Technical field
[0001]
　The present invention relates to a galvannealed steel sheet and a manufacturing method thereof, more particularly, to high-galvannealed steel sheet strength and a manufacturing method thereof having a tensile strength of at least 440 MPa.
Background technique
[0002]
　Recently, because of the reduction and reducing fuel consumption emissions of carbon dioxide, weight reduction of automobiles it has been required. Furthermore, there is an increasing demand for improvement in collision safety of automobiles. High strength steel is effective in improving the weight reduction of automobiles and collision safety.
[0003]
　Among auto parts, in the panel part, a high press-formability (deep drawability) is also required. Therefore, conventionally, the panel parts, high workability mild steel sheets have been used. However, even in the panel component, high strength of the above is required.
[0004]
　Patent Documents 1-3, has a high strength, and proposes an excellent cold-rolled steel sheet deep drawability. In the cold-rolled steel sheet disclosed in these documents, the average r value r defined by the following equation m high.
　r m = (r L + 2 × r 45 + r C ) / 4
[0005]
　R in the formula L is the cold-rolled steel sheet, which is parallel to the direction of the r values in the rolling direction. r 45 is the r value in 45 ° direction relative to the rolling direction. r C is a direction perpendicular r values in the rolling direction. r value is referred to as the Lankford value, indicating the plastic anisotropy of the steel sheet.
[0006]
　Of the panel components, more in parts having a shape such as a side frame outer, average r value r m not only is high, also required is that the are small in-plane anisotropy Δr value defined by the following formula . =
　[Delta] r (r L + r C ) / 2-r 45
[0007]
　Of the side frame outer, the four corners of the site where the door is fit, r 45 is required to have high, the hinge attachment portion of the door, r L is higher is required. Smaller plane anisotropy Δr value, it is possible to satisfy these requirements. In Patent Document 4, the average r value r m and high strength cold rolled steel sheet for deep drawing having excellent in-plane anisotropy Δr value is proposed.
[0008]
　Incidentally, as a cold-rolled steel sheet for automobiles steel, there is a case where a galvannealed steel sheet is used. Galvannealed steel sheet after hot-dip galvanizing process, is prepared by carrying out the alloying process. Galvannealed steel sheet is usually produced by in-line annealing system represented by Sendzimir method. The in-line annealing system represented by Sendzimir method, the annealing equipment and galvanizing are continuously arranged. Therefore, after annealing, galvanizing treatment is carried out continuously. Incidentally, the coating layer of the galvannealed steel sheet is made poor in deformability intermetallic compound, to restrain the deformation of the base material steel plate. Therefore, the average r value of the galvannealed steel sheet is lower than the average r value of the steel sheet without the plating layer.
[0009]
　In the high deep drawability of a high strength cold rolled steel sheet, Si content is high in order to obtain a high strength. If the Si content is high, Si is enriched on the surface of the steel sheet, Si oxide (SiO 2 ) is produced. Si oxide, during galvanizing treatment, prone to non-plating.
[0010]
　Patent Documents 5 to 7, even at high Si content to propose a method for producing a high strength cold rolled steel sheet non-coating is less likely to occur. In these documents, implementing the Ni pre-plating with respect to pre-annealed cold-rolled steel sheet. Against steel sheet after Ni pre-plating, it is rapidly heated to 430 ~ 500 ° C. implementing the galvanizing treatment. After galvanizing treatment, by heating to 470 ~ 550 ° C. implementing the alloying process. In this manufacturing method, while maintaining the workability of cold-rolled steel sheet is the original sheet can be produced galvannealed steel sheet. Furthermore, by implementing the Ni pre-plating, even with a high Si content non-coating hardly occurs.
[0011]
　Incidentally, when the slab after casting is cooled to room temperature, it may crack the slab is generated by the internal stress (thermal stress and transformation stress, etc.). Further, when the slab for moving the slab handling, and, when stacked slabs are loaded external stress to the slab cracking slab may occur. Such cracking is referred to as a "slab cracking". Resistance to the slab cracking, referred to as "anti-slab cracking".
[0012]
　Further, in the cold-rolled steel sheet, when the new tension or impact force to the end portion of the molded article after deep drawing is applied, there is a case where brittle fracture along the aperture direction is generated. Such a phenomenon is referred to as a "secondary work embrittlement". Resistance to secondary work embrittlement, referred to as "resistance to secondary work embrittlement".
[0013]
　The galvannealed steel sheet manufactured by the manufacturing method disclosed in Patent Documents 4-7, may have low resistance to the slab cracking and resistance to secondary work embrittlement of the above.
