JPS5974235A - 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 the N contained therein as fine AlN. 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 crystallization temp. -800 deg.C whereby the recrystallization annealing is accomplished. The sheet is then quickly cooled down to <=200 deg.C. The sheet is reheated and is subjected to an overaging treatment at 250-450 deg.C, whereby the cold-rolled steel sheet for deep drawing having excellent formability is obtd.

Description

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

As is well known, the manufacturing method for cold-rolled steel sheets involves the following processes: pickling, cold rolling, surface cleaning, annealing after rolling a steel strip in a hot rolling mill and winding it up in a down coiler. Cold-rolled steel sheets are manufactured through a process, and in recent years the material used has changed from rimmed steel to aluminum-killed steel. This is not only a high-strength steel with little variation in properties within the coil due to component segregation and resistance to aging deterioration, but also has the advantage of not only being a high-strength steel, but also having the characteristics of being resistant to aging deterioration. is precipitated as fine AIN during the annealing process, and can bring the Lankford value (hereinafter referred to as Y value), which is used as an index of formability (deep drawability), to a very high level. It is.

However, in the continuous annealing method, Y due to the precipitation of fine AIN, which is a characteristic of this aluminum killed steel,
It is generally difficult to improve the value. This is because the heating rate is fast, and the order in which recrystallization proceeds after the precipitation of fine At'N is reversed or close to this order. This is because, in the continuous annealing method, not only no precipitation effect as AIN is obtained, but also recrystallized grain growth is inhibited.

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

1. High-temperature winding was chosen for the down coiler, and large precipitates of carbide N and AfN were precipitated to improve the T value and grow recrystallized grains.

2. The recrystallization annealing temperature in a continuous furnace is raised higher than usual to improve the texture of the steel sheet to the ferrite decaustenite region, thereby improving the Y value.

3. There are some products in which the Y value is improved by adding titanium.

However, above 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 cold-rolled base material is often degraded. or,
2. If the recrystallization annealing temperature is increased, the fuel consumption required for a continuous furnace will increase, resulting in a decrease in efficiency and an increase in cost. In addition, when titanium is added in step 3, titanium combines with carbon, so when adding titanium, a vacuum decarburization treatment must be performed at that time, and this treatment and the use of titanium increase costs. There were drawbacks.

This invention proposes a method for producing cold-rolled steel sheets for deep drawing with excellent formability that eliminates the above-mentioned drawbacks by partially changing the heat pattern during the continuous annealing process without using these methods. It is something to do.

In other words, this invention uses less than 0.10% carbon and 0 silicon.
．． 20% or less, manganese 0.10-0.40%, phosphorus 0
．． 0.03% or less, aluminum 0.02-0.15%, nitrogen 0.0025-002%, and the balance substantially iron, and then subjected to normal hot rolling to 300°C below 600'C.
After winding into a coil and pickling, the pressing ratio is 40% or more -1:
After cold rolling at a temperature of 80% or less, 1
Preheat and hold for 0 to 60 seconds, then continue to heat to 800' above the recrystallization temperature.
Recrystallization annealing is performed by holding the temperature in a temperature range below 200°C for a short time, then rapidly cooling it to below 200°C, and then reheating it to 250°C.
The gist of this is to perform overaging treatment in a temperature range of ~450°C.

This invention will be explained in detail below. Figures 1 and 2
As shown in the figure, 1a12a shows the heat pattern curve in the recrystallization annealing process of this invention method, lb,
2b shows the heat pattern curve in the recrystallization annealing process of the conventional method, in which the coil is continuously unwound in a continuous furnace and continuously annealed while passing through the annealing furnace in the form of a copper strip. However, once the copper strip has been cured, it is heated in an annealing preheating temperature range (PreRA) of io to 60°C, which is a temperature range of 350°C or higher and below the recrystallization temperature.
Preheat and hold for about seconds. By applying this annealing preheating temperature range (preRA), nitrogen N in the cold rolled steel sheet can be reduced.
'e fi, 1N so that it is finely precipitated.

In other words, aluminum killed steel has AI! during the initial stage of recrystallization annealing after cold rolling. It is known that N is easily precipitated finely, thereby improving the recrystallized texture and obtaining a high r value. Therefore, in the continuous annealing method, the inventor held the temperature in a temperature range where AIN easily precipitates (
By this, ten AINs are generated and the collective structure is improved.
The purpose is to increase the Y value and improve moldability.

