JPH0617517B2 - Manufacturing method of cold rolled steel sheet with excellent press workability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet for press working having high ductility and deep drawability.

[Conventional technology]

For example, cold rolled steel sheets with excellent workability are required as materials for automobile quarter panels, fenders, oil pans and the like. From the viewpoint of steel composition, the conventional technique for producing such deep-drawable cold-rolled steel sheet can be obtained by adding C or N in the steel by adding Ti or Nb alone or in combination.
To improve ductility and ensure non-aging and T
There has been proposed a deep drawing steel sheet in which a recrystallization texture of {111} orientation effective for improving deep drawing property is developed by the action of carbonitrides of i and Nb. For example, Japanese Patent Publication No. 44-18066, Japanese Patent Publication No. 59-67322 and Japanese Patent Publication No. 59-89727 disclose such Ti-added steels, and Japanese Patent Publication No. 54-1245 and Japanese Patent Publication No. 59-34778 and JP-A-58-81952 disclose Nb-added steel, JP-A-59-67319 discloses Ti-Nb-added steel, and JP-A-59-123720.
The publication discloses Ti, Nb, and Cr-added steel.

Next, focusing on the hot rolling process, the conventional technique for producing such a cold-rolled steel sheet for deep drawing is as follows.

Hot-rolled slabs are manufactured by continuous casting of molten steel or by ingot casting and then slabbing, and the slabs are once cooled to room temperature, maintained in the refining process, and then heated again to a prescribed temperature in a heating furnace. Usually, a cold piece charging method (cold charge method, hereinafter referred to as CCR method) of charging a cold slab into a heating furnace, which is followed by hot rolling, was usual. At that time, the slab heating temperature was selected to be a fairly high temperature range around 1200 ° C. The reason for heating to high temperature is T
The steel in which C is fixed by i or Nb has a higher Ar ₃ transformation point of about 900 ° C, which transforms from the γ phase to the α phase than ordinary low carbon steel, and does not terminate hot rolling in this γ phase temperature range. This is because the material deteriorates and it is necessary to keep the finish rolling exit temperature in hot rolling at a high temperature.

As another method, in recent years, a hot piece charging method (hot charging method, hereinafter referred to as HCR method) in which a continuously cast slab is charged into the heating furnace without being cooled to room temperature, further progresses,
A continuous casting-direct rolling method (hereinafter referred to as CC-DR method), in which a slab continuously cast without a heating furnace is subjected to hot-rolling as it is, or subjected to a heat treatment, has been developed and is already in practical use. . For example, Japanese Patent Publication No. 60-45692 discloses CC-
Applying the DR method to ultra-low carbon steel with addition of Ti, Nb, etc., 60
Disclosed is a method for starting hot rolling under a slab surface temperature of 0 ° C. or higher and lower than 900 ° C. In addition, Japanese Patent Publication No. 60-45689 discloses a method of soaking an ultra-low carbon steel slab containing Ti, Nb, etc. at a temperature of 800 ° C or higher and lower than 1000 ° C so as to hot-roll it. It is said that a steel plate with good press formability can be produced by only controlling the cooling rate without requiring a special heating furnace.

[Problems to be solved by the invention]

In ultra-low carbon steel containing carbonitride forming elements such as Ti and Nb, {111} after cold rolling annealing due to the action of carbonitride
Since it develops a recrystallized texture of orientation, it is important to appropriately control the size and distribution of carbonitride at the stage of hot rolling. CC- which seems to be the most preferable hot rolling technology from the viewpoint of energy saving and process saving
Even in the DR method, it is still important to appropriately control the size and distribution of carbonitrides in the hot-rolled sheet in order to produce a cold-rolled steel sheet for deep drawing with good possibility.

In the slab once cooled to room temperature by the CCR method, relatively large carbonitrides are sufficiently precipitated,
When this is heated to the heating temperature and held, part of it re-dissolves, but the unmelted part remains as nuclei, and re-precipitates in a favorable distribution state during subsequent hot rolling and after winding. It seems to be. This is explained by the fact that the value of the cold-rolled steel sheet is higher when the heating temperature is lower even in the CCR method, that is, when the amount of unmelted carbonitride is larger, as will be exemplified later.

