JPH09263880A - Cold rolled steel sheet excellent in aging resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a cold rolled steel sheet of low carbon killed steel, having excellent aging resistance while obviating the necessity of overaging treatment consisting of holding at the prescribed temp. by the use of overaging equipment and also temper rolling at high draft. SOLUTION: This steel sheet has a composition consisting of, by weight, 0.010-<0.06% C, <=0.05% Si, 0.05-0.50% Mn, 0.001-0.050% Al, 0.0005-0.0100% N, >0.15-1.00% Cr, B in the amount in the range of 0.5[N%] to 3.0[N%] in relation with N content [N%], and the balance iron with inevitable impurities. In this cold rolled steel sheet, Cr forms carbide and B forms nitride to become precipitation neucleus of cementite, etc., and the grain boundary precipitation of solid-solution C is accelerated. As a result, superior non-aging characteristic can be obtained.

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 sheet of low carbon killed steel, and is intended to propose a cold-rolled steel sheet that can obtain particularly good aging resistance.

[0002]

2. Description of the Related Art Cold-rolled steel sheets are widely used for automobiles, home appliances, various building materials, etc. because they have better dimensional accuracy, beautiful surface, and better workability than hot-rolled steel sheets. Has been done. Conventionally, as a cold-rolled steel sheet with good workability, a material that is soft and has a high ductility (elongation (El.)) And a high Rankford value (r value) can be used by adjusting various component systems. It has been proposed by a combination of system and manufacturing method. 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. All of such steel sheets can easily attain 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 that has been degassed below and added with Ti and Nb is an expensive component of Ti and Nb, so compared to ordinary low carbon steel (C: 0.02 to 0.06 wt%). Therefore, the manufacturing cost becomes extremely high. 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 raw material, a steel sheet having good aging resistance and workability is desired.

In a conventionally known method for producing a steel sheet having a good press formability and a good aging resistance from a low carbon killed steel as a raw material, the solid solution N is dissolved at a coiling temperature of 600 ° C. or more after hot rolling. The cementite grains can be fixed as AlN, or in the case of continuous annealing after cold rolling, by performing rapid cooling in the cooling process after the end of recrystallization and then holding it in the temperature range of 300 to 500 ° C for several minutes. Among them, a means for reducing the amount of solute C by precipitating at grain boundaries is adopted. However, even if such a method is adopted, a steel sheet with a good aging resistance of 40 MPa or less in aging index (AI; difference in tensile stress before and after aging treatment at 100 ° C for 30 minutes after pulling at 7.5%) is obtained. It was difficult.

Further, as described above, the current mainstream of cold-rolled steel sheets is ultra-low carbon steel, and it is considered that over-aging treatment equipment is not required metallurgically in the continuous annealing equipment constructed in recent years accordingly. In addition, due to problems such as equipment construction costs, overage treatment equipment is no longer always available.
As described above, it was extremely difficult to obtain a steel sheet having a good aging resistance of 40 MPa or less in the aging index, particularly when the low carbon killed steel was treated by the continuous annealing equipment which is not always equipped with the overaging treatment equipment.

[0007] In order to obtain a product having good aging resistance by a short overaging treatment, research and development have been promoted.
JP-A-126924 discloses that steel having C and Mn in a predetermined range is coiled at 400 ° C. or less during hot rolling to finely disperse cementite in a hot rolled sheet and to solidify very fine cementite. A method has been proposed in which the amount of dissolved C is reduced by using it as a precipitation nucleus of dissolved C. In addition, Japanese Unexamined Patent Application Publication No.
In Japanese Patent No. 141534, the solid solution N in steel is determined by determining appropriate hot rolling conditions including a slab heating temperature in a low carbon Al killed steel in which Al and N are slightly higher or in a steel sheet in which B is added thereto. A method has been proposed in which AlN 3 and BN are completely fixed, solid solution C is precipitated using the AlN 2 and BN as precipitation nuclei, and temper rolling is performed at a high pressure reduction rate.

[0008]

However, in the method described in JP-A-57-126924, since the coiling temperature is low, an increase in strength cannot be avoided and the rolling reduction in temper rolling is high. Moreover, the above-mentioned Japanese Patent Laid-Open No. 2-14153
According to the method described in Japanese Patent Publication No. 4, a cold-rolled steel sheet having good aging resistance is obtained, but temper rolling at a high pressure reduction ratio is essential, and excellent workability (especially ductility) by any method. )
It was difficult to satisfy both the requirement and the aging resistance.

Therefore, the present invention advantageously solves the problems remaining in the prior art as described above, and requires overaging treatment equipment without temper rolling at a high pressure reduction rate. Even if you do not perform overaging treatment for a long time
It is an object of the present invention to propose a cold-rolled steel sheet having excellent aging resistance and also excellent workability.