CITATION
Patent Document
[0014]
Patent Document 1: JP KOKOKU No. 8-30217
Patent Document 2: JP KOKOKU No. 8-26412
Patent Document 3: Laid-Open Patent Publication No. 2001-131695
Patent Document 4: JP Patent No. 4094498
Patent Document 5: Patent No. 2526320 Publication
Patent Document 6: JP Patent No. 2526322
Patent Document 7: JP Laid-open No. 2015-063729
Summary of the Invention
Problems that the Invention is to Solve
[0015]
　An object of the present invention has high strength and excellent deep drawability, further excellent in resistance to the slab cracking and secondary work embrittlement resistance, to provide a galvannealed steel sheet of high strength and a manufacturing method thereof it is.
Means for Solving the Problems
[0016]
　Galvannealed steel sheet according to the present invention includes a base steel sheet and a galvannealed layer. The base steel sheet is a mass%, C: 0.0080% or less, Si: 0.7% or less, Mn: 1.0 ~ 2.5%, P: 0.030 ultra ~ 0.048%, S: 0.025% or less, Al: 0.005 ~ 0.20%, N: 0.010% or less, Ti: 0.005 ~ 0.040%, Nb: 0.005 ~ 0.060%, and, B : containing 0.0005 to 0.0030%, the balance being Fe and impurities, having a chemical composition satisfying expressions (1) to (4). Galvannealed layer is formed on the surface of the base steel sheet, containing 7 to 15% by weight of Fe. Average r value of the galvannealed steel sheet is 1.2 or more, the in-plane anisotropy Δr value is -0.5 to 0.5, the tensile strength is at least 440 MPa.
　× P + 25 4 × Si ≦ 3.6
　(1) B-X1 ≧ 0.0005
　(2) C-(12/93) × Nb-X2 ≧ -0.0035
　(3) 110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
　wherein each element symbol in the formula, the content of the corresponding element (mass%) is substituted.
　Further, X1 in the formula (2) is defined as follows.
　For N- (14/48) × Ti> 0 ,
　it is X1 = (11/14) [N- (14/48) ×
　Ti], the case of N- (14/48) × Ti ≦ 0,
　X1 = 0.
　Further, X2 in the formula (3) is defined as follows.
　Ti- (48/14) × N>
　0, is X2 = (12/48) [Ti- (48/14) ×
　N], Ti- (48/14) × For N ≦
　0, X2 = 0.
[0017]
　Method for manufacturing a galvannealed steel sheet according to the present invention, a slab having the above chemical composition was carried out hot rolling at a finish rolling temperature of 800 ~ 950 ° C. to produce a hot-rolled steel sheets, winding 600 ~ 760 ° C. a step of winding the hot-rolled steel sheet at a temperature taken, a step of producing a cold-rolled steel sheet was performed to cold rolling with respect to hot-rolled steel sheet, the base steel sheet by performing an annealing process on cold-rolled steel sheet a step of manufacturing, the step of performing hot-dip galvanizing against the base steel sheet for galvanizing the treated base material steel plate, 470 ~ 620 ° C. alloying treatment temperature of 10 to 40 seconds alloys for holding and a step of carrying out the process.
Effect of the invention
[0018]
　Galvannealed steel sheet according to the present invention has high strength and excellent deep drawability, further excellent in slab cracking and resistance to secondary work embrittlement. The production method according to the invention can be produced the galvannealed steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019]
[1] Figure 1 shows the relationship obtained by the preliminary tests were performed in the same manner as the secondary work embrittlement resistance evaluation test described below, F1 = 25 × P + 4 × Si and brittle crack length and (mm) it is a diagram.
FIG. 2 is obtained in resistance to the slab cracking resistance evaluation test in the examples, the ductile brittle fracture transition temperature vTrs (° C.), P content of the base material steel plate in galvannealed steel sheets is a diagram showing the relationship between (mass%).
DESCRIPTION OF THE INVENTION
[0020]
　The present inventors have found that high-strength and in galvannealed steel sheet having excellent deep drawability (high average r value and a low plane anisotropy Δr value), excellent slab cracking and resistance to secondary processing the methods for obtaining the brittle, were investigated and studied. As a result, the present inventors have obtained the following findings.
[0021]
　(A) while having a high tensile strength of at least 440 MPa, excellent in order to obtain the secondary work embrittlement resistance it is necessary to suppress the content of P and Si in the base steel sheet during the galvannealed steel sheet is there.
[0022]
　It is defined as F1 = 25 × P + 4 × Si. Each element symbol in the F1, the content of the corresponding elements of the base material steel plate in the galvannealed steel sheet (mass%) is substituted. F1 is an indicator of the resistance to secondary work embrittlement.
