JPH04327A - Production of cold rolled steel sheet excellent in deep drawability and ageing characteristic by continuous annealing - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in deep drawability and ageing characteristic by rolling a cast slab of Al killed steel at a temp. in a specific region, exerting cold rolling, and then carrying out continuous annealing having specific thermal hysteresis. CONSTITUTION:A slab of an Al killed steel having a composition containing, by weight, 0.010-0.038% C, <0.04% Si, 0.05-0.25% Mn, 0.003-0.020% S, 0.010-0.10% Al, and <0.0060% N is hot rolled at >=1100 deg.C into a plate and rolling is performed at >50% reduction of area based on the plate thickness when the surface temp. of the hot rolled plate reaches the upper limit of the temp. range T1 represented by an inequality I, and hot rolling is finished at >=900 deg.C. This plate is coiled at 600-800 deg.C, cooled, and cold-rolled to the final sheet thickness, and the resulting sheet is annealed at 700-900 deg.C and successively cooled from the above annealing temp. down to 690-650 deg.C at 3-20 deg.C/sec cooling rate. Further, this sheet is cooled down rapidly to 200-340 deg.C at 50-200 deg.C/sec cooling rate, heated up to 345-450 deg.C, and cooled down slowly to 200-330 deg.C in >=100sec.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for producing At-killed cold-rolled steel sheets, in which a heated slab is rolled in a specific temperature range, and after cold rolling, a specific The present invention provides a method for producing cold rolled steel sheets with excellent deep drawability and aging properties by performing continuous annealing with a thermal history.

(Prior Art) When using Al-killed steel as a material for cold-rolled steel sheets for deep drawing and manufacturing them by continuous annealing, it has been difficult to achieve workability and aging characteristics equivalent to those of box annealing.

As a solution to this problem, there is a method, for example, as disclosed in Japanese Unexamined Patent Publication No. 58-48635, in which finish rolling is performed from a specific temperature expressed according to the amounts of C, Mn, and P in the steel, and then the steel is rapidly cooled. However, because MnS in the steel, which has a large effect on deep drawability and aging characteristics, is not sufficiently controlled, neither deep drawability nor aging characteristics are sufficient, and depending on the coiling temperature, the surface of the cold rolled steel sheet may become rough. may occur.

In addition, in order to control precipitates in steel and improve workability and aging properties, after heating a cold slab to 1020 to 1180°C, as disclosed in Japanese Patent Application Laid-Open No. 58-31034, There is a method of hot rolling, followed by cold rolling and annealing. In this method, the slab is heated to 1020-1100℃.
When the slab is heated to 1100° C. or higher, coarse grains in the hot rolled sheet tend to occur, which do not occur when the slab is heated to 1100° C. or higher, and therefore the surface of the cold rolled steel sheet tends to become rough.

In addition, it becomes difficult to maintain the finishing temperature at Ar3 or higher over the entire length of the coil. In order to avoid the problem of rough skin, there is a method of hot-rolling and then winding at 630°C or lower, as disclosed in JP-A No. 58-31033, but it is not possible to produce products with sufficient deep drawability. There remain problems such as difficulty in securing the finishing temperature.

(Problems to be Solved by the Invention) Therefore, in the present invention, low carbon At-killed steel is used, and when the slab is heated to 1100°C or higher, Ar
The aim is to manufacture cold-rolled steel sheets that have excellent deep drawability and aging characteristics, which were difficult to achieve with conventional techniques, while maintaining the advantages of being easy to secure on fishing gear and preventing rough skin. It is something to do.

(Means for solving the problem) The present inventors heated a slab to 1100°C or higher, and then
It has been found that the above object can be achieved by rolling at a reduction rate of 50% or more in a specific temperature range corresponding to the S content and the S content, and continuous annealing including rapid cooling and reheating overaging.

