MX2008008962A - Hot-dip zinc-coated steel sheets and process for production thereof - Google Patents

Abstract

The invention provides hot-dip zinc-coated steel sheets excellent in the balance between strength and ductility and in hardenability and a process for the production of the same. The sheets have a chemical composition which contains C:0.005 to 0.04%, Si:1.5%or below, Mn:1.0 to 2.0%, P:0.10%or below, S:0.03%or below, Al:0.01 to 0.1%, N:less than 0.008%, and Cr:0.2 to 1.0%and satisfies the relationship:2.1<Mn(mass%) + 1.29Cr(mass%)<2.8 with the balance being iron and unavoidable impurities, and a structure which consists of ferrite and 3.0 to less than 10%by volume of martensite and in which the mean grain diameter of the ferrite exceeds 6μm and is up to 15μm with at least 90%of the martensite existing in ferrite grain boundaries. In producing the hot-dip zinc-coated steel sheets, cold rolled steel sheets are annealed in the temperature range of Ac1 to Ac3.

Description

- - STEEL SHEETS COATED WITH ZINC BY HOT DIVING AND PROCEDURE FOR ITS PRODUCTION TECHNICAL FIELD The present invention relates to galvanized steel sheets that are applicable in fields including automobiles and home applications, which have favorable press conformability and which are excellent in terms of toughness-ductility balance and bake hardening ability as well as methods for producing said galvanized steel sheets.

