JP5256689B2 - High-strength hot-dip galvanized steel sheet excellent in workability and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in workability used in industrial fields such as automobiles and electricity, and a method for producing the same.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, the movement to increase the strength and thickness of the steel sheet, which is the body material, and to reduce the weight of the vehicle body has become active. However, in general, increasing the strength of a steel sheet causes a decrease in the ductility of the steel sheet, that is, a decrease in workability. Therefore, a hot dip galvanized steel sheet having both high strength and high workability and excellent corrosion resistance is desired. ing.

  In response to these demands, high strength molten zinc of composite structure type, such as DP (Dual Phase) steel composed of ferrite and martensite and TRIP (Transformation Induced Plasticity) steel using transformation induced plasticity of retained austenite, has been developed so far. Plated steel sheets have been developed. For example, Patent Document 1 includes mass%, C: 0.05 to 0.15%, Si: 0.3 to 1.5%, Mn: 1.5 to 2.8%, P: 0.03% or less, S: 0.02% or less, Al: 0.005 to 0.5. %, N: 0.0060% or less, the balance is Fe and inevitable impurities, and (Mn%) / (C%) ≧ 15 and (Si%) / (C%) ≧ 4 are satisfied. A high-strength galvannealed steel sheet with good workability containing 3 to 20% martensite and retained austenite has been proposed. However, such a high-strength hot-dip galvanized steel sheet of the composite structure type has a problem that it has a high elongation El obtained by uniaxial tension, but is inferior in stretch flangeability required for hole expansion processing.

Therefore, as a high-strength hot-dip galvanized steel sheet with excellent stretch flangeability, Patent Document 2 includes mass%, C: 0.02 to 0.30%, Si: 1.50% or less, Mn: 0.60 to 3.0%, P: 0.20%. Below, S: 0.05% or less, Al: 0.01 to 0.10%, the balance of Fe and inevitable impurities steel, hot rolled above the Ac ₃ transformation point, pickled, cold rolled, continuously annealed molten zinc In the plating line, it is heated and maintained above the recrystallization temperature and above the Ac ₁ transformation point, and then rapidly cooled below the Ms point until reaching the molten zinc bath to partially or fully martensite in the steel sheet. High strength hot dip galvanized steel sheet with excellent stretch flangeability that is generated and then heated to at least the hot dip zinc bath temperature and the alloying furnace temperature at a temperature equal to or higher than the Ms point to generate partially or fully tempered martensite A manufacturing method is disclosed.
JP 11-279691 A JP-A-6-93340

  However, in the high-strength hot-dip galvanized steel sheet described in Patent Document 2, excellent stretch flangeability is obtained, but not only the product of tensile strength TS and El obtained by uniaxial tension, that is, the TS-El balance is low, The yield ratio YR (= YS / TS), which is the ratio between the yield strength YS and TS, is high and there is a problem in workability.

  An object of the present invention is to provide a high-strength hot-dip galvanized steel sheet having a high TS-El balance, excellent stretch flangeability, and low YR workability, and a method for producing the same.

  The present inventors have a high TS-El balance, specifically TS × El ≧ 19000 MPa ·%, excellent in stretch flangeability, specifically, a hole expansion ratio λ ≧ 70% described later, and a low YR, Specifically, the present inventors have made extensive studies on a high-strength hot-dip galvanized steel sheet excellent in workability with YR <75%, and found the following.

  i) By optimizing the component composition, by making the microstructure containing 20 to 87% ferrite, 3 to 10% total martensite and residual austenite, and 10 to 60% tempered martensite in terms of area ratio In addition to excellent stretch flangeability, high TS-El balance and low YR can be achieved.

  ii) These microstructures are forcibly cooled from a heating temperature of 750 to 950 ° C. to a temperature range of (Ms point −100 ° C.) to (Ms point −200 ° C.) during annealing, and then reheated and hot dip galvanized. Can be obtained. Here, the Ms point is a temperature at which martensitic transformation of austenite starts, and can be obtained from a change in the coefficient of linear expansion of steel during cooling.

  The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.3%, Si: 0.01 to 2.5%, Mn: 0.5 to 3.5%, P: 0.003 to 0.100%, S: 0.02 %: Less than, Al: 0.010-1.5%, N: 0.007% or less, the balance is composed of Fe and unavoidable impurities, and in area ratio, ferrite is 20-87%, residual with martensite Provided is a high-strength hot-dip galvanized steel sheet excellent in workability having a microstructure containing a total of 3 to 10% austenite and 10 to 60% tempered martensite.

