CN117858973A - High-strength high-toughness steel sheet and method for producing same - Google Patents
High-strength high-toughness steel sheet and method for producing same Download PDFInfo
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- CN117858973A CN117858973A CN202280056098.5A CN202280056098A CN117858973A CN 117858973 A CN117858973 A CN 117858973A CN 202280056098 A CN202280056098 A CN 202280056098A CN 117858973 A CN117858973 A CN 117858973A
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 103
- 239000010959 steel Substances 0.000 title claims abstract description 103
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 27
- 229910001562 pearlite Inorganic materials 0.000 claims description 149
- 238000001816 cooling Methods 0.000 claims description 39
- 239000011572 manganese Substances 0.000 claims description 37
- 238000005097 cold rolling Methods 0.000 claims description 32
- 238000005096 rolling process Methods 0.000 claims description 32
- 239000011651 chromium Substances 0.000 claims description 30
- 238000010438 heat treatment Methods 0.000 claims description 29
- 229910001567 cementite Inorganic materials 0.000 claims description 20
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims description 20
- 238000005452 bending Methods 0.000 claims description 17
- 229910052748 manganese Inorganic materials 0.000 claims description 13
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 12
- 239000010941 cobalt Substances 0.000 claims description 12
- 229910017052 cobalt Inorganic materials 0.000 claims description 12
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims description 12
- 229910052717 sulfur Inorganic materials 0.000 claims description 12
- 239000011593 sulfur Substances 0.000 claims description 12
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 claims description 12
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 11
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 11
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 11
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 11
- 229910052782 aluminium Inorganic materials 0.000 claims description 11
- 229910052799 carbon Inorganic materials 0.000 claims description 11
- 229910052710 silicon Inorganic materials 0.000 claims description 11
- 239000010703 silicon Substances 0.000 claims description 11
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 claims description 10
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 claims description 10
- 229910052804 chromium Inorganic materials 0.000 claims description 10
- 239000010960 cold rolled steel Substances 0.000 claims description 10
- 229910052698 phosphorus Inorganic materials 0.000 claims description 10
- 239000011574 phosphorus Substances 0.000 claims description 10
- 239000012535 impurity Substances 0.000 claims description 8
- 238000003303 reheating Methods 0.000 claims description 7
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 6
- 230000001186 cumulative effect Effects 0.000 claims description 5
- 238000005554 pickling Methods 0.000 claims description 5
- 238000000034 method Methods 0.000 abstract description 18
- 239000000203 mixture Substances 0.000 description 12
- 230000000052 comparative effect Effects 0.000 description 11
- 229910045601 alloy Inorganic materials 0.000 description 10
- 239000000956 alloy Substances 0.000 description 10
- 230000015572 biosynthetic process Effects 0.000 description 9
- 239000000463 material Substances 0.000 description 9
- 238000004804 winding Methods 0.000 description 8
- 238000005204 segregation Methods 0.000 description 6
- 238000005482 strain hardening Methods 0.000 description 6
- 230000000704 physical effect Effects 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 229910001563 bainite Inorganic materials 0.000 description 3
- 239000013078 crystal Substances 0.000 description 3
- 230000007547 defect Effects 0.000 description 3
- 230000006866 deterioration Effects 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 229910000677 High-carbon steel Inorganic materials 0.000 description 2
- 230000006835 compression Effects 0.000 description 2
- 238000007906 compression Methods 0.000 description 2
- 238000005098 hot rolling Methods 0.000 description 2
- 229910000734 martensite Inorganic materials 0.000 description 2
- 238000007670 refining Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 238000005728 strengthening Methods 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- 229910052720 vanadium Inorganic materials 0.000 description 2
- 241000219307 Atriplex rosea Species 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 230000002159 abnormal effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000005336 cracking Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000002542 deteriorative effect Effects 0.000 description 1
- 238000000635 electron micrograph Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 150000003839 salts Chemical class 0.000 description 1
- 238000009628 steelmaking Methods 0.000 description 1
- 229910000859 α-Fe Inorganic materials 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
- B21C47/02—Winding-up or coiling
-
- 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
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
-
- 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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
- C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
- C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
Abstract
The present invention relates to a high-strength and high-toughness steel sheet and a method for manufacturing the same, and more particularly, to a high-strength and high-toughness steel sheet that can be used for automobile seat belt springs and the like and a method for manufacturing the same.
Description
Technical Field
The present invention relates to a high-strength and high-toughness steel sheet and a method for manufacturing the same, and more particularly, to a high-strength and high-toughness steel sheet that can be used for automobile seat belt springs and the like and a method for manufacturing the same.
Background
Typically, the final material thickness of the material for automotive seat belt springs is as thin as about 0.1-0.3mm and is used in the form of a spring having a width of about 3-25mm, thus requiring high toughness. In addition, the rewinding performance, which is an important characteristic of the spring, must be excellent, and thus, in order to secure a target restoring force and torque for each product, the tensile strength of the final cold-rolled steel sheet must be high.
