JPS5974234A - Production of cold-rolled steel sheet for deep drawing having excellent formability - Google Patents

Abstract

PURPOSE:To produce easily a cold-rolled steel sheet for deep drawing having excellent formability by subjecting the cold-rolled steel sheet to continuous annealing at the heat pattern under specific conditions thereby depositing fine AlN in the texture. CONSTITUTION:A billet of an aluminum killed steel contg. <0.10% C, <0.20% Si, 0.10-0.40% Mn, <0.03% P, 0.02-0.15% Al, 0.0025-0.020% N is hot-rolled to a plate material which is coiled in a 600-300 deg.C temp. range. The hot rolled coil is pickled and descaled, and is then cold-rolled at 40-80% draft, whereby a steel sheet having a required thickness is obtd. Such sheet is heated and held for 10-60sec in a temp. range of 350 deg.C- recrystallization temp., in succession to which the sheet is held for a short time in a temp. range of the recrystallization temp. -800 deg.C whereby the recrystallization annealing is accomplished. Such a sheet is subjected to an overaging treatment at 250-450 deg.C. The cold-rollled steel sheet for deep drawing having excellent formability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing cold-rolled steel sheets for deep drawing with excellent formability.

As is well known, the manufacturing method for cold-rolled steel sheets involves rolling a steel strip in a hot rolling mill, winding it up in a down coiler, and then pickling, cold rolling, surface cleaning, and annealing. Cold-rolled steel sheets are manufactured through various processing steps, and in recent years the material used has changed from rimmed steel to aluminum-killed steel. This is a high-strength steel with little variation in properties within the coil due to component segregation, and is resistant to aging. Fine AI during the annealing process! The Lankford value (r
This is because it can bring the value (referred to as "value") to a very high level.

However, in the continuous annealing method, the fine A/? which is a characteristic of this aluminum guild steel. Generally, the r-value improves significantly due to N precipitation. This is because the heating rate is fast, so the order in which recrystallization proceeds after precipitation of fine AJN is reversed or close to this order. This is AI! in the continuous annealing method. This is because not only no precipitation effect as N can be obtained, but also recrystallized grain growth is inhibited.

Therefore, in recent years, the following methods have been proposed for improving the r value in the continuous annealing method.

1. Winding with a down coiler is performed at a high temperature to cause agglomeration of carbides and precipitation of large precipitates of AJN, thereby improving the r value and growing recrystallized grains.

2. Reconsolidation in a continuous furnace 8 The annealing temperature is raised to a higher than usual temperature, and the texture of the steel sheet is raised to the ferrite + austenite region, thereby improving the texture.

The aim is to improve the r value.

3. Some products have attempted to improve the value of 7 by adding titanium. However, 1. When high-temperature winding is performed, descaling properties and surface properties deteriorate, coarsening of crystal grains, poor shape, etc. occur, and the quality of the material is often degraded as a cold-rolled base material. or,
2. If the annealing temperature is increased, the fuel consumption rate required for a continuous furnace will increase, resulting in a decrease in efficiency and an increase in cost. Also, if titanium is added in 3., titanium will not bond with carbon, so when adding titanium, vacuum decarburization treatment must be performed at that time, and this treatment and the use of titanium will increase costs. There were drawbacks.

The present invention does not use these methods, but instead partially changes the heat pattern during the continuous annealing process to produce a cold-rolled steel sheet for deep drawing with excellent formability that can overcome the above-mentioned drawbacks. It proposes a law.

That is, the present invention has a carbon content of 0.10- or less and a silicon content of 0.1- or less.
20 or less, manganese 0.10-0.40%, phosphorus 0.
0.03 inch or less, aluminum 0.02 to 0.15%, nitrogen 0.0025 to 0.02 inch, and the balance substantially iron, subjected to normal hot rolling to 300 °C below 600 °C.
The above is wound into a coil, and after pickling, cold rolling is performed at a rolling ratio of 40% to 80 mm, and then annealing is carried out by holding for a short time in a temperature range of 350°C or higher and below the recrystallization temperature. The gist of this invention is to perform an overaging treatment in a temperature range of 250 to 450°C.The present invention will be described in detail below. As shown in FIG. 1, 18 shows the heat pattern curve in the reconsolidation 8 annealing process of the present invention, and 1b shows the heat pattern curve in the reconsolidation 8 annealing process of the conventional method. The coil is continuously unwound in a continuous furnace, and the copper strip is passed through the annealing furnace for continuous annealing. ~
Annealing preheating temperature range, which is the temperature range below the recrystallization temperature (
Preheat for about 10 to 60 seconds. By applying this annealing preheating temperature range (PreRA), nitrogen N in the cold rolled steel sheet is finely precipitated as AJN.

