JP2503224B2 - Method for manufacturing thick cold-rolled steel sheet with excellent deep drawability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a thick cold-rolled steel sheet excellent in deep drawability.

[Conventional technology]

Conventionally, in the process of manufacturing a thick cold-rolled steel sheet, it is general that the material hot-rolled at a finishing temperature of the A ₃ transformation point or higher is pickled, then cold-rolled and annealed. However, this conventional method has a problem that the deep drawability is poor. That is, for example
When manufacturing a 3.2 mm cold-rolled steel sheet, even if the hot-rolled finished sheet thickness is finished to a maximum thickness of about 6 mm that can pass through the pickling line, only a 46% cold-rolling rate can be obtained and a sufficient cold-rolled set is obtained. The development of the structure was not expected, the texture effective for deep drawability did not develop after recrystallization annealing, and the Rank Ford value (hereinafter referred to as r value) was around 1.0, which was poor for deep drawability.

On the other hand, as a method for producing a hot-rolled steel sheet, conventionally, hot rolling is performed warmly and recrystallization annealing is performed to obtain a high r
There is a method of obtaining a value, and even with such a method, it is possible to manufacture a thick steel sheet having excellent deep drawability, but this method is inferior to a cold rolled steel sheet in terms of dimensional accuracy and surface roughness of the sheet.

[Problems to be solved by the invention]

As described above, in the conventional method for manufacturing a thick cold-rolled steel sheet,
Since the cold rolling ratio cannot be obtained and a sufficient cold rolled texture cannot be obtained, there is a problem that good deep drawability cannot be obtained.

In view of the above problems, the present invention provides a method for manufacturing a thick cold-rolled steel sheet having an extremely large plate thickness of 2.5 to 4.0 mm and good deep drawability.

The inventor of the present invention diligently researched to solve the above-mentioned problems, made a part or most of the texture of cold rolling in the hot rolling stage, and completed it in the subsequent cold rolling. The present invention has been made based on the knowledge that the shape accuracy required for a steel sheet can be produced in the cold rolling stage.

[Means for solving problems]

Therefore, the first invention of the present application is C: 0.03% by weight or less,
N: 0.01 wt% or less, Ti: 0.2 wt% or less, Nb: 0.3
Weight% or less, and (C / 12 + N / 14) <(Ti / 48 + Nb / 93)
For steel with addition of one or both of Ti and Nb in an amount to achieve a roll-induced precipitation of carbon and nitride of Ti and Nb by performing rough rolling in the temperature range of 900 ° C to 1000 ° C. By setting the waiting time from rough rolling to finish rolling to 4 minutes or more, Ti, Nb carbon and nitride are precipitated to reduce the solid solution (C, N) to 30ppm or less, and lubricate at 800 ℃ to 400 ℃. perform the finish rolling reduction ratio R ₁ using a rolling roll in which the roll diameter D ₁ relative to the finish thickness t ₁ is the D _1> 100 t ₁ while, after pickling,
Roll diameter D ₂ with respect to the finish thickness t ₂ by using a rolling roll to be D _2> 100 t ₂ reduction ratio R ₂ (provided that the total reduction ratio of R ₁ and _{R 2 R> 60%, R} 2> 5%) is cold-rolled and then annealed. The total rolling reduction R is calculated by the following formula.

_{R = 1- (1-R 1} ) (1-R 2) Further, in the conventional reheating rolling for the rough rolling may be directly rolled immediately perform rolling after casting.

The second invention of the present application is C: 0.03% by weight or less, N:
0.01% by weight or less, B: 0.0005 to 0.005% by weight, Ti:
0.2 wt% or less, Nb: 0.3 wt% or less, and (C / 12 + N / 1
4) Ti or Nb in an amount such that <(Ti / 48 + Nb / 93)
900 to 1000 for steel made by adding one or both of
By rough rolling in the temperature range of ℃, Ti, Nb charcoal,
By rolling-induced precipitation of the nitride and by setting the waiting time from the rough rolling to the finish rolling to 4 minutes or more, T
i, charcoal Nb, solid solution by precipitating a nitride (C, N) was used as a 30ppm or less, the roll diameter D ₁ is D ₁ with respect to the plate thickness t ₁ finishing while performing lubrication at 800 ° C. to 400 ° C.> Finish rolling with a reduction ratio R ₁ using a rolling roll with 100t ₁ and after pickling, use a rolling roll with a roll diameter D ₂ D ₂ > 100t ₂ for the finished plate thickness t ₂ After cold rolling of R ₂ (however, the total reduction ratio R> 60%, R ₂ > 5%), annealing is performed.