In order to secure the high strength characteristics of the thin material as described above, high carbon steel containing more carbon than eutectoid steel is most widely used. The use of the pearlite structure of the hypereutectoid high-carbon steel can ensure high toughness and high strength by controlling the morphology of the elongated pearlite structure obtained after cold rolling. This is more economical than the method using expensive alloy elements or using a low temperature transformation structure such as bainite or tempered martensite through an additional heat treatment process.
In order to make the number of rewinding of the spring 30 ten thousand times or more without breakage, breakage or the like during use, the uniform pearlite fraction in the microstructure of the cold rolled material of about 0.2t finally must be high.
[ Prior Art literature ]
(patent document 1) Korean patent laid-open publication No. 10-2018-0034885 (published on 05 month 04 in 2018)
Disclosure of Invention
Technical problem to be solved
According to one aspect of the present invention, an object is to provide a high-strength and high-toughness steel sheet and a method for manufacturing the same.
The technical problem of the present invention is not limited to the above. Additional technical problems of the present invention may be readily apparent to one skilled in the art from the entire contents of the present specification.
Technical proposal
One aspect of the present invention may provide a steel sheet comprising, in weight percent: carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): below 0.01% aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, the balance of iron (Fe) and other unavoidable impurities,
the steel sheet has a microstructure including a pearlite structure as a main phase and a grain boundary proeutectoid cementite in an amount of 4 area% or less in the balance,
the pearlite structure comprises, in terms of its area%, 40% or more of uniform pearlite (fibrous pearlite), 50% or less of jagged pearlite (bent pearlite), and 10% or less of non-uniform pearlite.
When the cross section of the microstructure is observed in the thickness direction of the steel sheet, the average thickness of the uniform pearlite may be 2.5 μm or less.
The steel sheet may have an a value of 1.2 or less in relation 1 below.
[ relation 1]
A＝[Mn]+[Cr]+[V]
(wherein [ Mn ], [ Cr ] and [ V ] are weight% of each element.)
The tensile strength of the steel sheet may be 2100MPa or more, the elongation may be 2% or more, and the bending property (R/t) may be 3.0 or less (R is a bending radius at which no crack occurs in a bending portion after 180 DEG bending test, and t is the thickness of the steel sheet).
The tensile strength of the steel sheet may be 2200 to 2350MPa.
The thickness of the steel sheet may be 0.1 to 0.6mm.
Another aspect of the present invention may provide a method of manufacturing a steel sheet, including the steps of: reheating a steel billet comprising, in weight-%: carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): below 0.01% aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, the balance of iron (Fe) and other unavoidable impurities;
rough rolling the reheated steel billet;
finish rolling the rough-rolled steel sheet to obtain a hot-rolled steel sheet;
cooling the hot rolled steel plate to a temperature range of 540-660 ℃ at a cooling rate of 5-50 ℃/sec, and then rolling;
a heat treatment step of heating the cooled and rolled steel sheet to a temperature range of 850-1050 ℃ for 5-20 minutes, then cooling to a temperature range of 520-590 ℃ at a cooling rate of 50-150 ℃/sec, and then maintaining for 30-120 seconds; and
the heat-treated steel sheet is cold-rolled at a cumulative rolling reduction of 80 to 96%.
The steel slab may have an a value of 1.2 or less in the following relation 1.
[ relation 1]
A＝[Mn]+[Cr]+[V]
(wherein [ Mn ], [ Cr ] and [ V ] are weight% of each element.)
The reheating may be performed at a temperature in the range 1100-1300 c,
the rough rolling may be performed at a temperature ranging from 1000 to 1100 c,
the finish rolling may be performed at a temperature ranging from 860 to 940 ℃.
After the winding, the step of pickling the steel sheet at a temperature range of 200 ℃ or less may be further included.
After the heat treatment, the method may further include the step of air-cooling the steel sheet.
The microstructure of the heat-treated steel sheet may include a pearlite structure as a main phase and 4 area% or less of grain boundary proeutectoid cementite as a remainder.
The thickness of the finish rolled steel sheet may be 1.5 to 2.6mm.
The thickness of the cold-rolled steel sheet after cold rolling may be 0.1 to 0.6mm.
Effects of the invention
According to an aspect of the present invention, a high-strength and high-toughness steel sheet and a method of manufacturing the same may be provided.
According to one aspect of the present invention, a high strength and high toughness steel sheet that can be used in high-end industry/tools, car seat belt springs, etc., and a method of manufacturing the same are provided.
Drawings
Fig. 1 is a photograph of the morphology of uniform pearlite (fibrous pearlite) observed with a scanning electron microscope (×20000).
Fig. 2 is a photograph showing a method of calculating the uniform pearlite (fibrous pearlite) fraction of inventive example 2.
Best mode for carrying out the invention
Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention may be modified in various forms and should not be construed as limiting the scope of the invention to the embodiments set forth below. This particular embodiment is provided to illustrate the present invention in more detail to those skilled in the art.
