Abstract:
A galvannealed steel sheet contains, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfies the relations, Si +Al &gt;0.6% and Cr +V +Mo =0.1 to 2%, the balance being Fe and inevitable impurities.

Description:
RELATED APPLICATION 
       [0001]    This is a §371 of International Application No. PCT/JP2006/307406, with an international filing date of Mar. 31, 2006 (WO 2006/104282 A1published Oct. 5, 2006), which is based on Japanese Patent Application Nos. 2005-103832, filed Mar. 31, 2005, and 2006-058458, filed Mar. 3, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to a low-yield-ratio, high-strength galvannealed steel sheet used in application as an automobile steel sheet and a method for producing the same. 
       BACKGROUND 
       [0003]    In recent years, from the viewpoint of conservation of the global environment, improvement in mileage of automobiles has become an important problem. Therefore, attempts have been made actively to increase the strength of car body materials to thin the materials and decrease the weights of car bodies. However, increases in strength of steel sheets decrease ductility, i.e., decrease workability, and thus there have been desired materials having both high strength and high workability. 
         [0004]    For such a requirement, there have been developed various composite structure steels such as ferrite-martensite dual phase steel (Dual-Phase steel) and TRIP steel (transformation induced plasticity steel) using transformation-induced plasticity of residual austenite. 
         [0005]    In some cases, the surfaces of these steel sheets are galvanized for improving rust proofness in practical use. As such galvanized steel sheets, galvannealed steel sheets subjected to heat treatment for diffusing Fe of the steel sheets into plating layers after hot-dip galvanization are widely used from the viewpoint of securing press property, spot weldability, coating adhesion. With respect to the galvannealed steel sheets, various proposals have been made. 
         [0006]    For example, in Japanese Unexamined Patent Application Publication No. 11-279691, there has been proposed a galvannealed steel sheet with excellent workability in which residual y is secured by adding a large amount of Si to the steel sheet, thereby achieving high ductility. However, Si decreases the plating properties, and thus a complicated process of Ni pre-plating, applying a special chemical, reducing an oxide layer on the surface of a steel sheet, and/or appropriately controlling the thickness of an oxide layer is required for adhesion of a zinc plating to such high-Si steel. 
         [0007]    In Japanese Unexamined Patent Application Publication No. 2002-030403, there has been proposed a galvannealed steel sheet with excellent ductility in which instead of Si, Al with a small adverse effect on the plating properties is added to the steel sheet, thereby improving wettability and anti-powdering property. In actual press forming, improvement in ductility as well as improvement in shape fixability has become a large problem. 
         [0008]    When the strength of a steel sheet is increased, yield strength is also increased, and the amount of spring back in press forming is increased to decrease the shape fixability. Such a decrease in shape fixability can be improved by decreasing yield ratio. In Japanese Unexamined Patent Application Publication No. 2002-317249, a low-yield-ratio cold-rolled steel sheet has been proposed. However, when this steel sheet is applied to an alloyed hot-dip steel sheet, it is difficult to achieve a low yield ratio because of the zinc bath temperature of as high as over 450° C. and the need for alloying at over 500° C. 
         [0009]    Further, in Japanese Unexamined Patent Application Publication No. 2004-115843, there has been proposed a hot-dip galvanized steel sheet in which the amounts of Si, Al, and Mn are balanced, and the steel sheet is maintained at a low temperature for a short time after annealing to form a martensite phase containing a large amount of C, thereby achieving a low yield ratio. However, the proposed technique relates to DP steel which cannot utilize improvement in ductility (TRIP effect) due to strain-induced transformation of residual austenite. Therefore, the steel sheet cannot be recognized as having sufficient ductility. 
       SUMMARY 
       [0010]    We provide a high-strength galvannealed steel sheet capable of achieving good alloying hot-dip galvanization properties without the need of a complicated process and achieving excellent ductility and a low yield ratio after alloying hot-dip galvanization, and a method of producing the steel sheet. As sometimes used hereinafter, “high strength” means a TS of 340 MPa or more. 
         [0011]    We found that the yield ratio of the galvannealed steel sheet can be significantly decreased by adding Cr, V, and Mo in combination with Al, thereby achieving a yield ratio of about 55% or less. In addition, when the amounts of C, Si, Mn, and Al are appropriately controlled, the amount of residual austenite can be increased without decreasing the alloying hot-dip galvanization properties, achieving excellent ductility. 
