KR20220055273A - Method of manufacturing ultra high strength hot rolled steel sheet having softening heat treatment and apparatus of manufacturing hot rolled steel sheet - Google Patents

Abstract

The present invention provides a method for manufacturing an ultra-high-strength hot-rolled steel sheet, including softening heat treatment for performing an easy cold-rolling process, and an apparatus for manufacturing an ultra-high-strength hot-rolled steel sheet. According to an embodiment of the present invention, the method for manufacturing an ultra-high-strength hot-rolled steel sheet includes the steps of hot-rolling a steel material to manufacture a hot-rolled coil; unwinding the hot-rolled coil to provide a hot-rolled strip; subjecting the hot-rolled strip to softening heat treatment; cooling the hot-rolled strip; and winding the hot-rolled strip.

Description

Method of manufacturing ultra high strength hot rolled steel sheet having softening heat treatment and apparatus of manufacturing hot rolled steel sheet

The technical idea of the present invention relates to a steel material, and more particularly, to a method for manufacturing an ultra-high tensile strength hot rolled steel sheet including a softening heat treatment for performing an easy cold rolling process, and to an apparatus for manufacturing an ultra high tensile strength hot rolled steel sheet.

Recently, the automobile industry is demanding higher strength of automobile steel as environmental regulations such as reduction of carbon dioxide emissions and collision-related laws that require stability in the event of a vehicle collision are strengthened. In the case of such high-strength steel, if it has the same strength even at a reduced thickness, it is possible to simultaneously achieve the effect of reducing the weight and improving the impact properties, thereby having the advantage of responding to environmental regulations and collision-related laws. With this trend, the strength of steel sheets for automobiles has been gradually increased, and after high-tensile steel sheets with a tensile strength of 400 to 600 MPa, ultra-high strength steel sheets with a tensile strength of 800 to 1200 MPa have been developed and mass-produced. Furthermore, a third-generation steel sheet having greatly improved formability such as elongation, bendability, and hole expandability while maintaining ultra-high strength performance is being developed.

However, when manufacturing hot-rolled coils of ultra-high-strength steel sheets, more alloying elements are added compared to conventional steel sheets, which increases the strength of the hot-rolled coils, and complex phase transformation behavior occurs. Due to the difference, serious material deviation may occur. The high strength and material deviation of the hot-rolled coil may cause thickness non-uniformity and shape defects while performing cold rolling to form a cold-rolled steel sheet, which is a final product.

In order to solve this problem, conventionally, by reducing the cooling rate by accommodating the hot-rolled coil in an insulating bank to reduce material deviation, or by heat-treating using a batch heat treatment furnace, a reduction in strength and material deviation is induced. However, in the case of using the insulating bank, since a long cooling time is required, it is necessary to secure a space for the insulating bank, and there is a limit to the material and the material deviation reduction effect of the hot rolled coil. In addition, in the case of using the batch heat treatment, there is a limitation in that the productivity decreases and the processing cost increases due to the heat treatment for a long time.

Korean Patent Application No. 10-2020-7016047

The technical problem to be achieved by the technical idea of the present invention is to provide a method for manufacturing an ultra-high tensile strength hot-rolled steel sheet including a softening heat treatment for performing an easy cold rolling process, and an apparatus for manufacturing an ultra-high tensile strength hot-rolled steel sheet

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided a method for manufacturing an ultra-high tensile strength hot rolled steel sheet including a softening heat treatment for performing an easy cold rolling process, and an apparatus for manufacturing an ultra high tensile strength hot rolled steel sheet.

According to an embodiment of the present invention, the method for manufacturing the ultra-high tensile strength hot-rolled steel sheet includes the steps of: manufacturing a hot-rolled coil by hot-rolling a steel material; providing a hot-rolled strip by unwinding the hot-rolled coil; softening the hot-rolled strip; cooling the hot rolled strip; and winding the hot-rolled strip.

According to an embodiment of the present invention, the softening heat treatment step may be performed at a temperature in the range of 500 °C to 800 °C.

According to an embodiment of the present invention, the softening heat treatment step may be performed for a time ranging from 10 seconds to 1200 seconds.

