JP2006299344A - High-strength hot-dip galvanized steel sheet with excellent formability and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet excellent in formability and a method for producing the same.
SOLUTION: In mass%, C: 0.13 to 0.30%, Si: less than 0.2%, Mn: 0.8 to 2.8%, P: 0.03% or less, S: 0.00. 3% or less, Al: 0.25 to 1.80%, Mo: 0.05 to 0.30%, N: 0.010% or less, and the metal structure is mainly composed of ferrite or bainite. %, The ferrite structure has a volume fraction of 15 to 70%, the ferrite average particle diameter is 10 to 20 μm, and the pearlite structure has a volume fraction of 5% or less. And the pearlite average particle size is 3 μm or less.
[Selection figure] None

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in formability and a method for producing the same.

  In recent years, in order to improve the fuel efficiency of automobiles, it has been required to further reduce the weight of the vehicle body. In order to reduce the weight of the vehicle body, a steel material having high strength may be used. However, since the elongation of steel materials generally decreases as the strength increases, press formability deteriorates. Thus, recently, retained austenitic steel (TRIP steel) that retains austenite to room temperature has high strength and elongation, and therefore has been used as a framework member for automobiles.

  By the way, since the conventional TRIP steel is a component system containing Si exceeding 1%, there is a problem that the hot dip galvanizing is difficult to adhere uniformly and the hot dip galvanizing property is poor. For this reason, TRIP steel is limited only to the industrialization of hot-rolled steel sheets, cold-rolled steel sheets, and electroplated steel sheets, and the industrialization of hot-dip steel sheets has not been performed at present.

  Therefore, in order to solve such problems, it has been proposed to improve plating wettability and alloying processability by reducing Si, adding Al as an alternative element, and further adding Cu. (See Patent Document 1). However, in the case of this method, the problem that the recyclability of steel is deteriorated by addition of Cu remains.

In addition, the hot dip galvanizing equipment generally has a slow cooling rate, and pearlite is formed even by the addition of Al. Therefore, the amount of retained austenite is reduced in TRIP steel, and the elongation is lowered compared to cold rolled steel sheet. End up. In order to solve such problems, it has been proposed that Mo is added and cooled at a cooling rate of 7 ° C./second or more to suppress the formation of pearlite and improve the material of the TRIP steel sheet ( (See Patent Document 2). However, in this method, since the isothermal holding treatment cannot be performed, the TRIP steel sheet is not as strong and ductile as the cold-rolled steel sheet.
Japanese Patent No. 2962038 JP 2003-105491 A

  By the way, since it has become technically possible to improve the cooling rate and to provide an isothermal holding time, even in hot dip galvanized equipment, it has been possible to improve the cooling rate and hold isothermal in a short time. It has not been elucidated whether the component-type TRIP steel sheet has the same strength and ductility as that of the cold-rolled steel sheet under the isothermal holding conditions.

Further, in the cold rolled steel sheet manufacturing equipment, there is a knowledge of a method for manufacturing a retained austenitic steel sheet in which the austenite phase is stabilized by holding at a temperature range of 350 to 550 ° C. for 30 seconds to 30 minutes during continuous annealing. The knowledge is not given to the invention of the steel sheet for galvanizing, in which Al is used as an alternative to Si, and the alloying treatment is essential thereafter.
Then, this invention is proposed in view of such a conventional situation, and it aims at providing the high strength hot-dip galvanized steel plate excellent in formability, and its manufacturing method.

  As a result of intensive studies to achieve the above object, the present inventors have improved wettability by reducing Si and using Al as an alternative element, enabling hot-dip galvanization. By identifying the relationship between the mass% and the relationship between the mass% of C and Mo, optimizing the ferrite and bainite as the metal structure, and detoxifying the pearlite, we have obtained the knowledge that the above objective is achieved. It was. That is, the present inventors have found an optimal steel component that becomes a high-strength hot-dip galvanized steel sheet excellent in formability.

  Moreover, the present inventors specify the relationship between C, Mn, Mo and the isothermal holding temperature and time of a continuous annealing process as a manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in such a formability. It has been found that the ductility is improved to the same level as the retained austenitic steel of the cold rolled steel sheet.

