JP3194120B2 - Manufacturing method of cold-rolled steel sheet for non-aging deep drawing excellent in material uniformity in coil by continuous annealing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a non-aging and stable baking hardening method for the entire length of a coil without using decarburization by vacuum degassing in steel making or using expensive elements such as Nb and Ti. Hereinafter, BH (Bake Hardenabili)
ty)) and a method for producing a cold-rolled steel sheet having excellent press workability by continuous annealing.

[0002]

2. Description of the Related Art When a cold-rolled steel sheet for deep drawing is manufactured by continuous annealing using a low-carbon Al-killed steel, after cold rolling, 7
It is known that good deep drawability cannot be obtained unless winding is performed at a high temperature of 00 ° C. or higher. This utilizes the effect that cementite is aggregated by winding at a high temperature, and AlN is precipitated and coarsened. However, even in this method, the agglomeration of cementite and the precipitation of AlN are insufficient at the inner and outer peripheral portions of the coil having a high cooling rate, and good deep drawability cannot be obtained.

For this reason, in the conventional method, the inner and outer peripheral portions of the coil, which are inferior in deep drawability, are cut off, or as described in JP-A-58-37128, to improve the uniformity of the material in the coil. After winding at a high temperature, there is a method of controlling a cooling rate by inserting a coil into a heat retaining furnace or putting a heat retaining cover on the coil. Although the method of cutting off both ends of the coil is a simple method, it has a drawback of lowering the yield, and the method of putting it in a furnace or covering with a heat insulating cover complicates the process or causes a significant increase in cost. And the like.

On the other hand, Japanese Patent Application Laid-Open No. 1-136 discloses a method in which a good material can be obtained even after winding at a relatively low temperature of 550 to 700 ° C. after hot rolling, aiming at uniformity of the material in the longitudinal direction of the coil.
No. 933. However, when the winding temperature is lowered as described above, the deterioration of the material is obvious, and
The effect of improving the material by winding at a high temperature of 00 ° C. or more is significant.

[0005] Also, JP-A-2-93023 discloses that
For the purpose of uniformity of the material in the longitudinal direction of the coil, first, the hot-rolled sheet is wound at 620 to 670 ° C., and then, in cold rolling and annealing, rapid cooling and reheating after soaking as overaging treatment of continuous annealing. A method for producing a cold-rolled steel sheet which is substantially non-aged by overaging is disclosed. However, when the winding temperature is 620 as described above.
At a temperature of ℃ 670 ° C., the material properties as a whole are deteriorated, and as a result, the material level at the coil end may not satisfy the standard.

Japanese Unexamined Patent Publication (Kokai) No. 63-72829 discloses a steel characterized by controlling C: 0.01 to 0.02%, winding a hot rolled sheet at 700 to 800 ° C., and performing continuous annealing. Accordingly, a method for manufacturing a cold-rolled steel sheet having a uniform material in the coil longitudinal direction is disclosed. However, under these conditions, the level obtained as the material of the coil end portion is low, and it is extremely important to optimize the slab heating temperature as described later. Also, in order to achieve non-aging up to the coil end,
Optimization of hot rolling conditions and continuous annealing conditions is indispensable.

[0007]

As described above, high-temperature winding is indispensable in order to obtain sufficient material properties. However, in this method, since the inner and outer peripheral portions of the coil have a high cooling rate, cementite has a disadvantage. There is a drawback that they do not sufficiently coagulate coarsely and that the precipitation of AlN is incomplete, so that the deep drawability required as a steel sheet for deep drawing cannot be obtained and the yield decreases. In addition, the method of putting in a furnace or covering with a heat insulating cover has several problems such as complicating the process and causing a remarkable increase in cost. Under such circumstances, when a steel sheet for deep drawing is manufactured by continuous annealing using a low-carbon Al-killed steel, development of a technique for improving the material of the inner and outer peripheral portions of the coil has been eagerly desired.

The present invention relates to a method for producing a non-ageing deep-drawn steel sheet by continuous annealing using a low-carbon Al-killed steel as a raw material. It is intended to solve the problem of deterioration.

