JPH06104862B2 - Manufacturing method of cold-rolled steel sheet for work excellent in bake hardenability and non-aging at room temperature - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention provides a cold rolled product that is required to have a high plastic deformation resistance for a finished product that has excellent deep drawability such as an automobile outer plate. Regarding the manufacturing method of steel materials,
In particular, the present invention relates to a method for producing a cold rolled steel sheet for working, which has both high bake hardenability (BH property) and aging resistance during storage at room temperature.

(Prior Art) In a press-formed product used for an outer plate, external damage due to physical force is most suitable, and the manufacturer of such a product desires to use a steel plate that is less likely to cause external damage, if possible.

The resistance to dent damage due to collision is called dent resistance, and such characteristics can be generally obtained by increasing the yield stress of the steel sheet.

On the other hand, at the time of working, a lower yield stress is desired from the viewpoint of energy required for working and accuracy of shape.

As a steel plate that solves these conflicting requirements, 10
There is a BH (Bake Hardening) steel sheet in which the yield stress increases when a coating process including holding at a high temperature of about 0 to 200 ° C is applied after processing. In this steel sheet, solid solution C or solid solution N, especially solid solution C, is present in the steel, so that the solid solution C or the like diffuses into the mobile dislocations generated by the working at a high temperature during coating baking, and It cures by taking advantage of the reduced mobility.

The problem with the above-mentioned hardening mechanism is that some dislocations are already fixed by the solid solution component before processing, and during processing, wavy surface defects called stretcher strains due to elongation at the yield point occur. At present, by using ultra-light reduction cold rolling for shape correction called temper rolling after annealing, movable dislocations are separated from solid solution components and strain concentrated parts are generated on the surface to trap solid solution C and the like. Therefore, such inconvenience is prevented by preventing the diffusion into mobile dislocations.

Although this method is effective in the short term, it does not completely suppress the progress of the age hardening phenomenon itself, and it is remarkable especially in the case of high BH steel plate with a BH amount of 3 kgf / mm ² or more at the falling yield point. ,
Even before processing, solid solution C or the like easily diffuses again to the mobile dislocations by long-term storage at room temperature or high-temperature treatment at a maximum of about 550 ° C. in a surface treatment line. Therefore, at present, the usage conditions of these steel sheets are limited.

On the other hand, the applicants have already proposed a completely new type of high BH-non-aging type steel sheet in Japanese Patent Publication No. 61-12008.

That is, when an ultra-low carbon steel sheet to which Nb and B are jointly added is heated to the α phase-γ phase coexistence temperature region of Ac ₁ transformation point or higher and rapidly cooled, a two-phase structure of acicular-ferrite + ferrite is formed. This structure has a high BH property containing solid solution C, but since most of the solid solution C is trapped in acicular-ferrite having a large strain, that is, a dense transition, there is almost no yield point elongation even after annealing. do not do.

However, according to the knowledge of the present inventors, there is a drawback that such a steel sheet causes a yield point elongation when it is aged at room temperature for a long period of time.

The steel sheets disclosed in Japanese Patent Publication No. 61-12008, etc., were excellent in workability such as r value and elongation value, but their workability deteriorated sharply as the annealing temperature increased. Therefore, the optimum annealing temperature is practically limited to about 10 ° C. just above the Ac ₁ transformation point, and it is difficult to obtain a stable material in factory production.

(Problems to be solved by the invention) The object of the present invention is to produce a cold-rolled steel sheet for automobile outer plates, etc., while having excellent workability and high bake hardenability (BH property), Proposal of a stable manufacturing method for cold-rolled steel sheet for working that has long-term storage at room temperature before working and aging resistance to temperature rise below recrystallization temperature without processing (this room temperature aging resistance) It is to be.