However, as described above, since the fraction of uniform pearlite (fibrous pearlite) of the reel spring material currently being manufactured is insufficient, durability is reduced, and there is a mass deviation between materials, it is difficult to secure a stable uniform pearlite (fibrous pearlite) structure. Further, since there is a reduction in quality due to proeutectoid cementite in a component system and process characteristics produced by cold rolling a pearlite single-phase structure of a eutectoid steel or more, improvement is required.
The present inventors have conducted intensive studies in order to manufacture a cold rolled steel sheet having excellent strength and toughness by controlling the composition and manufacturing process of the steel.
As a result, it was confirmed that the physical properties can be ensured by optimizing the alloy composition and the manufacturing conditions, controlling the grain boundary eutectoid cementite of the steel sheet before cold rolling, and strictly controlling the uniform pearlite (fibrous pearlite) structure of the final steel sheet, and the present invention was completed.
The present invention will be described in detail below.
The composition of the steel of the present invention will be described in detail.
In the present invention, unless otherwise indicated, the% content of each element is expressed on a weight basis.
The steel sheet according to an aspect of the present invention may include, in weight percent: carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): below 0.01% aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, the balance of iron (Fe) and other unavoidable impurities.
Carbon (C): 0.70-1.20%
Carbon (C) is an element that has a great influence on the strength and toughness of the pearlite structure, and is preferably added in an amount of 0.70% or more in order to ensure uniform pearlite (fibrous pearlite) of 40% or more after cold rolling. However, when the content of carbon (C) exceeds 1.20%, the fraction of grain boundary proeutectoid cementite increases after heat treatment, and toughness is deteriorated. The lower limit of the content of the carbon (C) is preferably 0.75%, more preferably 0.76%, further preferably 0.77%, and still further preferably 0.78%. The upper limit of the content of the carbon (C) is preferably 0.90%, more preferably 0.88%, further preferably 0.87%, and still further preferably 0.85%.
Manganese (Mn): 0.2-0.6%
In order to improve strength by solid solution strengthening, manganese (Mn) may be added in an amount of 0.2% or more. However, when the manganese (Mn) is excessively added, there is a risk of decreasing toughness due to carbide formation, and there is a risk of brittleness due to a low-temperature structure of a segregated portion due to center segregation, so that the upper limit of the content of the manganese (Mn) may be limited to 0.6%. The lower limit of the content of manganese (Mn) is more preferably 0.22%, still more preferably 0.24%, and still more preferably 0.25%. The upper limit of the content of manganese (Mn) is preferably 0.5%, more preferably 0.48%, still more preferably 0.46%, and still more preferably 0.45%.
Silicon (Si): 0.01-0.4%
For solid solution strengthening of ferrite structure in pearlite, 0.01% or more of silicon (Si) may be added. However, when the silicon (Si) is excessively added, primary scale generated in the heating furnace is excessively formed, red scale defects are induced, thereby deteriorating heat treatment and workability, and there is a risk of causing brittleness due to residual cementite, so the content of the silicon (Si) may be limited to 0.4% or less. The lower limit of the content of silicon (Si) is preferably 0.05%, more preferably 0.06%, further preferably 0.08%, and still further preferably 0.1%. The upper limit of the content of silicon (Si) is preferably 0.3%, more preferably 0.28%, further preferably 0.26%, and still further preferably 0.25%.
Phosphorus (P): 0.005-0.02%
When phosphorus (P) exceeds 0.02%, there may be a risk of brittleness due to segregation. Therefore, the content of phosphorus (P) is preferably 0.02% or less. The upper limit of the content of phosphorus (P) is preferably 0.015%, more preferably 0.014%, further preferably 0.013%, and still further preferably 0.012%. In addition, the lower limit of the content of phosphorus (P) may be limited to 0.005% in consideration of the case that it is inevitably contained in the manufacturing process.
Sulfur (S): less than 0.01%
Sulfur (S) is an element that causes deterioration of toughness due to formation of nonmetallic inclusions, and thus it is necessary to control the content of sulfur (S) to a level as low as possible. Therefore, the content of sulfur (S) is preferably 0.01% or less. In the present invention, the lower the sulfur (S) content, the lower the risk of brittleness due to segregation/inclusion, which is advantageous in ensuring toughness, and therefore the lower limit of the sulfur (S) content is not particularly limited. The sulfur (S) content is more preferably 0.008% or less, still more preferably 0.006% or less, still more preferably 0.005% or less.
Aluminum (Al): 0.01-0.1%
Aluminum (Al) may be added to refine austenite grains by forming AlN, thereby refining the pearlite structure. When the content of aluminum (Al) is less than 0.01%, it may be difficult to sufficiently obtain the above-described effect. On the other hand, when the content of aluminum (Al) exceeds 0.1%, there may be a risk of brittleness due to inclusions caused by oxide formation. The lower limit of the content of aluminum (Al) is more preferably 0.012%, still more preferably 0.014%, and still more preferably 0.015%. The upper limit of the content of aluminum (Al) is preferably 0.06%, more preferably 0.05%, further preferably 0.04%, and still further preferably 0.03%.
