KR20110010510A - Method for manufacturing high strength hot rolled steel sheet having excellent uniformity - Google Patents

Abstract

PURPOSE: A method for manufacturing a high strength hot rolled steel plate with excellent uniformity is provided to minimize the material property deviation in the coil length direction by controlling the temperature of the front and rear ends of a coil to be higher than a target rewinding temperature. CONSTITUTION: A method for manufacturing a high strength hot rolled steel plate with excellent uniformity comprises next steps. A steel slab is re-heated to a predetermined temperature. The wind process is performed at a desirable winding temperature. A finishing mill finish temperature is 860~900°C.

Description

Method for manufacturing high strength hot rolled steel sheet having excellent uniformity

The present invention relates to a method for manufacturing a high strength hot rolled steel sheet, and more particularly, to a method for manufacturing a high strength hot rolled steel sheet having improved material uniformity by optimizing finishing finishing temperature during hot rolling and winding temperature in a winding process after hot rolling. will be.

The main manufacturing factors that determine the material of the coiled steel sheet are known as the composition (component) of the steel, hot rolling finish temperature and winding temperature. The component system used in the production of hot rolled steel sheets requiring ductility, formability and processability is approximately 0.03 to 0.06% by weight of carbon (C) and 0.10 to 0.30% by weight of manganese (Mn). After finishing hot rolling in, the winding is carried out in the temperature range of 600 ~ 680 ℃, the hot rolled steel sheet produced by the above conditions will have a material with a tensile strength of 290 ~ 450MPa, elongation 25 ~ 45%.

In general hot rolled steel, longitudinal material deviations of the coil can cause defects in the processing and manufacturing of the final application. It is not a problem to apply the product if it has a uniform material more than the target material in the entire length of the hot rolled steel coil, but in the case of the hot rolled steel sheet having the material that is less than the target material in the length part of the coil, the crack and Defects such as breakage may be seen.

Looking at the causes of the longitudinal material deviation of the coil, it is described as the difference in the cooling history of each part in the cooling zone and the difference in the cooling history of the coiling coil by the hot rolling finish temperature deviation. In particular, in the winding process, the top of the coil is due to the difference in temperature between the winder mandrell and the strip, and the tail of the coil is in direct contact with the atmosphere after the winding. It cools faster than the inner portion, that is, the center portion.

The difference in cooling history for each part of the coiling coil changes the amount of precipitation of AlN in steel, the amount of solid solution C and N, the shape and size of ferrite grains, the size of cementite, and at the same time, causes the longitudinal material deviation of the coil.

In Japanese Patent Application Laid-Open No. 58-207335, a method of manufacturing a hot rolled coil for securing the material of the front end and the rear end of the coil, i.e., both ends, adds B to fix N in the steel to BN to prevent fine AlN precipitation. A method of lowering the coiling temperature is proposed. However, there is a limit in reducing the material deviation of both ends of the coil by this method.

Japanese Patent Application Laid-Open No. 10-195543 reheats a bar before finishing rolling to precipitate S in the steel as MnS and controls the hot rolling process so that B is precipitated as MnS and composite precipitation or BN alone during finishing rolling. It is suggested that the material deviation can be reduced. However, in this method, a facility (bar heater) for heating the bar in front of the finishing mill is required to invest in additional equipment in addition to the existing equipment.

In addition, although Korean Patent No. 10-0530072 proposes to wind the temperature of the front end and the rear end of the coiling coil to be higher than 40 ° C above the target coiling temperature, in this case, a run out table (ROT) There is a problem that does not present a problem about the variation in the longitudinal cooling history of the coil, and there is a problem that the winding temperature control range is also narrow.

The present invention is to solve the conventional problems as described above, the present invention is a method of manufacturing a high-strength hot rolled steel sheet improved material uniformity by optimizing the finishing temperature during hot rolling and the winding temperature in the winding process after hot rolling The purpose is to provide.

