JP2010043360A - High-strength and high-ductility hot-dip galvanized steel sheet superior in hole expandability, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength galvanized steel sheet having a tensile strength of 850 MPa or higher, and improved hole expandability together with improved ductility. <P>SOLUTION: The high-strength hot-dip galvanized steel sheet includes, by mass, 0.03 to 0.20% C, ≤1.0% Si, 0.01 to 3% Mn, 0.0010 to 0.1% P, 0.0010 to 0.05% S, 0.3 to 2.0% Al and 0.01 to 5.0% Mo, and further includes one or more selected from 0.001 to 0.5% Ti, 0.001 to 0.5% Nb, 0.0001 to 0.0050% B and 0.01 to 5% Cr, and the balance Fe with inevitable impurities, and has a microstructure containing ferrite of ≥30% by area ratio. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to a high-strength galvanized steel sheet having excellent hole expansibility and ductility suitable for building materials, home appliances, automobiles, etc., and a tensile strength of 850 MPa or more, and a method for producing the same.

  In recent years, there has been an increasing demand for high-strength steel sheets with good workability for the purpose of improving fuel efficiency and durability particularly in automobile bodies. In addition, it is desired to increase the strength of the steel sheets for the members because of the need for collision safety and expansion of cabin space. In fact, regulations regarding automobile collision safety are becoming stricter year by year, as represented by N-CAP. Against this background, there has been an active movement to use high strength steel sheets of 780 MPa class. However, in order to withstand stricter regulations, it is considered that a steel sheet of 980 MPa class with high tensile strength is required for some components such as reinforcement.

  When a member is assembled using such a high-strength material, ductility, bendability, hole expansibility, corrosion resistance, and the like become major problems, and measures for these are required.

  Although hole expansibility and ductility are contradictory characteristics, the following measures have been taken to improve individual characteristics.

  For example, as described in Non-Patent Document 1, with regard to hole expansibility, the main phase is bainite to improve the hole expansibility, and the extension property is formed by generating retained austenite in the second phase. It is disclosed that it exhibits the same stretchability as that of the remaining austenitic steel. Furthermore, it is also shown that when retained austenite having an area ratio of 2 to 3% is generated by austempering at a temperature equal to or lower than the Ms temperature, the tensile strength × the hole expansion ratio is maximized. However, these do not take into account the current continuous galvanizing process, and lack consideration for improving corrosion resistance by plating.

  Moreover, about the hole expansibility improvement of a high-strength galvanized steel plate, as it is in patent document 1, high hole expansion is attained by raising the space ratio of a bainite or a low carbon martensite in the microstructure of a steel plate in a continuous plating process. The rate is achieved. However, with regard to ductility, increasing the space factor of bainite or low carbon martensite in the microstructure of the steel sheet is extremely detrimental. In fact, the present invention achieves an excellent hole expansion rate exceeding 80% at a tensile strength level exceeding 800 MPa, but there is no consideration for ensuring ductility. As for the high-strength, high-ductile plated steel sheet, as disclosed in Patent Document 2, the main focus is on how to ensure retained austenite during the continuous plating process to improve ductility. Further, martensite as well as retained austenite is allowed as the second phase. However, since the strength level is at most 780 MPa and the main phase is ferrite, the high strength material at 850 MPa or more which is the object of the present invention is not considered.

JP 2003-193190 A JP 2003-105492 A

CAMP-ISIJ vol. 13 (2000) p. 395

  An object of the present invention is to solve the above-mentioned problems, and to provide a high-strength steel sheet having improved tensile strength and ductility of a high-strength steel sheet having a tensile strength of 850 MPa or more and mainly a 980 MPa class or more and a method for producing the same. To do.