(Means for Solving the Problems) The present invention is C: 0.01 wt% (hereinafter simply referred to as%), S
i: 0.1% or less, Mn: 0.2 to 2.0%, Nb: 0.005 to 0.1%, B: 0.0
005 to 0.01%, Al: 0.5% or less and N: 0.02% or less, and B- (11 / 14N-11 / 27 Al) ≧ 0.0005 (%) (1) is satisfied, and the balance is Fe and Hot rolling is performed using steel with a composition of unavoidable impurities as the raw material, the hot rolling finishing temperature is set to the range of Ar ₃ transformation point to 900 ° C, and the hot rolled sheet obtained by this hot rolling is cold rolled. Following this cold rolling, heating to a temperature range of Ac ₁ transformation point to Ac ₁ transformation point + 90 ° C at least
In the temperature range of Ar ₁ transformation point + 100 ° C to Ar ₁ transformation point -100 ° C, the rolling reduction is performed after heat treatment for cooling at a cooling rate of 7 ° C / s or more.
Method for producing cold-rolled steel sheet for processing excellent in bake hardenability and non-aging at room temperature (first invention) characterized by subjecting to 0.2-5% temper rolling, and C: 0.01% or less, Si: 0.1 % Or less, Mn: 0.2 to 2.0%, Nb: 0.00
5 to 0.1%, B: 0.0005 to 0.01%, P: 0.03 to 0.15%, Al: 0.5
% Or less and N: 0.02% or less, and satisfying B- (11 / 14N-11 / 27 Al) ≥ 0.0005 (%) (1) with the balance being Fe and inevitable impurities. Is used as a raw material, the hot rolling finishing temperature is set to the range of Ar ₃ transformation point to 900 ° C., the hot rolled sheet obtained by this hot rolling is subjected to cold rolling, and this cold rolling is followed by Ac _{At least 1} transformation point-Ac ₁ transformation point + 90 ℃
In the temperature range of Ar ₁ transformation point + 100 ° C to Ar ₁ transformation point -100 ° C, the rolling reduction is performed after heat treatment for cooling at a cooling rate of 7 ° C / s or more.
Method for producing cold rolled steel sheet for working excellent in bake hardenability and non-aging at room temperature (second invention), characterized by performing temper rolling of 0.2 to 5%, and C: 0.0005 to 0.01%, Si: 0.1% or less, Mn: 0.2 to 2.0%, Ti:
0.005-0.05%, Nb: 0.005-0.1%, B: 0.0005-0.01%,
Al: 0.5% or less S: 0.05% or less and N: 0.02% or less,
And B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≧ 0.0005 (%)…
(1) 'C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%)
(2) is satisfied, and the balance is made of steel having a composition consisting of Fe and inevitable impurities, hot-rolled, and the hot-rolling finishing temperature is set in the range of Ar ₃ transformation point to 900 ° C. Cold rolling is performed on the hot-rolled sheet thus obtained, and subsequently to this cold rolling, at least the temperature range of Ac ₁ transformation point to Ac ₁ transformation point + 90 ° C. is heated to at least
In the temperature range of Ar ₁ transformation point + 100 ° C to Ar ₁ transformation point -100 ° C, the rolling reduction is performed after heat treatment for cooling at a cooling rate of 7 ° C / s or more.
Method for producing cold-rolled steel sheet for working excellent in bake hardenability and non-aging at room temperature (third invention) characterized by subjecting to 0.2-5% temper rolling, and C: 0.0005-0.01%, Si: 0.1% or less, Mn: 0.2 to 2.0%, Ti:
0.005-0.05%, Nb: 0.005-0.1%, B: 0.0005-0.01%,
P: 0.03-0.15%, Al: 0.5% or less, S: 0.05% or less and N: 0.
02% or less and B- (11 / 14N-11 / 27Al-11 / 48Ti) ≧ 0.0005 (%) ...
(1) 'C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%)
(2) is satisfied, and the balance is made of steel having a composition consisting of Fe and inevitable impurities, hot-rolled, and the hot-rolling finishing temperature is set in the range of Ar ₃ transformation point to 900 ° C. Cold rolling is performed on the hot-rolled sheet thus obtained, and subsequently to this cold rolling, at least the temperature range of Ac ₁ transformation point to Ac ₁ transformation point + 90 ° C. is heated to at least
In the temperature range of Ar ₁ transformation point + 100 ° C to Ar ₁ transformation point -100 ° C, the rolling reduction is performed after heat treatment for cooling at a cooling rate of 7 ° C / s or more.
A method for producing a cold-rolled steel sheet for working excellent in bake hardenability and non-aging at room temperature (fourth invention), characterized by performing temper rolling of 0.2 to 5%.

The above heat treatment is performed as a continuous annealing step in industrial production.

(Function) An important point in the present invention is that by combining the improvement of the crystal structure and the temper rolling, it is possible to realize both high BH property and non-aging at room temperature which are superior to the conventional ones.

This effect will be shown by the following experimental results.

Steel A (% by weight (same below) C: 0.0029%, Si: 0.01%, M
n: 0.55%, Nb: 0.010%, B: 0.0015%, P: 0.024%, Al: 0.
056%, S: 0.008%, N: 0.0031%) and Steel B (C: 0.0040)
%, Si: 0.01%, Mn: 0.22%, Nb: 0.020%, P: 0.045%, A
(L: 0.050%, S: 0.012%, N: 0.0027%), each of which is composed of two components: continuous casting-hot rolling-cold rolling-continuous annealing. And

Steel A is a steel in the composition range of the present invention, the slab heating temperature is 1200 ° C, the Ar ₃ transformation point is 830 ° C, and the hot rolling end temperature is 860.
℃, hot-rolled sheet thickness 3.5mm, hot-rolled sheet winding temperature 550 ℃, Ac ₁ transformation point 910 ℃, annealing temperature 940 ℃, Ar ₁ transformation point 820 ℃ 920 ℃ to 720 ℃ Average cooling rate up to ℃ 2
It was 5 ° C / s.

