CN109563599B - Super-thick steel material having excellent brittle crack growth resistance and method for producing same - Google Patents
Super-thick steel material having excellent brittle crack growth resistance and method for producing same Download PDFInfo
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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
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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- 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
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
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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
- C22C38/02—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
- C22C—ALLOYS
- 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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- 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
- C22C38/08—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
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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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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Abstract
The present invention provides a high-strength super-thick steel material having excellent brittle crack growth resistance, which comprises, in terms of weight%, 0.03 to 0.09% of C, 1.4 to 2.2% of Mn, 0.2 to 0.9% of Ni, 0.005 to 0.05% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.5% of Cu, 0.05 to 0.5% of Si, 0.01 to 0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance of Fe and other unavoidable impurities, wherein a surface layer portion is composed of a mixed phase of polygonal ferrite and bainite, a portion of a thickness of 1/2t to 1/4t (where t is the thickness of the steel material) is composed of 50% by volume or more of acicular ferrite and 50% by volume or less of bainite, and a fraction of a region having a bainitic single-phase structure in the entire thickness of the steel material is 20% or less.
Description
Technical Field
The present invention relates to an ultra-thick steel material having excellent brittle crack growth resistance and a method for producing the same. More particularly, the present invention relates to an ultra-thick steel material for a steel structure excellent in brittle crack growth resistance and productivity, and a method for manufacturing the same.
Background
Recently, ultra-thick high-strength steel is required for designing structures such as ships at home and abroad.
When designing a structure, if high-strength steel is used, the weight of the structure can be reduced, so that economic benefits can be obtained, and the plate thickness can be reduced, so that the ease of processing and welding operations can be ensured.
In general, when a high-strength steel is made into an ultra-thick steel material, the total rolling reduction is reduced, the entire structure is not sufficiently deformed, and the structure becomes coarse, and when rapid cooling for ensuring the strength is performed, the thickness is excessively thick, and a difference in cooling rate occurs between the surface layer portion and the central portion, so that the surface layer portion forms a coarse low-temperature transformation phase such as bainite, and it is difficult to ensure the toughness.
In particular, when a high-strength ultra-thick steel material is applied to a main structure of a ship or the like, there are increasing cases in which it is required to ensure brittle crack growth resistance representing the stability of the structure.
However, when a high-strength steel is made into a super-thick steel material as described above, if a low-temperature transformation phase is generated, a phenomenon in which the brittle crack growth resistance is remarkably decreased occurs, and thus it is difficult to improve the brittle crack growth resistance of the super-thick high-strength steel material.
In addition, when manufacturing a high-strength super-thick steel, since finish rolling is performed at a very low temperature in order to improve toughness, it is necessary to wait for a long time of air cooling from a high temperature after rough rolling until before finish rolling, which causes problems of coarsening of grain size and reduction in productivity.
When a high-strength super-thick steel material having a yield strength of 500MPa or more is produced, it is known that the grain size of the surface layer portion is made finer to improve the brittle crack growth resistance.
As a conventional technique for making the grain size of the surface layer finer as described above, there is known a technique of controlling the grain size by applying a bending stress during surface cooling or rolling in finish rolling.
However, although the above-mentioned conventional techniques contribute to the refinement of the structure of the surface layer portion, they cannot solve the problem of the reduction in impact toughness due to the coarsening of the remaining structure, and therefore cannot be used as a fundamental measure against the brittle crack growth resistance.
In addition, a technique is known in which an element contributing to improvement of toughness, such as Ni or the like, is added in a large amount to improve the brittle crack growth resistance.
However, when an element such as Ni is added in a large amount, although brittle crack growth resistance can be improved, since Ni is an expensive element, it is difficult to apply it commercially from the aspect of manufacturing cost.
Disclosure of Invention
Technical problem
One aspect of the present invention provides a high-strength super-thick steel material having excellent brittle crack growth resistance.
Another aspect of the present invention provides a method for manufacturing a high-strength super-thick steel material excellent in brittle crack growth resistance with high productivity.
