JPH05186824A - Production of cold rolled steel sheet having high strength and high ductility and excellent in aging characteristic - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet having high strength and high ductility and excellent in ageing resistance by applying hot rolling and cold rolling to a steel slab with a specific composition and then subjecting the resulting steel sheet to heat treatment containing a stage for applying specific strain to the steel sheet. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.05-0.30% C, 0.5-2.5% Si, 0.5-2.5% Mn, <0.02% P, <0.01% S, 0.01-0.06% sol.Al, and <0.005% N or further containing <=0.05%, in total, of one or >=2 elements among Ti, Nb, and B is hot-rolled and cold-rolled, and the resulting steel sheet is heated up to a temp. in the range between the Ac1 and the Ac3 transformation point of this steel sheet and held at the above temp. for 20sec-3min. Subsequently, the steel sheet is cooled slowly down to 600-750 deg.C at (5 to 25 deg.C)/sec cooling rate and successively cooled rapidly down to 350-480 deg.C at a cooling rate VR satisfying inequality I. At this time, the operation where a water-cooled roll is pushed against the steel sheet to apply bending deformation in which strain epsilonR at the surface of the steel sheet satisfies inequality II is repeated, and then, the steel sheet is held at 350-480 deg.C for 30sec-10min and cooled down to room temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent aging resistance, which is a high-strength / high-ductility cold-rolled steel sheet which is stable against room temperature aging It is about technology.

[0002]

2. Description of the Related Art In recent years, there has been a demand for improvement in fuel efficiency of automobiles from the viewpoint of global environmental protection, and there is an active movement to reduce the weight of vehicle bodies for the purpose of achieving the improvement. That is, for this reason, the weight of the vehicle body is reduced by thinning the steel sheet used for automobiles, and the decrease in the vehicle body strength due to the thinning is compensated by the higher strength of the steel sheet. The demand for higher ductility is becoming more and more severe, and a material that has both high strength and high ductility is expected.

In response to such requirements, a steel sheet having a TS (tensile strength) of 80 to 100 kgf / mm ² and an El (breaking elongation) of about 30% is utilized by utilizing the work-induced transformation of retained austenite. It is proposed in JP-A-60-43430. Such steel sheets are made of C, Si, M
It is produced by subjecting a steel sheet containing n as a basic component to austenite, and then performing a so-called austempering treatment in which it is placed in the bainite transformation temperature range and kept isothermal.
As a concrete method for giving such a heat cycle to a steel sheet, "Iron and Steel, 71 (1985), S1936"
There is continuous annealing by the gas jet cooling system as shown in Fig. 4 and continuous annealing by the water cooling roll cooling system as shown in "Iron and Steel, 73 (1987), S1263". By optimizing the cycle, the balance between strength and ductility is improved, and it becomes possible to manufacture a steel sheet having excellent press formability. By applying these steel plates to members for which high strength materials cannot be applied due to their severe shapes, the weight of the automobile body can be reduced.

[0004]

However, as the inventors of the present invention proceeded with research on such a composite structure steel sheet containing retained austenite, a steel sheet of this type showed a recovery of yield point elongation due to room temperature aging. I noticed that it is easy to happen. When recovery of the yield point elongation occurs, defects called stretcher strains occur during press molding and the appearance of the molded product is significantly impaired. Therefore, the lower the room temperature aging, the better.

On the other hand, in the case of Dual-Phase steel which has been conventionally used as a high-strength steel sheet, dislocations can easily multiply inside ferrite due to stress concentration around martensite, so that solid solution C and N remain in ferrite. It is known that recovery of yield point elongation after room temperature aging is unlikely to occur even in the state of aging. However, in the above-mentioned steel sheet, the main components of the second phase are bainite and retained austenite, and the stress concentration around these second phases is due to Dual-Phas
Since it is smaller than e-steel, recovery of yield point elongation is not significantly suppressed. Even if the ratio of martensite is increased, recovery of yield point elongation is not as good as that of Dual-Phase steel.

No matter how excellent the press formability immediately after production is, if the yield point elongation is recovered and defects occur at the time of actually forming, the press formability is truly excellent. I can't say. However, in the above-described conventional steel sheet, only the press formability immediately after production was evaluated, and the stability with respect to room temperature aging, that is, the aging resistance was not satisfactory.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted extensive research on solving the technical problems of the conventional ones as described above, and have an excellent balance between strength and ductility of a composite structure steel sheet containing retained austenite. While making the most of
Various studies were conducted on the chemical composition and manufacturing process for improving the aging resistance.

