JPS5974237A - Production of galvanized steel sheet for deep drawing having excellent formability - Google Patents

Abstract

PURPOSE:To produce a galvanized steel sheet for deep drawing having excellent formability by subjecting a cold-rolled steel sheet for the galvanized steel sheet to a continuous annealing treatment under specific conditions then galvanizing the same, coiling the galvanized sheet and subjecting the same to an overaging treatment in a batch furnace. 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 then coiled at <=600 deg.C. The coiled sheet is pickled and descaled, and is then cold rolled at >=40% draft to a steel sheet having a required thickness. The sheet is held for a short time in the temp. region of 350 deg.C- recrystallization temp., in succession to which the sheet is subjected to a recrystallization annealing treatment at the recrystallization temp. -800 deg.C. Such a steel sheet is galvanized and coiled, and the coil is put in a batch furnace and is subjected to an overaging treatment at 250-450 deg.C. Fine AlN is deposited in the texture and the galvanized 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 galvanized steel sheet for deep drawing with excellent formability.

As is well known, the manufacturing method for galvanized steel sheets is to roll a steel sheet in a hot finishing mill, wind it up in a down coiler, and then pickle it.
Coated steel sheets are manufactured through the following processing steps: cold rolling, surface cleaning, and plating.In recent years, the base cold rolled steel sheets used to make the deep drawing plated steel sheets have been replaced by rimmed steel and aluminum killed steel. It's starting to happen. This is a high-strength steel material that is not only resistant to aging deterioration due to slight fluctuations in characteristics within the coil due to component segregation, but also has the characteristic that it is not easily deteriorated by aging, but it is also resistant to aging deterioration due to the patch annealing method. By precipitating the nitrogen in the form of fine Al'N during the annealing process, the formability (Lankford value used as an index of fine drawability, hereinafter referred to as 7 value) is brought to an extremely high level. This is because it can be done.

However, in the continuous annealing method, T 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, so the order in which recrystallization proceeds after precipitation of fine AIN is reversed or in a state close to this. 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, the following method has been proposed to improve the seven values in the continuous annealing method.

1. By using a high-temperature winding method for winding with a down coiler, carbide agglomeration and large AIN precipitates are precipitated to improve the value of 1 and recrystallize grain growth.0 2. Continuous annealing furnace The reconsolidation high-temperature annealing temperature is raised to a higher temperature than usual, and the texture of the steel sheet is raised to the ferritic ten austenite region to improve the texture as shown in Figure 9.

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

However, above 1. If high-temperature winding is carried out, descaling properties and surface properties deteriorate, crystal grains become coarser, shape defects occur, and the quality of the material is often degraded as a cold-rolled base material. or,
2. Increasing the reconsolidation high temperature annealing temperature increases the fuel consumption required for continuous furnaces, which reduces efficiency and increases costs. 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 eliminates the above-mentioned drawbacks of cold-rolled steel sheets, which are the base material for galvanized steel sheets, by partially changing the heat pattern during the continuous annealing process without using these methods. This paper proposes a method for manufacturing galvanized steel sheets for deep drawing with excellent formability through aging and overaging treatment.

That is, this invention has carbon content of 0.10% or less and silicon content of 0.10% or less.
20% or less, manganese 0.10-0.40%, phosphorus 0.
030% or less, aluminum 0.02 to 0.15%, nitrogen 0.0025 to 0.020%, and the remainder substantially iron, and then subjected to normal hot rolling to a temperature of 600°C to 800°C.
Wind it up into a coil, pickle it, and then reduce it to 40% or more and 80%.
After cold rolling in the temperature range of 350°C or higher and recrystallization lagoon or lower, preferably 450 to 550°C, 107 to 60
The coil is preheated for 2 seconds, then held for a short time in a temperature range above the recrystallization temperature and below 800°C to perform recrystallization and high temperature annealing. After plating, the coil is wound and reheated 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.