  As a result of various investigations, the present inventors have specified the steel plate components and the microstructure structure as a technique for simultaneously improving the hole expandability and ductility in the region where the tensile strength is 850 MPa or higher and mainly 980 MPa or higher. Thus, it was found that hole expandability and ductility can be secured while maintaining a high strength of 850 MPa or more.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.03-0.20%,
Si: 1.0% or less,
Mn: 0.01 to 3%
P: 0.0010 to 0.1%,
S: 0.0010 to 0.05%,
Al: 0.3 to 2.0%,
Mo: contains 0.01 to 5.0%,
B: 0.0001 to 0.0050%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%,
Cr: 0.01 to 5%
Hole expandability characterized by containing one or more of the following, the balance Fe and inevitable impurities, the microstructure contains ferrite with an area ratio of 30% or more, and the tensile strength is 850 MPa or more Hot-dip galvanized high-strength steel sheet with excellent ductility.
(2) Further, it is characterized by containing 0.001 to 0.50% in total of Vb, Zr, Hf, and Ta, including Nb and Ti, by mass% (1) ) High-strength, high-ductility hot-dip galvanized steel sheet excellent in hole expansibility.
(3) Further, it contains 0.001 to 5% of Ni, Cu, Co, or W in a total of 0.001 to 5% including Cr in mass% (1) or (2) ) High-strength, high-ductility hot-dip galvanized steel sheet excellent in hole expansibility.
(4) Further, 0.001 to 0.5% of one or more of Y, Rem, Ca and Mg is contained by mass%, and any one of (1) to (3) A high-strength, high-ductility hot-dip galvanized steel sheet excellent in hole expansibility described in the item.
(5) Fe: 5 to 20% in mass% in the plating phase on the surface of the steel sheet, characterized by high hole expansibility according to any one of (1) to (4) High strength ductile hot dip galvanized steel sheet.
(6) High strength excellent in hole expansibility according to any one of (1) to (5), wherein the plating phase on the steel sheet surface contains Fe: less than 5% by mass. High ductility hot dip galvanized steel sheet.
(7) A method for producing the high-strength and high-ductility hot-dip galvanized steel sheet according to any one of (1) to (6), wherein the component according to any one of (1) to (4) The cast slab made of is heated as it is or after being cooled again, and the hot rolling is finished at a finishing temperature of 850 ° C. to 970 ° C., and then cooled to a temperature range of 550 ° C. or lower at an average of 10 to 100 ° C./s. Then, the hot-rolled steel sheet wound at 550 ° C. or lower is pickled and cold-rolled, and the maximum temperature during annealing is 0.1 × (Ac 3 −Ac 1) + Ac 1 (° C.) or higher and 0.8 × (Ac 3 −Ac 1). After annealing at + Ac1 (° C.) or less, it is cooled to a temperature range of galvanizing bath temperature −20 ° C. to zinc plating bath temperature + 50 ° C. at an average cooling rate of 0.1 to 100 ° C./second, and then the same temperature range Holds for 1 to 1000 seconds including plating immersion Lower resistance and a manufacturing method excellent hot dip galvanized high strength steel sheet ductility.
(8) A method for producing the high-strength, high-ductility hot-dip galvanized steel sheet according to (7), wherein after the plating bath immersion and holding treatment, the alloying treatment is performed in a temperature range of 400 to 550 ° C. and cooled to room temperature. A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet excellent in workability, characterized in that:

  The high-strength hot-dip galvanized steel sheet of the present invention has good plating, hole expansion, and ductility, and is extremely effective for uses such as automobile frameworks and their reinforcing members as well as building materials and home appliances.

  Hereinafter, the present invention will be described in detail.

  The inventors have melted the steel ingot added with each alloy element, heated again as cast or once cooled, hot-rolled hot-rolled steel sheet after hot rolling, cold-rolled after pickling, and then annealed, A cold-rolled annealed plate was created.

  The steel sheet was subjected to microstructural observation, hole enlargement test specified by the Federation of Iron and Steel, and tensile test based on JIS, and each characteristic was compared and evaluated.