That is, as an example, wt% (hereinafter simply referred to as%), C: 0.11%, Si: 1.12%, Mn: 1.67%, P: 0.
009%, S: 0.004%, sol. Al: 0.035%, N: 0.00
Steel containing 26% was melted, cast into a slab, and then hot-rolled and cold-rolled according to a conventional method to obtain a 1.2 mm-thick cold-rolled steel sheet. Then, after heating and holding at 825 ° C for 1 minute, the material was gradually cooled to 640 ° C at a cooling rate of 10 ° C / sec, and then rapidly cooled to 425 ° C at a cooling rate of 60 ° C / sec.
After performing isothermal holding for 5 minutes, continuous annealing for cooling to room temperature was performed.

For the annealed sheet sample (A) obtained as described above, (B) 0.5% temper rolling, (C) 2
% Temper rolling, (D) 200 ° C. × 3 min overaging treatment,
(E) Each sample was overaged at 400 ° C. for 3 minutes. The as-annealed sample was designated as sample A, and (B) to (E)
Samples B to E were treated with the samples of Sample Nos. B to E, and the samples A to E were subjected to a tensile test and a bending test immediately after production and after room temperature aging treatment at 38 ° C. for 30 days, respectively. The results of examining the yield point elongation and the minimum bending radius are as shown in FIG.

That is, although part of the solid solution C is precipitated during the cooling process from 825 ° C. to 425 ° C. and the holding process at 425 ° C., it does not completely precipitate, so that the yield elongation of sample A is large. Is becoming On the other hand, when temper rolling is performed at an elongation rate that can suppress the recovery of the yield point, or overaging treatment that causes the solute C to be almost precipitated, TS
It can be seen that × El decreases. This is because part of the retained austenite is transformed into martensite by temper rolling, and bainite and martensite are decomposed to generate carbides during the overaging treatment process. In this way, by applying the method that was effective in suppressing the recovery of the yield point elongation in the production of conventional steel sheets, the excellent strength / ductility balance originally possessed by the composite structure steel sheet containing retained austenite is obtained. Therefore, it was concluded that it could not be compatible with aging resistance.

Therefore, the inventors of the present invention reduce the residual solid solution C in ferrite by intentionally imparting strain to the steel sheet during the annealing process to accelerate the concentration of solid solution C in austenite. In addition, by further refining the microstructure of the second phase, the recovery of the yield point elongation can be suppressed and the balance between strength and ductility can be further improved, and further research was conducted. Hereinafter, the present invention will be specifically described based on experimental results.

When performing continuous annealing using the above cold-rolled steel sheet under the same conditions as described above, 640 to 42 ° C.
In the process of rapid cooling to 5 ° C., (F) a constant tension is continuously applied, (G) a tension is applied, and then a series of operations of unloading and then re-applying the tension is performed four times, (H) 2 Performing a light rolling operation with one roll four times, (I) performing a bending deformation by pressing the roll against the surface of the steel sheet and then applying a bending deformation to the steel sheet from the opposite direction four times. Thus, a strain of 0.2% is applied to the steel sheet, and the samples subjected to these operations (F) to (I) are referred to as Samples F to I, respectively. The strain on the surface of the steel sheet is used as the strain amount, and the tension is applied for the operations (F) and (G), and the strain is applied to the rolls for the operations (H) and (I). Means strain. With respect to the steel sheet obtained by such continuous annealing, the results of performing the same tensile test and bending test as above after the room temperature aging treatment at 38 ° C. for 30 days are shown in FIG.

As for samples G, H, and I, TS × El was further increased as compared with the other samples, the minimum bending radius was small, and there was almost no recovery of elongation at yield. In particular, the sample I can be bent in close contact, and TS × El is the maximum. On the other hand, the sample F is not much different from the sample A even though the strain is applied. That is, simply applying a strain is not effective, and by repeatedly applying a strain, solid solution C in austenite is concentrated and the second phase structure is refined, and the strength / ductility balance and the aging resistance are improved. Both sex are improved. In particular, it has been found that bending property can be further improved by imparting non-uniform strain in the plate thickness direction due to bending deformation.

Therefore, the present inventors also examined the chemical composition of the steel sheet and the amount of strain to be applied. That is, as an example, C: 0.11%, Si: 1.12%, Mn: 1.67%, P: 0.00
9%, S: 0.004%, sol. Al: 0.035%, N: 0.0026
%, Various cold-rolled steel sheets were produced with varying amounts of C, Si, and Mn, and continuously annealed under the same conditions as in Sample I. Further, continuous annealing was performed by changing only the amount of strain, and the results of the tensile test and bending test are as shown in FIGS.

