KR101148921B1 - Cold rolled steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention comprises the steps of cooling the slab comprising at least one component of titanium, niobium and aluminum, and at least one component of manganese, phosphorus and silicon at a first rate after hot rolling, and after the cooling the slab 700 to 800 ℃ A method of manufacturing a cold rolled steel sheet, the method including: maintaining the temperature range, cooling the slab at a second speed greater than the first speed after the holding step, and winding the slab by cold rolling after the cooling. It provides a cold rolled steel sheet produced thereby.

Description

Cold rolled steel sheet and its manufacturing method {COLD ROLLED STEEL SHEET AND MANUFACTURING METHOD THEREOF}

The present invention relates to a cold rolled steel sheet and a method for manufacturing the same, which are produced through hot rolling and cold rolling sequentially.

Cold rolled steel sheet having high strength and high formability at the same time is widely used in general home appliances as well as automobiles. The ultimate goal of the quality of these cold rolled steel sheets is to satisfy the demands of the end user, the consumer. Furthermore, the steel sheet for automobiles should satisfy the goals of fuel efficiency and environmental regulations through weight reduction. In the case of cold rolled steel sheets for automobiles, the characteristics required for each part are varied, and among them, high formability, high strength, plating property, corrosion resistance, shape freezing property, etc. must be satisfied at the same time.

However, it is difficult to secure both high strength and high formability at the same time in steel sheet materials. In general, when the formability is excellent, high strength is not secured, and in the case of high strength steel, there is a trade-off relationship in which the formability is not secured.

SUMMARY OF THE INVENTION An object of the present invention is to provide a cold rolled steel sheet and a method of manufacturing the same, which can increase the elongation while increasing the strength.

According to an aspect of the present invention, there is provided a method for manufacturing a cold rolled steel sheet according to an aspect of the present invention, after a hot rolling of a slab including at least one component of titanium, niobium, and aluminum, and at least one component of manganese, phosphorus, and silicon. Cooling the slab at a temperature ranging from 700 ° C. to 800 ° C. after the cooling, cooling the slab at a second speed greater than the first speed after the holding step, and after the cooling Winding the slab to cold rolling.

The second speed may be in the range of 5 times to 20 times the first speed. Specifically, the first rate is 5 ~ 10 ℃ / s, the second rate may be 50 ~ 100 ℃ / s.

The maintenance may be set to be made for a time of 5 to 10 seconds.

The winding may be made in a temperature range of 400 ~ 550 ° C.

Carbon and nitrogen content of the slab may be 100ppm or less.

Cold rolled steel sheet according to another aspect of the present invention is a cold rolled steel sheet manufactured to have a tensile strength of 490MPa or more and an elongation of 35% or more according to the above manufacturing method.

The cold rolled steel sheet may be a cold rolled steel sheet manufactured to have a plastic deformation ratio of 1.5 or more.

The grain size of the cold rolled steel sheet may be a cold rolled steel sheet manufactured to be 7 ~ 20㎛.

Cold rolled steel sheet according to another aspect of the present invention by weight%, carbon 0.002 ~ 0.01, nitrogen 0.002 ~ 0.01, silicon 0.05 ~ 0.25, sulfur 0.01 or less, manganese 0.2 or less, phosphorus 0.05 ~ 0.01, aluminum 0.01 ~ 0.06, titanium 0.02 Contains ~ 0.07,

The content of titanium is the following formula:

4 x carbon +3.4 x nitrogen +1.5 x sulfur

Is controlled by more than the sum of the elements of,

It is a cold-rolled steel sheet manufactured by hot rolling, holding at a temperature for a certain time after primary cooling, secondary cooling at a higher speed than the primary cooling, winding up, and then cold rolling.

Niobium may further comprise 0.01 to 0.05 by weight, and niobium may be a cold rolled steel sheet manufactured to be adjusted to be equal to or more than the sum of elements of 0.5 x carbon x (93/12).

