JP2007231369A - High strength cold-rolled steel sheet excellent in formability and weldability, high-strength hot-dip galvanized steel sheet, high-strength galvannealed steel sheet, manufacturing method of high-strength cold-rolled steel sheet, and manufacturing method of high-strength hot-dip galvanized steel sheet , Manufacturing method of high strength galvannealed steel sheet - Google Patents

Abstract

A high-strength cold-rolled steel sheet having a maximum tensile strength (TS) of 980 MPa or more and excellent in ductility, hole expansibility, bendability and spot weldability is provided.
SOLUTION: In mass%, C: 0.075 to less than 0.1%, Si: 0.4 to 0.8%, Mn: 1.9 to 2.3%, Mo: 0.1 to 0.3. 35%, Ti: 0.014 to 0.029%, B: 0.0001 to 0.0045, P: 0.04% or less, S: 0.01% or less, Al: 0.1% or less, N: A high-strength cold-rolled steel sheet containing 0.01% or less and O: 0.001 to 0.0045%, with the balance being Fe and inevitable impurities.
[Selection figure] None

Description

  The present invention has a maximum tensile strength (TS) of 980 MPa or more, excellent formability such as ductility, bendability, hole expansibility, etc., and also has excellent spot weldability, a structural member for automobile, a reinforcing member, The present invention relates to a high-tensile cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength galvannealed steel sheet that are particularly suitable for suspension members. 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 980 MPa is considered, there is a problem that spot weldability deteriorates.

Alternatively, Patent Document 3 and Patent Document 4 disclose a cold-rolled steel sheet and a plated steel sheet in which addition of C to the steel sheet is suppressed in order to provide formability and spot weldability. However, in the methods described in these documents, when considering securing strength of 980 MPa or more, it is indispensable to add a large amount of C, and it is difficult to simultaneously provide spot weldability and excellent formability. It was.
On the other hand, Patent Document 5 and Patent Document 6 disclose a technique having excellent stretch flangeability and spot weldability by making the main phase a bainite structure. However, the methods described in these documents have a problem that the main phase is a bainite structure, so that the elongation is low and the moldability is poor.

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, can be softened by tempering, the hardness difference from a ferrite structure can be 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 the strength of 980 MPa or more was considered, the spot weldability was inferior. 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 carbonitride of Ti or Nb at the time of hot rolling. 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, but these elements greatly delay recrystallization and reduce ductility, and are difficult to control. Had the problem.
CAMP-ISIJ vol. 13 (2000), p391 CAMP-ISIJ vol. 13 (2000), p411 Japanese Patent Laid-Open No. 1-2230715 JP-A-2-217425 JP 2001-152287 A JP 2001-226742 A JP 2004-256906 A JP-A-11-279691

An object of the present invention is to provide a high-strength cold-rolled steel sheet having a maximum tensile strength (TS) of 980 MPa or more and excellent formability such as ductility, hole expansibility, bendability, and spot weldability, and a method for producing the same. To do.
The present invention provides a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip galvanized steel sheet having a maximum tensile strength (TS) of 980 MPa or more and excellent formability such as ductility, hole expansibility, and bendability and spot weldability. It aims at providing those manufacturing methods.

  As a result of earnest and examination to achieve the above object, the present inventors have added Si, Mo, B and Ti to the steel sheet in combination, and added other alloy elements in an appropriate amount, which is the main phase. By strengthening ferrite and reducing the hardness difference between ferrite and martensite or bainite structure that contributes to high strength as the second phase, tensile strength of 980 MPa or more, hole expansibility, bendability, ductility, etc. It has been found that a steel sheet having excellent formability and excellent weldability can be produced.

That is, the present invention is a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet that are excellent in formability and weldability, and methods for producing them. It is.
(1) The high-strength cold-rolled steel sheet excellent in formability and weldability of the present invention is in mass%, C: 0.075 to less than 0.1%, Si: 0.4 to 0.8%, Mn: 1.9 to 2.3%, Mo: 0.1 to 0.35%, Ti: 0.014 to 0.029%, B: 0.0001 to 0.0045%, P: 0.04% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, O: 0.001 to 0.0045%, with the balance being Fe and inevitable impurities, tensile strength 980 MPa It is the above.
(2) The high-strength cold-rolled steel sheet excellent in formability and weldability of the present invention is the high-strength cold-rolled steel sheet described in (1) above, and further in mass% in the steel, Nb: 0.001 It is characterized by containing 0.029%.
(3) The high-strength cold-rolled steel sheet excellent in formability and weldability of the present invention is the high-strength cold-rolled steel sheet described in (1) or (2) above, and further in mass% in the steel, Cr: It contains 0.005 to 3%.
(4) The high-strength cold-rolled steel sheet excellent in formability and weldability according to the present invention is the high-strength cold-rolled steel sheet according to any one of (1) to (3) above, and further in mass% in the steel. , Ca, Mg, La, Ce, Y is contained in a total of 0.005 to 0.04% of one or more selected from the group consisting of Ca, Mg, La, Ce and Y.
(5) The high-strength hot-dip galvanized steel sheet according to the present invention is characterized in that the surface of the high-strength cold-rolled steel sheet according to any one of (1) to (4) is subjected to hot-dip galvanization. To do.
(6) The high-strength galvannealed steel sheet of the present invention is galvanized on the surface of the high-strength cold-rolled steel sheet described in any one of (1) to (4) above. It is characterized by that.