[0009]

The gist of the present invention is as follows: C: 0.01 to 0.02% by weight;
5% or less, Mn: 0.03 to 0.15%, P: 0.02
%, S: 0.015% or less, Al: 0.04-0.
10%, N: 0.0025% or less, balance is low-carbon Al-killed steel slab made of Fe except unavoidable impurities, soaked to a temperature (ST) 1000 ° C ≦ ST ≦ 10Mn / S + 1050 ° C. Hot rolling at a finishing temperature of Ar ₃ temperature or higher,
Winding at 700 to 770 ° C, followed by cold rolling and continuous annealing, annealing and soaking at 750 to 880 ° C for 30 seconds to
After 2 minutes, 50 ° C / s from 600 ° C or more
At the above cooling rate, it is rapidly cooled to a temperature (T _E ) in the range of 250 to 320 ° C., kept at T _E for 0 to 5 seconds, and then continuously at 10 ° C. to a temperature (T _R ) between 300 to 370 ° C.
/ S and then cooled to a temperature (T _F ) between 250 and 300 ° C. to perform overaging treatment for 30 seconds or more. The present invention relates to a method for producing a cold-rolled steel sheet for non-aging deep drawing having excellent heat resistance.

Hereinafter, the present invention will be described in detail. In order to ensure the workability and strain aging resistance of the product sheet over the entire length including the coil end even in short-time annealing such as continuous annealing, it is necessary to follow the following basic concept based on new knowledge There is. That is, excellent workability is achieved by i) sufficient grain growth and ii) formation of {111} recrystallized texture which is favorable for deep drawability, and for that purpose, optimal (C, Mn), (Al , N), the combination of low-temperature heating and high-temperature winding of the slab of hot rolling is essential. on the other hand,
In order to achieve the strain aging resistance while imparting the BH property, it is essential to control the solid solution C to a narrow range of 2 to 6 ppm after a relatively short overaging treatment. This can only be achieved with an optimal heat cycle based on the kinetics of nucleation and growth within the grains.

The present invention will be described in more detail.
The reasons for limiting the chemical components are as follows. C is 0.
It must be at least 01% and at most 0.02%. If C exceeds 0.02%, the product becomes hard, and the average r value, which is an index of deep drawability, also deteriorates. This is because C is 0.
If the steel content is more than 02%, i) the volume fraction of cementite increases, the cementite hardens, and the average r value also deteriorates. ii) Further, when C exceeds 0.02%, even if the amount of Mn is 0.1
Even when the content is 5% or less, since the cementite precipitates finely at the coil end of the hot-rolled sheet where the cooling rate after winding is particularly high, a large amount of the Mn-C complex that inhibits the deep drawability is present during annealing. I do. As a result, the {111} texture does not develop, and the average r value deteriorates. On the other hand, when C is less than 0.01%, the strain aging resistance is deteriorated, which is not preferable.

Although Si improves the strength of the steel sheet, it deteriorates the surface properties, so the upper limit is set to 0.5% for exterior panels. Mn has a lower limit of 0.03% as a measure against hot embrittlement. When Mn exceeds 0.15%, even if C is 0.02% or less, Mn-C
The concentration of the complex increases and the average r-value degrades. Furthermore, when the Mn content is as low as 0.15% or less, the number of MnS, which is important as cementite nuclei in grains during overaging, is 0.1%.
Since it is remarkably increased as compared with the material exceeding 5%, it promotes the nucleation of cementite in the crystal grains at the time of overageing, and is extremely advantageous for non-aging.

