JPS61113724A - Manufacture of cold rolled steel sheet extremely superior in press formability - Google Patents

Abstract

PURPOSE:To manufacture the titled steel sheet, by hot rolling a steel having a specified compsn. composed of C, Si, Mn, Ti, P, Nb, S, Al, N, Fe, etc. under a suitable condition, and cold rolling said plate, then annealing continuously said sheet under a suitable temp. CONSTITUTION:The steel having an ingredient composed of 0.0005-0.0030% C, <=0.2% Si, 0.04-0.5% Mn, (3.43N%+1.2X4C%)-(3.43N%+8X4C%) Ti, <=0.030% P, 0.0025-0.015% Nb, <=0.030% S, 0.002-0.1% SolAl, 0.0005-0.0060% N, and the balance Fe with inevitable impurities is cast continuously to a steel slab. Next, the slab is not rolled continuously while finishing it at >=870 deg.C, and said plate is wound at <=850 deg.C, next, pickled, cold rolled then, said sheet is annealed continuously at temp. of recrystallization temp. or above to <=910 deg.C. By this way, the titled steel sheet is manufactured economically.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a cold rolled steel sheet with extremely excellent press formability.

(Prior art) In general, cold-rolled steel sheets for press working used for applications such as automobile exterior panels are often required to have not only aging resistance but also deep drawability and stretch formability (recently, In order to improve productivity, parts are increasingly being molded in one piece, and steel plates are often required to have deep drawability and strict stretchability at the same time.

In order to satisfy these demands, it is necessary to improve the r and r45° of the steel sheet as well as the El.

Conventionally, steel sheets made by box-annealing At-killed steel and At-guild decarburized steel sheets manufactured by the box-annealing decarburization annealing method have been used for materials such as automobile exterior panels. Parts that could sufficiently meet the requirements with steel plates began to be considered, and it became desirable to develop steel plates with extremely high r values, r45°, and Et.

Until now, the methods of manufacturing cold-rolled steel sheets with extremely excellent drawability and stretchability by continuous annealing have been studied, but the methods that have been studied are close to
- Examples include JP-A-58-81952 (hereinafter referred to as known example 1), JP-A-58-25436 (hereinafter referred to as known example 2), and JP-A-59-6731.9 (hereinafter referred to as known example). 3) etc.

These are the method of Known Example 1 in which Nb is added alone as a special alloy to ultra-low carbon At-killed steel, the method of Known Example 2 in which Nb, Ti, etc. are added as equivalents, and one or more of them are added in combination. It is divided into the method of known method 3, which makes use of the respective characteristics of Ti and Nb, divides their roles, and makes it essential to add Ti and Nb in combination.

In the method of known example 1, N is fixed as AtN and the teeth are
Since this is a method of containing C (%) or more and 0.01% or more, fixing C as NbC and ensuring non-aging properties, A and tN will be finely fractured during annealing during low-temperature winding, resulting in material deterioration. It's hard to avoid the bottom. Furthermore, in high-temperature winding, it is difficult to avoid material deterioration at positions corresponding to the inner and outer peripheries of the hot-rolled coil. Furthermore, for non-aging etc.

There are manufacturing problems such as the need to contain a relatively large number of teeth, which results in an increase in recrystallization temperature and high annealing temperature, as well as extremely high elongation values as targeted by the present invention. Hard to get.

In the method of Known Example 2, one or more special alloys of Ti, V, Zr, and W are used in a total amount of 0.002 to 0.002.
This method involves adding a very small amount of 0.10%. When using teeth in this method, since the amount of Nb added is small, it can be expected to avoid the deterioration of elongation value due to large addition at the corners as in the method of Known Example 1 above, but it is not possible to combine addition with Ti. Therefore, similar to Known Example 1, high-temperature winding and high-temperature annealing are required to coarsely precipitate AtN.

In addition, since the amount of alloy added is very small, complete non-aging as in the method of the present invention is impossible, and as shown in the examples, the aging index (hereinafter referred to as AI) is 2 to 3 Kg/rtrm. ”
However, with this level of AI, there is a problem in the use of extremely severe press forming, which is the goal of the present invention, such as deterioration of Et in summer and the like. .

