CN113166885B - High-strength steel material having excellent ductility and low-temperature toughness, and method for producing same - Google Patents
High-strength steel material having excellent ductility and low-temperature toughness, and method for producing same Download PDFInfo
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- CN113166885B CN113166885B CN201980077747.8A CN201980077747A CN113166885B CN 113166885 B CN113166885 B CN 113166885B CN 201980077747 A CN201980077747 A CN 201980077747A CN 113166885 B CN113166885 B CN 113166885B
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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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
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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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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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
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- 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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- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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Abstract
The present invention relates to a structural steel material suitable for ships, steel structures, and the like, and more particularly, to a high-strength steel material having excellent ductility and low-temperature toughness, and a method for manufacturing the same.
Description
Technical Field
The present invention relates to structural steel materials suitable for ships, steel structures, and the like, and more particularly, to high-strength steel materials having excellent ductility and low-temperature toughness, and a method for producing the same.
Background
A steel plate is broken by external impact such as collision on a ship, a steel structure, or the like, and thus accidents such as water immersion and sinking are caused. Further, in the production process of ships, steel structures, and the like, cracks may occur due to molding, and in this case, there are problems such as an increase in the construction period and an increase in the production cost.
In order to solve the above problems, it is necessary to improve the elongation while maintaining the strength of the steel sheet used for ships, steel structures, and the like at a desired level. The higher the elongation of the steel, even if the steel is deformed by external impact or the like, more deformation can be accommodated until fracture, so that the occurrence of fracture can be suppressed, and the effect of reducing the possibility of occurrence of cracks due to working can be obtained.
In general, strength of steel has an inverse relationship with elongation, and therefore, although there is a limitation in improving elongation while maintaining strength, the following techniques have been developed.
For example, patent document 1 discloses a steel sheet having excellent impact absorbability, in which ferrite is formed in 90% or more by controlling the average grain size of ferrite in the primary phase to 3 to 12 μm, and the average equivalent circle diameter of the secondary phase is made finer to 0.8 μm or less, thereby achieving a tensile strength of 490MPa or more and a uniform elongation of 15% or more.
Patent document 2 discloses a steel material in which a microstructure is composed of ferrite and a hard second phase, the volume fraction of the ferrite in the entire thickness of the plate being 75% or more, the hardness being Hv 140 or more and 160 or less, and the average crystal grain size being 2 μm or more, by applying a step composed of front end cooling, air cooling, and rear end cooling in the cooling process after rolling.
Patent document 3 discloses a thick steel sheet having a dual phase (dual phase) structure mainly composed of ferrite and pearlite for increasing the energy absorption capacity at the time of collision, wherein the hardness, fraction, average area, and average circumferential length of the phases satisfy predetermined conditions, and the average dislocation density of the ferrite is reduced to a predetermined value or less. Further, in order to obtain the above-described thick steel sheet, there is disclosed a process of heating a steel material to a high temperature higher than a normal reheating temperature, then performing controlled rolling, and performing air cooling or weak water cooling.
However, the above-described technique can be found to have several problems.
Specifically, in the fracture of the steel sheet, although the correlation of the total elongation (or fracture elongation) is larger than the uniform elongation, patent document 1 discloses only the uniform elongation, and therefore does not disclose the effect of substantially suppressing the fracture or the like due to external impact or the like. Patent document 2 similarly discloses only uniform elongation, and the total elongation and the like of the steel sheet disclosed in patent document 2 are not clear. On the other hand, patent document 3 describes the total elongation, but does not disclose any of them for ensuring toughness, which is very important as the characteristics of structural steel.
That is, the characteristics required for structural steels suitable for use in ships, steel structures, and the like are extremely important not only in strength and ductility (total elongation), but also in ensuring toughness, particularly low-temperature toughness, and development of structural steels that ensure all of these characteristics is urgently required.
(patent document 1) Korean laid-open patent publication No. 10-2006-0127762
(patent document 2) Korean laid-open patent publication No. 10-2016-0104077
(patent document 3) Japanese patent No. 5994819
Disclosure of Invention
Technical problem to be solved
One aspect of the present invention relates to providing a steel material suitable as a structural material, and aims to provide a steel material having high strength, excellent ductility, and excellent low-temperature toughness, and a method for manufacturing the same.
The subject of the present invention is not limited to the above. Any person skilled in the art to which the present invention pertains will have no difficulty in understanding additional objects of the present invention from the contents of the specification of the present invention.