The holding time may be a cold rolled steel sheet is set to 5 ~ 10 seconds.

The grain size of the cold rolled steel sheet may be a cold rolled steel sheet manufactured to have a value of 7 ~ 20㎛ or less.

Cold rolled steel sheet and its manufacturing method according to the present invention configured as described above can be improved while increasing the strength, it is possible to prevent a decrease in elongation due to the strength improvement. Furthermore, the strength can be improved and the elongation can be improved. As a result, it is possible to manufacture a product that must have a high strength and a complicated shape.

1 is a graph illustrating a cooling and winding process after hot rolling in a process for manufacturing a cold rolled steel sheet according to an embodiment of the present invention with respect to temperature and time;
Figure 2 is a schematic diagram for explaining the relationship between the characteristics of the cold rolled steel sheet manufacturing method according to an embodiment of the present invention and the change of the cold rolled steel sheet,
3 is a graph showing the change in tensile strength and elongation according to the cooling rate change,
4 is a graph showing the change in tensile strength and elongation according to the change of holding temperature,
5 is a graph showing the change in tensile strength and elongation with the change of holding time.

Hereinafter, a cold rolled steel sheet according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail with reference to the accompanying drawings.

In the case of automotive steel sheet requiring high formability, excellent deep drawing property is required. In the case of automotive materials or general structural materials, it is essential to ensure deep workability because it is judged whether the material is defective due to high deep workability. In order to secure deep workability, a basic plastic strain ratio (hereinafter, referred to as 'R value') must also be secured.

The mean plastic deformation ratio (Rm) and the planar anisotropy coefficient (ΔR), which are exponents corresponding to R values, are as follows.

Rm = (R ₀ + 2R ₄₅ + R ₉₀ ) / 4

_{△ R = (R 0 - 2R} 45 + R 90) / 2

(R ₀ is rolling direction value, R ₄₅ is rolling direction diagonal value, R ₉₀ is rolling direction vertical value)

The high plastic strain ratio indicates that the core can be easily processed because of its excellent deep workability, and the low value of planar anisotropy means that the strain distribution is uniform in the plane direction of the sheet, which means uniform deformation.

As a result, the high plastic deformation ratio and low planar anisotropy coefficient make it easy to manufacture the desired parts after molding and excellent shape freezing.

In order to manufacture an excellent automotive steel sheet having high strength and high molding, it is basically intended to ensure a high elongation, and also a high R value.

In order to increase the elongation and R value, titanium (Ti) and / or niobium (Nb), which form carbonitrides, are added to sufficiently remove carbon (C) and nitrogen (N), which are solid solutions existing in the steel. In addition, it may be possible to further increase the elongation by growing the grains in the hot rolling step.

Cold rolled steel sheet according to an embodiment of the present invention is at least one component of the elements that precipitate carbon, nitrogen, such as titanium, niobium and aluminum, and at least one component of the elements providing a solid solution effect, such as manganese, phosphorus and silicon It is formed by hot rolling a slab comprising a.

As illustrated in FIG. 1, after the hot rolling, the slab is cooled at a first speed, and after the cooling, the slab is maintained for a predetermined time in a temperature range of 700 to 800 ° C. This maintenance step provides time for the grain to grow sufficiently. The grain size of the cold rolled steel sheet preferably has a value of 7 ~ 20㎛. If the grain size is less than the above range, the elongation of the slab is reduced, and if it is above the above range, the strength is reduced.

After the holding, the slab is cooled at a second speed greater than the first speed. The determination of the second cooling rate against the first cooling rate is also associated with the growth of the grains. In other words, when the second cooling rate is excessively low, grains become coarse and the strength is lowered. If the second cooling rate is excessively high, the grains become fine and elongation decreases.

Following the cooling, the step of winding and cold rolling the slab is followed.

