JPS5993834A - Manufacture of cold rolled steel sheet with superior press formability - Google Patents

Abstract

PURPOSE:To obtain easily a cold rolled steel sheet with high deep drawability by a continuous annealing method by restricting the total amount of Ni, Cr and Cu in a dead soft steel having a specified composition and conditions during the hot rolling of a slab of the steel. CONSTITUTION:A molten steel consisting of, by weight, <=0.002% C, 0.05-0.20% Mn, 0.010-0.100% sol. Al, 0.06-0.20% Ni+Cr+Cu and the balance Fe is continuously cast to form a slab. The slab is hot rolled at >=95% total draft in finish rolling or at >=44% average draft at each stand, and the resulting hot rolled steel strip is coiled at <=580 deg.C. The strip is pickled, cold rolled, and continuously annealed to obtain a cold rolled steel sheet with superior press formability.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent press formability, and particularly to a method for manufacturing a cold rolled steel sheet with excellent deep drawability and anisotropy.

Conventionally, cold-rolled steel sheets with good drawability and ductility have been manufactured by box annealing. However, the box annealing method not only takes several days to complete the process, but also has difficulty in obtaining uniform heat and strength over the entire coil, as the heating and cooling rates differ in the radial direction of the coil as the coil is heat treated. Met. By using the continuous annealing method, it is possible to eliminate these drawbacks of the box annealing method. However, since continuous annealing involves rapid heating and rapid cooling, the growth of crystal grains is poor, and the precipitation of C dissolved in solid solution in the steel does not progress, resulting in hardness and poor drawability and aging resistance. In order to eliminate these drawbacks of the continuous annealing method, by winding at a high temperature during hot rolling, grain growth is promoted in a direction that is advantageous for drawability, and after rapid cooling during continuous annealing, rolling at 300 to 500 °C for several seconds By performing over-aging treatment for ~ several minutes, precipitation of unprecipitated solid solution C is promoted,
A method to improve aging properties has been proposed, but high-temperature coiling during hot rolling is accompanied by a decline in pickling properties, and steel sheets produced by this method still have poor drawability, ductility, and heat resistance. Inferior to annealed material.

On the other hand, since the main cause of deteriorating the aging resistance of continuously annealed materials is solid solution C, the C content was reduced to 0.005.
A method has been proposed to improve aging resistance using ultra-low carbon steel material with carbon content reduced to 0% or less. Furthermore, Japanese Patent Application Laid-open No. 58333/1983 is a representative technique for producing steel sheets with good deep drawability using ultra-low carbon steel. According to an example in the same publication, steel with C: 0.0020% is heated to 1100°C, hot-rolled at a finishing temperature of 865-870°C, and a coiling temperature of 550-610°C.
It is said that a steel plate with a silanc tsode value (lower value) of 175 to 2.44 can be obtained by rolling in a range of 1 and continuously annealing with rapid heating. However, it is a well-known fact that under such manufacturing conditions, although the lower value is relatively high, the anisotropy increases.

Therefore, in order to improve the large anisotropy of ultra-low carbon steel, N
A method of adding carbonitride-forming elements such as Ti, Ti, etc. has also been proposed, but this method has the serious drawback that carbonitrides cause surface defects.

The purpose of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for producing cold rolled steel sheets with excellent press formability by continuous annealing. It is.

That is, in terms of weight ratio, C: 0.002% or less, Mn:
0.05~0.20%, 5otAt: o, ol, 0~
0100%, (Ni+Cr+Cu): 0.06-0.
A step in which molten steel containing 20% Fe and unavoidable impurities is converted into a cis slab by continuous casting, and the slab is finished rolled with a total reduction rate of 95% or more or an average reduction rate of each stand of 44%. A cold-rolled steel sheet with excellent press formability, characterized by comprising a hot-rolling step of winding at 580° C. or lower, and a step of pickling the hot-rolled copper strip, continuously annealing it after cold rolling. This is a manufacturing method.

