JPH11158554A - Production of cold rolled steel sheet having aging resistance extremely small in dispersion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high baking and hardening quantity in a steel sheet while the dispersion of its aging resistance is suppressed by composing cooling in continuous annealing for a low carbon steel sheet having a specified chemical compsn. of rapid cooling and slow cooling respectively under specified conditions. SOLUTION: The compsn. of a steel is composed of, by mass, 0.01 to 0.06% C, <=0.1% Si, 0.05 to 0.40% Mn, <=0.10% P, 0.002 to 0.025% S, 0.01 to 0.10% sol.Al, 0.001 to 0.06% N, and the balance Fe. At the time of subjecting a cold rolled steel sheet having this compsn. to continuous annealing, after recrystallization and the growth of the grains, it is rapidly cooled from the temp. range of 720 to 600 deg.C to the temp. range of 450 to 300 deg.C at a cooling rate of 50 to 250 deg.C/sec. Next, it is cooled to the temp. range of 350 to 200 deg.C at a cooling rate of 5 to 40 deg.C/sec. After holding to this temp. range for 0 to 15 sec, it is subjected to reheating with the increase of the temp. at least by >=40 deg.C in the temp. range of 400 to 320 deg.C. While cooling is executed from this temp. range to the temp. range of 385 to 220 deg.C, it is subjected to overaging treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an excellent workability,
The present invention relates to a method for producing a cold-rolled steel sheet having extremely small variation in aging resistance by continuous annealing, and its main uses are automobiles, home appliances, building materials, and the like. Furthermore, when the high-strength steel sheet according to the present invention is used in an automobile, the thickness can be reduced, which contributes to the improvement of the fuel efficiency of the automobile, and also contributes to the improvement of the global environment, which has been a major issue in recent years. .

[0002]

2. Description of the Related Art A method of producing a cold-rolled steel sheet having good aging resistance by continuous annealing using a low-carbon steel sheet is disclosed in, for example, Japanese Patent Publication No. 555553/1993. That is, according to the present invention, in continuous annealing of a cold-rolled low carbon steel sheet, a hot-rolled sheet is cold-rolled by a usual method, and the cold-rolled steel sheet is recrystallized and grain-grown, and then quenched and cooled. The present invention relates to a continuous annealing method for performing cooling / reheating / inclined overaging treatment, wherein a low-carbon steel sheet whose carbon content changes in a range of 0.01 to 0.06% is 450 to 450-600 ° C. from a high temperature range of 720-600 ° C. 3
It cools all the way down to a low temperature range of 00 ° C. in one step at a cooling rate of 50 to 250 ° C./sec. This rapid cooling secures a large amount of solute carbon to promote the generation of carbide nuclei.

[0003]

However, in the invention described in Japanese Patent Publication No. 5-55573 of the prior art, when the carbon content changes in the range of 0.01 to 0.06%, the number of nuclei of carbides is reduced. Becomes unstable, and as a result, even if the overaging treatment is performed, the final amount of solute carbon is likely to vary, resulting in a problem that the aging resistance characteristics of the cold rolled steel sheet vary. As described above, when the cold-rolled steel sheet is annealed by the conventional continuous annealing method, generally, due to the fluctuation of the amount of components such as C and Mn of the steel sheet and the fluctuation of the process conditions such as hot rolling and cold rolling, the steel sheet after continuous annealing is generally used. A great variation occurs in the aging resistance characteristics of the cold-rolled steel sheet. This variation not only reduces product value, but also causes difficulty in shipping management.