         [0012]    Although the reason why the yield ratio can be decreased by adding Cr, V, and Mo in combination with Al is not entirely understood, a conceivable reason is as follows: Al discharges C dissolved in ferrite into a second phase and effectively functions to purify ferrite, thereby decreasing the yield ratio. On the other hand, the addition of Cr, V, and Mo permits residual austenite to be formed by austempering at a high temperature with a short time. Therefore, the formed residual austenite has a small amount of dissolved C and is transformed to martensite with small strain, forming a strain field around it and decreasing yield stress. It is thought that the yield stress is more effectively decreased due to the formation of a strain field around ferrite which is purified by adding Al to decrease the amount of dissolved C. 
         [0013]    Thus, we provide the following items (1) to (18):
       (1) A galvannealed steel sheet including, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si +Al≧0.6% and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities.   (2) A galvannealed steel sheet containing, by % by mass, about 0.05 to about 0.25% of C, about 2.0% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6%, N≦0.007%−(0.003×Al)%, and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities.   (3) The galvannealed steel sheet described in (1), further containing, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni.   (4) The galvannealed steel sheet described in (2), further containing, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni.   (5) The galvannealed steel sheet described in (1), further containing, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (6) The galvannealed steel sheet described in (2), further containing, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (7) The galvannealed steel sheet described in (3), further containing, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (8) The galvannealed steel sheet described in (4), further containing, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (9) The galvannealed steel sheet described in any one of (1) to (8), having a metal structure containing a residual austenite phase at a volume ratio of about 3 to about 20%.   (10) A method for producing a galvannealed steel sheet, comprising the steps of:
           annealing, in the temperature range of about 730° C. to about 900° C., a cold-rolled steel sheet containing, by % by mass, about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6% and Cr+V+Mo=0.1 to 2%, the balance being Fe and inevitable impurities;   cooling the annealed cold-rolled steel sheet at a cooling rate of about 3 to about 100° C./second;   retaining the cooled cold-rolled steel sheet in the temperature range of about 350° C. to about 600° C. for about 30 to about 250 seconds;   hot-dip galvanizing the cold-rolled steel sheet after the retention; and   alloying the hot-dip galvanized cold-rolled steel sheet at a temperature of about 470° C. to about 600° C.   
           (11) A method for producing a galvannealed steel sheet, comprising the steps of:
           annealing, in the temperature range of about 730° C. to about 900° C., a cold-rolled steel sheet containing, by % by mass, about 0.05 to about 0.25% of C, about 2% or less of Si, about 1 to about 3% of Mn, about 0.1% or less of P, about 0.01% or less of S, about 0.3 to about 2% of Al, less than about 0.005% of N, about 1% or less of Cr, about 1% or less of V, about 1% or less of Mo, less than about 0.005% of Ti, and less than about 0.005% of Nb, and satisfying the relations, Si+Al≧0.6%, N≦0.007%−(0.003×Al)%, and Cr+V+Mo=0.1 to 2%, the balance including Fe and inevitable impurities;   cooling the annealed cold-rolled steel sheet at a cooling rate of about 3 to about 100° C./second;   retaining the cooled cold-rolled steel sheet in the temperature range of about 350° C. to about 600° C. for about 30 to about 250 seconds;   hot-dip galvanizing the cold-rolled steel sheet after the retention; and   alloying the hot-dip galvanized cold-rolled steel sheet at a temperature of about 470° C. to about 600° C.   
           (12) The method for producing the galvannealed steel sheet described in (10), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni.   (13) The method for producing the galvannealed steel sheet described in (11), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of about 0.005% or less of B and about 1% or less of Ni.   (14) The method for producing the galvannealed steel sheet described in (10), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (15) The method for producing the galvannealed steel sheet described in (11), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (16) The method for producing the galvannealed steel sheet described in (12), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (17) The method for producing the galvannealed steel sheet described in (13), wherein the cold-rolled steel sheet further contains, by % by mass, at least one of Ca and REM in a total of about 0.01% or less.   (18) The method for producing the galvannealed steel sheet described in any one of (10) to (17), wherein the galvannealed steel sheet contains a residual austenite phase at a volume ratio of about 3 to about 20%.       
 
         [0042]    We can obtain sufficient alloying hot-dip galvanization properties without passing through a complicated process, and excellent ductility and a low yield ratio of about 55% or less can be achieved after alloying hot-dip galvanization. 