On the other hand, Steel B is a conventional BH steel sheet with the manufacturing conditions and components selected to have high BH properties, and the slab heating temperature is 12
00 ℃, Ar ₃ transformation point 840 ℃, hot rolling end temperature 900 ℃, hot rolled sheet thickness 3.5mm, hot rolled sheet winding temperature 700 ℃, Ac ₁ transformation point 900 ℃ annealing temperature 860 ℃, annealing temperature (830 ℃)
The average cooling rate from 1 to 600 ℃ was 25 ℃ / s.

Next, temper rolling (skin pass) for both A and B steel sheets is 0
(None) -1.5% applied.

When the BH properties of these steel sheets were examined, the BH content of A steel was 5.5 kgf / mm ² , and the BH content of B steel was
It was 3.5 kgf / mm ² . As for BH content, as shown in Fig. 4, nominal stress at 2% prestrain and 170 ° C after prestrain
The difference of the yield stress (nominal) after the aging treatment for 20 minutes was taken.

Figure 1 shows the effect of temper rolling on the aging properties of these steel sheets.

As can be seen from the figure, 0.2% or more temper-rolled material of A steel, which has a two-phase structure of acicular-ferrite + ferrite, shows almost all yield point elongation even after aging treatment at 30 ° C for 6 months. In the case of 0.5% or more temper-rolled material, yield point elongation does not occur at all even after aging treatment. However, without temper rolling, which is a conventional manufacturing method, aging treatment results in elongation at yield. On the other hand, the conventional high BH steel plate B having a single ferrite phase exhibits normal temperature aging regardless of presence or absence of temper rolling, and yield point elongation occurs.

Two types of steel sheets were prepared for each of the A and B steels, one without temper rolling and the other with 1% temper rolling (skin pass), and the aging by 100 ° C. aging treatment, that is, the occurrence of yield point elongation was investigated. These results are shown in FIG.

As is clear from the figure, the temper-rolled material of steel A does not cause elongation at yield even if it is aged at 30 ° C for 6 months. However, it is understood that without temper rolling, the yield point elongation does not occur immediately after annealing, but the yield point elongation begins to occur with time. On the other hand, B steel has a high yield point elongation without temper rolling and cannot be used as a steel sheet for forming outer panels. In addition, it can be seen that even if temper rolling is applied to temporarily suppress the elongation at yield point, it is aged immediately.

As described above, the steel sheet produced by the method of the present invention has a high BH property and an excellent non-aging property at room temperature. The reason for this is presumed as follows.

As mentioned above, the ultra-low carbon steel sheet in which Nb and B are jointly added is Ac ₁
When heated to the α phase-γ phase coexisting temperature range above the transformation point and rapidly cooled, a two-phase structure of acicular ferrite + ferrite is formed. This structure contains solid C and has a high BH property, but most of the solid C is trapped in acicular ferrite having a large strain, that is, dense dislocations.
After annealing, it has almost no yield point elongation.

It was thought that such a steel sheet did not require the elongation at yield point prevention by temper rolling, but the inventors have found that even with such a structure, the solute C is soft in the movable ferrite phase. Unable to prevent dislocations from diffusing with time and yielding elongation at yield, and only for steel sheets with such a structure, temper rolling unexpectedly gives a remarkable effect of suppressing aging. He found out. Temper rolling of acicular ferrite + ferrite into a two-phase structure
Not only does it introduce strain into the surface, but it also causes additional strain concentration inside the acicular ferrite grains or at the grain boundaries between acicular ferrite and ferrite, so that the solid solution C in the mobile dislocations in the soft ferrite phase is dissolved. It is considered that the diffusion is suppressed to such an extent that it does not become a problem, and the aging does not occur unless a large amount of dislocations are newly introduced into the ferrite phase by working.

Another important point in the present invention is the knowledge of manufacturing conditions in which the working characteristics such as the r value and the elongation value do not deteriorate even when annealed at a temperature far exceeding the Ac ₁ transformation point. It was revealed that the hot rolling finishing temperature had a remarkable improvement effect on this. The effect will be described below along with experimental facts.