From these results, it is understood that the chemical components and the amount of strain that can achieve both the strength / ductility balance and the aging resistance are limited. In particular, the optimum strain amount may be further limited depending on the amounts of Si and Mn. S
Since pearlite was observed in the structure when the amount of i and Mn was low and the amount of strain was large, it is considered that the pearlite transformation was promoted by the strain and the characteristics deteriorated.
Therefore, for the above base steel, the strain amount ε _R and 640
The effect of the cooling rate V _R when quenching from ℃ to 425 ° C. on the mechanical properties was investigated in detail. The result is shown in FIG.

That is, from FIG. 1, V _R <V _Crit or ε _R
In the region of> ε _Crit , pearlite transformation occurs in the cooling process and the mechanical properties are significantly deteriorated. On the other hand, V _R > 200 ° C /
Even in the region of sec or ε _R <0.05%, TS × E
It can be seen that either one or both of l and elongation at yield become worse. Here, FIG. 7 shows the results of detailed investigation in which the influences of the amounts of Si and Mn on the critical cooling rate V _Crit and the strain amount ε _Crit that deteriorate the mechanical properties are associated with the change of TS × El. When sorted by the parameter of 0.5Mn (%) + 0.1Si (%), this parameter is 0.3-
Within the range of 1.5, V _R ≧ V _Crit , ε _R ≦ ε
_It can be seen that TS × El is high when it is _Crit . In addition, Si <0.5 (%) or Si> 2.5 (%),
0.5 even if Mn <0.5 (%) or Mn> 2.5 (%)
The value of Mn (%) + 0.1Si (%) may fall within the range of 0.3 to 1.5. In this case, as shown in FIGS. 5 and 6, the mechanical properties are irrespective of V _R and ε _R. Will deteriorate.

As explained above, there is a close relationship between the amount of strain, the cooling rate, and the chemical composition when cyclic strain is applied, and the object of the present invention cannot be achieved unless all of these are optimized. all right. The above-mentioned "Iron and Steel, 73 (1987), S1263" shows a method for producing a high ductility high tensile strength steel by cooling with a water-cooled roll. According to this method, the steel sheet inevitably undergoes bending by rolls during the quenching process, but the amount of strain and cooling rate at that time are not optimized at all, and other continuous annealing conditions and chemical The components have also been determined only from the viewpoint of improving the strength / ductility balance,
Since the aging resistance is unfavorable, it is clearly different from the method according to the present invention for the purpose of achieving both strength / ductility balance and aging resistance.

As a result of further research on other chemical components and conditions of continuous annealing, the present inventors have found for the first time that a steel sheet having a good balance of strength and ductility and excellent aging resistance can be produced. The present invention has been achieved as follows.

(1) wt%, C: 0.05 to 0.30%,
Si: 0.5 to 2.5%, Mn: 0.5 to 2.5%, P: 0.
02% or less, S: 0.01% or less, sol. Al: 0.01 to 0.06
%, N: 0.005% or less, hot rolled to steel slab balance being Fe and unavoidable impurities, after subjected to cold rolling, 20 seconds and heated to a temperature range of A _C1 to A _C3 ~ After holding for 3 minutes, 5 ~ 25 ℃
At a cooling rate of / sec to a temperature range of 600 to 750 ° C., and then 3 at a cooling rate V _R in a range that satisfies the following formula (1).
When rapidly cooling to a temperature range of 50 to 480 ° C., a strain ε _R in the range satisfying the following formula (2) is applied to the steel sheet, and after unloading, a strain ε _R in the range satisfying the following formula (2) is again performed. Is repeated in this rapid cooling process, and then the temperature is kept in the temperature range of 350 to 480 ° C. for 30 seconds to 10 minutes and then cooled to room temperature, which is excellent in aging resistance. Manufacturing method of rolled steel sheet. Equation _{(1) V Crit ≦ V R} (℃ / sec) ≦ 200 Equation _{(2) 0.05 ≦ ε R (} %) ≦ ε Crit _{However, V Crit = 12exp {1.8 -0.5} Mn (%) - 0.1 Si (%) _{} ε Crit = 0.003 (V R} -V Crit) +0.2

(2) 350 to 48 at a cooling rate V _R in a range satisfying the above expression (1) from a temperature range of 600 to 750 ° C.
When the steel plate is rapidly cooled to a temperature range of 0 ° C., a roll is pressed against the steel plate to give bending deformation so that the strain ε _{R on the} steel plate surface satisfies the above formula (2), and then the roll is pressed against the steel plate from the opposite direction. A high strength with excellent aging resistance as described in the above item (1), characterized in that a series of operations for giving a bending deformation such that the strain ε _{R on the} surface of the steel sheet satisfies the above formula (2) is repeated in this quenching process. Manufacturing method of high ductility cold rolled steel sheet.

