JPS62139822A - Production of cold rolled steel sheet for deep drawing having excellent uniformity of material quality - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet for deep drawing having excellent uniformity of material quality by subjecting a specifically composed slab to soaking, holding, and hot finish rolling under specific temp. conditions then growing ferrite particles, quickly cooling the steel sheet and coiling the sheet at specific temp. CONSTITUTION:The slab contg., per weight %, 0.001-0.005% C, 0.05-0.20% Mn, 0.010-0.06% Al, and 0.0010-0.0040% N and consisting of the balance iron and inevitable impurities is subjected to soaking and holding at 950-1,100 deg.C prior to continuous hot rolling. The slab is then hot rolled at 750-930 deg.C and thereafter the rolled sheet is held for about 5-15sec in the finishing temp. range in order to allow the ferrite particles to grow substantially. Such sheet is cooled for about 15 seconds at a cooling rate of about <=5 deg.C/sec. The sheet is then quickly cooled down to the coiling temp. at a cooling rate of about 30-80 deg.C/sec and is coiled at 600-680 deg.C. The coiled sheet is pickled by the conventional method and is cold rolled at about 75-90% draft, then the cold rolled sheet is continuously annealed at the soaking temp. of the recrystallization temp. of above and about <=900 deg.C.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to the production of ultra-low carbon cold-rolled steel sheets for deep drawing that have excellent uniformity of material in the length and width directions within the wound coil. Regarding the method.

(Prior art) In recent years, the required properties of steel sheets for press forming in automobile parts have become more severe than ever before, and the deep drawability indicated by the "value (Lankford value)" Of course, it is required that the material in the wound coil be excellent in uniformity in the length direction and the plate width direction.

When manufacturing a low carbon/1 m2 kill F21 temporary with excellent press formability by continuous annealing, the method described in Japanese Patent Publication No. 5-1341, for example, is required when manufacturing the hot-rolled steel sheet that is the base sheet. In order to improve the lower value and improve the strain aging properties due to N by coagulating and coarsening the cementite and fixing solid solution N as /IN, conventional hatch type annealing was performed. It is known that high-temperature winding is required.

However, even if high-temperature winding is performed during hot rolling, the leading and trailing ends of the coil in the longitudinal direction become the inner and outer peripheral parts of the coil, respectively, and the ends of the coil in the width direction become the inner and outer peripheral parts of the coil. Since the cooling rate also necessarily differs in the central portion, the morphology of cementite and ferrite crystal grains differ between these portions, resulting in non-uniformity in the material inside the coil.

In order to solve this problem, a method has been proposed in which the front and rear ends of the coil are wound at a higher temperature than the center.
After all, it is difficult to make the material inside the coil uniform,
Concomitant problems arise, such as a decrease in pickling properties. To this end, Japanese Patent Application Laid-Open No. 59-219415 describes a method of slowly cooling the coil immediately after winding it at a high temperature, rather than allowing it to cool naturally. The value is still not high enough.

(Object of the invention) The present inventors have discovered that the r
As a result of intensive research in order to further improve the value and uniformity of the material, we have developed an ultra-low carbon AN quilt steel by heating the slab at a low temperature before hot rolling, and by employing controlled cooling after finish rolling. The inventors have discovered that it is possible to obtain a cold-rolled copper plate for deep drawing that has the following tensile properties and deep drawability, and has thus arrived at the present invention.

Therefore, an object of the present invention is to provide a method for manufacturing a cold-rolled steel sheet for deep drawing, which has excellent uniformity of material inside the coil.

(Structure of the Invention) The method for manufacturing a cold-rolled steel sheet for deep drawing with poor material uniformity according to the present invention includes C0.001 to o, o s% by weight, Mn 0.05 to 0.20%. , A1 o, oio ~ 0.06%, N 0.0010 ~ 0.0040%, balance iron and inevitable impurities A steel billet is soaked at a temperature of 950 ~ 1100 ° C. prior to continuous hot rolling. Hold and finish temperature 7
After hot rolling at 50 to 930 ° C., it is maintained in the above finishing temperature range for a sufficient period of time to sufficiently grow ferrite grains, or it is slowly cooled at a slow cooling rate,
It is then rapidly cooled to a coiling temperature, coiled at a temperature of 600 to 680°C, cold rolled, and continuously annealed at a recrystallization temperature or higher.