  As a result, it was found that a tensile strength of 850 MPa or more was obtained by the microstructure control finally obtained, and a high-strength steel sheet excellent in hole expansibility and ductility could be produced.

  A preferable microstructure of the steel sheet will be described.

  In general, it is effective to use a bainite phase (also referred to as bainite) or bainitic ferrite as the main structure in order to ensure sufficient hole expandability. However, when such a hard phase is increased, the ductility deteriorates. On the other hand, when the number of relatively soft ferrite phases (also called ferrite) is increased, the ductility tends to be improved, but it is not suitable for securing the strength and hole expandability. Therefore, it is important to ensure that these microstructures are well balanced in the current continuous plating process in order to ensure a sufficient level of strength, hole expansibility, and ductility.

For example, the space factor of each microstructure is 20% or more of the soft ferrite phase, 10% or more of the bainite phase, 5% or more of the martensite phase (also called martensite), and the residual austenite phase (residual). By setting the austenite) to 5% or less, a good material can be obtained. Strength-ductility is ensured by the balance between ferrite and martensite, and strength-hole expansibility is ensured by the balance between bainite and martensite. Here, retained austenite is effective in securing ductility, but is less effective in improving hole expansion, and rather tends to deteriorate, so it is desirable that it be less than 3%. Most preferably, it is 0%.
In order to obtain a good hole expansion-ductility balance while securing a tensile strength of 980-1180 MPa class, ferrite is 20-70%, bainite is 20-60%, martensite is 5-20%, residual Desirably, austenite is less than 3%.
In the present invention, in particular, the volume fraction of the soft ferrite that is the main phase is set to 30% or more that can ensure sufficient strength-ductility.

  Furthermore, the relatively high volume fraction of the soft ferrite, which is the main phase, is effective for improving ductility, and the fact that it is fine is effective for improving the balance between strength and hole expansibility. For this reason, it is preferable that the upper limit of the average particle diameter of ferrite is 30 μm.

  In addition to the above, the present invention includes a case in which one or two or more of carbides, nitrides, sulfides, oxides, and the like are contained as a remaining structure of the microstructure in a volume fraction of 1% or less.

In addition, identification of each phase of the above microstructure, ferrite, bainite, austenite, martensite, interfacial oxidation phase and remaining structure, observation of the existing position, and measurement of the space factor are performed using the Nital reagent and Japanese Patent Application Laid-Open No. 59-219473. Can be quantified by observing the cross section in the rolling direction of the steel sheet or the cross section in the direction perpendicular to the rolling direction with the reagent disclosed in, and observing with an optical microscope of 500 to 1000 times and an electron microscope (scanning type and transmission type) of 1000 to 100,000 times. . Observation of 20 or more fields of view can be performed, and the space factor of each tissue and the average particle size of the main phase can be obtained by a point counting method or image analysis. Moreover, the space factor of each microstructure can be obtained from the investigation of each phase transformation behavior from the expansion / contraction curve by Formaster.
The total of each phase of the microstructure is 100%, but phases that cannot be observed or identified by an optical microscope or an expansion curve such as carbides, oxides, and sulfides are included in the area ratio of the main phase.

  Next, the reason for limiting the preferable range of the steel plate component in the present invention will be described.

  C is an important additive element in order to ensure a good material balance. It is the most important additive element for controlling the fraction of ferrite, bainite and martensite. From the viewpoint of securing strength and ductility, the lower limit is set to 0.03% by mass or more, and when aiming at 980 MPa or more, addition of 0.1% or more is desirable. On the other hand, when the amount added increases, the hole expandability deteriorates. Therefore, the upper limit for maintaining the hole expandability is preferably 0.20%. In order to obtain a good balance between hole expansibility and ductility at a strength level of 980 to 1180 MPa, a range of 0.05 to 0.15% is desirable.