[0023]
　Figure 1 was generated based on the secondary work embrittlement resistance evaluation test described below, is a graph showing the relationship between F1 and brittle crack length (mm). Figure 1 was obtained by the following method. It was performed the secondary work embrittlement resistance evaluation test described below. The chemical composition of the test materials within the scope of this invention except F1, formula (2) to (4) was also satisfied. As a result of observation of the specimen after the test, if the brittle crack is generated was measured brittle crack length. As a result of the measurement, the relationship between the F1 and brittle crack length was as follows. Brittle crack length of the sample F1 = 1.8 was 0. Brittle crack length of the sample F1 = 2.5 was 0. Brittle crack length of the sample of F1 = 3.1 was 0. Brittle crack length of the sample of F1 = 3.6 was 0. Brittle crack length of the sample F1 = 3.8 was 85 mm. Brittle crack length of the sample F1 = 4.2 was 168 mm. Brittle crack length of the sample F1 = 4.6 was 210 mm. Based on the obtained brittle crack length was created to FIG.
[0024]
　Referring to FIG. 1, F1 is more than 3.6, brittle crack length with increasing F1 increases significantly. On the other hand, F1 is equal to 3.6 or less, brittle crack does not substantially occur. Therefore, galvannealed steel sheet satisfies a formula (1), while having a high strength of at least 440 MPa, it is possible to enhance the secondary work embrittlement resistance.
　25 × P + 4 × Si ≦ 3.6 (1)
[0025]
　(B) if Takamere the grain boundary strength, it is possible to increase the resistance to slab cracking and resistance to secondary work embrittlement. If the appropriate amount of solid solute B and solute C, increases the grain boundary strength. Specifically, it satisfies the equations (2) and (3), it is possible to increase the appropriate amount of solid solute B and can be secured solute C, resistant slab cracking and resistance to secondary work embrittlement.
　-X1 ≧ B 0.0005
　(2) C-(12/93) × Nb-X2 ≧ -0.0035 (3)
　wherein each element symbol in the formula, the content of the corresponding element (mass% ) it is assigned.
　X1 in the formula (2) is defined as follows.
　For N- (14/48) × Ti> 0 ,
　it is X1 = (11/14) [N- (14/48) ×
　Ti], the case of N- (14/48) × Ti ≦
　0, X1 = 0.
　X2 in the formula (3) is defined as follows.
　Ti- (48/14) × N>
　0, is X2 = (12/48) [Ti- (48/14) ×
　N], Ti- (48/14) × For N ≦
　0, X2 = 0.
[0026]
　It is defined as F2 = B-X1. F3 = C- (12/93) is defined as × Nb-X2. F2 is a measure of the solute B amount, F3 is an indication of solid solution C content. If F2 is 0.0005 or more, the grain boundary contributes solute B amount to strengthen exists sufficiently in the steel. Also, F3 is equal to -0.0035 least contributes amount of solute C in the grain boundary strength is present sufficiently in the steel. Accordingly, the base steel sheet of the galvannealed steel sheet satisfying the equation (1) is further formula (2) and satisfies the equation (3), it increases the grain boundary strength, resistance to slab cracking and resistance to secondary work embrittlement It increases.
[0027]
　(C) above of the formula (1) while satisfying ~ expression (3), the tensile strength to be at least 440 MPa, the base material steel plate of the galvannealed steel sheet satisfies equation (4).
　110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
[0028]
　Defined as F4 = 110 × Si + 48 × Mn + 550 × P. F4 is a measure of the tensile strength of the steel sheet. If F4 is 120 or more, a tensile strength of galvannealed steel sheets satisfying expressions (1) to (3) is equal to or greater than 440 MPa.
[0029]
　(D) The present inventors have further by the P content of the base material steel plate in galvannealed steel sheets with less 0.048%, the galvannealed steel sheet, marked resistance to the slab cracking resistance was found that the increase in. This point will be described below.
[0030]
　Figure 2 is a diagram showing the relationship obtained in resistant slab cracking resistance evaluation test described below, the ductile brittle fracture transition temperature vTrs (° C.), P content of the galvannealed steel sheet and (mass%) it is.
[0031]
　Referring to FIG. 2, until the P content is 0.048%, in accordance with reduction of P content, ductile brittle fracture transition temperature vTrs is significantly reduced, it becomes -20 ° C. or less. On the other hand, the P content is when it becomes less 0.048%, even if the reduced P content, ductile brittle fracture transition temperature vTrs is not significantly reduced from -20 ° C.. That is, in FIG. 2, an inflection point is present near the P content = 0.048%.
[0032]
　Accordingly, P content is not more than 0.048%, excellent slab cracking resistance. Meanwhile, P is to enhance the strength of the steel, if the P content is 0.030% or less, the tensile strength is less than 440 MPa. Accordingly, satisfying the above (A) ~ (D), and, if P content is 0.030% or less super-0.048%, has a higher tensile strength and excellent resistance to secondary work embrittlement 440MPa while, galvannealed steel sheet excellent in slab cracking resistance.
