High Strength Galvanized Steel Sheet And Method For Producing The Same

Abstract

A high-strength hot-dip galvanized steel sheet that even on the premise of ordinary CGL heat cycle, has a low yield stress and excels in resistance to natural aging and baking hardenability without reliance on the use of expensive Mo; and a process for producing the same. The constituent composition thereof comprises 0.01 to less than 0.08% C, 0.2% or less Si, more than 1.0 to 1.8% Mn, 0.10% or less P, 0.03% or less S, 0.1% or less Al, 0.008% or less N and more than 0.5% Cr so that the relationship 1.95.ltoreq.Mn(mass %)+1.3Cr(mass %).ltoreq.2.8 is satisfied and comprising the balance iron and unavoidable impurities. The structure thereof has a ferrite phase and a martensite phase of 2 to 15% area ratio, and the cumulative area ratio of pearlite phase and/or bainite phase is 1.0% or less. In the production of this hot-dip galvanized steel sheet, the temperature and cooling rate are controlled during the annealing/plating operation subsequent to cold rolling.

Description

RELATED APPLICATIONS

[0001] This is a .sctn.371 of International Application No. PCT/JP2008/062878, with an international filing date of Jul. 10, 2008 (WO 2009/008553 A1, published Jan. 15, 2009), which is based on Japanese Patent Application Nos. 2007-181946, filed Jul. 11, 2007, and 2008-177466, filed Jul. 8, 2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

[0002] This disclosure relates to a galvanized steel sheet which is suitably used, for example, in an automobile and a home electric appliance field and which has a low yield stress and superior anti-aging property and bake hardenability and to a method for producing the galvanized steel sheet.

BACKGROUND

[0003] In recent years, reduction in thickness of a steel sheet to improve fuel consumption by lightening an automobile body and increase in strength of a steel sheet to improve safety have been pursued. However, the increase in strength of a steel sheet generally causes deterioration of press-formability, i.e., wrinkles, which are called surface distortions, on the order of approximately several tens of micrometers are generated, so that degradation in the appearance of body shape disadvantageously occurs.

[0004] To solve the above problem, a steel sheet (BH steel sheet) which has low strength in press forming to be easily pressed and which exhibits high bake hardenability in a paint baking step performed after press formation has been developed. This steel sheet is obtained by controlling a dissolved C amount by addition of Ti and Nb to ultralow-carbon steel used as a base material, has superior surface distortion resistance since a yield stress (hereinafter referred to as "YP" in some cases) is low, such as approximately 240 MPa at a strength level of 340 MPa, and ensures dent resistance by increasing a yield stress (YP') after press forming and paint baking to approximately 300 MPa.

[0005] However, in view of the weight reduction, a steel sheet having a thickness smaller than that of a current 340BH steel sheet having a thickness of 0.65 to 0.80 mm has been desired and, for example, to reduce the thickness by 0.05 mm, the yield stress (YP') after press formation and paint baking must be increased to approximately 350 MPa or more. In addition, to ensure a high YP' while a low YP is maintained, a steel sheet is required which has high paint bake hardenability (hereinafter referred to as "BH" in some cases) and work hardenability (hereinafter referred to as "WH" in some cases).

[0006] From the situation as described above, for example, in Japanese Unexamined Patent Application Publication No. 6-122940, a method for obtaining a steel sheet which has a low yield stress besides high WH and BH properties and which further has superior anti-aging property has been disclosed in which annealing and cooling conditions of steel containing 0.005% to 0.0070% of C, 0.01% to 4.0% of Mn, and 0.01% to 3.0% of Cr are appropriately controlled, and in which the microstructure obtained by annealing is made to be a single phase transformed in low temperature.

[0007] In addition, in Japanese Unexamined Patent Application Publication No. 2005-281867, a method for obtaining a steel sheet having superior anti-aging property, shape fixability, and dent resistance besides high WH and BH properties and a low yield stress of 300 MPa or less has been disclosed. In that method, the steel sheet is formed by appropriately controlling annealing and cooling conditions of steel containing 0.04% or less of C, 0.5% to 3.0% of Mn, and 0.01% to 1.0% of Mo so that after annealing, a composite microstructure is obtained which includes 0.5% to less than 10% of retained austenite on a volume fraction basis and the balance being ferrite and a hard phase composed of bainite and/or martensite.

[0008] In Japanese Unexamined Patent Application Publication No. 2006-233249, a method for obtaining a steel sheet having high strength and high bake hardenability (BH) has been disclosed, which is obtained by appropriately controlling cooling conditions after annealing for steel containing 0.01% to less than 0.040% of C, 0.3% to 1.6% of Mn, 0.5% or less of Cr, 0.5% or less of Mo, and 1.3% to 2.1% of Mn+1.29 Cr+3.29 Mo so that the microstructure after annealing includes, on a volume fraction basis, 70% or more of ferrite and 1% to 15% of martensite.

[0009] In Japanese Unexamined Patent Application Publication No. 2006-52465, a method for obtaining a steel sheet having superior bake hardenability, anti-aging property, and press-formability has been disclosed, which is obtained by the steps of, after steel containing 0.0025% to less than 0.04% of C, 0.5% to 2.5% of Mn, and 0.05% to 2.0% of Cr is annealed at a temperature between the Ac1 transformation point and less than the Ac3 transformation point, performing cooling at a cooling rate of 15 to 200.degree. C./s in the temperature range of 650 to 450.degree. C., and further performing cooling at a cooling rate of less than 10.degree. C./s in the temperature range defined by the amounts of C, Mn, and Cr.

[0010] However, the above conventional techniques have the following problems.