  The present invention has been created based on the above knowledge, and the gist of the present invention is as follows.

(1) By mass%, C: 0.13 to 0.30%, Si: less than 0.2%, Mn: 0.8 to 2.8%, P: 0.03% or less, S: 0.03 %: Al: 0.25-1.80%, Mo: 0.05-0.30%, N: 0.010% or less, and the metal structure is mainly ferrite or bainite, 5% In the steel sheet containing the above retained austenite, the ferrite structure has a volume fraction of 15 to 70%, the ferrite average particle diameter is 10 to 20 μm, and the pearlite structure has a volume fraction of 5% or less. A high-strength hot-dip galvanized steel sheet excellent in formability, characterized in that the average particle size of pearlite is 3 μm or less.

(2) The high-strength hot-dip galvanized steel sheet having excellent formability according to (1), wherein the pearlite structure is 1/10 or less in volume ratio with respect to the bainite structure.

(3) The high-strength hot-dip galvanized steel sheet having excellent formability according to (1) or (2), wherein the pearlite structure has a hardness Hv of 225 or more.

(4) The total of the ferrite structure and the bainite structure is 50 to 80% in terms of volume fraction with respect to the entire structure, as described in any one of (1) to (3) High strength hot dipped galvanized steel sheet with excellent formability.

(5) Any one of (1) to (4), wherein the Al and Mo are in mass% and satisfy a relationship of 0.10-Al / 16 <Mo <0.33-Al / 16. A high-strength hot-dip galvanized steel sheet excellent in formability as described in the item.

(6) The C and Mo are in mass%, and satisfy the relationship of 0.1 × C <Mo. The excellent excellent formability according to any one of (1) to (5) Strength hot dip galvanized steel sheet.

(7) Further, by mass%, at least one or more of V: 0.01 to 0.10%, Ti: 0.01 to 0.20%, Nb: 0.005 to 0.050% The high-strength hot-dip galvanized steel sheet having excellent formability as described in any one of (1) to (6).

(8) In the method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to any one of (1) to (7), annealing is performed at a temperature of 750 to 890 ° C. after cold rolling, and 350 to A method for producing a high-strength hot-dip galvanized steel sheet excellent in formability, wherein the steel sheet is cooled to a temperature of 500 ° C and then held at a temperature of 350 to 500 ° C for a time of 50 to 300 seconds.

(9) When C, Mn, and Mo are in mass%, the cooling end point temperature after annealing is T ° C., and the holding time in the temperature range is t seconds, 0.32 <(2 × C-0 0.1 × Mn−0.2 × Mo + T (logt) / 7000) <0.43 is satisfied, The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to (8) .

(10) The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability as described in (8) or (9), wherein the coiling temperature for hot rolling is 450 to 600 ° C.

(11) After annealing at a temperature of 750 to 890 ° C. after cold rolling, cooling is performed at a cooling rate of 10 ° C./second or more, according to any one of claims (8) to (10). Method for producing high-strength hot-dip galvanized steel sheet with excellent formability

  As mentioned above, according to this invention, since ductility can be improved like the retained austenitic steel of a cold-rolled steel plate, the high-strength hot-dip galvanized steel plate excellent in formability can be provided.

  Hereinafter, a high-strength hot-dip galvanized steel sheet excellent in formability to which the present invention is applied and a method for producing the same will be described in detail with reference to the drawings.

  The high-strength hot-dip galvanized steel sheet excellent in formability to which the present invention is applied is in mass%, C: 0.13 to 0.30%, Si: less than 0.2%, Mn: 0.8 to 2.8. %, P: 0.03% or less, S: 0.03% or less, Al: 0.25-1.80%, Mo: 0.05-0.30%, N: 0.010% or less And, in a steel sheet mainly composed of ferrite or bainite and containing 5% or more of retained austenite, the ferrite structure has a volume fraction of 15 to 70%, and the ferrite average particle diameter is 10 to 20 μm. Furthermore, the pearlite structure has a volume fraction of 5% or less, and the pearlite average particle size is 3 μm or less.