Technical scheme
The present invention provides a super-thick steel material having excellent brittle crack growth resistance, which comprises, in terms of weight%, 0.03 to 0.09% of C, 1.4 to 2.2% of Mn, 0.2 to 0.9% of Ni, 0.005 to 0.05% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.5% of Cu, 0.05 to 0.5% of Si, 0.01 to 0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance of Fe and other unavoidable impurities, wherein a surface layer portion is composed of a mixed phase of polygonal ferrite and bainite, a portion of a thickness of 1/2t to 1/4t (where t is the thickness of the steel material) is composed of acicular ferrite of 50% by volume or more and bainite of 50% by volume or less, and a fraction of a region having a bainite single-phase structure in the entire thickness of the steel material is 20% or less.
The steel preferably has a central microstructure having large-angle grain boundaries, and the average grain size of the steel is 20 μm or less.
The steel material preferably has a yield strength of 500MPa or more.
The steel preferably has a central impact transition temperature of-40 ℃ or lower.
The steel material may preferably have a thickness of 50mm or more.
Another aspect of the present invention provides a method for manufacturing an ultra-thick steel material having excellent brittle crack growth resistance, including: reheating a steel slab at 1150-1000 ℃, the steel slab comprising, in weight%, 0.03-0.09% of C, 1.4-2.2% of Mn, 0.2-0.9% of Ni, 0.005-0.05% of Nb, 0.005-0.04% of Ti, 0.1-0.5% of Cu, 0.05-0.5% of Si, 0.01-0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance Fe and other unavoidable impurities;
a step of rough rolling the reheated slab at a temperature of 1150-900 ℃;
a step of cooling the roughly rolled Bar (Bar) by means of a cooling means;
a step of reheating the cooled bar to a temperature of Ac3 or higher, based on the surface of the bar;
carrying out finish rolling on the reheated bar at a temperature of more than Ar3 with 1/4t as a reference; and
cooling to a temperature of 600 ℃ or lower at a cooling rate of 3 ℃/s or higher after finish rolling,
the cooling of the rod is performed so that the surface layer portion of the rod has a temperature lower than Ac3 and the 1/4t (where t is the thickness of the rod) region has a temperature higher than the finish rolling start temperature by 50 ℃ or more.
Effects of the invention
According to the present invention, a high-strength super-thick steel material excellent in brittle crack growth resistance can be provided with high productivity.
Detailed Description
Preferred examples of the present invention are described in detail below.
According to one aspect of the present invention, there is provided a super-thick steel material having excellent brittle crack growth resistance, comprising, in weight%, 0.03 to 0.09% of C, 1.4 to 2.2% of Mn, 0.2 to 0.9% of Ni, 0.005 to 0.05% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.5% of Cu, 0.05 to 0.5% of Si, 0.01 to 0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance Fe and other unavoidable impurities, wherein a surface layer portion is composed of a mixed phase of polygonal ferrite and bainite, 1/2t to 1/4t (where t is the thickness of the steel material) portion is composed of acicular ferrite of 50% by volume or more and bainite of 50% by volume or less, and a fraction of a region having a bainite (bainitite) single-phase structure in the entire thickness of the steel material is 20% or less.
The composition and content of the steel material will be described below.
C: 0.03 to 0.09% (hereinafter, the content of each component is% by weight)
C is an important element for ensuring the basic strength in the present invention, and therefore, it is necessary to include C in an appropriate range in the steel. If the C content is more than 0.09%, the weld heat affected zone is promoted to generate a large amount of island-like martensite and low-temperature transformation phase, resulting in a decrease in toughness, and if the C content is less than 0.03%, resulting in a decrease in strength, the C content is limited to 0.03 to 0.09%. The C content is preferably limited to 0.04 to 0.09%, and more preferably limited to 0.05 to 0.08%.
Mn：1.4～2.2％
Mn is a useful element for improving strength and hardenability by solid-solution strengthening to form a low-temperature transformation phase, and Mn needs to be added in an amount of 1.4% or more to satisfy a strength of 500MPa or more. If more than 2.2% of Mn is added, hardenability is excessively increased to promote the formation of Upper bainite and martensite, thereby greatly reducing impact toughness and brittle crack growth resistance, so that the Mn content is limited to 1.4-2.2%. The Mn content is preferably limited to 1.5 to 2.1%, and more preferably may be limited to 1.6 to 2.0%.