[0011] For example, in the technique disclosed in Japanese Unexamined Patent Application Publication No. 6-122940, as the evaluation of the anti-aging property, the restoration amount of yield point elongation (hereinafter referred to as "YPEl" in some cases) after an artificial aging treatment at 100.degree. C. for 1 hour was used. However, when an equivalent aging time at 30.degree. C. is calculated using Hundy's equation shown by equation (1) (disclosed by Hundy, B. B "Accelerated Strain Ageing of Mild Steel", J. Iron & Steel Inst., 178, pp. 34 to 38, (1954)), it is approximately 18 days at 30.degree. C., and the technique described above cannot be always said to be superior in terms of anti-aging property. In addition, to obtain a single phase microstructure transformed in low temperature, for example, annealing was performed at an extremely high temperature region of 860 to 980.degree. C. However, in that case, troubles, such as sheet breakage, may occur in some cases. Accordingly, development is required to form a steel sheet having superior anti-aging property without performing high temperature annealing.

Log.sub.10(t.sub.r/t)=4,400(1/T.sub.r-1/T)-log.sub.10(T/T.sub.r) (1) [0012] T: Acceleration aging temperature (K) [0013] T.sub.r: Temperature (K) to be evaluated [0014] t: Aging time (Hr) at acceleration aging temperature T [0015] t.sub.r: Equivalent aging hour (Hr) converted at temperature T.sub.r (K) to be evaluated

[0016] According to the technique described in Japanese Unexamined Patent Application Publication No. 2005-281867, to enhance work hardenability (WH), 0.01% to 1.0% and preferably 0.1% to 0.6% of Mo is contained, and as a microstructure, retained austenite is used. However, since Mo is a very expensive element, and when 0.18% to 0.56% of Mo is added as disclosed in the example, the cost is considerably increased. On the other hand, in the comparative example among the examples in which the addition amount of Mo is extremely low, YR is high, and WH is extremely low. Accordingly, development of a steel sheet having a low YR and a high WH must be performed without using expensive Mo.

[0017] According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-233249, the volume fraction of martensite and the solute C in ferrite are controlled, and as cooling after annealing to obtain high bake hardenability, cooling is performed from a temperature of 550 to 750.degree. C. to a temperature of 200.degree. C. or less at a cooling rate of 100.degree. C./s. However, to satisfy the cooling conditions as described above, a specific method must be performed in which, for example, quenching is performed in jet water, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-233249, and it is difficult to perform manufacturing in a current continuous galvanizing line.

[0018] According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-52465, when cooling is performed after annealing, cooling in the temperature range of 650 to 450.degree. C. is performed at a cooling rate of 15 to 200.degree. C./s, and cooling in the temperature range defined by the amounts of C, Mn, and Cr is performed at less than 10.degree. C./s. According to the example, cooling from the annealing temperature to 680.degree. C. is performed at 3.degree. C./s, rapid cooling is performed at a rate of 80.degree. C./s to a temperature represented by Ts, slow cooling is performed at a rate of less than 10.degree. C./s to a temperature represented by Tf, and subsequently, cooling to 180.degree. C. and then to room temperature are performed at 15.degree. C./s and 100.degree. C./s, respectively. The technique described above can be performed in a CAL which performs no galvanizing treatment and which is provided with an overaging zone. However, the technique is difficult to perform in a CGL which performs a galvanizing treatment during cooling and which is not generally provided with an overaging apparatus (when a galvanizing treatment is performed, a steel sheet must be dipped in a galvanizing bath at a temperature of approximately 460.degree. C. for several seconds, and when alloying is further performed, a steel sheet must be heated to 500 to 600.degree. C. and maintained for several tens of seconds). In addition, when a CGL, which has a galvanizing treatment apparatus, is provided with an overaging zone, the line length is extremely increased. Hence, in general, an overaging zone is not provided therefor, and after a galvanizing treatment, gas cooling is performed. Hence, the case shown in the example in which the temperature range of 650 to 450.degree. C. is cooled at a rate of 15.degree. C./s or more, and the temperature range of 390.degree. C. or less is cooled at an extremely low cooling rate of approximately 1.3.degree. C./s is difficult to perform by a heat cycle performed in a current CGL. After cooling is performed to room temperature in accordance with the above cooling pattern, galvanizing may be performed. However, the cost is seriously increased in this case. Hence, it is necessary to develop a method for obtaining a superior material by a general heat treatment cycle in a CGL without using the thermal history as described above.

[0019] It could therefore be helpful to provide a high strength galvanized steel sheet having a low yield stress and superior anti-aging property and bake hardenability and a method for manufacturing the high strength galvanized steel sheet and, even if a general heat treatment cycle in a CGL is performed, this high strength galvanized steel sheet can be obtained without using expensive Mo.

SUMMARY

[0020] We found that, by controlling Mn and Cr, which have high hardening properties in a specific region, pearlite and bainite are suppressed even in a heat treatment cycle in a CGL, which has a low cooling rate. Hence, a low yield stress and high work hardenability can be obtained.

[0021] In addition, at the same Mn equivalent, as the Mn content is decreased, the A1 and A3 lines in a Fe--C phase diagram are shifted to a higher temperature side and a higher C content side. Hence the amount of solute C in ferrite is increased. Accordingly, when the Mn content is decreased, the BH property, which is a strain aging phenomenon of solute C, is improved. However, when the Mn content is excessively decreased, the aging property is degraded. Hence, it is important to control the Mn content in an appropriate range to simultaneously obtain the anti-aging property and the bake hardenability.

[0022] That is, we found that when the anti-aging property and the bake hardenability are well balanced at a high level by an appropriate control of the Mn content, and when the Mn equivalent (=Mn+1.3Cr) is controlled in an appropriate range by adjustment of the Cr content, a high strength galvanized steel sheet having a low yield stress and a high work hardenability can be manufactured.