Means for solving the problems
One aspect of the present invention provides a high-strength steel material having excellent ductility and low-temperature toughness, comprising, in weight%, carbon (C): 0.05 to 0.12%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.8%, phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, aluminum (Al): 0.01 to 0.06%, titanium (Ti): 0.005 to 0.02%, niobium (Nb): 0.01 to 0.03%, and nitrogen (N): 0.002 to 0.006%, nickel (Ni): less than 0.5%, the balance Fe and inevitable impurities,
the fine structure is a polygonal ferrite having an average grain size (equivalent circle diameter) of 2 to 8 μm as a main phase, and the secondary phase comprises pearlite and bainite and has a thickness of 8 to 15mm.
Another aspect of the present invention provides a method of manufacturing a high strength steel material having excellent ductility and low temperature toughness, including: heating a steel billet satisfying the alloy composition at a temperature of 1100-1200 ℃; a step of manufacturing a hot-rolled steel sheet by roughly rolling and finish rolling the heated slab; and a step of cooling the hot-rolled steel sheet; the finish rolling is performed at a temperature ranging from Ar3+70 ℃ to Ar3+170 ℃.
Effects of the invention
According to the present invention, a steel material having not only high strength and high ductility but also excellent low-temperature toughness can be provided.
In addition, the steel material of the present invention has an effect that it can be advantageously used as a steel material for structural use.
Detailed Description
Generally, if the strength of steel is increased, ductility is relatively decreased, and thus it is not easy to manufacture steel having high strength and excellent elongation. In addition, even if the elongation of steel is high, the low temperature toughness is not necessarily excellent, and it is more difficult to ensure excellent high strength and high ductility and low temperature toughness at the same time.
However, the present inventors have conducted extensive studies to develop a steel material capable of ensuring high strength, high ductility and low-temperature toughness at the same time, and as a result, have confirmed that a steel material having desired mechanical properties can be provided by examining alloy compositions and production conditions as shown below, thereby completing the present invention.
The present invention will be described in detail below.
The high-strength steel material excellent in ductility and low-temperature toughness according to one aspect of the present invention may include, in% by weight, carbon (C): 0.05 to 0.12%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.8%, phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, aluminum (Al): 0.01 to 0.06%, titanium (Ti): 0.005 to 0.02%, niobium (Nb): 0.01 to 0.03%, and nitrogen (N): 0.002 to 0.006%, nickel (Ni): less than 0.5 percent.
The reason for limiting the alloy composition of the steel material provided by the present invention as described above will be described in detail.
On the other hand, the content of each element is based on weight and the ratio of the structure is based on area, as long as not specifically mentioned in the present invention.
Carbon (C): 0.05 to 0.12 percent
Carbon (C) has an influence on the pearlite fraction in the steel structure, and is an element advantageous for securing strength. In order to secure the strength at the target level of the present invention, 0.05% or more may be included. In particular, in a series of steps (rolling and cooling steps) for producing the steel material of the present invention, it is preferable that C is contained by 0.05% or more. However, if the content exceeds 0.12%, the pearlite fraction in the steel structure becomes excessive, and the low-temperature toughness deteriorates.
Therefore, in the present invention, the C may be contained in an amount of 0.05 to 0.12%, and more advantageously, may be contained in an amount of 0.06 to 0.10%.
Silicon (Si): 0.2 to 0.5 percent
Silicon (Si) may be contained in an amount of 0.2% or more as an element contributing to the elasticity and the hardening ability of steel in order to secure a desired level of strength. However, if the content exceeds 0.5%, the strength excessively increases, and there is a problem that the total elongation and the low-temperature impact toughness are impaired.
Therefore, in the present invention, the Si may be contained in an amount of 0.2 to 0.5%.
Manganese (Mn): 1.2 to 1.8 percent
Manganese (Mn) is an element useful for improving strength without greatly decreasing the elongation of steel. In order to secure the strength of the level targeted by the present invention, mn may be contained by 1.2% or more, but if the content thereof exceeds 1.8%, the strength of the steel is greatly increased and it becomes difficult to secure ductility.
Therefore, in the present invention, the Mn may be contained in 1.2 to 1.8%, more advantageously, 1.4 to 1.7%.
Phosphorus (P): 0.012% or less
Phosphorus (P) as an impurity inevitably mixed in the steel reduces ductility and low-temperature impact toughness of the steel, and thus needs to be minimized. In the present invention, even if the content of P is 0.012% or less, it is not so disadvantageous to ensure desired physical properties, and therefore the upper limit of P may be limited to 0.012%. However, in consideration of the burden in the steel-making process, 0% may be excluded.