On the other hand, in the HCR method and the CC-DR method, since the slab is not cooled to a low temperature region, it is charged into the heating furnace before the precipitation of carbonitride is insufficient (HCR method) or hot rolled. (CC-DR method). As a result,
Many very fine carbonitrides are precipitated in the hot-rolled steel sheet. Needless to say, these carbonitrides are by no means desirable in the sense that they provide a favorable texture for deep drawability. Therefore, the hot rolled sheet obtained by the HCR method or CC-DR method is
It is an undeniable fact that it is inferior to the hot-rolled sheet obtained by the CR method as a base material for deep-drawing steel sheets.

By the way, as a reason for heating the slab to a high temperature, it was necessary to finish the hot rolling in the γ region, but this also makes the recrystallization texture of the cold-rolled steel sheet preferable for deep drawability. Is a means of. In other words, it is considered that the hot rolled steel sheet with the residual work structure remains in the α-region rolling, and the work structure in this hot rolled steel sheet significantly suppresses the formation of the recrystallization texture in the {111} orientation after cold rolling annealing. Is.
However, in the CC-DR method up to now, a finish hot rolling technique in the α region has been proposed. For example, the previous Japanese Patent Publication Sho 60-45689
Japanese Patent Laid-Open Publication No. 7-200700 has an Ar of ₃ points or less at 700 ° C.
It is shown how to perform finish rolling. The present inventors have proposed a steel having the steel composition proposed by the present invention to this Japanese Patent Publication No. 60-4.
CC-D at temperatures below ₃ Ar points as taught by the 5689 publication
I investigated and researched finish rolling according to R method,
Probably due to the difference in steel composition,
The value after cold rolling annealing decreased significantly. Further, the same thing was applied to the low-temperature rolling technique by the CC-DR method disclosed in the above Japanese Patent Publication No. 60-45692, but it was still difficult to obtain a high value.

According to the CC-DR method, the above facts are as described above.
(1) The size and distribution of carbonitrides precipitated on hot-rolled sheet are not appropriate, (2) The work structure remains due to low-temperature finish rolling, and these two points are synergistically synergistic. Is considered to have decreased.

An object of the present invention is to solve such a problem, and to provide a method for manufacturing a cold-rolled steel sheet having a sufficiently high value and high ductility as a deep-drawing steel sheet.

[Means for solving problems]

The present invention provides a method for producing a cold-rolled steel sheet that achieves the above object, in% by weight, C: 0.001 to 0.01%, Si: 0.1 or less, Mn: 0.5% or less, Sol.Al: 0.01 to 0.10%, P : 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: [effective Ti amount] according to the following formula (1) is 4 × C% or more and this [effective Ti amount] Is 0.15% or less, [effective Ti amount] = total Ti amount- [N% x (48/14) + S% x (48/32) + 0% x (48/16) x 1/2] (1) Nb: 0.03% or more and in relation to [effective Ti amount], [effective Ti amount] + Nb ≤ 0.20% is a basic component, and if necessary, Cr: 0.06 to 0.20%, B : 0.0005 to 0.0020%, the balance: Fe and the inevitable impurities in the steel continuously cast slab without heating to room temperature without heating to room temperature in the slab width direction ≧ 800 ℃ Charge into Then, hot rolling is performed under the conditions of 1100 ° C ≥ slab heating temperature ≥ 1000 ° C, 930 ° C ≥ finish rolling exit temperature ≥ 850 ° C, 730 ° C ≥ hot rolling temperature ≥ 600 ° C, and then a rolling reduction of 60%. The present invention provides a method for producing a cold-rolled steel sheet having excellent press workability, which comprises performing the above cold rolling and annealing at a temperature not lower than the recrystallization temperature and not higher than 920 ° C.

The details of the present invention will be described below.

In order to solve the above-mentioned problems, the present inventors hot-rolled an ultra-low carbon steel containing a carbonitride-forming element by the HCR method to obtain a cold-rolled steel sheet after cold-rolling annealing. Extensive research has been conducted with the aim of improving workability to the same level as the conventional CCR method. The target of the HCR method instead of the CC-DR method is that the current slab surface temperature on the outlet side of the continuous casting machine drops to about 860 ° C even in the widthwise central part in the current continuous casting technology. This is because it is judged that it is difficult to achieve finish rolling of ₃ points or more even if hot rolling is performed.