[0023] We thus provide: [0024] [1] A high strength galvanized steel sheet has a composition containing, on a mass percent basis, 0.01% to less than 0.08% of C, 0.2% or less of Si, more than 1.0% to 1.8% of Mn, 0.10% or less of P, 0.03% or less of S, 0.1% or less of Al, 0.008% or less of N, more than 0.5% of Cr, and the balance being iron and inevitable impurities, in which 1.95.ltoreq.Mn (mass percent)+1.3Cr (mass percent).ltoreq.2.8 holds, wherein the microstructure includes a ferrite phase and 2% to 15% of martensite on an area ratio basis, and the total area ratio of pearlite and/or bainite is 1.0% or less. [0025] [2] According to the above [1], in the high strength galvanized steel sheet having excellent press formability, on a mass percent basis, the Cr content is more than 0.65%, and the Mn content is more than 1.0% to 1.6%. [0026] [3] According to the above [1] or [2], in the high strength galvanized steel sheet having excellent press formability, the composition further contains, on a mass percent basis, 0.01% or less of B. [0027] [4] According to one of the above [1] to [3], in the high strength galvanized steel sheet having excellent press formability, the composition further contains, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb. [0028] [5] A method for manufacturing a high strength galvanized steel sheet, comprises the steps of: performing hot rolling and cold rolling of a steel slab having the composition according to one of the above [1] to [4]; then performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and then performing cooling at an average cooling rate of 5.degree. C./s or more. [0029] [6] According to the above [5], the method for manufacturing a high strength galvanized steel sheet having excellent press formability further comprises the step of, after the step of performing galvanizing, performing an alloying treatment of a galvanizing layer.

DETAILED DESCRIPTION

[0030] % indicating the composition of steel is always on a mass percent basis. In addition, the high strength galvanized steel sheet is a galvanized steel sheet having a tensile strength of 340 MPa or more.

[0031] A high strength galvanized steel sheet having a low yield stress and superior anti-aging property and bake hardenability can be obtained. As a result, when the above steel sheet is used for automobile inner and outer panel application, weight reduction can also be achieved by thickness reduction.

[0032] In addition, since the high strength galvanized steel sheet has the superior properties described above, besides an automobile steel sheet, it can be widely used for home electric appliance application and the like. Hence, the steel sheet has industrial advantages.

[0033] The composition is defined such that the Mn content is more than 1.0% to 1.8% and the Cr content is more than 0.5, and in addition, the Mn equivalent is controlled in an appropriate range that satisfies 1.9.ltoreq.Mn (mass percent)+1.3Cr (mass percent).ltoreq.2.8. In addition, the microstructure is designed such that a ferrite phase and 2% to 15% of martensite on an area ratio basis are included, and that the total area ratio of pearlite and/or bainite is 1.0% or less. These are the most important features. When the composition and the microstructure as described above are prepared, as a result, a high strength galvanized steel sheet having a low yield stress and superior anti-aging property and bake hardenability can be obtained.

[0034] In addition, to manufacture the high strength galvanized steel sheet as described above which has a low yield stress and superior anti-aging property and bake hardenability, annealing/galvanizing conditions must be controlled, and annealing is performed at an annealing temperature of more than 750.degree. C. to less than 820.degree. C., cooling is performed at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed, and after galvanizing is performed, cooling is performed at an average cooling rate of 5.degree. C./s or more.

[0035] Hereinafter, our steels and methods will be described in detail.

[0036] First, the reasons for selecting chemical compositions of the steel will be described.

C: 0.01% to Less than 0.08%

[0037] C is effective to increase strength and is one of important elements. The content is set to 0.01% or more to ensure a predetermined amount or more of martensite. On the other hand, when the C content is 0.08% or more, since the amount of martensite is excessively large, YP is increased, the BH amount is decreased, and in addition, the weldability is degraded. Hence, the C content is set to less than 0.08% and, to obtain a lower YP and a higher BH, the C content is preferably set to less than 0.06% and more preferably set to 0.05% or less.

Si: 0.2% or Less

[0038] Si has a high solid-solution strengthening ability, and a lower Si content is preferable in terms of decrease in yield strength (decrease in YP). However, since a Si content of up to 0.2% is permissible, the Si content is set to 0.2% or less.

Mn: More than 1.0% to 1.8%

[0039] Mn is the most important element. When the Mn content is more than 1.8%, the amount of solute C in ferrite is decreased, and the BH property is degraded. In addition, when the Mn content is 1.0% or less, a high BH property is obtained since the amount of solute C in ferrite is large. However, on the other hand, the anti-aging property may be degraded in some cases. Hence, to simultaneously obtain the BH property and the anti-aging property, the Mn content is set in the range of more than 1.0% to 1.8% and is preferably set in the range of more than 1.0% to 1.6%.

P: 0.10% or Less

[0040] P is an effective element to increase strength. However, when the P content is more than 0.10%, the yield strength (YP) is increased, and surface-distortion resistance is degraded. Furthermore, an alloying speed of a galvanizing layer is decreased, surface defect occur, and in addition, resistance against secondary work-embrittlement is degraded due to segregation in grain boundaries of a steel sheet. Accordingly, the P content is set to 0.10% or less.

S: 0.03% or Less

[0041] S degrades ductility in hot rolling and enhances sensitivity of cracking in hot rolling. Hence, the content is preferably decreased. Further, when the S content is more than 0.03%, the ductility of the steel sheet is degraded due to precipitation of fine MnS, and the press formability is degraded. Hence, the S content is set to 0.03% or less. In addition, in view of the press formability, the S content is preferably set to 0.015% or less.

Al: 0.1% or Less

[0042] Al decreases inclusions in steel as a deoxidizing element and, in addition, it also functions to fix unnecessary solute N in steel in the form of a nitride. However, when the Al content is more than 0.1%, alumina inclusions in the form of clusters are increased, the ductility is degraded, and the press formability is also degraded. Hence, the Al content is set to 0.1% or less. To effectively use Al as a deoxidizing element and to sufficiently decrease oxygen in steel, 0.02% or more of Al is preferably contained.

N: 0.008% or Less

[0043] Since N in a solid solution state is not preferably present in view of anti-aging property, the content is preferably decreased. In particular, when the N content is more than 0.008%, the amount of a nitride forming element necessary to fix N is increased. Hence, manufacturing cost is increased. In addition, due to excessive generation of nitrides, the ductility and toughness are degraded. Hence, the N content is set to 0.008% or less. The N content is preferably set to less than 0.005% to ensure ductility and toughness.

Cr: More than 0.5%

[0044] Cr is a hardenability improving element and is a very important element for formation of martensite. In addition, since having a high hardenability and a low solid-solution hardenability as compared to those of Mn, Cr is effective to decrease YP, and Cr is positively added. However, when the Cr content is 0.5% or less, the hardenability and YP decreasing effect may not be obtained in some cases. Hence, the content is set to more than 0.5% and is preferably more than 0.65%.