Sulfur (S): less than 0.005%
Sulfur (S) is an impurity which is inevitably mixed into steel like the P, forms sulfides to greatly reduce ductility, and thus needs to be minimized. In the present invention, even if the S content is 0.005% or less, it is not so disadvantageous to ensure desired physical properties, and therefore the upper limit of the S content may be limited to 0.005%. However, in consideration of the burden in the steel-making process, 0% may be excluded.
Aluminum (Al): 0.01 to 0.06 percent
Aluminum (Al) may be contained in an amount of 0.01% or more in order to ensure the purity of the steel as an element necessary for deoxidizing the steel. However, when the content thereof is excessive, there is a concern that toughness of the weld portion is impaired, so in consideration of this, it may be limited to 0.06% or less.
Titanium (Ti): 0.005 to 0.02 percent
Titanium (Ti) is an element that suppresses excessive austenite growth during heating in the steel production process and is advantageous for refining ferrite grains during austenite-ferrite transformation. In order to sufficiently obtain the above effect, ti may be contained at 0.005% or more, but if the content thereof exceeds 0.02%, coarse nitrides are formed, the effect of refining crystal grains is reduced, and impact toughness is also deteriorated.
Therefore, in the present invention, the Ti may be contained in an amount of 0.005 to 0.02%.
Niobium (Nb): 0.01 to 0.03 percent
Niobium (Nb) precipitates as carbonitride during rolling in the steel production process, is effective for refining austenite grains, and contributes to improvement of strength. In order to sufficiently obtain such an effect, nb may be added in an amount of 0.01% or more, but if the content exceeds 0.03%, the strength is excessively increased, and it becomes difficult to secure ductility, and there is a concern that toughness of the welded portion may be impaired.
Therefore, in the present invention, the Nb may be contained in a range of 0.01 to 0.03%.
Nitrogen (N): 0.002 to 0.006 percent
Nitrogen (N) bonds to the Ti, nb, and the like, suppresses the growth of austenite grains during heating of the steel, and forms fine carbonitrides during rolling, thereby contributing to obtaining a grain-refining effect. For this reason, N may be added in an amount of 0.002% or more, but if the content exceeds 0.006%, the surface quality of the cast product or steel product is impaired.
Therefore, in the present invention, the N may be contained in a range of 0.002 to 0.006%.
Nickel (Ni): less than 0.5% (including 0%)
Nickel (Ni) is an element that does not significantly reduce the elongation while refining ferrite grains and improving strength, similar to the Mn. By adding such a predetermined amount of Ni, the strength, ductility and low-temperature toughness desired in the present invention can be more advantageously ensured. However, if the content exceeds 0.5%, the elongation is lowered, resulting in an increase in manufacturing costs, and therefore the Ni may be contained by 0.5% or less.
In the present invention, even if Ni is not added, it is not disadvantageous in terms of securing physical properties, and therefore 0% may be used.
The remainder of the composition of the invention is iron (Fe). However, in the usual production process, undesirable impurities cannot be completely eliminated because they are inevitably mixed in from the raw materials or from the surrounding environment. Since the impurities are known to those skilled in the art of ordinary manufacturing processes, they are not specifically mentioned in the present specification.
The steel material of the present invention having the alloy composition may contain polygonal ferrite as a main phase and pearlite and bainite as a secondary phase as a fine structure.
When the fine structure of the steel material such as the present invention is ferrite single phase, the average grain size (grain diameter) of the ferrite needs to be very small in order to secure the strength of the present invention at the target level, and in this case, the uniform elongation of the steel is greatly reduced, and the desired total elongation cannot be achieved. Even when the microstructure is composed of a single phase of granular ferrite or bainite, it is difficult to ensure high ductility although the strength is excellent.
Further, even when ferrite is used as a main phase and a second phase is used as a hard phase (bainite or martensite), uniform elongation is excellent, but post-elongation representing ductility after necking (necking) is deteriorated, and it is difficult to secure total elongation.
Therefore, in the present invention, in order to ensure a balance between the strength and ductility of a steel material, a ferrite-pearlite complex structure is formed by the fine structure of the steel material, and the fraction of bainite that may be partially included in the steel material in the production process is minimized, thereby ensuring desired physical properties.
