JP3224732B2 - Cold rolled steel sheet having good aging resistance and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the technical field of cold-rolled steel sheets of low-carbon killed steel, and in particular, proposes a cold-rolled steel sheet which can obtain good aging resistance, together with its advantageous production method. Is what you do.

[0002]

2. Description of the Related Art Cold-rolled steel sheets are widely used in automobiles, home appliances, various building materials, etc. because of their better dimensional accuracy, better surface and better workability than hot-rolled steel sheets. Have been. Conventionally, as a cold-rolled steel sheet having good workability, a soft, ductile material (typically represented by elongation (El.)) And a material having a high Rankford value (r-value) have been prepared by adjusting various component systems. It has been proposed by a combination of systems and manufacturing methods. A typical example is an ultra-low carbon cold rolled steel sheet in which the carbon content in steel is reduced to 50 ppm or less at the steelmaking stage and a carbonitride forming element such as Ti or Nb is added. Such steel sheets can easily achieve excellent properties such as a yield point (YS) of 200 MPa or less, an elongation (El.) Of 50% or more, and an r-value of 2.0 or more. In addition, since such a steel sheet completely fixes solid solution (C, N), which causes material deterioration such as aging, as carbides and nitrides, it goes without saying that aging hardly occurs. .

[0003] However, as described above, the amount of C is 50 ppm.
The ultra low carbon cold rolled steel sheet degassed below and added with Ti and Nb is compared with ordinary low carbon steel (C: 0.02-0.06wt%) because these Ti and Nb are expensive components. The manufacturing cost is significantly increased. In addition, the addition of Ti or Nb increases the recrystallization temperature, so that the recrystallization temperature during annealing after cold rolling must be 700 ° C. or higher. Therefore, in order to obtain the required material, the recrystallization temperature must be raised to a high temperature of 800 ° C. or more, which also increases the manufacturing cost.

[0004] In the production of cold-rolled steel sheets, not only steel sheets having excellent properties but also low production costs are required in recent years. , N) is extremely poor in profitability if it is manufactured using a material containing a large amount in a quantity that can completely fix it. Therefore, even when a low-carbon killed steel having a low molten steel cost is used as a material, a steel sheet having good aging resistance and good workability has been desired.

In a conventionally known method for producing a steel sheet having good press formability and good aging resistance from a low-carbon killed steel as a raw material, the winding temperature after hot rolling is set to 600 ° C. or higher, and solid solution N is dissolved. Means for fixing as AlN, and in the case of continuous annealing after cold rolling, rapid cooling is performed in the cooling process after recrystallization, and then the cementite is kept in a temperature range of 300 to 500 ° C for several minutes to reduce the cementite grain size. Of these, means for reducing the amount of solute C by precipitating at the grain boundaries is employed. However, even if such a method is adopted, a steel sheet having good aging resistance with an aging index (AI; tensile stress difference before and after aging treatment at 100 ° C. for 30 minutes after 7.5% tension) of 40 MPa or less can be obtained. It was difficult.

Further, as described above, the current mainstream of cold rolled steel sheet is ultra-low carbon steel, and it is considered that continuous aging equipment constructed in recent years does not require overaging treatment equipment in terms of metallurgy. In addition, due to problems such as facility construction costs, overage treatment facilities have not always been provided.
In particular, when a low-carbon killed steel is treated in a continuous annealing facility in which an overaging treatment facility is not always provided, it has been extremely difficult to obtain a steel sheet having good aging resistance of 40 MPa or less in aging index.

In order to obtain a product having good aging resistance by a short overaging treatment, research and development have been promoted.
No. 141534 discloses that a low-carbon Al-killed steel in which Al and N are slightly higher, or a steel sheet to which B is added, is provided with appropriate hot rolling conditions including a slab heating temperature, so that solid solution N in steel is reduced. There has been proposed a method in which AlN and BN are completely fixed, solid solution C is precipitated using the AlN and BN as precipitation nuclei, and temper rolling is performed at a high pressure reduction rate.
Japanese Patent Application Laid-Open No. 2-267227 discloses that the cooling rate of a continuous cast slab of steel having a specified O content is limited to a specific range, so that the oxide having a specific size distribution during solidification and cooling of the slab. It has been proposed to obtain good workability and aging through the effects of crystal rotation during cold rolling and recrystallization during annealing exerted by this oxide.