According to an embodiment of the present invention, the manufacturing of the hot-rolled coil comprises: reheating the steel material at a reheating temperature in the range of 1,150°C to 1,250°C; manufacturing a hot-rolled steel sheet by hot finish rolling the reheated steel at a finish rolling end temperature in the range of 880° C. to 930° C.; cooling the hot-rolled steel sheet to a temperature in the range of 550°C to 650°C; and manufacturing the hot-rolled coil by winding the hot-rolled steel sheet in the range of 550°C to 650°C.

According to an embodiment of the present invention, the cooling step may be performed to cool the hot-rolled strip to a temperature in the range of 0° C. to 40° C. at a cooling rate of 10° C./sec to 100° C./sec.

According to an embodiment of the present invention, the cooling step may be performed by performing an air cooling step on the hot-rolled strip, and then performing a water cooling step on the hot-rolled strip.

According to an embodiment of the present invention, in the air cooling step, the hot-rolled strip is cooled to a temperature in the range of 200 °C to 300 °C, and in the water cooling step, the hot-rolled strip is cooled to a temperature in the range of 0 °C to 40 °C. can

According to an embodiment of the present invention, the ultra-high tensile strength hot-rolled steel sheet, by weight, carbon (C): 0.15% to 0.20%, silicon (Si): 1.5% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.06%, sum of at least one of niobium (Nb) and titanium (Ti): 0.01% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S) ): more than 0% to 0.01%, and the balance may contain iron (Fe) and other unavoidable impurities.

According to an embodiment of the present invention, the ultra-high tensile strength hot rolled steel sheet, the ultra high strength cold rolled steel sheet, yield strength (YP): 500 MPa to 850 MPa, tensile strength (TP): 750 MPa to 1200 MPa, and elongation ( El): 15% to 25% can be satisfied.

According to an embodiment of the present invention, the apparatus for manufacturing the ultra-high tensile strength hot-rolled steel sheet includes: an unwinding unit for unwinding a hot-rolled coil to provide a hot-rolled strip; a softening heat treatment unit for softening and heat-treating the hot-rolled strip at a temperature in the range of 500° C. to 800° C. for a time in the range of 10 seconds to 1200 seconds; a cooling unit for cooling the hot-rolled strip to a temperature in the range of 0° C. to 40° C. at a cooling rate of 10° C./sec to 100° C./sec; and a winding unit for winding the hot-rolled strip.

In the method for manufacturing an ultra-high-tensile hot-rolled steel sheet according to the technical idea of the present invention, the wound ultra-high-tensile hot-rolled steel sheet is unwound to perform softening heat treatment in the form of a strip. Accordingly, it is possible to improve the productivity by reducing the heat treatment time required for the softening heat treatment. Specifically, the heating time, the heat treatment time, and the cooling time of the hot-rolled steel sheet can be dramatically reduced.

The method for manufacturing an ultra-high-strength hot-rolled steel sheet according to the technical idea of the present invention reduces the material deviation of the third-generation hot-rolled steel sheet and softens the material to reduce thickness hunting and shape defects that occur during cold rolling, thereby improving the recovery rate and stable operability can be obtained During the process for softening hot-rolled coil, it is possible to stably secure the effect of softening heat treatment while significantly shortening the heat treatment time to improve productivity, and to significantly reduce processing costs due to fuel and utility savings due to the shortened time.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method for manufacturing an ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.
2 is a schematic diagram illustrating an apparatus for manufacturing a hot-rolled steel sheet performing a method for manufacturing an ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.
3 is a graph exemplarily showing the temperature profile and time during the softening heat treatment process in the apparatus for manufacturing a hot-rolled steel sheet performing the method for manufacturing the ultra-high tensile strength hot-rolled steel sheet of FIG. 2 according to an embodiment of the present invention.
4 is a graph showing the change in hardness according to the softening heat treatment temperature of the ultra-high tensile strength hot-rolled steel sheet manufactured by using the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.
5 is a graph showing the change in hardness according to the softening heat treatment holding time of the ultra-high tensile strength hot-rolled steel sheet manufactured by using the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

According to the technical idea of the present invention, in order to manufacture an ultra-high-strength steel sheet, it is to prevent the thickness non-uniformity and shape defect occurring during cold rolling due to material deviation and high strength of the ultra-high-strength hot-rolled steel sheet. To this end, it may be implemented by performing softening heat treatment before cold rolling the ultra-high tensile strength hot-rolled steel sheet. Conventionally, since this softening heat treatment is performed in a state in which an ultra-high tensile strength hot-rolled steel sheet is wound as a hot-rolled coil, heat is transferred to the inside of the hot-rolled coil and a long period of time is required to secure the softening effect.

