JP4500197B2 - Manufacturing method of high-strength cold-rolled steel sheet, high-strength hot-dip galvanized steel sheet, and high-strength galvannealed steel sheet with excellent formability and weldability - Google Patents

Description

  In the present invention, the maximum tensile strength (TS) is 780 MPa or more, and the yield ratio (YR = YS / TS) which is the ratio of the yield stress (YS) and TS in the tensile test is 0.5 to 0.7, High-tensile cold-rolled steel sheet and high-strength hot-dip galvanized steel that are particularly suitable for automotive structural members, reinforcing members, and suspension members that have excellent formability such as ductility and bendability and spot weldability The present invention relates to a steel plate and a high-strength galvannealed steel plate. The high-strength steel sheet in the present invention includes various plated steel sheets represented by galvanized steel sheets, Al-plated steel sheets, and electroplated steel sheets, in addition to ordinary cold-rolled steel sheets. In addition to pure zinc, the plating layer may contain Fe, Al, Mg, Mn, Si, Cr, and the like.

  Lightweight automotive cross members and side members are being studied to meet the recent trend of lighter fuel consumption, and in terms of materials, the strength and collision safety can be ensured even if they are made thinner. Since then, the strength of steel sheets has been increasing. However, since the formability of the material deteriorates as the strength increases, it is necessary to manufacture a steel sheet that satisfies both the press formability and high strength in order to realize the weight reduction of the member. Steel plates having a maximum tensile strength of 780 MPa or more, which are mainly used for structural members and reinforcing members of automobiles, are required to be excellent in bendability, hole expansibility, and ductility. In general, since the total elongation (El) and the stretch formability of a tensile test are correlated with the n value of the steel sheet, steel sheets having a low yield ratio have been aimed at for improving the formability. Such a low yield ratio can be achieved by forming a composite structure composed of a soft structure (ferrite) and a hard structure (martensite and retained austenite). However, when martensite or retained austenite is used for the second phase, there is a problem that hole expansibility is significantly lowered (for example, Non-Patent Document 1). Since the structure capable of ensuring excellent ductility and the structure capable of ensuring excellent bendability and hole expansibility conflict with each other, it has been required to satisfy both of these characteristics.

  On the other hand, even if the strength of the steel sheet is sufficient, if it breaks at the welded part at the time of collision, the collision energy cannot be sufficiently absorbed, and the predetermined collision energy absorption performance cannot be obtained. Therefore, automobile parts are required to have excellent joint strength such as spot welding, arc welding, and laser welding. However, as the strength of the steel plate increases, the content of C, Si, Mn, etc. increases, and the strength of the welded portion decreases accordingly, and the strength is increased without increasing the content of alloying elements as much as possible. It was desired to make it.

  In order to deal with such problems, there is TRIP (Transformation Induced Plasticity) steel using martensitic transformation of retained austenite described in Patent Document 1 and Patent Document 2 as a technique for obtaining high strength and high formability in recent years. Applications are expanding. However, since this steel uses martensite transformation during forming to ensure excellent formability, a large amount of retained austenite is necessary to ensure formability. Requires a large amount of C addition. As a result, when securing strength exceeding 780 MPa is considered, there is a problem that spot weldability deteriorates.

  Alternatively, Patent Document 3 and Patent Document 4 disclose cold-rolled and plated steel sheets in which the addition of C into the steel sheet is suppressed in order to provide formability and spot weldability. However, this method has a problem that it is indispensable to add a large amount of C when securing strength of 780 MPa or more, and it is difficult to simultaneously provide spot weldability and excellent formability.

  On the other hand, Patent Document 5 discloses a method having excellent stretch flangeability and spot weldability by making the main phase a bainite structure. However, this method has a problem that since the main phase is a bainite structure, the yield ratio is high and the elongation is low, so that the formability is inferior. In addition, when considering securing the strength of the 780 MPa class, it is necessary to suppress the addition amount of C and Mn, so that the hardenability deteriorates and it is difficult to make the main phase into a bainite structure. .

  On the other hand, there is a method of reducing the hardness difference between ferrite as a main phase and martensite as a hard structure, and improving the hole expandability of a composite structure steel plate made of ferrite and martensite. Non-Patent Document 1 shows that martensite, which is a hard structure, is softened by tempering, the hardness difference from the ferrite structure is reduced, and the hole expandability can be improved. However, the improvement in hole expansibility due to softening of the martensite structure means a decrease in the strengthening ability of the martensite structure, so the decrease in strength must be compensated by an increase in the volume ratio of martensite by increasing the amount of C added. When securing strength of 780 MPa or more was considered, there was a problem of poor spot weldability. In addition, considering both high strength and excellent hole expansibility, there is a problem that the martensite volume ratio increases and the ductility is greatly deteriorated. On the other hand, instead of reducing the hardness of the hard tissue, a technique for improving hole expansibility by increasing the hardness of the soft tissue using solid solution strengthening of Si (for example, Non-Patent Document 1), or There is a technique (for example, Non-Patent Document 2) in which Ti and Nb are added singly or in combination to enhance the precipitation structure and improve the hole expansibility. However, in order to reduce the hardness difference and improve the hole expansibility by solid solution strengthening of Si, it is necessary to add a large amount of Si, and there is a problem that chemical conversion property, plating property and weldability are deteriorated. . On the other hand, the improvement of hole expansibility by addition of Ti or Nb is caused by precipitation of Ti or Nb carbonitride during hot rolling, so that it can be used in hot rolled steel sheets, but then cold rolling-heat treatment In cold-rolled steel sheets and plated steel sheets, the precipitation strengthening ability decreases, so a large amount of Ti and Nb addition is necessary. However, these elements significantly delay recrystallization and reduce ductility, so control It had the problem of being difficult.

