JPH0250978B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a non-aging cold rolled steel sheet having high ductility and ultra-deep drawability, and a method for manufacturing the same. [Prior Art] For example, automobile quarter panels, fenders, oil pans, etc. must be formed by extremely severe press working. Therefore, cold-rolled steel sheets used for such molded products are required to have high ductility and ultra-deep drawability. It also needs to be non-prescription. In response to these demands, conventionally, cold-rolled steel sheets that have been decarburized by batch open coil annealing have been applied, or carbonitride-forming elements have been used.
By adding Ti, Nb, Cr, etc. alone or in combination, C and N in the steel are fixed, increasing ductility and ensuring non-aging properties. {111} Effective for improving deep drawability
It has been proposed to use non-aging cold-rolled steel sheets for deep drawing that have developed oriented recrystallized textures. Regarding the latter carbonitride-forming element-added steel, for example, Ti-added steel is disclosed in Japanese Patent Publication No. 44-18066, Japanese Patent Application Laid-Open No. 59-67322, and Japanese Patent Application Laid-Open No. 59-89727, etc. - Publication No. 1245, Special Publication 1987
-34778 and JP-A-58-81952 disclose Nb-added steel, while JP-A-50-30572 and JP-A-58-185752 disclose Nb-added steel.
Cr-added steel was disclosed, and Ti and Nb-added steel was disclosed in JP-A No. 59-67319.
―123720 discloses Ti, Nb, and Cr added steel. [Problems to be Solved by the Invention] Even if the above-mentioned requirements are met, decarburized annealed steel tends to have a large grain size and is prone to rough skin such as orange peel during press forming. There is. In Ti-added steel and Ti, Cr steel, Ti has a strong bonding force with O and S and forms oxides and sulfides, so it is difficult to ensure non-aging properties by fixing C and N. It is necessary to add an amount of Ti that is several times the stoichiometric equivalent of , C, and N. In order to lower manufacturing costs and increase ductility, it is desirable to lower the Ti content as much as possible within the range that ensures non-aging properties. Ti
However, if the amount of C and the amount of solid solute Ti are reduced too much, the in-plane anisotropy of the γ value [Δγ = (γ ₀ ° +γ ₉₀゜−2γ ₄₅゜)/2] becomes larger,
_γ45 °, which is γmin, decreases, leaving problems with press workability. In Nb-added steel, with the recent progress in steel degassing technology, it has become possible to reduce C relatively easily to a range of C < 50 ppm. A manufacturing method for Nb-added steel with very low amounts of C and Nb has been proposed, such as in Therefore, the annealing temperature during annealing, particularly during continuous annealing, must be made higher than that for Ti-added steel.
Further, even if the hot-rolled coil is coiled at a high temperature, there is a problem that a homogeneous material cannot be obtained at the longitudinal ends of the hot-rolled coil because the cooling rate is fast. In addition, by reducing the amount of Ti and increasing the ductility,
By adding Nb and Cr in addition to γ
It is stated in the above-mentioned Japanese Patent Application Laid-Open Nos. 59-67319 and 1982-123720 that the in-plane anisotropy of the value is improved, but when actually investigated and researched,
In such a manufacturing method, the in-plane anisotropy of the γ value appears significantly even if a high cold rolling rate of 70 to 80% is used, although this may be due to the Nb content being too low.
Since γmin, _γ45 °, is too small, there is still a problem with press formability. It is possible to improve the in-plane anisotropy of the γ value by increasing γmin by increasing the cold rolling rate to 80% or more, but for cold rolled steel products with large plate thickness, thermal It becomes necessary to increase the thickness of the rolled sheet beyond the routine thickness, which causes problems in actual operation and also causes problems in the capacity of the cold rolling mill. [Means for Solving the Problems] The present invention aims to solve the above-mentioned problems by adding carbon to Ti, Nb, and Cr-added steel.
While reducing the amount of Ti and Ti below the specified value,
This method adopts a treatment method in which Ti is added in a predetermined amount or more and in an amount related to the [effective amount of Ti]. That is, the present invention has the following properties in weight%: C: 0.001 to less than 0.01%, Si: 0.1% or less, Mn: 0.5% or less, Sol.Al: 0.01 to 0.10%, Cr: 0.06 to 0.20%, P: 0.03% Below, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: [effective Ti amount] according to the following formula (1) is 4 × C% or more, and this [effective Ti amount] is 0.15% Hereinafter, Nb: 0.05% or more and in relation to [effective Ti amount], the range that satisfies [effective Ti amount] + Nb≦0.20%, remainder: Fe and unavoidable impurities, [effective Ti amount] = total Ti amount - [N%×(48/14)+S%