[0008]

However, according to the method described in Japanese Patent Application Laid-Open No. 2-141534, a cold-rolled steel sheet having good aging resistance can be obtained. Is essential, which makes it difficult to achieve both excellent workability (particularly ductility) and aging resistance.
Further, in the method described in Japanese Patent Application Laid-Open No. Hei 2-267227, the overaging treatment is performed for a long time of 2 minutes or more, and the method is applied to continuous annealing equipment where overaging treatment equipment is not always provided. hard.

Therefore, the present invention advantageously solves the problems left in the prior art as described above. Even if temper rolling at a high reduction rate is not performed, an overage treatment facility is required. Even without long-term overage treatment that would
An object of the present invention is to propose a cold rolled steel sheet having excellent aging resistance.

[0010]

According to the results of the inventors' intensive studies, the cold-rolled steel sheet having good aging resistance according to the present invention, which achieves the above object, has a C content of 0.010 to 0.10 wt% and a Si content of 0.001 to 0.001 wt%.
0.05wt%, Mn: 0.05 ~ 0.50wt%, S: 0.005 ~ 0.05wt
%, Al: 0.001 to 0.020 wt%, N: 0.001 to 0.0100 wt%
And O: 0.002 to 0.010 wt%, containing Si content and Al
The sum with the content satisfies 0.005 wt% or more.
0.5 [N%] to 3.0 [N%] in relation to the content [N%]
In a range that satisfies the following conditions. If necessary, one or two of Ti and Nb are contained in a total amount of 0.001 to 0.050 wt%, and further, if necessary, Cr: 0.05 to 1.00 wt%. Composition of iron and unavoidable impurities, average particle size 0.01-0.5 μm
A cold-rolled steel sheet having good aging resistance, characterized by having a structure having the following oxides, sulfides or nitrides at an average interval of 0.5 to 5 µm.

The method for producing a cold-rolled steel sheet having good aging resistance according to the present invention is as follows: C: 0.010 to 0.10 wt%, Si: 0.001 to 0.001%
0.05wt%, Mn: 0.05 ~ 0.50wt%, S: 0.005 ~ 0.05wt
%, Al: 0.001 to 0.020 wt%, N: 0.001 to 0.0100 wt%
And O: 0.002 to 0.010 wt%, containing Si content and Al
The sum with the content satisfies 0.005 wt% or more.
0.5 [N%] to 3.0 [N%] in relation to the content [N%]
In a range that satisfies the following conditions. If necessary, one or two of Ti and Nb are contained in a total amount of 0.001 to 0.050 wt%, and further, if necessary, Cr: 0.05 to 1.00 wt%. A steel slab having a composition of iron and unavoidable impurities is cast by a continuous casting method, and during a cooling process between 1400 and 1100 ° C, is cooled at an average cooling rate of 10 to 100 ° C / min. When the hot rolling is performed after heating or holding, the relationship of 0.02 ≦ R / T ≦ 0.08 is satisfied with respect to the relationship between the temperature T (° C.) and the rolling reduction R (%) in the rough rolling final pass, and 85
Finish hot rolling at 0 ° C or less, wind up at 500-700 ° C,
After cold rolling, the steel sheet is continuously annealed at a temperature in the range of recrystallization temperature to 850 ° C for 5 minutes or less, and then immediately starts cooling, and in the cooling process in the temperature range of 500 to 300 ° C for less than 5 to 120 seconds. This is a method for producing a cold-rolled steel sheet having good aging resistance, characterized by allowing the steel sheet to stay for a while.

[0012]

In the present invention, B is contained in a low-carbon killed steel, and BN is finely dispersed in crystal grains. As a result, the amount of solute N can be reduced, and the BN acts as a precipitation nucleus of cementite to form a solute C.
The amount can be reduced. In order to sufficiently exert the function of BN, the present invention uses SiO ₂ , Al ₂ O
An oxide such as ₃ or a sulfide or nitride such as MnS or TiS is finely dispersed in steel in advance, and these are allowed to act as the above-mentioned BN precipitation nuclei.

