KR20130110632A - Method of manufacturing steel sheet - Google Patents

Abstract

PURPOSE: A method for manufacturing a steel sheet is provided to obtain an r-bar value of 2.0 or greater and an elongation ratio of 47% or greater by alloy component controlling, slab reheating, and hot-rolling processes. CONSTITUTION: A method for manufacturing a steel sheet includes the following steps of: reheating a slab plate comprising 0.002-0.01 wt% of carbon, 0.002 wt% or less of silicon, 0.2-0.5 wt% of manganese, 0.025-0.065 wt% of phosphorus, 0.01 wt% or less of sulfur, 0.01-0.05 wt% of soluble aluminum, 0.002 wt% or less of nitrogen, and the rest of Fe and unavoidable impurities at a reheating temperature of 1250-1300°C; hot-rolling the slab plate under at a starting temperature of 950-1000°C and a finishing temperature of 890-930°C; coiling the slab plate by cooling and air-cooling the slab plate; and annealing the slab plate at a temperature of 800-850°C by drawing out and cold-rolling the slab plate. [Reference numerals] (AA) Start; (BB) End; (S210) Reheat a slab (1250-1300°C); (S220) Hot rolling (FET: 950-1000°C, FDT: 890-930°C); (S230) Cooling/winding (CT: 680-720°C); (S240) Cold rolling (draft percentage: 75-85%); (S250) Annealing (800-850°C)

Description

[0001] METHOD OF MANUFACTURING STEEL SHEET [0002]

The present invention relates to a steel sheet manufacturing method, and more particularly, to a method for producing a steel sheet excellent in deep drawing properties, so that it can be used as a material such as a vehicle side outer.

Among the steel sheets for automobiles, the outer plate member requires excellent workability and an excellent surface property. In particular, steel sheets applied to side outer parts may require extreme properties in elongation and deep drawability, depending on the size of the parts and the difficulty of molding.

Therefore, the steel company is used to design the steel sheet applied to the automotive side outer ultra-low carbon steel with a very low carbon content, and specially managed steel grades in the EDDQ (Extra Deep Drawing Quality) class.

Therefore, special rolling conditions for manufacturing a steel sheet applied to the side outer for automobiles are needed.

Background art related to the present invention is a steel sheet excellent in surface properties disclosed in the Republic of Korea Patent Publication No. 10-2011-0022340 (2011.03.07. Publication) and a method of manufacturing the same.

An object of the present invention is to provide a method for producing a steel sheet excellent in deep drawing through the control of the alloy component and process.

Steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is carbon (C): 0.002-0.01% by weight, silicon (Si): 0.002% by weight or less, manganese (Mn): 0.2-0.5% by weight, phosphorus (P): 0.025 to 0.065% by weight, sulfur (S): 0.01% by weight or less, soluble aluminum (S-Al): 0.01 to 0.05% by weight, nitrogen (N): 0.002% by weight or less and the rest of iron (Fe) Reheating the slab plate made of unavoidable impurities at a reheating temperature of 1250 to 1300 ° C; Hot rolling the reheated plate at a starting temperature of 950 to 1000 ° C. and a finishing temperature of 890 to 930 ° C .; And cooling and cooling the hot rolled plate, followed by air cooling.

The slab plate may further include boron (B): 0.007% by weight or less.

It is preferable that the said winding is performed at 680-720 degreeC.

Steel sheet manufacturing method according to an embodiment of the present invention for achieving the above object is carbon (C): 0.002-0.01% by weight, silicon (Si): 0.002% by weight or less, manganese (Mn): 0.2-0.5% by weight, phosphorus (P): 0.025 to 0.065% by weight, sulfur (S): 0.01% by weight or less, soluble aluminum (S-Al): 0.01 to 0.05% by weight, nitrogen (N): 0.002% by weight or less and the rest of iron (Fe) Reheating the slab plate made of unavoidable impurities at a reheating temperature of 1250 to 1300 ° C; Hot rolling the reheated plate at a starting temperature of 950 to 1000 ° C. and a finishing temperature of 890 to 930 ° C .; Cooling and winding the hot rolled sheet material, followed by air cooling; After unwinding the wound sheet, cold rolling; And annealing the cold rolled sheet at 800 to 850 ° C.