Here, the reasons for limiting the components and the production conditions of the present invention will be described.

First, the reason for limiting the components in the first invention will be described.

C, N: If C, N is added in excess of 0.03% and 0.01%, the workability of the product will deteriorate and the amount of Ti, Nb for fixing this will increase and the cost will increase. ≤0.03%, N≤
0.01%.

By adding this, Ti, Nb: Ti, Nb can form carbon and nitride, reduce the solid solution (C, N) in the steel, and further increase the ferrite recrystallization temperature significantly, It also makes it possible to delay recovery during processing. From a metallurgical point of view, the same state as cold rolling is used in the high temperature range.
It can be extended to 0 ° C or lower, and most of the cold texture can be formed during hot rolling. And this T
The amount of i, Nb added forms a solid solution (C,
The amount necessary to fix N), that is, (C / 12 + N / 14) <(Ti /
48 + Nb / 93) and Ti: 0.2 wt% or less and Nb: 0.3 wt% or less in consideration of economy.

Other components: Other components are not limited, but components usually contained as cold-rolled steel sheet for processing, such as Mn <
1.0%, Si <1.0%, P <1.0%, S <0.02%, Al <0.1%
Etc.

Next, the reason for limiting the components in the second invention will be described. Note that C, N, Ti, and Nb are the same as in the first aspect of the invention, so description thereof will be omitted.

B: By adding B, B can be improved in resistance to warp cracking. That is, by adding Ti and Nb, it is possible to reduce the solute C, but this reduction of the solute C weakens the bonding force of the grain boundaries and deteriorates the resistance to cracking during secondary working. Let Therefore, by adding B, B can be segregated at the crystal grain boundaries and the bonding force at the crystal grain boundaries can be strengthened. If the added amount of B is less than 0.0005%, the above effect cannot be obtained,
Addition of more than 0.005% is economically disadvantageous, and addition of excessive B adversely affects the deep drawability of the product.
05% to 0.005%

 Next, the manufacturing conditions will be described.

Points for Ti and Nb carbon and nitride precipitation treatment: Even if unrecrystallized rolling is performed in a state where a large amount of solid solution (C, N) is contained, it is parallel to the plate surface during recrystallization annealing (111 ) Texture does not develop, but deep drawability is adversely affected (200). In order to obtain high deep drawability by cold rolling, it is necessary to wind it at high temperature to precipitate carbon and nitride after completion of hot rolling.
Before unrecrystallized rolling in the temperature range of ~ 400 ℃, Ti, Nb charcoal,
It is necessary to precipitate the nitride to reduce the solid solution (C, N) in the steel. Before hot rolling finish, Ti (C, N), Nb
As a method of sufficiently precipitating (C, N), the rough rolling temperature is set to 9
By lowering the temperature to 00 ℃ -1000 ℃, Ti and Nb charcoal and nitride are induced by rolling, and the waiting time from the end of rough rolling to the hot rolling finish is 4 minutes or more. A method of depositing a substance can be adopted.

Points at which the warm rolling temperature was set to 800 to 400 ° C and points at which the total reduction ratio was set to 60% or less: Ferrite recrystallized at a temperature higher than 800 ° C, leaving no work structure. On the other hand, rolling at a temperature lower than 400 ° C is not practical because the deformation resistance of the material is too large. Therefore, the temperature should be 800-400 ℃.

Further, in order to obtain a texture effective for deep drawability by recrystallization annealing, the cold rolled texture should be sufficiently developed. For that purpose, the total rolling rate in the ferrite non-recrystallized region must be 60% or more. That is, the reduction ratio R ₁ in the warm rolling is 60% or more as the total reduction ratio R (= 1- (1-R ₁ ) (1-R ₂ )) with the reduction ratio R ₂ in the subsequent cold rolling. To be

In this case, for cold rolling, the texture produced by the above hot rolling may be perfected and a reduction ratio necessary for adjusting the surface shape may be secured, and it may be 5% or more.