Ni：0.2～0.9％
Ni is an important element for improving impact toughness by easily forming dislocation Cross slip (Cross slip) at low temperature and improving strength by improving hardenability, and in order to improve impact toughness and brittle crack growth resistance in high-strength steel having a yield strength of 500MPa or more, it is preferable to add 0.2% or more of Ni, and if the amount added is more than 0.9%, hardenability is excessively increased, resulting in a low-temperature transformation phase to be generated, toughness is lowered, and there is a problem of increase in production cost, so the upper limit of the Ni content is preferably limited to 0.9%. The Ni content is preferably limited to 0.3 to 0.9%, and more preferably limited to 0.4 to 0.8%.
Nb：0.005～0.05％
Nb improves hardenability and precipitates in the form of NbC or NbCN, thereby improving the base metal strength. In addition, Nb which is solid-dissolved at the time of high-temperature reheating is very finely precipitated as NbC at the time of rolling, and has an effect of suppressing recrystallization of austenite to refine the structure. Therefore, in order to obtain such an addition effect, Nb is preferably added at 0.005% or more, but when it is excessively added, brittle cracks may occur at the edge of the steel material, so the upper limit of the Nb content is limited to 0.05%. The Nb content is preferably limited to 0.01 to 0.04%, and more preferably limited to 0.015 to 0.03%.
Ti：0.005～0.04％
Ti is an element which is precipitated as TiN upon reheating to inhibit the grain growth of the base material and the weld heat affected zone and to greatly lower the low-temperature toughness, and Ti needs to be added in an amount of 0.005% or more in order to efficiently precipitate TiN. However, if the amount of Ti added is more than 0.04%, the Ti content is limited to 0.005 to 0.04% because excessive addition causes problems such as clogging of a continuous casting nozzle, crystallization of coarse TiN at the center part, or precipitation of coarse (TiNb), (C, N) and deterioration of toughness. The Ti content is preferably limited to 0.01 to 0.03%, and more preferably limited to 0.012 to 0.025%.
Cu：0.1～0.5％
Cu is an important element for improving hardenability, increasing steel strength by causing solid solution strengthening, and improving yield strength by the formation of ∈ — Cu precipitates when tempering (tempering) is employed, and in order to obtain such an addition effect, Cu is preferably added in an amount of 0.1% or more. However, when the amount is excessively added, slab cracking due to hot shortness (hot shortness) may occur in the steel making process, and therefore the upper limit of the Cu content is preferably limited to 0.5%. The Cu content is preferably limited to 0.1 to 0.4%, and more preferably limited to 0.2 to 0.4%.
Si：0.05～0.5％，Al：0.01～0.05％
Si and Al are alloy elements necessary for performing deoxidation operation by precipitating oxygen dissolved in molten steel as slag in steel making and continuous casting processes, and when a steel is produced by a converter, the Si content must be 0.05% or more and the Al content must be 0.01% or more. However, if it is excessively added, coarse Si and Al composite oxides may be formed or coarse island-like martensite may be formed in a large amount in the microstructure, so that Si is preferably added in an amount of 0.5% or less, and Al is preferably added in an amount of 0.05% or less.
P: 100ppm or less, S: less than 40ppm
P, S is an element causing brittleness at grain boundaries or forming coarse inclusions to cause brittleness, and the P content is preferably limited to 100ppm or less and the S content is preferably limited to 40ppm or less in order to improve the resistance to brittle crack growth.
The microstructure and properties of the steel are explained below.
The microstructure of the surface layer portion of the steel material of the present invention is composed of a mixed phase of polygonal ferrite and bainite, and the microstructure of a portion [ 1/2t to 1/4t (where t is the thickness of the steel material) ] from the central portion to 1/4t of the steel material is composed of acicular ferrite of 50% by volume or more and bainite of 50% by volume or less.
The surface layer portion of the steel material may be defined as, for example, a region from the surface to 10mm below the surface.
For example, the microstructure of the surface layer portion of the steel material is preferably composed of a mixed phase containing 70 to 90 vol% of polygonal ferrite and 10 to 30 vol% of bainite in a portion 2mm below the surface, and preferably composed of a mixed phase containing 20 to 30 vol% of polygonal ferrite and 70 to 80 vol% of bainite in a portion 10mm below the surface.