First, the reason why each composition of the high-strength hot-dip galvanized steel sheet excellent in formability is limited will be described.
C is an essential element as a basic element for stabilizing austenite, in addition to adding from the viewpoint of securing strength. However, if C is less than 0.13%, the strength is insufficient and residual austenite is not formed. On the other hand, if C exceeds 0.3%, the strength is excessively increased, the ductility is insufficient, and it cannot be used as an industrial material. Therefore, C is within a range of 0.13 to 0.30%. A preferred range is 0.15 to 0.27%, and a more preferred range is 0.17 to 0.24%.

  Si is an element that delays the formation of carbides more than Mn in addition to adding from the viewpoint of securing strength. Si is an element effective for the production of retained austenite, and is an element added to ensure ductility. However, if Si is added in an amount of 0.2% or more, the platability of hot dip galvanizing deteriorates. Therefore, Si is less than 0.2%. Furthermore, when emphasizing plating properties, Si is made 0.1% or less.

  Mn is an element that delays the formation of carbides in addition to adding from the viewpoint of securing strength. Moreover, Mn is an element effective for the production of retained austenite. However, if Mn is less than 0.8%, the strength is insufficient, the formation of retained austenite becomes insufficient, and the ductility deteriorates. On the other hand, when Mn exceeds 2.8%, the hardenability is improved, so that martensite is generated instead of retained austenite, and the strength is easily increased. In this case, the variation of the product becomes large and the ductility is insufficient so that it cannot be used as an industrial material. Therefore, Mn is set to a range of 0.8 to 2.8%. A preferable range is 1.0 to 2.2%, and a more preferable range is 1.2 to 1.8%.

  P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, when P exceeds 0.03%, the local ductility is deteriorated due to segregation to grain boundaries. Moreover, weldability also deteriorates. Therefore, P is set to 0.03% or less.

  S is an element that degrades local ductility and weldability by generating MnS, and is preferably an element that does not exist in the steel sheet. Therefore, the upper limit of S is 0.03% or less.

  Al is an element necessary for allowing austenite to remain, and has the same effect as Si, which stabilizes austenite by promoting the formation of ferrite and suppressing the formation of carbides. Al needs to be added in an amount of 0.25% or more in order to stabilize austenite. On the other hand, if Al is added excessively, the above effect is saturated, and the steel sheet is embrittled. Moreover, since the platability of hot dip galvanizing deteriorates, the upper limit is made 1.80%.

  Mo is an element that suppresses the formation of pearlite, and it is necessary to add 0.05% or more. When Mo is less than 0.05%, the formation of pearlite is not suppressed and the rate of retained austenite is reduced. On the other hand, when Mo is added excessively, ductility and chemical conversion are deteriorated, so the upper limit is made 0.3%. Furthermore, in order to obtain a higher balance between strength and ductility, the content is made 0.15% or less.

  N is an element that is inevitably contained. However, if it is contained in a large amount, not only the aging property is deteriorated, but also the precipitation amount of AlN is increased and the effect of adding Al is reduced. 01% or less. On the other hand, unnecessarily reducing N causes an increase in cost in the steel making process, so the lower limit is made 0.0010%.

  Furthermore, in the high-strength hot-dip galvanized steel sheet having excellent formability, V: 0.01 to 0.10%, Ti: 0.01 to 0.20%, Nb: 0.005 to 0.00. Of 050%, at least one kind or two or more kinds may be contained. These V, Ti, and Nb are elements that generate fine carbides, nitrides, or carbonitrides, and are effective in securing strength. Therefore, one or more of them are added as necessary. However, excessive addition of these V, Ti, and Nb increases the strength too much and decreases the ductility. Therefore, V is in the range of 0.01 to 0.10%, Ti is in the range of 0.01 to 0.20%, and Nb is in the range of 0.005 to 0.050%.

Next, the metal structure of the high-strength hot-dip galvanized steel sheet having excellent formability will be described.
In order to sufficiently ensure the ductility of the high-strength hot-dip galvanized steel sheet, it is necessary that the main structure is a ferrite structure and the ferrite structure is 15% or more in terms of volume fraction. On the other hand, when there is too much ferrite, retained austenite decreases and hardness decreases, so the ferrite structure needs to be 70% or less in volume fraction. Therefore, the ferrite structure is in the range of 15 to 70% in terms of volume fraction.