Particularly preferably, the pearlite in the second phase has an area fraction of 5 to 25% and the bainite area fraction is 2% or less (0% inclusive). Specifically, if the pearlite fraction is less than 5%, it is difficult to secure a desired level of strength, and if the fraction exceeds 25%, elongation is lowered, and desired toughness cannot be achieved. On the other hand, if the bainite fraction exceeds 2%, the post elongation is low, and it is difficult to ensure the total elongation desired in the present invention.
On the other hand, the smaller the average grain size (circle-equivalent diameter) of the polygonal ferrite, the more advantageous the improvement of the strength and low-temperature toughness of the steel, and conversely, the smaller the elongation, so it is necessary to appropriately control the average grain size of the polygonal ferrite.
The relationship between the average grain size of polygonal ferrite and the elongation is not linear, and when the average grain size of polygonal ferrite is less than 2 μm, the elongation tends to decrease sharply.
In the present invention, the average grain size of the polygonal ferrite is controlled to 2 to 8 μm, so that the balance between strength and ductility can be secured from an appropriate size reduction. If the average grain size of the polygonal ferrite is less than 2 μm, the uniform elongation is greatly reduced and it is difficult to secure the total elongation, whereas if the average grain size exceeds 8 μm, the fraction of pearlite needs to be increased in order to secure the strength of the target level, but the low temperature impact toughness is deteriorated.
More specifically, the steel material of the present invention having the above-mentioned fine structure has a yield strength of 355MPa or more, a tensile strength of 490MPa or more, and an elongation of 30% or more, and has impact toughness of 100J or more at-40 ℃, and therefore, not only strength and ductility but also excellent low-temperature toughness can be ensured.
The steel of the present invention may have a thickness of 8 to 15mm.
Next, a method for manufacturing a high-strength steel material having excellent ductility and low-temperature toughness according to another aspect of the present invention will be described in detail.
The high-strength steel material of the present invention can be produced by subjecting a steel slab having an alloy composition satisfying the requirements of the present invention to a series of steps of [ heating, hot rolling, and cooling ].
The respective process conditions will be described in detail below.
Heating of steel billets
In the present invention, the step of heating the billet and homogenizing it is preferable before hot rolling, and in this case, the heating step is preferably performed at 1100 to 1200 ℃.
If the heating temperature is less than 1100 ℃, the steel cannot be sufficiently homogenized, nb carbonitride and the like present in the center of the thickness of the billet cannot be sufficiently dissolved, and it is difficult to secure strength at a desired level. In contrast, if the temperature thereof exceeds 1200 ℃, the elongation and low-temperature toughness are lowered due to abnormal grain growth (abnormal grain growth) of austenite grains, and thus it is not recommended.
The heating time may be set differently depending on the thickness of the billet, and is preferably set so as to be sufficiently uniform from the billet surface portion to the thickness center portion. Generally, the billet can be heated for 1 minute or more per 1mm of thickness.
Hot rolling
The slab heated as described above is hot-rolled to produce a hot-rolled steel sheet, and in this case, 2 steps of rolling may be performed.
Specifically, the first rolling is rough rolling, which can be performed immediately after the heated slab is taken out from the heating furnace. The rough rolling includes widening rolling for securing the width of the final steel sheet, and may be rolled to a thickness at which finish rolling is started as a subsequent second rolling.
As described above, the second rolling is a finish rolling, and rolling can be performed so as to achieve a target thickness. In the present invention, the finish rolling is preferably performed at a temperature ranging from Ar3+70 ℃ to Ar3+170 ℃.
In general, as the temperature during finish rolling is lowered, the ferrite grain size of the final structure is reduced, and thus the strength and low-temperature toughness are improved, and conversely, the elongation is reduced.
Therefore, in order to simultaneously improve the strength, ductility and low-temperature toughness desired in the present invention, the finish rolling should be performed in an appropriate temperature range, but the temperature range is very narrow, and in this case, there is a problem that the industrial production of steel is difficult.
Accordingly, the present inventors have intensively studied the relationship between the alloy composition and the production process and found that the addition of Mn or Mn and Ni to the alloy composition as appropriate can expand the temperature range advantageous for ensuring the target physical properties in the finish rolling.
Specifically, the Mn and Ni lower the ferrite transformation temperature and induce ferrite grains to be finer, thereby improving the strength and low-temperature toughness without greatly impairing the elongation.
Thus, a steel material having excellent strength, ductility, and low-temperature toughness can be obtained by finish rolling at a temperature ranging from Ar3+70 ℃ to Ar3+170 ℃ within the Mn and Ni content range proposed in the present invention.