The fraction of a region having a bainitic (bainitite) single-phase structure in the entire thickness of the steel material is 20% or less.
In the present invention, the bar before finish rolling has an austenite structure, and the surface layer portion of the bar has a fine structure such as a bainite phase (phase), an acicular ferrite phase (acicular ferrite phase), or a fine austenite phase of a mixture thereof, which is inversely transformed by cooling and reheating the bar after rough rolling under appropriate conditions.
Since the air-cooled ferrite transformation temperature is increased by the refinement of austenite by reverse transformation of the surface layer portion as described above, at least a part of fine austenite is transformed into ferrite before the cooling process after the finish rolling, and austenite that is not transformed into ferrite is transformed into bainite by cooling.
Therefore, the microstructure of the surface layer portion of the steel material may have a mixed phase of ferrite and bainite.
By making the microstructure of the surface layer portion of the steel material consist of a mixed phase of ferrite and bainite in this manner, the fraction of the region having a bainite (bainite) single-phase structure in the entire thickness of the steel material can be made 20% or less.
If the fraction of the region having a bainite (bainite) single-phase structure in the entire thickness of the steel material is more than 20%, this results in a decrease in the brittle crack growth resistance.
The greater the C, Mn and Ni content, the greater the overall bainite fraction and hence the strength.
The steel preferably has a central microstructure having large-angle grain boundaries, and the average grain size of the steel is 20 μm or less.
If the grain size with large-angle grain boundaries is larger than 20 μm on average, it may result in a decrease in brittle crack growth resistance.
The steel material preferably has a yield strength of 500MPa or more.
The steel preferably has a central impact transition temperature of-40 ℃ or lower.
The steel material may preferably have a thickness of 50mm or more.
The method for producing a steel material of the present invention will be explained below.
The steel manufacturing method comprises the processes of slab reheating, rough rolling, Bar (Bar) cooling, backheating, final rolling and cooling.
Slab reheating temperature: 1150-1000 DEG C
The slab is reheated at a temperature of 1150-1000 ℃ before rough rolling.
The slab heating temperature is preferably 1000 ℃ or higher to solid-dissolve carbonitrides of Ti and/or Nb formed in casting.
More preferably, the heating is performed at a temperature of 1050 ℃ or higher so that the carbonitride of Ti and/or Nb is sufficiently solid-solved. However, since there is a possibility that austenite coarsens when the slab is reheated at an excessively high temperature, the slab reheating temperature is preferably 1150 ℃ or lower.
Rough rolling temperature: 1150-900 deg.C
The slab after reheating is subjected to rough rolling after heating in order to adjust its shape.
The rough rolling temperature is not lower than the temperature (Tnr) at which recrystallization of austenite stops. By performing rolling, the cast structure such as dendrites formed during casting can be destroyed, and the effect of reducing the grain size by recrystallization of coarse austenite can be obtained.
In order to obtain such an effect, the rough rolling temperature is preferably limited to 1150 to 900 ℃.
In order to induce sufficient recrystallization to refine the structure, the total cumulative reduction ratio during rough rolling is preferably 40% or more.
Cooling the Bar (Bar):
and (3) rapidly cooling the roughly rolled Bar (Bar) to be above the final rolling temperature by using a cooling means. Upon cooling, a fine structure is formed in the surface layer of the rod. For example, the surface layer of the rod material may be cooled to form a bainite phase, an acicular ferrite phase, or a mixture thereof.
The cooling completion temperature is preferably higher than the finish rolling start temperature by 50 ℃ or more based on 1/4t, and the cooling rate is preferably 0.5 ℃/s (sec) or more based on 1/4 t.
If the cooling end temperature is less than 50 ℃ higher than the finish rolling start temperature, the surface layer portion is not sufficiently reheated, and the fine structure formed in the surface layer portion during cooling, such as a bainite phase, a needle-like ferrite phase, or a mixture thereof, is not converted into austenite again, and there is a possibility that the toughness is lowered. Therefore, the cooling end temperature is preferably limited to a temperature higher than the finish rolling start temperature by 50 ℃ or more.