In the method for manufacturing an ultra-high-tensile hot-rolled steel sheet according to the technical idea of the present invention, the wound ultra-high-tensile hot-rolled steel sheet is unwound to perform softening heat treatment in the form of a strip. Accordingly, it is possible to improve the productivity by reducing the heat treatment time required for the softening heat treatment. Specifically, the heating time, the heat treatment time, and the cooling time of the hot-rolled steel sheet can be dramatically reduced.

Hereinafter, with reference to the accompanying drawings will be described with respect to the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet according to the present invention.

Manufacturing method of ultra-high-strength hot-rolled steel sheet

1 is a process flowchart schematically illustrating a method of manufacturing an ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the method of manufacturing an ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention comprises the steps of: manufacturing a hot-rolled coil by hot-rolling a steel material (S110), and unwinding the hot-rolled coil to provide a hot-rolled strip ( S120); softening and heat-treating the hot-rolled strip (S130); and cooling the hot-rolled strip (S140). and winding the hot-rolled strip (S150).

In the method for manufacturing an ultra-high tensile strength hot-rolled steel sheet according to the present invention, the semi-finished product to be subjected to the hot-rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

Hereinafter, the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet of the present invention will be described in detail for each step.

Hot-rolled coil manufacturing step (S110)

In the step (S110), a steel material having a predetermined composition is prepared, and the steel material is reheated, for example, at a reheating temperature (Slab Reheating Temperature, SRT) in the range of 1,150°C to 1,250°C. Through such reheating, re-dissolution of segregated components during casting and re-dissolution of precipitates may occur. Under the above temperature conditions, the segregation component may be sufficiently re-dissolved so that product quality may be excellent. When the reheating temperature is less than 1,150° C., there may be a problem in that the hot rolling load rapidly increases. When the reheating temperature exceeds 1,250° C., the amount of surface scale increases, which may lead to material loss, and energy may be wasted.

After the reheating, hot rolling is performed by a conventional method, for example, hot finish rolling is performed at a finish delivery temperature (FDT) in the range of 880° C. to 930° C. to manufacture a hot-rolled steel sheet. In the above temperature condition, the crystal grain refining effect is excellent, so that the target strength can be secured. When the finish rolling end temperature is less than 880° C., a band structure of ferrite and pearlite may be formed. When the finish rolling end temperature exceeds 930 ℃, scale generation is increased, the grain size is coarsened, it may be difficult to micro-uniform the structure.

Then, the hot-rolled steel sheet is cooled to a coiling temperature in the range of, for example, 500°C to 600°C. The cooling may be either air cooling or water cooling, for example, cooling at a cooling rate of 10° C./sec to 30° C./sec. A faster cooling rate is advantageous in reducing the average grain size. Preferably, the cooling is performed to a coiling temperature.

Then, the hot-rolled steel sheet is wound at a coiling temperature (CT) in the range of, for example, 550°C to 650°C to form a hot-rolled coil. The range of the winding temperature may be selected from the viewpoint of cold rolling properties and surface properties. Under the above temperature conditions, the surface quality and mechanical strength of the hot-rolled steel sheet may be excellent. When the coiling temperature is less than 550 °C, a hard phase such as martensite is excessively generated and the material of the hot-rolled steel sheet is excessively increased, so that the rolling load during cold rolling may be significantly increased. When the coiling temperature exceeds 650° C., the ferrite-pearlite band structure becomes severe and may result in non-uniformity of the microstructure of the final product.

The hot-rolled coil may also be cooled to room temperature, for example, at a temperature of 0°C to 40°C. Alternatively, the hot-rolled coil may be maintained at a temperature in the range of 0°C to 650°C. In this case, the temperature of the hot-rolled coil may be determined according to a treatment such as continuously performing a subsequent softening heat treatment on the hot-rolled coil, or loading it in a yard and storing it for a certain period of time.

Hot-rolled strip providing step (S120)

In the step S120, the hot-rolled coil is unwound to provide a hot-rolled strip. The hot-rolled strip may be loaded into the softening heat treatment apparatus 100 to be described below.

