KR101685838B1 - Cold worked steel sheet with batch annealing furnace thermal treatment and method for manufacturing the same - Google Patents

Cold worked steel sheet with batch annealing furnace thermal treatment and method for manufacturing the same Download PDF

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KR101685838B1
KR101685838B1 KR1020150116774A KR20150116774A KR101685838B1 KR 101685838 B1 KR101685838 B1 KR 101685838B1 KR 1020150116774 A KR1020150116774 A KR 1020150116774A KR 20150116774 A KR20150116774 A KR 20150116774A KR 101685838 B1 KR101685838 B1 KR 101685838B1
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heating
steel sheet
rolled steel
cold
annealing furnace
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KR1020150116774A
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Korean (ko)
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장건우
유수철
이경호
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현대제철 주식회사
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D11/00—Process control or regulation for heat treatments
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

A method of manufacturing a cold-rolled steel sheet thermally treated in a batch annealing furnace according to an embodiment of the present invention provides a cold-rolled steel sheet containing 0.050 to 0.080 wt% of carbon (C), more than 0 wt% and 0.03 wt% or less of silicon (Si), 1.5-1.7 wt% of manganese (Mn), more than 0 wt% and 0.03 wt% or less of phosphorus (P), more than 0 wt% and 0.006 wt% or less of sulfur (S), 0.01-0.06 wt% of aluminum (Al), more than 0 wt% and 0.006 wt% or less of nitrogen (N), 0.005-0.015 wt% of niobium (Nb), and the balance of Fe and inevitable impurities. With the cold-rolled steel sheet, a batch annealing furnace thermal treatment is carried out, which includes a first heating step, a first heat maintaining step, a second heating step, a second heat maintaining step, and a cooling step. In the first heat maintaining step, the heat is maintained at heat maintaining temperatures of 35010C for heat maintaining hours of 4 to 8 hours. In the second heat maintaining step, the heat is maintained at heat maintaining temperatures of 66010C for heat maintaining hours of 8 to 12 hours. As a result, the cold-rolled steel sheet thermally treated in the batch annealing furnace has a yield strength of 265 MPa or higher, a tensile strength of 440 MPa or higher, and an elongation rate of 30 or higher.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a cold annealed steel sheet,

The present invention relates to a box annealing furnace heat-treated cold-rolled steel sheet and a method of manufacturing the same.

The global warming problem is rapidly emerging, and regulation of atmospheric gas emission in vehicles is strengthening. In order to reduce the atmospheric gas of the vehicle, various measures have been realized by reducing the weight of the vehicle. Among them, attempts have been made to increase the strength of the automotive steel sheet while reducing the weight of the steel sheet used in the vehicle.

The general manufacturing process of such a cold rolled steel (CR) steel sheet for automobiles is performed by cold rolling a hot rolled steel sheet at room temperature and then subjecting it to a predetermined cold rolled steel sheet through a heat treatment suitable for the required mechanical properties. .

The heat treatment of the cold-rolled steel sheet is mainly divided into a continuous annealing line (CAL) and a batch annealing furnace (BAF). Recently, most of the annealing methods for cold rolled steel sheets for automobiles are mostly used for continuous annealing, and the results of mechanical properties of annealing conditions have been systematically studied. Recently, however, the production of automotive steel sheets has been increasingly carried out in box annealing furnaces, taking advantage of the advantages of low-cost investment through development of heat treatment techniques and improvement of compact equipment in box annealing.

A related prior art is Korean Patent Registration No. 10-1174615 (entitled BAF heat treated steel sheet for a bus roof panel having a low resistance ratio and a manufacturing method thereof)

The present invention provides a BAF heat treated high strength cold rolled steel sheet which can satisfy the mechanical properties required of a 440 MPa class high strength automotive steel sheet by using a hot rolled material.

A method of manufacturing a box annealed furnace heat-treated cold-rolled steel sheet according to one aspect of the present invention is disclosed. (P): more than 0 to 0.03%, more preferably more than 0.03%, more preferably more than 0.03%, more preferably more than 0.03% (S): more than 0 to 0.006%, aluminum (Al): 0.01 to 0.06%, nitrogen (N): more than 0 to 0.006%, niobium (Nb): 0.005 to 0.015% A cold rolled steel sheet comprising impurities. The cold-rolled steel sheet is subjected to box annealing furnace heat treatment comprising a first heating step, a first heating and holding step, a second heating step, a second heating and holding step and a cooling step. Wherein the first heating and holding step is maintained for a heating holding time of 4 to 8 hours at a heating holding temperature of 350 占 占 폚 and a heating holding time of 8 to 12 Time. As a result, the box annealed furnace heat-treated cold-rolled steel sheet has a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35%.