  Si is an element added for the purpose of suppressing the formation of relatively coarse carbides that deteriorate the strength-ductility balance, but significantly deteriorates the plateability. For this reason, you may add to 1.0% or less. Moreover, since excessive addition has a bad influence also on weldability, it is preferable to make an upper limit into 1.0 mass%. On the other hand, considering the refining capacity and raw material composition, the extreme decrease in Si leads to an increase in manufacturing cost. Therefore, it is desirable to add 0.005% or more, which does not greatly deteriorate the plating performance, and ensures plating quality. And from the point of reduction of alloying suppression, it is desirable to set it as 0.1% or less.

  Mn is added for the purpose of increasing the strength. Further, it is added for the purpose of suppressing carbide precipitation and pearlite formation, which are one cause of strength reduction and material deterioration. From these things, it was set as 0.01 mass% or more. On the other hand, 3 mass% was made the upper limit because it delayed the bainite transformation that contributes to improving hole expansibility. Preferably, a good balance between strength and hole expandability can be obtained by setting the content to 1.5 to 3.0%.

  In addition to being a deoxidizing element, Al is an important additive element for promoting ferrite transformation and improving the balance between hole expansion and ductility. For this reason, it is preferable to add 0.3 mass% or more. On the other hand, excessive addition not only impairs weldability and plating wettability, but also causes deterioration of hole expansibility, so the upper limit is preferably made 2.0%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.4% to 0.9% is desirable.

  Mo, like Al, is an important additive element that improves the ductility-hole expansibility balance. It is important not only to suppress the formation of carbides and pearlite, but also to form a relatively hard ferrite, so the lower limit is preferably 0.01% by mass. In particular, in order to suppress pearlite and carbide precipitation as much as possible even in a mild cooling process in a continuous plating process (average cooling rate after annealing is 10 ° C./s or less) and a high alloying temperature (520 ° C. or more). Addition of 0.05% or more is desirable. Further, excessive addition causes ductility deterioration, so the upper limit is made 5.0%. It is preferable to set it to 1.0% or less. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.1% to 0.3% is desirable.

  P is a strengthening element, and lowering P improves hole expansibility. However, since extremely low P is economically disadvantageous, 0.0010% by mass is preferable as the lower limit. Moreover, since addition in a large amount adversely affects weldability and manufacturability during casting or hot rolling, it is preferable to set the upper limit to 0.1%. Preferably, 0.03% or less is desirable.

  S is effective in improving hole expansibility by lowering S. On the other hand, since extremely low is economically disadvantageous, the lower limit is preferably 0.0010% by mass and the upper limit is preferably 0.05% by mass. This is because it adversely affects the productivity and manufacturability during casting and hot rolling.

In the present invention, in addition to the above components, one or more of B, Ti, Nb, and Cr are further contained.
B is an element that promotes fine homogenization of the hot-rolled sheet structure and, as a result, is effective for improving the hole expansion after annealing. For this reason, it was made into 0.0001% or more of addition. This effect is particularly effective when combined with Ti. On the other hand, excessive addition causes ductility deterioration, so the upper limit was made 0.0050%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.0003% to 0.0020% is desirable.

  Note that N is not a preferable element for exhibiting the effect of addition of B, and therefore is preferably 0.0070% or less.

  Nb forms fine carbides, nitrides or carbonitrides or is in a solid solution state, and is effective for strengthening the steel sheet. Further, it is an important additive element for forming hard ferrite, and 0.001% by mass or more is added, and 0.01% or more is desirable for forming hard ferrite. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 0.5 mass%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.005% to 0.020% is desirable.

  Ti forms fine carbides, nitrides or carbonitrides and is effective for strengthening the steel sheet. Further, since it is particularly effective for forming hard ferrite by composite addition with B, the addition was made 0.001% by mass or more. Further, in the case of composite addition with B, addition of 0.01% or more is desirable for forming hard ferrite. On the other hand, excessive addition deteriorates ductility and hot workability, so the upper limit was made 0.5 mass%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.005% to 0.020% is desirable.