BACKGROUND OF THE ART Recently, improvements in the fuel efficiency of automobiles has required a perspective of global environmental sustainability and an improvement in the safety of automobile bodies has also been a desired objective from the perspective of protection to people on board accidental damage. In order to satisfy these demands, a positive investigation has been carried out to reduce the weight of the car bodies along with the reinforcement thereof. It has been that increasing the tenacity of the materials of the components is effective to meet these demands, along with a reduction in weight of the bodies of the cars and their reinforcement. However, an increase in toughness often results in deterioration in the formability and therefore not only an improved toughness is needed but also an excellent press formability to produce steel sheets for automobiles that require complicated forming. In this way several approaches have been proposed to increase the tenacity of steel sheets and at the same time maintain the processing capacity thereof. In a representative approach, large amounts of solid solution reinforcing elements, Si and P, are added to interstitial free steel as the base material to obtain a tensile strength in the range of 340 to 490 MPa. For example, Patent Document 1 describes an example of methods for producing steel sheets with high tensile stress, with a tensile strength of 490 MPa degree by adding P in extra low carbon steel containing Ti. Also investigated as high conformability target of steel sheets together with high tenacity thereof were the double phase steel sheets, which include a second hard phase, such as martensite or bainite, in the ferrite structure. principal. For example, the patent document 2 describes a method for producing a steel sheet wherein the structure of the steel sheet consists of ferrite and a second phase, recovery of the processed ferrite structure is delayed by using the speed of heating at least 10 ° C / s for heating from 500 to 700 ° C during heating up to the annealing temperature, fine ferrite particles are measured from 2 to 6 μm in diameter are used to finely disperse the second hard phase so that it acts as the fracture starting points and in this way the steel sheet acquires a favorable tenacity-ductility balance of approximately 17,000 MPa *%, the product of toughness and ductility. In addition, Patent Documents 3 and 4 describe methods for producing a steel sheet wherein the structure of the steel sheet consists of ferrite and a second phase containing martensite, the rate of cooling after recrystallization is predetermined, the fraction of the second phase and the content ratio of martensite in the second phase is controlled and in this way the steel sheet acquires a toughness of 500 MPa or less and a favorable balance of toughness-ductility of approximately 17,000 MPa *%. In addition, when developed as sheets of Steel contain a favorable pressing formability together with a high tenacity postforming, the steel sheet with bake-hardening ability (hereinafter sometimes referred to as BH) are relatively soft and easily press-shaped into press-forming and then they can be hardened by a BH process to improve toughness as a component. These BH steel sheets are based on a hardening technique that uses natural aging during cold working that occurs in the presence of C and N dispersed in steel. For example, the patent document 5 discloses a steel sheet in which a solid dispersion C of approximately 30 ppm in ferrite structure is fixed to fix dislocations, whereby the hardening capacity by baking is increased. Additionally, the steel sheets described in the patent document 5 are usually used as external panels for automobiles. However, said steel sheets originally contain solid C in a small amount and therefore the BH is approximately in the range of 30 to 50 MPa maximum. In addition, the extra low carbon steel used as the base material makes it difficult to improve the toughness as a component at 440 MPa or higher. In response to this, research has been carried out on steel sheets of a double phase in which the martensitic transformation induces dislocations in the mother phase, ferrite and the solid C dispersed in ferrite fixes the dislocations, thus improving the BH. For example, the patent document 6 describes a method for producing a steel sheet, wherein the steel contains Mn, Cr and Mo so that the proportion of total content thereof (Mn + 1.29Cr + 3.29 Mo), a BH index, is in the range of 1.3 to 2.1%, the structure of the steel sheet containing at least 70% by volume fraction of ferrite and 1 to 15% by volume fraction of martensite and therefore the Steel sheet acquires tenacity in the range of 440 to 640 MPa and a BH equal to or greater than 60 MPa. Patent Document 1: Japanese Examined Patent Application Publication No. S57-57945 Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-235145 Patent Document 3: Publication of Japanese Unexamined Patent Application No. 2002-322537 Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-207237 Patent Document 5: Publication of Japanese Unexamined Patent Application No. S59-31827 Patent Document 6: Publication of Non-Patent Application No. examined Japanese No. 2006-233294 DESCRIPTION OF THE INVENTION However, the background of the technique described in the above have the following problems. For example, the techniques described in patent documents 1 and 5 involve solid solution hardening as an indispellable reinforcement mechanism to increase toughness. In the case where a tenacity is equal to or greater than 440 MPa, large amounts of Si and P must be added and, therefore, problems of deterioration over the surface characteristics such as alloy processing difficulties, red inlays become important. or electrolytic coating faults. Therefore, it is difficult to apply these techniques to external automotive panels that require rigorous control of surface quality. The technique described in Patent Document 2 uses ferrite particles with an average diameter that is in the range of 2 to 6 μm, although the reduction in diameter of each ferrite particle generates decreases in the n-value and uniform elongation. Thus, this technique can not be easily applied to external automotive panels formed mainly by stretch-forming, such as doors and awnings. The Patent documents 3 and 4 state that, in the techniques described herein, the primary cooling rate used in the production processes thereof for cooling from the annealing temperature to the electrolytic coating temperature is established in the range of 1 to 10 ° C / sec so that the ratio of martensite content in the second phase is improved and preferably is set in the range of 1 to 3 ° C / sec so that it reduces the volume fraction of the second phase to 10. % or less. However, for example, in the example where a primary cooling rate of 3 ° C / s is used from the annealing temperature of 800 ° C to the electrolytic coating temperature of 460 ° C, approximately 113 seconds are required for complete the cooling stage. This can alter productivity. In addition, the inventors actually cooled steel with Mn + 1.3Cr of 2.15 at a primary cooling rate of 3 ° C / sec according to the examples described in Patent Documents 3 and 4 (Sample 43, Examples, DESCRIPTION of the document Patent 3, Sample 29, Examples, DESCRIPTION of patent document 4) and evaluated the resulting microstructure. As a result, a pearlitic or vainitic transformation progressed during the cooling stage and it was difficult to obtain 90% or greater content proportion of martensite in the second phase consistently. This result indicates that steel sheets with an excellent balance of tenacity-ductility can not be easily obtained by using the components and production methods described in Patent Document 3 or 4 because it can decrease the ductility as a result of the generation of pearlite or bainite in the second phase. Regarding the techniques described in patent documents 2 to 4, the inventors in reality prepared materials GA 0.6 to 0.8 mmt for panels according to the examples thereof and carried out a test of pressing the materials in the door model. As a result, similar portions in the vicinity of the engraved areas, the formation of which is rather difficult, are fractured. In response to this, the representative characteristics of the materials are measured and then the TS is 443 MPa, El is 35.5% and TS El is 15727 MPa *%, which suggests that the balance between tenacity-ductility is not so good. This may be due to the fact that the thickness of the steel sheets used in the examples described in patent documents 2 to 4 is 1.2 mm and this large range probably contributes to the favorable balance between toughness and ductility. Therefore, the inventors verified this assumption using formula (2) derived from the formula of Oliver represented by the formula (1) (source: Puresu Seikei Nanni Handobukku (Textbook on difficulties in forming by pressing) 2nd Ed., P. 458, Usukouhan Seikei Gijutsu Kai), which is used virtually by skilled in the art to evaluate the ductility of thin steel sheets with different thicknesses. The =? (VA / LP (1) In formula (1),? And m are constants of material and in general, m for iron is 0.4, Parameter A represents the cross-sectional area and L represents the calibrated length Ela / Ela. = (t2 / t?) 0 2 (2) In formula (2), Elx and El2 represent the elongation (%) where the thickness of the sheet is ti (mm) and t2 (mm), respectively. verification, it is assumed that the thickness of the sheet is 0.75 mm, which is the thickness often used in the application of external panels for automobiles and the balance of tenacity-ductility and is not as good in any of the tested examples. More specifically, the example described in patent document 2 (sample 35, Example, DESCRIPTION) shows a TS of 446 MPa, El of 35.7% and TS El of 15922 MPa *%, the example described in the patent document 3 (sample 43, Example, DESCRIPTION) shows TS of 441 MPa, The of 35.6% and TS X The of 15700 MPa *% and the example described in the patent document 4 (sample 29, Example, DESCRIPTION) shows TS of 442 MPa, The one of 35.5 % and TS X The one of 15691) MPa *%. In addition, when considering the forming capacity by pressing, steel sheets having TS X equal to or greater than 16500 MPa *% can be used practically without any problem, and TS X is preferably 16500 MPa *% and more preferably 17000 MPa *%. Accordingly, it is difficult to apply the technique described in patent documents 2 to 4 to external automobile panels such as doors and awnings. Furthermore, in the technique described in the patent document 6, a second cooling rate is produced under the conditions wherein the cooling rate is 100 ° C / sec or above and the cooling stop temperature is 200 ° C or lower, in order to control the volume fraction of martensite and the amount of solid C dispersed in ferrite as well as to ensure a high BH. However, these cooling conditions can be satisfied only in an extraordinary method such as the jet in water described in the patent document 6, so that, as in practice, industrial processing using this technique is difficult. In addition, the patent document 6 describes only the conformability with reference to the results of a cylinder shaping test, omitting descriptions of parameters related to ductility such as total elongation, uniform elongation and local elongation. Therefore, steel sheets obtained using this technique may be insufficient in terms of toughness-ductility balance and therefore can not be easily applied to external automotive panels such as doors and awnings. The present invention is made in order to solve these problems and provide a galvanized steel sheet having a tensile strength in the range of 340 to 590 MPa, TS X El is equal to or greater than 16000 MPa *% considering the capacity of compression shaping and the elastic limit difference between a value measured after the application of 2% pre-tension and a value measured after hardening by subsequent baking by heating at 170 ° C for 20 minutes is equal to or greater than 50 MPa, in other words, a galvanized steel sheet that has a high conformability and is excellent in balance-tenacity-ductility and bake hardening capacity, as well as a method for the production thereof.