The gist of the present invention is that C: 0.01 in weight ratio
0 to 0.038%, Si: 0.04% or less, Mn: 0
．． 05-0.25%, S: 0.003-0.020%
, At: 0.010 to 0.10χ.

A slab containing N: 0.0060% or less, with the balance consisting of Fe and unavoidable impurities, is heated to 1100°C or higher and hot rolled, and the surface temperature of the slab is within the temperature range of T1 shown in the following formula. When the surface temperature of the slab reaches the upper limit of the T1 temperature range in After winding at a temperature below ℃, cold rolling, 700
Annealed at a temperature of ℃ to 900℃, and from this temperature range 6
Heat to a temperature range of 90 to 650°C for 3 to 50°C, then heat to a temperature range of 200 to 340°C to a temperature of 50 to 50°C, and then gradually heat from this temperature range to a temperature range of 200 to 330°C over a period of 100 seconds or more. The present invention provides a method for producing a cold rolled steel sheet with excellent deep drawability and aging properties by continuous annealing.

(However, the lower limit of T1 is 900° C.) The present invention will be described in detail below.

First, the reason for limiting the chemical composition of steel to which the method of the present invention is applied will be explained.

If C is less than 0.010χ, aging deterioration after continuous annealing will be large, which is not desirable. Moreover, when it exceeds 0.038χ, the deep drawability of the product deteriorates. Therefore, the amount of C is 0.010
It was limited to ~0.038χ.

A trace amount of Si causes no problem, but if the content increases, the processability decreases. Therefore, it must be 0.04% or less.

Mn is a necessary component to prevent hot embrittlement, but
If it is less than 0.05χ, FeS will be generated and its effect will not be achieved. Moreover, when o exceeds 25χ, deep drawability deteriorates.

Therefore, the Mn amount was limited to 0.05 to 25χ.

When S is less than 0.003χ, the amount of MnS produced is small, and the crystal grains of the hot-rolled sheet tend to become coarse, causing surface roughness. Moreover, exceeding 0.020χ causes hot embrittlement. Therefore, the amount of S was limited to 0.003 to 0.020χ.

AI is an element necessary to control the amount of oxygen and nitrogen in steel, and in order to precipitate N as AIN after winding up a hot rolled sheet, a minimum of 0.010χ is required. However, when it exceeds 0.10χ, workability deteriorates. Therefore, the amount of AI was limited to 0.010-0.10K.

In particular, in order to coarsen the size of precipitated AIN to the extent that roughness does not occur and improve workability, 0.030
A range of ~o, osoχ is desirable.

Furthermore, the amount of precipitated AIN deteriorates the workability, so the smaller the amount, the better, and therefore the amount of N must be 0.0060X or less.

Next, hot rolling conditions will be explained.

In order to improve deep drawability and aging properties, M in steel is
It is necessary to control the precipitation state of nS, and the present invention is characterized in that this control is performed during hot rolling. To do this, the heated slab must be heated to T1, which is expressed by the Mn content and the S content.
It has been found through many experiments that it is necessary to apply rolling at a reduction rate of 50% or more to the plate thickness when the surface temperature of the slab reaches the upper limit of the temperature range of T1 within the temperature range of T1.

Table 2 shows that after heating slabs of steels A, B, C, D, E, and F shown in Table 1 to 1250''C, rolling was started at various temperatures, and rolling was applied at various reduction rates. The rolling is finished at 900°C or higher, and the hot-rolled sheet is further wound up at 720°C.
The rolling conditions, f-value, and aging index of the cold-rolled steel sheet are shown when 75χ cold rolling and continuous annealing with the heat pattern shown in FIG. 1 are performed.

Figure 2 shows a slab of steel B in Table 1 heated to 1300°C.
Rolling was started at 1150°C, and after forming hot-rolled sheets at various rolling reduction rates up to 1070°C, the sheets were wound up at 720°C, followed by 80χ cold rolling and continuous annealing in the heat pattern shown in Figure 1. This figure shows the effect of the hot rolling reduction ratio on the f-value and aging index when the hot rolling reduction was carried out.