Chromium (Cr): 0.1-0.8%
In order to ensure strength and miniaturization of the pearlite layer spacing, chromium (Cr) is preferably added at 0.1% or more. On the other hand, when the content of chromium (Cr) exceeds 0.8%, there is a risk of deterioration in toughness due to excessive carbide formation. The lower limit of the content of chromium (Cr) is more preferably 0.12%, still more preferably 0.14%, and still more preferably 0.15%. The upper limit of the content of chromium (Cr) is preferably 0.4%, more preferably 0.35%, further preferably 0.33%, and still further preferably 0.30%.
Vanadium (V): 0.02-0.25%
Vanadium (V) is an element necessary for refining pearlite grains to secure strength by work hardening after cold rolling. In order to secure the above effect, in the present invention, 0.02% or more of vanadium (V) may be added. On the other hand, when the content of the vanadium (V) is excessive, coarse carbide/nitride is formed, so that there may be a risk of brittleness, and thus the upper limit of the content of the vanadium (V) may be limited to 0.25%. The lower limit of the content of vanadium (V) is more preferably 0.03%, still more preferably 0.04%, still more preferably 0.05%. The upper limit of the content of vanadium (V) is more preferably 0.22%, still more preferably 0.20%, still more preferably 0.18%.
Cobalt (Co): 0.01-0.2%
Cobalt (Co) is an element necessary to promote formation of uniform pearlite and to increase the degree of orientation of pearlite to ensure uniform pearlite (fibrous pearlite) after cold rolling, and the content of cobalt (Co) may be 0.01% or more. On the other hand, when the content of cobalt (Co) is too large, hardenability is lowered, so that a faster cooling rate is required, and thus there is a risk of lowering heat treatability. Therefore, the upper limit of the content of cobalt (Co) can be limited to 0.2%. The lower limit of the content of cobalt (Co) is more preferably 0.02%, still more preferably 0.03%, and still more preferably 0.05%. The upper limit of the content of cobalt (Co) is more preferably 0.18%, still more preferably 0.16%, and still more preferably 0.15%.
In addition to the above composition, the steel sheet of the present invention may contain the balance of iron (Fe) and unavoidable impurities. Unavoidable impurities may be inadvertently mixed in during conventional manufacturing processes, and therefore cannot be removed. These impurities are well known to those skilled in the art of conventional steel making and therefore are not specifically described in this specification in their entirety.
The steel sheet according to one aspect of the present invention may have an a value of 1.2 or less in relation 1 below.
In the present invention, it is intended to prevent deterioration of bendability and excessive carbide formation due to segregation by the following relational expression 1. When excessive Mn, cr, and V are added, macro (macro) segregation and micro (micro) segregation are induced in the continuous casting process step, and a large amount of carbide is formed in the heat treatment process step, so that toughness and bendability of the final product may be reduced. Therefore, in the present invention, in order to prevent the above-described problem, the a value may be controlled to 1.2 or less. The lower limit of the A value may be the sum of the lower limits of the contents of the respective elements Mn, cr and V.
[ relation 1]
A＝[Mn]+[Cr]+[V]
(wherein [ Mn ], [ Cr ] and [ V ] are weight% of each element.)
The microstructure of the steel of the present invention will be described in detail below.
In the present invention, unless otherwise indicated, the% of the fraction representing the microstructure is based on the area.
The steel sheet according to one aspect of the present invention may have a microstructure including a pearlite structure as a main phase and a balance of 4 area% or less of grain boundary proeutectoid cementite. And, the pearlite may include, in terms of area% of itself, 40% or more of uniform pearlite (fibrous pearlite), 50% or less of saw-tooth pearlite (bent pearlite), and 10% or less of non-uniform pearlite, and the average thickness of the uniform pearlite may be 2.5 μm or less.
By the cold rolling, the sheet material having a pearlite structure before the cold rolling eventually has three forms of pearlite structures due to compression deformation in the thickness direction. The fibrous pearlite is elongated in a state where the layered structure is parallel to the rolling direction, and takes a form as shown in the central part of fig. 1. The zigzag pearlite (bent pearlite) is bent more than once in the vertical direction of rolling, so that the lamellar structure of the pearlite takes on a zigzag-zag shape, and the heterogeneous pearlite is bent, bent and broken at intervals of several micrometers (μm) after cold rolling, so that it is presented that it is difficult to clearly observe the form of fibrous pearlite or bent pearlite. The ratio of the final pearlite structure after cold rolling may vary depending on the component system and the production conditions.
In addition, when the fraction of the grain boundary proeutectoid cementite exceeds 4 area% in the entire microstructure, there may be a problem of brittle fracture due to the grain boundary proeutectoid cementite.
