KR101225793B1 - Method for manufacturing steel sheet - Google Patents

Abstract

The slab is hot rolled at a temperature lower than the Ar3 temperature, the hot rolled material is first cooled at a cooling rate of 20 ° C./sec. To 50 ° C./sec., And the first cooled hot rolled product. After maintaining in the temperature range of 720 ℃ to 650 ℃, the second cooling at a cooling rate of 20 ℃ / sec (sec) to 50 ℃ / sec (sec), and the second cooled hot rolled product 500 ℃ to 300 ℃ After winding with a coil (coil) in the temperature range, a method for producing a steel sheet to be air-cooled.

Description

Method for manufacturing steel sheet {Method for manufacturing steel sheet}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet manufacturing technology, and more particularly, to a method for manufacturing a steel sheet excellent in workability using a mini mill.

For materials requiring workability, high deep drawing is required. Automotive materials and general structural materials are required to have a high deep drawing property for processing, and it is determined whether or not the material is defective by the degree of deep drawing property. The deep drawability may be determined to be increased in proportion to the r value in relation to the plastic deformation ratio r. Therefore, increasing r value at the time of manufacture of a steel plate is calculated | required. In particular, when manufacturing a steel sheet by hot rolling using steelmaking and cast slab using a mini mill, it is required to secure excellent workability.

The present invention is to provide a method for producing a steel sheet excellent in workability when manufacturing a steel sheet using a mini-mill.

One aspect of the invention, the step of hot rolling the slab (slab) at a temperature lower than the Ar3 temperature; A first cooling step of cooling the hot rolled product at a cooling rate of 20 ° C./sec to 50 ° C./sec; Maintaining the first cooled hot rolled product in a temperature range of 720 ° C. to 650 ° C .; A second cooling step of cooling the hot rolled product at a cooling rate of 20 ° C./sec to 50 ° C./sec; Winding the second cooled hot rolled product with a coil in a temperature range of 500 ° C. to 300 ° C .; And it provides a steel sheet manufacturing method comprising the step of air cooling the wound hot rolled product.

According to the embodiment of the present invention, the increase in the plastic strain ratio r may be implemented, thereby improving the deep drawing property and the moldability. Due to such excellent formability, steel sheet production having excellent surface properties is effectively possible.

1 is a view showing a steel sheet manufacturing method according to an embodiment of the present invention.

Embodiment of the present invention proposes a steel sheet manufacturing method that can ensure a high elongation and plastic deformation ratio (r) to ensure excellent workability. The steel sheet manufacturing method according to an embodiment of the present invention can be applied to a method for manufacturing a hot rolled steel sheet for structural use or a structural hot rolled steel sheet. In order to more easily secure the aggregate structure in the structure of the steel sheet in the cold rolling annealing step, a method of performing hot rolling in the ferrite region is proposed. By carrying out the finish rolling of hot rolling in the ferrite region, recrystallized microstructure can be obtained and excellent texture can be obtained. In addition, the formation of titanium (Ti), niobium (Nb), and molybdenum (Mo) precipitates in the ferrite region is relatively excellent, and thus the content of solid solution carbon and solid solution nitrogen can be lowered as compared with austenitic hot rolling.

Since the content of solid solution elements (C, N) in the steel can be lowered more effectively, a higher plastic strain ratio (r) can be ensured, and the r value can be increased to further improve deep drawing properties, thereby forming excellent molding. The castle can be secured. In the steelmaking process, ultra-low carbon steel is manufactured by performing decarburization and denitrification, and then titanium (Ti) or niobium (Nb), which is a carbonitride-forming element, is added to control solid solution elements present in the steel. . Elements such as Ti, Nb, and Mo cause precipitated solid solution elements (C, N) present in the steel as precipitates. In the embodiment of the present invention, titanium (Ti) is performed by performing hot rolling in a ferritic region. ), Niobium (Nb) and molybdenum (Mo) precipitates can be induced relatively well. Therefore, the content of solid solution carbon and solid solution nitrogen can be lowered compared to the austenitic hot rolling, and thus sufficient moldability can be secured by securing a sufficiently high r value and thereby excellent deep drawing property.