  In the high-strength hot-dip galvanized steel sheet of the present invention, further, in mass%, Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005- It is preferable that at least one element selected from 2.00% is contained. Furthermore, by mass%, at least one element selected from Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, B: 0.0002 to 0.005%, Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% More preferably, at least one selected element is contained.

  In the high-strength hot-dip galvanized steel sheet of the present invention, the galvanizing can be alloyed galvanizing.

  The high-strength hot-dip galvanized steel sheet of the present invention is, for example, a slab having the above component composition, hot-rolled, cold-rolled to obtain a cold-rolled steel sheet, and the cold-rolled steel sheet has a temperature range of 750 to 950 ° C. And then cooled to a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at an average cooling rate of 750 ° C to 10 ° C / s or more, and 350 to 600 ° C. It can manufacture by the manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the workability which hot-dip galvanizes, after giving it the conditions which reheat to a temperature range and hold | maintain for 1 to 600 s.

  In the manufacturing method of the high-strength hot-dip galvanized steel sheet of the present invention, after hot-dip galvanizing, galvanization can be alloyed.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having a high TS-El balance, excellent stretch flangeability, and excellent YR processability can be produced. By applying the high-strength hot-dip galvanized steel sheet of the present invention to an automobile body, not only weight reduction and corrosion resistance improvement of the automobile but also collision safety improvement can be achieved.

  Details of the present invention will be described below. “%” Representing the content of component elements means “% by mass” unless otherwise specified.

1) Component composition
C: 0.05-0.3%
C is an element that stabilizes austenite, and is an element that is necessary for generating a second phase such as martensite other than ferrite to raise TS and improve the TS-El balance. If the C content is less than 0.05%, it is difficult to secure a second phase other than ferrite, and the TS-El balance is lowered. On the other hand, if the C content exceeds 0.3%, the weldability deteriorates. Therefore, the C content is 0.05 to 0.3%, preferably 0.08 to 0.15%.

Si: 0.01-2.5%
Si is an effective element for improving the TS-El balance by solid-solution strengthening steel. In order to obtain such an effect, the Si amount needs to be 0.01% or more. On the other hand, if the amount of Si exceeds 2.5%, the El decreases, the surface properties, and the weldability deteriorate. Therefore, the Si content is 0.01 to 2.5%, preferably 0.7 to 2.0%.

Mn: 0.5-3.5%
Mn is an element that is effective in strengthening steel and promotes the formation of a second phase such as martensite. In order to obtain such effects, the Mn content needs to be 0.5% or more. On the other hand, if the amount of Mn exceeds 3.5%, the ductile deterioration of ferrite due to excessive increase of the second phase or solid solution strengthening becomes remarkable, and the workability deteriorates. Therefore, the Mn content is 0.5 to 3.5%, preferably 1.5 to 3.0%.

P: 0.003-0.100%
P is an element effective for strengthening steel. In order to obtain such an effect, the P amount needs to be 0.003% or more. On the other hand, if the P content exceeds 0.100%, the steel is embrittled by grain boundary segregation, and impact resistance is deteriorated. Therefore, the P content is 0.003 to 0.100%.

S: 0.02% or less
Since S exists as inclusions such as MnS and deteriorates impact resistance and weldability, the amount is preferably reduced as much as possible. However, the amount of S is 0.02% or less from the viewpoint of manufacturing cost.

Al: 0.010-1.5%
Al is an element effective in generating ferrite and improving the TS-El balance. In order to obtain such an effect, the Al content needs to be 0.010% or more. On the other hand, if the Al content exceeds 1.5%, the risk of slab cracking during continuous casting increases. Therefore, the Al content is 0.010 to 1.5%.

N: 0.007% or less
N is an element that degrades the aging resistance of steel. In particular, when the N content exceeds 0.007%, deterioration of aging resistance becomes remarkable. Therefore, the N content is 0.007% or less, but the smaller the amount, the better.

  The balance is Fe and inevitable impurities, but for the following reasons, Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00% Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, B: 0.0002 to 0.005%, Ca: 0.001 to 0.005%, REM: 0.001 to 0.005% are preferably contained.

Cr, Mo, V, Ni, Cu: 0.005 to 2.00% each
Cr, Mo, V, Ni, and Cu are effective elements for suppressing the formation of pearlite during cooling from the heating temperature during annealing and promoting the formation of martensite and strengthening the steel. In order to obtain such an effect, the content of at least one element selected from Cr, Mo, V, Ni, and Cu needs to be 0.005%. On the other hand, when the content of each element of Cr, Mo, V, Ni, and Cu exceeds 2.00%, the effect is saturated and the cost is increased. Therefore, the contents of Cr, Mo, V, Ni, and Cu are each 0.005 to 2.00%.

