KR101330929B1 - Method of manufacturing hot rolled coil for preventing form transforming - Google Patents

Abstract

Disclosed is a method for manufacturing a hot rolled coil for preventing deformation caused by self-weight after winding up, that is, duckbill deformation.
The present invention is a method for producing a hot rolled coil for deformation prevention, the slab manufacturing step of producing a steel slab containing hardenable element, the reheating step of reheating the slab, and the hot rolling step of hot rolling the reheated slab to form a steel sheet And a cooling step of cooling the hot rolled steel sheet by shear control cooling in a water cooling stand and a winding step of winding the cooled steel sheet.

Description

Hot rolled coil manufacturing method to prevent deformation {METHOD OF MANUFACTURING HOT ROLLED COIL FOR PREVENTING FORM TRANSFORMING}

The present invention relates to a method for manufacturing a hot rolled coil for preventing deformation, and to a method for manufacturing a hot rolled coil that can reduce generation of a duckbill coil, which is a shape defect that is crushed up and down by its own weight after winding up the hot rolled coil.

In general, the hot rolled coil produced in the hot rolling process is transferred to the yard through various transfer means, and then cooled to room temperature in the atmosphere.

The cause of the duckbill coil is known to be due to the metamorphic superplasticity that causes permanent plastic deformation even when a stress lower than the yield strength of the material is applied when steel is subjected to an external stress during phase transformation.

That is, if the phase transformation is not completed to a certain level until the winding of the steel, the coil undergoes external force due to the coil's own weight while the phase transformation is continuously performed after the winding is completed, and the coil is crushed up and down.

An object of the present invention is to provide a method for manufacturing a hot rolled coil that can minimize the generation of duckbill coils by minimizing the amount of phase transformation after winding, in order to prevent the deformation caused by the weight up and down after winding up.

According to the spirit of the present invention, (a) slab manufacturing step of producing a steel slab; (b) a reheating step of reheating the slab; (c) hot rolling to form a steel sheet by hot rolling the reheated slab; (d) a cooling step of cooling the hot rolled steel sheet by shear controlled cooling in a water cooling stand; And (e) a winding step of winding the cooled steel sheet.

In the step (a), the steel slab, in weight%, carbon (C): 0.20 ~ 0.25%, manganese (Mn): 1.0 ~ 1.8%, silicon (Si): more than 0 0.40%, phosphorus (P ): Greater than 0 and less than 0.03%, sulfur (S): greater than 0 and 0.005% or less, niobium (Nb): 0.01 to 0.03%, aluminum (Al): greater than 0, 0.05%, nitrogen (N): greater than 0 and 0.006% and less It may be composed of the remaining iron (Fe) and other unavoidable impurities.

In the step (b), the reheating temperature (SRT: slab-reheating temperatures) is preferably 1150-1250 ° C.

In addition, in step (c), the hot rolling finishing temperature (FDT: Finishing Delivery Temperature) is preferably 790 to 850 ° C.

In addition, in the step (d), during shear control cooling in the water cooling zone, cooling may be performed at a threading speed of 5 m / s or less, and the front end water cooling speed is 70 to 150 ° C / s. It is preferable that the front end water cooling end temperature is 620-700 degreeC.

In the step (e), the coiling temperature (CT) may be 550 to 620 ° C.

According to the present invention, by suggesting the optimum rolling and cooling conditions for preventing the deformation caused by the weight up and down after winding the hot rolled coil, it is possible to improve the level of duckbill coils while maintaining the steel material It is effective in improving quality.

In particular, according to the present invention, for steel having a high degree of phase transformation delay (for example, (C + Mn / 6) steel of about 0.5 level), the degree of phase transformation during cooling is maintained until the winding, thereby maintaining the material intact while inducing deformation. It is effective to prevent.

1 is a view schematically showing a process sequence to explain a method for manufacturing a hot rolled coil for preventing deformation according to an embodiment of the present invention.
2 is a graph illustrating a method for manufacturing a hot rolled coil for preventing duckbill coils according to an embodiment of the present invention.
Figure 3 is a graph of the first comparative example corresponding to the case where the shear water cooling unit cooling is not sufficiently for comparison with the embodiment of the present invention.
4 is a graph of a second comparative example corresponding to the case where the plate speed is set too fast for comparison with the embodiment of the present invention.
5 is a graph of a third comparative example corresponding to the case where the winding temperature is set too high for comparison with the embodiment of the present invention.
6 is a diagram schematically showing a cross-sectional shape of a hot rolled coil manufactured according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and how to achieve them, will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings.