The present inventors have discovered that by limiting the chemical composition and hot rolling conditions, it is possible to easily produce a steel sheet with good deep drawing properties. The basic experiment that led to this result will be explained. In other words, steel with the chemical composition shown in Table 1 is melted using a bottom blowing converter and an RH degassing device, cast into a slab using a continuous casting machine, reheated to 1.100°C, and then passed through a four-high rough rolling mill and a seven-stage rough rolling mill. A hot-rolled steel strip of Jii 3.2 tries was produced using a hot rolling apparatus such as a high-speed finishing mill under the hot rolling conditions shown in Table 2. That is, the finishing thickness is 3.2mm, and the rolling finishing temperature is 78mm.
0°C and the winding temperature were kept constant at 550°C, and the total rolling reduction rate of finish rolling was changed by changing the thickness of the sheet par. For sample material A5, the latter two stands of the seven-stage finishing mill were not used, and the first stage was used. Finish rolling was performed using only five stands, increasing the rolling reduction per stand.

Next, these hot rolled steel strips were pickled and cold rolled to a thickness of 0.8 mm.
After short-time annealing at 0° C. for 30 seconds, 0.8% temper rolling was performed, and the material properties were investigated. The results are also shown in Table 2. In the investigation yield stress (YS), tensile strength (TS)
, elongation (Ez), and Lankford value (lower value) were all measured in the rolling direction (c), at 45 degrees (D), and at 90 degrees (C) in the rolling direction, and their average values were calculated.

From Table 2, sample material 1 is made of ultra-low carbon steel A rolled at a normal rolling reduction, and this material has an Et of 290 MPa.
However, the anisotropy of Et and r values was Δ
Et: 6%, Δr = 0.8, which was very large and could not be used as a steel plate for drawing.

Sample material A2 used A steel like A1, the thickness of the sheet bar was 90fIrln, and all conditions were the same as A1 except for strong reduction, and the lower value was slightly improved compared to &1. However, ΔEt and Δr were not improved at all.

Sample material 53, A4 is B containing a large amount of Cu, Nj, and Cr.
Steel A3 is rolled under the same normal rolling conditions as A1, and steel A4 is rolled under the same high rolling reduction conditions as Flat 2. Arai 3, which is obtained by rolling B steel under normal hot rolling conditions, has △Et almost the same as AI and A2.
, Δr was very doglike.

However, steel B was rolled at a high reduction rate of 96%.
△El and △r became very small, and Et was also good.

Next, for the sample material/f65, B steel was finish rolled from a sheet bar thickness of 58 mm using only the first half of the 5 stands as described above, so the rolling reduction of each stand was as high as 44% on average, and in this case as well, the anisotropic Improvements in ΔEt and Δr were observed.

In addition, sample material A6, which was obtained by rolling a C steel with a high C content (both Ni%Cr%Cu) at a high reduction rate, had a low reduction value and large anisotropy, so it was not suitable for press use.

As mentioned above, like B steel, C is very low and Cu,
If ultra-low carbon steel containing appropriate amounts of Ni, Cr, etc. is hot-rolled at a high reduction rate and then continuously annealed after cold-rolling, a steel plate with high Et and r values and very small anisotropy suitable for pressing can be obtained. It has become clear that it can be manufactured.

Based on these basic experiments, we repeated similar experiments on many types of ultra-low carbon steel using the composition of Steel B as a reference.
It has been found that by limiting the steel components as described below, the effect of the hot rolling high-rolling finish is remarkable and an excellent cold rolled steel sheet for deep drawing can be obtained.

Next, the reason for limiting the components of the cold rolled steel sheet of the present invention will be explained.

As can be seen from the above basic experiment results, if there is too much C, the deep drawing properties will deteriorate and the effect of high-pressure finishing will disappear, so it is better to have less C, and it is especially suitable for press working. In order to obtain a high lower value, it is necessary to limit the value to 0.0 (12X or less).

Mn: 0.05% or more of Mn is required to prevent hot embrittlement caused by S, but since a content exceeding 0.20X deteriorates the material, it was limited to a range of 0.05 to 0.20X.

5olAt: 5otkt is an element useful for fixing N, 0.010
If the content is less than 0.1%, the effect will be too great, and if the content exceeds 0.100%, the surface quality will be impaired, so the content was limited to a range of 0.010 to 0.100%.