[0033]
　(E) the formula (1) When to Formula slab having a chemical composition satisfying the (4) hot rolling, finishing temperature FT and (℃) and 800 ~ 950 ° C., and 600 a coiling temperature CT (℃) if ~ 760 ° C., excellent deep drawability is obtained. In other words, the average r value becomes 1.2 or more, and in-plane anisotropy Δr value is -0.5 to 0.5.
[0034]
　Galvannealed steel sheet according to the present invention has been completed based on the above findings includes a base steel sheet and a galvannealed layer. The base steel sheet is a mass%, C: 0.0080% or less, Si: 0.7% or less, Mn: 1.0 ~ 2.5%, P: 0.030% ultra ~ 0.048%, S 0.025% or less, Al: 0.005 ~ 0.20%, N: 0.010% or less, Ti: 0.005 ~ 0.040%, Nb: 0.005 ~ 0.060%, and, B: 0.0005 ~ 0.0030% the balance being Fe and impurities, having a chemical composition satisfying expressions (1) to (4). Galvannealed layer is formed on the surface of the base steel sheet, containing 7 to 15% by weight of Fe. Average r value of the galvannealed steel sheet is 1.2 or more, the in-plane anisotropy Δr value is -0.5 to 0.5, the tensile strength is at least 440 MPa.
　× P + 25 4 × Si ≦ 3.6
　(1) B-X1 ≧ 0.0005
　(2) C-(12/93) × Nb-X2 ≧ -0.0035
　(3) 110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
　wherein each element symbol in the formula, the content of the corresponding element (mass%) is substituted.
　Further, X1 in the formula (2) is defined as follows.
　For N- (14/48) × Ti> 0 ,
　it is X1 = (11/14) [N- (14/48) ×
　Ti], the case of N- (14/48) × Ti ≦ 0,
　X1 = 0.
　Further, X2 in the formula (3) is defined as follows.
　Ti- (48/14) × N>
　0, is X2 = (12/48) [Ti- (48/14) ×
　N], Ti- (48/14) × For N ≦
　0, X2 = 0.
[0035]
　The chemical composition, C: less than 0.0040%, Ti: 0.005 ~ 0.035%, and, Nb: may contain from 0.005 to 0.035%.
[0036]
　The chemical composition, C: 0.0040 ~ 0.0080%, Ti: 0.005 ~ 0.040%, and, Nb: may contain 0.005 to 0.060%. In this case, the value of the expression (3) is preferably -0.0002 more.
[0037]
　Method for manufacturing a galvannealed steel sheet according to the present invention, a slab having the above chemical composition was carried out hot rolling at a finish rolling temperature of 800 ~ 950 ° C. to produce a hot-rolled steel sheets, winding 600 ~ 760 ° C. a step of winding the hot-rolled steel sheet at a temperature taken, a step of producing a cold-rolled steel sheet was performed to cold rolling with respect to hot-rolled steel sheet, the base steel sheet by performing an annealing process on cold-rolled steel sheet a step of manufacturing, with respect to the base steel sheet, a step of performing hot-dip galvanizing, the molten galvanizing the treated base material steel plate, holding alloying treatment temperature of 470 ~ 620 ℃ 10 ~ 40 seconds and a step of performing an alloying treatment.
[0038]
　For the step of performing the process and galvanizing manufacturing the base material steel plate, after performing in-line annealing such as the so-called Sendzimir method, the base material steel plate (steel strip) in molten zinc plating bath under a protective atmosphere it may be introduced. Further, cold-rolled steel sheet (i.e. the base steel sheet) was taken out of the furnace (so-called out-of-line annealing) after annealing, after performing the appropriate precleaning, carried out pretreatment such as Ni pre-plating treatment if necessary and, after heating to 430 ~ 480 ° C. under a protective atmosphere may be introduced base steel sheet to hot-dip galvanizing bath.
[0039]
　When performing in-line annealing of the oxidizable elements such as Si are likely concentrates on the surface. Therefore, Si content is provided by carrying out the in-line annealing with respect to cold-rolled steel sheet of 0.25% or more, non-coating is likely to occur. Therefore, if the Si content is 0.25% or more, Preferably a line out annealing to produce a base steel sheet, conducted Ni pre-plating processing on the manufactured base material steel plate by the line out annealing to. In this case, while suppressing the occurrence of non-plating can be performed galvanizing treatment. Incidentally, Si content relative to the base steel sheet is less than 0.25%, no adverse effects in any way be carried out Ni pre-plating process.
[0040]
　It described in detail below galvannealed steel sheet according to the present invention. "%" Related elements, unless otherwise specified, it means mass%.
[0041]
　[Configuration of the galvannealed steel sheet]
　galvannealed steel sheet according to the present invention includes a base steel sheet and a galvannealed layer. Galvannealed layer is formed on the surface of the base material steel plate.