The underlined conditions in Table 2 are outside the scope of the present invention.

As can be seen from Table 1, Table 2, and Figure 2, in order to obtain good deep drawability and aging characteristics, the reduction must be at least 50% within the temperature range T1 determined depending on the Mn content and S content. It is clear that a rate of rolling is required. It has been found that the temperature T1 and the rolling reduction ratio are deeply related to the precipitation of MnS, and when the MnS of the hot rolled sheet was investigated using an electron microscope when rolling was performed according to the conditions of the present invention, it was found that the MnS of the hot rolled sheet was investigated using an electron microscope. When manufactured according to the conditions, 0.05-0.2-MnS is uniformly dispersed, and less than 0.05% fine Mn
It was found that S was not generated. On the other hand, in the case where either or both of the rolling temperature range and rolling amount deviate from the conditions of the present invention, the amount of 0.05 to 0.2-MnS is extremely low, and is 0.051! It was found that a large number of fine MnS of less than m in size were precipitated. Previous tests have shown that a large amount of MnS less than 0.05% inhibits grain growth, so it is necessary to slow down the growth of crystal grains on the (ILI) plane to improve deep drawability during continuous annealing and to lower the f value. Furthermore, when the precipitates of cold-rolled steel sheets were investigated using an electron microscope, it was found that most of the MnS of 0.05 to 0.2n became precipitation nuclei of cementite, and that most of the MnS of less than 0.05r+a It was found that the parts do not become precipitation nuclei of cementite, and it was confirmed that the aging characteristics are good when manufactured within the scope of the present invention, and that the aging characteristics are deteriorated when outside the scope of the present invention. ing.

As mentioned above, the temperature T1 is determined depending on the amount of Mn and the amount of S.
By performing rolling at a reduction rate of at least 50% in a temperature range of A feature of the present invention is that the formation of MnS can be promoted to improve deep drawability and aging characteristics.

In addition, in the above investigation, the composition, hot rolling temperature range, coiling temperature, annealing temperature of continuous annealing, cooling rate, and overaging treatment conditions were changed within the scope of the present invention, or the rolling reduction rate was 50 within the temperature range of T1. It has been found that good deep drawability and aging properties can be obtained even when further rolling is carried out at 900° C. or above and outside the TI temperature range on the premise that rolling is carried out at a temperature of 900° C. or above.

The coiling temperature is 60°C in order to cause sufficient precipitation of AIN.
A temperature of 0°C or higher is required, but if the temperature exceeds 800'C, the crystal grains of the hot-rolled sheet will become coarse, and the surface of the continuously annealed sheet will likely become rough. Therefore, in order to precipitate AIN, prevent coarsening of crystal grains in the hot-rolled sheet, and further improve workability by coagulating cementite in the hot-rolled sheet, the lower limit is 6.
00°C and the upper limit must be 800°C. Figure 3 shows a slab of steel B shown in Table 1 heated to 1200"C,
Rolling was performed at a rolling reduction rate of 70χ with respect to the plate thickness at the time of reaching 1150℃ between 150 and 1080℃, and further rolling of 502 Rolling is carried out at a reduction rate of 920
When the rolling was finished at ``C'' and coiled at 550 to 850℃, followed by 80% cold rolling and continuous annealing in the heat pattern shown in Fig. 1, the f value at the above coiling temperature, the aging This shows the effect on the index.

The reduction rate of cold rolling may be the same as usual, but in order to develop the (111) texture after continuous annealing and to improve deep drawability, a high reduction cold rolling rate of 70% or more is required. preferable.