In addition, if the cooling end temperature is higher than the finish rolling start temperature by 100 ℃, the austenite grows to increase the grain size due to an excessively high temperature after the reheating, or a long waiting time is required due to a large temperature difference between the finish rolling after the reheating, and the productivity may be lowered. Therefore, the cooling end temperature is preferably limited to a temperature 100 ℃ or less higher than the finish rolling start temperature.
If the cooling rate is less than 0.5 ℃/s (sec) at 1/4t, the recrystallized austenite structure at the center of the bar may coarsen, and the grain size of the microstructure at the center of the finish rolled steel after cooling has a large angle grain boundary of more than 20 μm on average, so the cooling rate is preferably 0.5 ℃/s (sec) or more at 1/4t, more preferably 1 to 10 ℃/s (sec) at 1/4t, and still more preferably 2 to 5 ℃/s (sec).
As described above, by cooling the rod material, the recrystallized austenite structure is prevented from becoming coarse during the air cooling process, and the effect of making the microstructure fine is finally obtained.
Further, by preventing long air-cooling waiting from occurring before finish rolling, an effect of improving productivity can be obtained.
Heat regeneration: a surface layer part having a temperature of quasi Ac3 or higher
After rough rolling, the bar cooled by the cooling means is air-cooled for a certain period of time to reheat the temperature of the excessively cooled surface layer portion. In order to convert the microstructure, such as a bainite phase, a needle-like ferrite phase, or a mixture thereof, which is formed in the surface layer portion during cooling of the rod material, into austenite, i.e., reverse transformation, during the heat recovery, the temperature is preferably heated to a temperature at which the surface layer portion temperature is equal to or higher than Ac3, more preferably the surface layer portion temperature is Ac3 ℃ to Ac3+100 ℃, and still more preferably the surface layer portion temperature is Ac3+20 ℃ to Ac3+70 ℃.
The surface layer portion of the rod may be defined as, for example, a region from the surface to 10mm below the surface.
As described above, the fine structure of the surface layer portion of the rod material produced during the cooling, such as the bainite phase, the acicular ferrite phase, or the mixture thereof, is inverted into austenite with the heat recovery to produce fine surface layer portion austenite, whereby the air-cooled ferrite transformation temperature is increased, and the effect of reducing the production of the bainite single-phase structure in the steel material can be obtained.
The grain size of the austenite of reverse transformation in the fine structure such as a bainite phase, an acicular ferrite phase, or a mixture thereof may be 50 micrometers (μm) or less, for example.
The finishing temperature is as follows: 1/4t is quasi-Ar 3 or more
The bar after the rough rolling is subjected to finish rolling in a non-recrystallization region. The finishing temperature is not less than the ferrite generation temperature (Ar 3). If rolling is performed at a temperature lower than Ar3, a large amount of air-cooled ferrite is generated throughout the microstructure in the thickness direction, and it may be difficult to ensure a yield strength of 500MPa or more.
Cooling conditions after finish rolling: cooling to below 600 deg.C at a cooling rate of 3 deg.C/s or more, and finishing cooling
Since the air-cooled ferrite transformation temperature is increased by the refinement of austenite by the reverse transformation of the surface layer portion as described above, at least a part of fine austenite is transformed into ferrite before the cooling process after the finish rolling, and austenite that is not transformed into ferrite is transformed into bainite by cooling.
Therefore, the microstructure of the surface layer portion of the steel material may have a mixed phase of ferrite and bainite.
And cooling the steel after finish rolling to below 600 ℃ at a cooling rate of above 3 ℃/s.
When cooling is performed after finish rolling, if the cooling rate is lower than 3 ℃/s or the cooling is finished at a temperature higher than 600 ℃, an appropriate microstructure is not formed, and there is a possibility that the yield strength is less than 500 MPa.
The steel material may preferably have a thickness of 50mm or more.
The following steel materials can be manufactured by the manufacturing method: the surface microstructure of the steel material is composed of a mixed phase of polygonal ferrite and bainite, the portion from the center portion to 1/4t of the steel material is composed of 50% or more of acicular ferrite and 50% or less of bainite, and the fraction of a region having a bainite single-phase structure in the entire thickness of the steel material is 20% or less.