× (48/32) + O% × (48/12) × 1/2]
...(1) Provides a non-aging cold-rolled steel sheet for deep drawing, and in weight percent, C: 0.001 to less than 0.01%, Si: 0.1% or less, Mn: 0.5% or less, Sol. Al: 0.01 to 0.10%, Cr: 0.06 to 0.20%, P: 0.03% or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: according to the following formula (1) [effective Ti amount ] is 4×C% or more and this [effective Ti amount] is 0.15% or less, Nb is 0.05% or more, and in relation to [effective Ti amount], satisfies [effective Ti amount] + Nb ≦ 0.20%, Remaining part: Fe and unavoidable impurities, [effective Ti amount] = total Ti amount - [N% x (48/14) + S%
× (48/32) + O% × (48/12) × 1/2]
...(1) Deep drawing, which consists of hot rolling a steel slab, then cold rolling at a rolling reduction of 40% to 90%, and then annealing at a temperature above the recrystallization temperature and below 900°C. The present invention provides a method for manufacturing a non-aging cold rolled steel sheet for use. The excellent properties of the present invention will be specifically shown in Examples below, but the reasons for limiting the chemical component values will be summarized as follows. The lower the content of C, the more preferable it is for improving the ductility of the cold-rolled steel sheet, and if it exceeds 0.01%, a large amount of carbonitride-forming elements are required and the amount of carbonitride precipitation increases, resulting in poor press formability. begins to deteriorate. On the other hand, it is difficult to reduce the C content to less than 0.001% in a practical-scale steelmaking furnace. For these reasons, C; 0.001~
Less than 0.01%. Si is added primarily to deoxidize molten steel, and Mn is added to prevent hot embrittlement, but if too large amounts of both Si and Mn are added, they reduce ductility. In the steel of the present invention, the amount of Si contained in ordinary cold-rolled steel sheets is
≦0.1% and Mn≦0.5% are permissible, and within this range, the stated purpose can be fully achieved. Al is added for the purpose of deoxidizing molten steel, but this purpose cannot be fully achieved if the amount is less than 0.01% of Sol.Al (acid-soluble Al) in the steel. Furthermore, if the amount of Sol.Al exceeds 0.10%, its effect will be saturated and the non-metallic inclusions will increase, causing surface flaws.
The amount is 0.01-0.10%. When Cr is used alone, it is not possible to obtain the desired results aimed at by the present invention, but by adding it in combination with Ti and Nb, it exhibits the effect of improving deep drawability and stretchability. However, if the Cr content is less than 0.06%, there is no such effect, and if the content exceeds 0.20%, this effect will be saturated and the manufacturing cost will only increase. Therefore, Cr is contained in the range of 0.06 to 0.20%, and this Cr plays an important role in the steel of the present invention. If too much P is added, it will increase the descending strength and tensile strength, and in ultra-low C steel, it will segregate to the grain boundaries and cause secondary work cracking, so the content should be adjusted accordingly. The upper limit is set at 0.03%. The smaller the amount of N, the more desirable it is because the amount of Ti added can be reduced, and if the amount of N is too large, it will reduce the [effective amount of Ti] and deteriorate the press formability of the final product, so the allowable limit is N. ≦0.007%. S and O both reduce [effective Ti amount],
If these amounts increase, the total Ti amount to ensure [effective Ti amount] increases and the surface quality deteriorates, so the allowable limits for S and O are set to S≦0.015% and O≦0.01%, respectively. shall be. By fixing C and N, Ti ensures the non-aging properties of cold-rolled steel sheets, and also
TiC provides the effect of creating a [111]-oriented recrystallized texture, which is effective for improving deep drawability. For this purpose, the [effective Ti amount] shown in the above equation (1) must be 4
×C% or more is required. However, when the amount of Ti exceeds 0.15%, it becomes a solid solution in the ferrite.
As the amount of Ti increases, yield strength increases and ductility decreases. This also increases manufacturing costs. Therefore, for Ti, [effective Ti amount] is 4×C
% or more and 0.15% or less. In addition, in the steel of the present invention, by adding Nb in addition to Ti, the in-plane anisotropy of the γ value can be improved even with a small amount of Ti added. There is an upper limit for the amount of Ti, and the total amount of [effective Ti amount] + Nb is up to 0.20% as described below. By adding Nb in combination with Ti, even if the C content and Ti content are reduced as in the present invention, it can be significantly improved. Such an effect is achieved when the amount of Nb is less than (0.2% - [effective amount of Ti]), and adding more than this amount causes an increase in the recrystallization temperature and a decrease in ductility.