[0045] In addition, as described above, to simultaneously obtain the BH property and the anti-aging property, the content of Mn is limited. Hence, even in a heat treatment cycle in a CGL in which the cooling rate is low, to suppress pearlite and bainite and to decrease YP, the Mn equivalent must be controlled to be a predetermined level by adjusting the Cr content. Accordingly, the Cr content is set to more than 0.5% and is preferably set to more than 0.65%.

Mn+1.3Cr: 1.9% to 2.8%

[0046] The value of Mn+1.3Cr is one index indicating the hardenability and it is important to control the value to form martensite. When the value of Mn+1.3Cr is less than 1.9%, the hardenability becomes insufficient, and pearlite and bainite are liable to be generated during cooling performed after annealing, so that YP is increased. On the other hand, when the value of Mn+1.3Cr is more than 2.8, the hardenability effect is saturated, and by excessive addition of alloying elements, manufacturing cost is increased. Hence, the value of Mn+1.3Cr is set in the range of 1.9% to 2.8% and is preferably set in the range of more than 2.3% to 2.8%.

[0047] Targeted properties of the steel can be obtained by those essential addition elements described above. However, besides those elements, whenever necessary, the following elements may also be added.

B: 0.01% or Less

[0048] B is a hardenability improving element and can be added in an amount of 0.0005% or more to stably form martensite. Furthermore, when 0.0015% to 0.004% of B is added, besides improvement in grain growth properties of ferrite, BH can be improved, and balance between decrease in YP and increase in BH can be further improved. However, when more than 0.01% of B is added, adverse influence on the mechanical properties and the productivity in casting are enhanced. Hence, when B is added, the content thereof is set to 0.01% or less. At least one of Mo: 0.15% or less, V: 0.5% or less, Ti: 0.1% or less, and Nb: 0.1% or less

Mo: 0.15% or Less

[0049] Mo is an expensive element and is an element to increase YP. However, Mo is also an effective element which improves zinc coating surface quality, or improves hardenability and stably obtains martensite, and 0.01% or more of Mo may be added. However, when the Mo content is more than 0.15%, the effects thereof is saturated, and cost is seriously increased. Hence, when Mo is added, 0.15% or less of Mo may be added so that an adverse influence thereof, increase in YP, is not so significant. In view of reduction in cost and decrease in YP, the content of Mo is preferably decreased as small as possible, and Mo is preferably not to be added (0.02% or less of Mo being present as an inevitable impurity).

V: 0.5% or Less

[0050] V is a hardenability improving element and may be added in an amount of 0.01% or more to stably form martensite. However, even when V is excessively added, an effect corresponding to the cost cannot be obtained. Hence, when V is added, the content thereof is set to 0.5% or less.

Ti: 0.1% or Less, and Nb: 0.1% or Less

[0051] Ti and Nb each form carbide, nitride and carbonitride and decrease the amounts of solute C and N, and to prevent degradation of mechanical properties during aging, each element in an amount of 0.01% or more may be added. However, even when the element in an amount of more than 0.1% is excessively added, the effect is saturated, and an effect corresponding to the cost cannot be obtained. Hence, when Ti and/or Nb is added, the content of each element is set to 0.1% or less.

[0052] In addition, the balance other than those elements described above includes Fe and inevitable impurities. As the inevitable impurities, for example, since O forms non-metal inclusions and has an adverse influence on the quality, the content of O is preferably decreased to 0.003% or less.

[0053] Next, the microstructure of the high strength galvanized steel sheet having excellent press formability will be described.

A Ferrite Phase and 2% to 15% of Martensite on an Area Ratio Basis

[0054] The galvanized steel sheet has a dual phase microstructure containing a ferrite phase and 2% to 15% of martensite on an area ratio basis. When the martensite is controlled in the range described above, the surface-distortion resistance and work-hardenability are improved, so that a steel sheet usable for automobile outer panel application can be obtained. When the area ratio of the martensite is more than 15%, the strength is significantly increased, and for example, as a steel sheet for an automobile inner/outer plate panel, that is typically intended, sufficient surface-distortion resistance and press formability cannot be obtained. Hence, the area ratio of martensite is set to 15% or less. On the other hand, when the area ratio of martensite is less than 2%, YPEl is liable to remain and, in addition, YP is increased, so that the surface-distortion resistance is degraded. Hence, the area ratio of martensite is set in the range of 2% to 15% and is preferably set in the range of 2% to 10%.

Total Area Ratio of Pearlite and Bainite: 1.0% or Less

[0055] In the case in which slow cooling is performed after annealing and, in particular, an alloying treatment is performed, when the Mn equivalent is not optimized, fine pearlite or bainite is generated primarily adjacent to martensite, so that YR is increased. That is, since YP can be decreased when the total area ratio of pearlite and/or bainite is set to 1.0% or less, this total area ratio is set to 1.0% or less.

[0056] In addition, besides the ferrite phase, martensite, pearlite, and bainite, retained .gamma. and/or inevitable carbides having an area ratio of approximately 1.0% may be contained.

[0057] Here, the area ratio can be obtained by the steps of polishing an L cross-section (vertical cross-section parallel to a rolling direction) of a steel sheet, etching the cross-section using nital, observing 12 visual fields at a magnification of 4,000 times power using a SEM, and performing image analysis of an obtained microstructure photograph. In the microstructure photograph, a blackish contrast region indicates ferrite, a region in which carbides are generated in the form of lamellas or points is regarded as pearlite and bainite, and particles having a white contrast are regarded as martensite.

[0058] When the Mn equivalent and the cooling conditions after annealing are appropriately controlled, the microstructure can be controlled in the above area ratio range.

[0059] Next, conditions for manufacturing the high strength galvanized steel sheet will be described. The high strength galvanized steel sheet is manufactured by the steps of forming a slab by melting steel adjusted in the above chemical composition range; then performing hot rolling, followed by (pickling) cold rolling; then, after annealing, performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; and after galvanizing, performing cooling at an average cooling rate of 5.degree. C./s or more.

