JPS60258428A - Manufacture of cold rolled steel sheet having good aging property by continuous annealing - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet for press working having good aging property by hot and cold rolling a steel ingot contg. a specified compsn. of C, Mn, S, SolAl, O, then annealing said material continuously under a specified condition. CONSTITUTION:The steel ingot or slab contg. 0.005-0.025wt% C, 0.02-0.25% Mn, 0.001-0.015% S, 0.025-0.10% SolAl, <=0.01% O is heated to 980-1,200 deg.C, hot rolled, and wound at 600-780 deg.C. Next, when annealing continuously the steel obtained by cold rolling the hot rolled plate, the temp. is rised at about 1- 100 deg.C/sec heating rate, recrystallization annealing is applied at 680-850 deg.C temp. range for 20-200sec, then said sheet is cooled to 250-350 deg.C temp. by <=1.0- 100 deg.C/sec cooling rate, thereafter, immediately or after reheating, over aging treatment is performed at 250-350 deg.C temp. range for 1-12min to obtain the cold rolled steel sheet having good aging property.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing cold rolled steel sheets for press working which have good aging properties through continuous annealing.

Prior Art and Problems As a problem of cold-rolled steel sheets for press working, many energetic studies have been made regarding aging properties. Aging here refers to a phenomenon in which mechanical properties deteriorate due to changes over time after the steel sheet is manufactured, causing press defects such as stretcher strain and press cracking. In the case of At-killed continuously annealed materials that have become popular in recent years, the cause of such aging deterioration is said to be due to the residual solid solution C due to rapid cooling in a shorter time than in box-annealed materials, and therefore recrystallization. After cooling from temperature, it is customary to attach an overaging furnace and perform carbide precipitation treatment within the time required for economic reasons or production efficiency. However, even after such treatment, it is difficult to avoid residual solid solution C in actual VC exceeding the solid solubility limit, that is, aging deterioration, and this is considered to be the biggest cause of hindering the expansion of the application of continuously annealed materials.

In order to increase the non-aging effect under time constraints such as continuous annealing, in principle, after recrystallization annealing, water quenching from a state with a large amount of solid solute C is performed at a cooling rate of 1000℃/cap or more. , and then tempering, but in this case, the introduction of various defects such as quenching distortion, freezing, and depending on the case, the formation of a rapidly cooled structure, and the effects of carbides precipitated within the ferrite grains are also superimposed on these. This will lead to extreme hardness and, in turn, a significant deterioration in workability. On the other hand, if the steel plate is cooled at a cooling rate of 1-b using gas or a mixture of it and water after recrystallization and over-aged at about 400℃, the steel sheet will become soft, but the aging properties will be considerably inferior to the upper side. Become. To avoid this, if the overaging temperature is lowered,
The time required for over-aging treatment becomes longer, leading to lower production efficiency and higher costs.

As mentioned above, in the case of continuous annealing materials of general At-killed steels, it is considered impossible to satisfy both workability and aging properties at the same time. Although the so-called I-F (interstitial free) steel added with Ti, Nb, etc., can be mentioned, it requires a large increase in the cost of raw materials and decarburization, and is currently far from being called a general-purpose steel.

Taking these circumstances into consideration, the present inventors have investigated various continuous annealing conditions in order to obtain good aging properties without impairing workability. I will explain.

First, after recrystallization annealing, the cooling rate must be relatively slow (hereinafter referred to as slow cooling in 1 to b) in order to avoid hardening of the material due to extremely rapid cooling until the overaging temperature is reached. In this case, according to conventional knowledge, the statute of limitations should inevitably deteriorate. Next, as mentioned earlier, in order to improve the aging property, it is appropriate to use a lower temperature than usual (250 to 350°C: hereinafter referred to as low-temperature overaging) where the equilibrium solid solution C is small.

Therefore, in order to achieve the desired aging properties by slow cooling after recrystallization and low-temperature overaging within an allowable time for production, it is necessary to maintain the minimum degree of C supersaturation necessary for carbide nucleation at the start of overaging. , some measure is required to secure sufficient nucleation sites.

The present inventors conducted various studies based on the above-mentioned slow cooling after recrystallization and low-temperature overaging, and found that under specific components and process conditions, despite these limitations, it is possible to We found that non-aging is possible.

That is, according to the experiments conducted by the present inventors, Mns becomes the preferential precipitation site for intragranular carbides during overaging, and when a slab containing an appropriate amount of Mn and S is hot-rolled at a low temperature of about 1 oso℃, part of the MIIS Some of the dissolved Mn and S remain, but the dissolved Mn and S re-precipitate into other undissolved precipitates or inclusions, and the dissolved Mn and S, together with the remaining Mn, become carbide precipitation sites in the subsequent process. It was found that an appropriate size and distribution state could be obtained.