The present invention is characterized in that, in order to secure high strength and high toughness, when a pearlite structure before cold rolling is formed into uniform pearlite (fibrous pearlite), zigzag pearlite (bent pearlite) and nonuniform pearlite by cold rolling, the fraction of each pearlite formed is controlled at this time. Specifically, the present invention is characterized in that the pearlite comprises, in terms of area% of the pearlite itself, 40% or more of uniform pearlite (fibrous pearlite), 50% or less of saw-tooth pearlite (bent pearlite), and 10% or less of nonuniform pearlite after cold rolling. In order to secure the bendability for high toughness, it is preferable to include 40% or more of fibrous pearlite in terms of area% of itself, and in order to secure the desired physical properties of the present invention, it is preferable to limit the bent pearlite and the nonuniform pearlite to 50% or less and 10% or less, respectively. More preferably, more than 50% of uniform pearlite may be contained. In the present invention, the fraction of fibrous pearlite may be 100%, and the fraction of bent pearlite and the fraction of heterogeneous pearlite may be 0%, respectively. In the present invention, the fraction of the pearlite phase may be represented by calculating an average value of the microstructure fractions measured when any 10 to 15 positions are observed on a cross section in the thickness direction of the entire steel sheet, and the thickness of the fibrous pearlite phase may be represented by calculating the average value.
When the average thickness of the uniform pearlite exceeds 2.5 μm, coarse uniform pearlite is formed according to the same principle as that of the larger crystal grains and the lower strength, so that the strength of a desired level cannot be ensured, and brittleness increases, and thus the bendability cannot be ensured.
The method for manufacturing a steel sheet according to the present invention will be described in detail.
The steel sheet according to one aspect of the present invention may be manufactured by reheating, rolling, cooling, rolling, heat treating, and cold rolling a steel slab satisfying the above alloy composition.
Reheat of
The steel slab satisfying the alloy composition of the present invention may be reheated to a temperature in the range 1100-1300 c.
When the reheating temperature is less than 1100 ℃, it may be difficult to sufficiently secure the temperature of the slab required for the passage of the slab. On the other hand, when the reheating temperature exceeds 1300 ℃, abnormal austenite growth and excessive scale-induced surface defects may occur.
Rough rolling
The reheated steel slab may be rough rolled at a temperature in the range of 1000-1100 c.
When the rough rolling temperature is lower than 1000 ℃, the rolling load increases, and thus there may be a disadvantage in that the sheet passing property is deteriorated. On the other hand, when the rough rolling temperature exceeds 1100 ℃, excessive scale is formed, and thus, a disadvantage in that the surface quality becomes very poor may occur.
Finish rolling
The rough rolled steel sheet may be finish rolled at a temperature ranging from 860 to 940 c to obtain a hot rolled steel sheet.
When the finish rolling temperature is less than 860 ℃, the rolling load is excessive, and thus the hot rolling property may be greatly reduced. On the other hand, when the finish rolling temperature exceeds 940 ℃, austenite grain size becomes very coarse, and thus there is a risk of brittleness. In the present invention, the thickness of the finish rolled steel sheet may be 1.5 to 2.6mm. The upper limit of the thickness of the more preferable hot rolled steel sheet may be 2.5mm, and the lower limit of the thickness of the more preferable hot rolled steel sheet may be 1.6mm.
Cooling and winding
The hot rolled steel sheet may be cooled to a temperature range of 540-680 c at a cooling rate of 5-50 c/sec and then coiled.
When the cooling rate at the time of cooling is less than 5 ℃/sec, the pearlite structure becomes coarse, and thus there is a risk of brittleness. On the other hand, if the cooling rate at the time of cooling exceeds 50 ℃/sec, the rolling may become difficult due to the material deviation in the width direction caused by excessive cooling of the edge (edge) portion in the width direction.
When the winding temperature is lower than 540 ℃, a bainite structure or a martensite structure is formed as a low-temperature transformation structure, and thus it may be difficult to obtain a uniform hot rolled structure. In addition, the upper limit of the winding temperature may be limited to 680 ℃. However, surface defects may be caused by the formation of an internal oxide layer and a decarburized layer at the surface portion, and therefore, the upper limit of the winding temperature may be more preferably limited to 660 ℃ or less for this purpose.
After the winding, the invention can further comprise a process of pickling the hot rolled steel plate. The pickling may be performed after naturally cooling the rolled steel sheet to 200 ℃ or lower, and the scale formed on the surface of the steel sheet may be removed by the pickling.
Heat treatment of
The heat treatment may be performed in which the cooled and rolled steel sheet is heated to a temperature range of 850-1050 deg.c for 5-20 minutes, and then cooled to a temperature range of 500-650 deg.c at a cooling rate of 50-250 deg.c/sec for 30-180 seconds. More preferably, the upper limit of the cooling rate may be 150 ℃/sec, the lower limit of the more preferred cooling temperature range may be 520 ℃, and the upper limit of the more preferred cooling temperature range may be 590 ℃. The upper limit of the more preferred holding time may be 120 seconds.
