JPH04276023A - Manufacture of cold rolled steel sheet for working excellent in surface property by continuous annealing - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet having excellent surface properties by regulating the valley depth and average roughness in the center line of low carbon steel having a specified compsn. to specified value by cold rolling and successively executing continuous annealing under specified averaging conditions and continuous annealing at a specified draft. CONSTITUTION:A slab constituted of, by weight, 0.010 to 0.055% C, 0.05 to O.40% Mn, <=0.080% P, 0.002 to 0.020% S, 0.010 to 0.070% Al, <=0.0070% N, 0.0010 to 0.0070% B and the balance Fe with inevitable impurities is subjected to hot rolling and cold rolling to regulate the center line valley depth Rv and the center line average roughness Ra on the surface of the steel sheet respectively to 0.5 to 10mum and >=0.5mum. This steel sheet is furthermore subjected to continuous annealing with averaging in such a manner that it is annealed at 680 to 850 deg.C, is cooled to the range of 200 to 400 deg.C at >=50 deg.C/sec and is subjected to heating by >=20 deg.C over the cooling end temp. to regulate the heating arrival temp. to the range of 250 to 450 deg.C and is cooled to the range of 380 to 200 deg.C for >=120sec and is subjected to skinpass rolling at 0.7 to 3.5% elongation percentage.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a method for producing cold rolled steel sheets for processing by continuous annealing, and in which the centerline average roughness Ra and the center of the steel sheet surface are By setting the line valley depth Rv within a specific range, performing continuous annealing with a specific overaging, and then performing skin pass rolling in a specific elongation range, the surface texture of the cold rolled steel sheet after press working can be improved. The present invention provides a method for producing an excellent low carbon cold rolled steel sheet.

0002

BACKGROUND OF THE INVENTION From the viewpoint of aesthetics, it is necessary to prevent stretcher strain caused by the yield elongation phenomenon during processing for cold-rolled steel sheets used for press working. In order to produce cold-rolled steel sheets made of low carbon steel with good stretcher strain resistance, it is necessary to reduce the amount of solid solute carbon after annealing.
Furthermore, it is necessary to introduce dislocations into the steel sheet by light processing such as temper rolling. As a method of reducing solid solution carbon in a low carbon steel sheet, there is a method of performing overaging treatment by rapidly cooling to 200 to 310 ° C. after recrystallization annealing, as described in JP-A-2-93025, for example. Solute carbon in the steel plate can be sufficiently reduced. However, depending on the unevenness of the surface of the steel sheet after cold rolling, it may be difficult for the dislocations introduced by temper rolling to be sufficiently uniform, and some areas may be formed where dislocations are not introduced by temper rolling. be. When such a cold-rolled steel sheet is pressed, minute stretcher strains occur over the entire surface of the steel sheet, impairing its aesthetic appearance.

0003

[Problems to be Solved by the Invention] The present invention provides a cold-rolled steel plate stretcher manufactured by continuous annealing, which is advantageous in terms of productivity and manufacturing cost, using low-cost B-containing low carbon steel as a material. The object of the present invention is to provide a method for improving surface appearance by completely eliminating strain.

0004

[Means for Solving the Problems] The present inventors have studied manufacturing conditions for improving the surface quality in processing a B-containing low carbon cold rolled steel sheet manufactured by a continuous annealing method. As a result, after performing cold rolling so that the centerline valley depth (Rv) and centerline average roughness (Ra) of the steel sheet surface were within a specific range, and further performing continuous annealing with a specific overaging, It has been discovered that stretcher strain can be completely eliminated by skin pass rolling with an elongation rate within a specific range, and that a cold rolled steel sheet with excellent surface properties free of stretcher strain even after aging can be produced.