In order to secure excellent moldability, a high average plastic strain ratio (rm) and a low planar anisotropy coefficient value (Δr) should be secured. The average plastic strain ratio is obtained by rm = (r0 + 2r45 + r90) / 4, and the plane anisotropy coefficient value Δr = (r0-2r45 + r90) / 2. At this time, r0 means a rolling direction, r45 means a diagonal direction of the rolling direction, r90 means a plastic strain ratio value in the vertical direction of the rolling direction. The high plastic strain ratio (r) means that the deep drawing property is excellent, so that complex shapes can be easily processed, and the low planar anisotropy value means that the strain distribution in the plane direction of the sheet is uniform. It means that uniform deformation is possible. After all, the high plastic strain ratio and low planar anisotropy coefficient means that it is easy to manufacture the desired parts after molding, so that the shape freezing property is excellent.

Therefore, in order to secure sufficient molding, a high plastic strain ratio (r) should be increased. For this purpose, in the embodiment of the present invention, by performing finish rolling of hot rolling in a ferrite region, titanium (Ti), niobium (Nb), By relatively inducing the formation of molybdenum (Mo) precipitates, it is possible to lower the content of solid solution carbon and solid solution nitrogen as compared to austenitic hot rolling. That is, the effect of depositing carbon (C) and nitrogen (N), which are solid solution elements in the steel, as a precipitate can be enhanced to ensure a higher r value.

The hot rolling finish of the present invention is carried out below the Ar3 temperature. In other words, it is carried out by hot rolling in the ferrite region, thereby controlling the texture of the steel sheet to secure a high r value of 2.1 or more, thereby ensuring excellent deep drawing property. In addition, by securing a high elongation of 43% or more, the r value can be further supplemented to improve workability. Accordingly, the ferritic reverse hot rolling method according to the embodiment of the present invention can implement excellent mechanical properties compared to the conventional austenite reverse hot rolling method. In the austenitic hot rolling method, the r-value may be lowered by the influence of the (110) texture that has been previously formed by the transformation during cooling and by the influence of the texture formed by the ferrite transformation. In the case of the ferritic hot rolling method according to the present invention, similar to the annealing process of cold rolling, phase transformation may be substantially excluded, and the formation of (111) texture may be induced, which is advantageous for the development of texture. As a result, it is possible to secure an excellent r value and exhibit excellent deep drawing properties.

In the ferrite reverse hot rolling method according to an embodiment of the present invention, it is possible to secure an excellent r value and to implement excellent deep drawing property, thereby obtaining molten steel through a steelmaking process performed in a mini mill process, and ingot Or it can be applied to the production of steel sheet using a steel slab (slab) manufactured through a continuous casting process.

In the embodiment of the present invention, when producing a hot rolled steel sheet using a mini mill process using scrap scrap, the slab for the hot rolled steel sheet may contain the following alloying elements.

A) carbon (C): 0.005 to 0.01 parts by weight (wt%)

Carbon (C) is present in the steel sheet as a solid element. Carbon (C) may inhibit the formation of the (111) planar texture in favor of processability in the process of forming the texture of the steel sheet during cold rolling and annealing, thereby reducing workability and formability. In addition, when carbon (C) is present in steel, it may cause aging problems and cause strainer strain problems. Therefore, when the carbon (C) content is more than 100ppm, the amount of addition of titanium (Ti) or niobium (Nb), which is a carbide forming element, may be increased to induce precipitation of carbon (C). In this case, not only the production cost of steel is increased, but also the material and surface properties of the steel sheet may be lowered by the addition of a large amount of titanium (Ti) and niobium (Nb). Therefore, the lower the carbon (C) content is advantageous to the steel sheet properties, but the strength is lowered, there is a limitation of steelmaking technology and grain boundary embrittlement may occur. Accordingly, the content of carbon (C) is more effectively controlled to 0.005 parts by weight (wt%) to 0.01 wt%. More preferably, it is effective to contain 0.007 to 0.008 wt%.

s) nitrogen (N): 0.005 to 0.01 parts by weight (wt%)

Nitrogen (N), like carbon (C), exists as a solid element in steel, and can reduce elongation to reduce workability and formability of the steel sheet. The lower the content of nitrogen (N), the better the formability, but considering the steelmaking level and cost, it is difficult to reduce the content below the lower limit of 0.005wt%. As the content of nitrogen (N) increases, the degree of decreasing workability increases, so that it is effective not to contain 0.01 wt% or more.