The method of Known Example 3 is an attempt to improve the problems of Known Examples 1 and 20 by adding Ti and Nb in combination. This method converts A, tN to T
It is characterized by fixing it as iN and precipitating C as a composite carbide of (Ti, Nb)C, thereby improving the problems when adding corners alone and producing steel sheets for ultra-deep drawing. It is. However, the copper plate obtained by the method of Known Example 3 has an El'' of 47% as shown in the example.
, r':: 1.9, which is only slightly higher than the At-killed steel produced by the conventional box annealing method, which is insufficient to obtain the material quality targeted by the method of the present invention.

The above has described typical conventional manufacturing methods for manufacturing 6-rolled copper sheets with excellent press formability using continuous annealing, but regardless of whether the method is It is difficult to manufacture a steel plate that has more severe press forming ability.

(Problems to be Solved by the Invention) The object of the present invention is to provide a method for economically manufacturing a copper plate having extremely high degrees of 7, r45° and Et by a continuous annealing method.

(Means for Solving the Problems) As a result of a detailed study of the advantages and disadvantages of tooth-added steel, corners, and Ti composite added steel, the present inventors found that the corners are known as known example 1.
NbC1 such as or ('f'i
, Nb) not as a double carbide of C, but as a solid solution [
It has been found that the most effective way to maximize the benefits of teeth is to allow them to act solely as teeth.

Up until now, steels with oxides have the advantage of reducing Zr content, but on the other hand, they raise the recrystallization temperature, resulting in lower E.
It had the disadvantage of being difficult to go to due to the inferiority of l. The inventors believe that this disadvantage is due to NbC (Ti, Nb
') We found that this is due to the formation of fine carbides and double carbides such as C, and as an improvement, by adding sufficient Ti in a composite manner, C can be reduced to a relatively large TIC due to sufficient Ti. It is now possible to completely precipitate Nb as 0.0025%, avoiding a decrease in Et, and achieving complete non-aging.
A very small amount of ~0.015% is added to harden a small amount of Nb.
We found that by acting as b, it was possible to significantly improve r45 and r aη while avoiding the deterioration of El, and succeeded in preserving the advantages of pump-added steel and ameliorating the disadvantages. .

The method of the present invention will be described in detail below.

Extremely high temperatures such as Et and 7, r as the object of the present invention.
In order to achieve the angle of 45°, it is first necessary to use a component with a moderately regulated C content as the base composition.

In the method of the present invention, C is precipitated as a relatively large TiC with sufficient Ti, and the solid solution C is made fine (at the same time, the crystal grains are refined, and the Et concentration is not deteriorated and the temporary TiC is
However, if the total amount of TiC increases, deterioration of 5Et becomes difficult to avoid, so the C content must be regulated to 0.0030% or less. The lower limit of the C content was set at 0.0005% since the industrially obtainable limit is 0.0005%, but there is no lower limit from a metallurgical standpoint, and lower degrees Celsius is better.

The amount of Ti added is sufficient to fix N and C, and to maintain a relatively large amount of T.
It is necessary to add Ti in the amount necessary to precipitate it as iC, and the lower limit of the amount is Ti: [3.43N (pre-H
-1,2X 4. Cf%)]. Also, the upper limit is set because if too much Ti is added, the amount of Ti in solid solution will increase, the recrystallization temperature will gradually rise, the grains will become finer, and 21 will deteriorate, and the alloy cost will also increase. , [3.43N (%
l+8X4C (set as U).

Figure 1 shows C: 0.0020%, Excitation: 0.007%,
Si: 0.01%, Mn: 0.2%, P: 0.0
08%, S; 0.007%, N: 0.0020%, So
lAl: 0.02% T
Hot rolling was carried out under normal conditions at 680°C by varying the amount of i added.
This figure shows the influence of the addition amount of '1゛i on El, r, and r45° of a steel plate that was rolled, cold-rolled, and continuously annealed in the heat cycle shown in Figure 2, and the scope of the present invention is Make it clear.

As is clear from Fig. 1, the amount of Ti added is [(Ti(
%l-3,43N(%))/4C(%)] is 1.2, that is, the Ti addition amount is [3.43N(%)+1.2 x 4
C (%)], FJ, r, ′"°""sudden"
It is clear that a 9,5 property with a direction of 1" and a polar "1"16"' can be obtained, and the excessive Ti c"N and C of the present invention can be completely coarsely precipitated and a trace amount of excitation can be obtained. The method of addition clearly indicates that it is pickled.