Japanese Patent Laid-Open No. 1-2230715 JP-A-2-217425 JP 2001-152287 A JP 2001-226742 A Japanese Patent Application Laid-Open No. 11-296991 CAMP-ISIJ vol. 13 (2000), p391 CAMP-ISIJ vol. 13 (2000), p411

The present invention is a high strength cold-rolled steel sheet having a maximum tensile strength (TS) of 780 MPa or more, a yield ratio (YR) of 0.5 to 0.7, and excellent formability such as ductility and bendability and spot weldability , and to provide a method for manufacturing strength galvanized steel sheet and high strength galvannealed steel sheet.

  As a result of earnest and examination to achieve the above object, the present inventors have added Si, Nb and Ti to the steel sheet in combination, and added other alloy elements in an appropriate amount, and ferrite as the main phase is added. Reducing the hardness difference between ferrite and martensite or bainite structure, which contributes to strengthening as the second phase, has an appropriate yield ratio (0.5 to 0.7) and a tensile strength of 780 MPa or more. It has been found that a steel sheet having excellent formability such as strength, bendability and ductility and excellent weldability can be produced.

  That is, the present invention is a high-strength cold-rolled steel sheet excellent in formability and weldability, and the gist thereof is as follows.

(1) By mass%, C: 0.06 to less than 0.1%, Si: 0.4 to 0.8%, Mn: 1.8 to 2.2%, Nb: 0.014 to 0.029 %, Ti: 0.014 to 0.029%, P: 0.04% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, O: 0.0010 The cast slab containing ~ 0.0045% and the balance consisting of chemical components of Fe and inevitable impurities is directly or once cooled and then heated to 1200 ° C or higher, and the hot rolling is completed at the Ar3 transformation point or higher, 630 ° C. After winding in the following temperature range, pickling, cold rolling with a rolling reduction of 40 to 70%, and then passing through a continuous annealing line, a temperature range of 580 to 750 ° C. with an average heating rate of 1. Heat at 4 ° C./second or more, anneal at 750 ° C. or more and 900 ° C. or less, and then 600 ° C. to 5 ° C. It is cooled at an average cooling rate of 4 to 200 ° C./second between 0 ° C. and maintained for 30 seconds or more in a temperature range between 400 ° C. and 200 ° C., and the yield ratio is 0.5 to 0.7, and A method for producing a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and excellent formability and weldability.

(2) The yield ratio according to (1) above , wherein the cast slab further contains Cr: 0.005 to 3% by mass% , and A method for producing a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more and excellent formability and weldability .

(3) The above- mentioned (1) or (2), wherein the cast slab further contains 0.005 to 0.6% of one or two selected from V and W in mass%. described), yield ratio 0.5-0.7 and, the method of producing a high strength cold rolled steel sheet having excellent formability and weldability in tensile strength 780MPa or more.

(4) The yield according to any one of (1) to (3), wherein the cast slab further contains B: 0.0001 to 0.1% by mass%. A method for producing a high-strength cold-rolled steel sheet having a ratio of 0.5 to 0.7 and a tensile strength of 780 MPa or more and excellent in formability and weldability .

(5) The above-mentioned cast slab further contains 0.0005 to 0.04% in total of one or more selected from Ca, Mg, REM, and Y by mass%. (1) A method for producing a high-strength cold-rolled steel sheet according to any one of (4) , having a yield ratio of 0.5 to 0.7 and a tensile strength of 780 MPa or more and excellent in formability and weldability .

( 6 ) The cast slab composed of the chemical component according to any one of (1) to (5) above is directly or once cooled and then heated to 1200 ° C or higher, and the hot rolling is completed at the Ar3 transformation point or higher. In the temperature range of 630 ° C. or less, after pickling, cold rolling with a rolling reduction of 40 to 70% is performed, and then when passing through a continuous hot dip galvanizing line, the temperature range of 580 to 750 ° C. Heated at an average heating rate of 1.4 ° C./second or more, annealed at 750 ° C. or more and 900 ° C. or less, and an average cooling rate of 4 to 200 ° C./second between 600 ° C. and 500 ° C. from the galvanizing bath temperature. It is cooled to a temperature range from a low temperature to a temperature 50 ° C. higher than the galvanizing bath temperature, and then immersed in a galvanizing bath . The yield ratio is 0.5 to 0.7, and the tensile strength formability and weldability at least 780MPa Method for producing a high strength galvanized steel sheet.