C) sulfur (S): 0.01 parts by weight (wt%) or less

Sulfur (S) may cause the formation of sulfide-based inclusions, such as manganese sulfide (MnS), and may form iron sulfide (FeS) to cause cracks at the edges of the steel sheet, that is, edge cracks. It is preferable that sulfur (S) is not contained, but since this is practically impossible, it is more effective to control within 0.01 wt% or less.

D) manganese (Mn): 1.0 parts by weight or less (wt%)

Manganese (Mn) may be added to form a manganese sulfide (MnS) together with securing strength to induce desulfurization and to prevent cracking caused by sulfur (S). When manganese (Mn) is excessively contained, workability and moldability may decrease due to grain boundary segregation of manganese (Mn), and may adversely affect plating properties of the steel sheet surface after steel sheet production. Accordingly, it is effective to control the manganese (Mn) to 1.0wt% or less, and may be added to about 0.5wt%.

ㅁ) Phosphorus (P): 0.02 parts by weight (wt%) or less

Phosphorus (P) is a component that realizes an excellent solid solution strengthening effect, and is added a lot for the purpose of increasing the strength, and the effect of the addition of a relatively small amount is quite high. Nevertheless, when phosphorus (P) is added in a large amount, phosphorus (P) may segregate at grain boundaries, causing secondary processing brittleness. Thus, phosphorus is added above 0 wt% and below 0.02 wt%. More preferably, it may be added in an amount of about 0.008 wt% to 0.009 wt%. Secondary work brittleness by phosphorus can be suppressed by adding a trace amount of boron (B).

Iii) aluminum (Al): 0.01 to 0.06 parts by weight (wt%)

Aluminum (Al) is added as a deoxidizer component and lowers the amount of dissolved oxygen in the steel to keep the amount of dissolved oxygen low. When the addition of more than the upper limit as a role of the deoxidizer, problems such as a defect during the play may be caused, it is effective to add in the range of 0.01wt% to 0.06wt%. More preferably, about 0.04 wt% is contained. Aluminum (Al) reacts with nitrogen (N), which is a solid solution element, to form aluminum nitride (AlN) precipitates, and is effective for removing nitrogen solid solution elements. Therefore, when the continuous annealing method is applied, it is possible to induce precipitation of AlN in advance in the hot rolling step by maintaining a high winding temperature.

S) Titanium (Ti): 0.02 to 0.07 parts by weight (wt%)

Titanium (Ti) may increase the plastic deformation ratio (r), which is an index indicating moldability as the addition amount increases, thereby increasing moldability and inducing non-aging characteristics. Carbon (C) and nitrogen (N) present as solid elements in the steel can be precipitated in the form of precipitates such as titanium nitride (TiN) and titanium carbide (TiC), and the r value can be improved by removing the solid solution elements in the steel. . When solid element C remains in the steel, C moves and constrains the dislocation seeker aging phenomenon over time, which induces precipitation of C and suppresses this aging phenomenon. Since the addition amount of Ti can be set so that C can be removed sufficiently, the lower limit of the addition range is set to the amount which can stoichiometrically precipitate the solid solution element. As the Ti content increases, the r value tends to increase. However, when the Ti content increases, it is confirmed that the nozzle may be clogged during playing and surface defects may occur during cold rolling. It is effective to add in the range 0.07wt%.

It is effective that Ti is added so as to satisfy the formula of 4 x C (carbon) + 3.4 x N (nitrogen) + 1.5 x S (sulfur). That is, it is effective to control the solid solution element in which Ti is added in an amount more than satisfying this formula. Ti varies in form of carbonitride precipitation according to temperature, and is deposited in the order of titanium nitride (TiN), titanium sulfide (TiS), and titanium carbide (TiC). When Ti is added alone, when Ti, which is a carbonitride-forming element, is not sufficiently added, C and N, which are solid solutions, may remain in the steel, thereby deteriorating r value and workability. Therefore, in order to ensure a sufficient value of r in order to realize excellent workability, it is effective to add Ti enough to satisfy the above formula.