However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various forms, and provided to fully inform the scope of the invention to those skilled in the art. It is intended that the invention be defined only by the scope of the claims.

In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

Hereinafter, a hot rolled coil manufacturing method for preventing deformation of the present invention will be described with reference to the accompanying drawings.

Hot rolled coil manufacturing method to prevent deformation

1 is a view schematically showing a process sequence to explain a method for manufacturing a hot rolled coil for preventing deformation according to an embodiment of the present invention. Referring to Figure 1 will be described with respect to the hot rolled coil manufacturing method for preventing deformation according to an embodiment of the present invention.

Slab manufacturing stage (ST100)

This step is to produce a steel slab containing hardenable elements.

The steel slab at this time is, by weight%, carbon (C): 0.20 to 0.25%, manganese (Mn): 1.0 to 1.8%, silicon (Si): more than 0 and 0.40% or less, phosphorus (P): more than 0 and 0.03% Sulfur (S): more than 0 and 0.005% or less, niobium (Nb): 0.01 and 0.03%, aluminum (Al): more than 0 and 0.05%, nitrogen (N): more than 0 and 0.006% or less and the remaining iron (Fe) and It is composed of other unavoidable impurities.

Here, the role and content of each component included in the hot rolled coil according to the present invention will be described.

Carbon (C)

Carbon (C) is added to ensure strength.

The carbon is preferably added in 0.20 ~ 0.25% of the total weight of the hot rolled steel sheet according to the present invention.

If the content of carbon is less than 0.20% by weight, it may be difficult to secure sufficient strength. On the contrary, when the content of carbon exceeds 0.25% by weight, it may cause a decrease in toughness.

Manganese (Mn)

Manganese (Mn) is an element that is effective for enhancing hardenability of steel by strengthening solid solution strength. In addition, manganese is an austenite stabilizing element, which delays ferrite and pearlite transformation and contributes to grain refinement of ferrite.

The manganese is preferably added in an amount ratio of 1.0 to 1.8% of the total weight of the hot rolled steel sheet according to the present invention in consideration of the strength improving effect and the center segregation.

If the content of manganese is less than 1.0% by weight, the effect of solid solution strengthening may be insignificant. On the contrary, when the content of manganese exceeds 1.8% by weight, weldability is greatly reduced. In addition, there is a problem of greatly reducing the ductility of the steel sheet by the generation of MnS inclusions and the generation of center segregation.

On the other hand, the carbon (C) in the present invention corresponds to the hardenable element is likely to cause deformed craters during winding after hot rolling with manganese (Mn) to be described later, the carbon (C) to the content level of the following formula (1) It is preferable that the content of) and the content of manganese (Mn) be limited.

Equation 1: [C] + [Mn] / 6 ≒ 0.5

(Where [] is the weight percentage of each element)

Silicon (Si)

Silicon (Si) is added to enhance the function and solid solution of the deoxidizer to remove oxygen in the steel.

The silicon is preferably added in an amount ratio of more than 0 to 0.40% by weight of the total weight of the hot rolled steel sheet according to the present invention.

If the silicon content exceeds 0.40% by weight, the weldability may be degraded and red scale may be generated during reheating and hot rolling, which may adversely affect surface quality. In addition, it may adversely affect the plating property after welding.

Phosphorus (P)

Phosphorous (P) is added to inhibit cementite formation and increase strength. However, phosphorus deteriorates weldability and causes a final material deviation by slab center segregation.

Therefore, in the present invention, the content of phosphorus (P) may be limited to 0.03% by weight or less of the total weight.

Sulfur (S)

Sulfur (S) inhibits the toughness and weldability of steel, and forms an MnS non-metallic inclusion by binding with manganese, thereby generating cracks during steel processing.

Therefore, the content of sulfur (S) is preferably limited to 0.005% by weight or less of the total weight of the hot rolled steel sheet according to the present invention.

Niobium (Nb)

Niobium (Nb) is one of the elements that have the greatest influence on the strength of steel as a precipitate forming element. It is an element that improves the strength of steel by precipitating carbonitride in steel or strengthening solid solution in Fe. In particular, niobium-based precipitates are solidly precipitated during hot rolling after being heated in a heating furnace of 1200 ° C. or more when reheating the slab, thereby effectively increasing the strength of the steel.

The niobium is preferably added in an amount ratio of 0.01 to 0.03% by weight of the total weight according to the present invention.

In the present invention, when the niobium content is added in less than 0.01% by weight of the total weight, the niobium addition effect can not be properly exhibited. On the contrary, if the content of niobium exceeds 0.03% by weight, the playability, rolling property and elongation may be lowered due to excessive precipitation.