Therefore, a first object of the present invention is to provide a method of manufacturing a cold-rolled steel sheet which can reduce the variation in aging resistance. Aging index (Ag
The relationship shown in FIG. 5 exists between the ing Index (hereinafter also simply referred to as AI) and the bake hardening amount (Bake-hardenability, hereinafter simply referred to as BH). There is an upper limit to the aging index in the management of steel sheet shipment, and steel sheets with an aging index higher than this cannot be shipped. Usually, 30 MPa is used as the upper limit of the aging index.
There is an upper limit to the aging index in managing the shipment of steel sheets,
Nevertheless, there is a demand on the other hand to increase the bake hardening amount. Therefore, a second aim of the present invention is to provide a method for producing a cold-rolled steel sheet having a substantially high bake hardening amount by suppressing variation in aging resistance.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have developed an anti-aging property even if the components of the steel sheet and the process conditions such as hot rolling and cold rolling vary within a certain range. The continuous annealing thermal cycle that can sufficiently reduce the variation of the temperature was studied. As a result, as shown in FIG. 2, in contrast to the conventional method of linearly cooling from the quenching start temperature (T1) to the quenching end point temperature (T3) at once, the quenching is performed from T1 to the quenching intermediate temperature T2, Thereafter, a new effect has been found that if the cooling is performed slowly between the temperatures T2 and T3, the aging resistance characteristics of the cold-rolled steel sheet have extremely small variations. If the steel sheet is continuously annealed using the new continuous annealing heat cycle which can be called this two-stage cooling method, even if there is a variation within a certain range in the component system of the steel sheet and the process conditions such as hot rolling and cold rolling. In addition, variations in aging resistance can be sufficiently reduced.

[0006] The technical role of the cooling process from T1 to T3 is (1) securing a large amount of solute carbon by subcooling;
(2) Two points of induction of carbide nucleation prior to overaging treatment. First, the securing of a large amount of solute carbon in (1) is indispensable for generating a driving force for easily generating nuclei of carbides. In (2), in order to rapidly reduce solid solution carbon in the subsequent overaging treatment, the number of precipitation sites of solid solution carbon is sufficiently ensured, and it is uniformly generated in the plate. Is necessary for

[0007] The thermal cycling conditions from T1 to T3 to achieve this technical object were studied in detail. As a result, in order to sufficiently secure the amount of dissolved carbon in (1), a high cooling rate is required particularly in a temperature range from 720 ° C. to 600 ° C. where the diffusion rate of carbon is high to 450 ° C. to 300 ° C. at an intermediate temperature. I found that. That is, the cooling rate required in this case is a high cooling rate of 50 ° C./sec or more. On the other hand, the upper limit cooling rate is 250 ° C./sec, and a high cooling rate exceeding this is unnecessary in terms of increasing the amount of dissolved carbon. In addition, in order to uniformly generate sufficient carbide nuclei of (2) in the plate, the quenching intermediate temperature (T2) of 450 to 300 ° C. to the quenching end point temperature (T2) of 350 to 200 ° C.
It has been found that it is effective to cool 3) at a slightly slower rate of 5 to 40 ° C./s.

When the method of the present invention is applied to a continuous annealing method in which quenching, supercooling, reheating, and gradient overaging are performed after a recrystallization / grain growth treatment, the method has an aging resistance characteristic with extremely little variation compared to the conventional method. Cold rolled steel sheet can be manufactured. Furthermore, by reducing the variation in aging resistance,
The steel sheet can have substantially higher bake hardening.