A slab obtained by small-scale melting of steel consisting of 0.004% C, 0.12% Mn, 0.05% Al, 0.0035% N, and the balance iron and unavoidable impurities is heated to 1050°C (low temperature heating) or 1250°C. (high temperature heating), finish rolling at a finishing temperature of 900 to 910°C, cooling to the coiling temperature under various conditions, coiling at 640°C, cold rolling according to conventional methods, and continuous annealing at 800°C. A cold-rolled steel sheet was manufactured by performing the following steps. Table 1 shows the tensile properties, 7 values, and ferrite grain sizes of these steel plates.

Obviously, when a slab is heated at a low temperature, the resulting cold-rolled steel sheet has excellent tensile properties and 7 values, regardless of the cooling conditions. In addition to low-temperature heating of such slabs, the present invention provides
By controlling the cooling conditions after hot rolling, we succeeded in significantly improving not only the tensile properties but also the lower value.

That is, after finishing rolling, the method is described in more detail below, in which the material is slowly cooled within the finishing temperature range as in the manufacturing method, followed by rapid cooling and winding.

First, in order to investigate the material behavior of the finished hot-rolled coil in the sheet width direction, we hot-rolled a slab heated at a high or low temperature and then cooled it normally at a cooling rate of 30°C/sec. Relationship between hot-rolled finishing temperature, tensile properties, and lower values for hot-rolled plates and hot-rolled plates obtained by hot rolling a slab heated at a low temperature, then slowly cooling it, and then cooling it normally. I asked for The results are shown in Figure 1.

As is clear from these results, when a slab is heated to a high temperature and then cooled normally after hot rolling, the yield strength and elongation of the resulting hot rolled plate not only strongly depend on the finishing temperature, but also , 7 value is also low, so it is not suitable as a steel plate for deep drawing. On the other hand, according to the method of heating a slab at a low temperature and then cooling it normally after hot rolling, the obtained hot rolled plate is
Although the dependence of yield strength and elongation on finishing temperature is weakened compared to the above-mentioned high-temperature heating method, the value of 7 is still low.

However, according to the present invention, by heating the slab at a low temperature, hot rolling, slow cooling, and then normal cooling, the dependence of the tensile properties on finishing temperature is extremely weakened, the material is homogenized, and the 7 value is significantly reduced. improve. However, the lower value generally decreases as the finishing temperature decreases, regardless of the manufacturing conditions. This decrease in the lower value is disadvantageous for deep drawability (20
0) This is because the surface texture develops. However, according to the method of the present invention, when the finishing temperature is set to 750° C. or higher, a lower value of 1.6 or higher, which is required for a cold-rolled steel sheet for deep drawing, can be ensured.

In this way, by low-temperature heating of the slab and controlled cooling after hot rolling until coiling, it is possible to homogenize the material inside the steel sheet and further improve deep drawability. Although not limited in any way, A I N
The state of precipitates such as MnS, MnS, etc. will be different in proportion to 11, and furthermore, by performing the slow cooling after hot rolling, the growth properties of ferrite crystal grains will be improved, and the grain growth will be controlled throughout the interior of the steel sheet. As a result, the uniformity and deep drawability of the steel sheet material are expected to improve.

Next, the chemical composition of the steel used in the method of the present invention will be explained.

C generally has an important influence not only on the tensile properties but also on the deep drawability. In particular, in the continuous annealing method, unlike batch annealing, the steel sheet undergoes rapid heating, short-time soaking, and rapid cooling in this order, so there is not enough time for ferrite grain growth. For this reason, in the method of the present invention employing a continuous annealing method, it is essential to reduce Cl, and the upper limit of the amount added must be 0°005% or less. However, excessively reducing C to an extremely low level will unnecessarily increase steel manufacturing costs, so the lower limit of the amount added is set at 0.001%.

Mn needs to be added in an amount of at least 0.05% in order to fix free S as M n S and prevent red brittleness during hot rolling. However, when added in excess, the steel becomes hard and the deep drawability deteriorates, so the upper limit of the amount added is set at 0.20%.