Softening heat treatment step (S130)

In the step (S130), the hot-rolled strip, for example, at a temperature in the range of 500 ℃ ~ 800 ℃, for example, softening heat treatment for a time in the range of 10 seconds ~ 1200 seconds. By performing the softening heat treatment at a temperature in the range of 500° C. to 800° C., the cementite is not coarsened, and is uniformly fine and densely present in the hot-rolled steel sheet. As a result, the structure finally obtained after cold rolling becomes fine grains, and excellent stretch flangeability and bendability can be obtained.

When the softening heat treatment temperature is less than 500° C., the hot-rolled steel sheet is not sufficiently softened, and the influence of the structure after hot rolling on the finally obtained cold-rolled steel sheet cannot be removed. In addition, the structure after the softening heat treatment is formed as a non-uniform structure. That is, since the structure of the center portion and the edge portion of the steel sheet is non-uniform and the physical properties are also different, the shape and thickness quality may be adversely affected during cold rolling.

When the softening heat treatment temperature exceeds 800 ° C., since the microstructure becomes two phases of ferrite and austenite, the strength of the hot-rolled steel sheet after cooling increases. The steel sheet structure becomes a structure in which the carbon (C) concentration is non-uniform, and during the subsequent secondary heat treatment, austenite is coarsely and non-uniformly distributed, so that a uniform fine structure is not obtained. Here, the structure in which the carbon concentration is non-uniform refers to a structure in which coarse cementite having a high carbon concentration is distributed in a ferrite phase having a low carbon concentration. In addition, phosphorus (P) segregates at grain boundaries, and the steel sheet becomes brittle, so that stretching and stretching flangeability are remarkably reduced. In addition, even when the internal oxide layer is not observed in the hot rolling process, the internal oxide layer may be formed on the surface layer of the steel sheet after the softening heat treatment due to the high heat treatment temperature.

In the softening heat treatment step, it is possible to effectively control the influence of the microstructure of the hot-rolled steel sheet before cold rolling after hot rolling. By the softening heat treatment, the hot-rolled steel sheet can be softened, the rolling load is reduced in the subsequent cold rolling, and thickness deviations that often occur when cold rolling ultra-high-strength steel can be reduced, and shape control can be achieved This can be done easily.

In addition, in the case of performing the softening heat treatment on a hot-rolled coil by a batch annealing process (BAF) in the conventional manner, a long heat treatment time is required, ranging from 1 hour to 96 hours, and the edge portion and the center portion of the coil are uniform. It is difficult to secure performance, and the growth of cementite may be excessively progressed, and there is a limit to inhibiting productivity. However, if the hot-rolled coil is unwound and the hot-rolled strip is subjected to softening heat treatment according to the technical idea of the present invention, the heat treatment time can be reduced to about 20 minutes or less, and the uniformity and reliability of the quality of the final hot-rolled steel sheet can be secured. there is.

Cooling step (S140)

In the step (S140), the hot-rolled strip is cooled to room temperature at a cooling rate of 10°C/sec to 100°C/sec, for example, to a temperature in the range of 0°C to 40°C. After performing the air-cooling step for the hot-rolled strip for a predetermined time, the hot-rolled strip may be made by performing a water-cooling step for a predetermined time. In the air cooling step, the hot-rolled strip may be cooled to a temperature in the range of, for example, 200°C to 300°C. In the water cooling step, the hot-rolled strip may be cooled to, for example, a temperature in the range of 0°C to 40°C.

Winding step (S150)

In the step (S150), the cooled hot-rolled strip is wound to form a softening heat-treated hot-rolled coil. Accordingly, an ultra-high-strength hot-rolled steel sheet subjected to a softening heat treatment may be manufactured.

Cold-rolled steel sheet manufacturing stage

Then, if necessary, a cold-rolled steel sheet may be manufactured using the ultra-high tensile strength hot-rolled steel sheet. A cold-rolled steel sheet may be formed by performing a pickling treatment of washing the hot-rolled steel sheet with an acid and performing cold rolling at an average reduction ratio of 40% to 70%, for example. As the average reduction ratio is higher, there is an effect of increasing the formability due to the effect of refining the tissue. When the average reduction ratio is less than 40%, it is difficult to obtain a uniform microstructure. When the average reduction ratio exceeds 70%, the roll force is increased to increase the process load.