In one embodiment, the first heating and holding step and the second heating and holding step may include supplying 25 to 30 m 3 / h of hydrogen in the furnace.

In another embodiment, the first heating step includes supplying 20 to 25 m 3 / h of hydrogen into the furnace and raising the temperature to 350 ° C at a heating rate of 80 to 180 ° C / h, The second heating step may include, after the first heating and holding step, supplying 25 to 30 m 3 / h of hydrogen in the furnace and raising the temperature to 50 to 80 ° C / h to 660 ° C to 10 ° C.

In another embodiment, the cooling step may include a first cooling step of cooling from the heating holding temperature of the second heating and holding step to a temperature of 550 占 폚 to 10 占 폚 at a cooling rate of 30 to 40 占 폚 / h, A third cooling step of cooling from a temperature of 350 캜 to a temperature of 350 캜 to 10 캜 at a cooling rate of 38 캜 to 42 캜 / h, and a third cooling step of cooling from 350 캜 to 10 캜 to a room temperature.

The box annealing furnace heat treated cold rolled steel sheet according to another aspect of the present invention comprises 0.050 to 0.080% of carbon (C), more than 0 to 0.03% of silicon (Si), 1.5 to 1.7% of manganese (Mn) (P): more than 0 to 0.03%, sulfur (S): more than 0 to 0.006%, aluminum (Al) 0.01 to 0.06%, nitrogen (N) (Fe) and unavoidable impurities, and has a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35%.

According to the embodiment of the present invention, it is possible to provide a high-strength cold-rolled steel sheet having a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35% through control of the alloy component control and box annealing furnace heat treatment process . Accordingly, the high strength cold rolled steel sheet can be easily applied to automotive steel sheets requiring a tensile strength of 440 MPa.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a flow chart schematically showing a method of manufacturing a box annealed furnace annealed cold-rolled steel sheet according to an embodiment of the present invention;
2 is a graph showing detailed steps of a box annealing furnace heat treatment according to an embodiment of the present invention.

Hereinafter, a box annealing furnace heat-treated cold-rolled steel sheet according to an embodiment of the present invention and a method of manufacturing the same will be described in detail. The terms used below are appropriately selected terms in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout this specification.

Box annealing furnace  Heat-treated cold-rolled steel plate

The box annealed annealed cold-rolled steel sheet according to the embodiment of the present invention comprises 0.050 to 0.080% of carbon (C), more than 0 to 0.03% of silicon (Si), 1.5 to 1.7% of manganese (Mn) (P): more than 0 to 0.03%, sulfur (S): more than 0 to 0.006%, aluminum (Al) 0.01 to 0.06%, nitrogen (N) And the balance iron (Fe) and unavoidable impurities.

Accordingly, the cold-rolled steel sheet has a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35% after heat treatment in a box annealing furnace.

Hereinafter, the content of each component added to the box annealed furnace annealed cold-rolled steel sheet according to the embodiment of the present invention and the reason for addition thereof will be described.

Carbon (C)

Carbon (C) is added to ensure strength of the steel. The carbon is preferably added in an amount of 0.050 to 0.080 wt% of the total weight of the steel. When the added amount of carbon is less than 0.050 wt%, it is difficult to secure sufficient strength in the steel sheet according to the present invention. When the carbon content exceeds 0.080 wt%, the strength is increased but the elongation can be lowered.

silicon( Si )

Silicon (Si) acts as a deoxidizer in the steel and contributes to securing strength and elongation. The silicon is preferably added in an amount of more than 0 to 0.03 wt% of the total weight of the steel. When the added amount of silicon exceeds 0.03% by weight, a red scale may be generated at the time of hot rolling, and surface color difference may be generated at the time of annealing the box, thereby deteriorating the surface quality.