The gist of the invention is as follows. C: 0.010-0.055%, Mn: in weight ratio
0.05-0.40%, P: 0.080% or less, S: 0
．． 002-0.020%, Al: 0.010-0.07
0%, N: 0.0070% or less, B: 0.0010 to 0
．． After hot-rolling a slab containing 0.0070% and the remainder consisting of Fe and unavoidable impurities, it is cold-rolled to determine the centerline valley depth (Rv) and centerline average of the steel plate surface after cold rolling. The roughness (Ra) is 0.5 to 10 μm and 0.5 μm, respectively.
5 μm or more, and after annealing at 680 to 850°C 2
00 to 400 °C at a cooling rate of 50 °C/s or more, heating to a temperature of 20 °C or more with respect to the cooling end point temperature to reach a heating temperature in the range of 250 to 450 °C, and 380 to 2
Continuous annealing with overaging is performed in the range of 0.00°C over a period of 120 seconds or more, followed by continuous annealing with an overaging temperature of 0.7 to 3.0°C.
This is a method for producing cold rolled steel sheets with excellent surface properties, characterized by performing temper rolling with an elongation rate of 5%.

First, the reasons for limiting the chemical composition of steel to which the method of the present invention is applied will be explained. If C is less than 0.010%, it is not desirable because aging deterioration after continuous annealing is large. Moreover, if it exceeds 0.055%, the processability of the product will deteriorate. Therefore, the amount of C was limited to 0.010% to 0.055%. Mn is a necessary component to prevent hot embrittlement, but if it is less than 0.05%, FeS is generated and there is no effect. Moreover, if it exceeds 0.40%, deep drawability deteriorates. Therefore, the amount of Mn was limited to 0.05% to 0.40%. P is an element that does not greatly affect the aging properties, but if the content increases, the workability decreases, so the lower the content, the better, and the upper limit should be 0.080%. When S is less than 0.002%, the amount of MnS produced is small and the crystal grains of the hot-rolled sheet tend to become coarse, causing surface roughness. Also 0.
If it exceeds 0.020%, it causes hot embrittlement. Therefore, the amount of S was limited to 0.002% to 0.020%.

[0007] In the present invention, N in steel is mainly fixed by B, but Al is required to fix the small amount of N that cannot be fixed by B. If the amount of nitrogen and B are within the range of the present invention, Al is 0.010
% or more is necessary. However, when it exceeds 0.070%, workability deteriorates. Therefore, the amount of Al is 0.010
It was limited to ~0.070%.

[0008] N not only hardens the cold-rolled steel sheet and deteriorates workability, but also causes stretcher strain, so the less the better, and the content should be 0.0070% or less. B is an element necessary to fix N in steel as BN and prevent material deterioration due to N. For this purpose, a minimum content of 0.0010% is required. However, 0.00
If it exceeds 70%, the amount of solid solution B increases, causing material deterioration. Therefore, the amount of B is 0.0003 to 0.00
Limited to 70%.

Next, cold rolling conditions will be described. The cold rolling rate may be as usual, but is preferably 60% or more in order to obtain a steel plate with better workability. In addition, the present inventors have found that minute irregularities on the surface formed by cold rolling affect the surface quality during processing, and in particular, centerline valley depth (Rv), centerline average roughness (
It has been found that the elongation rate during temper rolling is closely related to stretcher strain.

[0010] Figure 1 shows a steel having a composition within the range of the present invention shown in Table 1, which is hot-rolled and has a center line average roughness (Ra) of 0.8 μm.
Total rolling reduction rate 72 with various centerline valley depths (Rv)
% cold rolling, annealing at 770°C and then rolling to 300°C.
Cooled at a cooling rate of 00°C/s, reheated to 350°C,
A stretch test was conducted on a cold-rolled steel sheet manufactured by continuous annealing with overaging in which the steel was cooled at a constant rate of 140 seconds to 280°C, followed by skin pass rolling at an elongation rate of 1.2%, and the effects on the surface texture were evaluated. This figure shows the influence of centerline valley depth (Rv).

[0011]

[Table 1]

From FIG. 1, the center line valley depth (Rv) is 0.5~
It can be seen that a good surface quality is obtained within the range of 10 μm, and stretcher strain occurs outside this range. This is because when the centerline valley depth (Rv) is less than 0.5 μm, the density of mobile dislocations varies, and the centerline valley depth (Rv)
This is because if v) exceeds 10 μm, the mobile dislocation density in the valley portions becomes insufficient in temper rolling with an elongation rate that does not cause material deterioration, and both conditions are conditions for stretcher strain to occur.