[0010]

The cold-rolled steel sheet according to the present invention, which achieves the above-mentioned object, is the result of intensive studies by the inventors.
C: 0.010 to less than 0.06 wt%, Si: 0.05 wt% or less, Mn: 0.
05 to 0.50 wt%, Al: 0.001 to 0.050 wt%, N: 0.0005 to
0.0100 wt% or less and Cr: more than 0.15 wt% and 1.00 wt% or less, and B is 0.5 [N] in relation to the N content [N%].
%] To 3.0 [N%], with the balance being iron and inevitable impurities, which is a cold rolled steel sheet with good aging resistance.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, low carbon killed steel is allowed to contain Cr to form carbides and thus to reduce solid solution C, and further B is added to finely disperse BN in crystal grains. To reduce the amount of solid solution N, and
By utilizing N as a precipitation nucleus of cementite or the like to promote the precipitation of carbides, the solid solution C is further reduced.

The reason why the component composition range is limited in the cold rolled steel sheet of the present invention having good workability and aging resistance will be described below. [C: 0.010 to less than 0.06 wt%] The content of C is 0.010 to 0.
The reason why the range is less than 06 wt% is that the amount of C is reduced to less than 0.010 wt% because the decarburization treatment cost in the steelmaking stage increases remarkably, and the crystal grains are significantly small when 0.06 wt% or more. This is because the elongation (El.) Value becomes small and the workability deteriorates. Preferably 0.01 to 0.04 wt
% Range.

[Si: 0.05 wt% or less] Si content of 0.050
The reason why the content is less than or equal to wt% is that if the Si content exceeds 0.05 wt%, the material is hardened and the workability is deteriorated. When Si is used as a deoxidizing agent in the steelmaking stage, 0.001 wt% or more is preferably contained in order to make the deoxidizing sufficient.

[Mn: 0.05 to 0.50 wt%] The content of Mn is 0.05
The reason for setting the range to 0.50 wt% is that if it is less than 0.05 wt% it is not sufficient to fix S that causes red heat embrittlement as MnS, and if it exceeds 0.50 wt% the material is hardened to improve workability. This is because it not only deteriorates, but also increases the steel cost.

[Al: 0.001 to 0.05 wt%] Since Al is used as a deoxidizing agent in the steelmaking stage, it is necessary to add Al so as to contain 0.001 wt% or more in order to make the deoxidizing sufficient. However, with a large amount of addition exceeding 0.05 wt%
Since AlN is preferentially precipitated, which leads to a decrease in the precipitation of BN that should become a precipitation nucleus of cementite, and unnecessarily increases the steel cost, it is necessary to set it to 0.05 wt% or less. It is preferably 0.03 wt% or less, and
It is more preferable to use deoxidation with Si in combination with the amount of Al in the range of 0.001 to 0.010 wt%.

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

[Cr: more than 0.15 wt% and not more than 1.00 wt%] The reason for setting the Cr content in the range of more than 0.15 wt% and not more than 1.00 wt% is to precipitate Cr _X C _Y and reduce the solid solution C. However, in order to improve the aging resistance, it is necessary to contain at least Cr in excess of 0.15 wt%, while a content in excess of 1.00 wt% increases the material cost. The range was set to more than wt% and 1.00 wt% or less. Cr within this range effectively contributes to improving the elongation (El.) And r value. The range is preferably more than 0.30 wt% and 0.80 wt% or less.

[B: In relation to the amount of N [N%], 0.5 [N
%] To 3.0 [N%]] The reason for setting the amount of B added to the range of 0.5 [N%] to 3.0 [N%] with respect to the amount of N is that the amount of B is less than 0.5 × [N%]. In this case, the amount of residual solid solution N increases and the aging due to this N easily occurs, and the absolute amount of BN as the precipitation nuclei of cementite decreases, and the B content is more than 3.0 x [N%]. Soluble B in quantity
Cause material deterioration.

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

Next, a method of manufacturing the cold rolled steel sheet according to the present invention will be described. Molten steel prepared to have a predetermined composition is made into a slab by the continuous casting method or the ingot-casting method. When hot rolling this slab, the slab once cooled to room temperature may be reheated to 900 ° C or higher, and the slab may be hot or warm at 1100 ° C or lower without being cooled to room temperature. A hot direct rolling (HDR) method or a hot charge rolling (HCR) method of inserting in a heating furnace and holding or reheating may be used.

The lower the temperature of the final pass of the hot rough rolling,
Further, the higher the pass reduction rate, the more advantageous it is to make the γ grain size finer and promote the precipitation of BN to form a carbide precipitation site. Moreover, it is possible to prevent the occurrence of defects called ridging. Therefore, rough rolling is performed at low temperature,
It is preferable to apply a large reduction.

Regarding the hot rolling finishing temperature, in order to finely disperse BN in the steel to promote the precipitation of cementite, and to reduce the scale amount on the surface of the steel sheet to improve the yield amount of the steel material, the α + γ range is set. It is preferable to carry out rolling below.

The coiling temperature after hot rolling is determined by taking into consideration the precipitation of solid solution N to prevent the deterioration of the material due to N aging, the amount of scale formed on the plate surface and the reduction of pickling efficiency.
The temperature is preferably 500 to 800 ° C. More preferably 500
~ 700 ℃ is good.