     
    
     DETAILED DESCRIPTION  
       [0043]    First, the reasons for specifying the composition of the galvannealed steel sheet will be described. Hereinafter, “%” represents “% by mass.” C: about 0.05 to about 0.25% 
         [0044]    C is an element for stabilizing austenite and a necessary element for securing residual austenite. When the C amount is less than about 0.05%, it is difficult to simultaneously secure the strength of the steel sheet and the amount of residual austenite to achieve high ductility. On the other hand, when the C amount exceeds about 0.25%, a welded portion and a heat-affected portion are significantly hardened, thereby impairing weldability. Therefore, the C amount is in the range of about 0.05 to about 0.25%. 
         [0000]    Si: about 2.0% or less 
         [0045]    Si is an element effective in strengthening steel. Si is also a ferrite forming element which promotes the concentration of C in austenite and suppresses the formation of a carbide and thus has the function of promoting the formation of residual austenite. The Si amount is preferably about 0.01% or more. However, when the Si amount exceeds about 2.0%, plating properties are degraded. Therefore, the Si amount is about 2.0% or less and preferably about 0.5% or less. 
         [0000]    Mn: about 1 to about 3% 
         [0046]    Mn is an element effective in strengthening steel. Mn is also an element for stabilizing austenite and an element necessary for increasing residual austenite. However, when the Mn amount is less than about 1%, these effects cannot be easily obtained. On the other hand, when the Mn amount exceeds about 3%, a second phase fraction is excessively increased, and the amount of solid-solution strengthening is increased, thereby significantly increasing strength and decreasing ductility. Therefore, the Mn amount is in the range of about 1 to about 3%. 
         [0000]    P: about 0.1% or less 
         [0047]    P is an element effective in strengthening steel. However, when the P amount exceeds about 0.1%, embrittlement is caused by grain boundary segregation to impair impact properties. Therefore, the P amount is about 0.1% or less. 
         [0000]    S: about 0.01% or less 
         [0048]    S forms an inclusion such as MnS and causes deterioration in impact resistance and cracking along a metal flow of a welded portion. Therefore, the S amount is preferably as small as possible. However, from the viewpoint of production cost, the S amount is about 0.01% or less. 
         [0000]    Al: about 0.3 to about 2% 
         [0049]    Al effectively functions to purify ferrite and decrease the yield ratio of steel. However, when the Al amount is less than about 0.3%, the effect is insufficient. On the other hand, when the Al amount exceeds about 2%, the amount of the inclusion in a steel sheet is increased to degrade ductility. Therefore, the Al amount is in the range of about 0.3% to about 2%. 
       Si+Al≧0.6% 
       [0050]    Like Si, Al is a ferrite forming element which promotes the concentration of C in austenite and suppresses the formation of a carbide and thus has the function of promoting the formation of residual austenite. When the total of Al and Si is less than about 0.6%, the effect is insufficient, and sufficient ductility cannot be obtained. Therefore, the total of Si+Al is about 0.6% or more and preferably about 3% or less. 
         [0000]    N: less than about 0.005% 
         [0051]    N is an inevitable impurity and forms a nitride. When the N amount is about 0.005% or more, ductility at high and low temperatures is decreased by the formation of a nitride. Therefore, the N amount is less than about 0.005%. 
       N≦0.007%−(0.003×Al)% 
       [0052]    When the amount of AlN precipitate is increased with an increase in the N amount, cracking in a slab easily occurs in continuous casting. When it is necessary to avoid such cracking in a slab in continuous casting, in order to avoid this, the N amount is less than about 0.005%, and the relational expression, N≦0.007%−(0.003×Al)%, is satisfied. 
         [0000]    Cr, V, Mo: each about 1% or less
 
Cr+V+Mo: about 0.1 to about 2%
 
         [0053]    Cr, V, and Mo are elements effective in decreasing the yield ratio of steel. The effect becomes significant when these elements are added in combination with Al. Even when each of these elements is added in an amount of over 1%, the effect is saturated. In addition, the effect is insufficient when the total of Cr, V, and Mo is less than about 0.1%. Conversely, when the total exceeds about 2%, strength may be excessively increased to decrease ductility and degrade the plating properties. Therefore, the amount of each of Cr, V, and Mo is about 1% or less, and the total is about 0.1 to about 2% and preferably about 0.15 to about 1.3%. 