[0060] In this case, the method for melting and refining steel is not particularly limited, and an electric furnace may be used, or a converter may be used. In addition, as a method for casting steel after the melting and refining, a cast slab may be formed by a continuous casting method, or an ingot may be formed by an ingot-making method.

[0061] When the slab is hot-rolled after continuous casting, rolling may be performed after the slab is re-heated in a heating furnace, or direct rolling may be performed without heating the slab. In addition, after blooming is performed for the ingot thus formed, hot rolling may be performed.

[0062] Hot rolling may be performed in accordance with an ordinary method, for example, such that the temperature for heating the slab is set to 1,100 to 1,300.degree. C., the finish rolling temperature is set to the Ar3 point or more, the cooling rate after the finish rolling is set to 10 to 200.degree. C./s, and the coiling temperature is set to 400 to 750.degree. C. The reduction ratio of cold rolling may be set to 50 to 85% which is the range performed in a general operation.

[0063] Hereinafter, annealing and galvanizing steps (CGL process) will be described in detail.

Annealing Temperature: More than 750.degree. C. to Less than 820.degree. C.

[0064] The annealing temperature must be increased to an appropriate temperature to obtain a microstructure containing a ferrite phase and martensite. When the annealing temperature is 750.degree. C. or less, since austenite is not sufficiently formed, a predetermined amount of martensite cannot be obtained. Hence, for example, due to remaining YPEl, increase in YP, the surface-distortion resistance is degraded. On the other hand, when the annealing temperature is 820.degree. C. or more, the amount of solute C in ferrite is decreased, and a high BH amount may not be obtained in some cases. In addition, enrichment of C in austenite is not sufficiently performed, and pearlite and bainite are liable to be generated during the subsequent cooling and alloying treatments, so that increase in YP ccurs. Hence, the annealing temperature is set to more than 750.degree. C. to less than 820.degree. C.

Primary Average Cooling Rate: 3 to 15.degree. C./s

[0065] In manufacturing the galvanized steel sheet, after the annealing, the primary average cooling rate from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed is set to 3 to 15.degree. C./s. When the cooling rate is less than 3.degree. C./s, since the pearlite and bainite significantly generate during cooling, YP is increased. In addition, since pearlite and bainite are generated, a predetermined amount of martensite cannot be obtained, and since YPEl remains, YP is increased. On the other hand, when the cooling rate is more than 15.degree. C./s, enrichment of C, Mn, and Cr in austenite is not sufficiently performed, and austenite is decomposed into pearlite and bainite during the subsequent cooling and alloying treatments, so that the amounts thereof are increased. Hence, YP is increased. In addition, enrichment of C in ferrite is suppressed, and a high BH amount may not be obtained in some cases. Hence, after the annealing, the primary average cooling rate is set to 3 to 15.degree. C./s from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed. A preferable average cooling rate is 5 to 15.degree. C./s. In addition, a galvanizing bath temperature in a galvanizing treatment may be a common temperature, such as approximately 400 to 480.degree. C.

[0066] In addition, after the galvanizing treatment is performed, whenever necessary, the alloying treatment may be performed. In this case, the alloying treatment after the galvanizing is performed, for example, such that after the dipping in a galvanizing bath is performed, whenever necessary, heating is performed to a temperature range of 500 to 700.degree. C., and the temperature is maintained for several seconds to several tens of seconds. According to a conventional steel sheet in which the Mn equivalent is not specified, the mechanical properties are seriously degraded by the alloying treatment as described above. However, according to our steels, the increase in YP is small even if the alloying treatment as described above is performed.

[0067] In addition, as the galvanizing conditions, a coating amount per one surface is preferably 20 to 70 g/m.sup.2, and when the alloying treatment is performed, the Fe content in the coating layer is preferably set to 6% to 15%.

Secondary Cooling Rate: 5.degree. C./s or More

[0068] In the secondary cooling to be performed after the galvanizing treatment or the alloying treatment, to obtain a predetermined amount of martensite, cooling is performed at an average cooling rate of 5.degree. C./s or more to a temperature of the Ms point or less. By slow cooling in which the secondary cooling rate is less than 5.degree. C./s, pearlite or bainite is generated at approximately 400 to 500.degree. C., so that YP is increased. On the other hand, although it is not necessary to limit the upper limit of the secondary cooling rate, when it is more than 100.degree. C./s, martensite is excessively hardened, so that the ductility is degraded. Hence, the second cooling rate is preferably 100.degree. C./s or less. Accordingly, the secondary cooling rate is set to 5.degree. C./s or more and is preferably set to 10 to 100.degree. C./s.

[0069] Furthermore, temper rolling may also be performed on the steel sheet after the heat treatment for shape flattening. In addition, a steel material is supposed to be manufactured by the steps including general steel making, casting, and hot rolling. However, by omitting part of the hot rolling step or all thereof, a steel material may be manufactured, for example, by thin-slab casting.

[0070] In addition, the surface of the galvanized steel sheet may be further processed by an organic film treatment.

EXAMPLES

Example 1

[0071] Hereinafter, our steel sheets and methods will be further described with reference to Examples.

[0072] Several types of steel having chemical compositions of steel A to Y shown in Table 1 were melted by vacuum melting, so that slabs were formed. After these slabs were heated to 1,200.degree. C. and were then hot rolled at a finish temperature of 850.degree. C., cooling was performed, and coiling was then performed at 600.degree. C., so that a hot-rolled band having a thickness of 2.5 mm was manufactured. After pickling was performed for the hot-rolled band thus obtained, cold rolling was performed at a reduction ratio of 70%, so that a cold-rolled steel sheet having a thickness of 0.75 mm was obtained.