Lubrication in hot rolling: As for lubrication, rolling without doing this creates a portion of the surface layer of the plate that undergoes significant shear deformation due to the effect of friction between the roll and the plate.
A texture different from the normal cold-rolled texture, that is, a texture with the (110) orientation parallel to the plate surface, develops and adversely affects the deep drawability. Therefore, lubricate the plate and roll,
The coefficient of friction should be at least 0.25 or less.

Point that regulates roll diameter: If the roll diameter is too small compared to the finished sheet thickness, plastic strain does not enter the center of the sheet even if the cold rolling rate is increased, and a sufficient cold rolled texture cannot be obtained. . Therefore, the roll diameter D is set to D> 100t with respect to the finished plate thickness t. This also applies to the cold rolling roll.

[Action]

In the first and second inventions of the present application, Ti, Nb carbon and nitride are precipitated to reduce the solid solution (C, N) in the steel, and a rolling roll having a predetermined roll diameter provides a predetermined rolling reduction ratio. Since warm rolling is performed, it is possible to extend the state almost the same as cold rolling from a metallurgical point of view to a high temperature range of 800 ° C or lower, and to develop a cold rolled texture that affects deep drawability. It becomes possible to manufacture at this stage, and high deep drawability can be obtained. Further, in the cold rolling, since the above-mentioned texture is perfect and the surface shape is adjusted, it is not necessary to increase the cold rolling reduction rate so much, and excellent surface properties and dimensional accuracy can be obtained.

In addition, in the second invention of the present application, since B is added, solid solution (C,
It is possible to prevent deterioration of vertical cracking resistance due to reduction of N).

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

 First, an embodiment of the first invention of the present application will be described.

Table 1 shows the chemical composition of the steels of the present invention (C and D in the table) and comparative steels (A and B in the table).

In this example, four steel types shown in Table 1 were melted in a converter, and a slab was produced by a normal process. Then, hot rolling was performed under the conditions shown in Table 2 and wound into a coil. After washing, cold rolling was performed under the conditions shown in Table 2 as well, and batch annealing was performed at 750 ° C. for 3 hours, and the r value of the material was measured. Table 2 shows the results. Table 2 also shows the surface properties of the materials.

According to this, it can be seen that in the present invention, the r value is as high as 1.75 or more, good deep drawability is obtained, and the surface quality is also good.

The present inventor also conducted an experiment on the relationship between the roll diameter and the deep drawability.

In this experiment, 0.0032% C-0.01% Si-0.18% Mn-0.
The hot-rolled sheet of 001% S-0.1% Ti-0.0030% N was cold-rolled with rolling rolls having various roll diameters, recrystallized and annealed at 850 ° C. for 2 minutes, and then the r value was measured. The cold rolling rate was fixed at 75% and the number of passes was also fixed at 15 passes. The hot rolled sheet of material is
What was rolled and finished at a hot rolling finishing temperature of 900 ° C was wound up at 700 ° C.

The relationship between roll diameter D and finished plate thickness t and r values is shown in the drawing. The numbers in the figure show r values, and the boundary line A shows D = 100t, and the right side portion is the result of using a roll within the scope of the present invention.

According to the figure, it can be understood that a high r value can be obtained in a range in which D> 100t.

 Next, an embodiment of the second invention of the present application will be described.

Table 3 shows the chemical composition of the steel of the present invention (E in the table) and the comparative steel (C in the table). In this example, two steel types shown in Table 3 were melted, and a slab was manufactured by an ordinary process, and then 1000 ° C.
It was reheated to γ region and roughly rolled to a thickness of 30 mm, and when it reached 800 ° C, it was finish rolled to a thickness of 5.5 mm. The finishing temperature was about 700 ° C for both steels. After winding this,
It was pickled and cold rolled. Since the finished sheet thickness of cold rolling is 4.0 mm, the total reduction ratio in the temperature range of 800 ° C or less is 87%.
Is. The roll diameter for hot rolling is φ800, and the roll diameter for cold rolling is φ580. After the obtained steel sheet was box annealed at 750 ° C. for 3 hours, the r value was measured and the test for the resistance to cracking during secondary working was performed.