( 7 ) The cast slab made of the chemical component according to any one of (1) to (5) above is directly or once cooled and then heated to 1200 ° C or higher, and the hot rolling is completed at the Ar3 transformation point or higher. In the temperature range of 630 ° C. or less, after pickling, cold rolling with a rolling reduction of 40 to 70% is performed, and then when passing through a continuous hot dip galvanizing line, the temperature range of 580 to 750 ° C. Heated at an average heating rate of 1.4 ° C./second or more, annealed at 750 ° C. or more and 900 ° C. or less, and an average cooling rate of 4 to 200 ° C./second between 600 ° C. and 500 ° C. from the galvanizing bath temperature. Cooling to a temperature range from a low temperature to a temperature higher by 50 ° C. than the galvanizing bath temperature, and then immersing in a galvanizing bath, alloying at a temperature of 460 ° C. or higher, and then cooling to room temperature. wherein, the yield ratio is 0. 0.7, and the method of producing a high strength galvannealed steel sheet excellent in formability and weldability in tensile strength 780MPa or more.

  INDUSTRIAL APPLICABILITY The present invention can provide a low-cost steel sheet that combines high strength of 780 MPa or more with excellent tensile strength suitable for automobile structural members, reinforcing members, and suspension members, and excellent formability and weldability.

The present inventors have intensively studied to solve the above problems. First, in order to ensure excellent spot weldability, it is important that the C content is less than 0.1%. Next, it is extremely important to add C, Si, Mn, Ti, and Nb at the same time and to control with a certain balance in order to set the tensile strength to 780 MPa or more and the yield ratio to the above appropriate range. I found out. By setting the yield ratio to an appropriate range, a high-strength steel sheet having a good balance of ductility, bendability, hole expansibility, shape freezing property, and impact characteristics can be obtained.
The present invention is described in detail below.

First, the reasons for limiting the chemical components of the cast slab (steel piece) will be described. In addition,% means the mass%.
C: C is an element that can increase the strength of the steel sheet. However, if it is less than 0.06%, it becomes difficult to achieve both a tensile strength of 780 MPa or more and molding processability. On the other hand, if it is 0.1% or more, it is difficult to ensure spot weldability. For this reason, the range was limited to 0.06 to less than 0.1%.

  Si: Si is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it is less than 0.4%, the decrease in formability due to the deterioration of elongation becomes remarkable, and if it exceeds 0.8%, the wettability of plating decreases in the case of chemical conversion treatment or galvanized steel sheet. Therefore, the Si content is limited to the range of 0.4 to 0.8%. A more preferable range is 0.5 to 0.7%.

  Mn: Mn is a strengthening element and is effective in increasing the strength of the steel sheet. However, if it is less than 1.8%, it is difficult to obtain a tensile strength of 780 MPa or more. On the other hand, if the amount is too large, co-segregation with P and S is promoted, and the bendability and the stretchability of the elongated hole are significantly deteriorated. In order to control the yield ratio in a narrower range, 1.9 to 2.1% is a more preferable range.

By containing Nb and Ti in combination, it is possible to have an appropriate yield ratio, ductility, bendability, and hole expandability.
Nb: Nb is a strengthening element. Addition is extremely important because it contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Further, since the main phase ferrite is hardened, the hardness difference between the strengthened phase bainite and martensite structure is reduced, and the bendability and hole expansibility are improved. Since these effects cannot be obtained if the effect is less than 0.014%, the lower limit is set to 0.014%. If the content exceeds 0.029%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.029%.

  Ti: Ti is a strengthening element. Addition is extremely important because it contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. Further, since the main phase ferrite is hardened, the hardness difference between the strengthened phase bainite and martensite structure is reduced, and the bendability and hole expansibility are improved. Since these effects cannot be obtained if the effect is less than 0.014%, the lower limit is set to 0.014%. If the content exceeds 0.029%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.029%. In addition, since Ti is a strong nitride-forming element, it binds preferentially to N compared to Al, thereby forming fine nitrides, suppressing the formation of coarse AlN, and bending properties. This is particularly important because it suppresses the deterioration of the material.

  P: P tends to segregate in the central part of the plate thickness of the steel sheet and embrittles the weld. When the content exceeds 0.04%, the weld becomes brittle, so the appropriate range is limited to 0.04% or less. Although the lower limit value of P is not particularly defined, it is preferable to set this value as the lower limit value because it is economically disadvantageous to set it to less than 0.0001% by mass.

  S: S adversely affects weldability and manufacturability during casting and hot rolling. For this reason, the upper limit value was set to 0.01% by mass or less. Although the lower limit of S is not particularly defined, it is preferable to set this value as the lower limit because it is economically disadvantageous to make it less than 0.0001%.