Next, continuous annealing conditions will be explained. The heating temperature needs to be higher than the recrystallization temperature, but in order to have sufficient workability, it is necessary to anneal at 700°C or higher. However, 90
When annealing is performed at a temperature exceeding 0° C., the steel becomes T-phase, the texture becomes random, and the deep drawability deteriorates significantly. Therefore, annealing at 700-900°C is required. 700~ for energy saving and prevention of surface scratches on cold rolled steel sheets
Annealing at 830″C is preferable. In order to make the material more homogeneous and to obtain a cold rolled steel sheet with good surface properties, it is preferable to change the annealing time depending on the annealing temperature.
For example, in the case of annealing at 700°C, about 60 seconds is good;
In the case of annealing at 0℃, about 20 seconds is good.After this recrystallization annealing, cooling to 690~650"C in 3~B further improves deep drawability and also has good aging characteristics. Cold-rolled steel sheets can be produced. If the cooling rate is slower than 3°C/sec, surface roughness is likely to occur, and if it is faster than 20°C/sec, the effect of improving deep drawability and aging properties by this treatment is lost. 650 even when cooling in 2o”67 seconds
If it is cooled to a temperature lower than 690℃, the aging characteristics will deteriorate.
If cooling is stopped at a temperature higher than "C", the effect of improving deep drawability and aging properties will be lost.

After that, 50 to b is an indispensable condition for improving aging characteristics. If the cooling rate is slower than 50° C./sec, the aging characteristics deteriorate because Fe and C numbers within the crystal grains are not sufficiently generated. Also, if the speed is faster than 200°C/sec, Fe in the grains,
The cooling rate must be lower than this because a larger number of carbon atoms than necessary will be generated, deteriorating workability, and making cooling control difficult at the temperature at which cooling ends. The cooling end temperature is important for producing cold rolled steel sheets that have both good workability and aging properties. In other words, the cooling end temperature is 340℃
If the value exceeds 1, the number of Fe5C in the crystal grains decreases, resulting in deterioration of aging characteristics. In addition, if the cooling end temperature is lower than 200°C, a larger number of Fe5C is generated in the crystal grains than necessary, so although the aging characteristics are good, the yield strength increases and the elongation deteriorates, resulting in insufficient workability. I can't get it. If the cooling end temperature is 200 to 340°C, sufficiently good aging characteristics and workability can be obtained. After this cooling 3
One of the features of the present invention is that the reheating treatment is performed at 45 to 450°C.

This reheating is intended to increase the diffusion rate of C and to speed up the growth of Fe5C that has started to precipitate around MnS at the end of rapid cooling, but if heated to a temperature lower than 345°C Since the diffusion rate of C is slow, this effect is small, and when heated to a temperature exceeding 450°C, Fe5G precipitated around MnS starts re-dissolving into the steel, reducing the number of Fe and C, and the number of Fe and C decreases. In some cases, products with good aging characteristics cannot be produced. After reheating, 100℃ to a temperature range of 200-330℃
The effect of promoting the precipitation of C can be maximized by slow cooling over a period of seconds or more. 330℃
If the steel sheet is slowly cooled to a temperature exceeding 200° C. or a temperature lower than 200° C., precipitation of C as Fe5C will be delayed, making it impossible to produce a steel sheet with good aging properties. Furthermore, even if the time required for this slow cooling is less than 100 seconds, C will not precipitate sufficiently, making it impossible to produce a steel sheet with good aging properties. By applying continuous annealing under specific conditions after rolling with a reduction rate of more than a specific amount in a specific temperature range determined by and excellent aging characteristics.

Examples of the present invention are shown below along with comparative examples.

Example 1 A! shown in Table 3! Slabs of killed sea bream are heated to various temperatures, rolled at various reduction rates as shown in Table 4 within the temperature range of T1, and then finished at 900"C or higher, 7
It was then rolled up at 00''C.Furthermore, it was subjected to cold rolling at 80χ and continuous annealing with a heat pattern as shown in Figure 1.The resulting cold rolled steel sheet was tested using a JISS No. tensile test piece. A tensile test was conducted to measure the 1 value and the aging index (A, 1.).The results of the investigation are shown in Table 4.