Niobium (Nb): 0.01 to 0.05 parts by weight (wt%)

Niobium (Nb) precipitates carbon (C) and nitrogen (N), which are solid solutions in steel, in the form of precipitates of niobium carbide (NbC) and niobium nitride (NbN). It can be added to enhance and remove C in the steel to induce non-aging characteristics. Such addition of niobium (Nb) is added in order to precipitate solid solution elements like Ti. When the addition of Nb above the upper limit, that is, the amount of the solid solution to form a solid solution, that is, the r-value may drop, and the manufacturing cost may increase due to the cost increase. Nevertheless, the addition of Nb can achieve a fairly good plating properties. Therefore, it is effective to add about 0.01 to 0.05 wt% of Nb by setting the minimum content of stoichiometrically precipitated solid solution in steel as the lower limit. More preferably, about 0.02 wt% is added. The content of Nb may be set to satisfy the formula of 0.5 × C × (93/12).

Silicon (Si): 0.03 parts by weight or less

Silicon (Si) increases the strength by the solid solution strengthening effect. The higher the content, the higher the strength, but may reduce the plating characteristics of the post-process, so that the upper limit of the content range may be set in consideration of this. In addition, it is possible to induce the formation of an oxide in the form of Fe-Al-O or Fe-Al-Si-O on the surface of the slab or steel sheet to suppress the formation or growth of the scale layer. The lower limit of the content range can be set in consideration of the effect of suppressing such scale formation. Therefore, silicon (Si) may be included from 0.02 to 0.03 wt%.

Copper (Cu): not more than 0.15 parts by weight (wt%)

Copper (Cu) is a tramp element that can remain by using scrap metal scram as a raw material in a mini mill process and is an impurity that cannot be completely removed in a steelmaking process. Although copper (Cu) mainly acts as an element to increase strength, it acts as a factor for lowering elongation, r value and surface quality of steel, and therefore it is preferable to regulate it to 0.015 wt% or less. When the copper (Cu) is excessively contained in a predetermined amount or more, since it is concentrated or concentrated on the surface of the steel during casting and annealing heat treatment, causing red brittleness, the surface cracks and surface properties of the cast steel may be reduced.

Tin: Sn: 0.15 parts by weight or less

Tin (Sn), like copper (Cu), may also be present as impurities, i.e., circulating elements, which cannot be removed in a minimill steelmaking process using scrap as a raw material. Unlike other cyclic elements, it can play a decisive role in degrading the mechanical properties of steel. If the content of tin (Sn) is greater than 0.015wt%, not only the strength of the steel is rapidly increased but also the elongation and r value are lowered, which adversely affects the formability. Therefore, tin (Sn) is preferably regulated to a content of 0.015 wt% or less.

The remaining components of the steel of the composition according to the embodiment of the present invention may be made of iron (Fe), and may further include impurities, such as oxygen (O), which are inevitably contained according to the environment of raw materials, materials, manufacturing facilities, and the like. have.

In the process of manufacturing a hot rolled steel sheet using the steel of the composition according to the embodiment of the present invention, the molten steel is obtained through a steelmaking process according to the mini-mill process, and then a slab is obtained through an ingot or a continuous casting process. The slabs are heated to temperatures above 1100 ° C. in a furnace to reclaim segregated components during casting. When the reheating temperature is relatively low, such as this temperature, it is easy to precipitate MnS and AlN, which can effectively precipitate sulfur (S) and nitrogen (N) in the form of precipitates, thereby contributing to the improvement of workability of the steel. have.