         [0000]    Ti, Nb: each less than about 0.005% 
         [0054]    Ti and Nb precipitate as carbonitrides to strengthen steel. However, such precipitation strengthening increases yield stress and is thus disadvantageous for decreasing the yield ratio. When the amount of each of the elements added is about 0.005% or more, the yield stress is increased. Therefore, the amount of each of Ti and Nb is less than about 0.005%. 
         [0000]    B: about 0.005% or less 
         [0055]    B is effective in strengthening steel and can thus be added according to demand. When the B amount exceeds about 0.005%, strength is excessively increased to decrease workability. Therefore, when B is added, the amount is about 0.005% or less. 
         [0000]    Ni: about 1% or less 
         [0056]    Ni is an austenite-stabilizing element which causes austenite to remain and is effective in increasing strength, and thus can be added according to demand. However, even when the amount of Ni exceeds about 1%, the effect is saturated, and conversely the cost is increased. Therefore, when Ni is added, the amount is about 1% or less. 
         [0000]    Ca and REM: at Least One in Total of about 0.01% or less 
         [0057]    Ca and REM have the function to control the form of a sulfide inclusion and thus have the effect of improving elongation and flange properties of a steel sheet, and thus can be added according to demand. When the total of these elements exceeds about 0.01%, the effect is saturated. Therefore, when Ca and REM are added, the total of at least one of the elements is about 0.01% or less. 
         [0058]    Besides the above-described elements and Fe in the balance, various impurities in the production process and trace amounts of essential elements added in the production process are inevitably mixed. However, these inevitable impurities are permissible because they have no particular influence on the advantage of our steel sheets. 
         [0059]    Next, the metal structure of the steel sheet will be described. 
         [0000]    Residual Austenite Phase: Volume Ratio of about 3 to about 20% 
         [0060]    A residual austenite phase is essential for effectively utilizing strain-induced transformation and obtaining high ductility. Therefore, it is very important to control the volume ratio of the residual austenite. From the viewpoint of securing high ductility, the ratio of the residual austenite phase is preferably at least about 3% or more. On the other hand, when the ratio of the residual austenite phase exceeds about 20%, a large amount of martensite is formed after molding to increase brittleness. Since it may be necessary to suppress brittleness in a permissible range, therefore, the ratio of the residual austenite phase is preferably about 20% or less. The metal structure of the steel sheet includes a ferrite main phase and a second phase including a residual austenite phase. However, the volume ratio of the ferrite phase is preferably about 40 to about 90% from the viewpoint of securing high ductility. Examples of a metal structure other than the residual austenite phase in the second phase include a bainite phase, a martensite phase and a pearlite phase. The total volume ratio of these phases is preferably about 7 to about 50%. 
         [0061]    Next, the conditions for producing the galvannealed steel sheet will be described. 
         [0062]    Steel having the above-described composition is melted and continuously cast to form a cast slab, and then the slab is hot-rolled and cold-rolled. However, the conditions for these processes are not particularly limited. Then, in a continuous hot-dip plating line, the steel sheet is annealed in a temperature range of about 730° C. to about 900° C., cooled at about 3 to about 100° C./s, retained in a temperature range of about 350° C. to about 600° C. for about 30 to about 250 seconds, hot-dip galvanized, and then alloyed at about 470° C. to about 600° C. Annealing temperature: about 730 to about 900° C. 
         [0063]    Annealing is performed in an austenite single-phase zone or a two-phase zone including an austenite phase and a ferrite phase. When the annealing temperature is lower than about 730° C., in some cases, a carbide is not sufficiently dissolved in the steel sheet, or recrystallization of ferrite is not completed, thereby failing to obtain intended properties. On the other hand, when the annealing temperature exceeds about 900° C., austenite grains are significantly grown, and the number of ferrite nucleation sites formed from the second phase by subsequent cooling may be decreased. Therefore, the annealing temperature is about 730° C. to about 900° C. 
         [0000]    Cooling Rate: about 3 to about 100° C./s 
         [0064]    When the cooling rate is less than about 3° C./s, a large amount of pearlite precipitates, the amount of C dissolved in untransformed austenite is significantly decreased, and thus the intended structure cannot be obtained. When the cooling rate exceeds about 100° C./s, growth of ferrite is suppressed to significantly decrease the volume ratio of ferrite, and thus sufficient ductility cannot be secured. Therefore, the cooling rate is preferably about 3 to about 100° C./s. 