The present invention relates to a method for manufacturing a high strength hot rolled steel sheet in which a steel slab is reheated to a predetermined temperature, followed by hot finishing rolling at a predetermined finishing rolling finishing temperature of an Ar3 transformation point or more, followed by cooling to be wound at a target winding temperature. The finishing rolling finish temperature is 860-900 degreeC, and winds up the temperature of the front end part and the rear end part of a winding coil to 30-80 degreeC higher than target winding temperature.

In addition, the steel slab, carbon (C): 0.03 to 0.06wt%, manganese (Mn): 0.10 to 0.30wt%, silicon (Si): more than 0 and 0.3wt% or less, phosphorus (P): 0 More than 0.015 wt% or less, sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, nitrogen (N): more than 0 and 0.006 wt% or less, and remainder iron (Fe) and It may be made of other unavoidable impurities.

In addition, the front end portion may be an area of less than 70m in the rear end direction from the top end of the coil, the rear end may be an area of less than 70m in the front end direction of the coil.

According to the present invention, the finishing rolling temperature during hot rolling is controlled to 860 to 900 ° C, and the temperature of the front end and the rear end of the winding coil is 30 to 80 ° C higher than the target winding temperature, thereby minimizing the coil longitudinal material deviation and Due to this, there is an advantage of manufacturing a high strength hot rolled steel sheet having improved uniformity of material.

Hereinafter, a preferred embodiment of the method for producing a high strength hot rolled steel sheet having improved material uniformity according to the present invention will be described in detail.

Hot-rolled steel sheet according to an embodiment of the present invention, carbon (C): 0.03 ~ 0.06wt%, manganese (Mn): 0.10 ~ 0.30wt%, silicon (Si): more than 0 0.3wt%, phosphorus (P): more than 0 and 0.0015 wt% or less, sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, nitrogen (N): more than 0 and 0.006 wt% or less, remainder Consisting of iron (Fe) and other unavoidable impurities.

The function and content of the alloying elements which are the basic components of one embodiment of the present invention are as follows.

Carbon (C): 0.03-0.06wt%

Carbon (C) is an essential element for giving high strength to steel. It is necessary to limit the content of carbon in order to obtain appropriate strength and ductility in manufacturing hot rolling for processing. When it exceeds 0.06 wt%, the tensile strength range of the present invention steel is exceeded, and at the same time the elongation is lowered. Less than 0.03 wt% ductility is ensured but additional post processing is required to meet the target strength. Therefore, in order to ensure sufficient elongation, it is preferably in the range of 0.03 to 0.06 wt%.

Manganese (Mn): 0.10 to 0.30 wt%

Manganese (Mn) is known as a solid solution strengthening element that prevents hot shortness caused by solid sulfur by precipitating sulfur (S) dissolved in steel as MnS. It is effective in preventing mixed tissues by reducing metamorphosis. If the content is less than 0.10 wt%, the effect is insignificant. If excessively added, the steel sheet is hardened and the moldability is lowered, so the content is limited to 0.30 wt% or less.

Silicon (Si): more than 0 and less than 0.3 wt%

Silicon (Si) is regulated to 0.3% or less because excessively added, the strength is increased to deteriorate formability. Preferably it is 0.01 to 0.1% of range.

Phosphorus (P): more than 0 and less than 0.015 wt%

Phosphorus (P) is inevitably present as an impurity in the steel, but has a high solid solution strengthening effect and a small decrease in the plastic strain ratio. However, if it exceeds 0.015 wt%, it segregates at the grain boundaries and generates secondary processing brittleness. Therefore, in the present invention, the content is limited to 0.015 wt% or less for strength improvement.

Sulfur (S): more than 0 and less than 0.015 wt%

Sulfur (S) inevitably exists as an impurity in steel, but reacts with manganese (Mn) to form fine MnS precipitates. When the content of sulfur exceeds 0.0015wt%, the content of solid solution is high, so the ductility and formability are greatly lowered, and there is a fear of red brittleness, so the content is limited to 0.015wt% or less.