The reason why the composition range of the cold-rolled steel sheet of the present invention is limited will be described below. [C: 0.010 to 0.10 wt%] The content of C is set to 0.010 to 0.10 wt%.
The reason why the content of C is reduced to less than 0.010 wt% is that the cost of the decarburization treatment in the steel making stage is significantly increased, and that the crystal grain size is significantly reduced when the content exceeds 0.10 wt%. This is because the value of elongation (El.) Becomes small and workability deteriorates. Preferably, it is in the range of 0.01 to 0.05 wt%.

[Si: 0.001 to 0.05% by weight]
The reason for setting the range of 001 to 0.050 wt% is that if Si is used as a deoxidizing agent in the steelmaking stage, deoxidation is sufficient, and that Si oxide is used as a nucleus for precipitation of BN. This is because it is necessary to contain 0.001 wt% or more. On the other hand, if the Si content exceeds 0.05 wt%, the material is hardened and the workability is deteriorated. Note that, as described later, the amount of Si is contained so that the sum with the amount of Al becomes 0.005 wt% or more.

[Mn: 0.05 to 0.50% by weight]
The reason for setting the range to 0.50 wt% is that if it is less than 0.05 wt%, S which causes red hot embrittlement is not sufficient to be fixed as MnS, and MnS which is effective as a precipitation nucleus of BN cannot be utilized. On the other hand, if the content exceeds 0.50 wt%, not only does the material harden, thereby deteriorating the workability, but also increases the steel cost. Preferably, Mn / S> 10 is good.

[S: 0.005 to 0.05 wt%]
The reason for setting the range of 005 to 0.05 wt% is that if it is less than 0.005 wt%, it becomes impossible to utilize MnS effective as BN precipitation nuclei, and if it exceeds 0.05 wt%, the material is hardened to deteriorate workability. It is because.

[Al: 0.001 to 0.020 wt%] Not only is Al used as a deoxidizing agent in the steelmaking stage, but in the present invention, Al-based oxides are positively used as nuclei for the precipitation of BN. Soluble C acting as precipitation nucleus
Of solid solution C after cold rolling and annealing by promoting precipitation of
Decrease the amount. For that purpose, it is necessary to add Al so as to contain 0.001 wt% or more. However, when added in a large amount, AlN is preferentially precipitated, and the precipitation of BN, which is to become a precipitation nucleus of cementite, is reduced.
wt% or less. It is preferably 0.03 wt% or less, and the Al content in the range of 0.001 to 0.010 wt% is
It is more desirable to use the combination of deoxidation and the action of using Si-based oxides as precipitation nuclei of BN by the inclusion of. Note that, as described later, the Al content is included such that the sum of the Al content and the Si content is 0.005 wt% or more.

[N: 0.0010 to 0.0100 wt%]
The reason for setting the content in the range of 0.0010 to 0.0100 wt% is that BN is positively used as a precipitation nucleus of cementite in the present invention. Therefore, if the N content is less than 0.0010 wt%, the effect cannot be expected. If the content exceeds 0.0100 wt%, BN is dispersed in a large amount and rolling cracks are easily caused.

[O: 0.002 to 0.010 wt%]
The reason for setting the content in the range of 0.002 to 0.0100 wt% is that in the present invention, BN is positively used as a precipitation nucleus of cementite, and that an oxide is positively used as a precipitation nucleus of BN. Therefore, O is at least 0.00
2 wt% is contained. If the content exceeds 0.010 wt%, press cracking due to inclusions may be caused during press working.
The range was 0 wt%.

[The sum of the Si content and the Al content is 0.005 wt%
As described above, the sum of the Si content and the Al content is 0.
It is necessary to contain it so as to be at least 05 wt%.
This is because if the sum of the Si content and the Al content is less than 0.005 wt%, the absolute amount of the Si-based or Al-based oxide that should become the precipitation nucleus of BN becomes small, and finally after cold rolling and annealing. Solid solution C
This is because the amount cannot be reduced.