At this time, the slab plate may further include boron (B): 0.007% by weight or less.

Moreover, it is preferable that the said winding is performed at 680-720 degreeC.

In addition, the cold rolling is preferably carried out at a reduction ratio of 75 to 85%.

According to the method for manufacturing a steel sheet according to the present invention, a steel sheet capable of producing an elongation of at least 47% and a rankford value (r-bar) of at least 47% by controlling alloy components and controlling slab reheating and hot rolling, etc. Can be.

Therefore, the steel sheet produced by the method according to the present invention can be utilized as a material for automotive side outer through excellent deep drawing characteristics.

Figure 1 schematically shows a steel sheet manufacturing method according to an embodiment of the present invention, it shows a hot rolled steel sheet manufacturing method.
Figure 2 schematically shows a steel sheet manufacturing method according to another embodiment of the present invention, it shows a cold rolled steel sheet manufacturing method.
3 and 4 are photographs showing the precipitates of the specimens according to Example 1 and Comparative Example 1.
Figure 5 shows the final microstructure photographs and analysis results of the specimens according to Example 1 and Comparative Example 1.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

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

Steel plate

Steel sheet according to the present invention is carbon (C): 0.002-0.01% by weight, silicon (Si): 0.002% by weight or less, manganese (Mn): 0.2-0.5% by weight, phosphorus (P): 0.025-0.065% by weight, sulfur (S): 0.01 wt% or less, soluble aluminum (S-Al): 0.01 to 0.05 wt% and nitrogen (N): 0.002 wt% or less.

In addition, the steel sheet according to the present invention may further include boron (B): 0.007% by weight or less for reducing solid solution nitrogen.

The rest of the alloy components are made of inevitable impurities generated during iron (Fe) and steelmaking.

Hereinafter, the role and content of each component included in the steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) forms carbide or carbonitride.

The carbon is preferably contained in 0.002 to 0.01 wt% of the total weight of the steel sheet according to the present invention. If the carbon content exceeds 0.01% by weight, excessive aging rise is a problem. On the contrary, when the carbon content is less than 0.002% by weight, carbide or carbonitride formation is insignificant, making it difficult to secure strength.

Silicon (Si)

In the present invention, silicon (Si) corresponds to impurities in the steel to lower the surface properties of the steel.

The silicon is preferably limited to 0.002% by weight or less of the total weight of the steel sheet according to the present invention. When the content of silicon exceeds 0.002% by weight, deterioration of the surface properties of the steel is a problem.

Manganese (Mn)

Manganese (Mn) is a solid element in steel, and serves to promote the assembly of gamma fiber during annealing during cold rolled steel sheet manufacturing.

The manganese is preferably added in 0.2 ~ 0.5% by weight of the total weight of the steel sheet according to the present invention. If the manganese content is less than 0.2% by weight, it is difficult to secure the strength in view of the fact that the present invention contains 0.01% by weight or less of carbon. On the contrary, when the content of manganese exceeds 0.5% by weight, segregation is difficult to be suppressed in spite of slab reheating temperature and hot rolling start temperature control.

Phosphorus (P)

Phosphorus (P) in the present invention contributes to the strength improvement together with the above manganese, the strength of the steel may increase as the phosphorus content is increased.

The phosphorus is preferably contained in 0.025 to 0.065% by weight of the total weight of the steel sheet according to the present invention. If the content of phosphorus is less than 0.025% by weight, the effect of addition thereof is insufficient. On the contrary, if the content of phosphorus exceeds 0.065% by weight, excessive strength increase may adversely affect the properties of the soft steel sheet, there is a problem that segregation in the steel is increased, and may cause delayed fracture.

Sulfur (S)

Sulfur (S) combines with manganese to form non-metallic inclusions such as MnS to significantly reduce the mechanical properties of the steel, so it is better to lower the content as much as possible, but to manage the sulfur to a very small amount of complex processes and excessive costs are required.

Thus, in the present invention, the sulfur content is limited to 0.01% by weight or less of the total weight of the steel sheet according to the present invention.

Soluble Aluminum (S-Al)

Soluble aluminum (Al) has excellent deoxidation ability compared to silicon (Si) and manganese (Mn), and is used as a main deoxidizer in the present invention. If the deoxidation is not sufficient, due to the solid solution oxygen of the molten steel in the liquid phase, poor bubble, crack generation and finally the mechanical properties of the steel sheet can be reduced.

