JPS62253733A - Method for manufacturing thin steel sheets with excellent deep drawability - Google Patents

Abstract

PURPOSE:To economically obtain a thin steel sheet having excellent deep drawability by hot rolling an Al killed steel contg. C, Mn, N, and T respectively at prescribed ratios at the Ar3 point or above and cooling the same, then subjecting the steel to warm rolling at a specific draft and to recrystallization annealing immediately thereafter. CONSTITUTION:The Al killed steel respectively contg. <=0.01% C, <=0.5% Mn, <=0.0050% N, and Ti at the ratio expressed by the formula is treated in the following manner to obtain the objective thin steel sheet: Said Al killed steel is first hot rolled at the Ar3 transformation point or above down to the sheet thickness subtracted by the warm draft prior to the warm rolling. The steel after the hot rolling is then cooled at a cooling rate of 5-70 deg.C/sec to thoroughly precipitate TiC and in succession, the steel is subjected to the warm rolling at >=40% total draft in a 300-650 deg.C region at <=0.12 coefft. of friction. The steel sheet after said warm rolling is immediately coiled or coiled once at <=300 deg.C and is subjected to the recrystallization annealing at about 700 deg.C or above and below the Ac1 point, by which the objective thin steel sheet having the excellent deep drawability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a hot rolled thin steel sheet having excellent deep drawability by a warm rolling method.

[Conventional technology]

The deep drawability of thin steel sheets is evaluated by the Lankford value (r value), and the T value is improved by annealing recrystallization texture (2221/1
This is achieved by increasing the plate surface component ratio of 2001. The value 7 of hot-rolled steel sheets is usually low, 1 or less, and the deep drawing properties in terms of T value cannot be said to be good.

In an attempt to improve this T value, technology that utilizes warm rolling has recently attracted attention, and several methods of manufacturing thin steel sheets with good deep drawing properties by annealing warm rolled materials have been proposed.

For example, JP-A-56-25930 is characterized in that ordinary low carbon aluminum killed steel is cooled from an Ar point of 4 or below to 50°C or below, and then warm rolled and annealed in the range of 150 to 450°C. In addition, in the Japanese Patent Publication No. 50-28056, the rolling temperature was lowered to 200℃ in hot rolling of low carbon steel.
It is characterized in that finish rolling is completed at a temperature below 550°C, and then decarburization annealing or normal annealing is performed.

In addition, in 18967 of 1896, the cold rolling of thin steel sheets was
It is characterized by being annealed immediately at a temperature of 50 to 350°C.

[Problem that the invention seeks to solve]

In the above method for manufacturing thin steel sheets with good deep drawability by warm rolling, ■ Adverse effects on rolling load and texture due to dynamic strain aging during warm rolling due to solid solution C and NK, ■ Rolling lubrication There are problems such as intra-thickness non-uniformity of the texture and economic efficiency.

For example, in JP-A-56-25930, the temperature is 150 to 450℃.
During rolling, dynamic strain aging occurs and the rolling load increases significantly, making rolling even more difficult than cold rolling, which is impractical. Unfavorable for drawing properties. In addition, the non-uniformity of the texture within the plate thickness caused by rolling lubrication has not been resolved. Special Publication No. 50-28056 targeted rimmed steel, but the situation is different now that U-killed steel due to its continuous rusting properties is the standard, and its deep drawing properties are affected by rolling load and rolling lubrication problems. Not enough. Also, special public introduction 53-18
967 is essentially related to cold rolling, and has a different technological base from the present invention, but if it were to be rolled into a warm rolling process, it would be energy-efficient because it would heat the material at room temperature. Not on point.

[Means and effects for solving the problem] The inventor has developed a method for economically producing thin steel sheets with good deep drawability by adding warm rolling to the extension of conventional hot rolling. After a lot of research, I was unable to come up with a new idea.

In other words, the gist of the present invention is that C: 0.02 cm or less, Mn: 0
．． 5% or less, i:o, ooso% or less, Ti is 4 (
(%C)+-C%N))≦(:%Ti]≦0.05+
4([%c) + 141-%N)) The remainder is substantially F
・After hot-rolling A-killed steel with impurity elements at an Ar point of 5 or more, cooling at a rate of 5°C/s or more and 70°C/s or less, followed by a total rolling reduction of 40 inches or more in a temperature range of 300 to 650°C. The present invention provides an artificial method for producing a thin steel sheet, which is characterized in that warm rolling is performed with a friction coefficient of 0.12 or less, followed by immediate recrystallization annealing, or once coiling at 300° C. or less and then recrystallization annealing.