[B: 0.5 [N] in relation to the amount of N [N%]
%] To 3.0 [N%] The reason why the amount of B added is in the range of 0.5 [N%] to 3.0 [N%] with respect to the N amount is that the B amount is less than 0.5 × [N%]. In this case, the amount of residual solid solution N increases, and aging due to this N easily occurs. In addition, the absolute amounts of BN and B _x O _y as precipitation nuclei of cementite decrease, and 3.0 × [N %] More than B
This is because solid solution B causes deterioration of the material in an amount. Preferably, it is in the range of 0.0010 to 0.0100 wt%.

[One or two of Ti and Nb are used in a total of 0.001
0.00.050 wt%] In the cold-rolled steel sheet of the present invention, one or two or more of Ti and Nb can be contained, if necessary, in addition to the components described above. Both Ti and Nb are components that form carbonitrides and oxides.However, in the present invention, it is not necessary to add an amount sufficient to completely fix solid solution C, and it may cause precipitation nuclei of cementite in the cooling process after annealing. Contains a certain amount. In order to bring out the effect, the content of at least 0.001 wt% is necessary. On the other hand, if the content exceeds 0.050 wt%, in addition to increasing the molten steel cost, fine TiC and NbC are precipitated to deteriorate workability and increase the recrystallization temperature. The content of Nb was set to 0.001 to 0.050 wt% in total of one or two kinds. Preferably, it is 0.003 to 0.030 wt%.

[Cr: 0.05 to 1.00 wt%] The cold-rolled steel sheet of the present invention may contain Cr, if necessary, in addition to the components described above. Cr is a component that forms carbide,
Since it is a very effective component for reducing the amount of solute C causing aging deterioration and improving elongation (El.) And r-value, it can be contained in the present invention in the range of 0.05 to 1.00 wt%. . 0.05wt% to bring out the effect of Cr
Although the above content is necessary, if the content exceeds 1.0 wt%, the cost of molten steel will increase significantly,
The content is in the range of 0.05 to 1.00 wt%. Preferably 0.10
~ 0.80 wt%.

[Structure having oxides, sulfides or nitrides having an average particle size of 0.01 to 0.5 μm at an average interval of 0.5 to 5 μm]
The cold rolled steel sheet of the present invention has a structure having oxides, sulfides or nitrides having an average grain size of 0.01 to 0.5 μm at an average interval of 0.5 to 5 μm. Oxides such as Si-based oxides, Al-based oxides, and MnS, sulfides, or nitrides are preferentially finely precipitated to become nuclei for BN precipitation, thereby accelerating the deposition of BN.
Is useful for fixing solid solution C through promotion of precipitation of cementite. If the average particle size of these oxides, sulfides or nitrides is less than 0.01 μm, it is too fine, while if it is more than 0.5 μm, it is too coarse to suppress the precipitation of BN. If the average spacing of these oxides, sulfides or nitrides is less than 0.5 μm, they are too densely dispersed,
This suppresses crystal grain growth and degrades properties such as elongation. On the other hand, if the average interval is more than 5 μm,
This is because it is disadvantageous for the precipitation of N and the precipitation of solid solution C. The oxide is Ti _x O _y , Al _x O _y , and Ti
_{x O y, Nb x O y} , in particular problems not effectively contribute be an B _x O _y. Further, oxides and nitrides of V and Zr mixed as unavoidable impurity components can expect the same effect.

Next, a method for manufacturing a cold-rolled steel sheet according to the present invention will be described. A molten steel prepared to a predetermined composition is formed into a slab by a continuous casting method. In the cooling process in this continuous casting, the average cooling rate in the range of 1400-1100 ° C is 10-100 ° C / m
Control to in. If the average cooling rate is less than 10 ° C./min, the size of oxides, sulfides and nitrides becomes coarse, and the oxides, sulfides or nitrides have an average particle size of 0.01 to 0.5 μm and an average interval of 0.5 to 5 μm. On the other hand, if the average cooling rate exceeds 100 ° C./min, the cooling rate is too fast,
The sulfides or nitrides become ultrafine and hardly become nuclei for precipitation of BN or cementite. As a result, solid solution C after cold rolling and annealing
This is because the amount increases. The reason why the temperature range of the cooling rate is set to 1400 to 1100 ° C is that the generation, growth and distribution of oxides, sulfides and nitrides are influenced by 1400 to 1100 ° C.
In the temperature range outside this range, oxides, sulfides,
This is because there is almost no effect of the cooling rate on the morphology of the nitride.