Manganese (Mn)

Manganese (Mn) is a solid solution strengthening element and contributes to the strength of steel. In addition, manganese serves as an austenite stabilizing element and contributes to the refinement of ferrite grains by delaying ferrite and pearlite transformation. The manganese is preferably added in an amount of 1.5 to 1.7% by weight based on the total weight of the steel. When the content of manganese is less than 1.5% by weight, the solid solution strengthening effect is insufficient and it is difficult to secure the strength. On the other hand, if the addition amount of manganese exceeds 1.7 wt%, the toughness of the steel may be largely lowered due to deterioration in weldability and occurrence of MnS inclusions or center segregation. In addition, the surface selective oxidation reaction for forming an oxide having a high manganese content may occur, and the surface quality may be deteriorated.

In (P)

Phosphorus (P) contributes to the enhancement of strength by inhibiting cementite formation. The phosphorus is preferably added in an amount of more than 0 to 0.03 wt% of the total weight of the steel. If the content of phosphorus is over 0.03 wt%, it may weaken the weldability and cause low temperature brittleness.

Sulfur (S)

Sulfur (S) inhibits the toughness and weldability of steel and increases MnS nonmetallic inclusions in steel to cause cracks during steel processing. Accordingly, in the present invention, the content of sulfur is limited to not less than 0 and not more than 0.006% by weight of the total weight of the steel in consideration of the above.

Aluminum (Al)

Aluminum (Al) is an element used as a deoxidizer. It serves to stabilize the austenite by improving the elongation rate of the ferrite and increasing the carbon concentration in the austenite. The aluminum is preferably added in an amount of 0.01 to 0.06% by weight based on the total weight of the steel. When the content of aluminum is less than 0.01% by weight, it is difficult to sufficiently exert the above-mentioned effects. In addition, the solid solution strengthening effect by nitrogen may be accompanied, which may be disadvantageous to the material deviation. On the other hand, when the content of aluminum exceeds 0.06% by weight, performance is deteriorated and AlN is formed in the slab, which may lead to hot cracking and ductility deterioration.

Niobium ( Nb )

Niobium (Nb) is a ferrite stabilizing element, which inhibits the formation of carbides (carbides) in ferrite during ferrite transformation, which is advantageous for securing elongation. The niobium bonds with carbon and nitrogen to form a precipitate. The precipitate acts as a nucleation site at the time of recrystallization after cold rolling to finally cause crystal grain refinement, thereby generating a strength increasing effect.

The niobium is preferably added in an amount of 0.005-0.015 wt% of the total weight of the steel. When niobium is added in an amount of less than 0.005% by weight, the amount of precipitate formed is decreased, which is disadvantageous in securing strength. When niobium is added in an amount exceeding 0.015% by weight, toughness is deteriorated due to excessive precipitate formation.

Nitrogen (N)

Nitrogen (N) can contribute to grain refinement. The nitrogen is preferably added in an amount of 0 to 0.006% by weight based on the total weight of the steel sheet. When nitrogen is added in an amount exceeding 0.006% by weight, the elongation and target characteristics of the steel sheet are deteriorated.

In the embodiment of the present invention, strength control can be ensured by controlling the manganese and niobium components in the alloy component, and the decrease in elongation can be suppressed. In this embodiment, alloying elements such as chromium and molybdenum for improving the strength are not separately added to the alloying elements.

The cold rolled steel sheet according to the present invention comprising the above alloy components may have a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35% in terms of mechanical properties.

Box annealing furnace  Manufacturing method of heat-treated cold-rolled steel sheet

Hereinafter, a method for producing a box annealed furnace heat-treated cold-rolled steel sheet having the above-described characteristics will be described. BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a flow chart schematically showing a method of manufacturing a box annealed furnace annealed cold-rolled steel sheet according to an embodiment of the present invention;

Referring to FIG. 1, in step S110, a cold-rolled steel sheet is prepared. Wherein said cold rolled steel sheet comprises 0.050 to 0.080% of carbon (C), more than 0 to 0.03% of silicon (Si), 1.5 to 1.7% of manganese (Mn) (S): more than 0 to 0.006%, 0.01 to 0.06% of aluminum (Al), 0 to 0.006% of nitrogen (N), 0.005 to 0.015% of niobium (Nb) . The slab having the predetermined alloy composition is manufactured as a semi-finished product through a casting process, and the slab is continuously subjected to a hot rolling process and a cold rolling process to manufacture the cold rolled steel sheet.