Aluminum (Al): 0.01 ~ 0.05wt%

Aluminum (Al) is an element necessary for deoxidation of steel, and is usually added at 0.05 wt% or less, and excess aluminum combines with nitrogen (N) in the steel to form fine AlN precipitates to refine crystal grains and improve strength by precipitation strengthening. Has an effect.

If the content of aluminum is less than 0.01w%, the amount of AlN precipitates is reduced, making it difficult to secure sufficient strength. If the content of aluminum is more than 0.05w%, it is difficult to play, which lowers the productivity and yield strength may be excessively increased. Limit to 0.05 wt%.

Nitrogen (N): more than 0 and less than 0.006wt%

Nitrogen (N), when combined with aluminum in steel, finely precipitates with AlN, inhibits ferrite grain growth and refines grains. If the content of nitrogen exceeds 0.006wt%, it is difficult to guarantee aging by solid solution nitrogen. Therefore, in the present invention, the content is limited to 0.006wt% or less in consideration of the microprecipitation and the solute nitrogen effect of AlN.

The present invention includes the components of the alloy steel, the remainder is iron (Fe) and unavoidable elements, and fine incorporation of unavoidable impurities may be allowed as elements contained according to the situation of raw materials, materials, manufacturing facilities, and the like.

In addition, in the above embodiment, the content of the steel slab component is limited, but the present invention is not limited to the content of the steel slab component, and the lengthwise direction of the coil is controlled by controlling the hot rolling and winding conditions to be described later in all steel grades. To improve the material deviation.

In the present invention, all steel grades, preferably the steel slabs, which are formed as described above, obtain molten steel through a steelmaking process and then manufacture them through a continuous casting process, and after reheating the steel slabs to a predetermined temperature through a heating furnace, A high strength hot rolled steel sheet is produced through a hot rolling process for rolling into a thickness, a cooling process for cooling the steel sheet at room temperature, and a winding process.

Hereinafter, the manufacturing process of the hot rolled steel sheet of the present invention will be described in detail.

[Heating process]

In the heating process of the steel slab, reheating is performed in the furnace to reclaim segregated components during casting.If the reheating temperature is low, the segregated components are not reusable. As it coarsens, its strength decreases.

In addition, it is necessary to adjust the reheating temperature holding time according to the thickness of the steel slab, that is, it is necessary to keep the reheating time longer as the thickness is thick, and to shorten the holding time as the thickness is thinner.

Therefore, in one embodiment of the present invention, the substrate is reheated for a predetermined time, for example, 3 hours, at a temperature in the range of 1150 to 1250 ° C. for the homogenization treatment.

If the reheating temperature is lower than 1150 ° C, the coarse precipitates produced during continuous casting remain completely insoluble, leaving a large amount of coarse precipitates after hot rolling, resulting in a large rolling load. This coarsening makes it difficult to secure strength.

[Hot Rolling Process]

The slab reheated in a temperature range of 1150 to 1250 ° C. finishes hot rolling at a predetermined finishing rolling finishing temperature of Ar3 transformation point or more.

As described above, filament rolling is a process of manufacturing slab worked in rough rolling to the final size required by the customer. It secures materials, secures a good surface and shape according to the temperature of each application, and improves productivity. The tandem rolling mill is equipped with six to seven rolls in series.

In addition, the latest facilities such as AGC (Automatic Gauge Control) to secure the accuracy of thickness, Pair Cross and Shift Mill for shape manufacturing, and measuring machine facilities to check the size and shape of the manufactured coil are installed. The material of hot rolled products is very influenced by rolling temperature together with chemical composition.

The Run Out Table (hereinafter referred to as "ROT") is a product that transfers hot rolled products in a finishing mill, and is installed to be driven by section for efficient transportation of hot rolled products. Rolling finish temperature means the temperature before the ROT injection.

In the present invention, the predetermined finishing rolling finishing temperature, that is, the target finishing rolling finishing temperature is limited to 860 to 900 ° C.

When the finishing rolling finish temperature is less than 860 ℃ rolling edge edge of the coil is generally low temperature to generate a mixed structure, and grow during the subsequent cooling or winding process to form coarse grains on the surface There is a problem that adversely affects mechanical properties such as hardness and elongation.