Also, ('i'i(%+ -3,43N(@) / 4
When ct@ exceeds 3, El, r gradually deteriorates, and when ct@ exceeds 8 times, not only is there little improvement in material quality, but also the cost of Ti alloy becomes high (which makes the present invention less economical). Since the effect would be weakened, the upper limit was set to 8 times.

Next, we added a small amount of Nb, which is another important point of the present invention.
The effect of suppressing the deterioration of El and significantly improving r45°, 7 will be explained.

Figure 3 shows C: O, 0O 16%, Si: 0.01%
, iV stone, 015%, P: 0.006%, Ti: 0
．． 019%, S: l), 006%, SolAl:
0.010%, N: 0.0019 The corners were added to the common ingredients within the scope of the present invention, and a cold-rolled steel plate was made under the same manufacturing conditions as the experimental conditions shown in Figure 1, and the material and i'Jbns ,7
The relationship between the amounts of 1fJ is shown in a diagram.

As is clear from FIG. 3D, when Ti of the present invention is added in excess and 0.0025% or more is added to the corner where all C becomes coarse TiC precipitates, El is degraded. It is clear that the method of the present invention is superior because r45° and r were significantly improved without increasing the temperature. Note that if a large amount of Nb is added, Et and r will deteriorate, and if Nb exceeds 0.015%, the effectiveness of the method of the present invention will be diminished and excellent El and r will not be obtained. Upper limit 0.015%
And so.

If a large amount of Si is contained, the paintability becomes poor (or becomes hard), so the upper limit was set at 0.2%.

(2) Stone needs to be added in an amount of 0.04% or more from the viewpoint of hot brittleness, but since adding a large amount causes hardness, the upper limit was set at 0.5%. Since ductility deteriorates when P and S are contained in too large amounts, the upper limit of both P and S is set to 0.030%.

The amount of 5otAt is at least 0.002 because when adding Ti, if there is a large amount of free element in the stone pot, TiO2 will be generated and +[1i, which is effective for fixing C and N, will be reduced.
% is required. Furthermore, if too large a quantity of 5otAt is included, El will be degraded, so the upper limit thereof was set at 01%.

N is fixed by Ti in the form of coarse precipitates of TiN, but if a large amount is still included, the amount of Ti required to fix it as TiN increases, which increases the cost of the Ti alloy. Furthermore, as the total amount of TiN increases, the quality of the material deteriorates, so the upper limit was set at 0.0060%.

In addition, for applications where secondary workability is particularly required, B2: "-
When 10 ppm is contained, secondary processability is improved.

The ingredients of the steel used in the method of the present invention have been described in detail above, but the manufacturing conditions other than the ingredients will be described below.

The molten steel adjusted to have the composition range detailed above is continuously cast into steel slabs, hot-rolled at a finishing temperature of 870°C or higher, and rolled up at a temperature of 850°C or lower to form a hot coil. During this hot rolling, the steel billets are heated at a finishing temperature of 870°C, whether in the process of reheating them in a heating furnace or in the process of directly hot rolling the pieces from a continuous casting machine at high temperatures. If I can do more than Usuka, it will be better.

In particular, when a large amount of Ti is contained, if the reheating temperature of the slab is set to 1100° C. or lower, TiC becomes coarser, which is a preferable condition for improving Wt. If the finishing temperature drops to 870°C or lower, the grains of the hot rolled sheet will become coarser and the r45° of the finished product will decrease, so the finishing temperature must be in the austenite range, which is 870°C or higher.

In addition, even with the usual 650℃ or so for winding the pot, a sufficiently excellent El, r, and r of 45 degrees can be obtained, but high-temperature winding requires T
The size of the precipitate of iC becomes larger, and the Et of the product,
This is a preferable method as it further improves r and r45°. However, in high-temperature excavation at 850°C or higher, the structure of the hot-rolled sheet undergoes abnormal grain growth, and the rate at which r45° decreases increases.
It is necessary to wind it up at a temperature below 850°C.

The hot coil hot-rolled under the above hot-rolling conditions is subjected to normal cold rolling, continuous one-bath dulling at a temperature higher than the recrystallization temperature, and if necessary, temper rolling to press-form it. This makes it possible to produce cold-rolled steel sheets with extremely excellent properties. In addition, during continuous annealing ←, the upper limit of the annealing temperature was set to 910℃.
This is because when annealing is performed at a temperature exceeding .degree. C., it transforms into austenite, resulting in a significant decrease in 〒 and r45.degree.