From Table 4, slabs whose hot rolling conditions are within the range of the present invention have better deep drawability and aging characteristics especially at slab heating temperatures of 1100°C or higher, compared to slabs whose hot rolling conditions are within the range of the present invention. It can be seen that there is little dependence and it is excellent.

Example 2 Slabs of steel of various compositions shown in Table 5 are heated to 1200°C and at least 50°C within a temperature range of 11.
% or more rolling and continued rolling to 900% or more.
After finishing at 720°C, it was wound up. Furthermore, cold rolling at 80° C. and continuous annealing in a heat pattern as shown in FIG. 1 were performed.

The resulting cold-rolled steel sheet was subjected to a tensile test using a JISS No. tensile test piece. value and aging index (
A, I,) were measured. The material investigation results are shown in Table 6.

From Tables 5 and 6, it can be seen that those having compositions within the range of the present invention are superior in both deep drawability and aging properties compared to those outside the range of the present invention.

Example 3 A slab of steel having the composition shown in Table 7 was heated to 1200°C,
Rolling was started at 1100°C, and rolling was carried out at a rolling reduction of 8 oz up to 960°C. After winding this at 700°C, it was further cold rolled at 80χ and heated at a temperature of 60 seconds as shown in FIG. 4, at T2. T3. T4. T5.
T6. Continuous annealing was performed with various CRI, CR2, and tb. Regarding the cold-rolled steel sheet obtained as a result, J
A tensile test was conducted using an IS No. 5 tensile test piece, and the 1 value and aging index (A, 1.) were measured.

Continuous annealing conditions and material investigation results are shown in Table 8.

From Table 8, it can be seen that specimens whose continuous annealing conditions are within the range of the present invention are superior in both deep drawability and aging properties compared to specimens whose continuous annealing conditions are within the range of the present invention.

No. table The conditions underlined in the middle are outside the scope of the present invention.

(Effects of the Invention) As explained above, the method of the present invention provides Mn in hot rolling.
To improve the deep drawability and aging properties of a cold rolled steel sheet by rolling with a reduction rate of a specific amount or more in a specific temperature range determined by the amount of steel and the amount of S, and further performing continuous annealing under specific conditions. I can do it.

[Brief explanation of drawings]

Figure 1 is a diagram showing the heat pattern of continuous annealing, Figure 2 is a diagram showing the effect of hot rolling reduction on F value and aging index,
FIG. 3 is a diagram showing the influence of the winding temperature on the f value and aging index, and FIG. 4 is a diagram showing the heat pattern of continuous annealing. Fig. 1 800'Cx 60sec Fig. 2

Claims (1)

[Claims] C: 0.010 to 0.038%, Si: 0.038% by weight.
04% or less, Mn: 0.05-0.25%, S: 0.0
03-0.020%, Al: 0.010-0.10%,
A slab containing N: 0.0060% or less, with the balance consisting of Fe and unavoidable impurities, is heated to 1100°C or higher and hot rolled, and the surface temperature of the slab is within the temperature range of T1 shown in the following formula. When the surface temperature of the slab reaches the upper limit of the T1 temperature range in After winding at a temperature below ℃, cold rolling, 700
Annealed at a temperature of ℃ to 900℃, and from this temperature range 6
Cooling at 3-20°C/sec to a temperature range of 90-650°C,
Subsequently, the temperature range is 50-200℃/200-340℃.
Cool for seconds, then heat to a temperature range of 345-450°C, and from this temperature range to a temperature range of 200-330°C for 1
A method for producing a cold-rolled steel sheet with excellent deep drawability and aging properties by continuous annealing, characterized by slow cooling over a period of 0.00 seconds or more. (8900)/(3.1-log(%Mn×%S))-
(%Mn×2000)-230≦T1(℃)≦(890
0)/(3.1-log(%Mn×%S))-230(
However, the lower limit of T1 is 900℃)