold time) Hold. Thereafter, from the annealing heating temperature to the primary cooling temperature of Ms to 550 ° C. is rapidly cooled at an average cooling rate (primary cooling rate) of 15 ° C./s or more, and the primary cooling temperature is 10 to 50 s (primary cooling holding time). Hold. Furthermore, from the primary cooling temperature to the secondary cooling temperature of [Ms point-100 ° C.] to [Ms point-30 ° C.] is rapidly cooled at an average cooling rate (secondary cooling rate) of 15 ° C./s or more. Hold at the next cooling temperature for 5 seconds or longer (secondary cooling holding time). Then, the annealed steel sheet (annealed material) was immersed in a hot dip galvanizing bath, and then held at an alloying treatment temperature of 450 to 550 ° C. for 10 to 60 s (alloying treatment time) to perform an alloying treatment. Then, cool to room temperature. Thereby, this invention steel plate (high-strength galvannealed steel plate based on this invention) is obtained.

-The steel sheet after cold rolling (cold rolled material) is heated to an annealing heating temperature of [Ac3 point + 50 ° C] to 930 ° C and then held for 30 to 1200 s (annealing holding time). Reducing the fraction is an important requirement for producing the steel sheet of the present invention. In order to reduce the ferrite fraction, it is necessary to have an austenite single phase structure during annealing. In order to suppress the ferrite transformation from the austenite single phase structure, it is effective to increase the austenite grain size and improve the hardenability. Therefore, annealing heating temperature shall be Ac3 point +50 degreeC or more.

The Ac3 point is based on the chemical composition of the steel sheet, by Lesley, “Iron & Steel Materials Science”, translation by Koda Narumi, Maruzen Co., Ltd., 1985, p. The following equation (1) described in H.273 can be used.
Ac3 (° C.) = 910−203 × √C−15.2 × Ni + 44.7 × Si-30 × Mn + 700 × P + 400 × Al-11 × Cr-20 × Cu + 31.5 × Mo + 400 × Ti + 104 × ∨ (1 )
Here, the element symbol in the above formula represents the content (% by mass) of each element.

  On the other hand, if the annealing heating temperature is too high, the austenite grain size may become excessively coarse, and the formability may deteriorate. Therefore, annealing heating temperature shall be 930 degrees C or less. Also, if the annealing heat holding time is too short, the austenite transformation does not proceed sufficiently, so that the ferrite is present in the final structure in excess of 2%. On the other hand, if the annealing heating holding time is too long, the heat treatment cost is high. It increases and productivity is remarkably deteriorated. Therefore, the annealing holding time is 30 to 1200 s.

-From the annealing heating temperature to the primary cooling temperature of the Ms point to 550 ° C is rapidly cooled at an average cooling rate (temporary cooling rate) of 15 ° C / s or more, and the primary cooling temperature is maintained for 10 to 50 s (temporary cooling holding time). In the primary cooling process, it is important that the austenite generated during annealing is not transformed into ferrite during cooling. For this reason, a primary cooling rate shall be 15 degrees C / s or more, More preferably, it is 30 degrees C / s or more.

  And after stopping this primary cooling, by holding for 10 to 50 s (primary cooling holding time) at a primary cooling temperature of Ms point to 550 ° C., bainite transformation occurs uniformly regardless of microsegregation, and C around the bainite. (Carbon) thickens. As a result, the MA produced around the bainite is uniformly dispersed in the final structure. If the primary cooling temperature is too high, ferrite may be generated. On the other hand, if the primary cooling temperature is too low, bainite cannot be uniformly generated due to the influence of microsegregation. Therefore, primary cooling temperature shall be Ms point -550 degreeC, and a more preferable upper limit shall be 520 degrees C or less.

  On the other hand, if the primary cooling holding time is too short, the amount of bainite transformation is insufficient, and it becomes impossible to achieve uniform MA dispersion. On the other hand, if the primary cooling holding time is too long, the bainite transformation proceeds excessively, increasing the MA fraction and inhibiting stretch flangeability. Therefore, the primary cooling holding time is 5 to 50 s, more preferably 10 to 35 s.