When the heating temperature, that is, the austenitizing heating temperature is lower than 850 ℃, there is a possibility that brittleness is induced because insufficient austenitizing remains of undissolved carbide. On the other hand, when the heating temperature exceeds 1050 ℃, austenite grains become coarse, so toughness may be reduced, and work hardening ability of the pearlite structure is reduced, so it may be difficult to secure a uniform pearlite (fibrous pearlite) structure thereafter. The heating method in the present invention is not particularly limited, but a method such as high-frequency induction heating or a BOX type (BOX type) heating furnace may be used.
When the holding time after heating is less than 5 minutes, it may be difficult to completely austenitize, and when the holding time after heating exceeds 20 minutes, crystal grains may become excessively coarse.
When the cooling rate is less than 50 c/sec at the time of heating and post-holding cooling, the proportion of the grain boundary proeutectoid cementite excessively increases, so brittleness may be induced, and it may be difficult to form a uniform pearlite (fibrous pearlite) structure. In addition, the upper limit of the cooling rate may be limited to 250 ℃/sec. However, since the control of the cooling rate is not easy, there may be a risk of forming a low-temperature structure other than pearlite, and thus the upper limit of the more preferable cooling rate may be 150 ℃/sec.
The lower limit of the cooling termination temperature may be 500 ℃. However, in order to prevent the risk of forming a low-temperature structure such as bainite other than pearlite, the lower limit of the cooling termination temperature may be 520 ℃. In addition, the upper limit of the cooling termination temperature may be 650 ℃. However, in view of the fact that it may be difficult to form uniform pearlite (fibrous pearlite) after cold rolling due to the coarsening of the crystal grains of the structure, the upper limit of the more preferable cooling termination temperature may be 590 ℃.
When the holding time after cooling is less than 30 seconds, a pearlite structure may not be sufficiently formed, and the upper limit of the holding time after cooling may be limited to 180 seconds. Further, since it may be difficult to secure sufficient strength by work hardening after cold rolling due to the decrease in strength, the upper limit of the more preferable holding time may be 120 seconds. In the present invention, the heat treatment method may use hydrogen gas, a salt bath, a lead bath, or the like, and may not be particularly limited. In the present invention, the steel sheet may be air-cooled after the heat treatment.
In the present invention, the microstructure of the heat-treated steel sheet preferably contains pearlite structure as a main phase and grain boundary proeutectoid cementite in an amount of 4 area% or less of the balance. In this case, in the present invention, cementite having very high strength is minimized by properly controlling the fraction of the grain boundary proeutectoid cementite, so that pearlite can be easily elongated at the time of cold rolling, and a uniform pearlite thickness of 2.5 μm or less can be ensured. In addition, when the fraction of the grain boundary proeutectoid cementite exceeds 4 area%, there may be a problem of brittle fracture caused by the grain boundary proeutectoid cementite during cold rolling.
Cold rolling
The heat-treated steel sheet may be cold-rolled at a cumulative reduction of 75 to 96%. The lower limit of the more preferable cumulative reduction may be 80%, and the upper limit of the more preferable cumulative reduction may be 95%.
In the present invention, cold rolling may be performed by applying a predetermined reduction ratio in order to manufacture a cold-rolled steel sheet having a desired thickness. The lower limit of the reduction ratio may be limited to 75%. However, it may be difficult to secure a fraction of uniform pearlite (fibrous pearlite), and thus a lower limit of a more preferable rolling reduction may be 80%. On the other hand, when the reduction exceeds 96%, there may be a risk of cracking due to excessive work hardening. The lower limit of the more preferable reduction ratio may be 95%. The detailed rolling pass arrangement such as reduction rate, speed, and width dimension of each pass varies depending on the apparatus and use, and is therefore not particularly specified in the present invention. In the present invention, more preferably, the thickness of the cold rolled steel sheet may be 0.1 to 0.6mm. More preferably, the thickness of the cold-rolled steel sheet may be 0.3mm or less.
As described above, by such cold rolling, the pearlite structure as the main phase of the microstructure constituting the sheet material can eventually have three forms of pearlite structure due to compression deformation in the thickness direction.
Accordingly, the steel sheet of the present invention may have a microstructure including a pearlite structure as a main phase and a balance of 4 area% or less of grain boundary proeutectoid cementite, and the pearlite structure may be formed into a structure including 40% or more of uniform pearlite (fibrous pearlite), 50% or less of jagged pearlite (bent pearlite), and 10% or less of nonuniform pearlite in terms of its area% by the cold rolling.
The steel sheet of the present invention manufactured as described above may have a thickness of 0.1 to 0.6mm, a tensile strength of 2100MPa or more, an elongation of 2% or more, a bending property (R/t) of 3.0 or less (R is a bending radius at which no crack occurs at a bending portion after a 180 ° bending test, t is the thickness of the steel sheet), and may have high strength and excellent toughness properties. The upper limit of the thickness of the more preferable steel sheet may be 0.3mm. More preferred tensile strength values may be 2200MPa or more, and more preferred upper limits of tensile strength values may be 2350MPa.
Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted, however, that the following examples are only for illustrating the present invention in more detail and are not intended to limit the scope of the claims.