The vertical cracking test was performed for the vertical cracking resistance test. That is, after blanking a steel plate with φ145, a test cup was prepared with a drawing ratio α: 2.0, and this was covered with a conical punch in a temperature range from liquid nitrogen temperature to normal temperature and destroyed by applying a load from the bottom surface of the cup. The transition temperature was measured from the brittle fracture rate at that time. Fruit The test results are shown in Table 4.

According to the table, in the present invention, the addition of B lowers the brittle transition temperature and improves the resistance to vertical cracking.

〔The invention's effect〕

As described above, according to the first and second inventions of the present application, Ti, N
The solid solution (C, N) in the steel is reduced by precipitating carbon and nitride of b, and part or most of the cold rolling texture is formed in the hot rolling stage, and this texture is generated in the cold rolling stage. Since it is made perfect and the surface texture is adjusted, there is an effect that a thick cold-rolled steel sheet having excellent surface texture and deep drawability can be manufactured.

Further, according to the second invention of the present application, since B is further added, in addition to the above-mentioned effects, there is an effect that resistance to cracking can be improved.

[Brief description of drawings]

The drawing is a diagram showing the relationship between the finished plate thickness, the roll diameter ratio and the r value.

Claims (2)

(57) [Claims] 1. C: 0.03% by weight or less, N: 0.01% by weight or less, Ti: 0.2% by weight or less, Nb: 0.3% by weight or less, and (C / 12 + N / 14) <(Ti / 48 + Nb / 93)
Alternatively, for steel consisting of balance Fe and unavoidable impurities to which one or both of Nb is added, rough-rolling is performed in a temperature range of 900 ° C to 1000 ° C to cause Ti or Nb carbon and nitride to undergo roll-induced precipitation. In addition, by setting the waiting time from the rough rolling to the finish rolling to 4 minutes or more, Ti or Nb charcoal,
The total amount of solute C and solute N in the steel is set to 30 by precipitating nitride.
and ppm or less, a finish rolling reduction ratio R ₁ using a rolling roll in which the roll diameter D ₁ lubricates the plate thickness t ₁ finish while performing in a temperature range of 800 ° C. to 400 ° C. is D _1> 100 t ₁ After that, pickling is performed, and the roll diameter D ₂ is D ₂ > 10 with respect to the finished plate thickness t ₂ .
Using a rolling roll of 0t ₂ , the rolling reduction R ₂ (however, R ₁ and R ₂
The total reduction ratio of R> 60%, R _2> subjected to cold rolling 5%), the manufacturing method of the thick matter cold-rolled steel sheet having excellent deep drawability, characterized in that to perform the annealing. 2. C: 0.03% by weight or less, N: 0.01% by weight or less, B:
Contains 0.0005 to 0.005% by weight, Ti: 0.2% by weight or less, Nb:
0.3% by weight or less, and (C / 12 + N / 14) <(Ti / 48 + Nb / 9
90% for steel consisting of balance Fe and unavoidable impurities with addition of Ti and / or Nb in amounts such that
By performing rough rolling in the temperature range of 0 ℃ ~ 1000 ℃, Ti
Alternatively, by inducing rolling-induced precipitation of Nb charcoal and nitride, and by setting the waiting time from the rough rolling to finish rolling to 4 minutes or more, Ti or Nb charcoal and nitride are precipitated to form solid solution C in steel. Also, the total amount of dissolved N is 30 ppm or less, and the roll diameter is applied to the finished plate thickness t ₁ while lubricating in the temperature range of 800 ° C to 400 ° C.
After finishing rolling with a reduction ratio R ₁ using a rolling roll with D ₁ of D ₁ > 100t ₁ , pickling is performed, and roll diameter D ₂ of D ₂ > 100t is applied to the finished plate thickness t ₂ . Using a rolling roll of ₂ , the reduction ratio R ₂ (however, the total reduction ratio of R ₁ and R ₂ R> 60%, R ₂ > 5
%) Cold-rolled and then annealed. A method for manufacturing a thick cold-rolled steel sheet having excellent deep drawability, which is characterized in that it is annealed.