The steel preferably has a central microstructure having large-angle grain boundaries, and the average grain size of the steel is 20 μm or less.
The steel material preferably has a yield strength of 500MPa or more.
The steel preferably has a central impact transition temperature of-40 ℃ or lower.
Thus, a high-strength super-thick steel material having a yield strength of 500MPa or more and a central impact transition temperature of-40 ℃ or less can be provided, the microstructure of a product excellent in brittle crack growth resistance can be ensured by controlling the steel composition and the production conditions, and the waiting time for air cooling after rough rolling until finish rolling can be shortened by cooling and reheating of a bar material, thereby improving productivity and ensuring the miniaturization of grain size.
In particular, in the present invention, the Bar (Bar) after rough rolling is cooled by cooling means, so that the air cooling waiting time is reduced and the austenite growth is prevented, thereby improving the productivity, and the grain size of the microstructure having large angle grain boundaries in the central part of the steel material can be kept to 20 μm or less on average.
As described above, by controlling the steel composition and performing the cooling and reheating processes under appropriate conditions before the manufacturing process, particularly the finish rolling process after rough rolling, it is possible to provide a high-strength ultra-thick steel material excellent in brittle crack growth resistance in a high productivity manner.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
The present invention is described more specifically by examples below. It should be noted that the following examples are only for illustrating the present invention and are not intended to limit the scope of the present invention. For the scope of the present invention depends on the contents recited in the claims and those reasonably derived therefrom.
The invention is described below by way of examples.
(examples)
After reheating at 1070 c, a steel slab having a thickness of 400mm and having the composition of table 1 below was subjected to rough rolling at 1025 c, thereby manufacturing a bar. The same cumulative reduction rate was used for rough rolling, and the cumulative reduction rate was 50%.
The thickness of the bar after rough rolling is 200mm, the bar is cooled after rough rolling and then is subjected to heat regeneration, and the surface heat regeneration temperature in the following table 2 is the measured value of the surface temperature at the time point when the temperature difference between 1/4t and 1/2t is less than 20 ℃ based on the thickness of the bar after the bar is cooled. The cooling of the rod is performed so that the surface layer portion of the rod has a temperature lower than Ac3 and the 1/4t (where t is the thickness of the rod) region has a temperature higher than the finish rolling start temperature by 50 ℃ or more. In this case, the cooling rate of the rod material during cooling is 1 to 5 ℃/sec.
And immediately performing finish rolling after the heat recovery to obtain a steel plate with the thickness of the following table 2, and then cooling the steel plate to the temperature of 500-300 ℃ at a cooling speed of 3.5-5 ℃/s.
[ TABLE 1 ]
[ TABLE 2 ]
The results of the microstructure analysis and the results of the yield strength/center impact transition temperature for the steels manufactured as shown in tables 1 and 2 are shown in table 3 below.
Further, the steel material was evaluated by a Crack Arrest Test (CAT, Crack Arrest Test) at-10 ℃ using an ESSO apparatus, and whether or not Crack propagation (Propagate)/Crack Arrest (Arrest) was observed is shown in table 3.
The grain size at the center in table 3 below is a value measured by the EBSD method, and the grain boundary having a large angle of 15 degrees or more is calculated from the measurement result.
[ TABLE 3 ]
In table 3 above, PF represents Polygonal Ferrite (Polygonal Ferrite), P represents Pearlite (Pearlite), AF represents Acicular Ferrite (Acicular Ferrite), and B represents Bainite (Bainite).
As shown in table 3 above, in comparative examples 1 and 2, since the rod material has a heat regeneration temperature after cooling of Ac3 or less, the bainite structure of the surface layer portion formed when the rod material is cooled is not transformed into austenite again, leaving coarse bainite, and therefore the bainite single-phase structure region exceeds 20%, and cracks are not cracked but are propagated in the-10 ℃ crack arrest test exhibiting brittle crack growth resistance.
In comparative examples 1 and 2, since the temperature was returned to Ac3 or less, 100% transformation from bainite generated during cooling to austenite was not generated, and only a part of transformation was generated, and thus the grain size of austenite reversely transformed after returning to heat could not be measured.