(3) From the temperature range of 600 to 750 ° C., the average cooling rate V _R in the range satisfying the above expression (1) is 350 to
When rapidly cooling to the temperature range of 480 ° C., a water-cooled roll is pressed against the steel sheet to cause bending deformation so that the strain ε _{R on the} steel sheet surface satisfies the above equation (2), and subsequently the steel sheet is water-cooled from the opposite direction. The bending and quenching of the steel sheet are performed by repeating a series of operations in which the strain ε _{R on the} surface of the steel sheet satisfies the above equation (2) by pressing the rolled roll, and the bending deformation is repeated. The method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent aging resistance according to 1.

(4) Ti: 0.005 to 0.05%, Nb: 0.005
To 0.05%, B: 0.0005 to 0.003%, and one or more elements selected from the total of 0.05% or less are contained in (1) or (2) or (3). A method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent aging resistance as described above.

[0023]

The operation of the present invention as described above will be explained. First, the reasons for limiting the content range of alloying elements in the present invention are as follows. C: 0.05 to 0.30%, C concentrates in the austenite in the process of transforming the supercooled austenite into bainite, and stabilizes the austenite to generate residual austenite. The presence of this retained austenite improves the balance between strength and ductility, but it is necessary to add 0.05% or more of C in order to obtain retained austenite sufficient to exert its function. On the other hand, when the content exceeds 0.30%, the retained austenite amount increases, but the martensite generated by the work-induced transformation becomes hard, and the deterioration of local ductility such as bendability is avoided even with the manufacturing method according to the present invention. I can't. Therefore, the upper limit of C is set to 0.30%.

Si: 0.5 to 2.5% Si has the function of promoting the concentration of C in austenite and facilitating the formation of retained austenite, and therefore 0.5% or more is required to be added. However, excessive addition causes embrittlement of steel and deteriorates the balance of strength and ductility. Therefore, the amount added should be limited to 2.5% or less.

Mn: 0.5-2.5% Mn is an element essential for the formation of retained austenite by bainite transformation, since it shifts the nose of ferrite-pearlite transformation to the long side. Moreover, like C, it is an austenite stabilizing element and is necessary to obtain an excellent balance of strength and ductility. These effects are 0.5
% Is not exhibited, so 0.5% is the lower limit. On the other hand, if added excessively, a band-like structure is generated and the ductility of the steel sheet deteriorates. Therefore, the upper limit is set to 2.5%.

P: 0.02% or less, P is preferable because it deteriorates aging resistance. Therefore, in the present invention, it is limited to 0.02% or less.

S: 0.01% or less, S is desired to be as small as possible because it significantly reduces the ductility of steel. Therefore, it is limited to 0.01% or less.

Sol. Al: 0.01 to 0.06%, sol. Al is a solid solution N by fixing N in steel.
It has an effect of suppressing recovery of elongation at yield point. This effect is not exhibited with the addition of less than 0.01%, so 0.01% is made the lower limit. However, even if added excessively, not only the effect will be saturated, but also the ductility will be deteriorated, so 0.06
% Is the upper limit.

N: 0.005% or less N is an element that most deteriorates the aging resistance of steel. The smaller the content, the more preferable. In the present invention, N is limited to 0.005% or less.

The basic elements in the present invention are as described above, but by further adding the following elements, it becomes possible to further improve the strength / ductility balance and the aging resistance. Both Ti and Nb are strong carbonitride-forming elements, and fine carbonitrides make the structure finer. It also has the effect of fixing C or N. B has the effect of refining the structure by solid solution in steel and fixing N. These actions improve the strength / ductility balance and prevent room temperature aging. In order to obtain such effects, 0.005% or more for Ti and Nb, and 0 for B.
Addition of 0005% or more is required. However, if it is added excessively, a large amount of carbonitrides such as TiC and BN are precipitated, and these precipitates deteriorate the ductility of the steel. Therefore, 0.05% or less for Ti and Nb and 0.003% or less for B, totaling 0.
It must be added within the range of less than 05%.

Next, the reasons for limitation in the manufacturing conditions will be described. The steel slab containing the above chemical components is formed into a steel plate having a desired plate thickness by ordinary hot rolling and cold rolling. After that, the obtained cold-rolled steel sheet is subjected to continuous annealing, but if any one of them is not appropriate under the conditions of this continuous annealing, the amount of retained austenite is insufficient or solid solution C remains. For this reason, the strength / ductility balance and aging resistance are deteriorated. In addition, bendability may deteriorate.