  Al: Al may be added because it promotes ferrite formation and improves ductility. It can also be used as a deoxidizer. However, excessive addition forms Al-based coarse inclusions, which causes surface damage and deterioration of hole expansibility. From this, the upper limit of Al addition was set to 0.1%. The lower limit is not particularly limited, but since it is difficult to make the lower limit 0.0005 or less, this is a practical lower limit.

  N: N forms coarse nitrides and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. This is because when N exceeds 0.01%, this tendency becomes remarkable. Therefore, the range of N content is set to 0.01% or less. In addition, it is better to use less because it causes blowholes during welding. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased, and this is a substantial lower limit.

  O: O forms an oxide and degrades bendability and hole expansibility, so it is necessary to suppress the addition amount. In addition, since O preferentially forms a compound with Ti rather than N, if the O content is large, Ti is present in a large proportion as an oxide and promotes the formation of coarse AlN. Further deteriorates bendability and hole expandability. This is because when O exceeds 0.0045%, this tendency becomes remarkable. Therefore, the upper limit of the O content is set to 0.0045% or less. Since setting it as less than 0.0010% invites excessive cost and is not economically preferable, this was made into the minimum.

  Cr: Cr is a strengthening element and is important for improving hardenability. However, if the content is less than 0.005%, these effects cannot be obtained, so the lower limit is set to 0.005%. On the other hand, if the content exceeds 3%, the manufacturability during production and hot rolling is adversely affected, so the upper limit was made 3%.

  Furthermore, the steel plate which this invention makes object can contain the 1 type or 2 types chosen from V and W which are strong carbide forming elements for the purpose of the further improvement of intensity | strength. Since these elements form fine carbides, nitrides, or carbonitrides, and are extremely effective for strengthening steel sheets, one or two kinds are added in total in an amount of 0.005% by mass as necessary. The above can be added. On the other hand, since a large amount of addition causes deterioration of ductility, the upper limit of addition is set to 0.6% by mass in total.

  B can also be added as needed. B is effective for strengthening grain boundaries and strengthening steel by addition of 0.0001% by mass or more, but when the addition amount exceeds 0.1% by mass, the effect is not only saturated but also heat The upper limit is set to 0.1% by mass because the production at the time of stretching is lowered.

  One or two or more selected from Ca, Mg, Y, and REM can be added in a total amount of 0.0005 to 0.04%. Ca, Mg, Zr, Y, and REM are elements used for deoxidation, and it is preferable that one or two of them are contained in a total amount of 0.0005% or more. However, when the content exceeds 0.04% in total, it causes deterioration of molding processability. Therefore, the total amount range is set to 0.0005 to 0.04% in total. In the present invention, REM refers to La and lanthanoid series elements, which are often added by misch metal, and contain complex series elements such as La and Ce. However, the effects of the present invention are exhibited even when metal La or Ce is added.

  In addition, as other inevitable impurities, Mo, Ni, and Cu may be included in total less than 0.03%.

  The fraction of each structure contained in the steel sheet is not particularly defined and the effects of the present invention are exhibited. However, in order to ensure excellent ductility, it is desirable that the main phase is ferrite. In addition, as a form of the ferrite phase, in addition to polygonal ferrite, it is assumed that it includes acicular ferrite and recovered non-recrystallized ferrite. For the purpose of increasing the strength of the steel sheet, one or two of martensite and bainite structures may be contained. With respect to the fraction of the bainite structure and martensite structure, the effect of the present invention is particularly demonstrated without limitation, but when the volume fraction of bainite structure is large compared to martensite, more excellent ductility and bendability. And a balance of hole expansibility. An austenite phase may be included for the purpose of further improving ductility.

  In addition, each phase of the above microstructure, ferrite, martensite, bainite, austenite, and the remaining structure are identified, the observation of the existing position and the measurement of the area ratio are performed by the Nital reagent and the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473. The steel plate rolling direction cross section or the rolling direction perpendicular direction cross section is corroded and can be quantified by observation with a 1000 times optical microscope and 1000 to 100000 times scanning and transmission electron microscopes. The recovered unrecrystallized ferrite and the lath bainite structure are separated by the crystal orientation mapping of the FESEM-EBSP method, and the ferrite phase in which the orientation continuously changes in the grains or the recovered fine cell structure is obtained. The ferrite phase possessed is judged as an unrecrystallized ferrite phase. It is possible to obtain an area ratio of each tissue by observing 20 fields of view or more and using a point counting method or image analysis.

  The crystal grain size of ferrite is not particularly limited, but it is preferably 7 μm or less in terms of nominal grain size from the viewpoint of balance of strength elongation.

  Yield ratio is a value defined by the ratio (YS / TS) of maximum tensile strength (TS) and yield strength (YS), and bendability and ductility are controlled within the range of 0.5 to 0.7. Can be achieved. Generally, since the ferrite phase is soft, when it is mixed with hard martensite, the yield ratio is lowered. However, by combining precipitation strengthening with Nb and Ti and solid solution strengthening with Si and hardening the ferrite phase, the yield ratio is controlled to an appropriate range, and while maintaining excellent ductility, It can be secured. As a strengthening method of the ferrite phase, in addition to solid solution strengthening and precipitation strengthening, fine grain strengthening by suppressing grain growth with Nb or Ti, or dislocation strengthening using non-recrystallized ferrite can be used at the same time.