Comparative steels 3 and 4, since they were not cooled by the bar proposed in the present invention, although the microstructure fraction fell within the range proposed in the present invention, the center portion average grain size reached 20 μm or more as the center portion microstructure became coarse during air cooling after rough rolling, so the center portion impact transition temperature was-40 ℃ or more, and cracks were not crack arrest but propagated in the-10 ℃ crack arrest test exhibiting brittle crack propagation resistance.
Comparative example 5 since having a value higher than the upper limit of C proposed in the present invention, excessive hardenability resulted in the formation of a large amount of bainite structure, the center portion impact transformation temperature was-40 ℃ or more, and cracks were not stopped but propagated in the-10 ℃ crack arrest test exhibiting brittle crack propagation resistance.
Comparative example 6 since having a value higher than the upper limit of Mn proposed in the present invention, excessive hardenability resulted in the formation of a large amount of bainite structure, the center portion impact transformation temperature was-40 ℃ or more, and cracks were not stopped but propagated in the-10 ℃ crack arrest test exhibiting brittle crack propagation resistance.
Comparative example 7 has a yield strength of 500MPa or less because of insufficient hardenability resulting in the generation of a large amount of polygonal ferrite and pearlite structures due to having a value lower than the lower limit of C, Mn proposed in the present invention.
Comparative example 8 since having a value higher than the upper limit of Ni proposed in the present invention, excessive hardenability resulted in the formation of a large amount of bainite structure, the center portion impact transformation temperature was-40 ℃ or more, and cracks were not stopped but propagated in the-10 ℃ crack arrest test exhibiting brittle crack propagation resistance.
Comparative example 9 since having a value higher than the upper limit of Ti, Nb proposed in the present invention, excessive hardenability resulted in the formation of a large amount of bainite structure and the precipitation of coarse TiN or (TiNb), (C, N), cracks were not crack arrested but propagated in the-10 ℃ crack arrest test exhibiting brittle crack growth resistance.
In contrast, in the invention examples 1 to 5, which satisfied the range of the components proposed in the present invention, the bar was cooled after rough rolling, and then the surface layer portion was reheated to a temperature higher than Ac3, the grain size of the center portion of the invention examples 1 to 5 was 20 μm or less, the bainite single-phase region in the entire thickness region of the steel material was 20% or less, and the microstructure of the surface layer portion, i.e., the center portion to 1/4t portion was composed of 50% or more of acicular ferrite and 50% or less of bainite. Therefore, the invention examples 1 to 5 had a yield strength of 500MPa or more and a central impact transformation temperature of-40 ℃ or less, and exhibited excellent brittle crack growth resistance to crack arrest in the-10 ℃ crack arrest test.
Claims (13)
1. An ultra-thick steel material excellent in brittle crack growth resistance, characterized in that:
the steel comprises, in weight%, 0.03 to 0.09% of C, 1.4 to 2.2% of Mn, 0.2 to 0.9% of Ni, 0.005 to 0.05% of Nb, 0.005 to 0.04% of Ti, 0.1 to 0.5% of Cu, 0.05 to 0.5% of Si, 0.01 to 0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance of Fe and other unavoidable impurities,
the surface layer part is composed of a mixed phase of polygonal ferrite and bainite,
the 1/2 t-1/4 t part of the thickness is composed of more than 50 volume percent of acicular ferrite and less than 50 volume percent of bainite, t is the thickness of the steel,
the fraction of the region having a bainite single-phase structure in the entire thickness of the steel material is 20% or less,
the surface layer portion is defined as a region from the surface to 10mm below the surface, and the microstructure of the surface layer portion is composed of a mixed phase containing 70-90 vol% of polygonal ferrite and 10-30 vol% of bainite if the microstructure is a portion 2mm below the surface, and is composed of a mixed phase containing 20-30 vol% of polygonal ferrite and 70-80 vol% of bainite if the microstructure is a portion 10mm below the surface.
2. The super thick steel material excellent in brittle crack growth resistance according to claim 1, characterized in that:
the grain size of the microstructure of the central part of the steel material, which has large-angle grain boundaries, is 20 microns or less on average.