Detailed Description
Example (example)
Billets having the alloy compositions shown in table 1 below were heated for 2 hours to 1200 c, and then cold rolled steel sheets were manufactured under the conditions shown in table 2 below. At this time, the rough rolling temperature was 1080℃and the finish rolling temperature was 900 ℃. The cooling rate after hot rolling and before coiling was 20 ℃/sec, and coiling was performed under the coiling temperature conditions shown in table 2. The hot rolled steel sheet thus obtained was pickled, then heated at 950 ℃ for 10 minutes, then cooled at a cooling rate of 70 ℃/sec, and then cold rolled under the conditions of table 2.
TABLE 1
TABLE 2
The microstructure and physical properties of the produced steel sheet were measured and are shown in table 3 below. For the microstructure, it was observed and shown after heat treatment and after cold rolling, respectively. First, the area fraction of the grain boundary proeutectoid cementite of the heat-treated steel sheet was measured and shown by using an x 3000 times electron micrograph before cold rolling. In the microstructure of the steel sheet before cold rolling in table 3 below, all the fractions other than the grain boundary proeutectoid cementite contained pearlite. The cross section of the cold rolled steel sheet in the thickness direction was photographed several times by an electron microscope of x 4300 times, about 10 to 15 sheets were photographed, and the thickness length occupied by the microstructure was measured, and then the thickness was expressed in proportion, and the average value was expressed as a fraction of the microstructure. In addition, each thickness was measured for a uniform pearlite (fibrous pearlite) structure, and then the average value thereof is shown in table 3 below. At this time, the fractions of uniform pearlite, jagged pearlite, and non-uniform pearlite represent fractions relative to the total fraction of pearlite.
In addition, tensile test and bending test were performed on the manufactured cold rolled steel sheet, and physical properties and whether or not cracks were generated were shown. For the tensile test, the normal temperature tensile test was performed in accordance with JIS No. 5, and the tensile strength and elongation were measured and it was shown that, for whether or not cracks were generated, when R/t (R is a bending radius at which no crack is generated at a bending portion after 180 ° bending test, t is a thickness of a steel sheet) after 180 ° bending test was 3.0 or less, O was represented, and otherwise X was represented.
TABLE 3
As shown in table 3, in the case of the inventive examples satisfying the alloy composition and the manufacturing conditions of the present invention, the characteristics of the microstructure proposed in the present invention were satisfied, and the desired physical properties of the present invention were ensured.
Fig. 2 is a photograph showing a method of calculating the fraction of the microstructure and the thickness of uniform pearlite (fibrous pearlite) in inventive example 2. When a photograph of a microstructure is taken in the thickness direction of the steel sheet, uniform pearlite (fibrous pearlite) is characterized as a portion where the layered structure is not bent or segmented, as shown in fig. 2, and may be represented by a broken line. In addition, the zigzag pearlite (bent pearlite) is characterized in that the lamellar structure is bent more than once to be zigzag-shaped, and as shown by a solid line in fig. 2, it is possible to distinguish from uniform pearlite (fibrous pearlite) by mixing the zigzag shape and the wavy shape and measure the thickness. The portions other than the solid line and the broken line in fig. 2 represent non-uniform pearlite. After measuring the thickness of each microstructure, their sum may be calculated and expressed as a fraction, and the thickness of the uniform pearlite (fibrous pearlite) may be expressed as an average value of the measured thickness values.
On the other hand, comparative example 1 satisfies the alloy composition of the present invention, but the winding temperature is too low to form a low-temperature structure, so that the strength cannot be sufficiently ensured by work hardening at the time of cold rolling, and thus the tensile strength does not satisfy the level desired by the present invention.
Comparative example 2 satisfies the alloy composition of the present invention, but the winding temperature is too high to form a coarse pearlite structure, which hinders the formation of a uniform pearlite (fibrous pearlite) structure at the time of cold rolling, and thus the fraction of uniform pearlite (fibrous pearlite) does not satisfy the level desired by the present invention. As a result, the desired strength cannot be ensured.
Comparative example 3 satisfies the alloy composition of the present invention, but the heat treatment temperature is too low to form a part of a low temperature structure, and thus the tensile strength does not satisfy the level desired in the present invention.
Comparative example 4 satisfies the alloy composition of the present invention, but the heat treatment temperature is too high to form a coarse pearlite structure, and thus the fraction of uniform pearlite (fibrous pearlite) does not satisfy the level desired in the present invention. As a result, the strength is deteriorated.
In comparative example 5, the holding time after cooling in the heat treatment was not within the range of the present invention, and since the holding time was insufficient, sufficient uniform pearlite (fibrous pearlite) could not be formed, and as a result, the increase in strength due to work hardening in the cold rolling was insufficient.
In comparative example 6, the holding time after cooling during heat treatment exceeded the range of the present invention, and sufficient uniform pearlite (fibrous pearlite) could not be formed, and softening occurred after pearlite formation, and the strength was lowered, so that the strength at the level desired by the present invention was not satisfied.