In this research, the present inventors have obtained two important findings.
One of them is that even in the HCR method, the lower the slab heating temperature, the larger the value after cold rolling annealing. The second is
Even if the slab heating temperature is the same, the lower the finishing temperature is, the higher the value after cold rolling annealing is until the hot rolling finishing temperature is 850 ° C. These contents will be verified in the examples described below,
First, the steel components and manufacturing conditions adopted in the present invention will be individually described.

About steel composition.

The lower the content of C, the better the ductility of the cold-rolled steel sheet, and when it is more than 0.01%, a large amount of carbonitride-forming elements are required and the amount of carbonitride precipitates increases. The press formability is deteriorated. On the other hand, in a steelmaking furnace of practical scale, the C content was 0.001
It is difficult to reduce it to less than%. For this reason, the C content is 0.001 to 0.01%.

Si is added for the purpose of deoxidizing molten steel, and Mn is mainly added for the purpose of preventing hot brittleness. However, if too much Si and Mn are added, ductility is lowered. In the steel of the present invention, the amounts contained in ordinary cold-rolled steel sheets are allowed to be up to Si ≦ 0.1% and Mn ≦ 0.5%, and the above-mentioned objects are sufficiently achieved within this range.

Al is added for the purpose of deoxidizing molten steel, but if the amount is less than 0.01% in Sol.Al (acid-soluble Al) in the steel, the purpose cannot be sufficiently achieved. If the amount of Sol.Al exceeds 0.10%, the effect is saturated and the non-metallic inclusions are rather increased to cause surface defects. Therefore, the amount of Sol.Al is set to 0.01 to 0.10%.

If too much P is added, the yield strength and tensile strength will increase, and in extremely low C steels, segregation at grain boundaries will cause secondary work cracking. The upper limit is 0.03%.

The smaller the amount of N, the smaller the amount of Ti to be added, which is desirable. When the amount of N is too large, [effective Ti
Amount] and deteriorate the press formability of the final product, so the allowable limit is N ≦ 0.007%.

S and O both decrease the [effective Ti amount], and when they increase, the total Ti amount for securing the [effective Ti amount] increases, and the surface quality deteriorates. The allowable limits of O are S ≦ 0.015% and O ≦ 0.
01%

Ti secures the non-aging property of the cold-rolled steel sheet by fixing C and N, and the generated TiC serves to form a [111] -oriented recrystallized texture effective for improving deep drawability. . For this purpose, the [effective Ti amount] shown by the above formula (1) needs to be 4 × C% or more. However,
If the amount of Ti exceeds 0.15%, the amount of Ti that forms a solid solution in the ferrite increases, resulting in an increase in yield strength and a decrease in ductility. It also increases the manufacturing cost. Therefore, the Ti is [effective Ti amount] not less than 4 × C% and not more than 0.15%. In the steel of the present invention, Ti
In addition to Nb, the in-plane anisotropy of the γ value can be improved even with a small Ti addition amount.
There is an upper limit of [effective Ti amount] related to
The total amount of i] + Nb is up to 0.20% as described later.

By adding Nb in combination with Ti, the in-plane anisotropy of the γ value can be significantly improved even if the C content and the Ti content are reduced as in the present invention. Such an effect is achieved when the amount of Nb is (0.2%-[effective Ti amount]) or less, and when the amount of Nb is more than this amount, the recrystallization temperature rises and the ductility decreases. Therefore [effective Ti
Amount] + Nb ≦ 0.2% or less. However, if the amount of Nb is less than 0.03%, the effect of improving the in-plane anisotropy of the γ value cannot be obtained.

Although Cr alone cannot obtain the preferable results aimed at by the present invention, the addition of Ti and Nb in combination makes it possible to improve the ductility and lower the yield stress, which are preferable for deep drawability. Therefore, it is an effective element for obtaining a particularly soft material. However, if the Cr content is less than 0.06%, this effect does not occur, and 0.20%
If the amount exceeds, the effect will be saturated and only the manufacturing cost will be increased. Therefore, when a material having a low yield stress is required, Cr is contained in the range of 0.06 to 0.20%.