3. The super thick steel material excellent in brittle crack growth resistance according to claim 1, characterized in that:
the yield strength of the steel is more than 500 MPa.
4. The super thick steel material excellent in brittle crack growth resistance according to claim 1, characterized in that:
the steel material has a central impact transition temperature of-40 ℃ or lower.
5. The super thick steel material excellent in brittle crack growth resistance according to claim 1, characterized in that:
the thickness of the steel is more than 50 mm.
6. A method for producing an ultra-thick steel material having excellent brittle crack growth resistance, the method comprising:
reheating a steel slab at 1150-1000 ℃, the steel slab comprising, in weight%, 0.03-0.09% of C, 1.4-2.2% of Mn, 0.2-0.9% of Ni, 0.005-0.05% of Nb, 0.005-0.04% of Ti, 0.1-0.5% of Cu, 0.05-0.5% of Si, 0.01-0.05% of Al, 100ppm or less of P, 40ppm or less of S, and the balance Fe and other unavoidable impurities;
a step of rough rolling the reheated slab at a temperature of 1150-900 ℃;
cooling the roughly rolled bar by using a cooling means;
a step of reheating the cooled bar to a temperature of Ac3 or higher, based on the surface of the bar;
carrying out finish rolling on the reheated bar at a temperature of more than Ar3 with 1/4t as a reference; and
cooling to a temperature of 600 ℃ or lower at a cooling rate of 3 ℃/s or higher after finish rolling,
the cooling of the rod is performed in such a manner that the surface layer portion of the rod has a temperature lower than Ac3 and the 1/4t region has a temperature higher than the finish rolling start temperature by 50 ℃ or more, where t is the thickness of the rod.
7. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 6, characterized in that:
the thickness of the steel is more than 50 mm.
8. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 6, characterized in that:
the surface temperature of the bar subjected to heat recovery in the heat recovery step is Ac3+ 20-Ac 3+70 ℃.
9. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 6, characterized in that:
in the step of cooling the rod material, a bainite phase, an acicular ferrite phase, or a mixed phase thereof is formed in a surface layer portion of the rod material.
10. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 9, characterized in that:
in the step of reheating the cooled rod material, the bainite phase, the acicular ferrite phase, or the mixed phase thereof in the surface layer portion is reversely transformed into austenite.
11. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 10, characterized in that:
the grain size of the reverse transformed austenite is 50 micrometers (μm) or less.
12. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 6, characterized in that:
and when the bar after rough rolling is cooled, the cooling speed is 1-10 ℃/s (second) based on 1/4 t.
13. The method for producing a super-thick steel material excellent in brittle crack growth resistance according to claim 6, characterized in that:
and when the bar after rough rolling is cooled, the cooling speed is 2-5 ℃/s (second) based on 1/4 t.
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| KR1020160100735A KR101819356B1 (en) | 2016-08-08 | 2016-08-08 | Ultra thick steel having superior brittle crack arrestability and method for manufacturing the steel |
| PCT/KR2017/008520 WO2018030737A1 (en) | 2016-08-08 | 2017-08-07 | Ultra-thick steel material having excellent resistance to brittle crack propagation and preparing method therefor |
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| KR102200222B1 (en) * | 2018-12-19 | 2021-01-08 | 주식회사 포스코 | High strength steel for a structure having excellent cold bendability and manufacturing method for the same |
| KR102200224B1 (en) * | 2018-12-19 | 2021-01-08 | 주식회사 포스코 | Steel for a structure having excellent resistance to brittle fracture and manufacturing method for the same |
| KR102255818B1 (en) * | 2019-06-24 | 2021-05-25 | 주식회사 포스코 | High strength steel for a structure having excellent corrosion resistance and manufacturing method for the same |
| KR102237486B1 (en) * | 2019-10-01 | 2021-04-08 | 주식회사 포스코 | High strength ultra thick steel plate having excellent very low temperature strain aging impact toughness at the center of thickness and method of manufacturing the same |
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| JP2019527773A (en) | 2019-10-03 |
| KR101819356B1 (en) | 2018-01-17 |
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| JP6868092B2 (en) | 2021-05-12 |
| CN109563599A (en) | 2019-04-02 |
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