JPH05222460A - Production of cold rolled steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet extremely excellent in press formability by removing the strain inevitably introduced in the ordinary process of cold rolled steel sheet production into a steel sheet and making the most of the workability inherent in a material. CONSTITUTION:The cold rolled steel sheet can be obtained by subjecting a steel which has a composition consisting of <=0.0025% C, <=0.30% Si, <=0.30% Mn, <=0.010% P, 0.010-0.100% Al, Ti by the amount satisfying 4C+3.43N<=Ti<=0.080%, <=0.005% N, and the balance iron with inevitable impurities and further containing, if necessary, <=0.020% Nb and/or <=0.0015% B to hot rolling, to coiling at 600-750 deg.C, to picking, and to cold rolling and subjecting the resulting sheet to continuous annealing at a temp. between 800 deg.C and the Ac3 transformation point and then to tight coil box annealing at 500-750 deg.C. The sheet can be subjected, after continuous annealing, to temper rolling at <=5.0% and then to tight coil box annealing at 550-750 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolling excellent in press formability, which is suitable for processing large integrally formed parts such as automobile side panels and difficult-to-form parts such as oil pans. The present invention relates to a method for manufacturing a steel plate.

[0002]

2. Description of the Prior Art Ductility,
As a cold-rolled steel sheet with excellent deep drawability, carbonitride-forming elements such as Ti or Nb have been added to ultra-low carbon steel,
So-called I in which solid solution C and N in steel are fixed as precipitates
The method of producing F steel by continuous annealing is well known.

For example, Japanese Patent Laid-Open No. 61-246344 proposes a Ti-added ultra-low carbon steel sheet as a cold-rolled steel sheet for super deep drawing which is excellent in secondary work embrittlement resistance, and Japanese Patent Publication No. 61-32375. Japanese Patent Publication proposes a method for producing an ultra-low carbon steel sheet to which Ti and Nb are added in combination.

Further, in recent years, press formability higher than that of conventional materials has been required for materials, and as a method for improving workability by making these ultra-low carbon IF steels into higher purity steels, it has been a special method. Japanese Patent Publication No. 1-40895 and Japanese Patent Laid-Open No. 1-177.
No. 322 publication is proposed.

However, when these high-purity ultra-low carbon IF steels are manufactured in a continuous annealing line, particularly in the case of a vertical type line which is generally used, it is possible to perform, for example, an overaging zone when rolling. Bending stress caused by rolls in a furnace in a low temperature range causes an increase in yield point (YP), resulting in deterioration of formability of the steel sheet. That is,
In press molding, a minimum wrinkle holding pressure must be applied so that wrinkles do not occur, but if YP is high, it is necessary to increase this wrinkle holding pressure, which not only narrows down the formable condition but also causes material inflow. Since it is suppressed, the molding itself may not be possible.

In the vertical continuous annealing furnace, as shown in FIG.
By bending the strip with the upper and lower rolls, a longer heat-treatable strip length is secured relative to the furnace length.
Although heating, soaking, and cooling are performed, each time the strip is bent, the strip is subjected to bending stress determined by the roll diameter, tension, plate thickness, and the like. Usually, the problem is in a low temperature furnace where strain is not released, for example,
It is the bending stress due to the hearth roll in the over-aged zone, and in terms of equipment design, a large diameter roll with a certain diameter or more is adopted so as not to adversely affect the material.

Particularly, in a line for treating low carbon Al killed steel sheets, for example, "Iron and Steel", 61st year (1975).
No. 10, p. 2363, "Study on Continuous Annealing of Low Carbon Capped Thin Steel Sheets", the roll diameter is set so as to satisfy bending stress σ ≦ 13 kgf / mm ² in order to suppress material deterioration. A roll with a bending stress of 13 kgf / mm ² or less and as close as possible to 13 kgf / mm ² is used, but if the roll diameter is too large, the equipment will become large and the cost of the roll itself will increase. It is usual to adopt a diameter close to the lower limit critical value.