-From the primary cooling temperature to the secondary cooling temperature of [Ms point -100 ° C] to [Ms point -30 ° C] is rapidly cooled at an average cooling rate (secondary cooling rate) of 15 ° C / s or more, and the secondary cooling Holding at a temperature of 5 seconds or more (secondary cooling holding time) If the cooling rate is too low in the secondary cooling process, bainite transformation occurs during cooling, the MA fraction increases, and stretch flangeability is inhibited. Therefore, the secondary cooling rate is 15 ° C./s or more, more preferably 30 ° C./s or more.

The Ms point is calculated based on the chemical composition of the steel sheet by Lesley, “Iron & Steel Materials Science”, translated by Kouta Naruse, Maruzen Co., Ltd., 1985, p. The following equation (2) described in H.231 can be used.
Ms (° C.) = 516-474 × C-33 × Mn-17 × Ni-17 × Cr-21 × Mo (2)
Here, the element symbol in the above formula represents the content (% by mass) of each element.

  If the secondary cooling temperature exceeds [Ms point −30 ° C.], martensite cannot be generated sufficiently. Further, the bainite transformation does not proceed sufficiently, untransformed austenite remains during the alloying treatment, and is transformed into martensite or bainite by subsequent cooling. As a result, since the as-quenched structure exists in the final structure, the number of movable dislocations increases and the yield strength decreases. On the other hand, when the secondary cooling temperature is less than [Ms point−100 ° C.], the martensite fraction increases and the bainite fraction decreases. As a result, it is not possible to obtain a sufficient MA fraction to ensure elongation. Therefore, the secondary cooling temperature is set to [Ms point −100 ° C.] to [Ms point −30 ° C.].

  If the secondary cooling holding time is too short, the bainite transformation is not completed, and soft bainite is generated during the alloying treatment, so that the yield strength is lowered. For this reason, the secondary cooling holding time is set to 5 seconds or more. The upper limit of the secondary cooling holding time is not particularly limited, but if held for a long time, carbides may be precipitated at the grain boundaries and the ductility may be hindered.

-Hold for 10 to 60 s (alloying time) at an alloying treatment temperature of 450 to 550 ° C Generally, the alloying treatment is performed in a temperature range of 450 to 600 ° C with a holding time of 60 s or less. In the recommended manufacturing method of the present invention, tempering is performed during the alloying process. If the alloying treatment temperature is too low, carbides cannot be sufficiently precipitated and high yield strength cannot be obtained. On the other hand, when the alloying treatment temperature is too high, the dislocation density is excessively lowered and the strength is insufficient. Therefore, the alloying treatment temperature is 450 to 550 ° C, more preferably 480 to 520 ° C.

  If the alloying treatment time is too short, alloying is not sufficient, and if it is too long, decomposition of MA occurs. Therefore, the alloying treatment time is 10 to 60 seconds.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

〔Test method〕
Steel having each of the components A to K shown in Table 1 was melted to prepare an ingot having a thickness of 120 mm, and hot rolling was performed using the ingot to obtain a thickness of 2.8 mm. After pickling, this was cold-rolled to a thickness of 1.4 mm to obtain a test material, and the test material was subjected to heat treatment and plating treatment under the conditions shown in Table 2 below.

〔Measuring method〕
Using each of the obtained steel plates, the average area ratio of ferrite, the average area ratio of MA, the dispersed state of MA, and the dislocation density at 1/4 part of the steel plate thickness were measured. Further, in order to evaluate the mechanical properties of the steel sheet, the yield strength (YS), tensile strength (TS), elongation (EL), and stretch flangeability (λ) were also measured. These measurement methods are shown below.