When the slab is hot-rolled, the slab which has been cooled to room temperature may be reheated to 900 ° C. or higher. A hot direct rolling (HDR) or a hot charge rolling (HCR) method of inserting into a heating furnace at a temperature of not more than ° C and holding or reheating may be used.
In any method, before hot rolling, the material is heated or held at a temperature of 1100 ° C. or less. This is because it is necessary to finely disperse BN in the steel. If the BN is heated or maintained at a temperature exceeding 1100 ° C., the BN dissolves and solid solution N remains.

In the hot rough rolling, the final pass satisfies the condition of 0.02 ≦ R / T ≦ 0.08 with respect to the relationship between the temperature T (° C.) and the rolling reduction R (%) in the final rough rolling pass. By satisfying the relationship between the temperature and the rolling reduction, the γ grain size is reduced, BN precipitation is promoted, and carbide precipitation sites are formed. In addition, when hot rolling is performed in the α + γ region or lower, in which the finishing temperature is 850 ° C. or lower, as in the present invention, it is possible to prevent the occurrence of defects called ridging, which are likely to occur during processing after cold rolling and annealing. Can be.
When the value of R / T is less than 0.02, the above-described effect is lost, and when the value of R / T exceeds 0.08 when the rough rolling pass is performed at extremely low temperature or under high pressure, the BN becomes It precipitates too finely and hardly becomes a precipitation nucleus of cementite, and also causes a decrease in elongation (El.) And r value.

The hot rolling finish temperature is 850 ° C. or less. The reason for terminating hot rolling at 850 ° C or less is that BN is finely dispersed in steel and is effective in promoting the precipitation of cementite. This is because the amount can be reduced, the yield of the steel material can be improved, and the cost can be reduced.

The winding temperature after hot rolling is 500 to 700 ° C. This is to completely precipitate solid solution N and prevent deterioration of the material due to N aging. If the temperature is lower than 500 ° C., this effect is poor. On the other hand, if the temperature is higher than 700 ° C., the amount of scale generated on the surface of the steel sheet increases, causing a reduction in the yield of the steel material and a reduction in the pickling efficiency, leading to an increase in the production cost.
Depending on the process up to this winding, the average particle size is 0.01 to 0.5
μm oxide, sulfide or nitride with an average spacing of 0.5 to 5μ
The tissue having m is obtained.

For the purpose of eliminating the temperature difference between the front and rear ends of the sheet bar before the hot rolling finish rolling, the sheet bar is temporarily removed by using a coil box provided between the hot rolling rough rolling equipment and the finish rolling equipment. Even if the rolling direction is reversed by rewinding and rewinding this, the rear end of the preceding sheet bar and the front end of the following sheet bar are joined together for the purpose of improving the yield of steel material to continuously Even if the sheet bar is rolled, and even if the lubricating rolling is performed for the purpose of reducing the rolling load, there is no adverse effect on the mechanical properties after cold rolling and annealing. It is also possible to apply

Annealing after cold rolling is performed by a continuous annealing method at a recrystallization temperature of 850 ° C. for 5 minutes or less. The reason for the continuous annealing is that the continuity with the cleaning equipment and the temper rolling equipment is easy, the cost and time required for handling can be significantly reduced, and the production time is greatly reduced as compared with the box annealing. . At the time of this continuous annealing, if the annealing temperature is lower than the recrystallization temperature, processing strain remains, resulting in a high-strength, low-elongation steel sheet, which makes it difficult to perform forming. On the other hand, 850
At an annealing temperature exceeding ℃, the (111) recrystallized texture is randomized, and press cracking is likely to occur when subjected to press forming. Therefore, in the present invention, the annealing temperature is set in the range from the recrystallization temperature to 850 ° C. If the annealing time exceeds 5 minutes, it may be a precipitation nucleus of the cementite precipitated during the cooling, and the cementite precipitated during the hot rolling is completely dissolved. Preferably 1s
120120 s.