Ti, Nb: 0.01 ~ 0.20% each
Ti and Nb are effective elements for forming carbonitride and increasing the strength of steel by precipitation strengthening. In order to obtain such an effect, the content of at least one element selected from Ti and Nb needs to be 0.01% or more. On the other hand, if the content of each element of Ti and Nb exceeds 0.20%, the strength becomes excessively high and the ductility decreases. Therefore, the contents of Ti and Nb are 0.01 to 0.20%, respectively.

B: 0.0002-0.005%
B is an element effective in suppressing the formation of ferrite from the austenite grain boundaries and generating a second phase such as a martensite phase to increase the strength. In order to obtain such effects, the B content needs to be 0.0002% or more. On the other hand, when the amount of B exceeds 0.005%, the effect is saturated and the cost is increased. Therefore, the B amount is 0.0002 to 0.005%.

Ca, REM: 0.001 to 0.005% each
Ca and REM are both effective elements for improving workability by controlling the morphology of sulfides. In order to obtain such an effect, the content of at least one element selected from Ca and REM must be 0.001% or more. On the other hand, if the content of each element of Ca and REM exceeds 0.005%, the cleanliness of steel may be adversely affected. Therefore, the Ca and REM contents are 0.001 to 0.005%, respectively.

2) Microstructure Area ratio of ferrite: 20-87%
Ferrite improves TS-El balance. In order to satisfy TS × El ≧ 19000 MPa ·%, the area ratio of ferrite needs to be 20% or more, preferably 50% or more. Note that the total area ratio of martensite and retained austenite is 3% or more in total and the area ratio of tempered martensite is 10% or more, so the upper limit of the area ratio of ferrite is 87%.

Martensite and retained austenite area ratio: 3-10% in total
Martensite and retained austenite not only contribute to strengthening of steel, but also improve TS-El balance and decrease YR. In order to obtain such an effect, the total area ratio of martensite and retained austenite needs to be 3% or more. However, when the area ratio of martensite and retained austenite exceeds 10% in total, stretch flangeability deteriorates. Therefore, the total area ratio of martensite and retained austenite is 3 to 10%.

Tempered martensite area ratio: 10-60%
Tempered martensite is a second phase that is effective in increasing strength while maintaining excellent stretch flangeability because it has less adverse effects on stretch flangeability than martensite before tempering. In order to obtain such an effect, the area ratio of tempered martensite needs to be 10% or more. However, when the area ratio of tempered martensite exceeds 60%, TS × El ≧ 19000 MPa ·% cannot be obtained. Therefore, the area ratio of tempered martensite is 10 to 60%.

  In addition, as a second phase other than martensite, retained austenite, and tempered martensite, pearlite and bainite can also be included. The object of the present invention can be achieved. Further, from the viewpoint of stretch flangeability, the area ratio of pearlite is preferably 3% or less.

  Here, the area ratio of ferrite, martensite, retained austenite, and tempered martensite is the ratio of the area of each phase in the observed area, and after corroding the plate thickness section of the steel sheet, it corrodes with 3% nital, The position of the plate thickness 1/4 was observed with a SEM (scanning electron microscope) at a magnification of 1000 to 3000 times, and obtained using commercially available image processing software.

3) Manufacturing conditions The high-strength hot-dip galvanized steel sheet of the present invention is, for example, a slab having the above component composition subjected to hot rolling and cold rolling to form a cold-rolled steel sheet. After heating to a temperature range of ℃ and holding for 10 s or more, it is cooled to a temperature range of (Ms point -100 ℃) to (Ms point -200 ℃) with an average cooling rate of 750 ℃ to 10 ℃ / s, 350 It can be manufactured by applying hot dip galvanization after annealing in a temperature range of ˜600 ° C. and annealing for 1 to 600 s.

Heating conditions during annealing: If the heating temperature during holding annealing is less than 750 ° C or holding time is less than 10 s in the temperature range of 750 to 950 ° C for more than 10 s, austenite formation will be insufficient, and sufficient amount for subsequent cooling The second phase such as martensite cannot be secured. On the other hand, when the heating temperature exceeds 950 ° C., austenite becomes coarse, and the formation of ferrite during cooling is suppressed, and ferrite with an area ratio of 20% or more cannot be obtained. Therefore, the heating at the time of annealing is maintained for 10 seconds or more in the temperature range of 750 to 950 ° C. The upper limit of the holding time is not particularly defined, but even if holding for 600 s or more, the effect is saturated and the cost is increased, so the holding time is preferably less than 600 s.