In comparative example 7, the rolling reduction during cold rolling was not within the range of the present invention, and the desired fraction of uniform pearlite (fibrous pearlite) and tensile strength could not be ensured due to the low rolling reduction.
In comparative example 8, the cold rolling reduction was too high, and the tensile strength was not satisfied in the range desired in the present invention because the strength was excessively increased.
In comparative example 9, since coarse pearlite is formed when the C content does not fall within the range of the present invention, the fraction of uniform pearlite (fibrous pearlite) does not fall within the desired range, and the strength does not fall within the desired range of the present invention.
In comparative example 10, the Mn content was not within the range of the present invention, the fraction of uniform pearlite (fibrous pearlite) did not satisfy the level desired in the present invention, and it was difficult to secure the strength of the desired level.
In comparative example 11, the Mn content exceeded the range of the present invention, and the strength was excessively increased, so that the Mn content exceeded the desired range of the present invention.
The invention has been described in detail with reference to the examples, but other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims are not limited to the embodiments.
Claims (14)
1. A steel sheet comprising, in weight percent: carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): below 0.01% aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, the balance of iron (Fe) and other unavoidable impurities,
the steel sheet has a microstructure including a pearlite structure as a main phase and a grain boundary proeutectoid cementite in an amount of 4 area% or less in the balance,
the pearlite structure contains, in terms of its area%, 40% or more of uniform pearlite, i.e., fibrous pearlite, 50% or less of saw-tooth pearlite, i.e., bent pearlite, and 10% or less of non-uniform pearlite.
2. The steel sheet according to claim 1, wherein the average thickness of the uniform pearlite is 2.5 μm or less when a cross section of a microstructure is observed in a thickness direction of the steel sheet.
3. The steel sheet according to claim 1, wherein the steel sheet has an A value of 1.2 or less in relation 1,
[ relation 1]
A＝[Mn]+[Cr]+[V]
Wherein [ Mn ], [ Cr ] and [ V ] are weight% of each element.
4. The steel sheet according to claim 1, wherein the steel sheet has a tensile strength of 2100MPa or more, an elongation of 2% or more, and a bending property R/t of 3.0 or less, wherein R is a bending radius at which no crack occurs in a bending portion after a 180 ° bending test, and t is a thickness of the steel sheet.
5. The steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 2200-2350MPa.
6. The steel sheet according to claim 1, wherein the steel sheet has a thickness of 0.1-0.6mm.
7. A method of manufacturing a steel sheet, comprising the steps of:
reheating a steel billet comprising, in weight-%: carbon (C): 0.70-1.20%, manganese (Mn): 0.2-0.6%, silicon (Si): 0.01-0.4%, phosphorus (P): 0.005-0.02%, sulfur (S): below 0.01% aluminum (Al): 0.01-0.1%, chromium (Cr): 0.1-0.8%, vanadium (V): 0.02-0.25%, cobalt (Co): 0.01-0.2%, the balance of iron (Fe) and other unavoidable impurities;
rough rolling the reheated steel billet;
finish rolling the rough-rolled steel sheet to obtain a hot-rolled steel sheet;
cooling the hot rolled steel plate to a temperature range of 540-660 ℃ at a cooling rate of 5-50 ℃/sec, and then rolling;
a heat treatment step of heating the cooled and rolled steel sheet to a temperature range of 850-1050 ℃ for 5-20 minutes, then cooling to a temperature range of 520-590 ℃ at a cooling rate of 50-150 ℃/sec, and then maintaining for 30-120 seconds; and
the heat-treated steel sheet is cold-rolled at a cumulative rolling reduction of 80 to 96%.
8. The method for producing a steel sheet according to claim 7, wherein the steel slab has an A value of 1.2 or less in relation 1 below,
[ relation 1]
A＝[Mn]+[Cr]+[V]
Wherein [ Mn ], [ Cr ] and [ V ] are weight% of each element.
9. The method of manufacturing a steel sheet according to claim 7, wherein the reheating is performed at a temperature range of 1100-1300 ℃, the rough rolling is performed at a temperature range of 1000-1100 ℃, and the finish rolling is performed at a temperature range of 860-940 ℃.
10. The method of manufacturing a steel sheet according to claim 7, wherein after the rolling, further comprising a step of pickling the steel sheet at a temperature range of 200 ℃ or less.
11. The method of manufacturing a steel sheet according to claim 7, wherein after the heat treatment, further comprising the step of air-cooling the steel sheet.
12. The method for producing a steel sheet according to claim 7, wherein the microstructure of the heat-treated steel sheet contains a pearlite structure as a main phase and a grain boundary proeutectoid cementite of 4 area% or less of the balance.
13. The method of manufacturing a steel sheet according to claim 7, wherein the thickness of the finish rolled steel sheet is 1.5-2.6mm.
14. The method of manufacturing a steel sheet according to claim 7, wherein the thickness of the cold rolled steel sheet after cold rolling is 0.1 to 0.6mm.
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