Aluminum (Al)

Aluminum (Al) is added for deoxidation during steelmaking.

The aluminum is preferably added in more than 0 ~ 0.05% by weight of the total weight of the hot rolled steel sheet according to the present invention.

If the content of aluminum exceeds 0.05% by weight, there is a problem of inhibiting weldability.

Nitrogen (N)

Nitrogen (N) is an unavoidable impurity.

When such a large amount of nitrogen (N) is added, the solid solution nitrogen is increased to significantly reduce the impact characteristics and elongation of the steel sheet as well as the toughness of the welded portion.

Therefore, the content of nitrogen in the present invention can be limited to 0.006% or less of the total weight.

The rest other than the above substantially consists of iron (Fe). Here, the remainder substantially made of iron (Fe) means that it may be included in the scope of the present invention to include other trace elements, including inevitable impurities, as long as the action effect of the present invention is not impeded.

As described above, when a steel slab having a predetermined content is provided, the step of reheating the slab is performed.

Slab Reheating Step (ST200)

Steel slab having the composition is reheated to SRT (Slab Reheating Temperature): 1150 ~ 1250 ℃.

Here, the steel slab may be obtained through a continuous casting process after obtaining a molten steel of a desired composition through a steelmaking process.

In this step, by reheating the steel slab obtained through the continuous casting process, the segregated components are cast again.

If the slab reheating temperature (SRT) is less than 1150 ° C., there is a problem in that the segregated components are not reusable sufficiently during casting.

On the other hand, if the SRT reheating temperature (SRT) exceeds 1250 ° C, the austenite crystal grain size may increase and the strength of the steel sheet may be difficult to secure, and the manufacturing cost of the steel sheet may be increased due to the excessive heating process.

Hot Rolling Step (ST300)

In the hot rolling step (ST300), the steel slab is finish rolled to a Finishing Delivery Temperature (FDT): 790 to 850 ° C.

If the finish rolling temperature (FDT) exceeds 850 ° C, the austenite grains are coarsened, thereby making it difficult to secure the strength. At this time, the coarsened austenite causes a decrease in the phase transformation speed, and thus, the phase transformation amount decreases until winding, thereby deepening the degree of cranial deformation.

That is, in terms of duckbill deformation prevention, the finish rolling temperature (FDT) is advantageously low temperature, but if the finish rolling temperature (FDT) is too low to less than 790 ℃, due to the generation of a mixed structure due to abnormal reverse rolling, etc. Problems may arise. As such a problem, a material in the width direction may be uneven, and a phenomenon in which uniformity occurs at an edge of the coil may be exemplified.

Cooling Steps (ST400)

In the cooling step (S130), shear-controlled cooling of the hot rolled steel sheet, as can be seen with reference to Figure 2, the sheet speed is 5 m / s or less, the front end water cooling rate is 70 ~ 150 ℃ / s, the end water cooling end The temperature is controlled to cool in the water cooler to 620 ~ 700 ℃.

In order for the phase transformation of the hot rolled steel sheet to proceed a large amount before winding, it is necessary to stay in the water cooling zone for a long time.

If the plate speed increases to more than 5 m / s, the steel sheet is wound up without reaching the phase transformation fraction at each temperature. As a result, crayon deformation occurs.

On the other hand, if the mailing speed is too slow, the productivity is directly lowered, and as the steel transformation approaches the equilibrium fraction, the degree of progression of phase transformation with time may be lowered, resulting in lowering of the duckbill deformation reduction efficiency.

Therefore, in the present invention, the plate speed is limited to 5 m / s or less, more preferably 5 m / s.

When the mail speed is determined, the time the steel sheet stays in the water cooling stand is also determined. At this time, it is advantageous to enter the temperature at which the phase transformation of the steel sheet can be made in the shortest time possible.

However, when the steel sheet is water-cooled at a too high speed, the temperature difference between the inside and the surface of the steel sheet is generated due to rapid cooling.

As a result, a reheating phenomenon is caused at the end of the water cooling, which makes it difficult to hit the set water cooling end temperature. In addition, it will not be advantageous to reduce the duckbill deformation.

For this reason, the shear water cooling rate can be presented as 70 ~ 150 ℃ / s.

In order to induce as much transformation as possible before winding, it is advantageous to maintain after completing the water-cooling of the shear section to a temperature at which the phase transformation of the steel sheet proceeds rapidly.

In particular, in the temperature range up to the winding, the phase transformation progresses faster as the temperature is lower. However, when too much water cooling, the low temperature phase such as bainite or martensite is excessively formed in the microstructure. As a result, the toughness and moldability are adversely affected.