To solve the problems described above, the inventors focused on a double phase steel consisting of a ferrite phase and a martensite phase. As a result, the following findings were obtained. First, the reinforcement transformation is used as a reinforcement mechanism and the fraction in volume of the martensite phase is reduced as much as possible and in this way the toughness range of 340 to 590 MPa, which is difficult to obtain using steel Free interstitial as base material, is what you get. In addition, the ferrite particle diameter and the position of the martensite phase are controlled so as to increase the ferrite forming capacity and therefore uniform elongation is improved. In addition, the second phase is uniformly dispersed to improve local elongation and therefore a galvanized steel sheet having an excellent balance between toughness and ductility can be obtained. Additionally, the content ratio of Mn and Cr, an index of the bake hardening capacity is appropriately controlled so that a high BH is obtained. The present invention is based on these findings and is summarized as follows. [1] A galvanized steel sheet containing C, Si, Mn, P, S, Al, N and Cr and ratios in% by mass content from 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10 % or less, 0.03% or less, 0.01 to 0.1%, less than 0.008% and 0.2 to 1.0%, respectively, with Mn (% by mass) + 1.29Cr (% by mass) that is in the range of 2.1 to 2.8 and contains iron and unavoidable impurities like the rest, where the structure thereof consists of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and less than 10%, the average particle diameter of the ferrite is greater than 6 μm and not less than 15 μm and 90% or greater of the martensite phase exists in a ferrite grain boundary. [2] A galvanized steel sheet containing C, Yes, Mn, P, S, Al, N and Cr and ratios in% by mass content from 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10% or less, 0.03% or less, 0.01 at 0.1%, less than 0.008% and 0.2 to 1.0%, respectively, with Mn (% by mass) + 1.29Cr (% by mass) which is in the range of 2.2 to 2.8 and contains iron and unavoidable impurities like the rest, wherein the structure thereof consists of a ferrite phase and a martensite phase with a volume fraction which is at least 3.0% and less than 10%, the average particle diameter of the ferrite is greater than 6 μm and is not greater than 15 μm and 90% or greater of the martensite phase exists in a ferrite grain boundary. [3] A galvanized steel sheet containing C, Si, Mn, P, S, Al, N and Cr and ratios in% by mass content from 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10 % or less, 0.03% or less, 0.01 to 0.1%, less than 0.008% and 0.2 to 1.0%, respectively, where Mn (% by mass) + 1.29Cr (% by mass) that is in the range of 2.3 to 2.8 and contains iron and unavoidable impurities like the rest, where the structure of it consists of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and less than 10%, the particle diameter Ferrite average is greater than 6 μm and not more than 15 μm and 90% higher than the martensite phase exists in a ferrite grain limit. [4] A galvanized steel sheet containing C, Si, Mn, P, S, Al, N and Cr in ratios in% by mass content from 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10 % or less, 0.03% or less, 0.01 to 0.1%, less than 0.008% and 0.35 to 0.8%, respectively, with Mn (% by mass) + 1.29Cr (% by mass) that is in the range of 2.3 to 2.8 and contains iron and unavoidable impurities like the rest, where the structure thereof consists of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and less than 10%, the particle diameter The average ferrite is greater than 6 μm and not more than 15 μm and 90% or greater of the martensite phase exists at a ferrite grain limit. [5] The galvanized steel sheet according to any of the items [1] to [4] as described above, which also contains one or more of Mo, V, B, Ti and Nb in proportions of the content in % mass of 0.5% or less, 0.5% or less, 0.01% or less, 0.1% or less and 0.1% or less, respectively. [6] The galvanized steel sheet according to any of the items [1] to [5] described in the above, wherein zinc is used to electrolytically coat the steel sheet that is alloyed. [7] A method for producing a galvanized steel sheet that includes a step of melting steel having the chemical composition described in any of [1] to [5] above, subsequent hot and cooled rolling steps and a step of annealing, the steel sheet obtained at an annealing temperature is at least at the Acl point and not greater than the Ac3 point. [8] A method for producing a galvanized steel sheet that includes a cold rolling stage for rolling and a hot rolled steel sheet having the chemical composition described in any of the indents [1] to [5] above and which also contains a transformation phase at low temperature in a volume fraction of 60% or greater and an annealing step of the steel sheet obtained at an annealing temperature which is therefore minus the Acl point and not greater than the Ac3 point. [9] The method for producing a galvanized steel sheet according to subsection [7] or [8] described above, wherein zinc is used to electrolytically coat the steel sheet that is alloyed after galvanization. In addition, the percentages represent components contained in steel in this description are all mass percentages. The present invention provides an excellent galvanized steel sheet in tenacity-ductility balance and bake hardening ability by proper control of the content ratio of Mn and Cr, the average ferrite particle diameter and the position, distribution profile and fraction in volume of a martensite phase. Furthermore, the galvanized steel sheets according to the present invention have such excellent characteristics and are applicable in fields of home electrical appliances, steel sheets for automobiles and other uses, and therefore are beneficial for the industry.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram showing the relationship between the content of Mn and Cr and TS X El. Figure 2 is a diagram showing the relationship between the content ratio of Mn and Cr and the capacity of bake hardening (BH).