If the heating temperature during annealing is lower than A _C1 , the reverse transformation to austenite does not occur and it is impossible to obtain retained austenite itself. On the other hand, when it is higher than A _C3 , the austenite is completely formed, so that the concentration of C does not occur, the residual austenite amount becomes insufficient, and the strength / ductility balance deteriorates. Therefore, the heating temperature during annealing is limited to A _{C1 to} A _C3 .

Regarding the holding time in the above temperature range,
If it is shorter than 20 seconds, a homogeneous two-phase structure cannot be obtained and 3
If it is longer than that, the structure becomes coarse, and the balance between strength and ductility deteriorates in any case. Therefore, it is necessary to set the heat retention time during annealing to 20 seconds to 3 minutes.

From the temperature range of A _{C1 to} A _C3 , 5 to 25 ° C./sec
By gradually cooling to a temperature range of 600 to 750 ° C. at a cooling rate of 1, the degree of supersaturation of solid solution C in ferrite becomes appropriate, and the solid solution C in austenite is obtained by repeatedly giving strain in the subsequent quenching process. Can be promoted. Here, if the cooling rate is less than 5 ° C./sec or if the slow cooling end temperature exceeds 750 ° C., the supersaturation degree of the solid solution C is insufficient and the solid solution C cannot be sufficiently concentrated. On the other hand, when the cooling rate exceeds 25 ° C / sec or the end temperature of slow cooling is less than 600 ° C, the degree of supersaturation of solid solution C is too high, and therefore ductility due to carbides finely precipitated in ferrite grains during the quenching process. Deteriorates. Therefore, the cooling rate is limited to 5 to 25 ° C./sec.

From the temperature range of 600 to 750 ° C., 350 to 4
Upon quenching to a temperature range of 80 ° C., quenching is performed at a cooling rate V _R within a range that satisfies the following formula (1), and then the following formula (2).
Giving the steel sheet a strain epsilon _R range satisfying, very in the present invention can be repeated in this quenching process the sequence of operations that provide a strain epsilon _R range satisfying the following expression (2) to the steel plate again and unloading Is important to. Equation _{(1) V Crit ≦ V R} (℃ / sec) ≦ 200 Equation _{(2) 0.05 ≦ ε R (} %) ≦ ε Crit _{However, V Crit = 12exp {1.8 -0.5} Mn (%) - 0.1 Si (%) _{} ε Crit = 0.003 (V R} -V Crit) is +0.2.

That is, V _R <V _Crit or ε _R > ε _Crit
In the case of, pearlite transformation occurs in the cooling process, and the strength / ductility balance, bendability, and aging resistance are significantly deteriorated.
Since the time until pearlite transformation occurs changes depending on the amounts of Si and Mn, V _Crit and ε _Crit differ depending on the amounts of Si and Mn. On the other hand, when ε _R <0.05%, the concentration of solute C due to strain is insufficient and the solute C remains even after annealing. Since the refinement of the second-phase structure due to the strain is also insufficient, it is not possible to obtain desirable characteristics. Further, when V _R > 200 ° C./sec, extremely supersaturated solid solution C is finely precipitated in the ferrite grains, so that the aging resistance is improved but the ductility is significantly deteriorated. Even if the cooling rate is changed within the range of the present invention, the characteristics may be improved without any problem.

It is important to repeatedly apply strain, since there is almost no effect of accelerating the concentration of solid solution C and refining the microstructure of the second phase when a constant strain is continuously applied. The number of repetitions is not particularly limited, but it is desirable to repeat three times or more to obtain a sufficient effect. However, excessive repetition should be avoided because the shape of the steel plate may deteriorate.

The above-mentioned rapid cooling is completed at 350 to 480 ° C., and it is necessary to hold it. If this temperature is lower than 350 ° C., it is difficult for C to diffuse, so it is difficult to concentrate C in the supercooled austenite, and solid solution C remains in the ferrite, which deteriorates the aging resistance. On the other hand, if the temperature is higher than 480 ° C., a large amount of carbide is precipitated, and the balance between strength and ductility is deteriorated. In addition, if the temperature is in the range of 350 to 480 ° C., it does not matter if the temperature changes during the holding.

When the holding time at 350 to 480 ° C. is shorter than 30 seconds, the concentration of C accompanying bainite transformation is insufficient. On the other hand, if it is longer than 10 minutes, most of the supercooled austenite is transformed into bainite. In any case, the amount of retained austenite decreases,
Since the strength / ductility balance deteriorates, the holding time is limited to 30 seconds to 10 minutes.