  When the yield ratio exceeds 0.70, the n value becomes too low and the ductility and the stretch formability become low, so the upper limit was set to 0.7. On the other hand, if the yield ratio is less than 0.5, sufficient collision safety cannot be ensured. In addition, since the extreme decrease in yield ratio also leads to deterioration of bendability, 0.5 was set as the lower limit. Also, controlling the yield ratio within a narrow range is very effective because it reduces the variation in shape during molding. A range of 0.55 to 0.65 is more preferable.

  In this invention, the steel plate excellent in spot weldability is provided. The spot weldability of the steel sheet obtained in the present invention is such that even if the welding current causes spilling, the CTS degradation allowance is related to the tensile load (CTS) by the cross-type tensile test when welding is performed immediately before the spilling. Is characterized as being small. That is, when welding accompanied by the occurrence of scattering is performed on a normal steel plate, the CTS is greatly reduced or the CTS variation is increased, whereas the CTS reduction rate and variation are small in the steel plate of the present invention. The welding current value in the scatter occurrence region is a current value obtained by adding 1.5 kA to the scatter occurrence current value (CE). When the average value of CTS when welding with CE as the welding current is performed 5 times is 1, the minimum value of CTS when the test with welding current as (CE + 1.5 kA) is performed 5 times is 0.7. That's it. Preferably it is 0.8 or more, More preferably, it is 0.9 or more. CTS is evaluated according to the method of JIS Z 3137.

  As described above, the steel of the present invention exhibits particularly excellent characteristics with respect to spot welding, and is also suitable for commonly used welding methods such as arc, TIG, MIG, mash seam and laser.

  In the present invention, a steel sheet having excellent ductility, bendability and hole expansibility and excellent formability is provided. Regarding the ductility, specifically, a material having 16000 (MPa ×%) or more in terms of TS (MPa) × tensile test total elongation (El) (%) is defined as having excellent ductility. With respect to bendability, a specimen having a thickness of 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction is sampled, and has a superior bendability when the crack generation limit bending radius at 90 ° bending is 0.5 mm. It is defined as a thing. The steel of the present invention has excellent hole expandability as well as bendability.

  The slab used for hot rolling is not particularly limited. That is, what was manufactured with the continuous casting slab, the thin slab caster, etc. should just be used. It is also compatible with processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

  The hot-rolled slab heating temperature needs to be 1200 ° C. or higher because it is necessary to redissolve carbonitride deposited during casting. Although the upper limit is not particularly defined, the effect of the present invention is exhibited. However, since it is not economically preferable to raise the heating temperature to an excessively high temperature, the upper limit of the heating temperature is preferably less than 1300 ° C.

When the finish rolling temperature is in the two-phase region of austenite + ferrite, the structure non-uniformity in the steel sheet and the material anisotropy increase, and the formability after annealing deteriorates. Therefore, the Ar3 temperature or higher is desirable.
The Ar3 temperature is calculated by the following formula.
Ar3 = 901-325 × C + 33 × Si-92 × (Mn + Ni / 2 + Cr / 2 + Cu / 2 + Mo / 2)

  The winding temperature needs to be 630 ° C. or lower. When the temperature exceeds 630 ° C., coarse ferrite and pearlite structures exist in the hot-rolled structure, so that the structure non-uniformity after annealing increases, and the bendability and hole expandability of the final product deteriorate. From the viewpoint of making the microstructure after annealing fine and improving the strength ductility balance, it is more preferable to wind up at 600 ° C. or lower. Although the lower limit is not particularly defined, the effect of the present invention is exhibited. However, since it is technically difficult to wind up at a temperature of room temperature or lower, this is the actual lower limit. Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once.

  The hot-rolled steel sheet thus manufactured may be pickled as necessary. Pickling is important for improving chemical conversion and plating properties because it can remove oxides on the surface of the steel sheet. Pickling may be performed inline or offline. Moreover, pickling may be performed once, or pickling may be performed in a plurality of times.

  The pickled hot-rolled steel sheet is cold-rolled at a rolling reduction of 40 to 70% and passed through a continuous annealing line or a continuous hot dip galvanizing line. If the rolling reduction is less than 40%, it is difficult to keep the shape flat. Moreover, since the ductility of the final product becomes poor, this is the lower limit. On the other hand, cold rolling exceeding 70% makes the cold rolling difficult because the cold rolling load becomes too large. It is a more preferable range than 45 to 65%. The effect of the present invention is exhibited without particularly specifying the number of rolling passes and the rolling reduction for each pass.