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is explained in detail in the following. First, the reason why the chemical composition of steel is limited as described in the above in the present invention is established. C: 0.005 to 0.04% In the present invention, C is one of the very important elements and is highly effective to form the martensite phase to increase the toughness. However, a content of C that exceeds 0.04% can generate a significant deterioration in the conformation capacity and decrease the susceptibility to welding. Therefore, the content of C should not exceed 0.04%. On the other hand, the martensite phase is required to take into consideration at least a fraction by volume necessary to ensure toughness and a high BH, and by - - so C must be limited to some extent. Accordingly, the content of C should be at least 0.005% and preferably greater than 0.010%. Yes: 1.5% or less If it is an effective element in increasing tenacity and consistently produces a composite structure. However, a content of Si that exceeds 1.5% can generate significant deterioration in surface characteristics and susceptibility to phosphating. Therefore, the content of Si must be 1.5% or less and preferably 1.0% or less. Mn: 1.0 to 2.0% Mn is one of the important elements used in the present invention. Mn has a very important role in the formation of a martensite phase and an ability to improve BH and acts to prevent blocks from breaking during the hot rolling stage due to the brittle effect in the grain limit of SI fixing the S contained in steel in the form of MnS. Therefore, the content of Mn must be at least 1.0%. However, an Mn content exceeding 2.0% can generate significant increases in the costs for plates and adding a large amount of Mn can promote the formation of band-like structures and in this way the susceptibility to formation is impaired. Therefore, the • - proportion in Mn content should not exceed 2.0%. P: 0.10% or less P is an effective element in increasing tenacity. However, a content of P that exceeds 0.10% can generate decreases in the alloy rate of a zinc coating layer and thus cause an insufficient electrolytic coating or a failure of the electrolytic coating and resistance to a brittle condition of secondary work of a steel sheet. Thus, the content of P must not exceed 0.1%. S: 0.03% or lower S deteriorates the susceptibility to hot forming and increases the susceptibility of the plates to fracture due to heating and the fine precipitation of MnS that is formed when the proportion in S content exceeds 0.03% degrades the susceptibility to conformation. Therefore, the content of S should not exceed 0.03%. Al: 0.01 to 0.1% Al is a deoxidizing element that has the effect of removing inclusions in the steel. However, Al content in a content less than 0.01% can not consistently provide this effect. On the other hand, an Al content exceeding 0.1% may result in an increased amount of alumina inclusion groups, which affects the susceptibility to training. Consequently, the content of Al should be in the range of 0.01 to 0.1%. N: less than 0.008% To improve the susceptibility to processing and aging characteristics, the lower the content of N is better. A content ratio of N that is equal to or greater than 0.008% may result in the formation of excessive amounts of nitrides, thereby degrading ductility and toughness. Therefore, the content of N must be less than 0.008%. Cr: 0.2 to 1.0% Cr is one of the important elements used in the present invention. Cr is an element that improves BH and is added to form a stable martensite phase. It improves the BH more effectively than Mn and helps the martensite phase to exist in a grain boundary and is therefore an advantageous element for structure formation according to the present invention. Furthermore, in the present invention, Cr is an indispensable element since it reinforces the solid solutions in only a slight degree and is suitable for low tenacity DP steel and therefore is added in a content of 0.2% or higher, preferably 0.35% or greater and more preferably more than 0.5% so that these advantageous effects are obtained. Do not However, a Cr content exceeding 1.0% may result not only in the saturation of said advantageous effects but also in the deterioration of the ductility due to the formation of carbides. Accordingly, the content ratio of Cr should be in the range of 0.2 to 1.0% and preferably 0.35 to 0.8% to ensure sufficient toughness and ductility.

Content ratio of Mn and Cr: Mn (% by mass) + 1.29Cr (% by mass) in the range of 2.1 to 2.8 Mn and Cr are elements that improve the BH, and this is extremely important to control them in the proportions of optimal content for the formation of a martensite phase. A ratio of total Mn content in Cr is less than 2.1% and can result in difficulties in the formation of a DP structure and makes it impossible to obtain the desired BH, thereby generating a decrease in toughness as a component. In addition, an increased yield ratio makes it difficult to carry out the pressing step and causes a defective shape. In addition, pearlite and bainite are likely to form in a cooling step after an annealing and crystallization step whereby BH is reduced. On the other hand, the ratio in total weighted content of Mn and Cr that exceeds 2.8% may result in not only the - - saturation of the advantageous effects described above but also decreases the formability due to the residual martensite in ferrite particles which increases with the increase in the volume fraction of martensite. Furthermore, the increases in the point of plastic deformation associated with the increase in toughness also significantly reduce the susceptibility to press-forming and cause an increase in manufacturing costs due to the need for addition of excessive amounts of alloying elements. Consequently, the proportion of weighted content of Mn and Cr, Mn + 1.29Cr, should be in the range of 2.1 to 2.8%. To obtain a high BH, the lower limit thereof is preferably 2.2% and more preferably 2.3%. In addition, to ensure conformation capacity, the upper limit thereof is preferably 2.6%. The essential elements mentioned in the above provide steel according to the present invention with desired characteristics, but one or more of the following elements can be added in addition to the essential elements mentioned above, as needed: Mo (0.5% or less), V (0.5% or less), B (0.01% or less), Ti (0.1% or less) and Nb (0.1% or less). Mo: 0.5% or less, V: 0.5% or less • - Mo and V are elements that each improves the BH and can be added to form a stable martensite phase. However, the content of Mo or V that exceeds 0.5% each can reduce the ductility and increase the cost. Therefore, the content of Mo or V must not exceed 0.5% each, if applicable. B: 0.01% or less B is an effective element in improving BH and can be added to form a stable martensite phase. However, a proportion of B content that exceeds 0.01% may not provide an effect that takes into account the cost. Therefore, the content of B should not exceed 0.01%, if applicable. Ti: 0.1% or less, Nb: 0.1% or less Ti and Nb are elements that effectively improve the characteristics of deep stretching by decreasing the amounts of dispersed solids of C and N through the formation of carbonitrides. However, the Ti or Nb content exceeding 0.1% each may result in the saturation of said advantageous effect and the increase of the recrystallization temperature for annealing so that the productivity deteriorates. Therefore, the content of Ti or Nb must not exceed 0.1% each, if applicable. In addition, the chemical components excluding the elements described in the above are Fe and impurities inevitable As an example of such unavoidable impurities, O forms non-metallic inclusions that affect the quality of the product so that it is preferably removed so that it constitutes a content of 0.003% or less. Next, the structure of the galvanized steel sheet according to the present invention is described in the following. The galvanized steel sheet according to the present invention consists of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and less than 10%, the average particle diameter of the ferrite is higher of 6 μm and not more than 15 μm and 90% or more of the martensite phase exists in a ferrite grain limit. These are essential requirements of the present invention and a structure that satisfies these requirements will provide a galvanized steel sheet with excellent tenacity-ductility balance according to the present invention.