As illustrated in FIG. 2, precipitation of carbon and nitrogen improves the elongation and plastic strain ratio of the slab, and the solid solution strengthening effect improves the strength of the slab. Furthermore, the process of cooling-maintenance-cooling after hot rolling of the slab further improves the elongation. By improving the strength, elongation and plastic strain ratio, a cold rolled steel sheet having a tensile strength of 490 MPa or more and an elongation of 35% or more can be obtained. Furthermore, the cold rolled steel sheet may be manufactured to have a plastic deformation ratio of 1.5 or more.

In another embodiment of the present invention the cold rolled steel sheet in weight%, carbon 0.002 ~ 0.01, nitrogen 0.002 ~ 0.01, silicon 0.05 ~ 0.25, sulfur 0.01 or less, manganese 0.2 or less, phosphorus 0.05 ~ 0.01, aluminum 0.01 ~ 0.06, titanium 0.02 And may include ˜0.07.

The content of titanium is adjusted to a sum of elements of 4 × carbon + 3.4 × nitrogen + 1.5 × sulfur and then hot-rolled and maintained at a temperature for a certain time after the first cooling, followed by secondary cooling at a higher speed than the first cooling. It is a cold rolled steel sheet manufactured by winding after cold rolling.

The cold rolled steel sheet may further include niobium 0.01 to 0.05 in weight%. Niobium is adjusted to be equal to or greater than the sum of the elements of the following formula: 0.5 x carbon x (93/12).

Description of the components contained in the cold rolled steel sheet of this embodiment is described below based on weight%.

Carbon [C]: 0.002-0.01%

Carbon is present as a solid solution in the steel sheet and inhibits the formation of (111) texture, which is advantageous for workability, during the formation of the texture of the steel sheet during cold rolling and annealing, thereby decreasing workability and formability. In addition, when carbon is present in steel, it causes aging problems, leading to problems with stretcher strains.

If the carbon content is higher than 100ppm, the addition amount of titanium and niobium carbide forming elements should be increased. In this case, not only induces a cost increase of the steel, but also a material and surface properties are degraded by adding a large amount of titanium and niobium.

The lower the carbon content, the more advantageous it is, but there are limitations in steelmaking technology and the possibility of generating grain boundary embrittlement is limited to the above range.

manganese[ Mn ]: 2.0 wt  % Below

Manganese is a component added to prevent cracking caused by sulfur by forming manganese sulfide with securing strength to some extent. The reason for setting the upper limit lowers the workability and formability due to the grain boundary segregation of manganese when it exceeds 2.0%. This is because it adversely affects the plating characteristics.

Phosphorus [P]: 0.05 ~ 0.09 wt % Below

Phosphorus is an excellent component to enhance the solid solution, and is added for the purpose of increasing strength. The reason for setting the upper limit is that when phosphorus is added in a large amount, phosphorus segregates at grain boundaries, causing secondary processing brittleness. However, it may be possible to suppress the occurrence of secondary processing brittleness by the addition of boron.

Sulfur [S]: 0.01 wt % Below

Sulfur forms sulfide-based (manganese sulfide and the like) inclusions, and forms iron sulfide to cause edge cracking and the like, and therefore, it is limited to within 0.01 wt% or less.

aluminum[ Al ]: 0.01-0.06 wt %

Aluminum is a component that functions as a deoxidizer and keeps the amount of dissolved oxygen in the steel sufficiently low. As the role of the deoxidizer, when the upper limit is added, problems may occur during playing.

Reaction with nitrogen, a solid solution, produces aluminum nitride (AlN) as a precipitate to remove the solid solution. In case of applying continuous annealing method, high winding temperature should be maintained to precipitate aluminum nitride in the hot rolling step.

silicon[ Si ]: 0.05 ~ 0.25 wt %

Silicon is present as a solid element in the steel sheet to increase strength. The higher the content, the higher the strength, but the upper limit to reduce the plating characteristics of the post-process. In addition, silicon has a lower limit for forming ferric silicate (2FeSiO ₂ ) to provide an effect of suppressing scale formation on the surface of the slab.