A method for producing a cold-rolled steel sheet having aging resistance with very little variation according to the present invention is as follows. (1) In mass%, C: 0.01 to 0.06%, Si:
0.1% or less, Mn: 0.05 to 0.40%, P: 0.
10% or less, S: 0.002 to 0.025%, sol.
Al: 0.01 to 0.10%, N: 0.001 to 0.0
Continuously hot-rolling a steel consisting of 06%, balance iron and unavoidable impurities, and subjecting cold-rolled cold-rolled steel sheet to recrystallization / grain growth treatment, rapid cooling / supercooling / reheating / inclined overaging treatment When performing annealing, after recrystallization and grain growth, the temperature range from 720 to 600 ° C. to 450 to 300 ° C. is 50 to 25.
Rapid cooling at a cooling rate of 0 ° C./sec.
After cooling to a temperature range of 0 ° C. at a cooling rate of 5 to 40 ° C./sec and maintaining the temperature range for 0 to 15 seconds,
Reheating with a temperature rise of at least 40 ° C. in a temperature range of 20 ° C., and performing overaging treatment while cooling from the temperature range to a temperature range of 385 to 220 ° C .; A method for producing a cold-rolled steel sheet having aging characteristics. (2) The steel has a ratio of B / N of B / N = 0.5.
Characterized by satisfying the relationship of ~ 2.0,
The method for producing a cold-rolled steel sheet having aging resistance with very little variation according to the above (1).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a cold-rolled steel sheet according to the present invention is a method for producing C, Si, Mn, P, S, Al, N
The present invention relates to a continuous annealing method in which the amount is limited, a hot-rolled sheet is formed by an ordinary method, and a cold-rolled cold-rolled steel sheet is subjected to recrystallization / grain growth treatment, followed by rapid cooling / supercooling / reheating / inclined overaging treatment. In particular, a heat cycle in which cooling is performed in two stages from the quenching start temperature (T1) to the quenching end point temperature (T3) is employed. With these, variations in aging resistance are suppressed, and a cold rolled steel sheet having a high bake hardening amount is manufactured. The reasons for limiting the present invention are described below.

First, the chemical components will be described. As for C, as its content decreases, ductility and deep drawability improve, and the aging resistance, which is too low in content, deteriorates.
In the range of 0.06%, a cold-rolled steel sheet excellent in workability and aging resistance can be obtained.

Since Mn and S prevent brittleness due to S during hot rolling and utilize MnS as a preferential precipitation site for carbides, their amounts need to have a limited range. From these viewpoints, Mn is set to 0.05 to 0.
40% and S were regulated to 0.002 to 0.025%. By the combination of this and the two-stage rapid cooling, a cold rolled steel sheet having more excellent aging resistance properties is manufactured.

[0013] Si not only dissolves in ferrite and hardens the matrix, but when added in a large amount, deteriorates surface quality due to surface scratches caused by scale,
To reduce the chemical conversion property, the addition amount is limited to 0.1% or less. However, if the amount of Si added is 0.00
Since it is difficult to control the content to less than 3% under actual manufacturing conditions, the lower limit of the amount of Si added is preferably set to 0.003%. P is an element that does not significantly affect the aging resistance. However, in the case of manufacturing a steel sheet for automobiles, the upper limit must be 0.10%. If P exceeds this, workability of the steel sheet is significantly deteriorated.

Sol. Al is an element necessary for controlling the amounts of oxygen and nitrogen in the steel, but if it is too much, it hardens, so the upper limit was made 0.10%. On the other hand, if the content is too small, the aging effect due to nitrogen cannot be suppressed, so the lower limit was made 0.01%. N in sol.
AlN in combination with Al (BN when B is added)
And the material is hardened, so the upper limit is 0.006
%. The lower limit is set to 0.001% because it is difficult to make the N amount lower than the current steelmaking technology.

In the present invention, B is contained as required. If B is contained in B / N of 0.5 or more, B is combined with N in the steel to form BN, and nitrogen aging can be prevented. However, if B / N exceeds 2.0, the amount of solid solution B increases and the material becomes hard. Therefore, the lower limit of B / N was set to 0.5 and the upper limit was set to 2.0. Next, the process from casting to hot rolling is carried out. Regardless of the method in which the slab is cooled and then reheated, the continuous casting-direct feed hot rolling (CC-DR) method is used. Good. Therefore, the temperature for heating the slab does not need to be particularly limited. Also,
The winding temperature after hot rolling does not significantly affect the aging resistance of the steel sheet, and the effect of the present invention can be sufficiently obtained even at a low temperature of about 600 ° C. When a high-temperature winding at 700 ° C. or more is performed, crystal grains after annealing become large and workability is improved. Next, the continuous annealing step will be described.