In the basic research of the present invention, the deep drawability, that is, the lower value, was increased by warm rolling, and the texture after annealing was 1222.
]/[2001 It has become clear that the important and fundamental points for increasing the component intensity ratio are as follows. In other words, (1) in order to develop the rolling texture, a total reduction rate of a certain value or more is required, similar to cold rolling, and (2) sufficient rolling lubrication (Ka to make the preferred rolling texture uniform in the thickness direction) is required. (3) To develop the [2221 component during annealing, dynamic and static recovery does not proceed during warm rolling, and sufficient energy for annealing recrystallization is required. (4) Dynamic strain aging due to solid solution C and N does not occur during warm rolling, and non-uniform deformation does not occur.

The reasons for limiting each condition of the present invention are based on the above points, and the details will be described below. First, regarding the components, C impairs the deep drawability of the steel, and as will be described later, the addition of Ti is intended to fix it as Tic, so it is better to have less C. However, unnecessarily lowering the carbon content would increase costs accordingly, so the upper limit was set at 0.01% as long as the economy in terms of steel manufacturing was not a problem.

Since Mn is also harmful to deep drawability, the upper limit was set to 0.5 inches. The lower limit is not particularly stipulated, but unnecessarily lowering Mn is not economical in terms of steel manufacturing technology and also causes problems in hot workability in relation to S, so 0.05% or more is desirable.

Like C, N is harmful to deep drawability, and moreover, it is fixed as Ti and TIN, so it is better to have less N, but unnecessarily low N
Since , , is a cost store, ft- yields, its upper limit is o, o
It was set as 6 so%.

One of the component characteristics of the present invention is the addition of Ti, which precipitates and fixes C and N as TiC and TIN during hot rolling and during cooling to the warm rolling temperature range, respectively, and solid solution C- ? N
Ti is indispensable for suppressing non-uniform deformation due to dynamic strain aging during warm rolling and the resulting formation of textures that are unfavorable for deep drawability, such as the development of 1101 components. Therefore, the lower limit of Ti The chemical equivalent ratio of C and N, 4 ((%c)+"-CAN)), is required as:

Regarding the upper limit of TiO, solid solution Ti in an amount more than equivalent is effective in suppressing dynamic and static recovery during warm rolling, but this recovery suppressing effect is caused by the precipitated TIN and Ti.
I can fully demonstrate my skills in e. On the other hand, if solid solution Ti becomes excessive, the recrystallization temperature during annealing will increase. In the present invention, T
The upper limit of I is 0.0 as the excess amount over the equivalent of C and N.
It was set at 5%.

Next, hot rolling conditions and warm rolling winding conditions will be explained. In the present invention, first, prior to warm rolling, hot rolling is performed at a temperature equal to or higher than the Ar5 transformation point to a sheet thickness obtained by subtracting the warm rolling rate. This is similar to normal hot rolling), forming fine-grained austenite,
This is to refine the ferrite structure immediately before warm rolling after transformation and develop the texture during warm rolling annealing in a direction favorable to deep drawability. This is because the 2001 component develops and is unfavorable for deep drawing properties.

The cooling rate from hot rolling to warm rolling is T during this cooling.
It is defined by controlling the precipitation of iC.

In other words, C and N are Tic or TIN before warm rolling.
It is important to reduce the solid solution amount of these components to almost zero, and the cooling rate range of the present invention is particularly focused on reducing C to Ti.
It is based on the conditions necessary for precipitation as c. 1st
The figure shows a steel with approximately 0.02% excess T1 added (A steel of Coating 1 described later), hot rolled at 880 tl, then warm rolled at 500°C for 70°C by varying the cooling rate to 500°C. After cooling to room temperature (finish thickness 2B) and then annealing at 750C for a short time, the T value is lower than that, indicating the influence of the cooling rate after hot rolling.When the cooling rate exceeds 701:/s The T value decreases significantly.This is because at such a high cooling rate, it is difficult for TIC to precipitate during cooling after hot rolling. This is thought to be due to the fact that a preferable texture is not formed due to the residual C.Also, the amount of precipitated TlC0 is small, and the accumulation of sufficient driving energy for recrystallization due to the suppression of dynamic recovery due to K does not occur. This is also considered to be the cause of the decrease in T value.In any case, it is important to generate enough TiC during the main cooling, and the upper limit of the cooling rate is set at 70"Cy'. s for the above reasons. On the other hand, the lower limit was set at 5°C/l from a practical standpoint in actual machines, in relation to the length of the cooling zone equipment and the sheet threading speed. In other words, from the viewpoint of T value, it is Although the cooling rate is preferable, this T straight improvement tends to be saturated.On the other hand, in order to achieve an unreasonably low cooling rate, it is necessary to increase the cooling length or slow down the threading speed.
This will hinder productivity.

Regarding N, it has an extremely strong affinity with TI, and most of the N precipitates out at high temperatures during hot rolling, so solid solution N during warm rolling will not be a problem if the steel is TI-added.