Since P increases the yield strength and degrades the average r-value, the upper limit is made 0.02%, preferably 0.01% or less. The upper limit of S is set to 0.015% from the viewpoint of preventing measures against hot embrittlement of low Mn steel. Al content, N
The balance of the amount is also extremely important from the viewpoint of ensuring workability.
That is, since AlN present during annealing impairs the grain growth, it is preferable to reduce the amount of precipitated AlN by reducing the total amount of N, or to make it as coarse as possible before cold rolling to make it harmless. . If the amount of N exceeds 0.0025%, the amount of AlN becomes too large, and the crystal grain size of the annealed plate becomes small and hardened. When the Al content is less than 0.04%, it is difficult to sufficiently precipitate AlN at the end of the hot-rolled sheet having a high cooling rate to fix and detoxify the AlN, the crystal grain size after annealing is small, and the average At the r value, the amount of the remaining solid solution C is large, and the non-ageing is not caused. On the other hand, when the Al content exceeds 0.10%, AlN is sufficiently precipitated and coarsened during hot rolling,
This leads to higher costs. Further, solid solution Al causes hardening.

The molten steel adjusted to the above chemical composition range is formed into a slab, and hot rolling-winding-cold rolling-continuous annealing-
The product is processed through the temper rolling process. Hot rolling conditions are extremely important in the present invention. First, the slab is soaked at a temperature (ST) determined by the following equation, and then hot-rolled. 1000 ° C. ≦ ST ≦ 10 Mn / S + 1050 ° C. (1) The finishing temperature of the rolling is set to ₃ points or more of Ar, and the hot-rolled sheet is wound by 7
00 to 770 ° C.

The reason for limiting the slab heating temperature as described above is as follows. The steel subject to the present invention is a steel having a lower Mn than conventional steels for the purpose of improving workability and non-aging property at the coil end. A problem with such steel is ear cracks that occur in the width direction of the hot-rolled sheet. As a result of a detailed study by the present inventors, it has been found that low-temperature slab heating represented by the above formula (1) is extremely effective in preventing ear cracks. Therefore, it is necessary to control the upper limit of the slab heating temperature as shown by the expression (1), and if it exceeds this, ear cracks occur. On the other hand, the lower limit is the lowest slab heating temperature at which the finishing temperature can be maintained at the Ar ₃ point or higher, and it depends on the hot rolling mill.

Further, if the slab is heated at a low temperature as shown in the equation (1), the precipitates of the hot rolled sheet become coarse. this is,
This is because coarse MnS present in the slab remains undissolved or in the case of steel containing high Al such as the steel of the present invention, AlN precipitates with MnS as a nucleus during low-temperature heating. . Such a composite precipitate has a size sufficiently larger than conventionally thought of AlN which is solely precipitated, and therefore does not hinder grain growth during annealing. Therefore, it also has the effect of improving the workability of the annealed plate.

If the winding temperature is lower than 700 ° C., particularly at the end of the hot-rolled coil, i) insufficient precipitation and coarsening of AlN, and ii) fine dispersion of cementite, the average of the annealed sheet is reduced. The r value decreases, and the aging property also deteriorates.
Therefore, the winding temperature must be at least 700 ° C.
On the other hand, if the winding temperature exceeds 770 ° C., the crystal grains of the hot-rolled sheet become abnormally coarse, which causes a decrease in the average r value and rough skin.

The conditions of the continuous annealing are extremely important to impart good press formability and strain aging resistance to the steel sheet of such a new component system over the entire length of the coil. Annealing and soaking conditions of continuous annealing are 750 to 880 ° C. and 30 seconds to 2 minutes. If the temperature is lower than this condition or if the time is too short, the annealing will be insufficient and the workability will deteriorate. High-temperature annealing exceeding 880 ° C. causes the austenite fraction to be too high during annealing, and the average r value is degraded. In addition, it is not preferable because it poses a problem in terms of sheet passing property and cost in industrial terms. If the soaking time exceeds 2 minutes, the passing speed will be significantly reduced, and the productivity will be extremely poor.

The temperature at which rapid cooling is started at a cooling rate of 50 ° C./s or more from the above annealing temperature needs to be 600 ° C. or more in order to secure supersaturated solid solution C. because,
Supersaturated solid solution C even if quenching is started from a temperature below 600 ° C
Is not secured, and the strain aging resistance is poor. 50 ° C /
The same is true for the need for a cooling rate of s or more. Under these conditions, the degree of supersaturation of C is sufficiently maintained, and cementite having a sufficient density can be precipitated in the crystal grains.

