JPS5940215B2 - High tensile strength cold rolled steel sheet with excellent formability and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength cold-rolled steel sheet with excellent formability and a method for manufacturing the same.
yns? The present invention relates to a high-strength cold-rolled steel sheet with excellent formability and non-aging properties of the same grade, and a method for producing the same.

Demand for high-tensile steel sheets is increasing to reduce the weight of automobile bodies in order to improve fuel efficiency.

Such steel plates must satisfy the following requirements.

■It is non-aging, 2 it has a high lower value, 3 it has a large elongation, 4. 5. The yield ratio is low. 6. Few inclusions and no surface flaws. Product cost is not high.

Dual phase steel, which has a martensitic and ferrite dual phase alloy structure, is a steel that meets these demands.
Lifous steel, which is made by adding P, Mn, Si, etc. to aluminum killed steel, has been developed, but because of its insufficient formability, it cannot be used in areas where deep drawing is performed, such as automobile fenders.

On the other hand, Ti or Nb, which has a strong bonding force with C and N in steel, is added in an amount stoichiometrically equivalent or more to the amount of C or C+N to fix C and N, and furthermore, as a solid solution strengthening element, M
A method of obtaining a steel plate with a high T value and tensile strength by blending n or Si has been proposed.

For example, 0.01% C in Tetsu to Hagane 1979 Stone 11 page 838
By continuously annealing the following ultra-low carbon steel containing 0.25% Ti, 1.5% Mn and 0.22% Si, the tensile strength (hereinafter abbreviated as TS) is 43 kg/
m7j, yield point (hereinafter abbreviated as YP) 22-25
It has been reported that a steel with good workability can be obtained with kg/mi, r value of 1.8, and elongation of 39%.

However, in this steel, in addition to Ti combining with C and N in the steel, S
, 0, it produces a large amount of inclusions and has the disadvantage of being prone to surface flaws.

In addition, the amount of Mn, Si, and Ti added is large, and it is necessary to reduce the addition of a large amount of alloy to 0 and decarburize the molten steel to 0.01% or less, which reduces alloy cost and decarburization. It has the disadvantage of high charcoal cost.

Also, in Japanese Patent Application Laid-Open No. 54-100920, CO,004
%, Si1.01%, Mn0.22%, AlO,025
%, NbO, 400° after continuous annealing to steel consisting of 049%
T S 4' by performing overaging treatment for C13 minutes
2-46kg/ma1YP 28-30kg/ma.

It has been proposed that a high tensile strength steel with a lower value of 1.6 to 1.8 and an elongation of 32 to 35% can be obtained, but it has the disadvantage that the lower value and elongation are low and the strength is insufficient.

The object of the present invention is to provide a high-strength cold-rolled steel sheet with excellent formability that eliminates and improves the above-mentioned drawbacks of conventional high-strength steel sheets and methods of manufacturing the same, and a method of manufacturing the same, and the scope of the claims is as follows: The above object can be achieved by providing a steel plate and a method for manufacturing the same.

Next, the present invention will be explained in detail.

The present inventors have investigated the influence of material composition and hot rolling conditions on the aging properties and deep drawability of ultra-low carbon steel, the amount of P, Si, and Mn used as additives to increase the tensile strength, and the depth of finish annealing conditions. As a result of detailed research on the effects on drawability and secondary force Loe embrittlement, we found that (1) steel containing Al with N equivalent of A [' “Elongation rate is 90% or more, rolling speed is 40ψin or more, coiling temperature is 600%
When processing at ~750°C, the amount of Nb added to C indicates the equivalent amount of NbC.

(2) When the amount of unbonded solid solution Nb is less than 0.020% with respect to C, the elongation is greater than when a large amount of Nb is included, and the lower value is about the same.

When P, Si, and Mn are individually added to a steel having 0.020% or less, the decrease in the lower value is most significant for Mn, followed by Si, and P has the smallest effect.

(4) Batch annealing of ultra-low carbon steel containing 0.05% or more of P causes secondary work brittleness after press working, but 1°
0.10% when continuously annealed at a cooling rate of C/sec or higher
Even if the following P is contained, secondary work embrittlement does not occur.

(5) When containing 0.03% or more of P, Mn or S
i is 0.8% or less, or the sum of Mn and Si is 1.2%
If it is contained below, the r value will be less deteriorated and high strength will be obtained.