5olAj2 is added to deoxidize the steel and fix free N due to the formation of Alx. In order to effectively obtain such an effect, it is necessary to add at least 0.010%, and if it is less than 0.0'10%, the fixation of N is insufficient, so the strain A statute of limitations will begin to apply. On the other hand, since adding an excessive amount leads to deterioration of deep drawability and economical efficiency, the upper limit of the amount added is set to 0.07%.

When N remains as free N before or after annealing, it reduces deep drawability and strain aging resistance.
The lower the content, the more preferable. However, in the method of the present invention, it can be assumed that N is completely fixed by low-temperature heating of the slab and slow cooling or retention treatment after hot rolling, so the content is between 0°0010% and 0.0040%. Any range is fine.

As chemical components other than the above, P and S deteriorate deep drawability and ductility when contained in large amounts, so the upper limit of the content of these elements is preferably 0.02% for each. .

Next, hot rolling conditions, cold rolling conditions, and annealing conditions in the method of the present invention will be explained.

In the method of the present invention, a slab having the above chemical components is soaked and maintained at a temperature in the range of 950 to 1100°C prior to continuous hot rolling. The uniform heating of this slab is
This is to improve deep drawability by allowing precipitates such as AAN and MnS to remain in undissolved form, causing them to aggregate and coarsen, thereby increasing the growth of ferrite grains. When the heating temperature exceeds 1100°C, at least a portion of the above precipitates dissolve and re-precipitate during hot rolling and/or annealing, but since the precipitates thus precipitated are fine, the grains Growth is suppressed, resulting in poor deep drawability.

However, if the billet heating temperature is too low, it will be difficult to secure the finishing temperature, so the heating temperature should be set at 950'.
A grade of C or higher. However, the method of obtaining the slab in the present invention is not limited at all, and may be any method using ingot formation or blooming, or continuous casting.

The finishing temperature of hot rolling is in the range of 750 to 930°C. As mentioned above, the finishing temperature is within this range,
In addition, by subsequently performing holding or slow cooling described later in this temperature range, the temperature dependence of the material of the obtained coil can be reduced. However, when the finishing temperature is too low, the processed structure remains and becomes hard.
The lower limit of finishing temperature is 750℃ as it will deteriorate deep drawability.
shall be.

In the method of the present invention, after this finish rolling, in order to allow the ferrite grains to grow sufficiently, the finishing temperature is maintained for a short time as shown by line I in FIG. Cool slowly as shown. For comparison, normal cooling is indicated by a broken line ■. A preferable holding time is in the range of 5 to 15 seconds, and preferable slow cooling is cooling for 15 seconds or less at a cooling rate of 5° C./second or less. However, holding the finish temperature in the above range for a short time includes not only maintaining the temperature at a constant temperature but also cooling the finish temperature range for 5 to 15 seconds. When the holding time is shorter than 5 seconds, or when the cooling rate during slow cooling is faster than 5° C./second, the ferrite grains do not grow sufficiently and the material inside the steel sheet remains non-uniform. On the other hand, if the above-mentioned holding and slow cooling times are too long, the effect of homogenizing the material will be saturated, and furthermore, there are restrictions on equipment, so 15 seconds or less is usually sufficient.

After holding or slow cooling in the above finishing temperature range, the steel plate is rapidly cooled, and from the viewpoint of improving productivity and deep drawability, the cooling rate is preferably 30 to 50 b.

Furthermore, after this rapid cooling, the steel plate is wound into a coil. The winding temperature is in the range of 600 to 680°C.

When the winding temperature exceeds 680℃, it not only shortens the life of the winder but also deteriorates the pickling property.
This is because, if it is lower than , the residual N cannot be completely dissolved as /IN.

The coil thus obtained is pickled, cold rolled, and continuously annealed according to a conventional method. These methods are not particularly limited, but since the steel plate in the present invention is an extremely low C steel plate, the cold rolling rate is 75 to 9.
It is preferable to have a high cold rolling rate of 0%, and the soaking temperature in continuous annealing is equal to or higher than the recrystallization temperature, and is 9%.
The temperature is preferably 00°C or lower.

The cold-rolled steel sheet after continuous annealing may be subjected to temper rolling, leveling, etc., as necessary, for the purpose of shape adjustment, elimination of elongation at yield point, and the like.