Next, the reason why the warm rolling temperature was set to 300°C to 650°C is as follows. In Figure 2, the same steel as in Figure 1 was used, and after the same hot rolling, it was cooled to various temperatures at 50°C/l, and warm rolling of 709b was carried out at a temperature drop of 20°C from that temperature ( The relationship between the T value and the warm rolling temperature is shown when the specimen is cooled to room temperature and then annealed for a short time at 750°C. When the rolling temperature exceeds 650°C, the T value decreases rapidly. This is because even though it is a T1-added steel, at such high temperatures, the amount of dynamic recovery is large and the static recovery progresses rapidly, which is favorable for deep drawability in annealing recrystallization [this is thought to be because the 1111 component does not develop]. . On the other hand, rolling at low temperatures improves the T value, although it tends to saturate; however, from the perspective of rolling load, the deformation resistance of low temperature steel increases, so the Unnecessary low-temperature rolling is not industrially advisable. The above is the reason why the upper and lower limits of the warm rolling source i were defined.

After warm rolling, the strip is immediately recrystallized annealed or
Alternatively, it is rolled up at 300°C or less, and then recrystallized and annealed. The selection of these methods is determined by the equipment layout, and the determining factor for this thermal history arises from the need to suppress static recovery of the temperature-rolled material. That is, if annealing is started immediately after warm rolling, static recovery can be ignored.

During winding and post-annealing, if the temperature exceeds 300°C, static recovery will proceed during the reheating and slow cooling process after winding 9, so the driving force for recrystallization will be small and the 11111 component will not develop. , high r[ cannot be expected. This is the reason for specifying the conditions of the present invention from warm rolling to annealing.

The recrystallization annealing may be rapid short-time continuous annealing or box annealing, and recrystallization is performed at a temperature of about 700° C. or higher and below the AeI point in accordance with the required lower value.

The basic conditions for developing a rolling texture favorable for deep drawing properties and an annealing recrystallization texture include the warm rolling rate and the friction coefficient during rolling. It is well known that cold-rolled material for deep drawing requires a required cold rolling rate to improve the lower value, and for the same reason, the warm rolling effect is exerted and the lower value of approximately 1.2, which is the target of the present invention, is cleared. In order to achieve this, the total rolling reduction in warm rolling must be 40q6 or more, so this was set as the lower limit of the total rolling reduction. As the rolling reduction increases, the lower value improves within a practical rolling reduction range, so no upper limit was specified.

Lubrication during warm rolling is an extremely important element in terms of material quality and rolling operation due to the uniform development of rolling texture (and recrystallization texture) within the sheet thickness. However, the regulation of the friction coefficient of the present invention is This was specified in particular from the perspective of materials. In other words, Figure 3 is the same as Figure 1.
Same as the figure - 500℃, 70 inch rolling of steel, 75 after rolling at room temperature
Regarding 0℃ annealed material, no lubrication during warm rolling at 500℃,
Changes in the lower value are observed by changing the friction coefficient by changing the type and amount of lubricant such as glass, graphite, or mineral oil. It can be seen that when the friction coefficient increases, the lower value is maintained at a high value up to about 0.12, but when it exceeds that value, it rapidly decreases. The reason for this decrease in value is that when the temperature-rolled material has a large friction coefficient, the area near the surface of the rolled material undergoes shear deformation.
1llll l with rolling texture and further annealing recrystallization texture
This is because the components decrease. Further, in this case, the (ilO) component also increases, which has the disadvantage of increasing the in-plane anisotropy Δr. The above is the reason why the friction coefficient f during warm rolling was set to 0.12 or less.

The thus warm-rolled strip is recrystallized and annealed. At this time, it may be annealed immediately, or it may be wound into a coil and then annealed.

In the case of immediate annealing, it is possible to perform continuous rapid heating annealing and then cooling again and winding, or it is also possible to perform rapid heating, winding, and subsequent slow cooling for annealing. The key point is to apply a thermal cycle uniformly within the strip 7a in order to grow the recrystallized grains so as to satisfy the required characteristics. By the way, as mentioned above, when rapid heating is performed immediately after warm rolling, the release of the strain energy accumulated during warm rolling in the process leading to annealing can be ignored, but the high temperature coiling in the process of annealing after coiling can be ignored. In this case, the above-mentioned strain energy is released during the slow cooling process after coiling, and formation of a preferable texture cannot be expected in the subsequent recrystallization annealing. In the present invention, the coiling temperature is set to 300°C or lower in order to reduce the release of strain energy due to the above-mentioned warm rolling and to form a recrystallized texture favorable for deep drawability during annealing. This is because the effect of suppressing the release of strain energy can be expected to be sufficient if it is wound.

[Embodiments of the invention]

Example 1 Using each steel type whose composition is shown in 1, it was hot rolled to 7I11 at 880°C, and then cooled to 500°C at 50°C/s. It was wound up to 9 with an or. The friction coefficient during warm rolling is approximately 0.0 when using a mineral oil system.
It was 09. This rolled material was rapidly heated and annealed at 750°C, and its lower value was investigated. The results are also shown in Table 1.