FIG. 2 also shows that among the above conditions, the centerline valley depth (Rv) is constant at 5.0 μm, and the centerline average roughness (Rv) is
This figure shows the influence of center line average roughness (Ra) on the surface quality when a) is varied. It can be seen from FIG. 2 that good surface quality can be obtained when the center line average roughness (Ra) is 0.5 μm or more.

The reason why a center line average roughness (Ra) of 0.5 μm or more provides a good surface quality is that mobile dislocations are uniformly introduced. In addition, the center line average roughness (Ra) is 0.
If the thickness is less than 5 μm, the passability of the continuous annealing equipment will deteriorate, and sliding scratches will easily occur on the surface, which is not preferable.

From the above, it is necessary to make the surface condition of the steel plate by cold rolling such that the centerline valley depth (Rv) is 0.5 to 10 μm and the centerline average roughness (Ra) is 0.5 μm or more. . Note that similar results were obtained when conditions other than the centerline valley depth (Rv) and centerline average roughness (Ra) were changed within the scope of the present invention. In addition, the above center line average roughness (R
a) and centerline valley depth (Rv) were obtained by adjusting the uneven state of the cold rolling roll surface.

Next, continuous annealing conditions will be described. Continuous annealing requires special overaging treatment to improve the aging properties after recrystallization annealing. The recrystallization annealing temperature is 680°C in order to provide good formability as a cold-rolled steel sheet for processing.
The above is necessary. However, when the temperature exceeds 850°C, abnormal grain growth tends to occur and the surface quality deteriorates. Therefore, the recrystallization annealing temperature was set at 680 to 850°C.

[0017] Furthermore, in order to improve the surface quality during processing, it is necessary to sufficiently reduce solute carbon to obtain good aging characteristics, and for this purpose, the following overaging conditions are required. First, in order to form cementite nuclei within the crystal grains, after recrystallization annealing, a cooling rate of 50°C/s or more is applied to
It is necessary to cool to 400°C. Next, in order to increase the precipitation rate of cementite,
Reheat at a high temperature of 250 to 450°C. Furthermore, in order to sufficiently precipitate the dissolved carbon as cementite, the overaging time is set to 120 seconds or more, and as the overaging progresses, the overaging temperature is lowered to 200 to 380°C.
ends the statute of limitations. The above-mentioned over-aging treatment reduces the solute carbon in the cold-rolled steel sheet containing the ingredients of the present invention, and when used in combination with the temper rolling conditions described below, it is possible to maintain good surface quality without stretcher strain even after aging. It can be made of steel plate.

[0018] Temper rolling is necessary to introduce movable dislocations into the steel sheet and to prevent stretcher strain from occurring during press forming. Figure 3 shows skin pass rolling under the conditions shown in Figure 1, with the centerline valley depth (Rv) constant at 5.0μm and the centerline average roughness (Ra) constant at 1.0μm, and the elongation rate varied. This figure shows the influence of the elongation rate of temper rolling on the surface texture and elongation when temper rolling is carried out. From FIG. 3, skin pass rolling with an elongation rate of 0.7% or more can prevent stretcher strain during press molding and provide good surface properties. However, when the elongation rate in temper rolling exceeds 3.5%, the elongation deteriorates significantly. Therefore, the elongation rate of temper rolling is 0.7 to 3.
It needs to be 5%. In addition, even when conditions other than skin pass rolling were changed within the range of the present invention, it was found that the elongation rate of skin pass rolling must be 0.7 to 3.5%, similar to the present results. As described above, the present invention improves the surface quality and workability of a cold rolled steel sheet by controlling the surface condition of the steel sheet during cold rolling, the solid solution carbon during continuous annealing, the elongation rate during temper rolling, etc. be able to.