The ultra-high-strength hot-rolled steel sheet manufactured by the method for manufacturing an ultra-high-tensile hot-rolled steel sheet according to an embodiment of the present invention may include steels of various compositions. For example, the ultra-high tensile strength hot-rolled steel sheet is, by weight, carbon (C): 0.15% to 0.20%, silicon (Si): 1.5% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum ( Al): greater than 0% to 0.06%, sum of at least one of niobium (Nb) and titanium (Ti): 0.01% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% ~ 0.01%, and the balance contains iron (Fe) and other unavoidable impurities.

The ultra-high tensile strength hot-rolled steel sheet may satisfy yield strength (YP): 500 MPa or more, tensile strength (TP): 750 MPa or more, and elongation (El): 15% or more. In addition, the ultra-high strength cold rolled steel sheet, yield strength (YP): 500 MPa to 850 MPa, tensile strength (TP): 750 MPa to 1200 MPa, and elongation (El): 15% to 25% can be satisfied.

Hereinafter, the role and content of each component included in the ultra-high tensile strength hot-rolled steel sheet according to the present invention will be described. At this time, the content of the component elements all mean weight % with respect to the entire steel sheet.

Carbon (C): 0.15% to 0.20%

Carbon is added to ensure the strength of the steel. When the carbon content is less than 0.15 wt%, it may be difficult to secure strength. When the carbon content exceeds 0.20 wt %, low-temperature impact toughness and weldability may be deteriorated. Therefore, it is preferable to add carbon in an amount of 0.15% to 0.20% of the total weight of the steel sheet.

Silicon (Si): 1.5% to 2.0%

Silicon is added as a deoxidizer to remove oxygen in steel in the steelmaking process. Silicon also has a solid solution strengthening effect. When the content of silicon is less than 1.5%, the effect of addition may be insignificant. When the content of silicon exceeds 2.0%, weldability and plating property may deteriorate. Therefore, it is preferable to add the content of silicon in an amount of 1.5% to 2.0% of the total weight of the steel sheet.

Manganese (Mn): 1.5% to 3.0%

Manganese (Mn) is an element contributing to the formation and stabilization of retained austenite. When the manganese content is less than 1.5%, the effect of adding manganese is insufficient. When the manganese content exceeds 3.0%, weldability and formability may deteriorate, and production cost may increase. Therefore, it is preferable to add the manganese content in an amount of 1.5% to 3.0% of the total weight of the steel sheet.

Aluminum (Al): >0% to 0.06%

Aluminum is used as a deoxidizer and, like silicon, suppresses cementite precipitation and stabilizes austenite. It makes the grain boundaries and carbides of the hot-rolled sheet fine, so unnecessary dissolved nitrogen in the steel is converted to aluminum nitride (AlN). to precipitate Therefore, it has the effect of increasing the strength. When the content of aluminum exceeds 0.06%, weldability and continuous castability may be deteriorated, and hot-rolled cracks may be caused by forming aluminum nitride in the slab. Therefore, it is preferable to add the content of aluminum to more than 0% to 0.06% of the total weight of the steel sheet.

Sum of at least one of niobium (Nb) and titanium (Ti): 0.01% to 0.05%

Niobium is an effective element for precipitation strengthening and grain refinement of steel sheets. Titanium prevents austenite grain growth during welding by generating Ti(C, N) precipitates with excellent high-temperature stability, thereby improving weld properties through refining the weld structure. When the sum of at least one of niobium and titanium is less than 0.01%, the effect of addition may be insignificant. When the sum of at least one of niobium and titanium exceeds 0.05%, precipitates may increase to impair the impact properties of the steel. Therefore, it is preferable to add the content of the sum of at least one of niobium and titanium in an amount of 0.01% to 0.05% of the total weight of the steel sheet.