Cooling conditions during annealing: After cooling and heating in the temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) with an average cooling rate of 750 ° C to 10 ° C / s or more, 750 ° C to 10 ° C / s It is necessary to cool at the above average cooling rate. However, if the average cooling rate is less than 10 ° C / s, a large amount of pearlite is generated, and the required amount of tempered martensite, martensite and retained austenite are obtained. It is because it is not possible. The upper limit of the cooling rate is not particularly specified, but it is difficult to control the cooling to the cooling stop temperature range from (Ms point -100 ° C) to (Ms point -200 ° C), although the steel plate shape deteriorates, It is preferable to be 200 ° C./s or less. The cooling stop temperature is one of the most important conditions in the present invention for controlling the amount of martensite, retained austenite, and tempered martensite generated during subsequent reheating, hot dip galvanizing, and alloying treatment of the plating phase. is there. That is, the amount of martensite and untransformed austenite is determined when cooling is stopped, and in the subsequent heat treatment, martensite becomes tempered martensite, untransformed austenite becomes martensite or retained austenite, and the strength of the steel, TS-El It affects the balance, stretch flangeability and YR. When the cooling stop temperature exceeds (Ms point -100 ° C), the martensitic transformation becomes insufficient, the amount of untransformed austenite increases, and finally the area ratio of martensite and residual austenite becomes 10% in total. Exceeding, stretch flangeability is reduced. On the other hand, when the cooling stop temperature is less than (Ms point -200 ° C), most of the austenite undergoes martensitic transformation, the amount of untransformed austenite decreases, and finally the total area ratio of martensite and residual austenite is 3 Less than%, TS-El balance deteriorates and YR increases. Therefore, it is necessary to perform cooling at the time of annealing in the temperature range of (Ms point-100 ° C.) to (Ms point-200 ° C.) at an average cooling rate of 750 ° C. to 10 ° C./s or more.

Reheating conditions during annealing: Hold for 1 to 600 s in the temperature range of 350 to 600 ° C
After cooling to the temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) with an average cooling rate of 10 ° C / s or more, reheat the sample for 1s or more in the temperature range of 350 to 600 ° C. Thus, the martensite generated during cooling is tempered to produce tempered martensite with an area ratio of 10 to 60%, and high strength can be achieved while maintaining excellent stretch flangeability. If the reheating temperature is less than 350 ° C. or the holding time is less than 1 s, the area ratio of tempered martensite is less than 10%, and stretch flangeability deteriorates. In addition, when the reheating temperature exceeds 600 ° C. or the holding time exceeds 600 s, the untransformed austenite generated during cooling is transformed into pearlite or bainite, and finally the total area ratio of martensite and residual austenite is 3 Less than%, TS-El balance deteriorates and YR increases. Therefore, it is necessary to perform reheating at the time of annealing in the temperature range of 350 to 600 ° C. for 1 to 600 s.

  The conditions for other production methods are not particularly limited, but the following conditions are preferable.

  The slab is preferably produced by a continuous casting method in order to prevent macro segregation, but can also be produced by an ingot-making method or a thin slab casting method. To hot-roll the slab, the slab may be cooled to room temperature and then re-heated for hot rolling, or the slab may be charged in a heating furnace without being cooled to room temperature. Can also be done. Alternatively, an energy saving process in which hot rolling is performed immediately after performing a slight heat retention can also be applied. When heating the slab, it is preferable to heat to 1100 ° C. or higher in order to dissolve carbides and prevent an increase in rolling load. In order to prevent an increase in scale loss, the heating temperature of the slab is preferably 1300 ° C. or lower.

When hot rolling a slab, the rough bar after rough rolling can be heated from the viewpoint of securing the rolling temperature. Moreover, what is called a continuous rolling process which joins rough bars and performs finish rolling continuously can be applied. Finish rolling is performed at a finishing temperature equal to or higher than the Ar ₃ transformation point in order to prevent the formation of a band structure that causes a decrease in workability after cold rolling / annealing and an increase in anisotropy. Further, in order to reduce the rolling load and make the shape and material uniform, it is preferable to perform lubrication rolling with a friction coefficient of 0.10 to 0.25 in all passes or a part of the finishing rolling.

  The steel sheet after hot rolling is preferably wound at a winding temperature of 450 to 700 ° C. from the viewpoint of temperature control and prevention of decarburization.