From the temperature range of 600 to 750 ° C., the above 35
In quenching to a temperature range of 0 to 480 ° C., a bending deformation is applied by pressing a roll on a steel plate, and subsequently a bending deformation is applied by pressing a roll on the steel plate from the opposite direction. By repeating the above, the strain applied to the steel sheet is repeatedly inverted, so that the effects of promoting the concentration of solid solution C in austenite and refining the second phase structure are further enhanced. In particular, the strain in the surface layer of the steel sheet changes greatly, so the effect of improving bendability is remarkable. Since the strain inside the steel sheet due to the bending deformation becomes maximum on the steel sheet surface, it is necessary to prevent the strain ε _{R on the} steel sheet surface from exceeding ε _Crit in the above-mentioned equation (2).

By pressing a water-cooled roll onto the steel sheet, it is possible to both rapidly cool the steel sheet and to apply bending strain. In this case, since the steel sheet is contacted with the roll and is subjected to strain and is cooled at the same time, the effect of accelerating the concentration of solid solution C and refining the second phase structure is maximized. Since this cooling is extremely effective, 600
In quenching from a temperature range of ˜750 ° C. to a temperature range of 350 to 480 ° C., the average cooling rate V _R is calculated by the formula (1).
If the above condition is satisfied, pearlite transformation does not occur and excellent properties can be obtained.

The steel sheet produced according to the present invention as described above is sufficiently excellent in aging resistance without temper rolling after annealing, but when temper rolling is performed due to surface finishing, It is desirable that the elongation rate be 0.5% or less. That is, if it exceeds 0.5%, a part of the retained austenite may be transformed into martensite,
This is because the strength / ductility balance deteriorates.

[0043]

EXAMPLES In describing specific examples of the present invention, a method for applying strain to a steel sheet will be described first. In the process of quenching from a temperature range of 600 to 750 ° C. to a temperature range of 350 to 480 ° C., a steel plate is formed by any of the following methods (V), (W), (X), (Y), and (Z). Repeated strain is applied. (V) Using a continuous annealing simulator with a tension mechanism, an operation of unloading after applying tension to the steel sheet is performed twice. (W) The same operation is performed 4 times using the simulator described above. (X) While forcibly cooling the steel sheet, a light reduction of 4 passes is performed with a hot rolling mill. (Y) Bending strain is applied by passing the steel sheet through a leveler while forced air cooling. 4 rolls on the top and 5 rolls on the bottom
In the book, the roll diameter is typically 50 mm. (Z) In a continuous cooling line of a water-cooled roll cooling system, bending deformation is applied while cooling the steel sheet with a water-cooled roll.
The flow rate of cooling water and the number of rolls are changed according to the target cooling rate. The number of rolls is in the range of 6 to 9, and the operation of bending in the forward direction and then in the reverse direction is performed three times or more. Further, the roll diameter and the line tension are changed according to the target strain amount.

Regarding (X) and (Y), after heating and holding in a salt bath, air cooling to a range of 600 to 750 ° C. is carried out, and then the operation of (X) or (Y) is carried out, followed by a predetermined operation. Put in a salt bath at temperature to keep isothermal.

(Example 1) The chemical compositions of representative steels according to the present invention and the comparative examples which were specifically adopted by the present inventors are as shown in Tables 1 and 2 below.

[0046]

[Table 1]

[0047]

[Table 2]

Each of the steels of Tables 1 and 2 as described above is melted and cast, and hot-rolled under the conditions of a heating temperature of 1200 ° C., a finishing temperature of 900 ° C. and a winding temperature of 620 ° C.,
After forming a steel plate having a thickness of 3.6 mm, it was subjected to pickling and cold rolling to obtain a cold rolled steel plate having a thickness of 1.6 mm. Then, after heating and holding at 825 ° C. for 1 minute, it was gradually cooled to 640 ° C. at a cooling rate of 10 ° C./sec, and then 425 at an average cooling rate of 60 ° C./sec.
In the process of rapid cooling to 0 ° C., bending / unbending deformation was repeatedly applied by the method (Z) described above so that the strain on the surface of the steel sheet during roll contact was 0.2%. Then 4
Continuous annealing was carried out by holding at 25 ° C for 5 minutes and then cooling to room temperature. Under these conditions, V _Crit <60 ° C./sec and ε _Crit > 0.2% for all the samples, which satisfies the formulas (1) and (2).