  On the other hand, when the average ferrite grain size exceeds 20 μm, the retained austenite becomes rough, so that the uniform TRIP effect cannot be obtained on the entire steel sheet, and the effect of improving the elongation becomes small. On the other hand, when the average ferrite grain size is less than 10 μm, the retained austenite becomes finer, and a stress-strain curve in which processing-induced transformation has occurred in the initial stage of the strain is taken, so that a sufficient TRIP effect cannot be obtained and the elongation is improved. The effect is reduced. Therefore, the average particle diameter of ferrite is in the range of 10 to 20 μm.

  Bainite helps increase the strength and contributes to the stabilization of retained austenite as well as the ferrite phase, and as a result helps increase the ductility. On the other hand, when bainite is produced excessively, ductility deteriorates. Therefore, the total of the ferrite structure and the bainite structure is in the range of 50 to 80% in terms of volume fraction with respect to the entire structure.

  In general, when pearlite is generated, the concentration of C in the retained austenite decreases and martensite increases, so that the strength increases but the ductility decreases. However, in a plated TRIP steel sheet having a high C concentration, pearlite is difficult to suppress due to alloying treatment.

  Therefore, in the high-strength hot-dip galvanized steel sheet having excellent formability, in order to make pearlite harmless, the pearlite average particle diameter is 3 μm or less, the hardness Hv is 225 or more, and the pearlite structure is 5% or less in volume fraction. And Moreover, when the pearlite structure is 1/10 or less in volume ratio with respect to bainite, a metal structure in which the decrease in the concentration of C in retained austenite is suppressed by utilizing the hardness due to C in pearlite. Therefore, in the present invention, by satisfying such conditions, even if a pearlite structure is included, a high-strength steel sheet having an excellent balance between strength and ductility can be obtained.

Moreover, in the high-strength hot-dip galvanized steel sheet having excellent formability, Al and Mo are in mass% and satisfy the relationship of the following formula (1).
0.10-Al / 16 <Mo <0.33-Al / 16 (1)
That is, in the high-strength hot-dip galvanized steel sheet with excellent formability, when Mo becomes 0.10-Al / 16 or less, retained austenite is not formed and ductility decreases, and when Mo becomes 0.33-Al / 16 or more, However, ductility deteriorates.

Here, the relationship between Al and Mo contained in the high-strength hot-dip galvanized steel sheet excellent in formability is shown in FIG. In addition, the enclosure part shown in FIG. 1 shows the preferable numerical range of Al and Mo mentioned above.
Al is an element that promotes the formation of the ferrite described above, and the ferrite fraction at the start of the bainite transformation becomes too large. On the other hand, Mo is the same ferrite former, but reduces the ferrite fraction by suppressing the speed of the transformation itself. Therefore, by setting Mo to be 0.10-Al / 16 or more, the bainite fraction can be increased and the retained austenite can be increased. On the other hand, when Mo becomes 0.33-Al / 16 or more, ductility deteriorates. This is presumably because the reaction rate of bainite decreases and the retained austenite decreases. Thus, the amount of retained austenite formed is determined by the interaction between Mo and Al. In addition, this said Formula (1) is the relationship obtained from the conditions with the low addition rate of Si like this invention especially, and the cooling rate and isothermal holding | maintenance of 10 degree-C / sec or more.

Moreover, in the said high intensity | strength hot-dip galvanized steel plate excellent in the formability, C and Mo are mass% and the relationship of following formula (2) is satisfied.
0.1 × C <Mo (2)
That is, in the high-strength hot-dip galvanized steel sheet having excellent formability, when Mo is less than 0.1 × C, pearlite having a hardness of Hv225 or less at 700 to 600 ° C. is generated during cooling, so that C decreases and remains. It is considered that the strength and ductility are lowered when the concentration of C in the austenite is lowered.