[0042]
　[Chemical composition of the base material steel plate]
　The chemical composition of base steel sheet contains the following elements.
[0043]
　C: 0.0080% or less
　carbon (C) is contained inevitably. C increases the strength of the steel. C is further segregated in the grain boundary as a solid solution C, suppresses secondary work embrittlement resistance and slab cracking resistance of the steel. However, if C content is too high, deep drawability of steel is lowered. Specifically, TiC and NbC are excessively generated in-plane anisotropy Δr value is increased by. TiC and NbC further suppress the grain growth during annealing, it decreases the average r value. Therefore, C content is less 0.0080%.
[0044]
　High strength and preferable lower limit of C content in order to further enhance the secondary work embrittlement resistance is 0.0008%, more preferably 0.0021%. The preferable upper limit of the C content for obtaining better deep drawability is 0.0070%, more preferably 0.0050%.
[0045]
　Particularly when improved high strength and resistance to secondary work embrittlement, the preferred C content is from 0.0040 to 0.0080%. Especially when improving the deep drawability, the preferred C content is less than 0.0040%.
[0046]
　Si: 0.7% or less
　Silicon (Si) is contained inevitably. Si increases the strength of the steel sheet by solid solution strengthening. However, if Si content is too high, with secondary work embrittlement resistance is deteriorated, non-coating is likely to occur in the galvanizing process (if in line annealing). Further if the Si content is too high, the scale peelability is lowered after hot working, also flash butt weldability is lowered. Therefore, Si content is 0.7% or less. If Si content exceeds 0.7%, it is carried out Ni pre-plating, there is a case where non-coating occurs. A preferable lower limit of Si content is 0.1%, more preferably from 0.2%, more preferably 0.3%. The preferable upper limit of the Si content is 0.6%, more preferably 0.5%.
[0047]
　Mn: 1.0 ~ 2.5%
　manganese (Mn) increases the strength of the steel sheet by solid solution strengthening. Mn further, A r3 decreases the transformation point. A r3 If decreases transformation point, it is possible to lower the finishing temperature (FT) at the hot rolling, the ferrite grains of hot-rolled steel sheet can be miniaturized. If the Mn content is too low not these effects can not be obtained. On the other hand, if the Mn content is too high, toughness of the steel sheet is lowered. Therefore, Mn content is 1.0 to 2.5%. The preferable lower limit of the Mn content is 1.2%, more preferably from 1.4%. The preferable upper limit of the Mn content is 2.3%, more preferably from 2.1%.
[0048]
　P: 0.030 ultra ~ 0.048%
　phosphorus (P) increases the strength of the steel sheet by solid solution strengthening. If the P content is too low, the chemical composition of the present invention, it is difficult to ensure a tensile strength of 440 MPa. On the other hand, if the P content is too high, P is segregated at grain boundaries to decrease the grain boundary strength. In this case, resistance to slab cracking and resistance to secondary working brittleness is reduced, in particular, as shown in FIG. 2, anti-slab cracking resistance decreases. Accordingly, P content is 0.030% or ultra-0.048%. The preferable lower limit of the P content is 0.033%, and still more preferably 0.035%. The preferable upper limit of the P content is 0.047%, more preferably 0.045%.
[0049]
　S: 0.025% or less
　Sulfur (S) is contained inevitably. S lowers the hot workability and toughness of the steel. Thus, S content is 0.025% or less. The preferable upper limit of the S content is 0.010%, still more preferably 0.005%. S content is preferably as small as possible. However, when considering the cost of desulfurization, the preferred lower limit of the S content is 0.0005%.
[0050]
　Al: 0.005 ~ 0.20%
　of aluminum (Al) is deoxidized steel. Al is further formed to suppress the coarsening of crystal grains AlN. Al is further the same ferrite stabilizing elements and Si, it can be included as an alternative to Si. If the Al content is too low not these effects can not be obtained. On the other hand, if the Al content is too high, toughness of the steel is lowered. Therefore, Al content is from 0.005 to 0.20%. A preferable lower limit of the Al content is 0.01%, more preferably 0.02%. The preferable upper limit of the Al content is 0.10%, more preferably 0.06%.
[0051]
　N: 0.010% or less
　Nitrogen (N) is inevitably contained. N inhibits coarsening of crystal grains to form a nitride (AlN). However, if the N content is too high, toughness of the steel is lowered. Therefore, N content is 0.010% or less. The preferable upper limit of the N content is 0.007%, more preferably 0.005%. N inhibits coarsening of crystal grains by forming a carbonitride or the like of Al or Ti. Therefore, lower limit of N content is 0.001%.