Regarding the steel sheet obtained by such continuous annealing, after JIS aging treatment at 38 ° C. for 30 days, JIS
A tensile test was carried out on a No. 13B test piece, and TS (tensile strength), El (total elongation) TS × El, and YPEl (yield point elongation) were investigated. In addition, the minimum bending radius Rmin at which cracks did not occur was investigated by a method of pressing and bending the steel sheet with a U-shaped punch and then bringing them into close contact. Furthermore, the volume ratio γ _R of the retained austenite was measured by the X-ray diffraction method. The results are shown in Tables 3 and 4 below.

[0050]

[Table 3]

[0051]

[Table 4]

That is, sample Nos. 2 to 4 and 7 to according to the present invention
13, 18 to 25, 31 to 35, 38 to 40 have T
S × El is 2400 or more, YPEl is 0.1% or less, R
Since min is 0.5 mm or less, it can be seen that the strength / ductility balance, aging resistance, and bendability are excellent. In particular, T
Samples Nos. 4, 10 to 13, 22 to 25, 34, and 35 in which i, Nb, and B were added in an appropriate range to further refine the structure had a further improved strength / ductility balance. Sample Nos. 1, 5, 6, 14 to 17, and 26 to 3 whose chemical components are outside the scope of the present invention with respect to the present invention
For 0, 36 and 37, TS × El is low or YP
Since the El or Rmin is high, the press working itself is difficult, or stretcher strain occurs, so it cannot be said that the press workability is truly excellent.

(Example 2) Steel f1 in Table 1 described above
The cold-rolled steel sheet according to 1. was continuously annealed under the conditions shown in Tables 5 and 6 below. The strain applying method is the above (W), and the strain ε _R of the steel sheet when tension is applied is also shown in Tables 5 and 6.

[0054]

[Table 5]

[0055]

[Table 6]

The sample after continuous annealing as described above was placed at 38 ° C.
After performing room temperature aging treatment for 30 days, the mechanical properties and the retained austenite volume ratio were examined in the same manner as in Example 1, and the results are shown in Tables 7 and 8 below.

[0057]

[Table 7]

[0058]

[Table 8]

That is, sample Nos. 2 and 3, which are examples of the present invention,
6, 7, 10, 13, 18-20, 22, 25, 26,
It is clear that the annealing conditions and strain amounts of Nos. 29, 30, and 32 to 40 are in the proper ranges, and the strength / ductility balance, aging resistance, and bendability are excellent. On the other hand, sample Nos. 1, 4, 5, 8, 9, 11, 12, which are comparative examples,
Nos. 14 to 17, 21, 23, 24, 27, 28, and 31 have problems in strength / ductility balance, aging resistance, and bendability. Especially, it can be seen from Sample Nos. 15 to 17, 21, and 23. As described above, it is clear that the characteristics deteriorate when the quenching rate V _R or the strain ε _R does not satisfy the equations (1) and (2).

(Example 3) The cold-rolled steel sheet according to the present invention shown in Tables 1 and 2 was subjected to continuous annealing under some conditions selected from Tables 5 and 6 above. In this continuous annealing, from the temperature range of 600 to 750 ° C., 35
In the process of rapid cooling to a temperature range of 0 to 480 ° C., the steel sheet was repeatedly strained by any of the methods (V), (W), (X), (Y), and (Z) described above. Further, as a comparative example, it was also tried to perform annealing by a continuous annealing simulator in a state where (U) a constant tension was continuously applied. The sample after continuous annealing was subjected to room temperature aging treatment at 38 ° C. for 30 days, and then the mechanical properties and the residual austenite volume ratio were investigated in the same manner as in Example 1. Steel No., Annealing No.,
The combinations of strain applying methods and the results are summarized in Tables 9 and 10 below.

[0061]

[Table 9]

[0062]

[Table 10]

Sample No. 1, 3 to 6, 8 to 14, 16 to 2
1, 23 to 25, 27 to 29, 31 to 39 are examples of the present invention, and show good values for TS × El, YPEl, and Rmin, and are high-strength steel sheets excellent in aging resistance and press workability. It is clear that there is. When the samples of the same steel type and the same annealing conditions are compared, the bendability is further improved by applying bending / bending back strain by the methods (Y) and (Z), and particularly the sample by the method (Z). Is the best in strength / ductility balance, aging resistance and bendability.