The slab subjected to this reheating process is rolled by a step hot rolling process as shown in FIG. 1 to prepare a steel sheet. The hot rolling process is performed at a temperature of 85 ° C. to 800 ° C., which is a temperature below Ar3, to finish the hot rolling (S1). Since hot rolling is performed in the temperature range below Ar3, that is, hot rolling is performed in the ferrite zone, phase transformation is substantially not generated and an (111) texture is formed which is advantageous for the development of the texture. As in the cold rolling annealing process, the (111) texture is formed. When hot rolling is performed in the existing austenite temperature range, phase transformation may occur during cooling, and r value is deteriorated due to the influence of the previously formed (111) texture and the texture formed during ferrite transformation. This can be caused. In the embodiment of the present invention, since the hot rolling is performed in the ferrite region, as in the cold rolling annealing process, the (111) texture is formed, which is advantageous for the development of texture without phase transformation. Thereby, it is possible to ensure the excellent r value.

After the hot rolling in the ferrite region, the steel sheet, which is a hot rolled workpiece, is first cooled (S2). The first cooling is a process of quenching with water cooling, so that the cooling to 20 ℃ / sec (sec) to 50 ℃ / sec cooling rate. At cooling rates lower than 20 ° C / sec, the grain size may be excessively increased, and at very fast cooling rates greater than 50 ° C / sec, the grain boundaries are too fine, leading to a sharp increase in strength. Thus, the elongation can be reduced. More preferably, cooling can be performed at a cooling rate of 20 ° C / sec (sec) to 40 ° C / sec, and considering the r value, a cooling rate of approximately 20 ° C / sec (sec) is more effective for increasing the r value.

After cooling the steel sheet, which is a hot rolled product by the first cooling, at a temperature of 700 ° C. to 720 ° C., it is maintained for a predetermined time (S3). This temperature holding step is performed for about 5 seconds to 10 seconds (sec) in the temperature range of about 720 ℃ to 650 ℃. More preferably, the temperature holding step is performed in the temperature range of 700 ° C to 670 ° C. This temperature holding step is introduced to induce carbon in the steel to be supersaturated to the maximum and precipitated as cementite (Fe ₃ C). As the carbon in the steel is precipitated with cementite, it is possible to effectively reduce the carbon in the steel to implement excellent non-aging characteristics. The temperature maintaining step may be performed by an air cooling process, unlike the previous first cooling process being performed by a water cooling process.

After the temperature holding step, the hot rolled steel sheet is second cooled again (S4). The second cooling is a process of quenching with water cooling, and is cooled at a cooling rate of 20 ° C / sec (sec) to 50 ° C / sec. Likewise, at cooling rates lower than 20 ° C./sec, the grain size may be excessively increased, and for very fast cooling rates greater than 50 ° C./sec, the grain boundaries may be too fine. Nevertheless, the second cooling may be performed at a faster cooling rate than the first cooling, for example, a cooling rate of approximately 40 ° C./sec.

The second cooled steel sheet is wound up to obtain a hot rolled coil (S5). The coiling temperature can be carried out in the temperature range of 300 ° C to 500 ° C. Subsequently, it is cooled by air cooling to lower the temperature of the hot rolled coil. It is confirmed that the hot rolled steel sheet manufactured by such a process may have a grain size of 5 μm to 15 μm. Accordingly, a high r value of 2.1 or more, a high firing rate of 50% or more, and a tensile strength of 270 MPa or more can be realized.

By performing the stepped hot rolling as described above in the process of rolling in the ferrite region, it is possible to secure a larger (111) texture in the structure of the hot rolled steel sheet. In addition, it is possible to implement an improvement in elongation by controlling the growth of grain boundaries by step hot rolling.

The results of evaluating the composition examples, process examples and physical properties of the steel sheet according to the embodiment of the present invention thus produced are shown in Table 1, Table 2 and Table 3, respectively.

division
Chemical composition (%) C Ti Nb N Cu Sn × 1000 × 1000 × 1000 × 1000 × 1000 × 1000 Comparative Example 1 8 35 20 6 12 8 Comparative Example 2 8 35 20 6 12 8 Comparative Example 3 8 40 20 6 15 12 Comparative Example 4 8 40 20 6 15 12 Example 1 8 35 20 6 12 8 Example 2 8 35 20 6 12 8 Example 3 8 35 20 6 12 8 Example 4 8 40 20 6 15 12 Example 5 8 40 20 6 15 12 Example 6 8 40 20 6 15 12