Next, the manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the formability to which this invention is applied is demonstrated.
The manufacturing conditions for TRIP steel in ordinary cold-rolled steel sheets are rolling in the hot rolling process, cold rolling after coil winding, and heat treatment in continuous annealing equipment. In the case of a hot dip galvanized steel sheet, annealing and plating are performed in the hot dip galvanizing process after cold rolling.

  The cold-rolled steel sheet is first annealed in a two-phase coexisting temperature range of austenite and ferrite. At this time, C is concentrated in austenite due to the effect of elements that improve the hardenability such as C and Mn, and the elements that leave residual austenite such as Al and Si, and it is easy to generate residual austenite by the subsequent heat treatment. To.

  The coiling temperature after hot rolling is an important condition for quickly reaching a two-phase equilibrium state in the annealing process. That is, it is necessary to make it easy to dissolve cementite in the annealing process by setting the structure after hot rolling to pearlite having a small interval or a mixed structure of pearlite and bainite. Therefore, the upper limit of the rolling coiling temperature is 600 ° C. or less. In addition, low-temperature winding is desirable in order to suppress the generation of scale and improve the desketing property. On the other hand, if the coiling temperature is too low, the hard phase increases and cold rolling becomes difficult. Therefore, the lower limit of the rolling coiling temperature is set to 450 ° C. or higher.

The hot-rolled steel sheet obtained as described above is pickled, cold-rolled and subjected to annealing.
Specifically, in the method for producing a high-strength hot-dip galvanized steel sheet to which the present invention is applied, after cold rolling, annealing at a temperature of 750 to 890 ° C., cooling to a temperature of 350 to 500 ° C., and then a temperature of 350 to 500 ° C. And holding for 50 to 300 seconds.

  As the annealing temperature in this annealing step becomes high, the austenite ratio increases or becomes a single phase of austenite, so that C in the austenite becomes dilute. For this reason, when the annealing temperature becomes high, stable retained austenite cannot be left by subsequent cooling. Therefore, the upper limit of annealing temperature shall be 890 degrees C or less. On the other hand, when annealing is performed at a low temperature, the carbide is not sufficiently dissolved, so that the concentration of C in the austenite becomes insufficient due to the lack of Sol. C, and the residual austenite ratio is remarkably reduced. Therefore, the lower limit of the annealing temperature is 750 ° C. or higher.

  When the cooling end point temperature and the holding temperature after annealing are less than 350 ° C., the formation of bainite is not promoted. On the other hand, when the temperature exceeds 500 ° C., the production of pearlite is promoted and ductility deteriorates. When the holding time is less than 50 seconds, the generation of bainite is not promoted. On the other hand, when it exceeds 300 seconds, the production of pearlite is promoted and ductility deteriorates. Therefore, the isothermal holding temperature is 350 to 500 ° C., and the holding time is 50 to 300 seconds.

  Moreover, in the manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the formability to which this invention is applied, it anneals at the temperature of 750-890 degreeC after cold rolling, Then, it cools with the cooling rate of 10 degree-C / sec or more. This is because when the cooling rate is less than 10 ° C./second, soft pearlite is formed even when Al is added, the amount of retained austenite decreases, and ductility decreases.

Moreover, in the manufacturing method of the high-strength hot-dip galvanized steel sheet with excellent formability to which the present invention is applied, C, Mn, and Mo are mass%, and the end point temperature of cooling after annealing is set to T ° C. and held in that temperature range. When the time is t seconds, the relationship of the following formula (3) is satisfied.
0.32 <(2 × C-0.1 × Mn-0.2 × Mo + T (logt) / 7000) <0.43 (3)
That is, when the condition of the above formula (3) is satisfied, TS × EI shows a high value, and a high-strength steel plate and hot-dip galvanized steel plate excellent in formability can be realized. On the other hand, when the above formula (3) is 0.32 or less, the concentration of C in the spherical austenite becomes insufficient, so that martensite is formed and residual austenite is not formed. On the other hand, when the above formula (3) is 0.43 or more, pearlite is formed, and the concentration of C in the retained austenite is lowered, so that the strength and ductility are lowered.

  The galvanized layer formed on the high-strength hot-dip galvanized steel sheet having excellent formability mentioned above refers to a plated layer containing zinc as a main component, and includes not only hot-dip galvanized but also alloyed hot-dip galvanized. Shall be.