In step S120, the cold-rolled steel sheet is subjected to box annealing furnace heat treatment. The continuous box annealing furnace heat treatment may include a first heating step, a first heating holding step, a second heating step, a second heating holding step and a cooling step.

2 is a graph showing detailed steps of a box annealing furnace heat treatment according to an embodiment of the present invention. Referring to Fig. 2, the box annealing furnace heat treatment process will be described in detail below.

In the first heating step, 20-25 m 3 / h of hydrogen is supplied into the furnace, and the temperature is raised to 350 ± 10 ° C at a heating rate of 80-180 ° C / h. The first heating step is a step of removing moisture and oxygen remaining in the furnace.

In the first heating and holding step, 25 to 30 m 3 / h of hydrogen is supplied into the furnace, and the heating and holding temperature is maintained within a range of 350 ± 10 ° C for 4 to 8 hours. The first heating and holding step is a step of additionally controlling the atmosphere in the furnace of the box annealing furnace and has an effect of lowering the dew point. Namely, due to moisture or oxygen generally remaining in the furnace, In the case of a steel having a content of 1.2% by weight or more), the oxidation easily occurs and the surface color difference may appear. In the present invention, since manganese is added in an amount of 1.5 to 1.7 wt% in order to secure strength, surface oxidation may occur under a conventional annealing furnace annealing furnace.

Therefore, in order to prevent the surface oxidation, by removing the moisture and oxygen remaining in the furnace through the heat treatment at 350 DEG C for 4 to 8 hours, the factor causing the oxidation can be sufficiently removed. Such a hydrogen supply can then be continued continuously until the second heating step and the second heating and holding step.

In the second heating step, 25 to 30 m 3 / h of hydrogen is supplied into the furnace and the temperature is raised to 660 ± 10 ° C at 50 to 80 ° C / h.

The second heating and holding step may be carried out by supplying 25 to 30 m 3 / h of hydrogen in the furnace and maintaining the heating and holding time in the range of 660 ± 10 ° C for 8 to 12 hours. The second heating and holding step is a step in which annealing annealing proceeds substantially.

The cooling step may include first to third cooling steps. First, the first cooling step may be performed by cooling from the heating and holding temperature of the second heating and holding step to a temperature of 550 ± 10 ° C at a cooling rate of 30 to 40 ° C / h. The second cooling step may be performed by cooling from the temperature of 550 占 폚 to the temperature of 350 占 폚 to 10 占 폚 at a cooling rate of 38 to 42 占 폚 / h. And the third cooling step may be a step of cooling the temperature from 350 占 폚 to 10 占 폚 to room temperature.

By carrying out the heat treatment step in the box annealing furnace described above, a cold rolled steel sheet having a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35% can be obtained.

Example

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

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

One. Appeal  Production of heat-treated cold-rolled steel sheets

The steel slab having the comparative composition and the composition shown in Table 1 was subjected to a hot rolling process, pickling treatment, and cold rolling to produce a cold-rolled steel sheet. Here, the composition to be used is an alloy composition according to an embodiment of the present invention, and the comparative composition is a composition as a comparative example in contrast to the embodiment of the present invention.

Next, in the case of test pieces of comparative compositions, the specimens of Comparative Examples 1-1, 1-2 and 1-3, which are subjected to the annealing heat treatment step shown in Table 2, and the other annealing heat treatment step The test pieces prepared in Comparative Examples 2-1 and 2-2 were prepared. The specimens of Comparative Examples 3-1, 3-2 and 3-3, which had undergone another annealing heat treatment process step shown in Table 4, and the annealing heat treatment according to the embodiment of the present invention shown in Table 5, Specimens of Examples 1-1, 1-2, and 1-3 which were subjected to the process steps were prepared.