When B is added in combination with Ti and Nb, the effect of improving the grain boundary strength is recognized. Therefore, it is effective in improving the secondary work cracking resistance, which is a problem with the deep-drawing steel. However, if the B content is less than 0.0005%, such an effect does not occur. If the B content exceeds 0.0020%, the crystal grains become finer and hardened, and the γ value is significantly deteriorated. Therefore, if secondary work crack resistance is required, 0.00
B is contained in the range of 05 to 0.0020%.

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

Heating furnace charging temperature: The present invention applies the HCR method of charging the continuous casting slab of the above-mentioned steel into a high temperature heating furnace without cooling it to room temperature. The surface temperature of the slab is about 860 ° C at the center of the slab width, and the slab surface temperature when charging the heating furnace is set to 800 ° C or higher in consideration of the temperature decrease due to the subsequent transfer time to the heating furnace. If the temperature drops below 800 ° C, it may be indistinguishable from the CCR method.

Slab heating temperature: The slab heating temperature in the heating furnace is an important requirement in the present invention together with the finish rolling outlet temperature described below. The inventors carried out hot rolling of a steel slab having the above-mentioned composition according to the present invention at various heating temperatures by the HCR method, winding the coil, and thereafter descaling by pickling, cold rolling and The mechanical properties of the cold-rolled and annealed material obtained by annealing were examined, and it was found that the lower the heating temperature, more specifically, the γ value and the total elongation increased at 1100 ° C or lower. The reason for this is not clear at this point in time, but the lower the heating temperature, the lower the solid solubility limit of carbonitrides, which is presumed to be related to the increase of carbides precipitated during heating. . The reason why ductility increases is not always clear, but it is speculated that this is an effect that accompanies it because the γ value increases. From such a thing,
In the method of the present invention, the upper limit of the slab heating temperature in the heating furnace is 1100 ° C. The lower limit of the heating temperature is not particularly limited in terms of precipitates, but if the temperature of the finish rolling start side, which will be described below, becomes lower than 850 ° C, the value after cold rolling annealing will decrease. The lower limit of the heating temperature must be regulated to 1000 ℃ or more to ensure it.
This seems to be inconsistent with the above-mentioned explanation of the prior art that heating at about 1200 ° C was usual, but this is not the case. Due to recent advances in hot rolling technology and equipment, for example, accelerated rolling, increasing the thickness of the hot bar on the entry side of finish rolling, and installing a hot bar heating device before finish rolling, the lower limit of the slab heating temperature becomes 1000. This is because the finish rolling temperature of 850 ° C can be secured even by heating at ° C. For the above reason, in the present invention, the heating temperature of the slab is set to 1000 ° C or higher and 1100 ° C or lower.

Finishing rolling outlet temperature: The present inventors, in combination with an appropriate heating temperature,
Mechanical properties of cold-rolled and annealed material obtained by changing the temperature of the finish rolling side in the range of 960 to 750 ℃, winding on a coil, descaling by pickling, cold rolling and annealing were investigated. It was found that the lower rolling temperature has a higher γ value in the range of 960 to 850 ° C, and that the lower rolling temperature lowers the γ value. The reason for this is not clear at this point in time, either, but for the former, it is thought that the carbonitride precipitation is promoted as the temperature at the exit side of finishing rolling becomes lower at the same heating temperature. The latter finish rolling temperature is 85
The reason why the γ value decreases below 0 ° C is that the effect of suppressing the development of {111} orientation texture by α rolling is greater than the effect of increasing the γ value by precipitation of carbonitrides. It is presumed that. From this, it is important to set the finishing rolling temperature to 850 ° C. or higher in order to achieve the object of the present invention. In the present invention, the upper limit of the finish rolling outlet temperature is set to 930 ° C because (1) the γ value is greatly reduced at a temperature higher than 930 ° C, and (2) the upper limit of the slab heating temperature is 1100 ° C. As a result, it becomes difficult to secure a stable finish rolling outlet temperature above 930 ° C.