C: 0.0025%, Si: 0.01%, Mn: 0.51%, Nb: 0.010%, B: 0.
0015%, P: 0.012%, Al: 0.060%, S: 0.008%, N: 0.0027
% Steel in the laboratory through vacuum casting-hot rolling-cold rolling-continuous annealing (heat cycle simulation)
m cold rolled steel sheet. As for the process conditions, the slab heating temperature was 1250 ° C, and the hot rolling end temperature was changed between 800 and 910 ° C. The hot rolling finishing temperature was 910 ° C., and the Ar ₃ transformation point (flat part of the air cooling curve) was about 850 ° C. in the measurement of the air cooling process of the hot rolled steel sheet. The hot-rolled sheet thickness is 3.5 mm, and the hot-rolled sheet winding temperature is equivalent to 500 ° C. The annealing temperature is Ac ₁ transformation point 910 ° C.
To about 860 ℃ to 1010 ℃.
Sec), and the average cooling rate from 910 ° C to 710 ° C was 20 ° C / s with respect to Ar ₁ transformation point (according to thermal expansion measurement) of 810 ° C.

Immediately after annealing, mechanical properties were measured without temper rolling. The mechanical properties were measured using JIS No. 5 test pieces, and the BH was measured in the same manner as in the case of FIG.

Bake hardenability (BH property) is 4.0 to 5.5 kgf / mm ² , yield strength (TS) is 18 to 20 kgf / mm ² , breaking strength (TS) is 32 to 33 kg.
It was f / mm ² .

Immediately after annealing, r value, elongation and yield point elongation characteristics without temper rolling are shown in FIG.

As can be seen from FIG. 3, the 910 ° C. hot-rolled finished material exhibited excellent workability just above the Ac ₁ transformation point, but the workability deteriorated rapidly as the annealing temperature increased. This was in line with conventional wisdom, but as in the present invention, the hot rolling finishing temperature was set to 840
When the temperature is lowered to a value close to just above the Ar ₃ transformation point, the workability is hardly deteriorated even if the annealing temperature rises, which is good. When the hot rolling finish temperature is further lowered to 800 ° C below the Ar ₃ transformation point, the elongation value does not deteriorate as compared with the hot rolling finish material, but r
The value is extremely low.

Under all hot rolling conditions, when the annealing temperature is below the Ac ₁ transformation point, the yield point elongation is large and the stretcher strain remarkably appears during processing, which is not suitable for processing. Also
In high-temperature annealing at about 950 ° C or higher, the yield point elongation, which has disappeared, reappears slightly, but this can be suppressed by temper rolling described later.

As is clear from FIG. 3, the steel sheet according to the present invention stably maintains a good material in a wider temperature range than the conventional one. The reason for this is not clear, but it is presumed as follows.

The mechanism by which a steel sheet annealed at a high temperature above the Ac ₁ transformation point exhibits high r-value and elongation characteristics is not clear from the beginning, but some α phase remains during high temperature annealing, and the distribution and aggregation of this residual α phase It is considered that the organization exerts such an effect. Here, when the annealing temperature usually exceeds the Ac ₁ transformation point by about 10 ° C. or more, the amount of residual α phase is remarkably reduced, and it is considered that the workability is deteriorated. On the other hand, the hot-rolled sheet hot-rolled and finished just above the Ar ₃ transformation point has a fine crystal grain size. Therefore, in the present invention, even if the amount of residual α phase is reduced, the number of crystal grains remains considerably even at high temperature because the grain size is fine, and the function as α phase growth nuclei during cooling deteriorates significantly. It is thought that the processability is maintained because it does not.

Needless to say, there is a limit temperature for maintaining workability by such a mechanism, and in fact, even in Fig. 3, at annealing temperatures exceeding 1000 ° C, workability deteriorates even in the steel sheet hot-rolled immediately above the Ar ₃ transformation point. Can be seen. That is, it is considered that the residual α phase has almost disappeared, but this phenomenon appears when the component system of the present invention exceeds approximately the Ac ₁ transformation point + 90 ° C.

The reasons for limiting the composition range of each component will be described below.

C: C is an important component for imparting BH property, but 0.0
If it exceeds 1%, it becomes difficult to maintain non-aging at room temperature even with the method of the present invention. Further, the lower the amount of C, the more advantageous the material is, and if it exceeds 0.01%, good workability cannot be obtained. Therefore, the C content is 0.01% or less. In order to obtain a high BH property, it is desirable that the C content be 0.0005% or more, which is a strong carbide forming component, as described later.
In the third and fourth inventions in which Ti is added, the solid solution C content is 0.0005.
% Or more is mandatory.

Si: Si does not hinder some addition for increasing the strength of the steel sheet, but if it is present in excess of 0.1%, the elongation and drawability of the steel sheet deteriorate, so it is made 0.1% or less.