However, in low carbon Al killed steel sheets and products, Y
Ultra-low carbon with relatively low purity P of 13 to 15 kgf / mm ²
If it is F steel plate, this bending stress does not cause any problem, but in high-purity IF steel, the YP of the material itself is 8 to 11 kgf.
/ Mm ² , the roll bending stress is larger than YP.
The problem arises that YP increases.

Recently, a continuous annealing furnace having no overaging zone has been constructed, but the situation is exactly the same as long as it is subjected to bending deformation by rolls in a low temperature region such as a cooling zone where strain is not released.

Further, it is considered that similar bending stress is also generated when the mandrel is wound while being tensioned. In this case, even in a horizontal continuous annealing furnace which is not subjected to bending stress by rolls in the furnace, It is thought that the above-mentioned problems will occur.

[0011] Further, temper rolling is a high-purity ultra-low carbon IF.
In the case of steel, the ductility is remarkably deteriorated, so it is usual to carry out rolling at the lowest elongation rate at which the required roughness can be obtained, but as the rolling itself introduces strain in the steel sheet, as a result, It only promotes the problem of.

The present invention solves the above-mentioned problems of the prior art, removes the strain that is inevitably introduced into the steel sheet in the ordinary cold-rolled steel sheet manufacturing process, and maximizes the original workability of the material. Thus, it is an object of the present invention to provide a method capable of producing a cold rolled steel sheet having extremely excellent press formability.

[0013]

Means for Solving the Problems The present inventor has completed the present invention as a result of earnest studies for solving the above problems.

That is, according to the present invention, C ≦ 0.0025.
%, Si ≦ 0.30%, Mn ≦ 0.30%, P ≦ 0.010
%, Al: 0.010 to 0.100%, 4C + 3.43N ≦
Ti ≤ 0.080%, N ≤ 0.005%, and if necessary, one or two of Nb ≤ 0.020% and B ≤ 0.0015%, with the balance being iron and unavoidable. Steel consisting of mechanical impurities is wound at 600 to 750 ° C after hot rolling,
After pickling and cold rolling, it is characterized by continuous annealing at 800 ℃ ~ Ac ₃ transformation point or less, then after tempering rolling at 5.0% or less, and then annealing at 500 ~ 750 ℃ tight coil box. The gist is a method for producing a cold-rolled steel sheet having excellent press formability.

The present invention will be described in more detail below.

[0016]

[Action]

First, the reasons for limiting the chemical composition of steel in the present invention will be explained.

C: C is preferably as small as possible in order to improve press workability, particularly ductility. If it exceeds 0.0025%, not only does the effect not appear sufficiently, but also solid solution C and N in the steel.
The amount of addition of Ti required to precipitate and fix the particles increases. Therefore, the C content is 0.0025% or less.

Si: Since Si deteriorates the chemical conversion treatment property,
The smaller the amount, the more preferable, and it is 0.30% or less.

Mn: Since a large amount of Mn deteriorates the workability, it is set to 0.30% or less.

P: P lowers the grain boundary strength in a solid solution state and causes secondary work embrittlement, and as a precipitate,
A hot rolled sheet which has been wound at a high temperature forms a compound with Ti and deteriorates the deep drawability, so it is an element to be reduced as much as possible, so in the present invention, it is set to 0.010% or less.

Al: Al is at least 0.010% for deoxidation.
Need to be added. However, the addition of more than 0.10% not only increases the cost but also deteriorates the surface properties, so the Al content is set in the range of 0.010 to 0.100%.

N: If the amount of N is too large, not only the amount of Ti necessary for precipitation and fixation increases but not only the cost increases, but also
Since the workability is deteriorated, it is set to 0.005% or less.

Ti: Ti is added to mainly fix and fix C and N in steel as precipitates. For that, 4
Addition of C + 3.43N or more is required. 4C + 3.43
When it is less than N, deep drawability and aging are deteriorated. On the other hand, if the amount of addition exceeds 0.080%, not only the effect is saturated but also the cost increases, so the upper limit is 0.080%.