Fraction in volume of the martensite phase: at least 3.0% and less than 10% A two-phase structure consisting of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and lower from 10% constitutes the galvanized steel sheet according to the present invention. A fraction by volume of the martensite phase that is 10% or greater can build a steel sheet for automotive exterior panels, a designed product of the present invention, insufficient in its susceptibility to press forming. Therefore, the volume fraction of the martensite phase should not exceed 10% and, to ensure a sufficient conformability susceptibility, the volume fraction of the martensite phase is preferably less than 8%. On the other hand, a volume fraction of the martensite phase that is less than 3.0 can cause mobile dislocation density, introduced with transformation, which is insufficient to thereby reduce BH and reduce nicking resistance. In addition, it increases YP and returns to YPEl more likely to remain, so that the susceptibility of shaping by pressing and the surface regularity of the obtained panels decreases, respectively. In addition, the volume fraction of the martensite phase can be at least 3.0%. In addition, the steel sheet according to the present invention may contain a perlite phase, a bainite phase, a phase and residual and unavoidable carbides to a maximum degree of about 3% in addition to the two phases mentioned above, the phases of ferrite and of martensite. However, a pearlite or bainite phase formed near the martensite phase can often provide the origins of voids and promote the growth of said voids. Therefore, to ensure sufficient conformation susceptibility, such perlite phase, bainite phase, phase and residual phase and inevitable carbides are preferably contained in less than 1.5% and more preferably in 1.0% or less.

Average ferrite particle diameter: greater than 6 μm and not larger than 15 μm The smaller the particle diameters of the crystals, the smaller the n-value and the uniform elongation that contributes to the susceptibility to stretch-forming. In the case where the average ferrite particle diameter is 6 μm or less, the decrease in the n-value and the uniform elongation are more significant. However, an average ferrite particle diameter exceeding 15 μm can cause surface roughness to be introduced during the press forming step and to deteriorate the surface characteristics and is therefore not recommended. Consequently, the average ferrite particle diameter should be greater than 6 μm and should not exceed 15 μm.

Position of the martensite phase: 90% at the ferrite grain boundary The position of the martensite phase is a very important factor of the present invention and is an essential requirement of the advantageous effects of the present invention. A phase of martensite that exists in a ferrite particle reduces the susceptibility to deformation of the ferrite and a percentage of said martensite phase in a ferrite particle that is 10% or greater can make this tendency stronger. Therefore, to obtain an excellent balance between toughness and ductility designed by the present invention, 90% or more of the martensite phase must be at the ferrite grain boundary. In addition, to further improve the balance between toughness-ductility, it is preferable that 95% or more of the martensite phase exists at the ferrite grain boundary. Next, the manufacturing conditions for the galvanized steel sheet according to the present invention which is excellent in balance of tenacity-ductility and BH are explained. The galvanized steel sheet according to the present invention is produced by casting steel of the content proportions whose chemical components are adjusted so that they are within the ranges described in the above, hot-rolled steel and subsequent cold-rolling steps and annealing of the steel sheet obtained at an annealing temperature that is at least the Acl point and not greater than the Ac3 point. In the cold rolling step, the hot-rolled steel sheet preferably contains a low temperature transformation phase in a volume fraction of 60% or greater. Moreover, it is more preferable that, during a galvanizing step after the annealing step, the galvanized steel sheet according to the present invention is subjected to annealing by recrystallization at an annealing temperature which is at least the Acl point and not greater than the point Ac3, primary cooling from the annealing temperature to a galvanization temperature with an average cooling speed exceeding 3 ° C / s and that is no higher than 15 ° C / s, and then secondary cooling with a Average cooling speed that is not less than 5 ° C / s. The alloy stage of the electrolytic coating can be added after the galvanization step. Said galvanizing process of the annealed steel sheets can be carried out using a continuous galvanization line. The preferred conditions and manufacturing conditions of the structure of the laminated steel sheet in hot are described in detail in the following.

Structure of a hot rolled steel sheet: low temperature transformation phase having a volume fraction of 60% or more (preferred range) In the procedure mentioned above, the hot rolled steel sheet obtained in the stage of hot rolling preferably has a structure that contains a low temperature transformation phase in a volume fraction of 60% or greater. A known hot rolled steel sheet having a structure consisting of ferrite and perlite phases is likely to retain insoluble carbides while annealing the biphasic regions a and y. This problem and the irregular distribution of the perlite phase in the hot rolled steel sheet result in an irregular distribution of the large phases. As a result a structure is formed consisting of rather large and irregularly distributed martensite phases. On the other hand, in the case of a hot rolled steel sheet containing a low temperature transformation phase in a volume fraction of 60% or greater, such as the hot rolled steel sheet according to the present invention , fine carbides are dissolved once in a ferrite phase during the heating stage of an annealing step and then uniform and fine phases are generated from the ferrite grain boundary while the biphasic + y regions are annealed. As a result, a uniform distribution of the martensite phase in the ferrite grain boundary is obtained, which is designed by the present invention, and the local elongation is improved. In addition, said low temperature transformation phase contained in the hot-rolled steel sheet is an acicular ferrite phase, a bainite ferrite phase, a bainite phase, a martensite phase or a mixed phase thereof. Meanwhile, a hot-rolled steel sheet containing a low temperature transformation phase in a volume fraction of 60% or greater can be obtained by suppressing the ferrite transformation or growth that occurs after the step of laminate finished. For example, it can be obtained by cooling the steel sheet at a cooling rate of 50 ° C / sec or more after a finishing rolling step to suppress the ferrite transformation and then capture the ferrite sheet at a temperature of 600 ° C or less. More preferably, the collection temperature is less than 550 ° C.

Heating rate: less than 10 ° C / s for the temperature range from the transformation point Acl, -50 ° C to annealing temperature (preferred range) The heating rate for annealing by recrystallization is not particularly limited. However, in order to facilitate the production of the steel sheet structure (with the preferred average ferrite particle diameter and the preferred position of the martensite phase) proposed by the present invention, it is preferable that the recrystallization be completely completed before that the temperature exceeds the Acl transformation point. Therefore, for example, the heating rate for the temperature range from the Acl transformation point, -50 ° C to the annealing temperature is preferably 10 ° C / s. In addition, at temperatures lower than this temperature range, the heating rate does not always need to be less than 10 ° C / s and may be much higher. Of course, a hot rolled steel sheet containing a low temperature transformation phase in a volume fraction of 60% or greater must provide the structure according to the present invention more efficiently.