This is because when the finishing rolling finish temperature exceeds 900 ° C., the ferrite grain size increases and the decarburization depth of the surface layer increases, thereby deteriorating mechanical properties such as strength.

Therefore, in the present invention, the temperature before ROT injection, that is, finishing finishing temperature is controlled to be uniform in the entire length direction of the coil in the range of 860 ~ 900 ℃.

[Cooling, winding process]

After performing hot finishing rolling on the steel slab to which the alloying element is added, at a predetermined finishing rolling finishing temperature, the steel slab is wound in a coil form at a predetermined target winding temperature to be cooled and convenient for storage and movement. Here, it is preferable to control the said predetermined target winding temperature to 600-700 degreeC.

The coiling temperature is an important control factor that determines the final structure of the hot rolled steel sheet and has a great influence on the tensile and impact toughness. The target coiling temperature is easily made of aluminum in the solid steel which is dissolved even after coarsening and finishing rolling of ferrite grains and cementite. In order to precipitate with nitride (AlN), the temperature should be 600 ° C or higher, and if the temperature is less than 600 ° C, the precipitation strengthening effect and the winding shape will be poor.

If the target winding temperature exceeds 700 ℃, the scale of oxidation of the surface of the hot rolled steel sheet becomes thick, it becomes difficult to remove the scale in the pickling process, and there is a problem that the longitudinal material deviation of the coil increases, so in the present invention, the target winding Limit the temperature to 600-700 ° C.

On the other hand, the present invention focuses on setting the temperature of the front end and the rear end of the coil higher than the target winding temperature to compensate for the rapid temperature drop of the front end and the rear end of the coil after the winding, the front end and the rear end of the coil The temperature is controlled so that the predetermined temperature is higher than the target winding temperature.

Specifically, in the present invention, the temperature of the front end portion and the rear end portion of the winding coil is wound at a temperature of 30 to 80 ° C. higher than the target winding temperature.

According to the experimental results of the present invention, the coiling temperature of the front end and the rear end of the coil should be 30 to 80 ° C. higher than the target winding temperature to minimize the coil longitudinal material deviation. Temperature of the longitudinal zone of the coil as described above can be adjusted by the control of the amount of cooling water.

Here, the front end part and the rear end part of the coil generally mean areas where material deviations occur frequently in the winding coil, which is usually 70 m in the front end region in the rear end direction of the coil and 70 m in the front end direction of the coil end. Within the rear end region, material deviation occurs more than the center portion.

Therefore, the coiling temperature may be 30 to 80 ° C. higher than the target winding temperature in the front end region within 70 m and the rear end region within 70 m, thereby minimizing the material deviation of the coil.

The hot rolled steel sheet manufactured by the manufacturing method as described above has a tensile strength of 300 to 450 MPa and an elongation of 25 to 45% in mechanical properties.

Hereinafter, a method of manufacturing a high strength hot rolled steel sheet having excellent material uniformity according to the present invention will be described in detail with reference to Examples.

Carbon (C): 0.03 to 0.06 wt%, manganese (Mn): 0.10 to 0.30 wt%, silicon (Si): more than 0 and 0.3 wt% or less, phosphorus (P): more than 0 0.015 wt % Or less, sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, nitrogen (N): more than 0 and 0.006 wt% or less, residual iron (Fe) and other unavoidable impurities After reheating the steel slab consisting of 3 hours at 1200 ° C, hot rolling was performed to produce a hot rolled steel sheet having a thickness of 1.6 mm. The finishing rolling temperature was 880 ° C and the target winding temperature was 620 ° C.

The finishing rolling temperature is in the range of 860 ~ 900 ℃, the invention method, the temperature other than the above range was divided by the comparative method, and considering the cooling history of the center and end after winding, the top and tail of the coil (tail) ) And the cooling method was controlled by dividing the invention method of controlling 30 ~ 80 ° C. higher than the target coiling temperature and the method of cooling the front and rear ends of the coil in the same way as the center part. Mechanical properties of the prepared hot-rolled steel sheet was evaluated by producing a JIS No. 5 tensile test specimen in a direction parallel to the rolling direction.