  The heating rate in the temperature range of 580 to 750 ° C when passing through the continuous annealing line needs to be 1.4 ° C / second or more. Since strengthening is performed using a combined addition of Ti and Nb, control of grain growth and recrystallization during heating is extremely important. Although the detailed mechanism is unknown, when the heating rate is less than 1.4 ° C./second, the grain growth or recrystallization proceeds excessively, and the yield ratio is greatly reduced. Or since the hardness difference with a hard structure becomes large, the deterioration of bendability and hole expansibility is caused. Therefore, the lower limit was set to 1.4 ° C./second. On the other hand, the effect of the present invention is exhibited without any particular upper limit, but setting the heating rate above 100 ° C. causes excessive capital investment and is not economically preferable. is there.

  The maximum heating temperature is in the range of 750 to 900 ° C. When the maximum heating temperature is less than 750 ° C., it takes too much time for the carbide formed at the time of hot rolling to re-dissolve, and the carbide or a part thereof remains, so that a strength of 780 MPa or more cannot be secured. Furthermore, coarse carbides remaining in the steel plate cause deterioration of hole expansibility. Therefore, 750 ° C. is the lower limit of the maximum heating temperature. On the other hand, excessively high temperature heating not only is economically undesirable because it leads to an increase in cost, but also induces troubles such as deterioration of the plate shape at the time of hot plate passing and reduction of roll life. Therefore, the upper limit of the maximum heating temperature is set to 900 ° C. The heat treatment time in this temperature range is not particularly limited, but a heat treatment of 10 seconds or more is desirable for dissolving the carbide. On the other hand, if the heat treatment time exceeds 600 seconds, the cost increases, which is not economically preferable. Regarding the heat treatment, the isothermal holding may be performed at the maximum heating temperature, or even if cooling is started immediately after the gradient heating is performed and the maximum heating temperature is reached, the effect of the present invention is exhibited.

  After the completion of the annealing, a process of continuously cooling between 600 ° C. and 500 ° C. at 4 to 200 ° C./second to form a hard phase is performed. Since austenite transforms into pearlite in the temperature range of 600 to 500 ° C., it is necessary to suppress it. If the average cooling rate during this period is less than 4 ° C./second, austenite is transformed into pearlite, a sufficient amount of martensite and bainite structure cannot be obtained, and it is difficult to ensure a strength of 780 MPa or more. The value was set to 4 ° C./second or more. Further, even if the cooling is performed at a temperature exceeding 200 ° C./second, no problem is caused in terms of the material. However, excessively increasing the cooling rate leads to a high manufacturing cost, so the upper limit is 200 ° C./second. It is preferable that As a cooling method between 600 ° C. and 500 ° C., roll cooling, air cooling, water cooling, and any method using these in combination may be used. The reason why the temperature range for limiting the cooling rate is 500 to 600 ° C. is that pearlite transformation occurs in this temperature range. Even if the cooling is started from a temperature range exceeding 600 ° C., the effect of the present invention is exhibited. However, if the cooling start temperature exceeds 750 ° C., the difference from the maximum heating temperature becomes too small and the cooling is started. This is a practical upper limit because it is difficult to ensure the starting temperature. The cooling lower limit for limiting the cooling rate is not particularly limited, and the effect of the present invention can be exhibited. However, since it is technically difficult to set the cooling temperature to be room temperature or lower, this is a practical lower limit.

  Moreover, in the case of a continuous annealing line, you may perform the heat processing for 30 second or more in a 200-400 degreeC temperature range using an overaging zone. If the average plate temperature in the overaged zone is more than 400 ° C., the martensite volume fraction contained in the steel sheet is reduced, so that it is difficult to achieve both a maximum tensile strength of 780 MPa and excellent weldability. On the other hand, the heat treatment at less than 200 ° C. makes the yield ratio less than 0.5, so the lower limit was set to 200 ° C. 250 degreeC or more is a more preferable range. The holding time means not only mere isothermal holding but also a residence time in a temperature range of 200 to 400 ° C., and includes cooling and heating in this temperature range.

  The reduction ratio of the skin pass rolling after the heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, the productivity is remarkably lowered, so this is the upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.

  The heating rate in the temperature range of 580 to 750 ° C. when passing through the hot dip galvanizing line after cold rolling is set to 1.4 ° C./second or more for the same reason as when passing through the continuous annealing line. The maximum heating temperature is also set to 750 to 900 ° C. for the same reason as when the continuous annealing line is passed. Regarding cooling after annealing, it is necessary to cool between 600 ° C. and 500 ° C. at a cooling rate of 4 ° C./second or more for the same reason as when passing through a continuous annealing line.

  The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature. If the bath immersion plate temperature is lower than the hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and part of the molten zinc may solidify and deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C. However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in. On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature +50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.

  Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. The upper limit is not particularly limited, but if it exceeds 600 ° C., pearlite transformation occurs and the volume fraction of hard structure (martensite, bainite, retained austenite) is decreased, and it becomes difficult to ensure a tensile strength of 780 MPa or more. It is. Moreover, you may perform additional heat processing in the temperature range of 500-200 degreeC before plating bath immersion. Skin-pass rolling may be applied to the hot-dip galvanized steel sheet.