Next, examples and comparative examples of the present invention will be described.
First, steel having each component composition shown in Table 1 is manufactured, and after cooling and solidification, it is reheated to 1200 ° C., finish-rolled at 880 ° C., and wound up at 550 ° C. after cooling to 70%. Each sample was produced by cold rolling. Among these samples, the samples 1 to 26 shown in Table 1 all have the component composition, the relationship between Al and Mo satisfies the above formula (1), and the relationship between C and Mo satisfies the above formula (2). This is a satisfactory example. On the other hand, Samples 27 to 31 shown in Table 1 are comparative examples that do not satisfy the above formulas (1) and (2) as component compositions.

  Next, after annealing at 750 ° C. to 930 ° C. for 50 seconds to 120 seconds in a continuous annealing and cooling to 100 ° C. to 500 ° C. at a cooling rate of 3 ° C./second to 50 ° C./second, 100 The temperature was maintained at 20 ° C. to 500 ° C. for 20 seconds to 5000 seconds. Next, in order to perform an alloying process, it reheated to 550 degreeC and further cooled to room temperature. Thereafter, 1% skin pass rolling was performed to prepare samples shown in Tables 2 and 3.

And each of these samples was evaluated. The evaluation results are shown in Table 2.
The residual γ rate shown in Table 2 is the residual austenite rate. Regarding the measurement of the retained austenite ratio, the surface of the specimen was polished to 1/4 t, then chemically polished and then subjected to X-ray diffraction using a Mo tube, and the ferrite (200) of the following formula (4) From the intensity ratio of diffraction intensity Iα (200) of ferrite, diffraction intensity Iα (211) of ferrite (211), diffraction intensity Iγ (220) of austenite (220), and diffraction intensity Iγ (311) of austenite (311) Asked.
Vγ (volume ratio%) = 0.25
× {Iγ (220) / (1.35 × Iα (200) + Iγ (220))
+ Iγ (220) / (0.69 × Iα (211) + Iγ (220))
+ Iγ (311) / (1.5 × Iα (200) + Iγ (311))
+ Iγ (311) / (0.69 × Iα (211) + Iγ (311))} (4)

  Further, the identification of ferrite, bainite, pearlite, martensite, and austenite structures in the metal structure, observation of the existing positions, and measurement of the average particle diameter (average circle equivalent diameter) and occupancy are performed using the Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. Quantification is possible by observing a steel plate rolling direction cross section or a cross section perpendicular to the rolling direction with the reagent disclosed in Japanese Patent Publication No.

  Among the evaluation results shown in Table 2, for tensile properties, JIS No. 5 tensile test pieces were prepared from each sample, and each test piece was evaluated by L-direction tension. A product having a TS [MPa] × EL [%] product of 19000 MPa% or more was considered good. Moreover, about the plating property, after performing hot dip galvanization to each sample, the adhesion state of plating was confirmed visually. And the case where the plating adhered uniformly in the area of 90% or more of the plating surface was judged as good (◯). Moreover, about alloying, the rating 3 or less was made favorable ((circle)) by the powdering test.

Moreover, another evaluation was performed about each of these samples. The evaluation results are shown in Table 3.
Among the evaluation results shown in Table 3, the pearlite structure was 1/10 or less in volume ratio with respect to the bainite structure, and the hardness Hv was 225 or more. Further, the total of the ferrite structure and the bainite structure was determined to be 50 to 80% in terms of volume fraction. Further, a ferrite structure having a volume fraction of 15 to 70% and an average ferrite particle diameter of 10 to 20 μm was considered good. A pearlite structure having a volume fraction of 5% or less and an average pearlite particle size of 3 μm or less was considered good.