Hot rolling coiling temperature: A slab having the composition according to the present invention is hot rolled under the heating furnace charging temperature, heating temperature and finish rolling outlet temperature according to the present invention, the coiling temperature is changed variously, and then the acid When the mechanical properties of the cold rolled annealed sheet obtained by descaling by washing, cold rolling and annealing were investigated, it was found that its γ value increased with the increase of the coiling temperature. It is speculated that the higher the winding temperature is, the higher the γ value is because the amount of carbonitrides that aggregate and grow after being wound around the coil increases. When the coiling temperature is 600 ° C or higher, the γ value is a value that substantially satisfies the requirements for deep-drawn molten steel sheet. Therefore, in the present invention, the lower limit of the winding temperature is 600 ° C. The upper limit of the coiling temperature does not seem to exist in terms of mechanical properties. However, if the temperature is too high, adverse effects such as deterioration of pickling performance and coil deformation after coiling may occur. There is.

Cold rolling reduction ratio: The reduction ratio in cold rolling should be at least 60% in order to obtain a high γ value like the steel of the present invention. Cold rolling reduction is 60
% Or more, sufficient strain can be accumulated for recrystallization even in a short time annealing such as continuous annealing. The upper limit of the cold rolling reduction ratio is not particularly limited, but considering the cold rolling workability, it is preferably about 85%.

Annealing temperature: The higher the annealing temperature, the better the mechanical properties. However, if the annealing temperature is too high, the α → γ transformation occurs and the γ value decreases, so the annealing temperature range should be between the recrystallization temperature and 920 ° C.

Hereinafter, the configuration and effects of the present invention will be described more specifically with reference to examples.

Example 1 This example is a manufacturing experiment on a commercial production scale for defining hot rolling conditions.

Steel having the chemical composition values shown in Table 1 was melted using a 180 ton LD converter and a vacuum degassing apparatus, and was continuously cast into a steel slab. This steel slab was hot-rolled under various conditions shown in Table 2 to obtain a 4.0 mm hot rolled coil. Each of the obtained hot rolled coils was descaled by pickling and then cold rolled to a sheet thickness of 0.8 mm, and each cold rolled sheet was subjected to continuous annealing at 900 ° C for 1 minute. After that, temper rolling with an elongation of 0.8% was performed. The mechanical properties of these cold rolled annealed sheets are also shown in Table 2.

The results shown in Table 2 show the following.

No. 1 with higher slab heating temperature than specified in the present invention
Compared with (Comparative example), the elongation of No. 2 to 4 invention examples
Not only 1.5-2.7% higher, but also 0.18-0.27 higher.

No. higher than the finishing rolling temperature range specified in the present invention.
In comparison with No. 5 (comparative example) and No. 6 to 7 (comparative example), the elongation is 1.6 to 3.9% higher in the inventive examples of No. 2 to 4,
Also, the value is 0.20 to 0.33 higher.

Compared with No. 9 to 10 (comparative example) having a lower winding temperature than that specified in the present invention, in No. 2 to 4 and No. 8 inventive examples, the elongation is 2.0 to 4.6% higher and the value is also higher. 0.08 to 0.31 higher.

Also in the case of the CCR methods of Nos. 11 to 13 shown as reference examples, the lower the heating temperature, the better the mechanical properties of the cold rolled steel sheet.

Example 2 Steels each having the chemical composition shown in Table 3 were melted in a 30 kg vacuum melting furnace, hot forged at a heating temperature of 1250 ° C., and test pieces of 24 mm × 50 mm × 150 mm size were cut out from each forging. Then, this test piece was heated to 1450 ° C. in a high-frequency induction heating furnace to dissolve carbonitrides of Ti or Nb, and then cooled to 900 ° C. and then charged into the heating furnace.

Hold the heating temperature at 1050 ℃ for 1 hour and finish rolling temperature 910
Hot rolling at ℃, cooled to 710 ℃ at 10 ℃ / S, then 710 ℃
Dipped in the salt bath furnace and subjected to heat treatment equivalent to winding, plate thickness 4.0 mm
It was a hot rolled sheet. These were pickled, cold-rolled to a sheet thickness of 0.8 mm, and each cold-rolled sheet was continuously annealed at 900 ° C for 1 minute. After that, 1.0% temper rolling was performed.