  Cr is an additive element effective for improving the hole expansion property in order to promote the formation of hard ferrite and the refinement of carbides, so it was added in an amount of 0.01% or more. Moreover, since excessive addition causes ductility fall, the upper limit was made 5%. In order to obtain a particularly good hole expansion-ductility balance, a range of 0.1% to 0.8% is desirable.

  Relationship between B, Al, O and N: When B is added, it is effective not to combine B with N as much as possible in order to fully exhibit the effect. Therefore, it is important for exhibiting the effect of B and improving the material balance that the sum of the amount of Al not bonded to O as a nitride-forming element stronger than B and the amount of B added is larger than the amount of N. is there. Therefore, it is desirable that the content of these elements satisfies the formula: B / 11 + (Al / 27−O / 16) −N / 14> 0, and the formula is a value of 0.02 or more. If there is, a better hole expansion-ductility balance can be obtained.

  Furthermore, the steel targeted by the present invention can contain one or two of strong carbide forming elements V, Zr, Hf, and Ta for the purpose of further improving the strength and refining the structure. These elements form fine carbides, nitrides or carbonitrides and are extremely effective for strengthening the steel sheet. Therefore, one or more of these elements may be added in a total amount of Nb and Ti as needed. Addition of 0.001% by mass or more.

  On the other hand, since it inhibits ductile deterioration and concentration of C in retained austenite, the upper limit of the total amount of addition of one or more of Nb and Ti is set to 0.50% by mass. Of these, Zr, Hf, and Ta, which are stronger nitride-forming elements than B, are also effective in utilizing the B addition effect, and it is desirable to add them in consideration of economy.

  Furthermore, the steel targeted by the present invention can contain one or more of Ni, Cu, Co, and W for the purpose of further improving the strength.

  Ni exhibits an effect by addition of 0.001% by mass or more for the purpose of strengthening by improving hardenability, and exceeds 5% by mass in total of one or more of Cr, Ni, Cu, Co, and W. The addition of the amount contributes to the workability, particularly the hardness increase of martensite, and has an adverse effect, so this was made the upper limit.

  Cu is effective when added in an amount of 0.01% by mass or more for the purpose of strengthening, and when added in an amount exceeding 5% by mass in total of one or more of Cr, Ni, Cu, Co, and W, it is processed. Adversely affects performance and manufacturability.

  Co exhibits an effect when added in an amount of 0.01% by mass or more in order to improve the strength ductility balance by controlling bainite transformation. On the other hand, since it is an expensive element, the addition of a large amount impairs the economy, so it is desirable that the total of one or more of Cr, Ni, Cu, Co, and W be 5% by mass or less.

  When W is added in an amount of 0.01% by mass or more, a strengthening effect appears. Addition of more than 5% by mass of one or more of Cr, Ni, Cu, Co, and W adversely affects workability. Effect.

  Y, an abbreviation for Rem (Rare Earth Metal), and a lanthanoid element starting from La. Industrially, it is added in the form of misch metal. In this case, it is mainly composed of La and Ce). Ca, Mg are added in appropriate amounts to control the form of inclusions, especially finely dispersed. From the viewpoint, the content is made 0.001% or more. On the other hand, excessive addition lowers the manufacturability such as castability and hot workability and the ductility of the steel sheet product, so the upper limit is made 0.5 mass%. Of these, La and Ce, which are stronger nitride forming elements than B, are also effective in utilizing the B addition effect, and it is desirable to add them while considering manufacturability.

  Inevitable impurities include, for example, Sn, but even if these elements are contained in the range of 0.02% by mass or less, the effect of the present invention is not impaired.

  Moreover, regarding a plating layer, Fe is taken in in a galvanization layer by alloying process, and the high intensity | strength hot-dip galvanized steel plate excellent in coating property and spot weldability can be obtained. When the Fe content of the galvanized layer is less than 5% by mass, spot weldability is insufficient. On the other hand, if the amount of Fe exceeds 20% by mass, the adhesion of the plating layer itself is impaired, and the plating layer breaks and drops during processing and adheres to the mold, thereby causing defects during molding. Therefore, the range of the amount of Fe in the galvanized layer when performing the alloying treatment is 5 to 20% by mass.