Mn: Mn is a component that can lower the transformation temperature of the steel sheet without deteriorating the deep drawability, and is also effective for increasing the strength of the steel sheet. If the amount of Mn is less than 0.2%, the α-γ phase coexisting temperature range becomes high and uneconomical high temperature annealing is unavoidable, and the cooling rate during annealing is 50 ° C for two-phase structure. / s or more is required. On the other hand, excessive addition of more than 2.0% of Mn amount deteriorates the balance between elongation and drawability of the steel sheet and strength, and Mn causes an endothermic reaction in the molten steel, so that vacuum degassing treatment is performed due to a decrease in molten steel temperature. It will be impossible. Therefore, the amount of Mn added is in the range of 0.2 to 2.0%.

Nb: Nb, when coexisting with B, lowers the γ → α transformation during cooling to the low temperature side and promotes acicular ferrite + ferrite two-phase organization. Also in the texture, coexistence of B causes remarkable accumulation of (111) orientation, which improves workability (especially r value). These effects are most effective between 0.005 and 0.1%. If the amount of Nb is less than 0.005%, the effect of two-phase organization and texture improvement is insufficient, and 0.1
If the addition amount exceeds%, not only the effect of addition is saturated but also the elongation is remarkably deteriorated and the strength-workability balance is lowered. Therefore, the amount of Nb added is 0.005 to 0.1%.

B: B reduces the γ → α transformation during cooling to the low temperature side by coexisting with Nb, as described above, and causes acicular ferrite +
Promotes the two-phase organization of ferrite. Also, in the texture, coexistence of Nb causes remarkable accumulation of (111) orientation, and improves workability (especially r value). Further, the addition of B is also effective for improving the secondary working brittleness resistance required for the working steel sheet. These effects are most effective between 0.0005 and 0.01%. When the amount of B is less than 0.0005%, the effect of forming a two-phase structure and texture is insufficient, and the addition of more than 0.01% not only saturates the effect of addition, but also causes remarkable deterioration of elongation, resulting in strength-working. Reduce sexual balance. Therefore, the B addition amount is 0.0005 to 0.01%.

Al: In the present invention, Al is added mainly for the purpose of deoxidizing and fixing N as AlN to prevent a decrease in B yield due to BN formation. In order to secure 0.0005% or more of solid solution B that is not bonded to N for the above-mentioned reason, the relationship between B and N is B- (11 / 14N-11 / 27 Al) ≧ 0.0005 (%) Furthermore, the third invention In the case of adding Ti of the fourth invention, N is added to Ti as well.
Since it has a fixing function, it is necessary to add Al in an amount satisfying B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≧ 0.0005 (%). On the other hand, A exceeding 0.5%
Since the addition of 1 adversely affects the surface properties, it should be kept to 0.5% or less, preferably 0.1% or less. Therefore, Al is 0.5% or less, and satisfies B- (11 / 14N-11 / 27 Al) ≧ 0.0005 (%) or B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≧ 0.0005 (%). The amount.

N: N deteriorates the deep drawability and, if not fixed with Al, binds with B and significantly reduces the effect of adding B. Therefore, the larger the amount, the more the amount of Al required is uneconomical. . Also, since solid solution N originally has a high aging property at room temperature, it is not used as a bake hardening component in the present invention. Therefore, it is desirable to keep the amount of N as low as possible, but the allowable amount is 0.02% or less from the economical aspect of the process. Preferably, the N content is 0.006% or less.

P: In the second and fourth inventions, P is added for the purpose of strengthening the steel sheet. If 0.15% or less of P is added as a steel plate reinforcing component, the effect of the present invention is not impaired. Meanwhile P
Since the effect of strengthening the steel sheet hardly appears unless 0.03% or more is added, the P amount needs to be 0.03 to 0.15% in the second and fourth inventions in which P is added.

Ti: In the third and fourth inventions, a small amount of Ti is added to improve workability and B yield. It is necessary to add 0.005% or more to obtain the effect of Ti, but 0.05
Addition in excess of 5% not only is uneconomical in terms of the effect of addition, but also leads to an increase in productivity cost due to an increase in the transformation point. In addition, in order to secure BH property, it is necessary to secure a solid solution C content of 0.0005% or more, and the effective Ti (T excluding TiN and TiS formation components)
Regarding i), C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%) must be satisfied. Regarding the N fixing effect, it is understood that it is sufficient to satisfy B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≧ 0.0005 (%) from the above-mentioned condition that the effective B amount is 0.0005% or more. Therefore, the optimum addition amount of Ti is 0.005 to 0.05% and C- (12 / 48Ti-12 / 32S-12 / 14N) ≧ 0.0005 (%) and B- (11 / 14N-11 / 27 Al-11 / 48Ti). ) ≧ 0.0005 (%).