In other words, it is known that aluminum killed steel tends to cause AJN to precipitate finely in the initial stage of reconsolidation 8 annealing after cold rolling, which improves the reconsolidation 8 texture and obtains a high 7 value. There is. Therefore, one inventor developed AI! in the continuous annealing method. By keeping the temperature in a temperature range where N is easy to precipitate for a short time, a sufficient amount of l'N is extracted, the texture is improved, the 7 value is improved, and the formability is improved.

Then, the subsequent temperature process is performed in the same manner as in a normal heat pattern. That is, A, recombination near the transformation point 8
The temperature is raised to the annealing temperature range (RA) (the temperature will be described later) and held for about 20 to 120 seconds, during which time it is softened through the process of recrystallization and grain growth, thereby improving moldability. Next, an overaging treatment temperature range of 250 to 450°C (
This is a method in which the temperature is lowered to 250 to 450° C. for 2 to 4 minutes to reduce the amount of solid solution carbon that causes aging.

The above annealing preheating temperature range (PreRA) was set from 350℃ or higher to recrystallization temperature or lower because AJN
This is because the amount of heat necessary for precipitation cannot be obtained, and if the temperature exceeds the recrystallization temperature, it becomes difficult to recrystallize fine AJN after precipitation and improve the texture, which is the object of the present invention. Therefore, l'N''f: For efficient precipitation, the annealing preheating temperature (PreRA) is preferably 450 to 550°C, and if the temperature is maintained for 10 seconds or more, sufficient AJN'i precipitation can occur. Although it is preferable that the AJN precipitation time be as long as possible, the practical upper limit is about 60 seconds since this leads to an increase in the length of the equipment.

The above-mentioned recrystallization 8 annealing temperature range (RA) may be above the recrystallization temperature and below 800°C, or within the same range of 700 to 850°C as usual, but it is as low as possible considering the fuel consumption rate of the furnace. It's better. In this regard, due to the improvement in the lower value described below, recrystallization 8 annealing is possible at a temperature from the recrystallization temperature to 850°C, preferably from 650 to 750°C. In addition, the longer the holding time, the better, but about 20 to 120 seconds, which is within the allowable range of equipment, will give about 10% of the annealing effect. The same temperature range of 250 to 450°C suitable for aging treatment is sufficient, and the usual aging treatment time of 2 to 4 minutes is also sufficient.

The preheating operation for annealing in the above-mentioned furnace can be easily performed by adjusting the temperature in the furnace, thereby providing the preheating process of the present invention. By only partially changing the heat pattern, it is possible to produce a high-quality product with improved 7 value, excellent formability, and suppressed strain aging. Along with this improvement in the lower value, various problems described below will also be resolved.

In other words, it is possible to reliably improve the value of 7 even if the coil winding temperature is lower than 660°C without requiring high-temperature winding of about 750°C in a down coiler. It is possible to suppress scaling, deterioration of surface properties, coarsening of crystal grains, poor shape, etc., and it is possible to obtain an optimal cold-rolled base material.

In addition, the re-annealing temperature range (RA) is usually 70% within a short time so that the texture can be improved and the 7 value can be improved.
Although the temperature is raised to 0 to 850°C, in this invention method, a new preheating stage is provided to improve the lower temperature, so the reconsolidation 8 annealing temperature range (RA) is increased to 650 to 750°C.
It can be lowered to about ℃. Therefore, the fuel consumption rate in the continuous reactor can be reliably reduced. Moreover, there is no need for addition of titanium or different atmosphere decarburization treatment, and it is possible to obtain an improvement in the value of 7 very easily and accurately.

Further, the cold-rolled steel sheet of the present invention is an aluminium-killed cold-rolled steel sheet manufactured by, for example, a continuous casting method or an ingot-forming method.

In this invention, the chemical composition of the steel is limited for the following reason.