  In the evaluation results shown in Table 2 and Table 3, Samples 1-2, 13-2, 16-2, 17-2, and 18-2 satisfy the component composition of the present invention in Table 1, but are annealed depending on the processing conditions. The temperature and the end-of-cooling temperature are outside the numerical range of the present invention. Samples 2-2, 3-2, 10-2, 22-2, and 25-2 satisfy the composition of the present invention in Table 1, but the retention time is outside the numerical range of the present invention depending on the processing conditions. is there. Samples 5-2, 20-2, and 21-2 satisfy the component composition of the present invention in Table 1, but the residual γ rate is outside the numerical range of the present invention depending on the processing conditions. Moreover, since the volume ratio and average particle diameter of each structure are out of the numerical range of the present invention, TS × EL does not satisfy the conditions of the present invention. Samples 29-1 and 30-1 do not satisfy the component composition of the present invention in Table 1, but TS × EL shows good results. However, the plating properties and alloying are getting worse. From the above, it can be seen that the sample of the example satisfying the conditions of the present invention is a high-strength hot-dip galvanized steel sheet having excellent formability as compared with the sample of the comparative example.

  INDUSTRIAL APPLICABILITY The present invention is widely applicable as a high-strength hot-dip galvanized steel sheet having excellent formability and a manufacturing method thereof, for example, steel materials used for automobile body parts and the like, and its industrial utility value is high.

FIG. 1 is a graph showing the relationship between Al and Mo contained in a high-strength hot-dip galvanized steel sheet excellent in formability to which the present invention is applied.

Claims (11)

% By mass
C: 0.13-0.30%,
Si: less than 0.2%,
Mn: 0.8 to 2.8%,
P: 0.03% or less,
S: 0.03% or less,
Al: 0.25 to 1.80%,
Mo: 0.05-0.30%,
N: 0.010% or less, and
In a steel sheet mainly composed of ferrite or bainite and containing 5% or more of retained austenite, the ferrite structure is 15 to 70% in volume fraction, and the ferrite average particle diameter is 10 to 20 μm. A high-strength hot-dip galvanized steel sheet excellent in formability, wherein the pearlite structure has a volume fraction of 5% or less and the pearlite average particle size is 3 μm or less.   The high-strength hot-dip galvanized steel sheet with excellent formability according to claim 1, wherein the pearlite structure is 1/10 or less in volume ratio with respect to the bainite structure.   The high-strength hot-dip galvanized steel sheet with excellent formability according to claim 1 or 2, wherein the pearlite structure has a hardness Hv of 225 or more.   The total of the ferrite structure and the bainite structure is 50 to 80% in terms of volume fraction with respect to the entire structure, and is excellent in formability according to any one of claims 1 to 3. High strength hot dip galvanized steel sheet. The Al and Mo are mass%,
0.10-Al / 16 <Mo <0.33-Al / 16
The high-strength hot-dip galvanized steel sheet excellent in formability according to any one of claims 1 to 4, wherein the relationship is satisfied. The C and Mo are mass%,
0.1 × C <Mo
The high-strength hot-dip galvanized steel sheet excellent in formability according to any one of claims 1 to 5, characterized by satisfying the relationship: In addition,
V: 0.01 to 0.10%,
Ti: 0.01-0.20%,
Nb: 0.005 to 0.050%
Among them, the high-strength hot-dip galvanized steel sheet excellent in formability according to any one of claims 1 to 6, comprising at least one kind or two or more kinds.   In the manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the moldability as described in any one of the said Claims 1-7, it anneals at the temperature of 750-890 degreeC after cold rolling, and is the temperature of 350-500 degreeC. A method for producing a high-strength hot-dip galvanized steel sheet excellent in formability, wherein the steel sheet is kept at a temperature of 350 to 500 ° C. for 50 to 300 seconds after cooling. When the C, Mn, and Mo are mass%, the cooling end point temperature after annealing is T ° C., and the time for holding in the temperature range is t seconds,
0.32 <(2 × C−0.1 × Mn−0.2 × Mo + T (logt) / 7000) <0.43
The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to claim 8, wherein:   The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability according to claim 8 or 9, wherein the coiling temperature for hot rolling is 450 to 600 ° C.   The high strength excellent in formability according to any one of claims 8 to 10, wherein the steel sheet is annealed at a temperature of 750 to 890 ° C after cold rolling and then cooled at a cooling rate of 10 ° C / second or more. Manufacturing method of hot dip galvanized steel sheet.

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