JPS6044377B2 - Method for producing soft cold-rolled steel sheets for drawing with excellent aging resistance through continuous annealing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a soft cold-rolled steel sheet for drawing that has excellent aging resistance by continuous annealing. The purpose is to provide a method for manufacturing the same.

In recent years, a technology for manufacturing soft cold-rolled steel sheets for drawing using a highly productive continuous annealing furnace has been developed, and is currently achieving considerable success.

On the other hand, as continuous casting in steel manufacturing progresses, the proportion of N-killed steel in cold-rolled products is rapidly increasing.

Therefore, from the viewpoint of cost and productivity, in order to manufacture soft cold-rolled steel sheets for drawing, low carbon Al-killed steel will be used in the future.
The method of manufacturing in a continuous annealing furnace can be said to be the most desirable.

By the way, in order to manufacture soft cold-rolled steel sheets for drawing by continuous annealing using this low-carbon Al-killed steel, various studies have been conducted on the composition, thermal cycle, etc. As a result, the material level is higher than that of the conventional It has become possible to produce low-carbon Al-killed steel that is equivalent to box-sintered steel, but continuous annealing
Because it is rapidly cooled after heating and soaking, steels that do not contain strong carbide-forming elements (such as Ti), such as low-carbon N-killed steels, tend to have more solid solution C remaining in the final product than box-annealed steels. However, it has a fatal drawback of poor aging resistance. For this reason, even if the material quality is the same as that of box annealed material immediately after manufacturing, during press processing several months later, material deterioration due to aging and recovery of drop point pressing occur, resulting in cracks, stretcher strain, etc. Press problems often occur. In other words, continuous annealing of low carbon N-killed steel with material quality equivalent to box-annealed Al-killed steel and superior aging resistance.
No definitive technology has been developed to stably produce blunt materials. The present invention was created after repeated studies in view of the above-mentioned circumstances, and includes: C; 0.01 to 0.05%;
Mn: 0.15% or less, SOI. Al: 0.020~0
．． A slab of steel consisting of 100% N: 0.0050% or less with the balance consisting of Fe and unavoidable impurities is hot rolled, coiled at 650°C or higher, cold rolled using a conventional method, and continuously annealed to a temperature higher than the recrystallization temperature. Heat and soak for more than m seconds to 600~500'
It is proposed that the material be rapidly cooled from the temperature of C at a cooling rate of 200° C. Isec or more and over-aged at 300 to 500° C.

That is, to further explain the present invention, the aging index (AI) is generally used to evaluate aging properties, and it is well known that the higher the AI, the faster the aging speed and the poorer the aging resistance. That's where it happened.

However, it is still not clear to what extent the AI value must be below to practically avoid the problem of aging. Therefore, the present inventors manufactured cold-rolled steel sheets with different AI using ordinary low-carbon Al-killed steel, and performed an aging test at ℃ for 8 days (equivalent to about 2 months at room temperature). To summarize, it is shown in Figure 1, and Al≦3kg1T1r1
It has been found that if t, recovery from depression point depression can substantially avoid the practical aging problem. Based on these results, the present inventors set N≦3k91Tn1t as a target and pursued various measures to improve the aging resistance of cold rolled steel sheets for drawing by continuous annealing made from low carbon A1 killed steel. By appropriately combining the amount of Mn, the rapid cooling start temperature after soaking in continuous annealing, and the subsequent cooling rate, as mentioned above, N≦3k91WrJi and without causing deterioration of ductility (E1>47%). , E1: total elongation) conditions were found. Reasons for limiting the composition of the steel used in the present invention. The following is an explanation of the manufacturing conditions as appropriate.

When C is less than 0.01%, special degassing treatment is required during steel manufacturing, which is undesirable as it increases the cost for mass-produced materials.

Moreover, if it exceeds 0.05%, it becomes hard, has low ductility, and also reduces deep drawability, which is not preferable. Mn is a main constituent factor in the present invention, together with the rapid cooling start temperature after soaking in continuous annealing and the subsequent cooling rate, and is set to 0.15% or less.

That is, Fig. 2 shows the relationship between the amount of Mn and N and El after continuous annealing at each quenching start temperature (the cooling rate after starting quenching is 200°C Isec or more).
℃, AI≦3k91wd for any Mn, but the El is 47%, which is insufficient ductility for a soft cold-rolled steel sheet for drawing. This is thought to be due to the formation of fine carbide. However, the quenching start temperature is 600-50
At 0°C, if Mn≦0.15%, AI≦3k9 no D. ．． El≧47%.

The reason why lowering the M field and starting quenching from a relatively low temperature results in lower AI and higher ductility is that lowering the Mn increases the amount of C dissolved in solid solution in the ferrite, which increases the temperature immediately after quenching. The large amount of solute C before over-aging increases the drying effect of C precipitation during the subsequent over-aging treatment, reduces the amount of solute C in the final product, and improves AI. In addition, since the quenching start temperature is low, fewer lattice defects, which become C precipitation sites, are introduced immediately after quenching than when the quenching start temperature is high, and therefore, carbide precipitates during overaging treatment. It is recognized that this is due to the fact that the grain size is not so fine as to cause significant deterioration of ductility. Further, when the quenching start temperature is 400° C. or lower, the steel has high ductility with E1>47% for any Mn content, but the N content is 3k91ml or more, resulting in poor aging resistance and is not preferred as a cold-rolled steel sheet for drawing. In other words, when the quenching start temperature becomes this low, the amount of lattice defects introduced immediately after quenching decreases significantly, so the C precipitation driving force during overaging treatment decreases, the AI value increases, and the amount of lattice defects introduced decreases. This is thought to result in high ductility due to the small amount of fine carbide. Based on the above results, h≦0.15%
In order to produce a soft cold-rolled steel sheet for drawing with excellent aging resistance, it is important to set the quenching start temperature to 600 to 500°C and to set the subsequent cooling rate to 2000 C1 sec or more.