The soluble aluminum is preferably added in 0.01 to 0.05% by weight of the total weight of the steel sheet according to the present invention. When the addition amount of soluble aluminum is less than 0.01% by weight, the deoxidation effect is insufficient. On the contrary, when the content of aluminum exceeds 0.05% by weight, it may cause surface defects of the steel sheet and lower the toughness.

Nitrogen (N)

Nitrogen (N) is an unavoidable impurity, and when it contains a large amount, despite the addition of nitrogen compound-forming elements such as aluminum and boron, solid solution nitrogen increases to deteriorate the formability of the steel sheet.

Thus, in the present invention, the content of nitrogen was limited to 0.002% by weight or less of the total weight of the steel sheet according to the present invention.

Boron (B)

Boron (B) serves to reduce solid solution nitrogen in steel through the formation of nitrogen compounds (BN). Through reduction of the solid solution nitrogen, it is possible to suppress the occurrence of yield point stretching due to the interaction between the nitrogen atom and the potential.

When boron is added, the content is preferably added at 0.007% by weight or less of the total weight of the steel sheet, and more preferably added at 0.002 to 0.007% by weight in order to exert a sufficient effect according to the addition of boron. If the amount of boron exceeds 0.007% by weight, there is a problem of inhibiting the toughness of the steel sheet.

The steel sheet according to the present invention may exhibit an elongation of 47% or more and a Rankford value (r-bar) of 2.0 or more through the composition and the process control described later.

Steel plate manufacturing method

Hereinafter, a steel sheet manufacturing method according to the present invention will be described.

 Figure 1 schematically shows a steel sheet manufacturing method according to an embodiment of the present invention, it shows a hot rolled steel sheet manufacturing method.

Referring to Figure 1, the steel sheet manufacturing method according to the present invention includes a slab reheating step (S110), hot rolling step (S120) and cooling / winding step (S130).

Reheat slab

The slab reheating step (S110) reheats the slab plate having the above-described composition, reclaims the components and precipitates segregated during casting, and homogenizes the steel.

Slab reheating is preferably carried out at a temperature of 1250 ~ 1300 ℃. This corresponds to a range of approximately 50-100 ° C. above the normal slab reheating temperature. In the present invention, the reason for raising the reheating temperature of the slab sheet is to secure sufficient amount of aging in the slab reheating step before the start of hot rolling to suppress precipitation of segregation elements such as manganese (Mn) and phosphorus (P) in the slab sheet. This is to allow these elements to be evenly diffused and dispersed in the slab plate.

On the other hand, when slab reheating temperature is less than 1250 degreeC, the slab plate material temperature will be low and the board | substrate and productivity of rolling will fall, and shape control of a board material is difficult. Conversely, when the slab reheating temperature exceeds 1300 ° C., the thickness of the hot rolled scale layer is increased to lower the surface quality of the steel sheet.

Hot rolling

In the hot rolling step (S120), the slab plate is hot rolled.

At this time, it is preferable that the start temperature of hot rolling is 950-1000 degreeC, and the finishing temperature of hot rolling is 890-930 degreeC. This is because the steel to be produced in the present invention has a very low carbon content, considering that the austenite-ferrite transformation temperature is relatively high compared to the high carbon content steel.

When hot rolling start temperature exceeds 1000 degreeC, productivity may fall by rolling time increase. On the contrary, if the hot rolling start temperature is less than 950 ° C., ferrite transformation may occur during hot rolling, and the segregation zones dispersed in the slab reheating step (S110) may be reformed before rolling starts.

In addition, if the hot rolling finish temperature exceeds 930 ℃ it is difficult to secure sufficient strength due to grain coarsening. On the contrary, if the hot rolling finish temperature is less than 890 ° C., ferrite transformation occurs during rolling, which causes problems such as hot roll structure unevenness due to abnormal reverse rolling and deterioration of final material after cold rolling.

Cooling / Winding

In the cooling / winding step S130, the hot rolled sheet is cooled and then wound at a winding temperature.

Cooling may be carried out at an average cooling rate of approximately 10 ~ 100 ℃ / sec, but is not necessarily limited thereto.