As shown in FIG. 1, curve 1a shows the heat pattern curve of the recrystallization high temperature annealing process and plating overaging treatment of the present method, and curve 1b shows the heat pattern curve of the recrystallization high temperature annealing process and plating overaging treatment of the conventional method. This shows the heat pattern curve of the treatment.In a continuous furnace, the coil is passed through the annealing furnace continuously and annealed continuously from the front. Preheating is maintained for about 10 to 60 seconds in an annealing preheating temperature range (Pre RA ) below the temperature range. This annealing preheating temperature range (Pre R
By applying A), nitrogen (N) in the cold rolled steel sheet can be
It is made to precipitate finely as IN.

In other words, aluminum killed steel, which is the base material of galvanized steel sheets, tends to precipitate fine AIN during the initial stage of recrystallization and high-temperature annealing after cold rolling, which improves the recrystallization texture.
It is known that high 7 values can be obtained. Therefore, the inventor proposed 1. in the continuous annealing method. By keeping the temperature in a temperature range where AIN is easy to precipitate for a short time, AIN is extracted sufficiently, 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, the temperature is raised to the recrystallization high temperature annealing temperature range (RA) near the AI transformation point (the temperature will be described later) and held for about 20 to 120 seconds. The coil is then plated in the plating temperature range (M), then wound into a coil, reheated, and over-aged in the over-aging temperature range of 250 to 450°C to eliminate the cause of aging. This is a method of reducing solid solution carbon.

Incidentally, in the over-aging treatment, even if the so-called post-annealing method is performed in which the over-aging treatment is carried out in a batch furnace due to equipment limitations, there is basically no difference, so boss annealing may be applied. That is, as shown in Fig. 1, after plating treatment is performed from the recrystallization temperature range (RA),
After cooling and winding into a coil, 2
It is reheated to an overaging treatment temperature range (OA) of 50 to 450°C and held at a cold spot for 80 minutes or more.

The reason why the above annealing preheating temperature range (Pre RA) is set to 350℃ or higher and lower than the recrystallization temperature is because AIN is lower than 850℃.
This is because if the amount of heat necessary for the precipitation of AIN cannot be obtained and the temperature exceeds the recrystallization temperature, it will be difficult to achieve the object of the present invention, which is to improve the texture by recrystallizing fine AIN after precipitation.

Therefore, in order to efficiently precipitate AIN, the most preferable annealing preheating temperature range (Pre RA) is 450 to 550°C, and if the temperature is maintained for 10 seconds or more, sufficient AI
N can be precipitated. The longer the AIN deposition time, the more preferable it is, but since this leads to an increase in the length of the equipment, the upper limit is practically about 60 seconds.

The recrystallization part annealing temperature range (RA) above is 8 above the recrystallization temperature.
The temperature may be lower than 00°C or within the same range of 700 to 850°C as usual, but it is better to keep it as low as possible considering the fuel consumption of the furnace. In this regard, by improving the T value, which will be described later, it becomes possible to smooth the recrystallized portion at a temperature above the recrystallization temperature and below 800°C, preferably between 650 and 750°C. The longer the holding time, the better, but a phosphorus annealing treatment effect of about 10 degrees 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 overaging treatment temperature (OA) Vi may be in the same temperature range as normal, 250 to 450°C, which is suitable for aging treatment.
Similarly, the usual aging treatment time of 2 to 4 minutes is sufficient.

The annealing preheating process of the present invention can be easily performed by adjusting the temperature inside the furnace.

In this way, by only partially changing the heat pattern during annealing of the recrystallized part, the 7 value of the finished product is improved, the formability is excellent,
It is possible to produce high-quality products in which the occurrence of strain aging is suppressed.As the T value improves, the various problems described below are also resolved.

In other words, high-temperature winding of about 750°C in a down coiler9
7 value can be reliably improved even if the coil winding temperature is set to 660 u or less. Therefore, it is possible to suppress descaling, 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 for plating.

In addition, the annealing temperature range (RA) of the recrystallized part is normally set to 70% within a short time so that the texture can be improved and the 7 value can be improved.
However, in this invention method, a new preheating stage is provided to improve the 7 value, so the annealing temperature range (RA) of the recrystallization part is increased from 650 to 750 t.
It can be lowered to about 'c'. Therefore, the fuel consumption rate in the continuous reactor can be reliably reduced. Furthermore, it is possible to accurately improve the value of 7 without requiring the addition of titanium or vacuum decarburization.