The present invention was completed by obtaining the above novel findings (1) to (5).

Next, the present invention will be explained with reference to experimental data and the reasons for limiting the component composition.

Steel having the composition shown in Table 1 was heated to 1250°C under pressure.

Work rate 90%, rolling speed 4 Q m/min, finishing temperature 8
A cold-rolled plate of final thickness obtained by hot rolling at a rolling reduction rate of 80% under conditions of 70°C and a winding temperature of 680°C was annealed for 40 seconds at 830°C by continuous annealing. Product characteristic values (AI value, El value) when subjected to

The relationship between parameter α3NFa/C and parameter βyoNb (to) - 80 (to) on
As shown in the figure.

From Figures 1 to 3, when the parameter α is 3 or more, the AI value, that is, the aging index is 1 kg/my? Below t, the lower value is 1
．． It can be seen that a substantially non-aging steel plate with a high lower value can be obtained above 9, and that the El value and elongation change according to the parameter β, and that a sufficiently high value can be obtained when β is 0.02% or less.

By repeating this experiment, Nb was reduced to 0%1. - Although the β3Nb(a)-8C value, that is, the % of Nb not bonded to C, is required to be 0.02 or less, although it is required to be three times or more.

Note that within the above range, the Nb content is 0.03 to 0.06%.
and within the range of 6XC to 8XC% 0.010%
It is more preferable that the value be within the range of .

C is an element that is effective in preventing grain boundary embrittlement when P coexists, and if it is less than 0.002%, there is no effect, while if it is more than 0.015%, the lower value and elongation will be significantly lowered. Therefore, C needs to be within the range of 0.002 to 0.015%.

Al is N for A7N,! = 0,02% to fix
It is necessary to add more than 4 times as much as NFa.

Otherwise, N in the steel will combine with Nb in the steel.
A large amount of C that is not fixed by b remains, resulting in an inability to reduce the AI value sufficiently.

However, the addition of Al in an amount of 0.1% or more should be avoided, as it will increase the strength of inclusions caused by alumina cracks in the steel and cause surface flaws.

≧ If the N content is high, it is necessary to increase the Al content, and if the N content is more than 0.01%, surface defects will increase due to an increase in inclusions caused by alumina clusters, so the N content is 0.01%. It is necessary to do the following.

P is used as the main reinforcing element.

Although P increases tensile strength, it has less effect on lowering the tensile strength than other reinforcing elements Si and Mn, and when P is reduced to 0.
It has been found through experiments that steel containing about 0.5% P has a higher lower value than steel with low P when alloyed with the same level of Mn or Si.

That is, po, oi%, Si, and Mn are each 0.1%.
The experimental results of changes in YP, TS, El, r and AI per unit are shown in Table 2.

If we calculate the amount of decrease below with respect to the amount of increase in TS based on Table 2, we will find that it is the smallest in the case of P, as shown in the rightmost column of the table.

Next, Si is added to the steel containing about 0.05% of P.

Table 3 shows the results of investigating the influence of Mn on various properties per 0.1% of Si and Mn.

According to Table 1 and Table 2, it can be seen that the rate of decrease in the lower value relative to the rate of increase in TS is smaller than when Si1 or Mn is added to steel with low P content.

Tensile strength 35 kg/wai, which is actually the desired strength level
In order to obtain t or more, it is necessary to make P 0.03% or more.

However, if it exceeds 0.1%, secondary stress embrittlement will occur, so it must be kept below 0.1%, and generally a range of 0.04 to 0.07% is more preferable, although it depends on the strength level. be.

Si is 0.2% or less as a reinforcing element, and Mn is S in steel.
It is used in an amount of 0.04 to 0.8% to fix P and to strengthen the steel, but as mentioned earlier, it tends to significantly lower the lower value and elongation compared to P, so it is rather used secondarily.

Table 4 shows the standard effects of P, Si, and Mn contents on tensile strength (TS), lower value, and elongation (El).

In the steel sheet of the present invention, C2Nb, Al
, N, P, Si, and Mn within the above range,
Regarding other elements, it is sufficient that they meet the conditions required for general cold-rolled steel sheets, that is, S is 0.02%.
Hereinafter, 0 may be approximately o, oos% or less.