The step of recrystallization and grain growth of the cold-rolled steel sheet does not need to be particularly limited, and may be performed by heating to a temperature higher than the recrystallization temperature in a usual method. Rapid cooling after soaking,
Cooling from a temperature range of 720 to 600 ° C. to a temperature range of 450 to 300 ° C. at a cooling rate of 50 to 250 ° C./sec,
Next, up to a temperature range of 350 to 200 ° C., 5 to 40 ° C./s
It is necessary to cool at a cooling rate of ec. This two-stage cooling method is the most important technical point in obtaining aging resistance with less variation.

After soaking, the steel sheet is cooled at a cooling rate of 20 ° C./sec or less in order to avoid introduction of distortion due to transformation into the steel sheet. For the same reason, the upper limit of the quenching start temperature is set to 720 ° C. If the quenching start temperature is lower than 600 ° C., the amount of solute carbon at the quenching intermediate temperature (T2) becomes insufficient, so
0 ° C. is the lower limit of the quenching start temperature. Rapid cooling intermediate temperature (T
The upper limit temperature in 2) is 450 ° C. If the temperature is higher than this, the amount of dissolved carbon is insufficient, and nucleation of carbides does not sufficiently occur. Also, the quenching intermediate temperature is 300 ° C.
If it is less than 300%, the variation in the nucleation of carbides becomes large, and as a result, the variation in the aging resistance becomes large. Therefore, 300 ° C. is set as the lower limit of the quenching intermediate temperature.

The lower limit of the cooling rate from the quenching start temperature (T1) to the quenching intermediate temperature (T2) is 50 ° C./sec. If it is later than this, a sufficient amount of solute carbon is not secured, and high aging resistance cannot be obtained. In addition, the cooling rate is 2
Even if it exceeds 50 ° C./sec, the controllability of the temperature is deteriorated and the aging resistance becomes unstable, so that 250 ° C./sec is set as the upper limit of the cooling rate.

In order to secure a sufficient precipitation density of carbides, the quenching end point temperature is set to 350 ° C. or less. On the other hand, if the quenching end point temperature is lower than 200 ° C., the density of carbide nuclei becomes too high and the steel sheet becomes hard, so 200 ° C. is set as the lower limit. From the quenching intermediate temperature (T2) to the quenching end point temperature (T
Until 3), it is necessary to cool at a cooling rate of 5 to 40 ° C./sec. If the cooling rate exceeds 40 ° C./sec, the variation in the aging resistance of the steel sheet becomes large.
The upper limit is 0 ° C./sec. On the other hand, if the cooling rate is less than 5 ° C./sec, the continuous annealing furnace becomes long,
sec was the lower limit.

After reaching the quenching end point temperature (T3), the temperature is maintained in this temperature range for 0 to 15 seconds. Although the longer the holding time, the more stable the aging resistance, the upper limit thereof is 15 seconds because the annealing furnace is unnecessarily long. Note that the object of the present invention is achieved even if the holding time is 0 second. After this, 40
It is reheated to a temperature range of 0 to 320 ° C (overaging start temperature T4), and is overaged up to a temperature range of 385 to 220 ° C (overaging end temperature T5) while being gradually cooled. When the overaging treatment is performed within these temperature ranges, the efficient diffusion speed of carbon can be utilized, and the amount of dissolved carbon can be rapidly reduced. Further, by setting the overaging end temperature to 385 to 220 ° C., it is possible to suppress the attained solid solution carbon amount to be low. Reheating from the quenching end point temperature (T3) to the overaging start temperature (T4)
If the heating is performed at a heating rate of 5 ° C./sec or more, the final characteristics are not significantly deteriorated. Preferably, in order not to excessively lengthen the line length of the continuous annealing furnace, 15 ° C./sec.
c or more, and preferably 60 ° C./sec or less in order to minimize an increase in the cost of reheating.

[0021]

The present invention will be further described with reference to the following examples. (Example 1) Table 1 shows the component compositions of hot-rolled steel sheets A to V collected from a hot-rolled steel sheet manufacturing site. The hot-rolled steel sheets B to J are cold-rolled in a laboratory to obtain 0.1 mm.
An 8 mm cold-rolled plate was subjected to the following heat treatment.