Table 4 shows the mechanical property values of the obtained cold rolled steel sheet. Table 4 also shows the secondary work cracking resistance temperature. This secondary work cracking resistance test method is as follows: after blanking the test material to 90mmφ, the primary aperture 50φ, the secondary aperture 40φ, the tertiary aperture
Cup-molded with 33φ three-stage drawing (final drawing ratio = 2.7),
After trimming the obtained cup to a height of 40 mm, the cup was placed in a coolant adjusted to each test temperature and a conical cone-shaped punch with an apex angle of 60 ° was pushed in to prevent brittle fracture called vertical cracking. The lower limit temperature that does not occur was measured, and this temperature was taken as the secondary work cracking resistance limit temperature.

As is clear from the results in Table 4, the steels Nos. 1 to 8 all have a high total elongation of 466% or more and a value of 1.87 to 2.2.
Not only is it as high as 8, but Δγ is 0.55 or less, and γ _min , γ _45, is 1.74 or more, significantly improving the in-plane anisotropy.

When comparing No. 3 and No. 6, which have almost the same Ti, Nb, and C, No. 6 with 0.15% Cr was added to No. 3 without Cr.
The YS is smaller and the total growth is larger than that of.
Regarding the secondary work crack resistance, when comparing No. 2 and 3 with almost the same steel composition other than B and No. 8, 0.0010%
The B-added steel No. 8 has a low secondary working crack resistance temperature of -90 ° C, and has excellent secondary working crack resistance.

In contrast, Comparative Steel Nos. 9 to 11 (Nb content lower than specified in the present invention) has a high total elongation of 49.7% or more, but Δγ
There large as from 0.62 to 0.71, gamma ₄₅ is gamma _min there is a problem in deep drawability in terms of there plane anisotropy at 1.57 or less and a low value.

In addition, No. 12 and No. 13 steels with large amounts of Nb and Ti, respectively, have high values of 2.24 to 2.31 and γ _min of 2.08 to 2.10.
Also, the deep drawability is sufficient because it is as small as 0.28 to 0.46, but the total elongation is 45.6 to 46.0%, which is lower than the steel of the present invention.

And, No. 14 steel, which has a large amount of C, has a low total elongation like No. 12 and No. 13.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takashi Matsumoto 11-1 Showa-cho, Kure-shi, Hiroshima Inside Kure Research Institute, Nisshin Steel Co., Ltd. (56) Reference JP-A-58-48633 (JP, A) JP Sho 61-3844 (JP, A)

Claims (1)

[Claims] 1. In% by weight, C: 0.001 to 0.01%, Si: 0.1% or less, Mn: 0.5% or less, Sol.Al: 0.01 to 0.10%, P: 0.03% or less, S: 0.015% or less, N : 0.007% or less, O: 0.01% or less, Ti: [effective Ti amount] according to the following formula (1) is 4 × C% or more and this [effective Ti amount] is 0.15% or less, [effective Ti amount] = total Ti amount- [N% x (48/14) + S% x (48/32) + 0% x (48/16) x 1/2]-(1) Nb: 0.03% or more and [effective Ti amount [Effective Ti content] + Nb ≦ 0.20% is the basic component, and, if necessary, one or two of Cr: 0.06 to 0.20 and B: 0.0005 to 0.0020% is included. , Remainder: Continuous cast slab of steel consisting of Fe and unavoidable impurities is loaded into a heating furnace at a surface temperature ≧ 800 ° C at the center of the slab width direction without cooling to room temperature, and 1100 ° C ≧ slab heating temperature ≧ 1000 ℃, 930 ℃ ≧ finish rolling Hot rolling is performed under the conditions of exit side temperature ≧ 850 ° C, 730 ° C ≧ hot rolling temperature ≧ 600 ° C, then cold rolling with a rolling reduction of 60% or more, and recrystallization temperature of 920 ° C or more. A method for producing cold-rolled steel sheets with excellent press workability, which consists of annealing at a temperature.