TABLE-US-00001 TABLE 1 Chemical Compositions (Mass Percent) Mn + 1.3Cr Steel C Si Mn P S Sol. Al N Cr Others (%) Remarks A 0.042 0.02 1.08 0.010 0.015 0.035 0.0045 0.70 -- 1.99 Invention Steel B 0.034 0.01 1.25 0.009 0.005 0.040 0.0043 0.71 -- 2.17 Invention Steel C 0.028 0.01 1.42 0.009 0.005 0.040 0.0043 0.71 -- 2.34 Invention Steel D 0.021 0.02 1.56 0.013 0.005 0.050 0.0040 0.72 -- 2.50 Invention Steel E 0.013 0.01 1.78 0.009 0.012 0.040 0.0040 0.71 -- 2.70 Invention Steel F 0.035 0.01 1.57 0.015 0.009 0.040 0.0040 0.57 -- 2.31 Invention Steel G 0.032 0.02 1.56 0.010 0.015 0.035 0.0050 0.72 -- 2.50 Invention Steel H 0.028 0.02 1.55 0.011 0.005 0.035 0.0050 0.92 -- 2.75 Invention Steel I 0.022 0.01 1.56 0.009 0.005 0.040 0.0043 0.71 -- 2.48 Invention Steel J 0.032 0.02 1.58 0.013 0.005 0.050 0.0040 0.73 -- 2.53 Invention Steel K 0.042 0.01 1.57 0.009 0.012 0.040 0.0040 0.71 -- 2.49 Invention Steel L 0.074 0.01 1.56 0.011 0.025 0.040 0.0040 0.72 B: 0.0017 2.50 Invention Steel M 0.032 0.01 1.58 0.011 0.009 0.030 0.0055 0.71 -- 2.50 Invention Steel N 0.031 0.05 1.57 0.011 0.015 0.035 0.0035 0.71 Mo: 0.08 2.49 Invention Steel O 0.033 0.05 1.56 0.011 0.005 0.050 0.0050 0.70 Mo: 0.05 2.47 Invention B: 0.002 Steel P 0.033 0.05 1.55 0.010 0.012 0.035 0.0040 0.71 Ti: 0.02 2.47 Invention V: 0.05 Steel Q 0.032 0.02 1.58 0.011 0.009 0.035 0.0045 0.71 Nb: 0.01 2.50 Invention Steel R 0.028 0.02 1.85 0.011 0.005 0.050 0.0040 0.45 -- 2.44 Comparative Steel S 0.050 0.02 0.95 0.010 0.005 0.050 0.0040 0.90 -- 2.12 Comparative Steel T 0.055 0.02 2.02 0.011 0.005 0.040 0.0035 0.58 -- 2.77 Comparative Steel U 0.050 0.02 1.12 0.009 0.005 0.050 0.0040 0.52 -- 1.80 Comparative Steel V 0.004 0.01 1.77 0.011 0.025 0.040 0.0040 0.77 -- 2.77 Comparative Steel W 0.040 0.02 1.02 0.026 0.012 0.081 0.0018 0.7 B: 0.0015 1.93 Invention Steel X 0.029 0.01 1.37 0.018 0.005 0.064 0.0037 0.6 B: 0.0031 2.15 Invention Steel Y 0.028 0.01 1.38 0.008 0.0008 0.056 0.0029 0.64 B: 0.0015, 2.21 Invention Ti: 0.004 Steel

[0073] Next, samples obtained by cutting off from the cold-rolled steel sheets, which were obtained as described above, were sequentially processed by the steps of performing annealing at annealing temperatures shown in Table 2 for 60 seconds in an infrared image furnace; performing primary cooling under conditions shown in Table 2; performing galvanizing (galvanizing bath temperature: 460.degree. C.); performing an alloying treatment (520.degree. C..times.15 s); performing secondary cooling to a temperature of 150.degree. C. or less; and performing temper rolling at an extension rate of 0.4%. In this case, the galvanizing treatment was adjusted to have a coating weight of 50 g/m.sup.2 per one surface, and the alloying treatment was adjusted so that the Fe content in the coating layer was 9% to 12%.

[0074] From the galvanized steel sheets obtained as described above, samples were obtained, and the area ratio of martensite and the total area ratio of pearlite and/or bainite were measured. In addition, the tensile properties, work hardening amount (WH), bake hardening amount (BH), and yield point elongation (YPEl) obtained after an acceleration aging test were measured. The detailed measurement methods are described below: [0075] (1) Area ratio of Martensite: After an L cross-section (vertical cross-section parallel to the rolling direction) was mechanically polished and was then etched with nital, 12 visual fields were observed by a scanning electron microscope (SEM) at a magnification of 4,000 times power, and quantification was performed using an obtained photograph (SEM photograph) of microstructure. In the photograph, particles having a white contrast were regarded as martensite, and remaining parts having a black contrast were regarded as ferrite, so that the ratio of martensite with respect to the overall area was obtained. [0076] (2) Tensile Properties: JIS No. 5 test pieces were obtained in a 90.degree.-direction (C direction) with respect to the rolling direction, and a tensile test in accordance with JIS Z2241 was performed, so that the yield stress (YP) and the tensile strength (TS) were measured. [0077] (3) Work Hardenability Amount (WH): The difference between a stress at a pre-strain of 2% and the yield stress (YP) was measured. [0078] (4) Bake Hardenability Amount (BH): The difference between a stress at a pre-strain of 2% and the yield stress obtained by a heat treatment corresponding to paint baking at 170.degree. C. for 20 minutes. [0079] (5) Yield Point Elongation (YPEl) after Acceleration Aging Test: After a heat treatment at 100.degree. C. for 24 hours, YPEl was measured by the tensile test (in accordance with HS Z2241). In consideration of the case in which a steel sheet crosses the red line for export, the acceleration aging conditions were set so that the equivalent aging times obtained from Hundy's equation were 1.2 years at 30.degree. C. and approximately 2 months at 50.degree. C.

[0080] The measurement results are shown in Table 2 together with the manufacturing conditions.