Titanium [Titanium]: 0.02-0.07 wt %

As the amount of titanium added increases, the R value is increased and non-aging characteristics are exhibited. Carbon and nitrogen present as solid solutions in the steel are precipitated in the form of precipitates such as titanium nitride (TiN) and titanium carbide (TiC) to remove the solid solution in the steel to improve the R value.

In addition, when carbon in solid solution remains in the steel, it moves over time to confine the dislocation and thus age. The amount of titanium added should be taken into account to ensure sufficient removal of carbon. The lower limit of the added amount was set to an amount that can stoichiometrically precipitate the solid solution element. Typically, the R value increases with increasing titanium content. However, if the titanium content is increased, the nozzle is clogged during playing and surface defects occur during cold rolling.

Niobium [ Nb ]: 0.01-0.05 wt %

Niobium, like titanium, precipitates carbon and nitrogen present as solid solutions in steel in the form of precipitates of niobium carbide (NbC) and niobium nitride (NbN) to remove solid elements in the steel, thereby improving R value and removing carbon in the steel. To exhibit non-aging characteristics.

The higher the content of the stoichiometric precipitation of the solid solution element, that is, the higher the upper limit, the lower the R value. In addition, the cost rises to raise manufacturing costs. However, when niobium is added, it shows excellent plating characteristics. The minimum content of stoichiometric precipitation of solid elements in steel is regulated by the lower limit.

Nitrogen [N]: 0.002 ~ 0.01 wt %

Nitrogen, like carbon, is present as a solid solution in steel, which lowers elongation, thereby degrading the workability and formability of the steel sheet. The lower the content of nitrogen, the more favorable the moldability, but the lower limit is 0.002% in consideration of steelmaking level and cost. In addition, the higher the nitrogen content, the lower the limit for the processability.

The slabs formed as described above are obtained by molten steel through a steelmaking process and then produced through ingot or continuous casting. The slab is subjected to reheating, hot rolling, winding, cold rolling and annealing to produce a steel sheet having a target mechanical property. Hereinafter, the manufacturing conditions of the characteristic parts for each process will be described.

<Reheating process>

The slabs are heated at a temperature of Ac ₃ or higher in a heating furnace to reclaim segregated components during casting. Low reheating temperature promotes titanium precipitation. In the case of titanium nitride (TiN) and titanium carbide (TiC), the precipitation tendency does not change significantly depending on the actual hot-rolled heating temperature.However, in the case of titanium dioxide disulfide (Ti ₄ C ₂ S ₂ ) and titanium sulfide (TiS), It changes with hot heating temperature. As the heating temperature is lowered, the fraction of precipitate increases, so that the carbon and nitrogen contents of the solid solution decrease.

<Hot Rolling Finishing Temperature>

Ar ₃ Hot rolling is finished above the temperature to produce a single-phase hot rolled coil. When the finish is at temperatures above Ar ₃ , austenite transforms into ferrite during the cooling process. In this metamorphosis process, after the cold rolling annealing, the aggregates that form the basis of the (111) aggregate development are formed.

As shown in FIG. 1, the cooling rate after hot rolling of the hot rolled steel sheet is slowly cooled to 5 to 10 ° C./sec to allow sufficient crystal grain growth.

After cooling for 10 seconds at 800 to 700 ° C. to further increase the texture and grain growth.

The cooling rate is maintained at 50 ~ 100 ℃ / sec. When the cooling rate is low, the grains become too coarse and cause a decrease in strength. If it is 100 ° C / sec or more, the grain size is too fine to increase the strength and reduce the elongation.

< Coiling temperature >

Winding is finished in coiling temperature (CT, winding temperature) of 550-400 degreeC, Preferably 500-450 degreeC, in order to make cold rolling easy. At temperatures above 550 ° C, grains grow coarsened to suppress the increase in strength.