Then, the subsequent temperature process is performed in the same manner as in a normal heat pattern. In other words, the recrystallization annealing temperature range (RA) near the A1 transformation point (temperature is (-'1))
The temperature is raised to 20 to 120 seconds and held for about 20 to 120 seconds. During this time, the material is softened through the process of recrystallization and grain growth, thereby improving moldability. Next, it is rapidly cooled to 200°C or less and then heated for 2 to 4 hours in an overaging treatment temperature range of 250 to 450°C.
This is a method to minimize the amount of solid solution carbon that causes aging.

Further, in the over-aging treatment, even if the over-aging treatment is carried out in a batch furnace due to the equipment, there is basically no difference in the so-called Bosto annealing method, and therefore Bosto annealing may be applied. That is, as shown in Fig. 2, after rapid cooling from the recrystallization annealing temperature range (RA,
OA) and held at the cold spot for 30 minutes or more.

The reason why the annealing preheating temperature range (preRA) was set from 350°C or higher to the recrystallization temperature is that if it is lower than 350°C,
This is because the amount of heat necessary for precipitation of IN cannot be obtained, and if the temperature exceeds the recrystallization temperature, it becomes difficult to recrystallize fine AIN after precipitation and improve the aggregated 4-weave, which is the object of the present invention.

Therefore, the preheating temperature (PreRA) for one-time annealing to efficiently precipitate AIN is preferably 450 to 550°C, and AIN can be sufficiently precipitated by holding the temperature for 10 seconds or more. The longer the AIN precipitation time is, the more preferable it is, but since this leads to an increase in the length of the equipment, the practical upper limit is about 60 seconds.

The recrystallization annealing temperature range (RA) is above the recrystallization temperature and below 800°C, or the same as normal 700-850°C.
O) Any value within the range is acceptable, but it is better to keep it as low as possible considering the fuel consumption rate of the furnace. In this regard, by improving the r value, which will be described later, recrystallization annealing becomes possible at temperatures above the recrystallization temperature and below 850°C, preferably at 650-75°C.
0'C is good. Further, although the longer the holding time is, the longer the holding time is, the better, but a crystal annealing treatment effect of about 10% can be obtained with a holding time of about 20 to 120 seconds, which is within the allowable range in terms of equipment.

In addition, the over-aging treatment moisture content (OA) is the same as normal, 250 to 4.
A temperature range suitable for aging treatment of 50° C. may be used, and a normal aging treatment time of 2 to 4 minutes is sufficient.

The ν゛^-like preheating operation in the above-mentioned furnace can be easily performed by adjusting the temperature inside the furnace. can be provided.

In this way, by only partially changing the heat pattern during re-knot annealing, the T value of the finished product improves, the formability is excellent,
It is possible to produce high quality products by suppressing the occurrence of strain aging. Along with this improvement in the 〒 value, the various intervals h:mouth shown below are also eliminated.

That is, it is not necessary to wind the coil at a high temperature of 750°C in the downcoat 1 arm, and even if the coil winding temperature is set to 660°C or less, it is possible to reliably improve Y@ by 1 degree. Therefore, it is possible to suppress descaling properties, deterioration of surface properties, coarsening of crystal grains, poor shape, etc. caused by high-temperature winding, and it is possible to obtain an optimal cold-rolled base material.

In addition, the recrystallization annealing temperature range (RA) is usually 70°C within a short period of time to improve the T value by modifying the texture.
Although the temperature is increased from 0 to 850°C, in this invention method, a preheating stage is provided at the new stage to improve the Y value, so the recrystallization annealing temperature range (RA) is increased to 650°C. ~7
The temperature can be lowered to around 50°C. Therefore, the fuel consumption rate in the continuous reactor can be reliably reduced. Furthermore, the Y value can be improved very simply and accurately without the need for addition of titanium or vacuum decarburization.

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 carbon exceeds 0.10%, the strength will increase, resulting in a decrease in ductility and deterioration in drawability due to coarse grains.
It was set to 0.10% or less.