[0052]
　Ti: 0.005 ~ 0.040%,
　titanium (Ti) is to form a fine Ti carbonitride and suppresses the coarsening of crystal grains. Ti further to form a TiN reduces solute N, suppresses the precipitation of BN. Thus, resistance to the slab cracking and resistance to secondary work embrittlement increases by solution B. If the Ti content is too low, these effects can not be obtained. On the other hand, if the Ti content is too high, Ti carbonitride decreases the toughness of the coarsened steel. Furthermore, the average r value and generates the TiP decreases. Therefore, Ti content is 0.005 to 0.040%. A preferable lower limit of the Ti content is 0.010%, more preferably 0.015%. The preferable upper limit of the Ti content is 0.035%, more preferably 0.030%. [0053]
　If C content is less than 0.0040%, the preferred range of the Ti content is from 0.005 to 0.035%. If C content is from 0.0040 to 0.0080%, the preferred range of Ti content is 0.005 to 0.040%.
[0054]
　Nb: 0.005 ~ 0.060%
　niobium (Nb) is to form a fine Nb carbonitrides, suppresses coarsening of crystal grains. This effect can not be obtained if the Nb content is too low. On the other hand, if the Nb content is too high, the NbC precipitated in grain boundaries, high slab cracking and secondary work embrittlement resistance is deteriorated. Therefore, Nb content is 0.005 to 0.060%. The preferable lower limit of Nb content is 0.010%, more preferably 0.015%. The preferable upper limit of Nb content is 0.040%, and still more preferably 0.035%.
[0055]
　If C content is less than 0.0040%, the preferred range of the Nb content is from 0.005 to 0.035%. If C content is from 0.0040 to 0.0080%, the preferred range of the Nb content is 0.005 to 0.060%.
[0056]
　B: 0.0005 ~ 0.0030%
　boron (B) is segregated at the grain boundaries as a solid solution suppresses the P and S are segregated at the grain boundaries. Thus, resistance to the slab cracking and secondary work embrittlement resistance is improved. If the B content is too low, the effect can not be obtained. On the other hand, if the B content is too high, coarse nitrides at grain boundaries (BN) is generated, anti slab cracking and resistance to secondary work embrittlement of steel is lowered. Coarse BN deposited at the grain boundaries further reduces the hot workability and toughness of the steel. Therefore, B content is 0.0005 to 0.0030 percent. The preferable lower limit of the B content is 0.0010%, more preferably 0.0015%. The preferable upper limit of the B content is 0.0025%, more preferably 0.0020%.
[0057]
　The remainder of the chemical composition of the base material steel plate of the galvannealed steel sheet of the present invention consists of Fe and impurities. Here, the impurity, when the industrial production of the base material steel plate, the ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, does not adversely affect to the base steel sheet ranges in means what is acceptable.
[0058]
　[Equation (1) to (4)]
　In addition the chemical composition of the base steel sheet satisfy the following equation (1) to (4).
　× P + 25 4 × Si ≦ 3.6
　(1) B-X1 ≧ 0.0005
　(2) C-(12/93) × Nb-X2 ≧ -0.0035
　(3) 110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
　wherein each element symbol in the formula, the content of the corresponding element (mass%) is substituted.
[0059]
　[For formula (1)]
　The chemical composition of the base steel sheet satisfy the equation (1).
　25 × P + 4 × Si ≦ 3.6 (1)
[0060]
　It is defined as F1 = 25 × P + 4 × Si. F1 is an indicator of the resistance to secondary work embrittlement. As shown in FIG. 1, if it exceeds F1 is 3.6, brittle crack length becomes remarkably long, secondary work embrittlement resistance is deteriorated. If F1 is at 3.6 or less, the occurrence of brittle cracks are sufficiently suppressed, secondary work embrittlement resistance is improved.
[0061]
　[Equation (2)]
　In addition the chemical composition of the base steel sheet satisfies equation (2).
　B-X1 ≧ 0.0005 (2)
　where, X1 in the formula (2) is defined as follows.
　For N- (14/48) × Ti> 0 , it is X1 = (11/14) [N- (14/48) × Ti]. For N- (14/48) × Ti ≦ 0 , it is X1 = 0.
[0062]
　It is defined as F2 = B-X1. F2 is a measure of the solute B amount, which is an index of resistance to the slab cracking and resistance to secondary work embrittlement. If X1 is greater than 0, in theory N is not sufficiently fixed by Ti, which means that B capable of binding N is present. If X1 is 0 or less, the theory, N are sufficiently fixed by Ti, capable of binding N means that there is no a B.
[0063]
　F2 is the case of less than 0.0005, the solid solution B can not be sufficiently segregated at the grain boundaries. In this case, since the P and S are segregated at the grain boundaries, slab cracking or secondary work embrittlement is likely to occur. If F2 is 0.0005 or more, the solid solution B is sufficiently segregated at the grain boundaries to suppress the grain boundary segregation of P and S. Therefore, increased grain boundary strength is obtained excellent slab cracking and resistance to secondary work embrittlement.