TABLE-US-00002 TABLE 2 Annealing and Galvanizing Conditions Temper Microstructure Primary Secondary Rolling Total Area Annealing Cooling Cooling Extension Primary Martensite Ratio of Steel Temperature Rate Alloying Rate Rate Microstructure Area Ratio Pearlite and No. No. (.degree. C.) (.degree. C./s) Conditions (.degree. C./s) (%) E* (%) Bainite (%) 1 A 770 12 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 2.9 0.95 2 B 770 12 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 2.8 0.73 3 C 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.6 0.69 4 D 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.6 0.58 5 E 800 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.5 0.54 6 F 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.2 0.72 7 G 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.0 0.58 8 H 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.1 0.52 9 I 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.9 0.58 10 J 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.6 0.60 11 K 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 5.6 0.59 12 L 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 13.3 0.51 13 M 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.2 0.61 14 N 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.4 0.63 15 O 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.3 0.64 16 P 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.3 0.62 17 Q 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.5 0.64 18 R 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.5 0.53 19 S 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.2 0.96 20 T 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 11.6 0.52 21 U 780 12 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 1.4 1.44 22 V 780 12 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 1.1 0.53 40 W 785 12 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 4.0 0.34 41 X 785 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 3.9 0.24 42 Y 785 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 3.8 0.26 Mechanical Properties YPEI After YP TS YR WH BH YP' Aging No. (MPa) (MPa) (%) (MPa) (MPa) (MPa) (%) Remarks 1 244 461 52.9 59 81 384 0.2 Invention Example 2 234 455 51.4 61 78 373 0.1 Invention Example 3 223 453 49.2 64 72 359 0 Invention Example 4 217 452 48.0 67 66 350 0 Invention Example 5 215 451 47.7 70 55 340 0 Invention Example 6 235 477 49.3 70 66 371 0 Invention Example 7 228 473 48.2 71 67 366 0 Invention Example 8 224 475 47.2 75 66 365 0 Invention Example 9 219 452 48.5 65 66 350 0 Invention Example 10 237 484 49.0 72 67 376 0 Invention Example 11 243 498 48.8 80 66 389 0 Invention Example 12 254 529 48.0 110 61 425 0 Invention Example 13 228 475 48.0 72 66 366 0 Invention Example 14 238 480 49.6 68 71 377 0 Invention Example 15 236 478 49.4 67 68 371 0 Invention Example 16 238 479 49.7 71 67 376 0 Invention Example 17 240 481 49.9 72 66 378 0 Invention Example 18 235 482 48.8 71 48 354 0 Comparative Example 19 230 443 51.9 63 82 375 1.8 Comparative Example 20 272 566 48.1 102 41 415 0 Comparative Example 21 245 441 55.6 62 78 385 1.6 Comparative Example 22 261 432 60.4 63 57 381 1.2 Comparative Example 40 219 469 46.7 77 87 383 0 Invention Example 41 207 454 45.6 73 79 359 0 Invention Example 42 210 458 45.9 72 80 362 0 Invention Example *F: Ferrite, M: Martensite, P: Pearlite, B: Bainite

[0081] In Table 2, the compositions and the manufacturing conditions of Nos. 1 to 17 and 40 to 42 are within our range, and the microstructures thereof are our examples in which the area ratio of martensite is in the range of 2% to 15%, and the total area ratio of pearlite and/or bainite is 1.0% or less. Compared to comparative examples, our examples have a low YR and a high BH, and YPEl after aging is also low, such as 0.2% or less.

[0082] On the other hand, according to Nos. 18 to 22 of the comparative examples manufactured using steel R to V which are outside our predetermined composition, at least one of YR, BH, and YPEl after aging are inferior.

[0083] As for No. 18 (steel R), the Mn content and the Cr content are outside our range and, since the Mn content is particularly high, the BH amount is low. As for No. 19 (steel S), since the Mn content is low, the amount of solute C in ferrite is large, and a high BH is obtained. However, on the other hand, YPEl after aging is high, so that the anti-aging property is inferior. As for No. 20 (steel T), since the Mn content is high, the amount of solute C in ferrite is small, so that BH is low. In addition, since ferrite is sold-solution strengthened, YP is relatively high, and the surface-distortion resistance is inferior. As for No. 21 (steel U), since the value of Mn+1.3Cr is low, pearlite and bainite are generated during cooling performed after annealing, and a predetermined amount of martensite can not be ensured. Hence, YR is relatively high, and YPEl after aging is also high. As for No. 22 (steel V), since the amount of C is small, a predetermined amount of martensite can not be obtained. Hence, YR is high, and YPEl after aging is also high.

Example 2

[0084] Several types of steel having chemical compositions of steel C, D, E, and G shown in Table 1 were melted by vacuum melting, and under conditions similar to those in Example 1, they were then processed by hot rolling, pickling, and cold rolling, followed by annealing at annealing temperatures shown in Table 3 for 60 seconds. Subsequently, after primary cooling under conditions shown in Table 3 and a galvanizing treatment (galvanizing bath temperature: 460.degree. C.) were performed, an alloying treatment was performed, and secondary cooling to a temperature of 150.degree. C. or less and temper rolling were then performed.

[0085] Samples were obtained from the galvanized steel sheets thus obtained, and by methods similar to those in Example 1, the area ratio of martensite and the total area ratio of pearlite and/or bainite were measured. In addition, the tensile properties, work hardenability amount (WH), bake hardenability amount (BH), and YPEl after an acceleration aging test were measured.

[0086] The obtained results are shown in Table 3 together with the manufacturing conditions.