C Si Mn P S Cr Mo Al N Nb Comparative composition 0.1085 0.0509 1.10 0.0186 0.0034 0.0166 0.0029 0.0253 0.0021 - practice
Furtherance 0.060 0.03 1.6 0.03 0.06 - - 0.03 0.006 0.010


step1 step2 step3 step4 step5 step6 The first heating step The second heating step Heating and holding step The first cooling step The second cooling step Third cooling step Target temperature (캜) 400 660 660 550 350 50 Process time (h) 3 5 10 3 5 19 Amount of hydrogen supplied (m 3 / h) 20 0 0 0 0 0

Referring to Table 2, Comparative Example 1 has the comparative composition shown in Table 1, and the annealing in the box annealing furnace was heated to 660 占 폚 without heating and holding at 660 占 폚 for 10 hours and then cooled. The temperature raising process proceeded to a first heating stage proceeding to 400 ° C with a hydrogen supply of 20 m 3 / h, and to a second heating stage proceeding to 660 ° C without a hydrogen feed.


 step1 step2 step3 step4 step5 step6 step7 The first heating step The second heating step Third heating step Heating and holding step The first cooling step The second cooling step Third cooling step Target temperature (캜) 300 670 710 710 630 350 50 Process time (h) 3 5 4 10 4 8 11 Amount of hydrogen supplied (m 3 / h) 20 0 0 0 0 0 0

Referring to Table 3, Comparative Example 2 has the comparative composition shown in Table 1, and the annealing furnace for box annealing was heated to 710 占 폚 without heating and holding at 710 占 폚 for 10 hours during the heating, and then cooled. However, the temperature raising process is performed by a first heating step to 300 ° C with a hydrogen supply of 20 m 3 / h, a second heating step to 670 ° C without hydrogen supply, and a third heating step to 710 ° C without hydrogen supply. Heating step.


 step1 step2 step3 step4 step5 step6 step7 The first heating step The first heating and holding step The second heating step The second heating and holding step The first cooling step The second cooling step Third cooling step Target temperature (캜) 350 350 630 630 550 350 50 Process time (h) 2 6 5 10 3 5 19 Amount of hydrogen supplied (m 3 / h) 20 20 30 30 0 0 0

Referring to Table 4, Comparative Example 3 had the composition shown in Table 1, and the box annealing furnace was heated to 630 占 폚 and maintained at 630 占 폚 for 10 hours while being subjected to the first heating / . In the first heating step and the first heating and holding step, 20 m 3 / h of hydrogen was supplied, and the first heating and holding step was carried out at 350 ° C. for 6 hours.


 step1 step2 step3 step4 step5 step6 step7 The first heating step The first heating and holding step The second heating step The second heating and holding step The first cooling step The second cooling step Third cooling step Target temperature (캜) 350 350 660 660 560 350 50 Process time (h) 2 6 5 10 3 5 19 Amount of hydrogen supplied (m 3 / h) 20 20 30 30 0 0 0

Referring to Table 5, the embodiment has the composition shown in Table 1, and the box annealing furnace is cooled after being heated to 660 占 폚 and maintained at 660 占 폚 for 10 hours while passing through the first heating and holding step during the heating. In the first heating step and the first heating and holding step, 20 m 3 / h of hydrogen was supplied, and the first heating and holding step was carried out at 350 ° C. for 6 hours.

2 . Mechanical properties evaluation

Comparative Example 1-1, 1-2, 1-3, 2-1, 2-2, 3-1, 3-2, 3-3, and Examples 1-1, 1-2, The yield strength, the tensile strength and the elongation of the heat treated specimens with respect to the temperature at which the annealing heat treatment was performed were evaluated, and the results are shown in Table 6 below.


 Annealing temperature (° C) Yield strength (MPa) Tensile Strength (MPa) Elongation (%) Target value: 265 or more Target value: 440 or more Target value: 30 or more Comparative Example 1-1 659 328 442 32.0 Comparative Example 1-2 659 346 435 33.0 Comparative Example 1-3 656 336 444 31.0 Comparative Example 2-1 707 315 420 36.0 Comparative Example 2-2 709 308 423 35.9 Comparative Example 3-1 628 344 470 32.0 Comparative Example 3-2 625 354 481 30.9 Comparative Example 3-3 626 352 466 31.7 Example 1-1 656 340 460 35.0 Examples 1-2 658 333 458 34.8 Example 1-3 658 332 457 34.2

The mechanical properties of Comparative Examples 1-1, 1-2, 1-3, 2-1, 2-2, 3-1, 3-2, 3-3, and Examples 1-1, 1-2, The target values were all satisfied in the case of the group of Comparative Examples 1-1, 1-2, and 1-3, but the tensile strength and the tensile strength of the group of Examples 1-1, 1-2, The elongation was low.