Further, in the present invention, if necessary, N
It is possible to add one kind or a combination of b and B, and thereby further improve the material quality.

Nb: Adding a small amount of Nb has the effect of reducing the grain size of the hot rolled sheet and improving the deep drawability after cold rolling annealing. However, the addition of more than 0.020% raises the recrystallization temperature and requires the annealing temperature to be higher, so the Nb content is set to 0.020% or less.

B: B is effective for improving the secondary work embrittlement resistance, but if it is added in an amount exceeding 0.0015%, the deep drawability deteriorates, so 0.0005% is the upper limit.

Next, the manufacturing conditions of the present invention will be described.

The steel having the above chemical composition is produced by continuously casting a slab according to a conventional method and hot rolling it. The heating temperature of the slab is not particularly limited, but is 1000
It suffices to carry out at ˜1300 ° C., and the lower the temperature, the better the workability. In the continuous cast slab, the cold pieces may be reheated or the high temperature slab may be directly charged into the heating furnace, or may be directly rolled without being charged into the heating furnace.

Further, the hot rolling finishing temperature has sufficient characteristics in high purity steel even if it is not necessarily higher than the Ar ₃ transformation point, but it is desirable to control it to 850 to 950 ° C.

However, it is necessary to set the coiling temperature to a high temperature of 600 to 750 ° C. and sufficiently precipitate carbonitrides in the steel as a compound of Ti, which is expected to improve workability, particularly deep drawability. it can. If the temperature is lower than 600 ° C, precipitation of carbonitrides is not sufficient, and winding at a temperature higher than 750 ° C is not preferable because the amount of scale produced increases.

Cooling until the winding is performed as quickly as possible, for example, 50 ° C./sec to 120 ° C./sec, and the hot-rolled sheet is made finer by cooling immediately after the finish rolling mill. It is desirable to improve the deep drawability after annealing and prevent abnormal coarsening of crystal grains.

Next, after pickling, cold rolling is carried out, but cold rolling is preferably carried out at a rolling ratio of 50 to 90% from the viewpoint of deep drawability.

After cold rolling, continuous annealing is carried out. The continuous annealing is carried out at a soaking temperature of 800 ° C. to Ac ₃ transformation point.
Improves ductility, deep drawability, and promotes grain growth to reduce YP. The heating rate and the cooling rate are not particularly limited, but from the viewpoint of productivity, a heating rate of 5 ° C./sec or more is preferable, and a heating rate of 30 ° C./sec or less is preferable in order to prevent deterioration of characteristics due to an increase in recrystallization temperature. When C is re-dissolved at high temperature and remains at room temperature during cooling, strain aging occurs due to strain and box annealing after continuous annealing, and the characteristics deteriorate. Keep in mind that cooling is done at a rate that is not too fast, preferably from the soaking temperature to 7
It is preferable that the temperature up to 00 ° C is 30 ° C / sec or less on average.

Temper rolling is carried out after continuous annealing, if necessary, in order to adjust the roughness of the steel sheet. When temper rolling is performed, since the steel of the present invention is soft, no special large reduction is necessary, and the rolling may be performed at the minimum elongation. After temper rolling, annealing is performed again to remove the strain, so there is no upper limit to the reduction ratio from the viewpoint of characteristics, but a reduction greater than 5.0% requires a large facility, so This is the upper limit.

In the present invention, the cold-rolled steel sheet produced in the above-mentioned ordinary process is annealed again to remove the strain which is inevitably introduced in the ordinary process to further improve the workability. Tight coil 500-75 for this annealing
Box annealing is performed at 0 ° C. Open coil box annealing or continuous annealing is not preferable because unwinding of the coil or bending with a roll during rolling, and distortion again when winding after processing, are not preferable. It is better to reanneal as it is. The tight coil annealing temperature is set to 500 to 750 ° C., but if it is less than 500 ° C., the effect of removing strain is not sufficient, and if it exceeds 750 ° C., seizure occurs, so the temperature should be controlled within this temperature range.