TABLE-US-00003 TABLE 3 Annealing and Temper Microstructure Plating Conditions Rolling Total Area Annealing Primary Secondary Extension Martensite Ratio of Steel Temperature Cooling Rate Alloying Cooling Rate Rate Primary Area Ratio Pearlite and No. No. (.degree. C.) (.degree. C./s) Conditions (.degree. C./s) (%) Microstructure* (%) Bainite (%) 23 C 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.6 0.69 24 740 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 1.4 0.32 25 760 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.5 0.66 26 800 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 3.2 0.72 27 840 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.3 1.06 28 D 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.6 0.58 29 780 5 520.degree. C. .times. 15 s 20 NONE F + M + P/B 2.7 0.56 30 800 5 NONE 20 0.4 F + M + P/B 3.3 0.34 31 E 800 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 2.5 0.54 32 800 2 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 1.1 1.45 33 770 20 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 1.3 1.31 34 780 5 520.degree. C. .times. 15 s 3 0.4 F + M + P/B 1.4 1.41 35 800 5 520.degree. C. .times. 15 s 40 0.4 F + M + P/B 3.2 0.55 36 G 780 5 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 4.0 0.58 37 760 5 520.degree. C. .times. 15 s 20 NONE F + M + P/B 4.2 0.57 38 800 10 520.degree. C. .times. 15 s 20 0.4 F + M + P/B 5.4 0.62 39 800 5 NONE 30 0.4 F + M + P/B 4.8 0.28 Mechanical Properties YPEI After YP TS YR WH BH YP' Aging No. (MPa) (MPa) (%) (MPa) (MPa) (MPa) (%) Remarks 23 223 453 49.2 64 72 359 0 Invention Example 24 248 428 57.9 52 83 383 1.0 Comparative Example 25 215 443 48.5 61 75 351 0 Invention Example 26 223 459 48.6 67 68 358 0 Invention Example 27 249 478 52.1 68 59 376 0 Comparative Example 28 217 452 48.0 67 66 350 0 Invention Example 29 197 448 44.0 83 68 348 0 Invention Example 30 217 461 47.1 69 66 352 0 Invention Example 31 215 451 47.7 70 55 340 0 Invention Example 32 232 411 56.4 44 86 362 2.0 Comparative Example 33 233 421 55.3 48 82 363 1.0 Comparative Example 34 236 429 55.0 49 80 365 0.8 Comparative Example 35 227 459 49.5 71 53 351 0 Invention Example 36 228 473 48.2 71 65 364 0 Invention Example 37 208 472 44.1 93 66 367 0 Invention Example 38 243 495 49.1 79 65 387 0 Invention Example 39 240 486 49.4 75 67 382 0 Invention Example *F: Ferrite, M: Martensite, P: Pearlite, B: Bainite

[0087] As shown in Table 3, the compositions and the manufacturing conditions of Nos. 23, 25, 26, 28 to 31, and 35 to 39 are within our range, and the microstructures thereof are our examples in which the area ratio of martensite is in the range of 2% to 15%, and the total area ratio of pearlite and/or bainite is 1.0% or less. Compared to comparative examples, our examples have a lower YR and a higher BH, and YPEl after aging is also smaller, such as 0.2% or less.

[0088] On the other hand, as for No. 24, since the annealing temperature is low, a predetermined amount of martensite can not be obtained, YR is high, and YPEl after aging is also high, so that the anti-aging property is inferior.

[0089] As for No. 27, since the annealing temperature is high, enrichment of elements in austenite during annealing is insufficient. Hence, pearlite and bainite are generated during the alloying treatment. As a result, compared to our example having the same strength as that of No. 27, YR is relatively high.

[0090] As for No. 32, since the primary cooling rate is low, its cooling curve come across pearlite and bainite noses, and the generation amounts thereof are increased, so that YP is increased. In addition, since pearlite and bainite are generated, a predetermined amount of martensite can not be obtained, and due to remaining YPEl, YP is increased. Hence, YR is relatively high, and YPEl after aging is also relatively high.

[0091] As for No. 33, since the primary cooling rate is high, enrichment of elements in austenite is insufficient, and pearlite and bainite are liable to be generated during the alloying treatment. As a result, the martensite area ratio obtained after cooling is decreased, YR is relatively high, and YPEl after aging is also high.

[0092] As for No. 34, since the secondary cooling rate is low, austenite is decomposed into pearlite and bainite in a temperature range of approximately 400 to 500.degree. C. during the secondary cooling, and the amounts thereof are increased. Hence, the martensite area ratio obtained after cooling is decreased. Accordingly, YR is relatively high, and YPEl after aging is also high.

INDUSTRIAL APPLICABILITY

[0093] Since our high strength galvanized steel sheet has a low yield stress and also has superior anti-aging property and bake hardenability, the steel sheet can be applied to parts which require high formability, such as automobile inner and outer plate application.

Claims

1. A high strength galvanized steel sheet having a composition which contains, on a mass percent basis, 0.01% to less than 0.08% of C, 0.2% or less of Si, more than 1.0% to 1.8% of Mn, 0.10% or less of P, 0.03% or less of S, 0.1% or less of Al, 0.008% or less of N, more than 0.5% of Cr, and the balance being iron and inevitable impurities, wherein 1.95.ltoreq.Mn (mass percent)+1.3Cr (mass percent).ltoreq.2.8 and the microstructure includes a ferrite phase and 2% to 15% of martensite on an area ratio basis, and the total area ratio of perlite and/or bainite is 1.0% or less.

2. The high strength galvanized steel sheet according to claim 1, wherein, on a mass percent basis, the Cr content is more than 0.65%, and the Mn content is more than 1.0% to 1.6%.

3. The high strength galvanized steel sheet according to claim 1, wherein the composition further contains, on a mass percent basis, 0.01% or less of B.

4. The high strength galvanized steel sheet according to claim 1, wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb.

5. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 1; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.

6. The method according to claim 5, further comprising, after performing galvanizing, performing an alloying treatment of a galvanizing layer.

7. The high strength galvanized steel sheet according to claim 2, wherein the composition further contains, on a mass percent basis, 0.01% or less of B.

8. The high strength galvanized steel sheet according to claim 2, wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb.

9. The high strength galvanized steel sheet according to claim 3, wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb.

10. The high strength galvanized steel sheet according to claim 7, wherein the composition further comprises, on a mass percent basis, at least one selected from 0.15% or less of Mo, 0.5% or less of V, 0.1% or less of Ti, and 0.1% or less of Nb.

11. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 2; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.

12. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 3; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.

13. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 4; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.

14. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 7; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.

15. A method for manufacturing a high strength galvanized steel sheet, comprising: performing hot rolling and cold rolling of a steel slab having the composition according to claim 10; performing annealing at an annealing temperature of more than 750.degree. C. to less than 820.degree. C.; performing cooling at an average cooling rate of 3 to 15.degree. C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; performing galvanizing; and performing cooling at an average cooling rate of 5.degree. C./s or more.