According to this condensed milk, in the present invention, the front end water cooling end temperature can be presented as 620 ~ 700 ℃.

Winding step (ST500)

In the winding-up step (S140), the cooled plate is wound at CT (Coiling Temperature): 550 ° C to 620 ° C.

If the coiling temperature is higher than 620 ° C, the amount of transformation until the coiling is increased due to a decrease in the equilibrium percentage that can be transformed until the coiling, and the generation of the duckbill coil becomes severe.

On the other hand, if the coiling temperature is less than 550 ℃, it is advantageous to prevent the duckbill coil from occurring, but the strength of the steel is too high, resulting in material defects such as reduced formability, exceeding yield strength and impaired toughness, and also increase the manufacturing cost of the steel. Can be.

Therefore, in the present invention, the winding temperature in the present winding step can be presented as 550 ~ 620 ℃.

The hot rolled coil manufactured through the above-mentioned steps has a long diameter-short diameter of 30 mm or less, and exhibits very good antiballistic deformation, and at the same time, yield strength of 551 MPa or less can be achieved.

Example

Hereinafter, the configuration and effect of the present invention through the preferred embodiment of the present invention will be described in more detail.

However, this is presented as a preferred example of the present invention, and in no sense can be construed as limiting the present invention.

The contents not described here are sufficiently technically inferior to those skilled in the art, and a description thereof will be omitted.

1. Manufacturing of hot rolled coil

The hot rolled coils according to Comparative Examples 1 to 4 and Examples 1 to 3 were prepared using the compositions shown in Table 1 and the process conditions described in Table 2. Thereafter, the long diameter and short diameter of each of the hot rolled coils manufactured according to Comparative Examples 1 to 4 and Examples 1 to 3 were measured, and the yield strength of each hot rolled coil was measured.

division SRT
(° C) FDT
(° C) CT
(° C) Mailing speed
(m / s) Shear
Water cooling speed
(° C / s) Shear
Water cooling end temperature
(° C) Comparative Example  One 1200 803 620 4.95 50 740 Comparative Example 2 1200 839 582 6.18 65 645 Comparative Example  3 1200 819 640 4.56 123 690 Comparative Example 4 1200 818 525 4.45 144 690 Example 1 1200 816 586 4.56 135 690 Example 2 1200 830 610 4.56 135 690 Example 3 1200 850 550 4.56 135 690

At this time, the compositions of Comparative Examples 1 to 4 and Examples 1 to 3 can be confirmed through the following [Table 2].

division C Mn Si P S Al Nb N (ppm) Comparative Examples 1 to 4 0.23 1.6 0.40 0.030 0.005 0.05 0.03 60 Example 1 0.23 1.6 0.40 0.030 0.005 0.05 0.03 60 Example 2 0.25 1.5 0.20 0.020 0.004 0.04 0.02 60 Example 3 0.25 1.5 0.20 0.010 0.005 0.05 0.02 60

(Unit: weight%)

division Long-radius
(mm) Yield strength
(MPa) Comparative Example 1 92 472 Comparative Example 2 63 530 Comparative Example 3 85 478 Comparative Example 4 11 585 Example 1 23 511 Example 2 23.1 514 Example 3 23.4 513

2. Evaluation of mechanical properties

Table 3 shows the evaluation results for the mechanical properties of the hot rolled steel sheet prepared according to Comparative Examples 1 to 4 and Examples 1 to 3.

In Table 3, the long diameter-radius can be seen with reference to the drawing shown in FIG. 8. In the inner hollow of the hot-rolled coil, the difference between the long diameter d1 and the short diameter d2, that is, the difference between d1-d2 is determined. Through this, it becomes a standard for checking the degree of buckling in the vertical direction.

Referring to Tables 1 to 3, it can be seen that for the hot rolled coils manufactured according to Examples 1 to 3, all of the target levels for the long-radius were satisfied, that is, 30 mm or less. That is, it can be confirmed that 551 MPa or less is satisfied.

In particular, a graph of shear controlled cooling for Example 1 can be seen through FIG. 3.

On the other hand, the mechanical properties for the hot rolled coil prepared according to Comparative Examples 1 to 4 will be evaluated through Tables 1 to 3 and accompanying drawings 4 to 7.

First, in the case of Comparative Example 1, the shear control cooling conditions can be confirmed through the graph shown in Figure 4, unlike the shear control cooling conditions of the present invention, the front end water cooling rate is 50 ℃ / s, the end water cooling end It can be seen that the temperature is out of the conditions of the present invention at 740 ° C.