In the finish rolling, if the temperature is less than Ar3+70 ℃, the strength of the steel is rapidly increased and the elongation is greatly reduced, whereas if the temperature exceeds Ar3+170 ℃, the austenite is coarsened and the crystal grains of the ferrite, which is the final structure, are coarsened, thereby causing a problem that the strength and the low-temperature toughness are deteriorated.
Wherein Ar3 can be represented by the following empirical formula.
[ Ar3=910-310C-80Mn-20Cu-55Ni-15Cr-80Mo (each element means a weight content) ]
In the finish rolling in the above temperature range, the cumulative reduction ratio is preferably 60 to 90%. If the cumulative reduction ratio during finish rolling is less than 60%, the ferrite average grain size becomes coarse, and it becomes difficult to secure the strength at the target level, whereas if it exceeds 90%, the ferrite average grain size becomes too fine, which is advantageous for securing the strength, but the elongation is deteriorated.
Cooling down
The hot-rolled steel sheet produced by hot rolling as described above can be cooled, and in this case, cooling to room temperature by air cooling means cooling in the atmosphere.
If water cooling is applied during the cooling, ferrite is excessively refined, or the fraction of hard phases such as bainite as second phases increases, the probability of non-uniformity of cooling increases, and it becomes difficult to secure the post-elongation, and as a result, it becomes difficult to secure the total elongation.
The steel material of the present invention produced through the above-described series of production steps has a thickness of 8 to 15mm, and the desired microstructure of the present invention can be uniformly formed in any thickness within the above-described thickness range.
The present invention will be described in more detail with reference to examples. It should be noted, however, that the following examples are only illustrative for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters recited in the claims and reasonably derived therefrom.
Detailed Description
(examples)
After molten steels having alloy compositions shown in the following Table 1 were prepared, slabs having a thickness of 250mm were obtained by a continuous casting method. Then, steel sheets having a thickness of 8 to 15mm were finally manufactured by heating, rolling, and cooling under the conditions shown in table 2 below. At this time, the cooling is divided into air cooling and water cooling, and the cooling is performed at a cooling rate of about 20 ℃/s in the case of water cooling, and after the water cooling is finished at 650 ℃, the air cooling is performed to normal temperature.
[ TABLE 1 ]
[ TABLE 2 ]
In order to observe the microstructure of each steel sheet manufactured as described above, a test piece was sampled at a point t/4 of the thickness of each steel sheet (where t means the thickness (mm)) and ground, etched with a nitric acid etching solution, and then observed with an optical microscope. Then, the average grain size (circle-equivalent diameter), pearlite fraction, and bainite fraction of the polygonal ferrite were measured by an image analyzer (image analyzer) connected to an optical microscope, and the results thereof are shown in table 3 below. In this case, pearlite and bainite fractions were measured based on the area.
Further, the tensile test piece and the impact test piece were sampled at 1/4 of the width of each steel sheet, and the mechanical properties were evaluated, and the results are shown in table 3 below.
At this time, the tensile test piece was processed into a proportional test piece, the length of the test piece was set to the width direction of the steel sheet, the width of the test piece was set to 25mm, the thickness of the test piece was set to the thickness of the steel sheet, and the gauge length was set to 5.65 × √ (test piece width × test piece thickness), and the Yield Strength (YS), tensile Strength (TS), and total elongation (El) values were measured by a room temperature tensile test.
Further, in the impact test piece, the length of the test piece was set to the width direction of the steel sheet, and the test piece was processed into an ASTM E23 Type A standard test piece (however, a steel sheet having a thickness of 8mm was processed into a small-sized test piece (10 mm. Times.7.5 mm)), and then, the impact test was carried out at-40 ℃ to show the average value of the energy absorbed from 3 test pieces.
[ TABLE 3 ]
(in Table 3 above, the remainder except for the pearlite and bainite fractions is polygonal ferrite.)
As shown in tables 1 to 3 above, it was confirmed that all of invention examples 1 to 11 satisfying the alloy composition and the production conditions proposed by the present invention ensured the strength, ductility and low-temperature toughness at the target levels or higher.
On the other hand, comparative example 1, which had an excessively high C content in the alloy composition and an excessively high temperature during heating of the billet, had a high pearlite fraction, a large average ferrite grain size, and a difference in elongation and impact energy value. In comparative example 2 in which the C content in the alloy composition was insufficient, the pearlite fraction was low, and the strength at the target level could not be secured.