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

A lower amount of manganese is desirable in order to improve 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 will deteriorate and the drawability will be significantly reduced, so the manganese content is preferably 0.10 to 0.40%.

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

Aluminum is necessary for the precipitation of AZN, and if it is less than 0.02%, it will have little effect, and if it exceeds 0.15%, the solid solution of AIN will be incomplete during slab heating, and the ductility will decrease due to the refinement of recrystallized grains. The content was set at 0.02 to 0.15% for the first round.

It is better to have less nitrogen in order to improve elongation, but
If it is less than 0.0025%, precipitation of AIN is insufficient,
If it exceeds 0.02%, the elongation decreases and the AZN competes with aluminum, making the solid solution of AZN incomplete during heating of the slab, so it is set at 0.0025 to 0.02%.

In addition, the coiling temperature after hot finish rolling should be 600°C or less.
If the temperature exceeds 600°C, large Ajl'N will precipitate during cooling after winding, and the original purpose of (PreRA) will be that fine AIN will not be precipitated. At temperatures below 300°C, the strength of the steel strip during winding is high, making winding difficult and causing problems in the manufacturing process, increasing the amount of water for cooling and reducing efficiency. This is because even if the temperature is 300°C or lower, the effect of improving the drawability remains the same.

In addition, cold rolling a steel plate whose surface has been descaled by pickling at a reduction rate of 40% to 80% is the same as for ordinary cold rolled steel plates, and improves the dimensional accuracy and shape of the finished product. This is to improve the recrystallization texture, and the rolling load of 80% or more and the total rolling load increase, resulting in decreased workability and deterioration of plate thickness accuracy, etc., and the required plate thickness for cold rolled steel sheets. This is because a rolling reduction ratio of 40% or more is required to ensure accuracy and shapeability, and good drawability cannot be obtained with a reduction ratio of 40% or less.

〔Example〕

Next, taking the manufacturing process of cold-rolled steel sheets for deep drawing as an example, we compared the method of this invention and the conventional method, and the results of implementation are shown in Table 1.
In addition, the composition of the product and annealing treatment conditions are also shown.

From Table 1, that is, Table 1 above, the sample of this invention ^1-1
ti 3 is compared to sample 44 v A 5 of subtone method,
Although there was not much difference in tensile strength, both were able to significantly improve the T value. It can be seen that this results in a cold-rolled steel sheet for deep drawing with excellent formability.

As mentioned above, this invention is most suitable for producing cold-rolled steel sheets for deep drawing of aluminium-killed copper by performing recrystallization annealing treatment with a preheating process without rapidly heating to high temperatures in the continuous annealing method. The result is recrystallization annealing, excellent formability, and easy production of cold rolled steel sheets for deep drawing.

[Brief explanation of the drawing]

FIG. 1 is a chart showing a heat pattern of the recrystallization annealing process of the present invention, and FIG. 2 is a chart showing an example of the heat pattern of another recrystallization annealing process of the invention. preRA... annealing preheating temperature range, h... recrystallization annealing temperature range, OA... overaging treatment temperature range, 1a,
2a... An example of a curve of a heat pattern in the recrystallization annealing process of this invention method, xb, 2'b... An example of a curve of a heat pattern in the recrystallization annealing process of the conventional method. Applicant Sumitomo Metal Industries Co., Ltd. Agent Press 1) Good Interval°')1

Claims (1)

[Claims] Carbon 0.1091; silicon 0.20% or less, manganese 0.10-0.40%, phosphorus 0.03% or less, aluminum 0.02-0.15%, nitrogen 0.0025-0
．． 020%, the remainder consisting essentially of iron and unavoidable impurities, is subjected to normal hot rolling to a temperature of 600"C or less.
Winding into a coil at a temperature of 'c or higher, pressure reduction rate of 40% after pickling
After performing cold rolling at a temperature of 80% or less, the preheating is held at a temperature range of 350'C or above and below the recrystallization temperature for 10 to 60 seconds, and then held for a short time at a temperature range above the recrystallization temperature and below soo'c. Cold rolling for deep drawing with excellent formability, characterized in that it is recrystallized annealed at 200°C, then rapidly cooled to below 200″C, and after reheating is subjected to overaging treatment in a temperature range of 250 to 450″C. Manufacturing method of steel plate.