A lower carbon content is desirable in order to improve drawability, and if the carbon content exceeds 0.10 mm, a decrease in ductility due to increased strength and a significant deterioration in drawability due to coarse grains will occur.
It was set to 0.10% or less.

If silicon exceeds 0.20, the steel sheet surface will be colored during annealing, and surface defects due to scale will occur.
20- or less is better.

A lower amount of manganese is desirable in order to improve the drawability, but if it is less than 0.10%, there is a risk of red heat embrittlement and manufacturing is also difficult. If it exceeds 0.40%, the recrystallized texture deteriorates and the drawability is significantly reduced, so manganese is preferably 0.10 to 0.4 (l).

When phosphorus exceeds 0.03%, ductility decreases due to solid solution strengthening, so the content is set to 0.03S or less.

Aluminum is Aj? Necessary for N precipitation, 0.02%
If it is less than 0.15 inch, there will be little effect, and if it exceeds 0.15 inch, the solid solution of l'N will be incomplete during slab heating, and the ductility will decrease due to the refinement of recrystallized grains. did.

It is better to have less nitrogen in order to improve elongation, but
Aj below 0.0025%? Precipitation of N is insufficient, and if it exceeds 0.02%, elongation decreases and it competes with aluminum, resulting in incomplete solid solution of l'N when heating the slab. %.

In addition, the coiling temperature after hot finish rolling should be 600℃ or less and 300℃.
The reason for this is that if the temperature exceeds 600°C, large AJN will precipitate during cooling after winding, making it impossible to precipitate fine AJN in (PreRA), which is the original purpose. ℃ or lower, the strength of the copper strip during winding is high, making winding difficult and causing manufacturing defects, increasing the amount of water for cooling, or reducing efficiency.
This is because even if the temperature is below 00°C, the effect on improving the drawing property remains the same.

In addition, the steel plate that has been pickled to remove the surface and undergo scale treatment is cold-rolled at a rolling reduction of 40 inches or more and 80% or less, as with ordinary cold-rolled steel plates, which improves the dimensional accuracy and shape of the finished product. In addition, reduction of more than 80% increases the total rolling load, resulting in decreased workability and poor plate thickness accuracy, and also reduces the plate required for cold-rolled steel sheets. This is because a rolling reduction ratio of 40% or more is required to ensure thickness accuracy and shapeability, and good drawing properties cannot be obtained with a thickness of 40 inches or less.

〔Example〕

Next, Table 1 shows the results of a comparison between the method of this invention and the conventional method, taking the manufacturing process of cold-rolled steel sheets for deep drawing as an example, and also shows the composition of the product and annealing treatment conditions. Indicated.

That is, according to Table 1 above, samples 1 to 3 of this invention
Compared to samples A4 and A5 of the conventional method, the tensile strength was not much different, but both were able to significantly improve the 7 value. It can be seen that this results in a cold-rolled steel sheet for deep drawing with excellent formability.

As described above, this invention produces cold-rolled steel sheets for deep drawing of aluminium-killed steel by performing recrystallization annealing treatment with a preheating process without rapidly heating to high temperatures in the continuous annealing method. It is the most suitable recrystallization annealing method, has excellent formability, and can easily produce cold-rolled steel sheets for deep drawing.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the heat pattern of the recrystallization annealing process of the present invention. Pre-A...Annealing preheating temperature range, RA...
..., Recrystallization annealing temperature range, OA... Overaging treatment temperature range, la... Curve example of heat pattern in the recrystallization annealing process of this invention method, 1b...・・・
An example of a heat pattern curve in the conventional recrystallization annealing process.

Claims (1)

[Claims] Carbon o, ios or less, silicon 0.20% or less, manganese 0.10-0.40%, phosphorus 0.03% or less, aluminum 0.02-0.15%, nitrogen 0.0025-0.0
20%, the balance being substantially iron and unavoidable impurities, is subjected to normal hot rolling to a temperature of 600°C or less and 300°C.
After winding into a coil at the above temperature, cold rolling at a rolling ratio of 401 to 80 inches after pickling, preheating and holding for 10 to 60 seconds at a temperature range of 350 ° C or more and below the recrystallization temperature, Excellent formability characterized by subsequent holding for a short time in a temperature range above the recrystallization temperature and below 800°C to annealing the recrystallized part, and then overaging in a temperature range of 250 to 450°C. A manufacturing method for cold-rolled steel sheets for deep drawing.