One is a typical one of the conventional heat treatments.
After soaking at 00 ° C. for 60 seconds, the temperature was rapidly cooled from 675 ° C. to 250 ° C. at a cooling rate of 100 ° C./sec.
It was reheated to 0 ° C. and gradually cooled while changing the overaging time from 350 ° C. to 270 ° C. to perform an overaging treatment. On the other hand, according to a typical method of the present invention,
After 0 second soaking annealing, 80 ° C from 675 ° C to 350 ° C
/ Sec, and then cool to 250 ° C for 2
Cooling was performed at a cooling rate of 0 ° C./sec. After that, 350
C., and overaged from 350 ° C. to 270 ° C. while changing the overage time.

The variation in the aging index (AI) of these steel sheets is shown in FIGS. 3 and 4 as a function of the overaging time. FIG.
Has a carbon content of 0.010 to 0.025%,
FIG. 4 shows the results when the carbon content is 0.025 to 0.055%. FIG. 1 shows a change in the range of variation of the aging index (AI). The aging index is an index substantially proportional to the amount of solute carbon remaining in the annealed steel sheet. This aging index (AI) is 10% after 10% tensile strain.
A heat treatment of 0 ° C. for 1 hour is performed, a tensile test is performed again, and the yield is expressed by an increase in yield stress.

As is clear from the results shown in FIGS. 3, 4 and 1, according to the two-stage cooling method of the present invention, the variation of the aging index is suppressed to a very low level regardless of the range of the carbon content of the steel sheet. It is possible. If the variation of the aging index can be suppressed to a low level, as can be seen from FIG. 5, it is possible to produce a steel sheet having a high bake hardening amount. (Example 2) Hot rolled steel sheets A to V shown in Table 1 were cold-rolled in a laboratory into steel sheets having a sheet pressure of 0.80 mm. After soaking them at 800 ° C for 60s, 5-10 ° C / sec
To the quenching start temperature T1, and to the quenching intermediate temperature T2 and thereafter to the quenching end point temperature T3,
Then, the mixture was cooled at the cooling rate shown in Table 2, kept at a predetermined isothermal temperature at the T3 temperature, and reheated at a heating rate of 10 to 30 ° C / sec until the overaging start temperature. Thereafter, the aging treatment was performed for a predetermined time while cooling from the start temperature to the end temperature of the overaging treatment. After the aging treatment was completed, the aging treatment was cooled to room temperature and the material of the steel sheet was investigated. The properties of the steel sheet thus obtained are shown in Tables 2 and 3 together with the annealing conditions. Conducted 10 experiments under each condition,
Each material showed an average value of 12 times. The variation in AI was shown in the range of the measured AI.

No. No. 1 indicates that not only AI is 30 MPa or more but also A
The variation of I is also a large value of 10 MPa or more. In addition, No. All 42 to 51 are steels whose chemical components are outside the scope of the present invention. First, no. Sample No. 42 has not only a hardened YP of 250 MPa or more but also a large variation in AI and AI due to an excessive amount of C. No. In No. 43, since the amount of Si added was outside the range of the present invention, scale-induced flaws occurred in the hot-rolled steel sheet, and the surface quality of the cold-rolled steel sheet was poor. Therefore, no other evaluation was performed. No. 44 and 47 are Mn and S
Is as small as 10 or less, cracking occurs in the slab stage, and the surface quality of the cold-rolled steel sheet is poor. Therefore, no other evaluation was performed. No. 45, 46, 49, 50, 51
In each case, since the added amounts of Mn, P, Al, N, and B were larger than the range of the present invention, the steel sheet was hardened and YP was 250M.
Exceeding Pa, the workability is degraded. No. 48 is
Since the amount of Al added is less than the range of the present invention, aging caused by N appears, and both AI and variation in AI increase.