  On the other hand, even if the Fe content of the hot dip galvanized layer is less than 5% by mass, an effect of improving the corrosion resistance other than the effect of the alloying described above can be obtained.

  The plating adhesion amount is not particularly limited, but is preferably 5 g / m 2 or more in terms of single-sided adhesion from the viewpoint of corrosion resistance. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.

  A method for producing a high-strength, high-ductility galvanized steel sheet having such a structure and excellent hole expansibility will be described below.

  When the steel sheet of the present invention is manufactured by cold rolling and annealing after hot rolling, the slab adjusted to a predetermined component is cast as it is or once cooled and then reheated for hot rolling. In this case, the reheating temperature is desirably 1150 ° C. or higher and 1250 ° C. or lower. When the reheating temperature is high, coarse grains and thick oxide scales are formed. On the other hand, low temperature heating increases the rolling resistance. Moreover, after hot rolling, if the surface scale is removed by using a high-pressure descaling device or pickling, the surface of the product is better cleaned, which tends to be advantageous for plating properties.

  In addition, it is particularly important to control the structure from hot rolling to achieve both ductility and hole expandability. Making the hot rolled sheet structure uniform and fine contributes greatly to improving the ductility and hole expandability of continuous hot-dip galvanized steel sheets. Effect. For this purpose, the hot rolling finishing temperature is set to 850 ° C. to 970 ° C.

  Subsequent cooling suppresses pearlite transformation, and in order to obtain a better material, it is desirable to cool the average cooling rate after finish hot rolling to a temperature range of 550 ° C. or lower at 10 to 100 ° C./s. The cooling stop was 550 ° C. or less up to the bainite generation temperature range.

  After that, it was decided to wind up to 550 ° C. or less for the purpose of homogenizing the hot rolled structure by bainite or martensite formation. The homogenization of the hot-rolled sheet is particularly important for ensuring the same hole expandability and ductility as well as the effect of adding B. On the other hand, it is desirable to wind as high a temperature as possible from the problem of an increase in reaction force during cold rolling, and winding at 500 to 400 ° C. is desirable.

  For the subsequent cold rolling, if the total reduction ratio set from the relationship between the final sheet thickness and the cold rolling load is 40% or more, it is sufficient from the viewpoint of recrystallization and transformation control, and the final steel sheet characteristics deteriorate. This is preferable because it is not allowed to occur.

Moreover, in order to obtain a favorable structure fraction in the annealing in the hot dip galvanizing and hot galvannealed galvanizing processes, 0.1 × (Ac ₃ −Ac ₁ ) + Ac _{1 to} 0.9 × (Ac ₃ −Ac ₁ ) Annealing in the temperature range of + Ac ₁ is desirable. Here, Ac ₁ and Ac ₃ are the Ac ₁ temperature (° C.) and Ac ₃ temperature (° C.) determined by the chemical composition of the steel (for example, “Steel Material Science”: WC Leslie, translated by Koyasu Naruyasu, Maruzen P273) is there.

Furthermore, in order to obtain a good material, it is preferable to set the upper limit of the annealing temperature to 0.8 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.). The annealing time in this temperature range is preferably 10 seconds or more in order to make the temperature of the steel plate uniform and to secure austenite. On the other hand, if it exceeds 30 minutes, formation of a grain boundary oxidation phase is promoted and cost increases, and therefore, it is preferably 30 minutes or less. Moreover, in order to ensure better hole expansibility, it is desirable to anneal in a temperature range of 0.3 × (Ac ₃ −Ac ₁ ) + Ac ₁ (° C.) or more for 60 to 200 seconds.