[0064]
　[Equation (3)]
　Moreover the chemical composition of the base steel sheet satisfies equation (3).
　C- (12/93) × Nb-X2 ≧ -0.0035 (3)
　where, X2 in formula (3) is defined as follows.
　Ti- (48/14) × N> 0, it is X2 = (12/48) [Ti- (48/14) × N]. Ti- (48/14) × For N ≦ 0, it is X2 = 0.
[0065]
　F3 = C- (12/93) is defined as × Nb-X2. F3 is an indicator of the amount of solute C, which is an index of resistance to the slab cracking and resistance to secondary work embrittlement. If X2 is greater than 0, in theory, it means that greater than that binds to N Ti is present, it means that capable of binding Ti is present as C. If X2 is less than or equal to 0, in theory, all Ti binds to N, which means that C capable of binding Ti is absent.
[0066]
　If F3 is lower than -0.0035, due to insufficient amount of solid solution C it is, is not increased sufficiently the strength of the grain boundary. In this case, resistance to slab cracking and secondary work embrittlement resistance is deteriorated. If F3 is greater than or equal -0.0035, amount of dissolved C for increasing the grain boundary strength is sufficiently present. Therefore, resistance to the slab cracking and secondary work embrittlement resistance is improved. A preferred lower limit of F3 is -0.0015, more preferably from 0.0002. Preferably, F3 is 0.0025 or less, more preferably 0.0018 or less. In this case, the deep drawability is further improved.
[0067]
　If C content is from 0.0040 to 0.0080 percent, preferable lower limit of F3 is -0.0002.
[0068]
　[Equation (4)]
　In addition the chemical composition of the steel sheet satisfy the equation (4).
　110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
[0069]
　Defined as F4 = 110 × Si + 48 × Mn + 550 × P. F4 is, C content, when the Ti content and the Nb content is within the above range, an indicator of the tensile strength of the steel sheet. If F4 is less than 120, the tensile strength of the steel sheet becomes less than 440 MPa, no high strength is obtained. If F4 is 120 or more, elements are in the above range, and the tensile strength of the steel sheet having a chemical composition satisfying the above formula (1) to (3) is equal to or greater than 440 MPa. Preferably, F4 is 130 or more.
[0070]
　[Galvannealed layer]
　galvannealed layer is formed on the surface of the base steel sheet. Galvannealed layer contains 7 to 15% by weight of Fe. If Fe content is less than 7%, the spot weldability is deteriorated. On the other hand, if the Fe content exceeds 15%, it decreases the adhesion of the galvannealed layer. If Fe content of 7 to 15 percent, excellent spot weldability and adhesiveness. By carrying out alloying treatment to be described later condition, Fe content of the galvannealed layer is within the above range. The preferable lower limit of the Fe content of the galvannealed layer is 8%, and the preferable upper limit is 12%.
[0071]
　The chemical composition of the galvannealed layer, and above Fe, contain and Al, the balance being Zn and impurities. Al is contained in the molten zinc plating bath described above. In the manufacturing process, when practicing the Ni pre-plating process to be described later, the chemical composition of the galvannealed layer may contain Ni.
[0072]
　Coating weight of the galvannealed layer is not particularly limited. From the viewpoint of corrosion resistance, the preferred coating weight of the galvannealed layer, 5 g / m on one surface adhered amount of Zn 2 is at least. From the viewpoint of workability and adhesion of the galvannealed layer, a preferable upper limit of the Zn single-side adhesion of the galvannealed layer is 100 g / m 2 is.
[0073]
　Average r value]
　The galvannealed steel sheet of the present invention, the average r value r is defined by the formula (A) m is 1.2 or more.
　r m = (r L + 2 × r 45 + r C ) / 4 (A)
[0074]
　R in the formula (A) L is the galvannealed steel sheet, which is parallel to the direction of the r values in the rolling direction. r 45 is the r value in 45 ° direction relative to the rolling direction. r C is a direction perpendicular r values in the rolling direction. r value is referred to as the Lankford value, indicating the plastic anisotropy of the steel sheet.
[0075]
　Furthermore the galvannealed steel sheet of the present invention, the in-plane anisotropy Δr value defined by the formula (B) is -0.5 to 0.5. =
　[Delta] r (r L + r C ) / 2-r　45 (B)
[0076]
　Represented by the side frame outer or the like, the processing degree of the high auto parts, are required to have excellent deep drawability. Average r value r m long is 1.2 or more, excellent deep drawability is obtained.