(Average area ratio of ferrite)
Regarding the average area ratio of ferrite, each steel plate was mirror-polished, its surface was corroded with 3% nital solution to reveal the metal structure, and then the plate was obtained using SEM (Scanning Electron Microscope). A structure having a thickness of ¼ part was obtained by observing a region of approximately 40 μm × 30 μm and 5 visual fields at a magnification of 2000 times. Specifically, among the regions observed in black, those that are polygonal (polygonal) and do not contain white carbide observed inside are defined as ferrite, and the area of ferrite from the area ratio to the entire observation region for each field of view The average area ratio of the ferrite was obtained by arithmetically averaging the area ratios of the five fields of view.

(Average area ratio of MA and dispersion state of MA)
Further, regarding the average area ratio of MA and the dispersion state of MA, each steel plate is mirror-polished, its surface is corroded with a repeller liquid to reveal a metal structure, and then the plate thickness is 1/4 using an optical microscope. It was determined by observing at 10 magnifications about 10 fields of an area of approximately 80 μm × 60 μm.
Specifically, an area having no contrast difference inside is defined as MA among the areas observed in white, and the area ratio of MA is obtained from the area ratio with respect to the entire observation area for each field of view. The area ratio was arithmetically averaged to obtain an average area ratio VM of _MA .
Further, the observation area is divided into small areas of 20 μm × 20 μm for each visual field, and the area ratio V _MA-L of MA in each small area is obtained, and the following is shown for the total area of the observation areas of all ten visual fields: The ratio of the total area of the small regions satisfying Equation (3) was calculated.
V _MA-L ≦ V _MA × 0.6 (3)

(Dislocation density)
The dislocation density is calculated by irradiating the steel plate to be measured with X-rays and measuring the half width of the obtained diffraction peak. Specifically, after adjusting the sample so that a ¼ depth position of the plate thickness could be measured, this was applied to an X-ray diffractometer (RAD-RU300, manufactured by Rigaku Corporation), and an X-ray diffraction profile was collected. Based on this X-ray diffraction profile, the dislocation density was calculated according to the analysis method proposed by Nakajima et al. (See Nakajima et al .: “Materials and Processes”, Vol. 17 (2004) p. 396-399).

(Yield strength, tensile strength and elongation)
Yield strength (YS) and tensile strength are obtained by using each steel plate to be evaluated and preparing a test piece No. 5 described in JIS Z 2201 with the long axis in the direction perpendicular to the rolling direction and measuring according to JIS Z 2241. (TS) and elongation (EL) were determined.

(Stretch flangeability)
Each steel plate to be evaluated was subjected to a hole expansion test according to the iron standard JFST1001, and the hole expansion rate was measured, and this was defined as stretch flangeability.

〔Measurement result〕
The measurement results are shown in Table 3 below. In this example, the yield strength (YS) is 900 MPa or more, the tensile strength (TS) is 1180 MPa or more, the elongation (EL) is 12% or more, and the stretch flangeability (λ) is 60%. The above was evaluated as “good” and judged as a high-strength galvannealed steel sheet excellent in yield strength, elongation and stretch flangeability. On the other hand, the yield strength (YS) is less than 900 MPa, the tensile strength (TS) is less than 1180 MPa, the elongation (EL) is less than 12%, or the stretch flangeability (λ) is less than 60%. It was determined to be rejected. In addition, what attached | subjected the shading to each item of Tables 1-3 shows that the requirements of this invention, the recommended manufacturing conditions, mechanical characteristics, etc. are not satisfied.

  As shown in Table 3, the invention steels (steel Nos. 1, 8, 11 to 17) satisfying the requirements of the present invention (the above component requirements and the above structural requirements) all have a yield strength YS of 900 MPa or more. Moreover, the tensile strength TS is 1180 MPa or more, the elongation EL is 12% or more, and the stretch flangeability λ satisfies 60% or more, and the high strength GA combines the yield strength with the elongation and stretch flangeability. A steel plate was obtained.

  On the other hand, comparative steels (steel Nos. 2 to 7, 9, and 10) that lack at least one of the requirements of the present invention (the above-described component requirements and the above-described structural requirements) are yield strength YS, tensile strength TS, and elongation At least one of EL and stretch flangeability λ is inferior

  For example, steel no. 2 is the production No. 2 in Table 2. As shown in FIG. 2, the primary cooling temperature after annealing is too high outside the recommended range, so the transformation proceeds during cooling, and as shown in Table 3, the ferrite is too much and the MA dispersion state is It becomes non-uniform, the dislocation density also decreases, and the yield strength YS and tensile strength TS are inferior.