※ Same as Si 0.02%, Mn 0.5%, P 0.009%, S 0.008%, Al 0.04%.

division
Manufacture process Rolling 1st cooling speed Holding temperature Retention time Coiling temperature 2nd cooling speed ℃ ℃ / s ℃ sec ℃ ℃ / s Comparative Example 1 910 - - - 700 - Comparative Example 2 910 - - - 700 - Comparative Example 3 910 - - - 700 - Comparative Example 4 910 - - - 700 - Example 1 830 20 670 7 400 40 Example 2 830 20 670 9 400 40 Example 3 810 40 670 7 400 40 Example 4 830 20 670 7 400 40 Example 5 830 20 670 9 400 40 Example 6 810 40 670 7 400 40

division
material
Yield Strength (YP) Tensile Strength (TS) Elongation (EI) Plastic strain ratio (r-bar) MPa MPa % Comparative Example 1 195 327 43 1.61 Comparative Example 2 186 331 42 1.59 Comparative Example 3 189 329 43 1.56 Comparative Example 4 196 340 43 1.65 Example 1 143 297 52 2.19 Example 2 146 286 54 2.30 Example 3 139 290 52 2.11 Example 4 135 289 51 2.22 Example 5 138 288 52 2.26 Example 6 139 296 49 2.20

Considering Table 3 showing the results of measuring the properties according to the composition examples of Table 1 and the process examples of Table 2, the final product of the steel sheet manufacturing method according to an embodiment of the present invention has a high plastic strain ratio (r) value of 2.1 or more It can be implemented, along with a tensile strength of at least 270Mpa or more. In particular, in the case of Inventive Example 2 and Inventive Example 5, by showing a relatively long temperature holding time to 9 seconds, it shows that the excellent increase in the plastic strain ratio (r) value.

According to this embodiment of the present invention, it is possible to secure a high plastic strain ratio (r) and thereby excellent deep drawing properties and formability compared to the existing ultra low carbon steel, steel sheet having excellent surface properties by such excellent formability Manufacturing is effectively possible.

S1 ... hot rolling 110 ... scale inhibiting layer
130 ... scale layer.

Claims (8)

Hot rolling the slab at a temperature lower than the Ar3 temperature;
A first cooling step of cooling the hot rolled product at a cooling rate of 20 ° C./sec to 50 ° C./sec;
Maintaining the first cooled hot rolled product in a temperature range of 720 ° C. to 650 ° C .;
A second cooling step of cooling the hot rolled product at a cooling rate of 20 ° C./sec to 50 ° C./sec;
Winding the second cooled hot rolled product with a coil in a temperature range of 500 ° C. to 300 ° C .; And
Air-cooling the wound hot rolled product,
The slab is
0.005 to 0.01 wt% of carbon (C); 0.005 to 0.01 wt% nitrogen (N); 0.02 to 0.07 wt% titanium (Ti); 0.01 to 0.05 wt% niobium (Nb); 0.012 to 0.015 wt% of copper (Cu); 0.008 to 0.015 wt% Tin (Sn); And a steel composition comprising a balance of iron (Fe),
The steel composition
0.02 to 0.03 wt% of silicon (Si); 0.5 to 1.0 wt% manganese (Mn); 0.008 to 0.02 wt% phosphorus (P); 0.008 to 0.01 wt% sulfur (S); And 0.01 to 0.06 wt% of aluminum (Al),
The cooling rate in the second cooling step is faster than the cooling rate in the first cooling step.
delete delete delete The method of claim 1,
The slab is
Steel sheet manufacturing method to form a mini mill (mini mill) process using the steel composition.
The method of claim 1,
The hot rolling
Steel sheet manufacturing method which is finished in the temperature range of 850 ℃ to 800 ℃.
The method of claim 1,
The temperature maintaining step
Steel sheet manufacturing method performed to maintain for 5 seconds to 10 seconds in the temperature range of 700 ℃ to 670 ℃.
The method of claim 1,
The first cooling comprises the step of water cooling at a cooling rate of 20 ℃ / sec (sec),
The second cooling is a steel sheet manufacturing method comprising the step of water cooling at a cooling rate of 40 ℃ / sec (sec).

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