SOL. Al is within the range of normal Al-killed steel, that is, 0.
020 to 0.100%, that is, S
Ol. If the Al content is 0.020%, the precipitation of AlN will be slow, and even if it does precipitate, it will be finely precipitated, so the growth of ferrite grains during continuous annealing will be poor and the material will become hard. On the other hand, this SOI
．． If Al is more than 0.100%, the cost will be high and N
This is not preferable because it becomes somewhat hard due to solid solution hardening. The lower N is, the better, but the maximum is 0.0.
050%.

That is, when this N exceeds 0.0050%, the amount of precipitated AlN increases and the material becomes hard, which is not preferable. In the present invention, steel having the above-mentioned composition is continuously cast or made into a slab by ingot rolling, and then hot rolled. In carrying out this hot rolling, the finishing temperature is The conventional high temperature finish rolling is carried out at a temperature higher than the A3 transformation point, and the coiling temperature is 650° C. or higher to complete coarsening of carbide and precipitation and agglomeration of AlN.

Further, cold rolling is performed after pickling or mechanical descaling treatment. This continuous annealing after cold rolling is performed by soaking at a temperature of 1@7 or more to achieve complete recrystallization at a temperature higher than the recrystallization temperature, and for the reason stated in the reason for limiting the amount of iron,
It is rapidly cooled from a temperature of 0 DEG C. to 200 DEG C. at a cooling rate of 1 sec or more, and then maintained at a temperature of 300 to 500' C for 3 or more times to precipitate solid solution C, which is a so-called overaging treatment.

The reason why the cooling rate from the quenching start temperature is specified is to ensure the amount of solid solution C and the amount of lattice defects immediately after quenching in order to promote C precipitation in the subsequent overaging treatment. As an example, FIG. 3 shows N after overaging when a sample with a 0.10% ■ content was rapidly cooled from a quenching start temperature of 55 (s) at various cooling rates.

That is, as is clear from this Figure 3, 20 (generations) Isec
It is clear that if the material is rapidly cooled at the above cooling rate, AI≦3k91i and excellent aging resistance can be obtained. Note that the overaging treatment temperature is set at 300 to 500° C., since it is not possible to sufficiently precipitate solid solution C at a temperature lower than 300° C., and energy cost increases at a temperature higher than 500° C. The treatment time needs to be 3@7 or more to ensure sufficient precipitation of solid solution C. Specific embodiments according to the present invention will be described below.

Each of the test steels shown in Table 1 below was melted and continuously cast into slabs.

That is, TfA2 to TfA6 marked with O are within the scope of the present invention, but these steels have a finishing temperature of 870°C and a finished plate thickness of 2.
It was hot rolled to 8T1$& and wound into a coil at a winding temperature of 680°C.

These hot-rolled coils were descaled by pickling with hydrochloric acid, then cold-rolled at 71% in a cold tandem mill to a plate thickness of 0.8WL, and then continuously annealed under the continuous annealing conditions shown in Table 2. . Continuity conditions other than those shown in Table 25 are as follows:
Line speed 100rT11min1 over-aging treatment:
400~300'CX2nllnl Temper rolling rate: 0.8
~1.0%. Mechanical test values are also shown in Table 2 above, and the invention materials 2B, 3A, 4B, 5A,
All 6A are AI≦3k91i1E1≧47%,
In addition, it is soft and has a high value, and should be fully satisfied as a soft cold-rolled steel sheet for drawing with excellent aging resistance, and is comparable to box-annealed material made of conventional A1 killed steel.

Among these materials of the present invention, samples 2B, 3A and In were
The mechanical test values after 8 days of accelerated aging treatment at ℃ are shown in Table 3, and it was confirmed that no recovery of falling elongation occurred and there was almost no material deterioration. is,
It is understood that the aging resistance of the material of the present invention is extremely excellent. Ru.

According to the present invention as described above, a soft cold-rolled steel sheet for drawing with excellent aging resistance can be stably and precisely produced by an industrially advantageous continuous annealing treatment, and it can be produced industrially. This is a highly effective invention.

[Brief explanation of the drawing]

The drawings show the technical content of the present invention, and Fig. 1 shows the relationship between recovery point elongation and N after accelerated aging treatment (38°C x 8 days) of low carbon AI killed steel. Diagram, 2nd
The figure is a chart showing the relationship between the cooling rate from the quenching start temperature and AI, and Figure 3 is a chart showing the influence of the Mn content and the quenching start temperature on AI and El. The applied range is N≦3k91Trd! T,. El
≧47% The relationship between the quantity and the quenching start temperature is shown, and the quenching start temperature for each measurement point is also shown above.

Claims (1)

[Claims] 1 C: 0.01-0.05%, Mn: 0.15% or less, sol, Al: 0.020-0.100%, N: 0.
A slab of steel with 0.050% or less and the balance consisting of Fe and unavoidable impurities is hot-rolled, coiled at 650°C or higher, cold-rolled by a conventional method, and heated to a recrystallization temperature or higher by continuous annealing.
Soak for more than seconds, then lower the temperature to 200℃ from 600 to 500℃.
A method for producing a soft cold-rolled steel sheet for drawing with excellent aging resistance by continuous annealing, characterized by rapid cooling at a cooling rate of /sec or more and overaging treatment at 300 to 500°C.