Annealing temperature: at least the Acl point and not greater than the Ac3 point. To obtain a consistent microstructure of ferrite and martensite phases, the annealing temperature should be suitably high. If the annealing temperature is less than the Acl point, the austenite phase is not formed and consequently the martensite phase is not formed. In such a situation, the ferrite particle diameter is too small so that the forming capacity by pressing is reduced in association with decreases in the n-value and uniform elongation. On the other hand, an annealing temperature that exceeds the Ac3 point may result in the austenite phase of the ferrite phase, so that characteristics such as the forming capacity obtained by recrystallization deteriorate. The ferrite particle diameter is too large in this situation that the surface characteristics also get worse. In addition, C is contained in a low content ratio in the steel according to the present invention so that annealing at a high temperature can result in insufficient concentration of C in the? Phase. This makes it difficult to form a DP structure and consequently reduces tenacity and BH. Furthermore, even if a DP structure is formed by increasing the extinguishing characteristic at a sufficient level a large amount of martensite precipitates in the particles and hence the ductility is deteriorated. Consequently, the annealing temperature should be at least the Acl point and must not exceed the Ac3 point. To ensure sufficient shaping capacity, the annealing temperature is preferably at least the Acl point and not higher than the temperature 100 ° C higher than the Acl point. Regarding the annealing time, to obtain a favorable average ferrite particle diameter and promote the concentration of the constituent elements in an austenite phase, the duration thereof is preferably at least 15 seconds and shorter than 60 seconds. In addition, the Acl and Ac3 points can be determined by actual or calculated measurement using the following formula (nLeslei Tekkou Zairyou Gaku "(The Physical Metallurgy of Steels), P. 273, MARUZEN Co., Ltd.): Acl = 723 - 10.7 Mn + 29.1 Si + 16.9 Cr Ac3 = 910 - 203 CA 0.5 + 44.7 Si + 104 V + 31.5 Mo - 30 Mn - 11 Cr + 700 P + 400 Al + 400 Ti Primary cooling speed: greater than 3 ° C / s no higher than 15 ° C / s (preferred range) In the production process of the galvanized steel sheet, the primary cooling rate for cooling from the annealing temperature to the galvanizing temperature is not particularly limited. To form martensite, the average cooling rate is preferably greater than 3 ° C / s and not higher than 15 ° C / sec. exceeding 3 ° C / sec can prevent the austenite from being transformed into perlite in the cooling step, thereby helping to form the martensite phase designed by the present invention. This improves the balance between tenacity and ductility and BH. On the other hand, the cooling rate is preferably 15 ° C / sec or less because in this range the steel sheet structure designed by the present invention can be formed in a consistent manner extending both in the lateral direction and in the direction Longitudinal (direction of sliding) of a steel sheet. Therefore, the average cooling rate for cooling from the annealing temperature to the galvanization temperature is preferably greater than 3 ° C / s and not higher than 15 ° C / s, and the most efficient average cooling rate is in the range of 5 to 15 ° C / s. In addition, the galvanization temperature is in the normal range, i.e., approximately in the range of 400 to 480 ° C. Secondary cooling speed: 5 ° C / s or greater (preferred range) The secondary cooling rate after the electroplating step or the additional alloying step of the non-electrolytic coating is not particularly limited. However, the cooling speed is 5 ° C / s or higher should prevent the austenite is transformed into pearlite or other phases, and in this way helps the martensite phase to form. Therefore, the secondary cooling rate is preferably 5 ° C / s or greater. On the other hand, the upper limit of the second cooling rate is not particularly limited as well, although preferably it is less than 100 ° C / sec for such purposes as to avoid deformation of the steel sheets. In addition, the electrolytic coating layer is allowed to continuously warm up typically at a temperature in the range of approximately 500 to 700 ° C and preferably in the range of 550 to 600 ° C from a few seconds to several tens of seconds. The conditions not described in the above are the following. The method for melting steel is not particularly limited and examples of such a method may include an electric furnace, a converter or the like. In addition, a method for casting molten steel can be continuous casting to form cast plates or cast ingots to form steel ingots. The continuously emptied plates can be reheated using a heating furnace before being hot rolled or sent directly to the hot rolling stage. Steel ingots can be rolled thick before they are hot rolled. The finished temperature Hot rolling is preferably the point Ar3 or higher. The cold rolling ratio is in the range of 50 to 85% of the value used in normal operations. Regarding the galvanization conditions, the electrolytic coating weight preferably is in the range of 20 to 70 g / m2 and Fe% in an electrolytic coating layer preferably is in the range of 6 to 15%. In addition, the present invention can include the step of having the steel sheets laminated and tempered according to the present invention to convert the steel sheets after the heat treatment step. Further, in the present invention, it is intended that the steel materials be subjected to customary steel processing, casting or hot rolling steps to produce steel sheets. However, the hot rolling step can be omitted partially or completely, for example with the use of thin plate casting. Of course, the electrogalvanization of steel sheets obtained in the aforementioned process also provides proposed advantageous effects. Said electrogalvanized steel sheets can be subsequently coated with an organic layer.

- - EXAMPLES The present invention is described in greater detail in the following with reference to the examples. Each of the steels A to Y have a different chemical composition that is included in table 1 and are melted by vacuum melting and then formed into plates by continuous casting. Steels A to S are examples of the present invention. As comparative examples, each of the steels T and U has a C content that is outside the range according to the present invention, each of steels V, X and Y has a content ratio of Mn and Cr which is outside the range according to the present invention and the steel has a content of Mn and Cr, each outside the range according to the present invention. Each of the plates obtained in the stages mentioned above is heated to 1200 ° C, subjected to finished rolling at a temperature equal to or greater than point Ar3, cooled in water and then captured at a temperature exceeding at 500 ° C and that is less than 650 ° C. In this manner, hot-rolled steel sheets having volume fractions of a low temperature transformation phase in the range of 5 to 100% are produced.