  On the other hand, Steel No. 3 is the production No. of Table 2. As shown in Table 3, since the primary cooling temperature after annealing is too low outside the recommended range, as shown in Table 3, the dispersed state of MA becomes uneven and the stretch flangeability λ is inferior.

  Steel No. 4 is the production No. of Table 2. As shown in FIG. 4, since cooling is not stopped or held in the middle of cooling after annealing, as shown in Table 3, the dispersed state of MA becomes uneven and the stretch flangeability λ is inferior.

  Steel No. 5 is the production No. of Table 2. As shown in Table 5, since the primary cooling holding time is too long outside the recommended range, MA is generated excessively and the stretch flangeability λ is inferior as shown in Table 3.

  On the other hand, Steel No. 6 shows the production No. in Table 2. As shown in Table 6, the secondary cooling temperature is too high outside the recommended range, and as shown in Table 3, MA is insufficient and the dispersed state of MA becomes uneven, yield strength YS, elongation EL, and elongation. Flangeability is inferior.

  On the other hand, Steel No. 7 shows the production No. in Table 2. As shown in FIG. 7, the secondary cooling temperature is too low outside the recommended range. Therefore, as shown in Table 3, the MA is insufficient and the elongation EL is inferior.

  Steel No. 9 shows the steel No. C in Table 1 and the C content is too low. As shown in Table 9, since the annealing heating temperature is too low, as shown in Table 3, while MA is insufficient, ferrite is generated excessively, dislocation density is insufficient, yield strength YS, tensile strength TS, and elongation. Flange property λ is inferior.

  Steel No. 10 shows the steel No. D in Table 1, the Mn content is too low, and the production No. As shown in Table 10, since the annealing heating temperature is too low, as shown in Table 3, while MA is insufficient, ferrite is generated excessively, and the dispersed state of MA becomes non-uniform, yield strength YS and tensile strength Strength TS and stretch flangeability λ are inferior.

  As described above, it was confirmed that a high-strength GA steel sheet having both yield strength and elongation and stretch flangeability can be obtained by satisfying the requirements of the present invention.

Claims (4)

% By mass
C: 0.05 to 0.30%
Si: 0.1 to 3.0%,
Mn: 1.0 to 3.0%
P: 0.1% or less,
S: 0.01% or less,
N: 0.01% or less,
Al: 0.001 to 0.10%
And the balance has a component composition consisting of iron and inevitable impurities,
The mixed structure MA consisting martensite and austenite, the average area ratio _{V MA} with respect to the entire organization is 2-5%,
Area MA area fraction of less than or equal to 60% of the locally the average area ratio V _MA is 10% or less in area ratio to the whole organization,
The average area ratio of ferrite to the entire structure is 2% or less,
The balance is bainite and martensite,
A structure having a dislocation density of 2.0 × 10 ^{15 to} 8.0 × 10 ¹⁵ m ⁻² ;
A high-strength galvannealed steel sheet with excellent yield strength and workability. Ingredient composition is further mass%,
Cu: 0.05 to 1.0%,
Ni: 0.05 to 1.0%,
B: 0.0002 to 0.0050%
Including one or more of
A high-strength galvannealed steel sheet excellent in yield strength and workability according to claim 1. In addition, the component composition is
Mo: 0.01 to 1.0%,
Cr: 0.01 to 1.0%,
Nb: 0.01-0.3%
Ti: 0.01 to 0.3%,
V: 0.01 to 0.3%
Including one or more of
The high-strength galvannealed steel sheet excellent in yield strength and workability according to claim 1 or 2. Ingredient composition is further mass%,
Ca: 0.0005 to 0.01%,
Mg: 0.0005 to 0.01%
Including one or two of the following:
The high-strength galvannealed steel sheet excellent in the yield strength and workability of any one of Claims 1-3.