Table 1 below shows the tensile properties of the high-strength hot-rolled steel sheet having the above composition, and summarizes the tensile properties of the coil tail, and the variation of the tensile properties between the tail and the middle.

Psalter

Hot rolled finish of coil rear end
Temperature (℃)

Coil winding temperature
(℃)
Coil rear end
Tensile Properties Difference in Tensile Properties between Rear End and Center Hot rolling finish temperature control method
Winding temperature control method
The tensile strength
(MPa)
Total elongation
(%) Tensile Strength Difference (MPa) % Total elongation One 851 622 434 25.6 49.3 10.1 Comparative method Comparative method 2 883 621 411 29.5 31.5 6.3 Invention Comparative method 3 852 674 430 27.7 25.5 5.1 Comparative method Invention 4 885 653 355 38.7 12.2 2.1 Invention Invention 5 894 671 367 36.8 8.9 1.4 Invention Invention 6 868 695 358 36.0 10.1 1.6 Invention Invention

In case of specimen 1, the hot rolling finish temperature of the rear end of the coil is about 30 ° C. lower than the target temperature (880 ° C.), and the winding temperature is the same as that of the intermediate part. It can be seen that the difference in tensile properties between the rear end and the center of the coil is quite large.

In case of specimen 2, the hot rolling finish temperature at the rear end of the coil was in accordance with the invention method, but the winding temperature was not controlled as in the comparison method. In specimen 3, the hot rolling finish temperature at the rear end of the coil was out of the target temperature according to the comparative method. This is the case where the coiling temperature is made higher than the target temperature according to the invention method.

From the results of specimens 2 and 3, if any of the hot rolling finish temperature and the winding temperature were not controlled according to the invention method, the tensile property deviation of the rear end and the center part was 20 MPa or more, and the deviation was large. do.

In specimens 4, 5, and 6, the hot rolling finish temperature and winding temperature of the coil rear end were controlled under the inventive method, and the difference in tensile properties between the rear end and the center part was significantly reduced.

From the above test results, according to the invention method, the hot rolling finish temperature is controlled within the target temperature (880 ℃) ± 20 ℃ range, that is, 860 ~ 900 ℃ in the longitudinal direction of the coil, the coil winding temperature of the front and rear ends 30 than the target temperature When controlled at ~ 80 ° C., it can be seen that the central and front end tensile property deviations are within about 12 MPa of tensile strength and within about 2% of elongation, and thus, it is possible to manufacture hot rolled steel sheets having material uniformity with minimum material deviation in the longitudinal direction.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Claims (3)

In the method for producing a high strength hot rolled steel sheet, after reheating the steel slab to a predetermined temperature, performing hot finish rolling at a predetermined finishing rolling finishing temperature of Ar3 transformation point or more, and then cooling and winding the steel slab at a target winding temperature. The predetermined finishing rolling finish temperature is 860 ~ 900 ℃, A method of manufacturing a high strength hot rolled steel sheet having excellent material uniformity, characterized in that winding is performed at a temperature of the front end portion and the rear end portion of the winding coil at a temperature of 30 to 80 ° C. higher than the target winding temperature. The method according to claim 1, The steel slab is carbon (C): 0.03 to 0.06 wt%, manganese (Mn): 0.10 to 0.30 wt%, silicon (Si): more than 0 and 0.3 wt% or less, phosphorus (P): more than 0 0.015 wt% or less, sulfur (S): more than 0 and 0.015 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, nitrogen (N): more than 0 and 0.006 wt% or less, balance iron (Fe) and other unavoidable Method for producing a high strength hot rolled steel sheet having excellent material uniformity, characterized in that consisting of impurities. The method according to claim 1, The front end is a region of less than 70m in the rear end direction from the top end of the coil, the rear end is a region of less than 70m in the shear direction from the end of the coil characterized in that the high-strength hot rolled steel sheet excellent in uniformity.