Note that the appropriate MnS size and distribution state referred to here are 0,
005~1.0μ, the average spacing is about 1.5~
Refers to those distributed at less than 5μ.

Structure of the Invention The present invention is constructed based on the above-mentioned new findings, and is a completely new method for manufacturing steel sheets that overcomes the technical bottleneck of achieving both aging properties and workability when dividing into continuously annealed materials. That is, the present invention is CO,005 to +1.025 cm in heavy f-chi,
Mn0.02~0.25 t4.80.001~0.0
15%, 5olA1.0.025-0.10%, 00.
Steel ingots or slabs containing 0.01 inch or less 980-1200
When continuously annealing steel that has been heated and hot rolled to 600 to 780C, then cold rolled to 680 to 8
After performing recrystallization annealing for 20 to 200 degrees in a temperature range of 50℃, annealing at a cooling rate of less than 1.0 to 100C/□□□
Cool to a temperature range of 50 to 350°C, then immediately or reheat to 1 to 1°C in a temperature range of 250 to 350°C.
This is a method for manufacturing a cold rolled steel sheet with good aging properties by continuous annealing, which is characterized by carrying out the over-aging treatment of 2-.

The present invention will be explained in detail below.

First, regarding the composition range of the target steel, if C exceeds 0.025%, it will be present as no-R-rite in the cooling process following recrystallization annealing or at grain boundaries or before cold rolling. Existing large precipitates of the order of magnitude precipitate as carbides at the inclusion interface, making it impossible to ensure sufficient C supersaturation at the start of low-temperature overaging, even if the appropriate size and dispersion state of MnS exists as a precipitation site. However, good aging properties cannot be obtained as a result. On the other hand, if the C content is less than o, oos%, the degree of C supersaturation necessary for C precipitation nucleation is insufficient, and good aging properties cannot be obtained. From the above, the upper limit of C is set at 0.025% and the lower limit is set at 0.005%, but if the most effective and stable way to improve aging is to be aimed at, 0.025% is set.
010 to 0.018% is desirable.

The amount of Mn is required to be at least 0.02 to prevent hot embrittlement and to form MnS, which is effective as a carbide precipitation site. Must not exceed 25%. Therefore, the effect on aging should be optimized by limiting it to 0.02-0.25%, and Mnf? Taking into consideration the deterioration of the r value due to an increase in the amount of carbon, it is desirable that the amount is less than 0.05 to 0.20%.

Regarding the amount of S, although there are slight differences depending on Mnt,
At least 0.001% is necessary for effective MnS formation, and if it exceeds 0.015%, large MnS on the μ order
A large number of ferrite grains remain and the ferrite grains 4 become smaller, resulting in deterioration of aging properties. Therefore 0.001-0.015%
limited to. In order to maximize the effect of improving aging properties, it is desirable that o, o o a be less than 0.010 h.

Regarding the amount of S o 111, a minimum of 0.025% is required in order to completely fix the N of 0.007 or less, which is commonly present in steel.If it exceeds 0.10%, strength increases and elongation occurs. 0.025 to 0.10 as it may cause deterioration.
%, but if you want to fully develop ferrite grains and improve aging properties the most, it is desirable to set it to 0.05 to 0.10%.

0 exists in the steel in the form of an oxide, and if it is too much, it will have a negative effect on the aging properties, as in the case of excess S. In order to completely eliminate this adverse effect, it is desirable to suppress it to 0.0.05% or less, but if it is 0.01% or less, the substantial adverse effect is tolerable, so it is set to 0.01% or less. St amount,
Pt and Nt are not particularly limited, but from the viewpoint of r-value and elongation deterioration, they are preferably 0.3% or less, 0.08% or less, and 0.007% or less, respectively.

When hot rolling a steel ingot or slab containing the above components, the heating temperature must be set lower than usual in order to ensure effective MnS. When heated above 1200°C, most of MnS dissolves, and during the cooling process it reprecipitates mainly at the γ grain boundaries.
'l, has no effect as a precipitation site. On the other hand, if the heating temperature is lower than 980° C., the material will harden, placing an excessive burden on the rolling mill.

Therefore, the upper limit of the heating temperature is 1200℃, and the lower limit is 980℃.
shall be. However, from the viewpoint of maximizing the effect of improving aging properties and ease of operation, the temperature is preferably 1020°C to 1120°C.

The winding temperature after hot rolling is set at 600° C. or higher because it is necessary to sufficiently fix N as AtN, but it must not exceed 780° C. from the viewpoint of scale pickling properties. The most desirable balance between these temperatures is between 700 and 750°C.

As for cold rolling, it is not necessary to particularly limit the rolling reduction, which is usually carried out at a rolling reduction of about 50 to 90.