  The steel sheet after winding is removed by scale pickling or the like, then cold-rolled preferably at a rolling reduction of 40% or more, annealed under the above conditions, and hot dip galvanized.

  In hot dip galvanizing, if the galvanizing is not alloyed, the amount of Al is 0.12-0.22%, or if the galvanizing is alloyed, the steel plate is placed in a 440-500 ° C plating bath containing 0.08-0.18% of Al. After dipping, the plating adhesion amount is adjusted by gas wiping or the like. In the case of alloying galvanizing, an alloying treatment is further performed at 450 to 600 ° C. for 1 to 30 seconds.

  The steel sheet after the hot dip galvanization or the steel sheet after the galvanization alloying treatment can be subjected to temper rolling for the purpose of shape correction, adjustment of surface roughness, and the like. Moreover, various coating processes, such as resin and oil-fat coating, can also be given.

Steels A to S having the composition shown in Table 1 were melted in a converter and made into a slab by a continuous casting method, and then hot-rolled to a sheet thickness of 3.0 mm at a finishing temperature of 900 ° C. and 10 ° C./s after rolling. And cooled at a winding temperature of 600 ° C. Next, after pickling, it is cold rolled to a plate thickness of 1.2 mm, and after annealing under the annealing conditions shown in Tables 2 and 3 by a continuous hot dip galvanizing line, it is immersed in a plating bath at 460 ° C., and the adhesion amount is 35 to 45 g. / m < ² > plating was formed, alloyed at 520 [deg.] C., and cooled at a cooling rate of 10 [deg.] C./sec to produce plated steel sheets 1 to 44. As shown in Tables 2 and 3, some of the plated steel sheets were not alloyed. And about the obtained plated steel plate, the area ratio of a ferrite, a martensite, a retained austenite, and a tempered martensite was measured by said method. In addition, a JIS No. 5 tensile test piece was taken in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241. Further, a test piece of 150 mm × 150 mm was collected and subjected to a hole expansion test three times in accordance with JFST 1001 (Iron Standard) to obtain an average hole expansion ratio λ (%), and the stretch flangeability was evaluated.

  The results are shown in Tables 4 and 5. Each of the plated steel sheets according to the present invention has a high TS-El balance with TS × El ≧ 19000 MPa ·%, excellent expansion flangeability with a hole expansion ratio λ ≧ 70%, and a low YR with YR <75%. Recognize.

Claims (8)

In mass%, C: 0.05-0.3%, Si: 0.01-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.02% or less, Al: 0.010-1.5%, N: 0.007% or less And the balance is composed of Fe and inevitable impurities, and the area ratio is 20 to 87% for ferrite, 3 to 10% in total for martensite and residual austenite, and 10 to 10 for tempered martensite. It has a 60% including microstructure, and high-strength galvanized steel sheet excellent in workability that have a tensile strength of at least 839MPa.   Furthermore, it contains at least one element selected from Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00% by mass%. 2. A high-strength hot-dip galvanized steel sheet excellent in workability according to claim 1.   The high-strength hot-dip galvanized steel sheet excellent in workability according to claim 1 or 2, further comprising at least one element selected from Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20% by mass%.   The high-strength hot-dip galvanized steel sheet with excellent workability according to any one of claims 1 to 3, further comprising B: 0.0002 to 0.005% by mass%.   The high-strength molten zinc excellent in workability according to any one of claims 1 to 4, further comprising at least one element selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% in mass%. Plated steel sheet.   6. The high-strength hot-dip galvanized steel sheet excellent in workability according to claim 1, wherein the galvanizing is alloyed galvanizing. A slab having the component composition according to any one of claims 1 to 5 is subjected to hot rolling and cold rolling to form a cold rolled steel sheet, and the cold rolled steel sheet is heated to a temperature range of 750 to 950 ° C. After holding for 10 s or more, cool to a temperature range of (Ms point -100 ° C) to (Ms point -200 ° C) at an average cooling rate of 750 ° C to 10 ° C / s or more, and then return to a temperature range of 350 to 600 ° C. After annealing under conditions of heating and holding for 1 to 600 s, hot dip galvanizing is performed , and the area ratio is 20 to 87% for ferrite, 3 to 10% in total for martensite and retained austenite, and tempered martensite. A method for producing a high-strength hot-dip galvanized steel sheet having a microstructure containing 10 to 60% and excellent workability having a tensile strength of 839 MPa or more .   8. The method for producing a high-strength hot-dip galvanized steel sheet having excellent workability according to claim 7, wherein the alloying treatment of galvanizing is performed after hot-dip galvanizing.