For the purpose of eliminating the temperature difference between the front and rear ends of the sheet bar before hot rolling and finish rolling, the sheet bar is temporarily fixed by using a coil box provided between the hot rolling rough rolling facility and the finish rolling facility. Even if it is wound up and rewound to reverse the rolling direction, the rear end of the preceding sheet bar and the leading end of the following sheet bar are joined continuously for the purpose of improving the yield of steel products. Even if the sheet bar is rolled on the steel sheet, or even if lubrication 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.

Applying continuous annealing to the annealing after cold rolling facilitates continuation of cleaning equipment and temper rolling equipment,
Moreover, the manufacturing time is greatly shortened as compared with the case of box annealing, and the temperature condition of the continuous annealing is preferably in the range of recrystallization temperature to 850 ° C.

In the cooling process following the heating and soaking of the annealing,
5 without maintaining a constant temperature in the temperature range of 500 to 300 ℃
It is preferable to allow the solution to stay for 60 seconds to accelerate the precipitation of the solid solution C. Such a treatment can be easily performed in the cooling zone even in continuous annealing equipment that is not always equipped with overaging treatment equipment, so that a cold rolled steel sheet with good aging is obtained without overaging treatment equipment. be able to. Of course, there is no problem with overaging treatment that keeps it at a constant temperature in the overaging treatment equipment.In this case, there is no problem if the holding time is longer than 60 seconds, but from the viewpoint of productivity, it should be less than 120 seconds. Is desirable.

After cold rolling, temper rolling is carried out mainly for the purpose of correcting the shape and preventing the occurrence of stretcher strain.
Since the cold-rolled steel sheet of the present invention has good aging resistance even before temper rolling, this temper rolling does not require high pressure. On the contrary, if the rolling reduction is 1.0% or more, work hardening may occur and the yield point may be increased, and the workability may be impaired. Therefore, the rolling reduction is preferably less than 1.0%.

[0028]

[Example] Thickness 300 having various composition shown in Table 1
After heating a steel slab of mm to 970 to 1100 ° C, various manufacturing conditions shown in Table 2, that is, rough rolling in 3 passes and variously changing the temperature and reduction rate of the final pass, the plate thickness 30 mm.
The sheet bar was subjected to hot rolling using a 7-stand finishing rolling mill so that the finishing temperature was 720 to 850 ° C. and the finishing plate thickness was 2.8 mm. Subsequently, it was wound at 660 ° C. or lower, pickled, and then cold rolled to obtain a cold rolled sheet having a sheet thickness of 0.7 mm. After that, recrystallization annealing was performed by a heat cycle as shown in FIG. 1 in a continuous annealing furnace, and then temper rolling with a reduction rate of 0.8% was performed.

[0029]

[Table 1]

[0030]

[Table 2]

From the steel sheet thus obtained, JIS No. 5 tensile test pieces were sampled in the directions of 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 each direction calculated by the following formula. X = (X ₀ + 2X ₄₅ + X ₉₀ ) / 4, where X ₀ : characteristic value in 0 ° direction with respect to rolling direction, X ₄₅ : characteristic value in 45 ° direction with respect to rolling direction, X ₉₀ : rolling direction Characteristic value in the direction of 90 °,

[0032]

[Table 3]

From Table 3, the cold rolled steel sheet having the composition according to the present invention has an elongation (El.) Of 45% or more and an aging index (AI).
Good aging resistance and workability of 40 MPa or less and r value of 1.6 or more were obtained. On the other hand, No. 9, No. 10,
Steel sheets No. 11 are steel symbols H, I, J, or Cr, respectively.
Since each of the steels had a small content, the aging index was high, and the elongation and the r value were low. Further, the No. 10 steel had a high steel code I, that is, an Al content, so that the precipitation of cementite having BN as a precipitation nucleus was small, the aging index was high, and the elongation and the r value were low. The steel plate of No. 11 had a steel symbol J, that is, the amount of solid solution B was too large, and thus the elongation and r value were low.
The steel plate of No. 12 had a high aging index because it was steel K, that is, a steel containing a large amount of solid solution N.

[0034]

INDUSTRIAL APPLICABILITY The cold-rolled steel sheet of the present invention can obtain a good non-aging property by making the carbide of Cr and BN act as precipitation nuclei of cementite for a low-carbon steel which is less expensive than an ultra-low carbon steel. . Thus, since the precipitation accelerating treatment of the solid solution C during the continuous annealing can be performed by the non-constant temperature treatment for an extremely short time, the stripability of the continuous annealing equipment is good, the line speed is easily increased, and the mass production is possible. Therefore, the manufacturing cost can be reduced. Further, since temper rolling at a high pressure reduction ratio is not required, workability and aging resistance can be compatible at a high level.

[Brief description of drawings]

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

Claims (1)

[Claims] 1. C: 0.010 to less than 0.06 wt%, Si: 0.05 wt% or less, Mn: 0.05 to 0.50 wt%, Al: 0.001 to 0.050 wt%, N: 0.0005 to 0.0100 wt% or less and Cr: 0.15 wt. % Over 1.
Aging resistance consisting of less than 00 wt% and B in a range satisfying 0.5 [N%] to 3.0 [N%] in relation to the N content [N%], with the balance being iron and inevitable impurities. Cold rolled steel sheet with good properties.