JP3261037B2 - Manufacturing method of cold rolled steel sheet with good aging resistance - 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 more particularly to an advantageous method for producing cold-rolled steel sheets having good aging resistance. It is.

[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.

[0007] In order to obtain a product having good aging resistance by a short overaging treatment, research and development have been promoted.
No. 768828 discloses that by setting the C content of the material to an appropriate range, the hot-rolling finishing temperature is set to an Ar ₃ point or lower, and the annealing temperature after cold rolling is set to an Ac ₁ point or lower. A method has been proposed in which undissolved carbide finely dispersed therein acts as an effective precipitation site of solid solution C to obtain a cold-rolled steel sheet having good workability and aging even for a short overage treatment. . In Japanese Patent Application Laid-Open No. 2-141534, Al, N
Carbon Al-killed steel with slightly higher
By adding appropriate hot-rolling conditions including the slab heating temperature to the steel sheet to which N is added, the solid solution N in the steel can be completely converted to AlN, B
N and fix the solid solution C with these AlN and BN as precipitation nuclei.
And a method of performing temper rolling at a high pressure reduction rate.

[0008]

However, according to the method described in JP-A-59-76828, the aging index of the obtained steel sheet exceeds 40 MPa, and further improvement in aging resistance is required. It is hard to say that it is no longer a good property today. In addition, the above-mentioned Japanese Patent Application Laid-Open No. Hei 2-
According to the method described in JP-A No. 4 (1999) -2004, a cold-rolled steel sheet having good aging resistance is obtained, but temper rolling at a high pressure reduction is essential, and thereby excellent workability (particularly ductility) and resistance to heat are obtained. Compatibility with aging has been difficult.

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]

SUMMARY OF THE INVENTION As a result of the inventors' earnest studies, the method for producing a cold-rolled steel sheet having good aging resistance according to the present invention, which achieves the above object, comprises: C: 0.010 to 0.10 wt%, Si: : 0.05 wt% or less, Mn: 0.05 to 0.50 wt%, Al: 0.001 to 0.05 wt% and N: 0.0005 to 0.0100 wt%, and B is 0.5 [N in relation to the N content [N%]. %] To 3.0
[N%] is contained within the range that satisfies [N%].
A steel slab containing 0.001 to 0.050 wt% in total of one or two of the following, and further containing 0.05 to 1.00 wt% of Cr if necessary, with the balance being iron and unavoidable impurities, at 1100 ° C or lower. When the hot rolling is performed after heating or holding at the temperature described above, the condition 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
Finish hot rolling at 850 ° C or less, and after cold rolling, recrystallization temperature ~
Cooling is started immediately after continuous annealing in a temperature range of 850 ° C for 5 minutes or less, and the cooling process is started at 500 to 300 ° C.
A cold-rolled steel sheet having good aging resistance, wherein the cold-rolled steel sheet is retained for 5 seconds or more and less than 120 seconds in the above temperature range.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, BN is finely dispersed in crystal grains by subjecting a low-carbon killed steel containing B to hot rolling under specific conditions. As a result, in addition to reducing the amount of solute N, such BN acts as a precipitation nucleus of cementite, thereby reducing the amount of solute C. In addition to the above-mentioned BN, it is more effective to coexist TiC, NbC or the like as a precipitation nucleus of cementite.

Hereinafter, the reason why the composition range of the cold rolled steel sheet of the present invention is limited will be described. [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: 0.05 wt% or less] The Si content is 0.050%
The reason for not more than wt% is that if the Si content exceeds 0.05 wt%, the material is hardened and the workability is deteriorated. In addition, when Si is used as a deoxidizing agent in the steelmaking stage, it is preferable to contain 0.001 wt% or more in order to sufficiently deoxidize.

[Mn: 0.05 to 0.50 wt%]
The reason for setting the range to 0.50 wt% is that if the content is less than 0.05 wt%, it is not enough to fix S which causes red hot embrittlement as MnS, while if it exceeds 0.50 wt%, the material is hardened to improve workability. This is because not only is the steel deteriorated, but also the steel cost is increased.

[Al: 0.001 to 0.05 wt%] Since Al is used as a deoxidizing agent in the steel making stage, Al is also used in the present invention in an amount of 0.001 to 0.05 wt%.
It is necessary to add so as to contain at least wt%. But,
If added in a large amount, AlN is preferentially precipitated, and the precipitation of BN, which is to be a nucleus for the precipitation of cementite, is reduced. Preferably 0.03wt
% Or less, and it is more desirable to use deoxidation by Si in combination with an Al content in the range of 0.001 to 0.010 wt%.

[N: 0.0005 to 0.0100 wt%]
The reason for setting the content in the range of 0.0005 to 0.0100 wt% is that BN is positively used as a precipitation nucleus of cementite in the present invention, so that the effect cannot be expected if the N content is less than 0.0005 wt%. If the content exceeds 0.0100 wt%, BN is dispersed in a large amount and rolling cracks are easily caused.

[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 solid solution N becomes large, so that aging due to this N easily occurs, and the absolute amount of BN as a precipitation nucleus of cementite decreases, and more than 3.0 × [N%] B This is because solid solution B causes deterioration of the material in an amount. Preferably 0.00
The range is 10 to 0.0100 wt%.

[One or two of Ti and Nb are added in a total amount 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, but in the present invention, it is not necessary to add an amount sufficient to completely fix solid solution C, and it is sufficient to form cementite precipitation nuclei in the cooling process after annealing. The amount is contained. 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%, the cost of molten steel is increased and the recrystallization temperature is increased. Therefore, the content of Ti or Nb in the present invention is 0.001 to 0.050 in total of one or two types. wt%. Preferably 0.003
-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%.