<Cold rolled>

It is more preferable to carry out at a reduction ratio of 30 to 80% and to 60 to 75% in a step of cold rolling to obtain a final desired thickness and to obtain a desired material. The cold rolling rate plays an important role in the development of the (111) texture. The higher the cold rolling rate, the more the (111) texture is developed. If the cold reduction rate is too high, the annealing recrystallized grain becomes too fine and the strength is increased, which adversely affects the workability.

< Annealing Process  or Alloying Hot dip galvanizing  Process>

The annealing is maintained at 800 ° C. for 30 seconds to 100 seconds, cooled to 20 ° C. per second, and maintained at 400 ° C. for 180 seconds to 300 seconds, followed by cooling to anneal the cold rolled steel sheet. After maintaining for 3 seconds at 500 ℃ or more and includes a step of maintaining and cooling for at least 5 seconds.

Hereinafter, the experimental results of the inventive steels (Invention Examples 1 to 5) according to the present invention in comparison with Comparative Steels (Comparative Examples 1 to 5) will be described with reference to Table 1.

division Chemical composition (wt%) Process conditions  material carbon silicon manganese sign sulfur aluminum titanium Niobium boron nitrogen maintain
(Temperature, time) Cold
bracket
genus
Degree The tensile strength Yield strength Elongation R value × 100 × 100 × 100 × 1000 × 1000 × 100 × 1000 × 1000 × 1000 × 1000 ℃, seconds ℃ / sec MPa MPa % Comparative Example 1 4 3 120 70 8 4 35 20 One 4 0 0 444 310 34 1.34 Comparative Example 2 3 3 120 70 8 4 35 20 One 4 0 70 472 330 32 1.21 Comparative Example 3 3 15 150 80 8 4 40 20 One 4 0 0 450 300 35 1.45 Comparative Example 4 3 15 150 80 8 4 40 20 One 4 0 70 491 288 32 1.29 Comparative Example 5 3 15 150 80 8 4 40 20 One 4 760,7 20 456 329 37 1.43 Inventory 1 4 3 120 70 8 4 35 20 One 4 760,7 70 492 348 40 1.82 Inventive Example 2 3 3 120 70 8 4 35 20 One 4 760,7 90 498 359 38 1.65 Inventory 3 4 20 150 80 8 4 40 20 One 4 760,9 70 509 375 38 1.77 Honorable 4 4 20 150 80 8 4 40 20 One 4 760,7 70 501 365 40 1.81 Inventory 5 4 25 150 80 8 4 40 20 One 4 760,7 90 511 379 39 1.82

Referring to Table 1, the content of solid solution strengthening elements, such as silicon, manganese, phosphorus in the examples 3 to 5 is higher than the examples 1 and 2 in the comparative examples and the invention examples, respectively. Furthermore, among the titanium, niobium, and aluminum for depositing carbon and nitrogen, only titanium content was changed in the above manner. In addition, the comparative examples did not go through the maintenance step except the example 5, all the invention is subjected to the maintenance step.

Although the tensile strength of the comparative examples is 444 ~ 491 MPa level, the tensile strength of the invention examples is 492 ~ 511 MPa. From this, it can be seen that the tensile strength of the inventive examples is improved by about 10% compared to the comparative examples. The cooling rate of the comparative example 5 is 20 degree-C / sec, and is 40% of the minimum of 50-100 degree-C / sec according to a present Example, and is excessively low. As a result, the grains of Comparative Example 5 became excessively coarse, and even though more manganese, silicon, phosphorus, and the like were added than Comparative Example 2, the tensile strength was rather low. However, the elongation of Comparative Example 5 is larger than Comparative Example 2 as the crystal grains are increased.

The elongation of the comparative examples is 32 to 37%, but the elongation of the inventive examples reaches 38 to 40%. Even in the elongation, the inventive examples show a 10% or more improvement over the comparative examples.