When performing continuous annealing, the thorn annealing heating rate [H normal passing tl
For items 1 to 1b, a minimum temperature of 680° C. is required for sufficient recrystallization. - Regarding the sumo wrestler limit h, it can be said that a somewhat high temperature is desirable from the point of view of increasing the ferrite grain size and improving aging properties, but if the temperature is too high and the grains become coarse, there is a risk of rough skin. Therefore, it is necessary to keep the temperature below 850°C. The annealing time is 20 minutes as the time required to complete recrystallization.
sec or more is required, and if it is too long, productivity will be hindered, so it must be within 200 (6). Considering the various points mentioned above, in order to efficiently produce balance tops, 7.
It is desirable to perform annealing at 70°C to 830°C for 30 to 120 seconds.

Regarding the cooling rate after recrystallization annealing, it is a factor that can be said to be a premise of the present invention in the first place, and rapid cooling of 100 ° C / 9 ec or more must be absolutely avoided in order to avoid deterioration of workability. The appropriate cooling rate is: Each component mentioned above and hot rolling heating source 1
Although it is related to the degree of aging, if it is less than IC/sec, the supersaturated C at the start of overaging becomes too small, and the effective MnS cannot be utilized, which does not lead to improvement in aging performance, so it is excluded.

Even if the cooling rate is changed at any time during cooling, as long as the cooling rate is within the above range, the desired effect will be obtained in any case, but this will not guarantee the optimal balance between aging properties and workability. /! The temperature range is 650 to 700℃, and above this temperature range is 5 to b. Below this temperature range is 10 to b. Even if the end point temperature of the cooling rate limited here matches the overaging temperature, or is supercooled to below the overaging temperature. However, in any case, it is necessary to set the end point temperature to 350° C. or lower, preferably 300° C. or lower, to ensure a sufficient degree of C supersaturation. However, even if the temperature is lower than 250°C, another carbide (ε phase) other than the normally found 01% carbide will be formed, and the degree of supersaturation with respect to this will appear to be small, so temperatures below 250°C must be avoided. Next, regarding the overaging temperature, it is necessary to carry out the aging in a temperature range of 350° C. or lower (preferably 300° C. or lower) and 250° C. or higher for exactly the same reason as explained for the cooling end point temperature. Even if it is held in the specified temperature range immediately after cooling, whether it is supercooled and reheated and held, or it is not held at all,
A so-called gradient overaging method or step overaging method may be adopted in which the temperature is initially high in this temperature range and the temperature is lowered over time. If the time required from the end of cooling to the end of overaging (the final point when the temperature drops below 250°C) is less than 1°C, C precipitation will not be completed and 12jI! Exceeding I+ significantly inhibits productivity and is therefore excluded from the scope of the present invention. In order to most effectively obtain the effects of the present invention, basically 270 to 300
It is desirable to ensure that the total time corresponding to the temperature range of
It is also effective to provide a thermal cycle that lasts 30 cycles or more in the temperature range of °C.

0.5 to 2.
By performing 0-chi temper rolling, a steel plate with excellent aging properties and excellent workability can be obtained, although the cooling rate after annealing is relatively iI (as expected).

The effects of the present invention will be explained in more detail below based on Examples.

Examples Table 1 After 80% cold rolling, the product was subjected to continuous annealing, and the aging properties and mechanical properties of the finished product were examined.
(A, 1., kf/rxJ).

This gives a uniaxial strain of 10% to the finished board, 1oo""Cx
After artificial aging for 1 hr, tensile again, stress difference before and after aging &A, 1. 7. The smaller this value is, the better the aging property is. Normally 3.0-3.5 kf/I
If it is less than 2 kg/wj, it can be considered that the aging property is quite good (and if it is less than 2 kg/wj, it can be considered that there is no real aging. From this result, it is clear that the aging property of the steel sheet obtained by the present invention is extremely excellent.

Effects of the Invention As described above, according to the present invention, it is possible to produce a steel plate with excellent aging properties and workability even by continuous annealing.

Agent Patent Attorney Masaaki Akizawa 2 people outside

Claims (1)

[Claims] (1) CO, 005-0.025%, Mn in weight%
0.02-0.25%, SO,001-0,015%
, 5otAtO, 025~0.10%, 00.01% or less steel ingot or slab is heated and hot rolled to 980~1200℃, rolled at 6oo~780℃, and then cold rolled steel is continuously annealed. After recrystallization annealing of 20 to 200 (8) in the temperature range of 680 to 850 °C, cool to a temperature of 1.0 to 350 °C, and then immediately or reheat to 250 to 350 °C. A method for producing a cold-rolled steel sheet with good aging properties by continuous annealing, characterized in that an overaging treatment of 1 to 12 degrees Celsius is carried out in a temperature range of 1 to 12 degrees Celsius.