As a result, the yield strength is 472 MPa, which satisfies the target of 551 MPa or less in the present invention, but the long-radius value, which is a criterion for determining the degree of cranial deformation, is 92 mm, which is much out of the target 30 mm or less in the present invention. .

Next, in the case of Comparative Example 2, the shear control cooling conditions can be confirmed through the graph shown in Figure 5, unlike the shear control cooling conditions of the present invention, the plate speed is 6.18m / s, the shear water cooling rate is It turns out that it is outside the conditions of this invention as 65 degree-C / s.

As a result, the yield strength is 530 MPa, which satisfies the target of 551 MPa or less, but the long-radius value is 63 mm, which indicates that the present invention deviates much from the target 30 mm or less.

Next, in the case of Comparative Example 3, the shear control cooling conditions can be confirmed through the graph shown in Figure 6, unlike the shear control cooling conditions of the present invention, the winding temperature is 640 ℃ of the winding temperature conditions of the present invention You can see that it is out of the upper limit.

As a result, the yield strength is 478 MPa, which satisfies the target of 551 MPa or less, but the long-radius value is 85 mm, which is much deviated from the target of 30 mm or less.

Next, in the case of Comparative Example 4, the shear control cooling conditions can be confirmed through the graph shown in FIG. 7, Unlike the shear control cooling conditions of the present invention, the winding temperature is 525 ℃ of the winding temperature conditions of the present invention You can see that it is outside the lower limit.

As a result, the long-radius value was 11mm, which satisfies the target 30mm or less, but the yield strength was 585MPa, which was found to deviate from the target of 551MPa or less.

As described above, according to the present invention, by suggesting the optimum rolling and cooling conditions for preventing the deformation caused by the self-weight up and down after the winding of the hot rolled coil, the material of the steel grade is maintained while the deformation does not occur It is effective in improving product quality.

In particular, according to the present invention, for steel having a high degree of phase transformation delay (for example, (C + Mn / 6) steel of about 0.5 level), the degree of phase transformation during cooling is maintained until the winding, thereby maintaining the material intact while inducing deformation. It is effective to prevent.

The hot rolled coil manufacturing method for preventing deformation according to the exemplary embodiment of the present invention has been described above with reference to the accompanying drawings.

However, the present invention is not limited to the above embodiments, but may be manufactured in various forms, and a person of ordinary skill in the art to which the present invention pertains does not change the technical spirit or essential features of the present invention. It will be appreciated that the specific form may be practiced.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.

ST100: Slab Production Step
ST200: Reheating Step
ST300: Hot Rolling Step
ST400: cooling stage
ST500: winding stage

Claims (8)

(a)% by weight, carbon (C): 0.20 to 0.25%, manganese (Mn): 1.0 to 1.8%, silicon (Si): more than 0 and 0.40% or less, phosphorus (P): more than 0 and 0.03% or less, sulfur (S): more than 0 and less than 0.005%, niobium (Nb): 0.01 and 0.03%, aluminum (Al): more than 0 and 0.05%, nitrogen (N): more than 0 and less than 0.006% and the remaining iron (Fe) and other unavoidable impurities Slab manufacturing step of manufacturing a steel slab is composed of;
(b) a reheating step of reheating the slab;
(c) hot rolling to form a steel sheet by hot rolling the reheated slab;
(d) a cooling step of cooling the hot rolled steel sheet by shear controlled cooling in a water cooling stand; And
(e) a winding step of winding up the cooled steel sheet.
delete The method of claim 1,
In the step (b)
The slab-reheating temperatures (SRT)
Hot rolled coil production method for preventing deformation, characterized in that 1150 ~ 1250 ℃.
The method of claim 1,
In the step (c)
Hot Rolling Finishing Temperature (FDT)
Hot-rolled coil production method for preventing deformation, characterized in that 790 ~ 850 ℃.
The method of claim 1,
In the step (d)
Hot shear coil manufacturing method characterized in that the cooling at the shearing speed (threading speed) of 5m / s or less during the shear control cooling in the water cooling zone.
The method of claim 1,
In the step (d)
When the shear control cooling in the water cooling zone, the front end water cooling rate is 70 ~ 150 ℃ / s method for producing a hot rolled coil.
The method of claim 1,
In the step (d)
When the shear control cooling in the water cooling zone, the end water cooling end temperature is 620 ~ 700 ℃ characterized in that the hot rolled coil manufacturing method.
The method of claim 1,
In the step (e)
Coiling Temperature (CT)
Hot rolled coil production method for preventing deformation, characterized in that 550 ~ 620 ℃.

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