All steel materials A to C are Ω-killed steels with a trace amount of Ti added that satisfy the composition specifications of the present invention, and the lower value of the final annealed material is 1.
．． It can be seen that it has a sufficient deep drawing property of around 4. On the other hand, D steel is made of Ti71J- material, and its lower value is 1.
1 or less, and E steel is a Ti-added steel, but the addition of T
Since the amount of i is less than the equivalent of C and N, the 7 value after annealing is as low as about 91.1, and the effect of the present invention is not sufficient to ensure deep drawability.

Example 2 Steel C in Table 1 was hot rolled to an intermediate thickness at 880°C, and the subsequent cooling rate and warm rolling conditions were changed to 21! Rolled to l,
It was wound up at 250°C. The friction coefficient during warm rolling is approximately 0,
It was 09''. After that, it was rapidly heated and annealed at 750° C. and the lower value was examined. Table 2 shows the manufacturing conditions up to winding and the lower value measurement results.

Both A2.3 and 45 steel materials have warm rolling conditions and cooling rates up to the warm rolling temperature that are within the specifications of the present invention, and their lower value after rapid heating annealing is around 1.4, which is sufficient deep drawability. It shows. On the other hand, material No. 41 cannot obtain a recrystallized texture preferable for deep drawability because the dynamic recovery amount is large when the warm rolling temperature is too high, and it shows only a 7 value of about 1.1. Flat 4 material is a case where the reduction ratio in coercive rolling is too small and sufficient rolling texture does not develop, so
The lower value after final annealing is also low. Further, in the case of A6 material, the cooling rate during the process leading to warm rolling after hot rolling is too high as specified in the present invention, and a lower value sufficient to ensure deep drawability has not been obtained.

Example 3 A steel of Shishi 1 was hot rolled to an intermediate thickness of 711 at 880°C,
It was cooled to 500°C at a rate of 50°C/I, and then warm-rolled to 70 inches. The friction at that time was about 0.09. This warm rolled material was immediately subjected to rapid heating annealing at 750°C, or was wound up at different temperatures and subjected to rapid heating annealing and slow heating annealing at 750°C. The lower value of each annealed material was investigated, and the results are shown in Table 3 together with the manufacturing conditions.

Table 3 JI & 7 materials were rapidly heated and annealed immediately after warm rolling, then coiled and slowly cooled, and it can be seen that sufficient lower values can be obtained with this method as well. The AIO and Al materials were wound at 300° C. or lower and then subjected to rapid heating annealing and slow heating annealing, respectively, and both exhibited excellent deep drawability. On the other hand, A8 and A9 materials are coiled at a high temperature higher than the specification of the present invention and then subjected to rapid heating or slow heating annealing. A recrystallized texture favorable to the grain size did not develop, and a high 7 value was not obtained.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to reduce the lower value of hot rolled material, which was conventionally 1.0 or less, to 1.2 or more, and to achieve excellent deep drawing properties. Until now, hot rolling - cold rolling -
For many thin steel sheets that are relatively thick and require a high degree of deep drawability, which had to be manufactured by annealing, the finishing process can be expected to eliminate the cold rolling process, and its economic benefits are also immeasurable. It should be noted that applying mild cold rolling after the warm rolling and coiling of the present invention in consideration of adjusting the shape and thickness does not impair the effects of the present invention in any way, but rather improves the cold rolling aggregate composition, and therefore the annealing recrystallization. It is also clear that the texture develops in a direction favorable to deep drawability.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the relationship between the cooling rate and lower value after hot rolling until warm rolling, Figure 2 is an explanatory diagram showing the relationship between warm rolling temperature and lower value, and Figure 3 is during warm rolling. FIG. 3 is an explanatory diagram showing the relationship between the friction coefficient and the 7 values. 850"c-500"c throat sokuku broiled ("c/s") Fig. 1 Oh 4-kari (I series 4ji (Fig. 3)

Claims (1)

[Claims] C0.01% or less, Mn 0.5% or less, N0.0050
% or less, Ti: 4 ([%C]+12/14[%N]≦[
%Ti]≦0.05+4([%C]+12/14[%N
]), Al with the remainder consisting essentially of Fe and impurity elements
After hot-rolling the killed steel at Ar_3 points or higher, it is cooled at 5°C/s or more and 70°C/s or less, and then warm-rolled in a temperature range of 300 to 650°C with a total reduction of 40% or more to achieve a friction coefficient of 0. ．．
A method for manufacturing a thin steel sheet with excellent deep drawability, characterized in that the process is carried out at a temperature of 12° C. or lower, followed by immediate recrystallization annealing, or by winding at 300° C. or lower and then recrystallization annealing.