  Further, in order to further improve the plating adhesion, the present invention does not depart from the present invention even if the steel plate is plated with Ni, Cu, Co, or Fe alone or before the annealing.

Further, regarding annealing before plating, “after degreasing pickling, heating in a non-oxidizing atmosphere, annealing in a reducing atmosphere containing H ₂ and N ₂ , cooling to near the plating bath temperature, and invading the plating bath. Zenjimer method called “Kizuke”, a total reduction furnace method called “immersion in the plating bath after adjusting the atmosphere during annealing, first oxidizing the surface of the steel sheet, and then reducing it before cleaning by plating” Alternatively, there is a flux method such as “after degreasing and pickling a steel plate, and then fluxing it with ammonium chloride and soaking it in a plating bath”, etc. The effect of can be demonstrated. Regardless of the annealing method prior to plating, the dew point during heating is set to −20 ° C. or higher, which advantageously works on the wettability of the plating and the alloying reaction at the time of plating alloying.

In addition, even if this cold-rolled steel sheet is electroplated, the tensile strength, formability, and weldability of the steel sheet are not impaired at all. That is, the steel sheet of the present invention is also suitable as a material for electroplating.
In addition, the material of the high-strength and high-ductility hot-dip galvanized steel sheet having excellent workability according to the present invention is manufactured in principle through refining, steelmaking, casting, hot rolling, and cold rolling processes, which are ordinary steelmaking processes. However, even if manufactured by omitting a part or all of them, the effects of the present invention can be obtained as long as the conditions according to the present invention are satisfied.

  Next, the present invention will be described in detail with reference to examples.

  A slab having the components shown in Table 1 was heated to 1220 ° C., hot-rolled at a finish hot rolling temperature of 900 ° C., water-cooled in a water-cooled zone, and then wound up at a temperature shown in Table 2. . After pickling the hot-rolled sheet, a hot-rolled sheet having a thickness of 3 mm was cold-rolled to 1.4 mm to obtain a cold-rolled sheet. Thereafter, an annealing heat treatment is performed on these cold-rolled sheets under the conditions shown in Table 2, cooling between 600 ° C. and 500 ° C. to 0.5 to 50 ° C./second, and additional heat treatment is performed at each temperature for 250 seconds. And then cooled to room temperature. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.3%.

About some steel plates, it carried out to cold rolling by the method similar to the above, and performed the heat processing and the hot dip galvanization process in the continuous alloying hot dip galvanization equipment. About the steel plate which hot-dip galvanized, after annealing, between 600 degreeC-460 degreeC was cooled, and it passed through the zinc plating bath, and it cooled to room temperature with the cooling rate of 10 degree-C / sec to room temperature. For the alloying treatment, after passing through a galvanizing bath, the alloying treatment is performed at 500 ° C. for 30 seconds, cooled to room temperature at a cooling rate of 10 ° C./second, and finally obtained. The steel plate was subjected to skin pass rolling at a rolling reduction of 0.3%. About some steel plates, the alloying process was performed following the plating process. The basis weight at that time was about 50 g / m ^{2 on} both sides. The plated steel sheet was subjected to 0.3% skin pass rolling.

The obtained cold-rolled annealed plate or galvanized plate was subjected to a tensile test to measure YS, TS, and El. The yield stress was measured by the 0.2% offset method.
In the tensile test, a JIS No. 5 test piece was sampled in a direction perpendicular to the rolling direction from a 1.4 mm thick plate, and the tensile properties were evaluated.

  With respect to bendability, a test piece of 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction was sampled and evaluated by the crack generation limit bending radius of 90 ° bending. That is, the bendability was evaluated in increments of 0.5 mm from the bend radius of the punch tip to 0.5 to 5.0 mm, and the minimum bend radius without occurrence of cracking was defined as the limit bend radius.

  Spot weldability was performed under the following conditions. Electrode (dome type): tip diameter 6 mmφ, applied pressure 4.3 kN, welding current: current (CE) and (CE + 1.5) kA immediately before the occurrence of scattering, welding time: 15 cycles, holding time: 10 cycles. After welding, a cross tension test was performed according to JIS Z 3137. Welding with CE as the welding current was performed 10 times, and the lowest value among them was CTS (CE). On the other hand, CTS (CE + 1.5) was defined as the minimum value of CTS when welding was performed 10 times with the welding current being scattered (CE + 1.5) kA. The ratio of these values (= CTS (CE + 1.5) / CTS (CE)) was evaluated as x when less than 0.7, Δ when 0.7 or more and less than 0.8, and ○ when 0.8 or more.

Plating properties and alloying reactions were evaluated as follows.
Plating property ○: No unplating △: Slightly unplated ×: Many unplated ・ Alloying reactivity ○: No alloying unevenness on surface appearance △: Some alloying unevenness on surface appearance ×: Alloying on surface appearance Unevenness

  Table 2 shows the measured tensile properties, bendability, hole expandability, plating properties, alloying reactivity, and spot weldability. It can be seen that the steel sheets of the present invention are all excellent in weldability, and are excellent in reasonably high yield ratio, ductility and bendability.