Immediately after the heating and soaking of the annealing, cooling is started,
During the cooling process, it is kept at a temperature of 500 to 300 ° C. for less than 5 to 120 seconds. This is to retain C for a relatively long time in a temperature range advantageous for precipitating solid solution C in the heating process. That is, in the temperature range exceeding 500 ° C., the amount of solid solution of C in α-iron is large, so that the solid solution C hardly precipitates in the crystal grain boundaries and in the crystal grains. , And solute C hardly precipitates in the crystal grain boundaries and in the crystal grains. The time required for precipitation in such a temperature range of 500 to 300 ° C. must be at least 5 seconds. However, setting the time longer than 120 seconds is inevitable because the equipment must be lengthened or the line speed must be reduced, which inevitably increases the equipment cost or significantly reduces productivity. Must. For this reason, in the present invention, as a condition for cooling immediately after the heating and soaking during annealing, the cooling condition is set to stay in the temperature range of 500 to 300 ° C. for less than 5 to 120 seconds. Preferably, the residence time in this temperature range is
10 to 60 seconds. The term "retention" in the present invention can be used for any cooling, holding, and heating patterns as long as a specific temperature range of 500 to 300 ° C. is applied for a predetermined time, but application to a continuous annealing line is considered. It is preferable to appropriately cool without maintaining the temperature at a constant level. Such stagnation treatment of the present invention can be easily performed in the cooling zone even in continuous annealing equipment where overaging treatment equipment is not always provided, so that good aging properties can be obtained even without overaging treatment equipment. A cold rolled steel sheet can be obtained.

After cold rolling, temper rolling is performed mainly for the purpose of shape correction and prevention of the occurrence of stretcher strain.
Since the cold-rolled steel sheet of the present invention has good aging resistance even before temper rolling, high-pressure is not required in this temper rolling. Rather, if the rolling reduction is 1.0% or more, work hardening occurs to increase the yield point, which may impair workability. Therefore, it is preferable to reduce the rolling reduction to less than 1.0%.

[0034]

EXAMPLES The thicknesses of the various components shown in Table 1 are 250.
mm steel slab is cast by the continuous casting method, and the average cooling rate is 8 to 1400 to 1100 ° C in the cooling process by water cooling.
Cooling was performed at various cooling rates of 200 ° C./min. After that, this slab is led to a soaking furnace and heated to 900-1080 ° C.
Various production conditions shown in Table 2, namely, a three-pass rough rolling, a final pass temperature and a rolling reduction varied variously to obtain a sheet bar having a thickness of 30 mm, and the finishing temperature was reduced by a seven-stand finishing mill. Hot rolling was performed at 750 to 820 ° C and a finished plate thickness of 3.5 mm. Subsequently, it was wound at 580 to 700 ° C., pickled, and then cold rolled into a cold rolled sheet having a thickness of 0.8 mm. Thereafter, recrystallization annealing was performed by a heat cycle as shown in FIG. 1 in a continuous annealing furnace, and then temper rolling at a reduction of 0.8% was performed.

[0035]

[Table 1]

[0036]

[Table 2]

From the steel sheet thus obtained, JIS No. 5 tensile test specimens were sampled at 0 °, 45 ° and 90 ° with respect to the rolling direction, and the mechanical properties were examined. The results are shown in Table 3. In addition,
The morphologies of oxides, sulfides and nitrides shown in Table 3 are 10 magnifications.
Observed with a transmission electron microscope at a magnification of 10,000 times,
Further, the tensile properties are average values in the respective directions calculated by the following equations. X = (X ₀ + 2X ₄₅ + X ₉₀ ) / 4 where X _{0 is} a characteristic value in the 0 ° direction with respect to the rolling direction, X _{45 is} a characteristic value in the 45 ° direction with respect to the rolling direction, and X _{90 is a} rolling direction. Characteristic value in the 90 ° direction,

[0038]

[Table 3]

As can be seen from Table 3, the cold-rolled steel sheet according to the present invention has an elongation (El.) Of 45% or more, an aging index (AI) of 40 MPa or less, and r
The value was 1.5 or more. In particular, the steel sheet to which Cr was added in addition to Ti or Nb had considerably good aging resistance and workability.
On the other hand, a steel sheet in which the steel composition range and the manufacturing process conditions of the present invention are out of range has a low elongation, low r-value, and poor aging resistance.