Phosphorus (P): >0% to 0.02%

Phosphorus (P) is an element that is unavoidably included in the manufacture of steel, and can increase strength by solid solution strengthening. When added together with silicon, it suppresses cementite precipitation during annealing heat treatment and promotes carbon enrichment with austenite. do However, if phosphorus is included excessively, processing brittleness and formability may be deteriorated. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

Sulfur (S): >0% to 0.01%

Sulfur (S) is an element that is unavoidably contained in the manufacture of steel, and forms precipitates of manganese (Mn) and fine MnS to improve workability. However, if sulfur is included excessively, ductility and formability may be greatly reduced, and red hot brittleness may occur. Therefore, it is preferable to limit the content of sulfur (S) to more than 0% to 0.01% of the total weight of the steel sheet.

Nitrogen (N): >0% to 0.006%

Nitrogen (N) is an element that deteriorates aging resistance, and when the nitrogen content exceeds 0.006%, deterioration of aging resistance becomes remarkable. In addition, nitrogen (N) combines with boron (B) to form BN and consumes boron (B). For this reason, nitrogen (N) reduces the quenching property by solid solution boron (B), and makes it difficult to ensure the tempered martensite phase of a predetermined area ratio. In addition, nitrogen (N) exists as an impurity element in ferrite and reduces ductility by strain aging. Therefore, it is preferable that the content of nitrogen (N) is lower. Therefore, it is preferable to limit the content of nitrogen (N) to more than 0% to 0.006% of the total weight of the steel sheet.

The remaining component of the ultra-high tensile strength hot-rolled steel sheet is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal steelmaking process, it cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

2 is a schematic diagram illustrating an apparatus 100 for manufacturing a hot-rolled steel sheet performing a method for manufacturing an ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 2 , the apparatus 100 for manufacturing a hot-rolled steel sheet includes an unwinding unit 110 , a softening heat treatment unit 120 , a cooling unit 130 , and a winding unit 140 .

The unwinding unit 110 may provide a hot-rolled strip M by unwinding the hot-rolled coil. The unwinding part 110 may include a first winding roll 111 , a first cut part 112 , a welding part 113 , and a looper part 114 .

The first winding roll 111 may wind and store the hot-rolled coil, and may unwind the hot-rolled coil by rotation to provide it to the softening heat treatment unit 120 as a hot-rolled strip M. The first winding roll 111 may consist of one roll or two or more rolls.

The first cutting part 112 may cut the hot-rolled strip M unwound from the first winding roll 111 if necessary. The first cut portion 112 may be configured in one or a plurality of pieces corresponding to the first winding roll 111 . By such cutting, the hot-rolled strip M may be prepared to be welded.

The welding part 113 may connect the hot-rolled strip M unwound from the first winding roll 111 by welding an end thereof, if necessary. That is, the end portion of the hot-rolled strip M unwound from one first winding roll 111 and the start portion of the hot-rolled strip M unwound from the other first winding roll 111 may be connected by welding. . Accordingly, by connecting the plurality of hot-rolled strips M, the softening heat treatment process may be continuously performed.

The looper part 114 provides a movement path of the hot-rolled strip M, and accordingly, it is possible to control the time for which the hot-rolled strip M is charged into the softening heat treatment part 120 . In particular, since the looper part 114 is stopped for welding the hot-rolled strip M in the welding part 113 , it can perform a function of securing a necessary time to enable continuous softening heat treatment while performing the welding.

The softening heat treatment unit 120 may soften the hot rolled strip M at a temperature in the range of 500° C. to 800° C. for a time in the range of 10 seconds to 1200 seconds. The softening heat treatment unit 120 may include a preheating unit 121 , a heating unit 122 , and a cracking unit 123 . A heater (not shown) may be provided in each of the preheating unit 121 , the heating unit 122 , and the cracking unit 123 .

The hot-rolled strip M may be preheated from room temperature to 300° C. to 400° C. in the preheating unit 121 . Subsequently, the hot-rolled strip M may be heated to a target softening heat treatment temperature, for example, a temperature in the range of 500° C. to 800° C. in the heating unit 122 . Subsequently, the hot-rolled strip M may be maintained at the target softening heat treatment temperature in the cracking part 123 , for example, at a temperature in the range of 500° C. to 800° C. for a time in the range of 10 seconds to 1200 seconds, and the softening heat treatment is performed can be

The cooling unit 130 may cool the hot-rolled strip M to a temperature in the range of 0° C. to 40° C. at a cooling rate of 10° C./sec to 100° C./sec. The cooling unit 130 may include an air cooling unit 131 and a water cooling unit 132 .