Each of these hot-rolled steel sheets is treated with acid and then subjected to cold rolling at a lamination ratio of 75% so that cold-rolled steel sheets are obtained each with a thickness of 0.75 mm. In an infrared oven, samples cut from these cold-rolled steel sheets are each heated from the Acl transformation point, -50 ° C to the annealing temperature at a heating rate in the range of 5 to 20 ° C / sec. as shown in table 2, maintaining the annealing temperature indicated in table 2 for 30 seconds, cooled to a primary cooling speed in the range of 3 to 20 ° C / s and then galvanized in an electrolytic coating bath adjusted at 460 ° C. Subsequently, each of the samples is alloyed at 550 ° C for 15 seconds and then cooled to a secondary cooling rate in the range of 4 to 20 ° C / s. In this way, galvanized and alloy steel sheets are obtained. Samples are then taken from these galvanized and alloyed steel sheets. These samples are evaluated to determine the average ferrite particle diameter, the volume fraction of a martensite phase, the volume fraction of a second phase that excludes the The martensite phase and the percentage of the martensite phase in the grain limit as well as the mechanical properties and the BH thereof are measured as performance characteristics. Each sample is cut in the thickness direction in the middle part of it and then, according to the method described in JIS G 0552, the average ferrite particle diameter of each sample is measured using an optical microscope image (with a magnitude of 400) showing the structure of the section. The section of each cut sample is polished and corroded with nital and then the volume fraction of a martensite phase, the volume fraction of a second phase that excludes the martensite phase and the percentage of the martensite phase in the Grain limit is measured using an SEM image (scanning electron microscope) of the micro structure of the section. It should be noted that, in these measurement steps, the fields within the central area of the section, each with a size of 100 μm in length and 200 μm in width, generate an image continuously with a magnitude of 2000 and then the values Average of the parameters mentioned above are calculated from the obtained images. Regarding the mechanical properties, the YP (plastic deformation point), TS (resistance to traction), T-El (total elongation), U-El (uniform elongation) and L-El (local elongation) of the JIS-5 test pieces taken from the samples are measured in a stress test according to the test method specified in JIS Z 2241. The BH of each sample is also measured using the JIS-5 test pieces taken from the samples according to the method specified in JIS G 3135, where the increase in the deformation point plastic is measured as the BH of the tensile test performed after the application of a 2% pretension and subsequent heating at 170 ° C for 20 minutes. In the present invention, TS X El should be 16000 MPa *% or higher and preferably 16500 MPa *% or higher and more preferably 17000 MPa *% or higher. On the other hand, the BH must be 50 MPa or higher, and preferably it is 55 MPa or higher and more preferably 60 MPa or higher. This lower limit of BH is the value necessary to obtain the notch resistance required in the steel sheet manufacturing process for the thinner and lighter automotive exterior panels. The results of the tests mentioned above and the manufacturing conditions used are included in table 2.

- - In Table 2, samples 1, 4, 5, 7, a 13, 15, 17 to 35, 37 and 38 are examples of the present invention, each of which has the chemical composition and manufacturing conditions of according to the present invention and has a structure wherein the volume fraction of the martensite phase is at least 3.0% and less than 10%, the average ferrite particle diameter exceeds 6 μm and is not more than 15 μm and 90% or greater of the martensite phase in the ferrite grain boundary. These examples of the present invention show that TS X El of at least 16000 MPa *% and BH of at least 50 MPa, by which it is demonstrated that the galvanized steel sheets obtained are excellent in the balance of tenacity- ductility and the BH. On the other hand, as comparative examples, each of the samples 39 and 40 has the content of C that deviates from its ranges according to the present invention, each of the samples 41, 43 and 44 has the proportion in content of Mn and Cr which depart from their range according to the present invention and sample 42 have the contents of Mn and Cr, each departing from the range according to the present invention. In addition, each of the other comparative examples, samples 2, 3, 6, 14, 16 and 36 are annealed at a temperature which deviates from the annealing temperature range of according to the present invention and in these samples, at least one of the volume fraction of the martensite phase, the average ferrite particle diameter and the percentage of the martensite phase in the ferrite grain boundary are outside the range corresponding according to the present invention. Each comparative example presents TS X El sub-standards and BH values and therefore these comparative examples are considered insufficient in their information capacity by pressing and difficulty to elaborate in a thinner way than the existing steel sheets. In addition, the comparison between the examples of the present invention having the same chemical composition and different structures of the hot rolled sheet, ie the comparison between samples 1 and 4, 5 and 7, 10 and 11 and between the samples 25 to 27, suggests that samples 1, 5, 7, 10, 25 and 26 in which the content ratio of the low temperature transformation phase in the structure of the hot rolled steel sheet is in the preferred range , 60% or higher, is better in terms of tenacity-ductility balance compared to samples 4, 11 and 27. Furthermore, under the same chemical composition, comparisons were made between samples 5 and 9 and 10 and 12 heated at speeds of different heating, comparison between samples 5 and 8 and 32 and 35 annealed at different temperatures, comparison between samples 32 to 34 cooled at different primary cooling rates and comparison between samples 25, 28 and 29 cooled at different secondary cooling rates. As a result, samples 7 and 10, each heated at a heating rate in the preferred range, less than 10% C / s, samples 5 and 32, each annealed at a temperature in the preferred range not greater than 100. ° C higher than the Aci point, the sample 32 cooled at a primary cooling rate in the preferred range greater than 3 ° C / s and not higher than 15 ° C / s, samples 25 and 29, each cooled at a rate of secondary cooling in the preferred range, 5 ° C / sec or better, were better in terms of tenacity-ductility equilibrium than samples 9, 12, 8, 35, 33, 34 and 28. Excluding samples 39 and 40 whose content of C deviates from the range according to the present invention, Figure 1 shows the summary of the relationship between the content ratios of Mn and Cr and the TS x El values for samples 1, 5, 10, 13, 15, 17 to 25, 30 to 32, 37, 38 and 41 to 44 based on the results that are included in table 2. These examples those of the present invention and comparative examples each have a transformation phase at low temperature in the structure of the hot-rolled steel sheet in a percentage of 100% and contain Mn and Cr in different content proportions and heating temperature, annealing temperature, primary cooling speed and secondary cooling rate of these samples are in the preferred ranges of agreement with the present invention. As seen in the figure, TS X El is greater than 16000 MPa *% for all examples | of the present invention and greater than 16500 MPa *% for the examples under the preferred conditions, ie the examples containing Mn and Cr in a proportion in content in the range of 2.2 to 2.6%, confirming the favorable balance of tenacity- ductility. This drawing also shows that the examples under the most preferred conditions, ie, the samples in which the Cr content is in the range of 0.35 to 0.8% and the content ratio of Mn and Cr is within the range of 2.3 to 2.6%, have TS X The one that is at 17000 MPa *% or higher, so it is suggested that these conditions result in a more favorable balance between toughness-ductility compared to the other conditions. Figure 2 shows the summary of the relationship between the content ratio of Mn and Cr and BH of the steel samples mentioned above. As is evident in the Figure 2, the BH is greater than 50 MPa in the examples of the present invention under the condition wherein the content ratio of Mn and Cr is 2.1% or greater, greater than 55 MPa in some of the examples under the condition in where the content ratio of Mn and Cr is 2.2% or higher and 60 MPa or higher in some of the examples under the condition where the content ratio of Mn and Cr is 2.3% or higher. This suggests that BH is also favorable.