Although coil winding is a problem at the time of press forming, there is no problem if the leveler is used to correct the shape with very little distortion introduced into the steel plate and almost no deterioration in workability. ..

When various types of plating such as zinc plating are applied to the cold-rolled steel sheet obtained by the present invention, when a horizontal continuous line is subjected to stress during winding, it is subjected to temper rolling. Since the strain caused by rolling is larger, a plated steel sheet having workability superior to that of the conventional material can be obtained.

Next, examples of the present invention will be shown.

[0040]

Example 1

[Table 1] Steel with the chemical composition shown in Figure 3 was melted and continuously cast into a slab. This slab is heated to a slab heating temperature of 1150 to 1200
℃, finishing temperature 900 ~ 940 ℃ hot rolled to a thickness of 3.6mm,

[Table 2] It was wound at the winding temperature shown in. Then, after pickling, cold rolling was performed to a thickness of 0.8 mm, continuous annealing (overaging treatment: 400 ° C. × 220 sec) was performed under the soaking conditions shown in Table 2, and then temper rolling was performed to obtain an elongation of 0.5%. Conducted at a rate. Then, re-annealing was performed under the box annealing conditions shown in Table 2 to investigate the mechanical properties.
The results of the survey are also shown in Table 2.

As is clear from Table 2, the invention examples (No.
1, No. 3, No. 5, No. 7) are all 10 kgf / mm ²
It has the following low YP, high ductility of 50% or more, and super deep drawability of r value of 2.0 or more, while Comparative Examples (No. 2, N)
No. 4, No. 6, No. 8 to No. 12) cannot satisfy all of these values.

[0042]

Example 2 The slab of steel A in Table 1 was heated to slab temperature 1
It was hot-rolled to a thickness of 3.6 mm at 150 ° C and a finishing temperature of 900 to 920 ° C, and wound at 700 ° C. After pickling, board thickness 0.8m
After cold rolling to m and continuous annealing at 870 ° C,

[Table 3] The temper rolling and box annealing were carried out under the conditions shown in. A leveler was applied to the obtained cold rolled steel sheet, and the mechanical properties of each were investigated. The results are also shown in Table 3. From Table 3, it is shown that the presence or absence of temper rolling before box annealing and the presence or absence of leveler after box annealing have almost no adverse effect on the mechanical properties of the product.

[0043]

As described in detail above, according to the present invention,
The workability originally possessed by the material can be maximized, and a cold rolled steel sheet having extremely excellent press formability can be manufactured. It is suitable for processing large integrally molded parts such as automobile side panels and difficult-to-mold parts such as oil pans.

[Brief description of drawings]

FIG. 1 is a diagram showing a strip passing state in a vertical continuous annealing furnace.

FIG. 2 is an enlarged view showing a strip passing state in a vertical continuous annealing furnace.

Claims (3)

[Claims] 1. In weight% (hereinafter the same), C ≦ 0.002
5%, Si≤0.30%, Mn≤0.30%, P≤0.01
0%, Al: 0.000 to 0.100%, 4C + 3.43N
A steel containing ≦ Ti ≦ 0.080% and N ≦ 0.005%, the balance of which is iron and unavoidable impurities, is 600 after hot rolling.
After winding at ~ 750 ℃, pickling, cold rolling, 800 ℃ ~ A
c ₃ consecutive below the transformation point annealed, then, 500 to 750 ° C.
A method for producing a cold-rolled steel sheet having excellent press formability, which comprises annealing in a tight coil box. 2. The steel further comprises Nb ≦ 0.020% and B ≦
The method according to claim 1, comprising one or two of 0.0015%. 3. The method according to claim 1, wherein after continuous annealing, temper rolling is performed at 5.0% or less, and then tight coil box annealing is performed at 550 to 750 ° C.