In the present invention, it is not necessary to specify S as a general component in steel. However, it is desirable to reduce S within a range commensurate with the cost of the steel sheet for working, and it is preferably 0.05% or less. Further, in the case of the above Ti addition, the S addition amount has an important meaning in considering the effective Ti amount.

Next, the reasons for limiting the steel sheet manufacturing conditions of the present invention will be described below.

First, steelmaking may be carried out according to a conventional method. In particular, the present invention does not require the limitation of the conditions, but it is preferable to use the continuous casting method in terms of cost and quality.

Regarding hot rolling, as described above, the hot rolling finishing temperature is limited to Ar ₃ transformation because it is effective to limit the hot rolling finishing temperature in order to prevent the workability from being deteriorated even by annealing at a temperature far exceeding the Ac ₁ transformation point. It should be near the point. In particular
Ar ₃ object one good workability is of the present invention in transformation point to 900 ° C. is obtained, preferably if is to be obtained a stable material at high-temperature annealing is Ar ₃ transformation point to Ar ₃ transformation point Finish at + 50 ° C. Other conditions are not particularly specified, and may be performed according to a conventional method. The winding temperature is usually around 550 to 700 ° C, and a sufficiently excellent material can be obtained.

Cold rolling may be carried out according to a conventional method, and in particular, it is not necessary to limit the conditions in the present invention, but for rapid progress of recrystallization and α-γ transformation, a cold rolling reduction of 50% or more. Is desirable.

In this invention, the heat treatment is preferably a continuous annealing method because the box annealing method is insufficient for forming a two-phase structure. Further, in order to form a two-phase structure, it is naturally important to heat the annealing temperature of continuous annealing to at least the α phase-γ phase coexisting temperature range of the Ac ₁ transformation point or higher. Meanwhile workability deterioration seems to be due to loss of residual α phase exceeds Ac ₁ transformation point + 90 ° C. as described above because inevitable Mechanistically, the annealing temperature Ac ₁ transformation point to Ac ₁ transformation point + 90 ° C. and There is a need to. It should be noted that in the case of annealing just above the Ac ₁ transformation point, the fluctuation of the material value is rather large, and considering the composition, soaking, and variations in measuring equipment that are inherent in factory production, an annealing temperature of Ac ₁ transformation point + 10 ° C or higher should be secured. Is desirable.

Further, in order to turn at least a part of the γ phase into acicular ferrite during cooling after annealing, it is necessary to increase the cooling rate after annealing, but this rapid cooling is required only near the Ar ₁ transformation point. This quenching temperature range is Ar ₁ transformation point +
It shall be limited to the temperature range of 100 ° C to Ar ₁ transformation point -100 ° C.
If the cooling rate in this temperature range is less than 7 ° C./s, acicular ferrite is not generated from the γ phase, and therefore the cooling rate in the temperature range of Ar ₁ transformation point + 100 ° C. to Ar ₁ transformation point −100 ° C. is 7
℃ / s or more, but if the cooling rate in this temperature range exceeds 50 ℃ / s, the deterioration of both r value and elongation becomes a little noticeable.
The cooling rate in the temperature range of Ar ₁ transformation point + 100 ° C to Ar ₁ transformation point -100 ° C is preferably in the range of 7 ° C / s to 50 ° C / s.

By subjecting the thus obtained acicular-ferrite + ferrite dual-phase steel sheet to temper rolling at a rolling reduction of 0.2 to 5%, the object of the present invention is to obtain excellent bake hardenability and normal temperature non-aging A cold rolled steel plate can be obtained. The steel sheet produced by the method of the present invention originally has almost no yield elongation, but the rolling ratio cannot be sufficiently suppressed at a rolling reduction of less than 0.2%. Further, if the rolling reduction exceeds 5%, temper rolling causes the increase of movable dislocations due to processing strain, which results in the promotion of strain aging. Therefore, the rolling reduction of the temper rolling is set in the range of 0.2 to 5%. If excellent workability is important, it is desirable to avoid a reduction ratio exceeding [plate thickness (mm) x 1.5]% because the steel plate will be slightly hardened. That is, for such a purpose, it is desirable that the reduction ratio of the temper rolling is 0.2% to [sheet thickness (mm) × 1.5]%. On the other hand, if non-aging at room temperature is important, a reduction of less than [plate thickness (mm) x 0.7]% causes slight aging by holding at room temperature for an extremely long time, so [plate thickness (mm)]
A reduction of × 0.7]% or more is desirable. That is, for such purpose, the reduction ratio of temper rolling is [plate thickness (mm) x 0.
7]% to 5% is desirable.

(Example) Example 1 Steels having various compositional compositions shown in Table 1 were prepared.