Furthermore, the invention examples have improved values in comparison with the comparative examples. Specifically, the R value of the comparative examples is 1.21 ~ 1.45, but the R value of the invention examples is 1.65 ~ 1.82, it can be seen that about 20% improved than the comparative examples.

3 is a graph where the holding temperature is set at 760 ° C. and the holding time is set at 7 seconds. Referring to this figure, the cooling rate of 50 ~ 100 ℃ / sec is a section considering both the tensile strength (TS) and elongation (EI). If the cooling rate is 50 ° C / sec or less, the elongation is slightly improved, but the tensile strength is further lowered. At cooling rates above 100 ° C / sec, the elongation also drops and the tensile strength drops even more.

4 is a graph where the cooling rate is 80 deg. C / sec and the holding time is 7 seconds. This graph is the basis for setting the holding temperature to 700 to 800 ° C. If the holding temperature is 700 ° C. or lower, the tensile strength is high but the elongation is low. If the temperature is 800 ° C. or higher, the elongation is high, but the tensile strength is sharply lowered.

5 is a graph when the holding time is set at a holding temperature of 750 ° C. and a cooling rate of 80 ° C./second. This graph provides the basis for setting the retention time to 5-10 seconds. If the holding time is 5 seconds or less, the tensile strength is improved, but the elongation is lowered below the required level. Further, when the holding time is 10 seconds or more, the elongation is improved, but the tensile strength is sharply lowered.

Such a cold rolled steel sheet and a method of manufacturing the same are not limited to the configuration and operation of the embodiments described above. The embodiments may be configured so that all or some of the embodiments may be selectively combined so that various modifications may be made.

Claims (13)

Cooling the slab comprising at least one of titanium, niobium and aluminum and at least one of manganese, phosphorus and silicon at a first rate of 5-10 ° C./s after hot rolling;
Maintaining the slab at a temperature ranging from 700 to 800 ° C. after the cooling;
Cooling the slab at 50-100 ° C./s, a second speed greater than the first speed, after the holding step; And
Cold rolling steel sheet manufacturing method comprising the step of cold rolling the slab after the cooling.
The method of claim 1,
The second speed is a cold rolled steel sheet manufacturing method in the range of 5 times to 20 times the first speed.
delete The method of claim 1,
The holding method is a cold rolled steel sheet manufacturing method set to be made for a time of 5 ~ 10 seconds.
The method of claim 1,
The winding is cold rolled steel sheet manufacturing method made in the temperature range of 400 ~ 550 ℃.
delete The cold rolled steel sheet manufactured according to any one of claims 1, 2, 4, and 5, having a tensile strength of at least 490 MPa and an elongation of at least 35%, and having a plastic strain ratio of at least 1.5.
delete The method of claim 7, wherein
The size of the crystal grains of the cold rolled steel sheet is manufactured to have a value of 7 ~ 20㎛.
In weight%, carbon 0.002-0.01, nitrogen 0.002-0.01, silicon 0.05-0.25, sulfur 0 more than 0.01 or less, manganese 0 or more 0.2 or less, phosphorus 0.05-0.01, aluminum 0.01-0.06, titanium 0.02-0.07,
The content of titanium is the following formula:
4 x carbon +3.4 x nitrogen +1.5 x sulfur
Is controlled by more than the sum of the elements of,
A cold rolled steel sheet manufactured by hot rolling, holding at a temperature at a temperature after primary cooling, after secondary cooling at a higher speed than the primary cooling, winding up, and then cold rolling.
The method of claim 10,
By weight%, niobium further comprises 0.01 to 0.05, niobium is a cold rolled steel sheet manufactured to be adjusted to be equal to or more than the sum of the elements of the following equation: 0.5 × carbon × (93/12). The method of claim 10,
The holding time is set to 5 ~ 10 seconds cold rolled steel sheet manufactured. The method of claim 10,
The size of the crystal grains of the cold rolled steel sheet is manufactured to have a value of 7 ~ 20㎛.

Images