On the other hand, in comparative examples 3 to 5 in which water cooling was applied at the time of cooling after hot rolling, the bainite phase was excessively formed, and the strength was high, but the elongation rate was poor and was less than 30%. Among them, comparative example 4, in which the cumulative reduction ratio was insufficient in the finish rolling, was confirmed to have poor low-temperature toughness.
In comparative examples 6 and 7, the hot finish rolling temperature was out of the range of the present invention, and the ferrite grain size of comparative example 6 was very small and the strength was high, but the ductility was poor, whereas in comparative example 7, the ferrite grain size was large and the strength could not reach the target level.
In comparative example 8, the final steel sheet had a thickness of 23mm, and air cooling was applied after hot rolling, but the air cooling rate was relatively slow, and the strength at the target level could not be secured.
Claims (4)
1. A high-strength steel material having excellent ductility and low-temperature toughness,
comprises, in% by weight, carbon (C): 0.05 to 0.12%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.8%, phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, aluminum (Al): 0.01 to 0.06%, titanium (Ti): 0.005 to 0.02%, niobium (Nb): 0.01 to 0.03%, and nitrogen (N): 0.002 to 0.006%, nickel (Ni): less than 0.5%, the balance Fe and unavoidable impurities,
the fine structure is a polygonal ferrite having an average grain size of 2 to 8 μm expressed by an equivalent circle diameter as a main phase, and a pearlite and bainite as a secondary phase having a thickness of 8 to 15mm,
wherein the pearlite contained in the second phase has an area fraction of 5 to 25% and the bainite contained in the second phase has an area fraction of 2% or less and including 0%,
wherein the impact toughness of the steel at-40 ℃ is more than 100J.
2. The high-strength steel material excellent in ductility and low-temperature toughness as claimed in claim 1,
the steel has yield strength of 355MPa or more, tensile strength of 490MPa or more and elongation of 30% or more.
3. A method for producing a high-strength steel material having excellent ductility and low-temperature toughness, comprising:
comprising carbon (C) in weight%: 0.05 to 0.12%, silicon (Si): 0.2 to 0.5%, manganese (Mn): 1.2 to 1.8%, phosphorus (P): 0.012% or less, sulfur (S): 0.005% or less, aluminum (Al): 0.01 to 0.06%, titanium (Ti): 0.005 to 0.02%, niobium (Nb): 0.01 to 0.03%, and nitrogen (N): 0.002 to 0.006%, nickel (Ni): heating a billet containing not more than 0.5% of Fe and inevitable impurities in a temperature range of 1100 to 1200 ℃;
a step of producing a hot-rolled steel sheet by rough rolling and finish rolling the heated slab; and
a step of cooling the hot-rolled steel sheet;
the finish rolling is performed at a temperature ranging from 853 ℃ to Ar3+170 ℃, wherein the cooling is air cooling to normal temperature and has a thickness of 8-15 mm.
4. The method for producing a high-strength steel material excellent in ductility and low-temperature toughness according to claim 3, wherein,
the finish rolling is performed so that the cumulative reduction rate reaches 60 to 90%.
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| KR1020180150707A KR102164112B1 (en) | 2018-11-29 | 2018-11-29 | High-strength steel sheet having excellent ductility and low-temperature toughness and method for manufacturing thereof |
| KR10-2018-0150707 | 2018-11-29 | ||
| PCT/KR2019/016692 WO2020111856A2 (en) | 2018-11-29 | 2019-11-29 | High-strength steel sheet having excellent ductility and low-temperature toughness and method for manufacturing thereof |
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| CN113061811A (en) * | 2021-03-17 | 2021-07-02 | 攀钢集团江油长城特殊钢有限公司 | LNG (liquefied Natural gas) marine structural steel and preparation method thereof |
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| WO2020111856A2 (en) | 2020-06-04 |
| KR102164112B1 (en) | 2020-10-12 |
| KR20200064511A (en) | 2020-06-08 |
| EP3889296B1 (en) | 2023-10-18 |
| JP2022510212A (en) | 2022-01-26 |
| WO2020111856A3 (en) | 2020-08-13 |
| CN113166885A (en) | 2021-07-23 |
| EP3889296A2 (en) | 2021-10-06 |
| EP3889296C0 (en) | 2023-10-18 |
| JP7221475B6 (en) | 2023-02-28 |
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| JP7221475B2 (en) | 2023-02-14 |
| US20220042132A1 (en) | 2022-02-10 |
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