In the case of the group of Comparative Examples 2-1 and 2-2, the tensile strength did not reach the target value. In the case of the groups of Comparative Examples 3-1, 3-2 and 3-3, the target values were all satisfied, but the elongation was lower than that of the groups of Examples 1-1, 1-2 and 1-3.

As described above, in the case of the embodiment of the present invention, the oxidation of the steel sheet due to moisture and manganese can be effectively suppressed by subjecting the heat treatment step in the hydrogen atmosphere to the heating and holding step separately in the heating step of the heat treatment. As a result, excellent yield strength, tensile strength and elongation can be secured as compared with the comparative examples.

It is to be understood that the invention includes various modifications and equivalent embodiments that can be derived from the disclosed embodiments as well as those of ordinary skill in the art to which the present invention pertains. Accordingly, the technical scope of the present invention should be defined by the following claims.

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Claims (5)

(a) from 0.050 to 0.080% carbon (C), from 0.0 to 0.03% silicon (Si), from 1.5 to 1.7% manganese (Mn) (Fe) and inevitable impurities, and a content of iron (Fe) in excess of 0.006%, aluminum (Al) in an amount of 0.01 to 0.06%, nitrogen (N) in an amount of more than 0 to 0.006%, niobium (Nb) in an amount of 0.005 to 0.015% Providing a cold rolled steel sheet; And
(b) subjecting the cold-rolled steel sheet to a box annealing furnace comprising a first heating step, a first heating and holding step, a second heating step, a second heating and holding step and a cooling step,
Wherein the first heating and holding step is maintained for a heating holding time of 4 to 8 hours at a heating holding temperature of 350 占 占 폚 and a heating holding time of 8 to 12 Keeping the time,
Having a yield strength of 265 to 340 MPa, a tensile strength of 440 to 460 MPa, and an elongation of 30 to 35%
A method for producing a heat - treated cold - rolled steel sheet in a box annealing furnace.
The method according to claim 1,
wherein the first heating and holding step and the second heating and holding step of step (b)
Comprising the step of supplying a 25 ~ 30 m 3 / h with hydrogen in the
A method for producing a heat - treated cold - rolled steel sheet in a box annealing furnace.
The method according to claim 1,
In the step (b)
The first heating step
It supplies the 20 ~ 25 m 3 / h in a hydrogen to, and including the step of raising the temperature to a temperature of 350 ± 10 at a heating rate 80 ~ 180 ℃ / h,
The second heating step
After the first heating and holding step, supplying 25 to 30 m 3 / h of hydrogen into the furnace and raising the temperature to 660 ± 10 ° C at 50 to 80 ° C / h,
A method for producing a heat - treated cold - rolled steel sheet in a box annealing furnace.
The method according to claim 1,
The cooling step of step (b)
A first cooling step of cooling from the heating and holding temperature of the second heating and holding step to a temperature of 550 ± 10 ° C at a cooling rate of 30 to 40 ° C / h;
A second cooling step of cooling from a temperature of 550 占 폚 to a temperature of 350 占 폚 to 10 占 폚 at a cooling rate of 38 to 42 占 폚 / h; And
And a third cooling step of cooling the temperature from 350 占 폚 to 10 占 폚 to room temperature
A method for producing a heat - treated cold - rolled steel sheet in a box annealing furnace.
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Publication number Priority date Publication date Assignee Title
KR101897498B1 (en) 2018-03-30 2018-09-12 (주)영흥산업 method for low temperature annealing heat treatment of bar steel

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Publication number Priority date Publication date Assignee Title
KR101033394B1 (en) * 2011-01-04 2011-05-09 현대하이스코 주식회사 440mpa high strengh steel sheet and it's manufacturing method
KR20140002843A (en) * 2012-06-26 2014-01-09 현대하이스코 주식회사 Heat treating method for steel sheet using batch annealing furnace

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR101033394B1 (en) * 2011-01-04 2011-05-09 현대하이스코 주식회사 440mpa high strengh steel sheet and it's manufacturing method
KR20140002843A (en) * 2012-06-26 2014-01-09 현대하이스코 주식회사 Heat treating method for steel sheet using batch annealing furnace

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR101897498B1 (en) 2018-03-30 2018-09-12 (주)영흥산업 method for low temperature annealing heat treatment of bar steel

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