Cu+Ni+Cr: Although Cu, Ni, and Cr are sometimes added for the purpose of improving surface properties such as weather resistance and corrosion resistance, they have been considered as impurities that have no positive effect on deep drawing properties.

However, in steels like the steel of the present invention, which have a very low c content and do not contain carbonitride-forming elements, Cu, Ni, Cr
The role of elements that do not form carbonitrides, such as carbonitrides, becomes relatively important. In other words, to improve the material quality, C, N
It is known that it is effective to reduce , M, n, etc., and Mn, which is a main alloying element, is also at a very low level of 0.20% or less in the present invention. In this case,
Steel is highly susceptible to recrystallization at high temperatures, resulting in coarse grains and the development of a (200) texture that is unfavorable for drawability. Although the effects of non-carbonitride-forming elements Cu, Ni, and Cr are still unclear in ultra-low carbon as in the present invention,
What these elements have in common is that they are uniformly dissolved in solid solution within the grains and do not deteriorate the lower value. The addition of these elements makes it difficult for strain during hot working to be released dynamically during hot rolling, thereby preventing uneven deformation and suppressing recrystallization, thereby improving the material quality of ultra-low carbon steel. Conceivable. Cu, Ni,
The effects of Cr are the same in all cases, so the total amount of these elements is important. Usually, in ultra-low carbon steel, Cu, Ni, and Cr are each less than 0.015%, and the total of the three components is at most 0.005% or less. However, the effect of improving the material quality, especially the anisotropy, by increasing the hot rolling reduction ratio can be seen from about 0.06% (Cu+N i + cr), and preferably the content should be 0.08% or more. Also, if the total amount exceeds 0.20%, it will become hard, so
(Cu+Ni+Cr) Te was limited to a range of 0.06 to 0.02%.

Next, the manufacturing conditions of the cold-rolled steel sheet having the above-mentioned limiting ingredients of the present invention will be explained.

First, the steel manufacturing method is not particularly limited, but C: 0.0'
A combination of a converter and a degassing device is effective in achieving a reduction of 0.02% or less. Since the slab requires uniformity, it is manufactured by continuous casting. Finish rolling conditions when continuously hot rolling a slab are extremely important in the present invention.

In other words, the material for conventional hot-rolled steel sheets or cold-rolled steel sheets is mainly steel with C: 0.02% or more, which can be manufactured without degassing treatment, and naturally, the material for hot-rolled and cold-rolled steel sheets is Both conditions were designed for low carbon steel.

However, ultra-low carbon steel exhibits various behaviors that are different from low carbon steel, and in particular, rolling strain is easily released during hot rolling and recrystallization is easy, so the austenite grain size and the corresponding ferrite grain size are thicker. It was on point.

Increasing the hot rolling reduction rate in low carbon steel means that sufficient strain is applied to the center of the sheet thickness, and the strain becomes large and the grains become finer, resulting in a good lower value and small anisotropy. It is thought that this provides a sense of gender.

Based on this knowledge, we conducted the following basic experiment. That is, the first
The A steel and B steel shown in the table were rolled under the same hot rolling conditions as the test materials Flat 2 and Flat 4 shown in Table 2 except for the thickness of the sheet bar, and a cold rolled steel plate of 0.8 tnn was obtained. The relationship between the total finishing reduction ratio of hot rolling, Δr, and the lower value was investigated, and the results are shown in FIGS. 1 and 2.

From Fig. 1 and Fig. 2, the improvement effect of high rolling reduction on the lower value of △r- is not so noticeable in steel A, which contains very few alloying elements such as Cu, Ni, and Cr, and in steel B, which contains alloying elements to some extent. It can be seen that this is noticeable in steel. In addition, in steel B containing alloying elements to some extent, significant improvements in Δr and lower values are observed when the total rolling reduction in finish is 95% or more, so in the present invention, the total rolling reduction in finish rolling is limited to 95% or more. did.

In addition, with regard to high rolling reduction in finish rolling, setting the average rolling reduction ratio of each stand perforation to 44% or more has the same effect of improving △r and lower value as limiting the total rolling reduction ratio to 95% or more. Is recognized.