No. Nos. 11 and 12 have quenching start temperatures. Nos. 3, 4, 18, 31, and 39 have quenching intermediate temperatures. Nos. 24 and 25 have AIs as large as 30 MPa or more because the quenching end point temperature is outside the range of the present invention;
Further, the variation in AI is as large as 10 MPa or more. In addition, No. No. 24 is also outside the scope of the present invention in that reheating is not performed from the quenching end point temperature to overaging.

In No. 8, the cooling rate from T1 to T2 is N
o. In No. 10, since the cooling rate from T2 to T3 is too slow outside the range of the present invention, the AI is as large as 30 MPa or more, and the variation in AI is as large as 10 MPa or more. No. 9, 13, 14, 26, 40, and 41 indicate that although the cooling rate is too high outside the scope of the present invention or that the cooling is performed in one stage, the AI itself is low, but the variation in the AI is Became 10 MPa or more, and stable AI control was not performed.

No. Sample No. 22 has an AI variation of 1 because the holding time at T3 is 18 seconds, which is longer than the range of the present invention.
It is as large as 0 MPa or more. No. 35, 36,
In Nos. 37 and 38, since the overaging temperature is out of the range of the present invention, AI and variation in AI are large. All of the examples other than the comparative examples described above are examples of the present invention. When the cooling condition and the overaging condition after the soaking heat treatment of annealing are the two-stage cooling-reheating-overaging treatment of the present invention, the AI is 3
This indicates that a cold-rolled steel sheet with extremely small aging resistance variation within 0 MPa and variation in AI within 10 MPa can be manufactured.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

As described in detail above, according to the present invention,
Substantially reduces the variation in AI of cold-rolled steel sheets for processing,
As conceptually shown in FIG. 5, higher B
A non-ageing steel sheet having H property can be stably provided. This facilitates not only shipping management of the steel sheet, but also enables an automobile manufacturer or the like who uses the steel sheet to use a higher BH steel sheet under conditions where stretcher strain does not occur. Therefore, it is possible to contribute to energy saving, reduction of environmental load, and the like by improving the quality of the product, improving the safety, and further reducing the weight of the vehicle body.

[Brief description of the drawings]

FIG. 1 is a graph showing the range of the aging index for the present invention and the conventional method.

FIG. 2 is a graph conceptually showing a comparison between annealing cycles of the present invention and a conventional method.

FIG. 3 is a graph showing C: 0.
It is a graph which shows the dispersion of the aging index of a steel plate of 010-0.025%.

FIG. 4 is a graph showing C: 0.
It is a graph which shows the dispersion of the aging index of the steel plate of 025-0.055%.

FIG. 5 is a graph conceptually illustrating the effect of the present invention on the reduction of variation based on the relationship between the bake hardening amount (BH) and the aging index (AI).

Claims (2)

[Claims] C: 0.01 to 0.06% by mass%,
Si: 0.1% or less, Mn: 0.05 to 0.40%,
P: 0.10% or less, S: 0.002 to 0.025%,
sol. Al: 0.01 to 0.10%, N: 0.001
Hot-rolled steel consisting of 0.006%, balance iron and unavoidable impurities, and re-crystallization of cold-rolled cold-rolled steel sheet.
When performing continuous annealing for rapid cooling, supercooling, reheating, and gradient overaging after the grain growth treatment, 720 to 6 after recrystallization and grain growth.
5 from the temperature range of 00 ° C to the temperature range of 450 to 300 ° C
Quenching at a cooling rate of 0 to 250 ° C./sec.
After cooling to a temperature range of 0 to 200 ° C. at a cooling rate of 5 to 40 ° C./sec and maintaining the temperature range for 0 to 15 seconds,
Reheating with a temperature rise of at least 40 ° C. in a temperature range of 00 to 320 ° C., and performing overaging treatment while cooling from the temperature range to a temperature range of 385 to 220 ° C .; A method for producing a cold-rolled steel sheet having low aging resistance. 2. The steel according to claim 1, wherein the ratio of B and N is B / N.
2. The method for producing a cold-rolled steel sheet having aging resistance with very little variation according to claim 1, wherein the cold-rolled steel sheet has a content of 0.5 to 2.0.