         [0000]    Retention Temperature: about 350° C. to about 600° C. 
         [0065]    When the retention temperature exceeds about 600° C., a carbide precipitates from untransformed austenite. When the retention temperature is lower than about 350° C., a car-bide precipitates in bainitic ferrite due to lower bainite transformation, thereby failing to sufficiently obtain stable residual austenite. Therefore, the retention temperature is about 350° C. to about 600° C. In order to stably produce residual austenite, the retention temperature is preferably about 500° C. or less. 
         [0000]    Retention Time: about 30 to about 250 seconds 
         [0066]    The retention time pays a very important role for controlling residual austenite. Namely, when the retention time is less than about 30 seconds, stabilization of untransformed austenite does not proceed, and thus the amount of residual austenite cannot be secured, thereby failing to obtain desired properties. On the other hand, when the retention time exceeds about 250 seconds, an austenite phase containing a small amount of dissolved C cannot be obtained, and it becomes difficult to transform to a martensite phase with a small amount of strain and achieve low yield stress by a strain field formed around the martensite phase. Therefore, the retention time is about 30 to about 250 seconds. From the viewpoint of stabilization of untransformed austenite, the retention time preferably exceeds about 60 seconds and more preferably exceeds about 90 seconds. In order to decrease yield stress, the retention time is preferably about 200 seconds or less. 
         [0000]    Alloying Temperature: about 470° C. to about 600° C. 
         [0067]    The alloying temperature after the retention and hot-dip galvanization must be higher than the plating bath temperature, and the lower limit is about 470° C. When the alloying temperature exceeds about 600° C., like in the case where the retention temperature exceeds about 600° C., a carbide precipitates from untransformed austenite, and thus stable residual austenite cannot be obtained. Therefore, the alloying temperature is about 470° C. to about 600° C. 
         [0068]    In the production conditions, the specified annealing temperature, retention temperature, and alloying temperature need not be constant as long as they are in the above respective ranges. In addition, the cooling rate may be changed during cooling as long as it is in the above range. Further, the plating conditions may be in a usual operation range, i.e., METSUKE may be about 20 to about 70 g/m 2 , and the amount of Fe in a plating layer may be about 6 to about 15%. 
       EXAMPLE 
       [0069]    An example of our steel sheets will be described. 
         [0070]    Steel having each of the compositions shown in Table 1 was molten by a converter and continuously cast to form a cast slab. The occurrence of cracking in the slab is shown in Table 1. The occurrence of cracking was determined by visual observation as well as color check after the slab was cooled to room temperature. 
         [0071]    The resulting slab was heated to 1250° C. and then hot-rolled at a finish rolling temperature of 900° C. to prepare a hot-rolled steel sheet having a thickness of 3.0 mm. After hot-rolling, the hot-rolled steel sheet was pickled and further cold-rolled to prepare a cold-rolled steel sheet having a thickness of 1.2 mm. Then, in a continuous hot-dip galvanization line, each cold-rolled steel sheet was heat-treated under the conditions shown in Table 2, plated at 50/50 g/m 2 , and then alloyed so that the Fe amount in the plating layer was 9%. 
         [0072]    Further, each of the resulting steel sheets was temper-rolled by 0.5% to examine mechanical properties. As the mechanical properties, yield stress YS, tensile strength TS, and elongation EL were measured using a JIS No. 5 tensile specimen obtained from each steel sheet in a direction perpendicular to the rolling direction. A tensile test was conducted at a strain rate of 6.7×10 −3  s −1 . The measured values, yield ratios YR, and values of TS×EL are also shown in Table 2. 
         [0073]    Table 2 indicates that steel sheet Nos. 1, 2, 5 to 8, 11 to 16, 18, 21, 22, 24, and 28 satisfy our composition and production conditions and have yield ratios as low as about 55% or less and satisfactory values of tensile strength TS and elongation EL. On the other hand, comparative steel sheet Nos. 3, 4, 9, 10, 17, 19, 20, 23, 25 to 27, and 29 to 38 not satisfying our composition and production conditions are out of the preferred range of at least one of yield ratio YR, tensile strength TS, elongation EL, and balance therebetween. Table 1 indicates that among our steel sheets, steel sheet Nos. A to L satisfying N≦0.007%−(0.003×Al)% caused no cracking in the slabs.