On the other hand, it is preferable that winding temperature is 680-720 degreeC. The coiling temperature of hot rolled steel determines the formation and size of the hot rolled precipitate. When the coiling temperature is 680 ° C. or more, the precipitation behavior of Fe 3 C is sufficient, and workability can be improved. However, if the coiling temperature exceeds 720 ℃ to secure the strength may be insufficient.

 Figure 2 schematically shows a steel sheet manufacturing method according to another embodiment of the present invention, it shows a cold rolled steel sheet manufacturing method.

Referring to Figure 1, the illustrated steel sheet manufacturing method includes a slab reheating step (S210), hot rolling step (S220) and cooling / winding step (S230), further cold rolling step (S240) and annealing step (S250) ).

Of these, the slab reheating step (S210), hot rolling step (S220) and the cooling / winding step (S230) may be carried out in the same process as the steps shown in Figure 1, the detailed description thereof will be omitted below, In the cold rolling step (S240) and the annealing step (S250) will be described.

Cold rolling

In the cold rolling step (S240), after winding the wound sheet, it is cold rolled. Before cold rolling may further include a pickling process for removing the surface of the steel sheet using hydrochloric acid, etc. to remove the scale of the plate.

At this time, cold rolling is preferably carried out at a reduction ratio of 75 to 85%. When the reduction ratio of the cold rolling is 75% or more, the {111} orientation is preferentially made in the texture of the cold rolled steel sheet, thereby improving the deep drawing property. However, when the reduction ratio of cold rolling exceeds 85%, problems of shape control and thickness deviation may occur.

Annealing

In the annealing step (S250) by annealing the cold-rolled sheet material to adjust the grain size of the final sheet. In this invention, it is preferable to perform annealing so that the average grain size of a final board material may be 10-20 micrometers. If the average grain size of the final plate exceeds 20㎛ may decrease the strength and workability, etc., if the average grain size of the final plate is less than 10㎛ requires excessive addition of alloying components, in this case to solve the segregation of alloy elements it's difficult.

In consideration of the grain size of the final sheet, the annealing is preferably carried out at 800 ~ 850 ℃. When the annealing is carried out at a temperature exceeding 850 ° C, the average grain size of the final sheet exceeds 20 µm, which may lower the strength, workability, and the like. On the contrary, when annealing is performed at less than 800 degreeC, austenite recrystallization hardly takes place and hardly annealing effect is obtained.

After annealing, the plate may be further cooled to about 400 to 550 ° C. at a cooling rate of about 10 to 100 ° C./sec.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Preparation of specimens

The slab plates according to Examples 1 to 3 and Comparative Examples 1 to 3 were reheated under the compositions shown in Table 1 and the process conditions described in Table 2, and after hot rolling, cooled at an average cooling rate of 50 ° C./sec, and then wound up. . After the pickling treatment, cold rolling was performed at a reduction ratio of 80%, annealing at 820 ° C. for 120 seconds, followed by cooling to 500 ° C. at a cooling rate of 10 ° C./sec, followed by air cooling.

 [Table 1] (Unit: wt%)

 [Table 2]

2. Evaluation of mechanical properties

Table 3 shows the mechanical properties of each specimen prepared according to Examples 1-3.

[Table 3]

Referring to Table 3, in the case of specimens prepared according to Examples 1 to 3, which satisfy the conditions of the slab reheating temperature, hot rolling starting temperature, and hot rolling finishing temperature, the elongation of 47% or more, And a Rankford value (r-bar) of 2.0 or more, indicating excellent deep drawing characteristics.

However, in the case of specimens prepared according to Comparative Example 1 having the same composition as in Example 1, but having a relatively low slab reheating temperature and hot rolling initiation temperature, the elongation and the Rankford value (r-bar) did not reach the target values. It was.

In addition, Example 1 and the manufacturing process is the same, but the specimens according to Comparative Example 2 and Comparative Example 3 having a relatively low content of phosphorus and manganese, the strength was relatively low compared to the deep drawing characteristics.

3 and 4 are photographs showing the precipitates of the specimens according to Example 1 and Comparative Example 1.