On the other hand, sample Nos. 7, 15, and 30 strained by the above method (U) were made of YPE.
It can be seen that there is no improvement effect on the aging resistance and bendability by simply giving a constant strain because l and Rmin are large.
Furthermore, in the case of sample Nos. 2, 22, 26, and 40, although the chemical composition, annealing condition, and strain imparting method are each in accordance with the present invention, the combination thereof is unsuitable, so that the characteristics are It has been significantly deteriorated, and it is clear that it is very important to use a combination that satisfies the relationships of the expressions (1) and (2).

[0065]

According to the present invention described above, it becomes possible to manufacture a steel sheet having high strength, high ductility and excellent aging resistance, and it is possible to perform favorable press working without causing defects. It is obvious that the invention has a great industrial utility value and is extremely useful particularly for reducing the weight of automobile bodies and the like.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between the quenching rate during continuous annealing and the amount of strain applied, TS × El, and elongation at yield.

FIG. 2 is a table showing the effect of temper rolling and overaging treatment after continuous annealing on the mechanical properties of a steel sheet.

FIG. 3 is a chart showing changes in mechanical properties of steel sheets due to differences in a method of applying strain in a quenching process during continuous annealing.

FIG. 4 is a chart showing the relationship between the amount of C and the amount of strain and the mechanical properties of a steel sheet.

FIG. 5 is a chart showing the relationship between the amount of Si and the amount of strain and the mechanical properties of the steel sheet.

FIG. 6 is a chart showing the relationship between the amount of Mn and the amount of strain and the mechanical properties of the steel sheet.

FIG. 7 is a chart showing the effects of Si and Mn contents on the critical cooling rate and strain amount at which mechanical properties deteriorate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yuki Osaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Yasuyuki Takada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (4)

[Claims] 1. W: wt%, C: 0.05-0.30%, Si: 0.
5 to 2.5%, Mn: 0.5 to 2.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: 0.01 to 0.06%,
N: 0.005% or less, hot rolled to steel slab balance being Fe and unavoidable impurities, after subjected to cold rolling, 20 seconds to 3 minutes and heated to a temperature range of A _C1 to A _C3 After holding, 5 ~ 25 ℃
At a cooling rate of / sec to a temperature range of 600 to 750 ° C., and then 3 at a cooling rate V _R in a range that satisfies the following formula (1).
When rapidly cooling to a temperature range of 50 to 480 ° C., a strain ε _R in the range satisfying the following formula (2) is applied to the steel sheet, and after unloading, a strain ε _R in the range satisfying the following formula (2) is again performed. Is repeated in this rapid cooling process, and then the temperature is kept in the temperature range of 350 to 480 ° C. for 30 seconds to 10 minutes and then cooled to room temperature, which is excellent in aging resistance. Manufacturing method of rolled steel sheet. Equation _{(1) V Crit ≦ V R} (℃ / sec) ≦ 200 Equation _{(2) 0.05 ≦ ε R (} %) ≦ ε Crit _{However, V Crit = 12exp {1.8 -0.5} Mn (%) - 0.1 Si (%) _{} ε Crit = 0.003 (V R} -V Crit) +0.2 2. A cooling rate V _R of 350 to 480 ° C. in a range satisfying the above formula (1) from a temperature range of 600 to 750 ° C.
In quenching to the temperature range of 1, the steel sheet surface is bent by pressing the roll against the steel sheet so that the strain ε _{R on the} surface of the steel sheet satisfies the above equation (2), and then the roll is pressed against the steel sheet from the opposite direction. The high-strength, high-ductility cold-rolled sheet with excellent aging resistance according to claim 1, characterized in that a series of operations for giving a bending deformation such that the strain ε _{R in} (4) satisfies the above-mentioned formula (2) is repeated in this quenching process. Steel plate manufacturing method. 3. An average cooling rate V _R in a range satisfying the above formula (1) from a temperature range of 600 to 750 ° C. is 350 to 48.
When rapidly cooling to a temperature range of 0 ° C., a water-cooled roll is pressed against the steel sheet to cause bending deformation so that the strain ε _{R on the} steel sheet surface satisfies the above equation (2), and subsequently the steel sheet is water-cooled from the opposite direction. The bending and quenching of the steel sheet are performed by repeating a series of operations for giving a bending deformation such that the strain ε _{R on the} surface of the steel sheet satisfies the above equation (2) by pressing the rolled roll. A method for producing a high-strength, high-ductility cold-rolled steel sheet having excellent aging resistance. 4. Ti: 0.005 to 0.05%, Nb: 0.005 to 0.05
%, B: 0.0005 to 0.003%, and one or more elements selected from the total of 0.05% or less is contained, and the aging resistance according to claim 1 or 2 or 3. A method for producing an excellent high strength and high ductility cold rolled steel sheet.