The unavoidable impurities are not particularly limited. However, if the P content exceeds 0.03 wt%, a decrease in the aging index (aging index) tends to be obstructed, so that 0.03 wt% or less is preferable. More preferably 0.02
wt% or less. However, it is not necessary to excessively reduce P, which leads to an increase in cost.

The molten steel prepared to have the above-mentioned composition is formed into a slab by a continuous casting method or an ingot-bulking rolling method. When the slab is hot-rolled, the slab once cooled to room temperature may be reheated to 900 ° C. or higher, or the slab may be heated or warmed without cooling to room temperature.
A hot direct rolling (HDR) or a hot charge rolling (HCR) method of inserting into a heating furnace at a temperature of 00 ° C. or lower 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 for 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 finishing temperature of hot rolling 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 not particularly limited.
500-800 ° C is preferred. This is to completely precipitate solid solution N and prevent deterioration of the material due to N aging. 50
If the temperature is lower than 0 ° C., the effect is poor. On the other hand, if the temperature is higher than 800 ° 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 manufacturing cost. More preferably, the temperature is 500 to 700 ° C.

In order to eliminate 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 carried out by continuous annealing 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 steel sheet with high strength and low elongation, 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. Preferably 650-800
° C. If the annealing time is longer than 5 minutes, B precipitates during hot rolling, which may become a cementite nucleus in subsequent cooling.
The time is set to 5 minutes or less because N is completely dissolved. Preferably it is in the range of 1 s to 120 s.

Immediately after the heating and soaking of the annealing, cooling is started,
In the cooling process, at a temperature range of
Let dwell for less than a second. 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. Also, such a 500-300
At least 5 seconds are required for the time required for the precipitation in the temperature range of ° C. 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 temperature is kept in a temperature range of 500 to 300 ° C. for 5 seconds to less than 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 in which overage treatment equipment is not always provided, so that the aging property is good without using the overage 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%.

[0029]

EXAMPLES Thicknesses of various compositions shown in Table 1 were obtained.
After the steel slab having a thickness of 30 mm was heated to 900 to 1250 ° C., various production conditions shown in Table 2, that is, a three-pass rough rolling and varying the temperature and the rolling reduction of the final pass in various ways, to achieve a thickness of 30 mm The sheet bar is used, and the finishing temperature is reduced by a 7-stand finishing mill.
Hot rolling was performed at 700 to 900 ° C and a finished plate thickness of 3.5 mm. Subsequently, it was wound at various temperatures of 700 ° C. or less, pickled, and cold rolled to form a cold rolled sheet having a thickness of 0.7 mm. afterwards,
After recrystallization annealing was performed by a heat cycle as shown in FIG. 1 in a continuous annealing furnace, temper rolling was performed at a rolling reduction of 0.8%.

[0030]

[Table 1]

[0031]

[Table 2]

JIS No. 5 tensile test pieces were sampled from the steel sheet thus obtained 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 tensile properties shown in Table 3 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,

[0033]

[Table 3]

From Table 3, it can be seen that 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 further added with Ti, Nb or Cr had considerably good aging resistance and workability. On the other hand, the steel sheet in which the composition range of the steel composition and the conditions of the manufacturing process of the present invention are out of alignment has a low elongation, low r-value, and poor aging resistance. For example, No. 5 and No. 8 steel sheets were inferior in aging resistance and workability and generated ridging because the R / T of the final pass of rough rolling was less than 0.02. Further, the steel of No. 6 was inferior in aging resistance and workability since the finishing temperature was over 850. No. 13 is Ti,
Since the amount of Nb added was too large and fine TiC and NbC were precipitated in large amounts, the workability was reduced. The steel sheet of No. 14 had a high aging index because a large amount of dissolved N remained. The steel sheet of No. 15 had poor aging resistance and low elongation and r-value because B was not added.

[0035]

According to the method for producing a cold-rolled steel sheet of the present invention, a low-carbon steel which is inexpensive as compared with an ultra-low carbon steel can be used for BN
Not only obtains good non-aging properties by acting as precipitation nuclei of cementite, but also can promote precipitation of solid solution C during continuous annealing by non-constant temperature treatment for a very short time. Good threadability, easy line speed increase, mass production possible, and low manufacturing cost. 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 front page (56) References JP-A-7-3332 (JP, A) JP-A 8-85827 (JP, A) JP-A 8-109415 (JP, A) JP-A 8- 143968 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C21D 8/00-8/04

Claims (3)

(57) [Claims] C: 0.010 to 0.10 wt%, Si: 0.05 wt% or less, Mn: 0.05 to 0.50 wt%, Al: 0.001 to 0.05 wt% and N: 0.0005 to 0.0100 wt%, and B 0.5 [N%] to 3.0 in relation to N content [N%]
When the steel slab consisting of iron and unavoidable impurities is heated or held at a temperature of 1100 ° C or less and then hot-rolled, Temperature T (° C) and rolling reduction R
(%), The condition of 0.02 ≦ R / T ≦ 0.08 is satisfied, hot rolling is completed at 850 ° C. or less, and after cold rolling, maintained at a temperature range of recrystallization temperature to 850 ° C. for 5 minutes or less. Good aging resistance characterized by immediately starting cooling after performing continuous annealing and staying in a temperature range of 500 to 300 ° C. in the cooling process for 5 seconds or more and less than 120 seconds. Production method of simple cold rolled steel sheet. 2. The steel slab according to claim 1, further comprising Ti
And a method for producing a cold-rolled steel sheet having a good aging resistance and a composition containing one or two kinds of Nb in total of 0.001 to 0.050 wt%. 3. The steel slab according to claim 1 or 2,
Further, a method for producing a cold-rolled steel sheet having a good aging resistance and a composition containing Cr: 0.05 to 1.00 wt%.