3 and 4, in the case of the specimen according to Example 1 having a relatively high slab reheating temperature, it can be seen that the size of the precipitate is significantly larger than that of the specimen according to Comparative Example 1. Such coarse precipitates contribute to miniaturization and disorder of the hot rolled tissue. In the case of such coarse precipitates, dynamic recrystallization occurs during hot rolling, which is possible by disordering the orientation of grains and suppressing grains.

Such coarse precipitate formation can be achieved by increasing the finishing temperature of hot rolling and reducing the cooling time until the winding by increasing the winding temperature.

If it is considered only to suppress grain growth, an additional element such as niobium is effective. However, when the content of niobium is increased, recrystallization rolling occurs while recrystallization during hot rolling, which may affect the disorder of the hot rolled texture, which is undesirable.

Figure 5 shows the final microstructure photographs and analysis results of the specimens according to Example 1 and Comparative Example 1.

Through FIG. 5, due to the difference in the hot rolling end temperature, it is possible to know the misorientation map of the hot rolled texture and the final cold rolled sheet. In the specimen according to Comparative Example 1 to which the rolling end temperature of 840 ° C. was applied, the {100} plane developed in the hot rolled texture, and the deviation distribution of the final cold rolled sheet was compared with that of Example 1 to which the rolling end temperature of 910 ° C. was applied. You can see that it has gotten significantly worse.

The rolling end temperature is related to the end point of recrystallization of austenite and further to the start point of phase transformation of ferrite from austenite. As ideal conditions for the hot-rolled grains described above, formation of fine grains and disordered aggregates is possible only when the end of recrystallization of austenite is accompanied by the end of rolling, and the start of phase change of ferrite is only after the end of rolling. In general, a problem mainly occurring in the rolling process is that, while applying the low-temperature slab reheating, the rolling temperature drops too much, so that the phase transformation into the ferrite structure occurs even before the rolling is completed.

At this time, while the BCC slip system in the rolled ferrite is operated, the {110} or {100} orientation is developed parallel to the rolling surface. These bearings developed in hot rolled plates are more likely to remain intact in subsequent cold rolling processes and, if destroyed, reduce the sharpness of the {111} orientation. As a result, this problem is the cause of bending and moldability due to the difference in material inflow during the molding process.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110, S210: Slab reheating step
S120, S220: hot rolling step
S130, S230: cooling / winding step
S240: cold rolling stage
S250: Annealing Step

Claims (7)

Carbon (C): 0.002-0.01 wt%, Silicon (Si): 0.002 wt% or less, Manganese (Mn): 0.2-0.5 wt%, Phosphorus (P): 0.025-0.065 wt%, Sulfur (S): 0.01 wt% % Or less, soluble aluminum (S-Al): 0.01 to 0.05% by weight, nitrogen (N): 0.002% by weight or less step;
Hot rolling the reheated plate at a starting temperature of 950 to 1000 ° C. and a finishing temperature of 890 to 930 ° C .; And
And cooling and cooling the hot rolled sheet material, followed by air cooling.
The method of claim 1,
The slab plate
Boron (B): The steel sheet manufacturing method characterized in that it further contains 0.007% by weight or less.
The method of claim 1,
The winding
The steel sheet manufacturing method characterized by being carried out at 680 ~ 720 ℃.
Carbon (C): 0.002-0.01 wt%, Silicon (Si): 0.002 wt% or less, Manganese (Mn): 0.2-0.5 wt%, Phosphorus (P): 0.025-0.065 wt%, Sulfur (S): 0.01 wt% % Or less, soluble aluminum (S-Al): 0.01 to 0.05% by weight, nitrogen (N): 0.002% by weight or less step;
Hot rolling the reheated plate at a starting temperature of 950 to 1000 ° C. and a finishing temperature of 890 to 930 ° C .;
Cooling and winding the hot rolled sheet material, followed by air cooling;
After unwinding the wound sheet, cold rolling; And
Annealing the cold-rolled sheet at 800 ~ 850 ℃; steel sheet manufacturing method comprising a.
5. The method of claim 4,
The slab plate
Boron (B): The steel sheet manufacturing method characterized in that it further contains 0.007% by weight or less.
5. The method of claim 4,
The winding
The steel sheet manufacturing method characterized by being carried out at 680 ~ 720 ℃.
5. The method of claim 4,
The cold rolling
A steel sheet manufacturing method, characterized in that it is carried out at a reduction ratio of 75 to 85%.

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