In addition, there is no problem in containing or using trace amounts of rare earth elements or Ca as deoxidizing element J, and Mo
, Cu, Ni.

There is no problem in containing a small amount of Cr.

Next, the manufacturing method of the present invention will be explained.

In melting the steel plate of the present invention, any commonly used method can be used alone or in combination.

However, it is necessary to decarburize C in advance at the stage of molten steel, and it is advantageous to perform vacuum decarburization treatment by the Rf-(method), DH method, etc. as a means for this purpose.

It is also advantageous to directly produce ultra-low carbon steel using the pure oxygen bottom-blown converter method (Q-BOP method).

Furthermore, either the conventional ingot forming method or the continuous casting method can be used.

A slab obtained by continuous casting or a slab obtained by blooming a steel ingot produced by a conventional ingot-forming method is subjected to continuous hot rolling.

In this case, the rough thermal temperature of the slab should be 1150° C. or higher, which is necessary for solid solution of NbC in the steel, and a general temperature range of 1150 to 1300° C. is sufficient.

According to the present invention, it is necessary to limit the rolling reduction rate and rolling speed during continuous hot rolling.

That is, the rolling reduction ratio must be such that the total rolling reduction ratio from when the slab undergoes rough rolling until it leaves the finishing rolling stand group is 90% or more.

Also, the rolling speed of finishing stand group is minimum 4Q771
/min.

If the above conditions of downstream rate and rolling speed are satisfied,
During the rolling process, fine Nb of, for example, 1000λ or less
Composite precipitates that appear to be composed of (C, N), AlN, and MnS exist very densely, and C in the steel stably exists around these precipitates, making the steel sheet virtually non-aging. is obtained.

On the other hand, the reduction rate is lower than 90% or the reduction rate is 4
When the speed is slower than Q m/min, the above phenomenon does not occur, and a steel plate having non-aging properties cannot be obtained.

According to the present invention, the hot rolling finishing temperature needs to be 850°C or higher.

If a finishing temperature lower than this temperature is used, the value and elongation aging of the special order will deteriorate.

According to the present invention, the winding temperature must be 600'C or higher.

If the steel sheet is wound at a temperature lower than this temperature, the fixation of C by Nb and the fixation of N by Al will be insufficient, making it impossible to obtain a non-aging steel plate.

From the point of view of AI value, lower value, and E7 value, high winding temperature,
A range of 640 to 750°C is advantageous, and in order to achieve a coiling temperature within this temperature range, measures such as weakening the water cooling after finish rolling or omitting water cooling altogether are taken. be able to.

The hot-rolled coil thus obtained is then subjected to pickling to remove oxide scale according to a conventional method and then cold-rolled, or after cold-rolling, the scale is removed by pickling or grinding.

If the cold rolling rate during cold rolling is less than 60%, the desired lower value cannot be obtained, while if it exceeds 90%, the lower value will be high, but the anisotropy will increase, so according to the present invention, the cold rolling The rate is 70-8
A range of 5% is particularly preferred.

According to the present invention, the cold rolled steel strip obtained as described above is further subjected to continuous annealing.

The annealing temperature and time can be appropriately selected in the range of 700 to 900° C. and 10 seconds to 3 minutes depending on the target material.

Between 700 and 900°C, the higher the temperature, the lower the strength, but the lower value and elongation become larger.

Soaking at 750-850°C for 130-90 seconds is particularly suitable.

After the above soaking and recrystallization, the copper strip is cooled to room temperature*.

If the cooling rate at this time is at most 60°C/min up to 500°C, secondary work embrittlement will occur due to grain boundary segregation of P.

However, the cooling rate is 100°C/Sec due to water cooling etc.
If the temperature exceeds 100°C, the aging resistance will deteriorate, that is, the AI value will increase, but overaging treatment may be performed separately at 300 to 500°C.

After all, the cooling rate is 60°C/min or less, preferably 5~b. The steel sheet of the present invention is non-aging when continuously annealed and does not produce yield elongation. % or less, preferably 1% or less, is acceptable.

Next, the present invention will be explained with reference to examples.

Example Steel 1 whose composition is shown in Table 5. The three types of steel II and II are manufactured using the following manufacturing process 1'l)? (2) j Manufactured via (3).

(1) Steel production, ingot steel I, I was made in a pure oxygen top-blown converter (LD converter) at 100 tOrL, and steel (I) was made in a pure oxygen bottom-blown converter (Q-BOP) at 230 tOrL.