  Here, in order to improve the plating quality, it is desirable that the atmosphere at the time of temperature rise and annealing is an oxygen concentration of 50 ppm or less and a dew point of −20 ° C. or less. If the oxygen concentration exceeds 50 ppm or the dew point exceeds −20 ° C., the plateability of the steel sheet, particularly the wettability, deteriorates and may cause non-plating.

  Subsequent primary cooling is important for generating bainite or bainitic ferrite or martensite while suppressing the transformation from the austenite phase to the ferrite phase to some extent. Setting this cooling rate to less than 0.1 ° C./second may promote the formation of ferrite and pearlite and cause a decrease in strength, so the lower limit of the cooling rate was set to 0.1 ° C./second. On the other hand, when the cooling rate is higher than 100 ° C./second, the hard steel phase such as the martensite phase in the final steel sheet becomes large and operation is difficult.

  In the cooling after the annealing described above, if the cooling is performed to a temperature lower than −20 ° C., there are operational problems such as a large heat removal at the time of entering the plating bath. On the other hand, if the cooling stop temperature exceeds the plating bath + 50 ° C., in addition to operational problems, carbides are generated during subsequent holding, leading to a decrease in strength. For this reason, it is preferable that cooling stop temperature shall be plating bath temperature-20 degreeC-plating bath temperature +50 degreeC.

  Next, in order to promote the progress of the bainite transformation, the temperature range is maintained. If the retention time is long, it is not preferable in terms of productivity, and carbides are generated. Further, in order to cause the bainite transformation to proceed, it is desirable to hold for 1 second or longer, preferably 15 seconds to 10 minutes. If the holding temperature is lower than the plating bath temperature −20 ° C., the bainite transformation hardly occurs, and if it exceeds the plating bath temperature + 50 ° C., carbides are generated and the material is deteriorated.

  In order to generate the martensite phase, it is not necessary to cause the bainite transformation, but since it is necessary to suppress the formation of carbide and pearlite, 400 to 550 ° C in order to perform sufficient alloying treatment after the above holding. It is preferable to perform the alloying treatment in the temperature range.

  Moreover, when performing an alloying process, it was set as 400 degreeC or more and 580 degrees C or less. When the alloying treatment temperature is less than 400 ° C., the progress of alloying is slow and the productivity is poor. Moreover, when it exceeds 580 degreeC, a carbide | carbonized_material precipitation will accompany and material quality will deteriorate. Preferably it is set as 430 degreeC or more and 550 degrees C or less.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  The steel sheets having the compositions shown in Tables 1 and 2 were heated to 1100 to 1250 ° C., the hot rolling was completed at the Ar3 transformation temperature or higher, the wound steel strip was pickled, cold rolled, and then cold rolled to 1.2 mm. Thickness.

Thereafter, it was determined by calculating the Ac ₁ and Ac ₃ transformation temperature according to the following equation from a component of the steel (mass%).
Ac ₁ = 723-10.7 × Mn% + 29.1 × Si%,
Ac ₃ = 910−203 × (C%) ^1/2 −15.2 × Ni%
+ 44.7 × Si% + 104 × V% + 31.5 × Mo%
−30 × Mn% −11 × Cr% + 400 × Al%,
After raising and maintaining the annealing temperature calculated from these Ac1 and Ac3 transformation temperatures in a 10% H ₂ -N ₂ atmosphere, a temperature of 420 to around the plating bath temperature at a cooling rate of 0.02 to 10 ° C./sec. After cooling to 670 ° C., plating was performed by immersing in a galvanizing bath for 3 seconds. In that case, the steel plate was hold | maintained for 3 to 15000 second at each temperature of 450 to 465 degreeC and 650 degreeC including the immersion time at the time of plating.
Moreover, about some steel plates, as a Fe-Zn alloying process, the steel plate after plating was hold | maintained for 5 to 25 seconds in the temperature range of 500-650 degreeC, and Fe content rate in a plating layer was adjusted.