[0077]
　Furthermore, among the side frame outer, the four corners of the site where the door is fit, r 45 is required to have high, the hinge attachment portion of the door, r L is higher is required. If anisotropy Δr value is -0.5 to 0.5 surface, it is possible to meet these requirements, excellent deep drawability is obtained.
[0078]
　[Others]
　In the galvannealed steel sheet according to the present invention, from the viewpoint of coatability and weldability, the upper plating layer may be formed on the galvannealed layer described above. Further, with respect to the galvannealed steel sheet according to the present invention, the coating, corrosion resistance, lubricity, in view of improvement of such weldability, chromate treatment, phosphate treatment, be carried out various other chemical processes good.
[0079]
　[Production Method]
　An example of a method for manufacturing a galvannealed steel sheet according to the invention is as follows.
[0080]
　This manufacturing method includes a hot rolling step, a cold rolling step, the annealing step, the hot-dip galvanizing treatment step, the alloying treatment step. Below, it will be described in detail each of the steps.
[0081]
　[Hot Rolling Step]
　In the hot rolling process, first, to produce a molten steel having the above chemical composition (satisfying the formulas (1) to (4)) in a known manner. By casting a molten steel produced in a known manner, to produce the slabs. The method of cooling during casting, e.g., the well-known slow cooling or air cooling.

　By
　mass%, C: 0.0080% or
　less, Si: 0.7% or
　less, Mn: 1.0
　~ 2.5%, P: 0.030 ultrasonic
　0.048% ~, S: 0.025% or less ,
　Al: 0.005
　~ 0.20%, N: 0.010% or
　less,
　Ti: 0.005 ~ 0.040%, Nb: 0.005 ~ 0.060%, and,
　B: 0.0005 ~ containing 0.0030% the balance being Fe and impurities, and the base steel sheet having a chemical composition satisfying expressions (1) to (4),
　is formed on the surface of the base steel sheet, 7 ~ and a galvannealed layer containing 15 wt% of Fe,
　and the average r value is 1.2 or more,
　the in-plane anisotropy Δr value is -0.5 to 0.5,
　the tensile strength There is at least 440 MPa, galvannealed steel sheets.
　× P + 25 4 × Si ≦ 3.6
　(1) B-X1 ≧ 0.0005
　(2) C-(12/93) × Nb-X2 ≧ -0.0035
　(3) 110 × Si + 48 × Mn + 550 × P ≧ 120 (4)
　wherein each element symbol in the formula, the content of the corresponding element (mass%) is substituted.
　Further, X1 in the formula (2) is defined as follows.
　For N- (14/48) × Ti> 0 ,
　it is X1 = (11/14) [N- (14/48) ×
　Ti], the case of N- (14/48) × Ti ≦
　0, X1 = 0.
　Further, X2 in the formula (3) is defined as follows.
　Ti- (48/14) × N>
　0, is X2 = (12/48) [Ti- (48/14) ×
　N], Ti- (48/14) × For N ≦
　0, X2 = 0.
[Claim 2]
　A galvannealed steel sheet according to claim 1,
　wherein the chemical
　composition, C: less than%
　0.0040, Ti: 0.005 ~ 0.035%,
　and, Nb: 0.005 ~ 0. containing 035% galvannealed steel sheet.
[Claim 3]
　A galvannealed steel sheet according to claim 1,
　wherein the chemical
　composition,
　C: 0.0040 ~ 0.0080%, Ti: 0.005 ~ 0.040%,
　and, Nb: 0.005 containing ~ 0.060%,
　the lower limit of the formula (3) is -0.0002, galvannealed steel sheet.
[Claim 4]
　By carrying out hot rolling to produce a hot-rolled steel sheet slabs having the chemical composition according to claims 1 to 3 at a finish rolling temperature of 800 ~ 950 ° C., the heat at the coiling temperature of 600 - 760 ° C. a step of winding the rolled steel sheet,
　a step of performing a cold rolling to produce a cold-rolled steel sheet, with respect to the hot-rolled steel sheet
　to produce a base material steel plate by carrying out annealing treatment to the cold-rolled steel sheet step and,
　a step of performing hot-dip galvanizing process on the base material steel plate,
　to the melted zinc plated said base material steel plate, 10-40 seconds holding alloying treatment temperature of 470 ~ 620 ° C. and a step of performing an alloying treatment of method for producing a galvannealed steel sheet.
[Claim 5]
　Furthermore a method of manufacturing a galvannealed steel sheet according to claim 4,
　even after the step of producing the base material steel plate, before the step of performing said hot-dip galvanizing process, the base material steel plate a step of carrying out Ni pre-plating process,
　a post-process to carry out the Ni pre-plating process, before the step of performing said hot-dip galvanizing treatment step of heating the Ni pre-plated the base material steel plate comprising the door, the manufacturing method of the galvannealed steel sheet.
Drawing