Hereinafter, the present invention will be explained based on examples.

Example 1 Table 1 shows the chemical composition and hot rolling conditions of the invention steel and the test steel used for comparison.

The test steel shown in Table 1 was hot-rolled and rolled according to the process and hot-rolling conditions shown in the table, and the plate thickness was 4. Oal hot coil, 0
．． After cold rolling to 80+1III+, the f
It was continuously annealed in an annealing cycle and temper rolled to the 07% side to produce an electrically resistant coil.

Table iA2 shows the results of the material quality testing of the cold-rolled steel sheet thus rolled. Test steel l, which is the steel of the present invention, 2.3.7.8.1
1 and 12 are 7 or 2.1 with a temperature difference of 51% or more for B4.
The above clearly shows that a cold-rolled steel sheet with an extremely excellent material property value of r45° of 1.64 or more and extremely excellent press formability can be produced by the method of the present invention.

Compared to that, the relative mild steel is 4, 5, 6, 9, 10 °C
- The material quality of both cases is inferior to that of the steel of the present invention, and it is clear that the method of the present invention is ambiguous. Comparative Steel 4 does not contain islands, so as shown in Table 2, r45° is 1.
It is extremely small at 30. Comparative Steel 5 contains a large amount of teeth at 0.020%, so F and t are low at 47.6%.

Comparative steel 6 does not contain Ti, so r45° is 1.5
1, and the AI is 2.1 Kq@/rran2, so complete non-aging has not been achieved.

Comparative steel 9 contains 0.071% Ti and (Ti(@-3,4
3N(%)]/4C(@) is contained in a large amount of 10.08, i = 1.99, E/!, = 50.5%, despite adding a large amount of expensive Ti. First, the material quality is not very good. Comparative Steel 10 contains a large amount of C, 0.0036%, so 'fat = 47.1%, r =
It has a bad custom value of 1.89.

In the case of the present invention-2, the winding temperature is 780°C, but if high temperature winding is performed also in the method of the present invention, the material quality will be further improved, Et = 53.8%, i = 2.35
Extremely excellent material properties can be obtained. The invention steel 12 is
Implementation 5+ where the hot-rolled slab heating temperature was as low as 1080°C
However, in this case as well, extremely excellent material properties can be obtained as in the case of high-temperature treatment.

(Mechanical result of the invention) As described above, C and N are completely fixed as relatively large precipitates with excess Ti, and furthermore, a small amount of Nb is added on top of the precipitates, and a small amount of the same precipitates are added. By acting as Nb in the bath, Nb, Ti, added separately,
It is possible to produce a steel plate with significantly lower EA, r, and r45° than a copper plate with a composite addition of Ti and teeth in which Ti is added in an amount equal to or less than the equivalent of C+H.

[Brief explanation of the drawing]

Figure 1 shows ultra-low carbon At- containing 0.007% Nb.
An explanatory chart showing the effect of adding T1 excessively to K,
Fig. 2 is an explanatory diagram showing the heat cycle of continuous annealing of the test in Fig. 1, and Fig. 3 shows the effect of composite addition of steel KNb to ultra-low coal A/, - containing Ti: 0.019%. FIG. 4 is an explanatory diagram showing the heat cycle of continuous annealing in Example 1. Figure 1 tl 1 hour Figure 3 5'' Nb (%) Figure 4 → Time;

Claims (1)

[Claims] C: 0.0005 to 0.0030%, Si: 0.2% or less, Mn: 0.04 to 0.5%, Ti: [3.43N(
%)+1.2×4C(%)]～[3.43N(%)+8
×4C (%)], P: 0.030% or less, Nb: 0.0
025-0.015%, S: 0.030% or less, Sol
Al: 0.002-0.1%, N: 0.0005-0.
Steel with a composition consisting of 0060% residual iron and unavoidable impurities is made into steel billets by continuous casting, finished with continuous hot rolling at a temperature of 870 ° C. or higher, rolled up at a winding temperature of 850 ° C. or lower, and pickled, A method for producing a cold-rolled steel sheet with extremely excellent press formability, which comprises cold rolling and continuous annealing at a temperature of not less than a recrystallization temperature and not more than 910°C.