INDUSTRIAL APPLICABILITY Galvanized steel sheets according to the present invention are excellent in their balance of tenacity-ductility and BH and therefore can be used as components having a high conformability and are therefore used properly in production of internal and external panels for automobiles and other applications that require high conformation capacity. In addition, internal and external automotive panels using galvanized steel sheets according to the present invention can be made thinner and lighter using known steel sheets.

Table 1 I I fifteen fifteen co fifteen I fifteen

Claims (9)

1. Galvanized steel sheet comprising C, Si, Mn, P, S, Al, N and Cr in proportions of mass% content of 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10% or less, 0.03% or less, 0.01 to 0.1%, less than 0.008% and 0.2 to 1.0%, respectively with Mn (% by mass) + 1.29 Cr (% by mass) that is in the range of 2.1 to 2.8 and that also comprises iron and unavoidable impurities like the rest, where the structure of the galvanized steel sheet consists of a ferrite phase and a martensite phase with a volume fraction that is at least 3.0% and less than 10%, the particle diameter The average ferrite is greater than 6 μm and not more than 15 μm and 90% or more of the martensite phase exists at a ferrite grain boundary.

2. Galvanized steel sheet comprising C, Si, Mn, P, S, Al, N and Cr in proportions of% mass content of 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%, less than 0.008% and 0.2 to 1.0%, respectively with Mn (% by mass) + 1.29 Cr (% by mass) that is in the range of 2.2 to 2.8 and that comprises plus iron and inevitable impurities like the rest, where the structure of the galvanized steel sheet consists of a ferrite phase and a martensite phase • - with a volume fraction that is at least 3.0% and less than 10%, the average particle diameter of the ferrite is greater than 6 μm and not more than 15 μm and 90% or more of the martensite phase exists in a limit of ferrite grain.

3. Galvanized steel sheet comprising C, Si, Mn, P, S, Al, N and Cr in proportions of% mass content of 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10% or lower, 0.03% or lower, 0.01 to 0.1%, less than 0.008% and 0.2 to 1.0%, respectively with Mn (% by mass) + 1.29 Cr (% by mass) which is in the range of 2.3 to 2.8 and which comprises besides iron and unavoidable impurities like the rest, where the structure of the galvanized steel sheet consists of a phase of ferrite and a phase of martensite with a fraction in volume that is at least 3.0% and less than 10%, the diameter The mean particle size of the ferrite is greater than 6 μm and not more than 15 μm and 90% or more of the martensite phase exists in a ferrite grain boundary.

4. Galvanized steel sheet comprising C, Yes, Mn, P, S, Al, N and Cr in content proportions in% by mass from 0.005 to 0.04%, 1.5% or less, 1.0 to 2.0%, 0.10% or less, 0.03% or less, 0.01 to 0.1 %, less than 0.008% and 0.35 to 0.8%, respectively with Mn (% by mass) + 1.29 Cr (% by mass) which is in the range of 2.3 to 2.8 and which comprises besides iron and unavoidable impurities like the rest, where the structure of the galvanized steel sheet consists of a phase of ferrite and a phase of martensite with a fraction in volume that is at least 3.0% and less than 10%, the diameter The mean particle size of the ferrite is greater than 6 μm and not more than 15 μm and 90% or more of the martensite phase exists in a ferrite grain boundary.

5. Galvanized steel sheet as described in any of claims 1 to 4, further comprising one or more of Mo, V, B, Ti and Nb in proportions in mass% content of 0.5% or less, 0.5% or less, 0.01% or less, 0.1% or less and 0.1% or less, respectively.

6. Galvanized steel sheet as described in any of claims 1 to 5, wherein the zinc is used to coat the steel sheet is alloyed.

7. Method for producing a galvanized steel sheet, comprising a step of melting the steel having the chemical composition described in any of claims 1 to 5, subsequent hot and cold rolling steps and an annealing step and obtains the steel sheet at an annealing temperature that is at least the Acl point and not greater than the Ac3 point.

8. Method to produce a steel sheet galvanized, comprising a step of cold rolling for rolling a hot-rolled steel sheet having the chemical composition described in any of claims 1 to 5, and which further contains a transformation phase at low temperature in a volume fraction of 60% or higher and an annealing step of the steel sheet obtained at an annealing temperature that is at least the Acl point and not higher than the Ac3 point.

9. Method for producing a galvanized steel sheet as described in claim 7 or 8, wherein the zinc used to coat the steel sheet is alloyed after the galvanization.