That is, when melting and hot rolling various types of steel within the limited composition range of the present invention, the average rolling reduction of each stand was changed by changing the sheet bar thickness or the number of stands used in the finishing mill, and then cooling to 0.8. Inter-rolling and investigating Δr and lower value,
The results are shown in FIGS. 3 and 4. From FIG. 3 and FIG. 4, the anisotropy is reduced by setting the average rolling reduction to 44% or more, so the lower value rolling reduction is limited to 44% or more.

Although the heating temperature of the slab is not limited, the lower the heating temperature of the slab, the better the quality of the material, and especially good results were obtained at 1150° C. or lower. In ultra-low carbon steel, the change in material properties due to hot rolling temperature is small, so the hot rolling temperature is higher than the recrystallization temperature.
There is no particular limitation as long as the temperature is 900°C or less. The winding temperature was limited to 580° C. or lower, at which no grain growth occurs, because as the winding temperature increases, grain growth progresses and coarsens the winding diameter.

Cold rolling after pickling these hot rolled steel strips is not particularly limited, but the higher the rolling reduction, the higher the reduction value and the better the material quality can be obtained. When annealing after cold rolling, box annealing with a slow heating rate increases anisotropy, so continuous annealing is performed to take advantage of the features of the present invention.

Continuous annealing can produce excellent materials not only in a continuous annealing furnace but also in an in-line annealing surface treatment process such as hot-dip galvanizing.

Example Steel with the composition shown in Table 3 was melted using a converter and an RH degassing device, made into a slab by continuous casting, and after the slab was treated, it was roughly rolled into a sheet bar with a thickness of 40 to 90 mm. 3.2 under the finishing rolling conditions shown in Table 4 on a 7-stand finishing mill.
It was made of hot-rolled steel strip.

Note that in Tables 3 and 4, items that do not satisfy the limiting conditions of the present invention are underlined. Next, the above hot-rolled steel strip was pickled and then cold-rolled to a temperature of 0.8 degrees.
Continuous annealing was performed for 40 seconds, followed by 0.6% temper rolling to obtain a cold rolled steel sheet. Regarding these cold rolled steel sheets, the second
The materials were investigated in the same manner as in the table, and the results are also shown in Table 4.

Table 4 shows that Sample Materials 11, 12, and 14, which are examples of the present invention, all have high lower values and small Δr and excellent press formability, whereas Sample Materials A 13, 15, which are comparative examples.
It can be seen that No. 16 has a large Δr and cannot be used as a steel plate for pressing.

As is clear from the above examples, the present invention limits the components of the continuous casting slab and reduces the total reduction rate to 9 in hot finish rolling.
A cold-rolled steel sheet with excellent press formability can be produced by rolling the steel sheet at a temperature of 580° C. or lower with a reduction rate of 5% or more or an average rolling reduction of each stand of 44% or more, followed by continuous annealing after cold rolling.

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing the relationship between the total rolling reduction in hot finish rolling and △r and lower value of cold rolled steel sheets, respectively.
4 and 4 are diagrams respectively showing the relationship between the average rolling reduction of each stand in hot finish rolling and Δr and lower value of a cold rolled steel sheet. Agent Patent Attorney Takeo Nakaji Figure 1: Total f” x T ratio (%) Figure 2: Total finishing pressure (%) Figure 3: Equal pressure at each start (%) 4 Figures each star] Throat pressure T-tei (%)

Claims (1)

[Claims] (1) In terms of weight ratio, C: 0.002% or less, Mn: 0.
05-0.20%, 5otAA: 0.010-0.10
0%, (Ni-1-Cr+Cu): 0.06-0.2
0%, with the remainder consisting of Fe and unavoidable impurities, into a slab by continuous casting, and the slab is finished rolled with a total reduction rate of 95% or more or an average reduction rate of each stand of 44% or more. Production of a cold-rolled steel sheet with excellent press formability, characterized by comprising a hot-rolling step of winding at 580°C or less, and a step of pickling the hot-rolled copper strip, continuously annealing it after cold rolling. Method.