[0040]

According to the cold rolled steel sheet of the present invention, good non-aging property can be obtained by using BN as a precipitation nucleus of cementite in a low-carbon steel which is less expensive than an ultra-low carbon steel. In order to sufficiently exert the action of BN, the present invention uses oxides such as SiO ₂ and Al ₂ O ₃ , MnS, Ti
Sulfides such as S and nitrides are finely dispersed in steel in advance, and these are allowed to act as the above-mentioned BN precipitation nuclei.
Thus, the process of accelerating the precipitation of solid solution C during continuous annealing can be carried out by a non-constant temperature treatment in a very short time, so that the continuous annealing equipment has good plateability, the line speed can be easily increased, and mass production is possible. In addition, the manufacturing cost can be reduced. Further, since temper rolling at a high reduction rate is not required, workability and aging resistance can both be achieved at a high level.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat cycle of recrystallization annealing in an example.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-130431 (JP, A) JP-A-61-270354 (JP, A) JP-A-7-188851 (JP, A) JP-A 58-130 48636 (JP, A) JP-A-62-185834 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 9/46-9/48

Claims (6)

(57) [Claims] C: 0.010 to 0.10 wt%, Si: 0.001 to 0.05 wt%, Mn: 0.05 to 0.50 wt%, S: 0.005 to 0.05 wt%, Al: 0.001 to 0.020 wt%, N: 0.001 to 0.0100 wt% and O: 0.002 to 0.010 wt%, the sum of the Si content and the Al content satisfies 0.005 wt% or more, and further, B is 0.5% in relation to the N content [N%].
[N%] to 3.0 [N%], the balance is composed of iron and unavoidable impurities, and oxides, sulfides or nitrides having an average particle size of 0.01 to 0.5 μm have an average spacing of 0.5%. A cold-rolled steel sheet having good aging resistance, which has a structure having a thickness of about 5 μm. 2. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a composition further containing one or two of Ti and Nb in a total amount of 0.001 to 0.050 wt%. 3. The method according to claim 1, wherein Cr: 0.05 is further added.
A cold rolled steel sheet having a good aging resistance and a composition containing up to 1.00 wt%. 4. C: 0.010 to 0.10 wt%, Si: 0.001 to 0.05 wt%, Mn: 0.05 to 0.50 wt%, S: 0.005 to 0.05 wt%, Al: 0.001 to 0.020 wt%, N: 0.001 to 0.0100 wt% and O: 0.002 to 0.010 wt%, the sum of the Si content and the Al content satisfies 0.005 wt% or more, and further, B is 0.5% in relation to the N content [N%].
[N%] to 3.0 [N%] is contained in the range that satisfies the condition, and the balance is iron and slabs having an unavoidable impurity composition.
After cooling at an average cooling rate of 10 to 100 ° C / min, heating or holding to a temperature of 1100 ° C or less, and then performing hot rolling, the temperature T (° C) and rolling reduction in the final pass of rough rolling Rate R
(%), The condition of 0.02 ≦ R / T ≦ 0.08 is satisfied, hot rolling is completed at 850 ° C. or less, and 500 to 700
C., cold-rolled, and then continuously annealed at a temperature in the range of recrystallization temperature to 850 ° C. for 5 minutes or less, and immediately started cooling, and then cooled in the temperature range of 500 to 300 ° C. in the cooling process. A method for producing a cold-rolled steel sheet having good aging resistance, wherein the cold-rolled steel sheet is retained for less than 120 seconds. 5. The method for producing a cold-rolled steel sheet according to claim 4, wherein the steel slab further comprises one or two of Ti and Nb in a total content of 0.001 to 0.050 wt%. 6. The method for producing a cold-rolled steel sheet according to claim 4, wherein the steel slab further has a composition further containing Cr: 0.05 to 1.00 wt%.