The air cooling unit 131 may air-cool the hot-rolled strip M, for example, to a temperature in the range of 200°C to 300°C. The water cooling unit 132 may water-cool the hot-rolled strip M and cool it to a temperature in the range of 0°C to 40°C.

The softening heat treatment unit 120 and the cooling unit 130 may be provided in the form of one heat treatment furnace.

The winding unit 140 may wind the cooled hot-rolled strip (M). The winding unit 140 may be wound by applying tension to the hot-rolled strip (M). The winding unit 140 may include a second cutting unit 142 for cutting and dividing the hot-rolled strip M, if necessary, and a second winding roll 141 for winding the hot-rolled strip M.

The apparatus 100 for manufacturing such a hot-rolled steel sheet has a temperature range similar to that of the conventional softening heat treatment, but since the hot-rolled strip can be continuously softened and heat treated, the heat treatment temperature maintenance period and cooling time are very short.

3 is a graph exemplarily showing a temperature profile and time during a softening heat treatment process in the apparatus for manufacturing a hot-rolled steel sheet performing the method for manufacturing the ultra-high tensile strength hot-rolled steel sheet of FIG. 2 according to an embodiment of the present invention.

Referring to FIG. 3 , temperatures and durations corresponding to the preheating unit 121 , the heating unit 122 , the cracking unit 123 , the air cooling unit 131 , and the water cooling unit 132 are shown. However, this is exemplary and the technical spirit of the present invention is not limited thereto.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Steel materials having the composition (unit: weight %) shown in Table 1 below were prepared. The remainder is iron (Fe) and other unavoidable impurities. Comparative Examples and Examples all had the same composition.

element C Si Mn Al Ti Nb P S N content 0.18 1.7 2.8 0.03 0.02 0.02 0.01 0.005 0.003

Table 2 shows the softening heat treatment conditions of Comparative Examples and Examples.

division heat treatment target softening heat treatment temperature
(℃) softening heat treatment time
(candle) Comparative Example 1 hot rolled coil - - Comparative Example 2 hot rolled coil 620 21,600 Comparative Example 3 hot rolled coil 670 21,600 Example 1 hot rolled strip 650 100 Example 2 hot rolled strip 700 100

Table 3 shows the results of measuring the yield strength (YS), tensile strength (TS), and elongation (EL) of the manufactured steel, respectively, as mechanical properties.

division yield strength
(MPa) tensile strength
(MPa) elongation
(%) Comparative Example 1 900 1182 12.4 Comparative Example 2 745 849 18.5 Comparative Example 3 593 718 23.6 Example 1 776 923 15.4 Example 2 713 873 17.8

Referring to Table 3, Comparative Example 1 is a hot rolled coil manufactured by performing a hot rolling process, and is a case in which softening heat treatment is not performed, and exhibits high yield strength, tensile strength, and low elongation. The yield strength of Comparative Example 1 is 900 MPa, and in the case of the third-generation steel sheet during cold rolling, the work hardening degree is high, and when the yield strength is 800 MPa or more, problems related to thickness and shape may mainly occur. Accordingly, the physical properties of Comparative Example 1 may be inappropriate for performing a subsequent cold rolling process. Comparative Examples 2 and 3 are cases in which softening heat treatment is performed in the state of a hot-rolled coil, and the time required for softening heat treatment is reduced. It was very long as 21,600 seconds (ie, 6 hours), and the yield strength and tensile strength of the hot-rolled steel sheet were low below the target values. Also, there was a difference in strength between the central portion and the edge portion of the hot-rolled steel sheet.

In the case of the example in which the softening heat treatment was performed in the state of the hot-rolled strip, the softening heat treatment time was very short as 100 seconds, and the yield strength, tensile strength, and elongation of the hot-rolled steel sheet satisfied the target ranges. In other words, the yield strength, tensile strength, and elongation of the hot-rolled steel sheet could be maintained at an appropriate level for subsequent cold rolling. Therefore, the method for manufacturing an ultra-high tensile strength hot-rolled steel sheet according to the technical idea of the present invention can effectively soften the hot-rolled steel sheet.