  The present invention is suitable for structural members, reinforcing members and suspension members for automobiles, and steel sheets having high strength of 780 MPa or more and excellent ductility, bendability, hole expansibility and weldability at TS are inexpensive. It can be expected to contribute greatly to the weight reduction of automobiles, and the industrial effect is extremely high.

Claims (7)

In mass%, C: 0.06 to less than 0.1%, Si: 0.4 to 0.8%, Mn: 1.8 to 2.2%, Nb: 0.014 to 0.029%, Ti : 0.014-0.029%, P: 0.04% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, O: 0.0010-0. The cast slab containing 0045% and the balance consisting of chemical components of Fe and inevitable impurities is directly or once cooled and then heated to 1200 ° C. or higher, and the hot rolling is completed at the Ar3 transformation point or higher, and the temperature is 630 ° C. or lower. In the zone, after pickling, cold rolling with a rolling reduction of 40 to 70%, and then passing through a continuous annealing line, a temperature range of 580 to 750 ° C. with an average heating rate of 1.4 ° C. / Heat for more than a second, anneal at 750 ° C or higher and 900 ° C or lower, then 600 ° C-500 ° C It cooled at an average cooling rate of 4 to 200 ° C. / sec, and wherein the holding in a temperature range between 400 ° C. to 200 DEG ° C. 30 seconds or more, the yield ratio is 0.5 to 0.7 and a tensile strength A method for producing a high-strength cold-rolled steel sheet having excellent formability and weldability at 780 MPa or more . The cast slab further contains, by mass% Cr: according to claim 1, characterized in that it contains from 0.005 to 3%, the yield ratio is 0.5 to 0.7 and a tensile strength 780MPa A method for producing a high-strength cold-rolled steel sheet having excellent formability and weldability . The said cast slab further contains 0.005-0.6% of 1 type or 2 types chosen from V and W by mass% in total, The Claim 1 or Claim 2 characterized by the above-mentioned. A method for producing a high-strength cold-rolled steel sheet having a yield ratio of 0.5 to 0.7 and a tensile strength of 780 MPa or more and excellent formability and weldability . The yield ratio according to any one of claims 1 to 3, wherein the cast slab further contains B: 0.0001 to 0.1% by mass . A method for producing a high-strength cold-rolled steel sheet having a formability and weldability of 5 to 0.7 and a tensile strength of 780 MPa or more . The cast slab further contains 0.0005 to 0.04% in total of one or more selected from Ca, Mg, REM, and Y by mass%. Item 5. The method for producing a high-strength cold-rolled steel sheet according to any one of Items 4 having a yield ratio of 0.5 to 0.7 and a tensile strength of 780 MPa or more and excellent in formability and weldability . The cast slab comprising the chemical component according to any one of claims 1 to 5 is directly or once cooled and then heated to 1200 ° C or higher, and hot rolling is completed at an Ar3 transformation point or higher, and 630 ° C or lower. In the temperature range, after pickling, cold rolling with a rolling reduction of 40 to 70%, and then passing through a continuous hot dip galvanizing line, a temperature range of 580 to 750 ° C. with an average heating rate of 1 Heated at 4 ° C./second or higher, annealed at 750 ° C. or higher and 900 ° C. or lower, and between 600 ° C. and 500 ° C. at an average cooling rate of 4 to 200 ° C./second from a temperature lower by 40 ° C. than the galvanizing bath temperature. Cooling to a temperature range up to 50 ° C. higher than the galvanizing bath temperature, and then immersed in the galvanizing bath , forming at a yield ratio of 0.5 to 0.7 and a tensile strength of 780 MPa or more Excellent in weldability and weldability Method for manufacturing strength galvanized steel sheet. The cast slab comprising the chemical component according to any one of claims 1 to 5 is directly or once cooled and then heated to 1200 ° C or higher, and hot rolling is completed at an Ar3 transformation point or higher, and 630 ° C or lower. In the temperature range, after pickling, cold rolling with a rolling reduction of 40 to 70%, and then passing through a continuous hot dip galvanizing line, a temperature range of 580 to 750 ° C. with an average heating rate of 1 Heated at 4 ° C./second or higher, annealed at 750 ° C. or higher and 900 ° C. or lower, and between 600 ° C. and 500 ° C. at an average cooling rate of 4 to 200 ° C./second from a temperature lower by 40 ° C. than the galvanizing bath temperature. Cooled to a temperature range up to 50 ° C. higher than the galvanizing bath temperature, then immersed in a galvanizing bath, subjected to alloying treatment at a temperature of 460 ° C. or higher, and then cooled to room temperature , Yield ratio is 0.5-0. And, the method of producing a high strength galvannealed steel sheet excellent in formability and weldability in tensile strength 780MPa or more.