[0019]

[Example] [Example 1] 1 slab of chemical composition shown in Table 2
After heating to 150° C. and hot rolling, and further cold rolling, the continuous annealing conditions shown in FIG. 4 were applied. Samples A, B, C, D, E, F, G, H, I, J and K
are within the component range of the present invention, and samples L, M, N, O, P,
Any of the chemical components of Q, R, S and T is outside the scope of the present invention. The cold rolling rate of these steels was 72%, and the surface condition of the cold rolled steel sheets was within the range of the present invention.The centerline valley depth (Rv) was 5 μm, and the centerline average roughness (Rv) was 5 μm.
a) is 1.2 μm. Further, the temper rolling conditions after continuous annealing were also set to 1.0%, which is within the scope of the present invention.

[0020] Regarding the cold-rolled steel sheet obtained as a result, JI
A tensile test was conducted using a No. S5 tensile test piece to investigate surface properties, yield strength, and elongation. The results of the material investigation are shown in Table 3. From Table 3, it can be seen that those whose chemical components are outside the scope of the present invention are inferior in surface quality and/or elongation compared to those within the scope of the present invention.

[0021]

[Table 2]

[0022]

[Table 3]

[Example 2] Slabs having the chemical composition shown in Table 4 were heated to 1150°C and hot rolled to give the cold rolling surface condition and temper rolling elongation as shown in Table 5. Ta. The total reduction ratio of cold rolling was 72% in each case, and the surface condition of the steel in samples 1 to 9 was within the range of the present invention, and in sample 10.
~16 is centerline valley depth (Rv), centerline average roughness (Ra
) or temper rolling elongation are out of the scope of the present invention. In addition, both samples are shown in Figure 4.
The continuous annealing conditions shown below were given. The resulting cold-rolled steel sheet was subjected to a tensile test using a JIS No. 5 tensile test piece to investigate its elongation and surface properties. Table 6 shows the results of the material investigation. From Table 6, it can be seen that those whose components and conditions fell within the scope of the present invention were superior in both surface quality and workability compared to those outside the scope of the present invention.

[0024]

[Table 4]

[0025]

[Table 5]

[0026]

[Table 6]

[0027]

Effects of the Invention: When producing a low carbon cold rolled steel sheet containing B, according to the present invention, the centerline average roughness (R
a) Cold rolling is performed so that the centerline valley depth (Rv) is within a specific range, and then continuous annealing is performed with a specific overaging, followed by skin pass rolling with a specific elongation rate. Stretcher strain can be completely eliminated, and a cold-rolled steel sheet with excellent surface properties without stretcher strain even after aging can be produced.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the influence of centerline valley depth (Rv) on surface texture.

FIG. 2 is a chart showing the influence of center line average roughness (Ra) on surface texture.

FIG. 3 is a chart showing the influence of the elongation rate of skin pass rolling on the surface texture and elongation.

FIG. 4 is a chart showing the temperature history of continuous annealing.

Claims (1)

[Claims] Claim 1: C: 0.010 to 0.055%, Mn: 0.05 to 0.40%, P: 0.080% or less, S: 0.002 to 0.020%, in weight ratio. After hot rolling a slab consisting of Al: 0.010 to 0.070%, N: 0.0070% or less, B: 0.0010 to 0.0070%, the balance being Fe and unavoidable impurities, then cold rolling. Rolling is carried out, and the centerline valley depth (Rv) and centerline average roughness (Ra) of the steel plate surface after cold rolling are set to 0.5 to 10 μm and 0.5 μm or more, respectively, and further at 680 to 850 ° C. After annealing, cool to a range of 200 to 400 °C at a cooling rate of 50 °C / s or more, and heat to a temperature of 20 °C or more with respect to the cooling end point temperature to reach a heating temperature of 250 to 4.
50℃ range, 120s in the range of 380-200℃
Continuous annealing with overaging in which cooling takes a time of ec or more is performed, followed by skin pass rolling with an elongation rate of 0.7 to 3.5%. A method for producing cold-rolled steel sheets.