(7) The method for producing a high-strength cold-rolled steel sheet excellent in formability and weldability according to the present invention is obtained by directly or once casting slabs comprising the chemical component according to any one of (1) to (4) above. After cooling, it is heated to 1200 ° C. or higher, completes hot rolling at the Ar3 transformation point or higher, wound in a temperature range of 630 ° C. or lower, pickled, and then subjected to cold rolling with a rolling reduction of 40 to 70%, Next, when passing through the continuous annealing line, a temperature range of 580 to 750 ° C. is heated at an average heating rate of 0.8 ° C./second or more, and annealed at 750 ° C. or more and 900 ° C. or less, and 400 ° C. to 200 ° C. After cooling to a temperature range between, the temperature range between 400 ° C. and 200 ° C. is maintained for 30 seconds or more.
(8) The method for producing a high-strength hot-dip galvanized steel sheet having excellent formability and weldability according to the present invention is obtained by directly or directly forming a cast slab comprising the chemical component according to any one of (1) to (4) above. Once cooled, heat to 1200 ° C or higher, complete hot rolling above the Ar3 transformation point, wind up at a temperature range of 630 ° C or lower, pickle, and then cold-roll with a rolling reduction of 40-70%. Then, when passing through a continuous hot dip galvanizing line, a temperature range of 580 to 750 ° C. is heated at an average heating rate of 0.8 ° C./second or more, and annealed at 750 ° C. or more and 900 ° C. or less, and galvanized. It is characterized by being immersed in a galvanizing bath after cooling to a temperature range from a temperature 40 ° C. lower than the bath temperature to a temperature higher by 50 ° C. than the galvanizing bath temperature.
(9) The method for producing a high-strength galvannealed steel sheet having excellent formability and weldability according to the present invention is a casting slab comprising the chemical component according to any one of (1) to (4) above. Directly or once cooled and then heated to 1200 ° C or higher, completed hot rolling above the Ar3 transformation point, wound up in a temperature range of 630 ° C or lower, pickled, and then cold-rolled with a rolling reduction of 40-70% Then, when passing through a continuous hot dip galvanizing line, a temperature range of 580 to 750 ° C. is heated at an average heating rate of 0.8 ° C./second or more, and annealed at 750 ° C. or more and 900 ° C. or less, After cooling to a temperature range from 40 ° C. lower than the galvanizing bath temperature to 50 ° C. higher than the galvanizing bath temperature, after being immersed in the galvanizing bath and alloying at a temperature of 460 ° C. or higher, room temperature It is characterized by cooling to.

  The present invention is a high-strength cold-rolled steel sheet that combines high strength of 980 MPa or higher with excellent tensile strength suitable for structural members, reinforcing members, and suspension members for automobiles, and excellent formability and weldability. High-strength galvanized steel sheets and high-strength galvannealed steel sheets can be provided at low cost.

  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, in order to set the tensile strength to 980 MPa or more and to have excellent ductility, hole expansibility and bendability, C, Si, Mn, Mo, Ti and B are simultaneously added and controlled in a certain balance. We found that this is extremely important. By setting the steel plate component in an appropriate range, a high-strength steel plate having a good balance of ductility, bendability, hole expansibility and spot weldability can be obtained.

The present invention is described in detail below.
First, the reasons for limiting the components will be described. In the present specification, “%” means mass%.
C: C is an element that can increase the strength of the steel sheet. However, if it is less than 0.075%, it becomes difficult to achieve both a tensile strength of 980 MPa or more and moldability. 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 0.075 to less than 0.1% (0.075 or more and 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 a range of 0.4 to 0.8% (0.4% or more and 0.8% or less). 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 1.9%, it is difficult to obtain a tensile strength of 980 MPa or more. On the contrary, if the amount is too large, co-segregation with P and S is promoted, and the bendability and stretchability of the elongated hole are significantly deteriorated. Therefore, the upper limit is 2.3%. That is, the Mn content is in a range of 1.9% or more and 2.3% or less, and a more preferable range is 2.0 to 2.2%.