4 is a graph showing the change in hardness according to the softening heat treatment temperature of the ultra-high tensile strength hot-rolled steel sheet manufactured by using the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 4 , Vickers hardness of a hot-rolled coil and a hot-rolled steel sheet subjected to softening heat treatment by holding at 500° C. to 700° C. for 60 seconds according to the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet of the present invention is shown. Compared to the hot-rolled coil, when the softening heat treatment was performed, the Vickers hardness was lowered, and further decreased as the softening heat treatment temperature increased. However, there was no decrease in Vickers hardness at a temperature of 600° C. or higher. The difference in hardness between the edge and 1/4 of the width of the hot-rolled steel sheet increased as the softening heat treatment temperature increased at 500°C to 600°C, but decreased at temperatures above 600°C, and the difference in hardness at 700°C appeared the smallest. Therefore, a softening heat treatment temperature between 600° C. and 700° C. may be desirable.

5 is a graph showing the change in hardness according to the softening heat treatment holding time of the ultra-high tensile strength hot-rolled steel sheet manufactured by using the manufacturing method of the ultra-high tensile strength hot-rolled steel sheet according to an embodiment of the present invention.

Referring to FIG. 5 , at a softening heat treatment temperature of 600° C., the Vickers hardness decreased until the softening heat treatment time of about 100 seconds, and the decrease in hardness was hardly or very small until 500 seconds thereafter. At a softening heat treatment temperature of 700° C., the Vickers hardness decreased until the softening heat treatment time of about 60 seconds, and the decrease in hardness was hardly or very small until 500 seconds thereafter. That is, the softening effect was dulled and converged at a time of 100 seconds at 600°C and 60 seconds or longer at 700°C.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (10)

manufacturing a hot-rolled coil by hot-rolling a steel material;
providing a hot-rolled strip by unwinding the hot-rolled coil;
softening the hot-rolled strip;
cooling the hot rolled strip; and
Including; winding the hot-rolled strip
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The softening heat treatment step,
carried out at a temperature in the range of 500 ° C to 800 ° C,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The softening heat treatment step,
performed for a time ranging from 10 seconds to 1200 seconds,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The manufacturing of the hot-rolled coil comprises:
reheating the steel at a reheating temperature in the range of 1,150° C. to 1,250° C.;
manufacturing a hot-rolled steel sheet by hot finish rolling the reheated steel at a finish rolling end temperature in the range of 880°C to 930°C;
cooling the hot-rolled steel sheet to a temperature in the range of 550°C to 650°C; and
Manufacturing the hot-rolled coil by winding the hot-rolled steel sheet in the range of 550°C to 650°C; including,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The cooling step is
performed to cool the hot-rolled strip to a temperature in the range of 0°C to 40°C at a cooling rate of 10°C/sec to 100°C/sec,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The cooling step is
After performing the air cooling step of the hot-rolled strip,
Subsequently, the hot-rolled strip is formed by performing a water cooling step,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. 7. The method of claim 6,
In the air cooling step, the hot-rolled strip is cooled to a temperature in the range of 200 ° C to 300 ° C,
In the water cooling step, cooling the hot-rolled strip to a temperature in the range of 0 ℃ ~ 40 ℃,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. The method of claim 1,
The ultra-high-strength hot-rolled steel sheet,
By weight, carbon (C): 0.15% to 0.20%, silicon (Si): 1.5% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.06%, niobium the sum of at least one of (Nb) and titanium (Ti): 0.01% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.01%, and the balance is iron (Fe) and other unavoidable impurities,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. 9. The method of claim 8,
The ultra-high-strength hot-rolled steel sheet,
The ultra-high strength cold rolled steel sheet, yield strength (YP): 500 MPa ~ 850 MPa, tensile strength (TP): 750 MPa ~ 1200 MPa, and elongation (El): 15% ~ 25%,
A method for manufacturing an ultra-high-strength hot-rolled steel sheet. an unwinding unit for unwinding the hot-rolled coil to provide a hot-rolled strip;
a softening heat treatment unit for softening and heat-treating the hot-rolled strip at a temperature in the range of 500° C. to 800° C. for a time in the range of 10 seconds to 1200 seconds;
a cooling unit for cooling the hot-rolled strip to a temperature in the range of 0°C to 40°C at a cooling rate of 10°C/sec to 100°C/sec; and
Containing; a winding unit for winding the hot-rolled strip
A device for manufacturing ultra-high-strength hot-rolled steel sheet.

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