The end point temperature of the quenching is in the range of 250 to 320 ° C., and the temperature is maintained at 0 to 5 seconds. If the cooling end point temperature is lower than 250 ° C., temperature control becomes difficult. On the other hand, when the temperature exceeds 320 ° C., the effect of precipitation of cementite in the grains disappears. The effect of the retention at the quenching end point temperature is sufficient if 0 to 5 seconds. Such a retaining effect has a role of nucleating cementite in crystal grains. Also, this effect saturates in a few seconds of retention, and is inefficient if retained for longer. In addition, it is not preferable to keep the retention time longer than 5 seconds industrially because furnace equipment becomes longer, equipment costs increase, and line speed is lowered to lower productivity. Therefore, the retention time is set to 0 to 5 seconds.

After the retention, the steel sheet is reheated, but if the reheating rate is less than 10 ° C./s, the equipment of the furnace becomes too large to be industrially feasible. Next, the reheating temperature (T _R ). If T _R is less than 300 ° C., even if nuclei of cementite are formed in the bent crystal grains, C is not sufficiently diffused, so that cementite cannot grow. If the temperature exceeds 370 ° C., the diffusion of C becomes sufficiently fast and cementite can grow, but the reheating temperature range becomes too large, and the production cost and the equipment cost increase. Therefore, the upper limit is set to 370 ° C.

Next, regarding the end point temperature (T _F ) of overaging, when T _F is less than 250 ° C., if the overaging time is short, the amount of residual solid solution C becomes too large and the strain aging resistance is not obtained. . On the other hand, when the overaging time is sufficiently long, the amount of solid solution C decreases too much, and BH property cannot be imparted. Also, T _F is 3
If the temperature is higher than 00 ° C., the amount of residual solid solution C is too large and the strain aging resistance is lost. On the other hand, if the overaging time is less than 30 seconds, the strain aging resistance cannot be obtained even with a small-scale supercooling and reheating technique such as the present invention, and even with a gradient overaging technique.

[0023]

【Example】

Example 1 Steel having the chemical components shown in Table 1 was tapped in a converter, turned into a slab by continuous casting, and then heated to 1120 to 1150 ° C, with a finishing temperature of 880 to 920 ° C and a sheet thickness of 4 Hot rolling was performed so as to have a thickness of 0.0 mm, followed by cooling at an average cooling rate of about 35 ° C./s on the run-out table, and then winding at 710 to 720 ° C. After pickling, cold rolling was performed to 0.8 mm, followed by continuous annealing. The continuous annealing conditions are as follows: annealing temperature: 810 ° C., soaking: 50 s, initial slow cooling rate: 5 ° C./s to 675 ° C., rapid cooling rate: 1 to 290 ° C.
00 ° C / s, holding conditions: 0 s, reheating rate: 40 ° C /
s, gradient conditions: (reheating temperature (T _R ) = 350 ° C., gradient end temperature (T _F ) = 270 ° C., linear gradient of 150 seconds), water cooling from T _F. Thereafter, 1% temper rolling was added and subjected to a tensile test. Tensile test pieces were collected from positions corresponding to the innermost and outermost peripheral portions and the central portion of the coil when the hot-rolled sheet was wound.

The tensile test was carried out using JIS Z 2201 and No. 5 test pieces according to the method described in Z 2241. The average r value is an in-plane average obtained at a 15% tensile strain. The aging effect was evaluated by the aging index (AI). AI is 100 ° C.-6 after applying 10% tensile prestrain.
This is the amount of change in stress when a 0-minute accelerated aging treatment is performed and a tensile test is performed again.

Table 1 shows the test results.