Further, the cold-rolled aluminum-killed steel sheet of the present invention is applicable to a Sendzimir continuous galvanizing steel sheet of an aluminum-killed cold-rolled steel sheet produced 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.

It is desirable that the carbon content be low in order to improve the drawability.If the carbon content exceeds 0.10%, the ductility decreases due to the increase in strength and the deterioration of the drawability due to coarse grains becomes significant. .10% or less.

If it exceeds 0.20%, the steel plate surface will be colored during annealing and surface defects due to scale will occur.
0% or less is preferable.

A lower amount of manganese is desirable in order to improve drawability, but there is a risk of 010%10% 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%.

If it exceeds o, oao%, the ductility decreases due to its solid solution strengthening, so the o, os of 6% or less aluminum is necessary for the precipitation of AIN, and if it is less than 0.02%, it has little effect ( , 0.15%, A when heating the slab
Since the solid solution of IN becomes incomplete and the ductility decreases due to the refinement of recrystallized grains, 0.02 to 0.15% is preferable.

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 0.020%, the elongation will decrease and the AIN will compete with the aluminum, making the solid solution of AIN incomplete when the slab is heated.

In addition, the coiling temperature after hot finish rolling should be 600℃ or less and 800℃.
The reason for the sales is that when the temperature exceeds 600°C, large AIN will precipitate during cooling after winding, and the original purpose (Pre RA) of fine AIN will not come out. At temperatures below 800°C, the strength of the copper strip during winding is high, making winding difficult and causing manufacturing defects9, increasing the amount of water for cooling or reducing efficiency, and This is because the effect on improving drawing performance 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. The purpose of this is to improve the recrystallization texture.If the reduction exceeds 80%, the total rolling load will increase, resulting in a decrease in workability and deterioration in the flatness of the thickness accuracy, which is necessary for cold-rolled steel sheets. This is because a rolling reduction ratio of 40% or more is required to ensure shapeability, and good drawability cannot be obtained if it is less than 40%.

[Example 1] Next, Table 1 shows the results of comparing the method of this invention and the conventional method using 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. and are shown together.

(Leaving space below) According to Table 1 above, the tensile strength of this invented method is not much different than that of the conventional method, but it can significantly improve the 7 value and has excellent formability. It can be seen that a plated steel sheet for deep drawing can be obtained. Note that even when the overaging treatment was carried out in a batch furnace and Bost annealing was carried out at 800° C. for 14 hours, it was possible to obtain properties equivalent to those of the method of this invention shown in Table 1.

As described above, this invention enables the easy production of galvanized steel sheets for deep drawing of aluminum killed steel by performing a two-step process of preheating and recrystallization annealing in the continuous annealing method. Some 0

[Brief explanation of the drawing]

#! Figure 1 shows the recrystallization annealing and plating treatment of this invention,
It is a chart showing the heat pattern of each process of over-aging treatment in comparison with the conventional one. PreRA: Annealing preheating temperature range, RA: Recrystallization annealing temperature range, M: Plating processing temperature range, OA=Overaging processing temperature range, Ia: Recrystallization annealing plating processing of this invention method, overaging Example of curve of heat pattern in process of treatment, 11): Example of curve of heat pattern in process of conventional recrystallization annealing treatment and overaging treatment 0 Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] Carbon 0.10% or less, silicon 0.20% or less, manganese 0.10-0.40%, phosphorus o, oao% or less, aluminum 002-0.15%, nitrogen 0.0025-0.0
20%, the remainder being substantially iron and unavoidable impurities, is subjected to normal hot rolling, wound into a coil at a temperature of 600°C or less, and after pickling, cold rolled at a rolling rate of 40% or more. After that, preheating is held for a short time in a temperature range of 850°C or more and below the recrystallization temperature, and then held for a short time in a temperature range of above the recrystallization temperature and below 800°C to annealing the recrystallized high part, followed by plating. 1. A method for producing a galvanized steel sheet for deep drawing with excellent formability, which comprises applying the treatment, then winding it into a coil, reheating it in a batch furnace, and subjecting it to an overaging treatment in a temperature range of 250 to 450°C.