By adding Ti and B in combination, the steel sheet can be provided with strength, ductility, bendability and hole expandability.
Ti: Ti is a strengthening element. In addition to the strengthening of precipitates, fine grain strengthening by suppressing the growth of ferrite crystal grains and dislocation strengthening through suppression of recrystallization, which contributes to an increase in the strength of the steel sheet. Since it is a strong nitriding element, it is possible to secure B in the solid solution state in the steel sheet structure after hot rolling by preferentially binding with N in comparison with B. And in order to exhibit the B addition effect for the structure control at the time of annealing, the addition is essential. In addition, Ti is preferentially associated with N compared to Al, thereby forming fine nitrides, suppressing the formation of coarse AlN, and suppressing the deterioration of bendability. is important. 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%. That is, the Ti content is in the range of 0.014% or more and 0.029% or less.
B is effective for strengthening grain boundaries and strengthening steel by adding 0.0001% or more. However, when the added amount exceeds 0.0045%, the effect is not only saturated but also during hot rolling. The upper limit is set to 0.0045% because manufacturing is reduced. That is, the B content is in the range of 0.0001% to 0.0045%.
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%.
S: S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit is set to 0.01% 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 it is difficult to set the lower limit to 0.0005% or less, which 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 to less than 0.0010% invites excessive cost and is not economically preferable, this was made into the minimum.
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 these effects are less than 0.001%, the lower limit is set to 0.001%. If the content exceeds 0.029%, precipitation of carbonitrides increases and the formability deteriorates, so the upper limit was made 0.029%.

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%.
One or more selected from Ca, Mg, La, Ce, and Y can be added in a total amount of 0.0005 to 0.04% (0.0005% or more and 0.04% or less). Ca, Mg, La, Ce and Y are elements used for deoxidation, and it is preferable to contain one or more elements in total of 0.0005% or more. However, when the content exceeds 0.04% in total, it causes deterioration of molding processability. Therefore, the total content is set to 0.0005 to 0.04%. In the present invention, La and Ce are often added by misch metal, and in addition to La and Ce, a lanthanoid series element may be contained in combination. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. However, the effects of the present invention are exhibited even when metal La or Ce is added.
In addition, in this invention composition, you may contain less than 0.05% in total as Ni and Cu as another unavoidable impurity.

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.

In the present invention, the crystal grain size of ferrite is not particularly limited, but it is desirable that the nominal grain size is 4 μm or less from the viewpoint of balance of strength, elongation, and hole expansibility.
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 or more. 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 18000 (MPa ×%) or more in terms of TS (MPa) × tensile test total elongation (El) (%) is defined as having excellent ductility. With respect to hole expansibility, specifically, TS (MPa) × hole expansivity (λ) (%) having 30000 (MPa ×%) or more is defined as having excellent hole expansibility.
When TS (MPa) × hole expansion rate (λ) (%) is less than 30000 (MPa), cracks are likely to occur during stretch flange molding, so the lower limit was set to 30000 (MPa).
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 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.
When passing through the continuous annealing line, the heating rate in the temperature range of 580 to 750 ° C. needs to be 0.8 ° C./second or more. Since strengthening is performed using Ti, control of grain growth and recrystallization during heating is extremely important. Although the detailed mechanism is unknown, when the heating rate is less than 0.8 ° C./second, grain growth and recrystallization proceed excessively, leading to deterioration of bendability and hole expansibility. For this reason, the lower limit was set to 0.8 ° 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 980 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 in the life of the roll. 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 completion of the annealing, it is cooled to 200 to 400 ° C. The cooling rate or the cooling start temperature at this time is not particularly limited, and the effect of the present invention is exhibited. In particular, since the present steel contains a large amount of Mo and B, the structural change during the cooling process is suppressed, and even if the cooling rate is reduced, the structural change does not occur and no problem occurs in the material. Even if the cooling rate is increased, there is no problem in terms of the material. However, excessively increasing the cooling rate leads to an increase in manufacturing cost, so the upper limit is preferably set to 200 ° C./second. The cooling method may be roll cooling, air cooling, water cooling, or any combination of these methods.
In the case of a continuous annealing line, heat treatment for 30 seconds or more may be performed at a temperature range of 400 ° C. or lower 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 980 MPa and excellent weldability. On the other hand, the lower limit temperature is not particularly defined, but the effect of the present invention is exhibited. However, the heat treatment at less than 200 ° C. is not effective, and excessive cooling is not preferable. C. 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. Since productivity will fall remarkably when it exceeds 1.5%, this is made an 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 0.8 ° 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. With respect to cooling after annealing, the effects of the present invention can be exhibited without any particular definition for the same reason as when the continuous annealing line is passed.
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 ratio of hard structure (martensite, bainite, retained austenite) is decreased, and it becomes difficult to ensure a tensile strength of 980 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 sheet is plated with Ni, Cu, Co, or Fe alone or before plating.
Furthermore, 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”. 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 more, which advantageously works on the wettability of plating and the alloying reaction during alloying of plating.
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 some 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.