These test steel slabs were manufactured by continuous casting, heated to 1200 ° C, then rough rolled (reduction rate 88%), finish rolling (reduction rate 88%, hot rolling finish temperature: shown in Table 2). Thickness 3.5mm
Hot coil, and then cold rolled to 0.8 mm. After that, continuous annealing and temper rolling were performed under the conditions shown in Table 2.

The results of the material investigation of the cold rolled steel sheet thus obtained are shown in Table 3.
Shown in. The BH amount was taken to be the yield yield point as shown in FIG.

As is clear from Table 3, No. 1A, 2, 3, 4, 1 which is an example of the present invention
0 and 11 not only show excellent workability,
It can be seen that it has a high BH property and an excellent non-aging property at room temperature.

On the other hand, Comparative Examples No. 5,6,7,8, whose components do not fit this invention
Comparative example No. 1B whose process conditions do not conform to this invention,
1C, 1D, 1E, 1F are both inferior in BH property or room temperature non-aging property, many inferior in workability and strength, compared with the examples of the present invention,
It is clear that the method of the invention is excellent.

That is, since Comparative Example No. 5 does not contain Nb, or Comparative Example No. 6 does not contain B, the workability, especially the r value, is deteriorated, and the room temperature aging easily proceeds. . Comparative Example No. 9 also has less A1 and B- (11 / 14N-11 / 2
7 Al) ≧ 0.0005% is not satisfied, so that the effective amount of solid solution B is not secured, so that the workability and the non-aging at room temperature deteriorate as in Comparative Example No. 6 in which B is not added.

Since Comparative Example No. 7 contains a large amount of C as 0.0150%, it has a high BH property but is poor in workability, and is extremely inferior in non-aging at room temperature and shows a considerable elongation at yield before aging. On the contrary, due to the excessive addition of Ti, C- (12 / 48Ti-12 / 32S-12 / 14
N) ≧ 0.0005% is not satisfied and the solid solution C is insufficient. Comparative Example N
O.12 has no BH property, and the workability is deteriorated because the grain size of the hot-rolled sheet is difficult to be fine.

Comparative Example No. 8 has a low Mn content of 0.12%, so that not only high-temperature annealing is required, but also a sufficient two-phase structure cannot be obtained, resulting in a half-finished workability, BH property, and non-aging at room temperature. There is.

Comparative Example No. 1B has a ferrite single-phase structure because it is annealed at a temperature lower than the Ac ₁ transformation point, and the BH content and non-aging at room temperature are insufficient, and the workability is slightly inferior.

Since Comparative Example No. 1C has a low cooling rate after annealing, it does not form a two-phase structure sufficiently and is poor in workability and non-aging at room temperature.

Comparative Example No. 1D has no temper rolling, and Comparative Example No. 1E has a low rolling reduction of 0.1%. All of these are inferior in non-aging at room temperature.

Further, when the hot rolling finishing temperature is below the Ar ₃ transformation point as in Comparative Example No. 1F, the r value is remarkably poor.

Example 2 The cold-rolled steel sheets produced by the method 1A of the present invention and the comparative method 1D of Example 1 were annealed and temper-rolled, and then passed through a hot dip galvanizing line. The soaking cycle of the plating line was 550 ° C x 15 seconds.

Material of plated steel plate 1A is YS: 18.8kgf / mm ² , TS: 31.9kg
F / mm ² , elongation value 49.8%, r value 2.38, workability is almost the same as before plating, BH amount 5.5 kgf / mm ² , yield elongation 0.0
%, Excellent BH property and normal temperature non-aging property.

On the other hand, steel plate 1D has a BH of 5.0 kgf / mm ² and a yield point elongation after plating.
1.2%, and the material is YS: 21.3kgf / mm ² , TS: 3
The workability was slightly deteriorated at 2.0 kgf / mm ² , elongation value 45.6%, and r value 1.96.

(Effects of the Invention) This invention enables industrial stable production of cold-rolled steel sheet having both workability, high BH property, and non-aging at room temperature.
In particular, it has brought about easy processing and excellent strength as a steel plate for outer plate processing. This is for applications under conditions where conventional BH steel sheets for processing could not be used due to aging, such as long-term storage necessary to always maintain inventory, exports that require long-term cruises, and temperatures of around 500 ° C. It opens the way for applications such as alloy-plated original plates that pass high temperatures.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of temper rolling on non-aging at room temperature, FIG. 2 is a graph showing comparison between the steel of the present invention and conventional steel with respect to room temperature aging, and FIG. Immediately after annealing, a graph showing r value, elongation and yield point elongation characteristics without temper rolling, and FIG. 4 is a diagram showing a method for measuring bake hardenability (BH property).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division

Claims (4)