(Effect of the invention) Ultra-low carbon Al-killed steel produced by conventional continuous annealing requires high-temperature coiling in order to ensure deep drawability, which causes non-uniformity in the material within the steel plate, resulting in a decrease in yield. This results in a decrease in pickling properties and other problems.

However, according to the method of the present invention, hot rolling conditions for A1 killed steel in which only C is reduced as much as possible without the need for addition of expensive alloying elements such as Ti and Nb or addition of large amounts of alloys. By specifying , the material quality within the steel sheet can be made uniform, and a cold rolled steel sheet with improved deep drawability can be obtained.

Therefore, such a cold-rolled steel sheet not only has excellent press formability, but also is significantly improved in terms of uniformity of the formed part, material cost, and yield compared to conventional cold-rolled steel sheets.

(Examples) The present invention will be described in detail with reference to Examples below, but the present invention is not limited to these Examples in any way.

Example Steels of the present invention and comparative steels having the chemical compositions shown in Table 2 were melted into small slabs with a thickness of 30 l.

After holding this at a heating temperature of 1000°C or 1050'C for 30 minutes, it was finished hot rolled at a finishing temperature of 760 to 925°C (finishing plate thickness 3.2 mm), and then, as shown in Table 2, After holding or slowly cooling under the following conditions,
Rapid cooling to the coiling temperature at the cooling rate shown in Table 2, 620
℃ or 640℃.

The hot-rolled plate thus obtained was pickled and cold-rolled to a thickness of 0.8 mm, then soaked at 850'C for 1.5 minutes in a salt bath, and heated to 400'C for 3 minutes. An overaging treatment of heating for 1 minute was performed, followed by continuous annealing, and then 1.2% temper rolling was performed.

Table 2 shows the seven values of the tempered cold-rolled steel sheets obtained in this way, along with the tensile test results. Note that steels A-H are steels having chemical components within the range specified in the present invention, and steel 1
-M is comparative steel. wJI is Ci, steel J is Mn amount, steel K and L are 5olAj! It and NM are each 5olNl outside the range specified in the present invention.

As is clear from the results shown in Table 2, according to the method of the present invention, when the finishing temperature is in the range of 760 to 920°C,
It is possible to obtain a cold rolled steel sheet with excellent deep drawability, which has a low yield strength of 8 Jf/mm2 or less, a high total elongation of 49% or more, and a 7 value of 1.7 or more. On the other hand, even if the manufacturing conditions are the same, all comparative steels whose chemical compositions are not within the range specified by the present invention have poor yield strength, total elongation, and/or 7 value, and are cold for deep drawing. Does not meet the requirements for rolled steel plate.

Next, comparative steels A1 to A5 have chemical compositions within the range specified by the present invention, but holding or slow cooling conditions after finish rolling, or coiling temperatures are outside the range specified by the present invention. That is, the slab heating temperature for AI was too high, the finishing temperature for A2 was too low, and the slab heating temperature for A3 was neither maintained nor slowly cooled within the finishing temperature range. For A4, the slow cooling time is too short, and for A5, the winding temperature is too low.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the tensile properties and deep drawability (7 values) of a steel plate, slab heating temperature, hot rolling finishing temperature, and cooling conditions, and Figure 2 shows the cooling pattern in the method of the present invention. It is a graph. Figure 1: 37 and ↓ degrees (Ogo) Figure 2: Confirmation

Claims (1)

[Claims] (1) C 0.001-0.005% by weight, Mn 0.05-0.20%, Al 0.010-0.06%, N 0.0010-0.0040%, balance iron and unavoidable A steel billet containing impurities is soaked and maintained at a temperature of 950 to 1100°C prior to continuous hot rolling,
After hot rolling at a finishing temperature of 750 to 930°C, the finishing temperature is maintained at the above finishing temperature range for a sufficient period of time to sufficiently grow the ferrite grains, or it is slowly cooled at a slow cooling rate, and then rolled. Rapidly cool down to temperature 600-68
A method for producing a cold-rolled steel sheet for deep drawing with excellent material uniformity, characterized by coiling and cold rolling at a temperature of 0°C, and continuous annealing at a temperature higher than the recrystallization temperature.