Therefore, [effective Ti amount]+Nb≦0.2% or less. However, if the amount of Nb is less than 0.05%, the effect of improving the in-plane anisotropy of the γ value cannot be obtained. In this way, the present invention can reduce the amount of C and Ti to below a predetermined value while increasing the amount of Nb to a predetermined amount or more in Ti, Nb, and Cr added steel.
A non-aging cold-rolled steel sheet for deep drawing that has a high γ value with little in-plane anisotropy and ensures non-aging properties without reducing ductility by adding a compound in a related amount to However, when manufacturing this cold-rolled steel sheet, it is preferable to carry out the production under the following conditions. First, it is desirable to melt steel in a steel-making furnace and perform vacuum degassing treatment before ingot-forming or continuous casting. As a result, it is possible to advantageously lower the C and O content in the steel as described above and to adjust the content within the composition range as described above with a high yield. When performing this vacuum degassing treatment, Al can also be added for deoxidation treatment. After adjusting C and O and adjusting the composition to the above-mentioned range by adding ferroalloy, a slab is manufactured by ingot-forming, blooming rolling, or continuous casting, and after performing slab maintenance as necessary, Perform hot rolling. When carrying out this hot rolling, it is desirable to set the hot rolling finishing temperature to Ar ₃ points or higher from the viewpoint of improving deep drawability. Further, the hot rolling coiling temperature is preferably in the range of 650 to 750°C. Next, after pickling, cold rolling is performed, but this cold rolling requires a cold rolling rate of 40% or more in order to develop a [111] oriented texture that is advantageous for deep drawability. However, even at a high cold rolling rate of over 90%, the effect of improving deep drawability is saturated and the cost becomes extremely high, so it is preferable to set the upper limit of the cold rolling rate to 90%. Next, annealing is performed, but this annealing can be either batch annealing or continuous annealing, and the temperature is higher than the recrystallization temperature.
Excellent press formability can be obtained by carrying out the process at a temperature range of 900°C or lower. More specifically, for batch annealing, the annealing temperature may be 700°C or more and 850°C or less, and for continuous annealing, the annealing temperature may be 750°C or more and 900°C or less. In this way, according to the present invention, it is possible to maintain a high γ value while at the same time having a high γmin and low in-plane anisotropy. Provided is an economical non-aging cold-rolled steel sheet for deep drawing that can withstand even more severe press forming than Nb and Cr steels. [Example] Example 1 Each 30 kg of steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace, hot forged at a heating temperature of 1250°C, and then finished at a finishing temperature of 880 to 940°C and a coiling temperature of 700 to 700°C. 720℃
The sample was hot-rolled to obtain a hot-rolled plate with a thickness of 5 mm. After pickling, the sheets were cold rolled to a thickness of 1.2 mm, and each cold rolled sheet was batch annealed at 800° C. for 4 hours. Table 2 shows the mechanical property values of the obtained cold rolled product. As shown in Tables 1 to 2, No. 1 to No. 7
Not only do these steels have a high total elongation of 52.5% or more and a high value of 1.87 to 2.20, but also the in-plane anisotropy of the γ value (Δγ) has been significantly improved.
_γ45 °, which is γmin, is 1.70 or more. Therefore, it can be seen that the steel of the present invention is a non-aging cold rolled steel sheet having high ductility and ultra-deep drawability. On the other hand, comparative steels No. 8 to 12 (Nb content is less than 0.05% as defined in the present invention) have a total elongation of
Although it is high at 54.2% or more, Δγ is large at 0.98 to 1.29.
The value of _γ45 °, which is γmin, is as low as 1.52 or less, and there is a problem in deep drawability. In addition, steel No. 13 with a large amount of Ti has a value of 1.94,
Since γmin is high at 1.86 and Δγ is small at 0.32, the deep drawability is sufficient, but the total elongation is 50.8, which is lower than the steel of the present invention. Then, No. 14 with no Cr added and No. 15 with a high amount of C.
Both steels have low total elongation, similar to No. 13.