[Claims] 1. C: 0.01 wt% or less, Si: 0.1 wt% or less, Mn: 0.2 to 2.0 wt%, Nb: 0.005 to 0.1 wt%, B: 0.0005 to 0.01 wt%, Al: 0.5 wt% or less, and N: containing 0.02wt% or less, and satisfying B- (11 / 14N-11 / 27Al) ≥ 0.0005 (wt%), the balance being steel and a composition consisting of inevitable impurities. Rolling is performed, the hot rolling finishing temperature is set in the range of Ar ₃ transformation point to 900 ° C., the hot rolled sheet obtained by this hot rolling is subjected to cold rolling, and following this cold rolling, Ac ₁ transformation point to Ac ₁ transformation point + 90 ℃
Heating up to the temperature range of at least Ar ₁ transformation point + 100 ℃ ～ A
r ₁ transformation point -100 ° C temperature range: bake hardenability and tempering at room temperature, which is characterized by performing heat treatment of cooling at a cooling rate of 7 ° C / s or more and then temper rolling with a reduction rate of 0.2 to 5%. A method for producing a cold-rolled steel sheet for processing having excellent aging properties. 2. C: 0.01 wt% or less, Si: 0.1 wt% or less, Mn: 0.2 to 2.0 wt%, Nb: 0.005 to 0.1 wt%, B: 0.0005 to 0.01 wt%, P: 0.03 to 0.15 wt% , Al: 0.5 wt% or less and N: 0.02 wt% or less, and B- (11 / 14N-11 / 27 Al) ≧ 0.0005 (wt%) are satisfied, and the balance is Fe and inevitable impurities. Hot rolling is performed using steel of the composition as the raw material, the hot rolling finishing temperature is set to the range of Ar ₃ transformation point to 900 ° C., the hot rolled sheet obtained by this hot rolling is cold rolled, and this cold rolling is performed. continue to Ac ₁ transformation point ~Ac ₁ transformation point to + 90 ℃
Heating up to the temperature range of at least Ar ₁ transformation point + 100 ℃ ～ A
r ₁ transformation point -100 ° C temperature range: bake hardenability and tempering at room temperature, which is characterized by performing heat treatment of cooling at a cooling rate of 7 ° C / s or more and then temper rolling with a reduction rate of 0.2 to 5%. A method for producing a cold-rolled steel sheet for processing having excellent aging properties. 3. C: 0.0005 to 0.01 wt%, Si: 0.1 wt% or less, Mn: 0.2 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, Nb: 0.005 to 0.1 wt%, B: 0.0005 to 0.01 wt. %, Al: 0.5 wt% or less S: 0.05 wt% or less and N: 0.02 wt% or less, and B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≥ 0.0005 (wt%) C- (12 / 48Ti-12 / 32 S-12 / 14N) ≥ 0.0005 (wt%) is satisfied, and the balance is steel with a composition of Fe and unavoidable impurities. Ar ₃ transformation point to 900 ° C. is set, and the hot-rolled sheet obtained by this hot rolling is subjected to cold rolling. Following this cold rolling, Ac ₁ transformation point to Ac ₁ transformation point + 90 ° C.
Heating up to the temperature range of at least Ar ₁ transformation point + 100 ℃ ～ A
r ₁ transformation point -100 ° C temperature range: bake hardenability and tempering at room temperature, which is characterized by performing heat treatment of cooling at a cooling rate of 7 ° C / s or more and then temper rolling with a reduction rate of 0.2 to 5%. A method for producing a cold-rolled steel sheet for processing having excellent aging properties. 4. C: 0.0005 to 0.01 wt%, Si: 0.1 wt% or less, Mn: 0.2 to 2.0 wt%, Ti: 0.005 to 0.05 wt%, Nb: 0.005 to 0.1 wt%, B: 0.0005 to 0.01 wt. %, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less S: 0.05 wt% or less and N: 0.02 wt% or less, and B- (11 / 14N-11 / 27 Al-11 / 48Ti) ≥0.0005 (wt%) C- (12 / 48Ti-12 / 32 S-12 / 14N) ≥0.0005 (wt%), with the balance being Fe and inevitable impurities Then, the hot rolling finishing temperature is set in the range of Ar ₃ transformation point to 900 ° C., the hot rolled sheet obtained by this hot rolling is subjected to cold rolling, and following this cold rolling, Ac ₁ transformation point to Ac ₁ Transformation point + 90 ° C
Heating up to the temperature range of at least Ar ₁ transformation point + 100 ℃ ～ A
r ₁ transformation point -100 ° C temperature range: bake hardenability and tempering at room temperature, which is characterized by performing heat treatment of cooling at a cooling rate of 7 ° C / s or more and then temper rolling with a reduction rate of 0.2 to 5%. A method for producing a cold-rolled steel sheet for processing having excellent aging properties.