[Example 1]
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 1.2 mm to obtain a cold-rolled sheet. Thereafter, these cold-rolled sheets were subjected to annealing heat treatment under the conditions shown in Table 2, cooled from annealing temperature to 650 ° C. at 4 ° C./second, and then to each temperature at 40 ° C./second, at each temperature. An additional heat treatment was performed 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 0.4%.
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 carries out hot dip galvanization, between 650 degreeC-460 degreeC was cooled after annealing, Then, 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 0.4%. 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 0.4% 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 1.2 mm thick plate, and the tensile properties were evaluated. A high strength steel sheet having excellent hole expansibility was obtained when the balance of strength (TS) -total elongation (El) (TS × E1) exceeded 18000 (MPa ·%).
Hole expandability is 10mm diameter circular hole punched under the condition that the clearance is 12.5%, the burr is on the die side, molded with a 60 ° conical punch, hole expansion rate λ (%) It was evaluated by. A steel sheet having a strength-hole expansibility (λ) balance (TS × λ) exceeding 30000 (MPa ·%) was defined as a high-strength steel sheet having excellent hole expansibility.
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 splatter, welding time: 14 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 lowest 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 ○: Unplated △: Slightly unplated ×: Many unplated ・ Alloying reactivity ○: No alloying unevenness on surface appearance △: Some alloying unevenness on surface appearance ×: Alloy on surface appearance Many irregularities

  Tables 2, 3 and 4 show 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 excellent in ductility, hole expansibility and bendability.

  The present invention is suitable for automobile structural members, reinforcing members, and suspension members, and steel sheets having high strength of 980 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 (9)

  In mass%, C: 0.075 to less than 0.1%, Si: 0.4 to 0.8%, Mn: 1.9 to 2.3%, Mo: 0.10 to 0.35%, Ti : 0.014 to 0.029%, B: 0.0001 to 0.0045%, P: 0.04% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01 %, O: 0.0010 to 0.0045%, the balance being Fe and inevitable impurities, and having a tensile strength of 980 MPa or more, a high-strength cold-rolled steel sheet excellent in formability and weldability .   The high strength cold-rolled steel sheet according to claim 1, further comprising Nb: 0.001 to 0.029% by mass% in the steel.   Furthermore, Cr: 0.005-3% is contained by mass% in steel, The high strength cold-rolled steel plate of Claim 1 or Claim 2 characterized by the above-mentioned.   Furthermore, the steel contains 0.0005 to 0.04% in total of one or more selected from Ca, Mg, La, Ce, and Y in mass%. Item 4. The high-strength cold-rolled steel sheet according to any one of items 3.   A high-strength hot-dip galvanized steel sheet, wherein the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 4 is hot-dip galvanized.   A high-strength galvannealed steel sheet, wherein the surface of the high-strength cold-rolled steel sheet according to any one of claims 1 to 4 is galvannealed.   The cast slab comprising the chemical component according to any one of claims 1 to 4 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% is performed, and then when passing through a continuous annealing line, a temperature range of 580 to 750 ° C. is set to an average heating rate of 0.8. Heating at ℃ / second or more, annealing at 750 ° C or more and 900 ° C or less, cooling to a temperature range between 400 ° C and 200 ° C, and holding at a temperature range between 400 ° C and 200 ° C for 30 seconds or more. A method for producing a high-strength cold-rolled steel sheet having excellent formability and weldability.   The cast slab comprising the chemical component according to any one of claims 1 to 4 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 0 . Heating at 8 ° C / second or more, annealing at 750 ° C or more and 900 ° C or less, and cooling to a temperature range from 40 ° C lower than the galvanizing bath temperature to 50 ° C higher than the galvanizing bath temperature, A method for producing a high-strength hot-dip galvanized steel sheet excellent in formability and weldability, characterized by being immersed in a galvanizing bath.   The cast slab comprising the chemical component according to any one of claims 1 to 4 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 0 . Heating at 8 ° C / second or more, annealing at 750 ° C or more and 900 ° C or less, and cooling to a temperature range from 40 ° C lower than the galvanizing bath temperature to 50 ° C higher than the galvanizing bath temperature, A method for producing a high-strength galvannealed steel sheet excellent in formability and weldability, characterized by being immersed in a galvanizing bath, subjected to alloying treatment at a temperature of 460 ° C. or higher, and then cooled to room temperature.