  JIS No. 5 tensile test specimens were collected from these steel plates and measured for mechanical properties. In addition, a hole expansion test was performed in accordance with the Steel Federation standard, and the hole expansion rate was obtained. In addition, the plating test was evaluated by observing the appearance and giving a five-point score.

  Table 4 shows the microstructure and each material, and Table 3 shows each manufacturing condition. It can be seen that the invention steel that satisfies the outline of the present invention is excellent in ductility, strength (tensile strength of 800 MPa or more), and hole expandability.

  On the other hand, the comparative example which deviates from the conditions of the present invention is inferior in hole expansibility, ductility and plating properties.

Claims (8)

% By mass
C: 0.03-0.20%,
Si: 1.0% or less,
Mn: 0.01 to 3%
P: 0.0010 to 0.1%,
S: 0.0010 to 0.05%,
Al: 0.3 to 2.0%,
Mo: contains 0.01 to 5.0%,
Ti: 0.001 to 0.5%,
Nb: 0.001 to 0.5%,
B: 0.0001 to 0.0050%,
Cr: 0.01 to 5%
Hole expandability characterized by containing one or more of the following, the balance Fe and inevitable impurities, the microstructure contains ferrite with an area ratio of 30% or more, and the tensile strength is 850 MPa or more Hot-dip galvanized high-strength steel sheet with excellent ductility.   Furthermore, 0.001 to 0.50% of the total containing 1 type, or 2 or more types of V, Zr, Hf, Ta including Nb and Ti is contained by the mass%. High strength and high ductility hot dip galvanized steel sheet with excellent hole expandability.   Furthermore, 0.001-5% of the total containing 1 type (s) or 2 types or more of Ni, Cu, Co, and W including Cr is contained by mass%, The hole expansion of Claim 1 or 2 characterized by the above-mentioned. High strength and high ductility hot dip galvanized steel sheet with excellent properties.   Furthermore, 0.0001-0.5% of 1 type (s) or 2 or more types of Y, Rem, Ca, Mg is contained by the mass%, The hole of any one of Claims 1-3 characterized by the above-mentioned. High strength and high ductility hot dip galvanized steel sheet with excellent spreadability.   The high strength and high ductility molten zinc excellent in hole expansibility according to any one of claims 1 to 4, wherein the plating phase on the surface of the steel sheet contains Fe: 5 to 20% by mass. Plated steel sheet.   The high strength and high ductility hot dip galvanizing excellent in hole expansibility according to any one of claims 1 to 5, wherein the plating phase on the surface of the steel sheet contains Fe: less than 5% by mass. steel sheet.   A method for producing the high-strength and high-ductility hot-dip galvanized steel sheet according to any one of claims 1 to 6, wherein a cast slab comprising the component according to any one of claims 1 to 4 is cast as it is. Or after cooling once, it heats again, finishes hot rolling at a finishing temperature at 850 ° C. to 970 ° C., and then cools to a temperature range of 550 ° C. or lower at an average of 10 to 100 ° C./s, and then winds at 550 ° C. or lower. The picked hot-rolled steel sheet is pickled and cold-rolled and annealed at a maximum annealing temperature of 0.1 × (Ac3−Ac1) + Ac1 (° C.) to 0.8 × (Ac3−Ac1) + Ac1 (° C.). After cooling to a temperature range of galvanizing bath temperature −20 ° C. to galvanizing bath temperature + 50 ° C. at an average cooling rate of 0.1 to 100 ° C./second, and subsequently including plating immersion in the same temperature range 1 Hole expansibility characterized by holding for 2 seconds to 1000 seconds and A method for producing hot-dip galvanized high-strength steel sheets with excellent ductility.   A method for producing the high-strength, high-ductility hot-dip galvanized steel sheet according to claim 7, wherein after the plating bath immersion and holding treatment, the alloying treatment is performed in a temperature range of 400 to 550 ° C and cooled to room temperature. A method for producing a high-strength, high-ductility hot-dip galvanized steel sheet with excellent processability.