【table】

【table】

[Table] Example 2 180T converter and degassing equipment
The composition of the steel is adjusted to have the chemical composition values shown in the table, and each molten steel is continuously cast to form a slab. From this slab, the heating temperature is 1250-1280℃, and the finishing temperature is 900-
A hot-rolled coil with a thickness of 5.0 mm is prepared at 930℃ and a winding temperature of 700 to 730℃. After pickling, the coil is cold-rolled to a thickness of 1.2mm, and the coil is held at 800℃ for 4 hours.Batch type annealing. A cold-rolled steel sheet was manufactured on an actual line. Table 4 shows the mechanical property values of the obtained cold rolled steel sheets. As seen in the results in Table 4, the invention steel No.
A has a high total elongation of 53.6%, a high elongation of 2.14, a large γmin (γ ₄₅ °) of 1.85, and a small Δγ of 0.58, so it is a cold-rolled steel sheet with high ductility and ultra-deep drawability. I understand that. No.B in which the amount of Nb is less than the range specified in the present invention
Steel has a high total elongation of 54.0%, but γmin is too low at 1.40, and Δγ is also high at 0.96. Therefore, there is a problem with deep drawability. In No.C steel with Ti and Cr added and no Nb added,
γ is relatively large, 2.18, γmin is 1.96, and Δγ is also
Although it can be said that the deep drawability is good because it is small at 0.44, the total elongation is 50.2 because the Ti content is large (the [effective Ti content] is greater than the amount specified in the present invention), which is lower than the inventive steel. It's getting lower.

【table】

[Table] Example 3 A cold rolled steel plate was produced on an actual line in the same manner as in Example 2, except that the cold rolled coil was annealed continuously at 850° C. for 1 minute of soaking time. Table 5 shows the mechanical property values of the obtained cold rolled steel sheets. As seen in the results in Table 5, the invention steel No.
A is a cold-rolled steel sheet with a high total elongation of 52.8%, a relatively large elongation of 1.86, and a relatively large γmin (γ ₄₅ °) of 1.60, and has high ductility and ultra-deep drawability even through continuous annealing. I understand that. No.B where the amount of Nb is less than the range specified in the present invention
Steel has a high total elongation of 53.9%, but is very low at 1.19, and Δγ is also large at 0.85, causing problems in deep drawability. In No.C steel with Ti and Cr added and no Nb added,
The total elongation is low at 49.1 and there is a problem with ductility.

[Table] As is clear from the results of the above examples, Ti,
The steel of the present invention, in which Cr is added and Nb is added in an amount of 0.05% or more, with an appropriate composite addition in the range of [effective Ti amount] + Nb≦2.0%, has particularly high ductility due to the reduction in C and Ti content. While maintaining the same, the in-plane anisotropy of the γ value was significantly improved and γmin was successfully increased, indicating that this is a non-aging cold rolled steel sheet with high ductility and deep drawability. .

Claims (1)

[Claims] In 1% by weight, C: 0.001 to less than 0.01%, Si: 0.1% or less, Mn: 0.5% or less, Sol.Al: 0.01 to 0.10%, Cr: 0.06 to 0.20%, P: 0.03 % or less, S: 0.015% or less, N: 0.007% or less, O: 0.01% or less, Ti: [effective Ti amount] according to the following formula (1) is 4 × C% or more, and this [effective Ti amount] is 0.15 % or less, Nb: 0.05% or more and within the range that satisfies [effective Ti amount] + Nb≦0.20% in relation to [effective Ti amount], balance: Fe and unavoidable impurities, [effective Ti amount] = total Ti amount −[N%×(48/14)+S%
× (48/32) + O% × (48/12) × 1/2]
...(1) A non-aging cold rolled steel sheet for deep drawing. 2 In weight%, C: 0.001 to less than 0.01%, Si: 0.1% or less, Mn: 0.5% or less, Sol.Al: 0.01 to 0.10%, Cr: 0.06 to 0.20%, P: 0.03% or less, S: 0.015 % or less, N: 0.007% or less, O: 0.01% or less, Ti: [effective Ti amount] according to the following formula (1) is 4 × C% or more and this [effective Ti amount] is 0.15% or less, Nb: 0.05 % or more and satisfies [effective Ti amount] + Nb≦0.20% in relation to [effective Ti amount], remainder: Fe and unavoidable impurities, [effective Ti amount] = total Ti amount - [N% × ( 48/14)+S%
× (48/32) + O% × (48/12) × 1/2]
...(1) Deep drawing, which consists of hot rolling a steel slab, then cold rolling at a rolling reduction of 40% to 90%, and then annealing at a temperature above the recrystallization temperature and below 900°C. Manufacturing method for non-aging cold rolled steel sheets.