Thereafter, decarburization and deoxidation were performed in both cases by RH degassing treatment.

Processing time is 1. It was 25 minutes for III and 35 minutes for Steel ■.

P and Mn are added immediately before the start of degassing treatment, and Si, Al, Nb
The addition was made immediately before degassing.

Steel 1. Steel II was made into a slab with a thickness of 220 mm by a continuous casting method, and steel (I) was made into a slab with a thickness of 220 mm by an ingot-blowing rolling method.

(2) Hot rolling The above three slabs were subjected to surface treatment and then heated to 128 ml in a heating furnace.
Soaking was carried out at 0°C (surface temperature) for 35 minutes.

Continuous rolling was then carried out using a 4-row roughing mill and a 7-tandem finishing mill.

In the rough rolling mill, the slab was finally made into a 40 mm, 9-inch sheet bar, and further into a 3.2 mm thick hot rolled steel strip in the finishing mill.

At this time, the total rolling reduction from the sheet bar to the hot rolled steel strip was 92%.

Further, the rolling speed in the finishing mill (approximately corresponding to the passing speed on the Kundem roll exit side) was set to 98 m/min in the first stand and 660 m/min in the seventh stand.

The temperature of the sheet cover on the entry side of the finishing rolling machine is 1030-1050°C, and the finishing temperature is 880-910°C.
And so.

Thereafter, the hot rolled copper strip was wound at a winding temperature of 760°C for Steel I, 660°C for Steel ■, and 700°C for Steel ■.

(3) 0.7m by pickling and cold rolling cold rolled hot rolled steel strip
The rolling reduction ratio at this time was 78% to form a cold-rolled coil with a thickness of m.

(4) Recrystallization annealing After cleaning, the cold rolled coil was recrystallized and annealed in a continuous annealing line.

Soaking conditions are steel 800-830℃, 3.0 sec, steel 820-860℃, 405ec1 steel 800-830
It was 0C125sec.

The cooling rate after soaking was in the range of 15 to 20°C/sec in all cases.

The annealed coil that underwent the above steps was subjected to a 0.6% skin pass to produce a product.

The mechanical properties of the product are shown in Table 6.

TS35kg/m7j each from Steel I, II, and III
, class, 40kg/my? Class, 45kg/my? A non-aging, high-strength cold-rolled steel sheet with excellent formability of grade A has been obtained.

As a result of surface inspection, both steel plates were found to be comparable to general Al-killed steel plates, and there were no problems in using them as products.

According to the present invention, since P can be used as a reinforcing element as described above, the amount of Si and Mn added is small, and the cost of dephosphorizing molten steel is low, so the total cost of the material is low.
Furthermore, although it is a DDQ class drawable steel sheet, it does not require overaging treatment after continuous annealing, so it has the advantage of low annealing costs.

[Brief explanation of drawings]

Figure 1 shows the Nb%/C% and A I kg/my? of the steel plate.
Figure 2 shows the relationship between (Nb%-8C%) of the steel plate.
) and EA%, and FIG. 3 is a diagram showing the relationship between Nb%/C% and the lower value.

Claims (1)

[Claims] I C0,002~0.0'15%, Si1.2% or less, Mn 0.04~0.8%, PO,03~0.1
0%, Al is 0.02% or more and N% x 4 or more, Nb
is a high-strength cold-rolled steel sheet with excellent formability, consisting of C% x 3 to (0% x 8 + 0.020%), and the remainder substantially consisting of Fe. 2 C0,002-0.015%, Si 1.2% or less, Mn 0.04-0.8%, PO,03-0.10
%, Al is 0.02% or more and Nb is C
% x 3 ~ (0% x 8 + 0.020%), the remainder is substantially F
A steel slab made of
0% or more and the rolling speed of finish rolling is 4 Q 771
/min or higher and coiled at a temperature of 600°C or higher to obtain a hot-rolled coil, and cold-rolled the hot-rolled coil by a conventional method to obtain a cold-rolled steel strip with a final thickness. 10 sec to 5 m1 at 700 to 900°C to the belt
After continuous annealing at 500°C for 60°C
A method for producing a high-strength cold-rolled steel sheet with excellent formability, characterized by cooling at a rate of ℃/min or more.