[0026]

[Table 1]

The steels of the present invention are steels B, E, H and I, which are not aged (AI ≦ 35 MPa) over the entire length of the coil and have excellent deep drawability ( average r value ≧ 1.5). A cold-rolled steel sheet for non-aging deep drawing excellent in uniformity of the material in the coil can be obtained. On the other hand, steels A and K have an excessively low C content, and thus have poor strain aging resistance even with the overaging treatment as in the present invention. Further, steel K has an excessively low Al content and an excessive N content, so that the average r value is inferior and N aging also occurs. Further, the uniformity in the longitudinal direction is poor. Steels C and J have an excessively high Mn content, and therefore have a low average r value and poor uniformity in the longitudinal direction. Furthermore, due to the high C content than steel J steel K Rights
The average r value is further deteriorated, and the uniformity is poor. Steel D is
Steel F has a too high C content due to low Al content, and steel G has a high C content.
Since the amount of N is too large, the average r value is deteriorated and the uniformity is poor.

Example 2 A steel having the components shown in Table 1B was melted and formed into a slab by continuous casting, and heated to a temperature of 1140 ° C.
Hot rolling at a temperature of 10 ° C., winding at a temperature of 650 to 720 ° C. to form a coil, pickling, cold rolling at a rolling reduction of 80%, and continuous annealing and control in exactly the same manner as in Example 1. Quality rolling was performed to obtain a product. Tensile test pieces were taken from each part in the coil longitudinal direction and subjected to a tensile test. The test conditions are the same as in Example 1.

FIG. 1 shows the change in the average r value of the product plate in the coil longitudinal direction. The position in the longitudinal direction of the horizontal axis is a converted value in the hot rolled sheet. FIG. 1 shows that the winding temperature was set to a value within the range of the present invention.
When the temperature is 0 ° C. or more, the uniformity of the material in the longitudinal direction of the coil is remarkably improved, and the outermost and inner peripheral portions are targeted.
It can be seen that an average r value ≧ 1.5 can be secured.

[0030]

According to the present invention, it is possible to manufacture a steel sheet for deep drawing excellent in uniformity of the material inside the coil including the both ends of the coil. Therefore, the present invention is extremely advantageous because it can solve the problem of a decrease in yield due to the deterioration of the material at both ends of the coil in the conventional method.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a change in a coil longitudinal direction obtained by converting an average r value of a product plate into a hot rolled plate.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C22C 38/06 C22C 38/06 (72) Inventor Tosumi Ueda 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Kimitsu Corporation Inside the steelworks (72) Inventor Noritoshi Kimura 1 Kimitsu, Kimitsu-shi, Chiba Prefecture Inside the Kimitsu Works, Nippon Steel Corporation (56) References JP-A-3-2329 (JP, A) JP-A-2-93025 (JP, A) JP-A-2-285029 (JP, A) JP-A-4-314849 (JP, A) JP-A-4-327 (JP, A) (58) Fields investigated (Int. Cl. ⁷⁾ , DB name) C21D 9/46-9/48 C21D 8/04 C22C 38/00-38/60

Claims (1)

(57) [Claims] 1. C: 0.01 to 0.02% by weight, S
i: 0.5% or less, Mn: 0.03 to 0.15%, P:
0.02% or less, S: 0.015% or less, Al: 0.0
4 to 0.10%, N: 0.0025% or less, the balance being a low-carbon Al-killed steel slab consisting of Fe except for inevitable impurities at a temperature (ST) satisfying the following equation: 1000 ° C ≦ ST ≦ 10Mn / S + 1050 ° C Soaking, hot rolling at finishing temperature above Ar ₃ temperature,
Winding at 700 to 770 ° C, followed by cold rolling and continuous annealing, annealing and soaking at 750 to 880 ° C for 30 seconds to
After 2 minutes, 50 ° C / s from 600 ° C or more
At the above cooling rate, it is rapidly cooled to a temperature (T _E ) in the range of 250 to 320 ° C., kept at T _E for 0 to 5 seconds, and then continuously at 10 ° C. to a temperature (T _R ) between 300 to 370 ° C.
/ S and then cooled to a temperature (T _F ) between 250 and 300 ° C. to perform overaging treatment for 30 seconds or more. Method of producing cold-rolled steel sheet for non-aging deep drawing excellent in quality.