WO2017105109A1 - High-strength steel material having excellent low-temperature strain aging impact properties and method for manufacturing same - Google Patents

Abstract

The present invention relates to a steel material for pressure vessels, offshore structures and the like and, more specifically, to a high-strength steel material having excellent low-temperature strain aging impact properties and a method for manufacturing same, the high-strength steel material comprising 0.04-0.14 wt% of carbon (C), 0.05-0.60 wt% of silicon (Si), 0.6-1.8 wt% of manganese (Mn), 0.005-0.06 wt% of soluble aluminum (sol. Al), 0.005-0.05 wt% of niobium (Nb), 0.01 wt% or less (not including 0 wt%) of vanadium (V), 0.001-0.015 wt% of titanium (Ti), 0.01-0.4 wt% of copper (Cu), 0.01-0.6 wt% of nickel (Ni), 0.01-0.2 wt% of chromium (Cr), 0.001-0.3 wt% of molybdenum (Mo), 0.0002-0.0040 wt% of calcium (Ca), 0.001-0.006 wt% of nitrogen (N), 0.02 wt% or less of phosphorus (P) (not including 0 wt%), and 0.003 wt% or less of sulfur (S) (not including 0 wt%), with a balance of Fe and other inevitable impurities, and comprising a mixed structure of ferrite, pearlite, bainite and a martensite-austenite (MA) composite as a microstructure, wherein the fraction of the MA composite is 3.5% or less (not including 0%).

Description

High-strength steel with excellent low-temperature strain aging impact characteristics and its manufacturing method

The present invention relates to steel materials used as materials for pressure vessels, offshore structures, and more particularly, to high-strength steels having excellent low-temperature strain aging impact characteristics and a method of manufacturing the same.

Recently, due to the depletion of energy resources, mining areas are gradually moving to deep sea regions and extreme cold regions, and as a result, mining and storage facilities are becoming more complicated and complex. The steel used for this purpose is required to have excellent low temperature toughness in order to secure high strength and facility stability in order to reduce weight.

On the other hand, the cold deformation in the process of manufacturing a steel and other complex structures to secure the strength and toughness as described above is increasing significantly, the steel is required to minimize the reduction of toughness due to the deformation aging by cold deformation There is.

Mechanisms for reducing toughness by strain aging are as follows. The toughness of the steel measured by the Charpy impact test is explained by the correlation between the yield strength and the fracture strength at the test temperature. If the yield strength of the steel at the test temperature is greater than the fracture strength, the steel will be brittle without fracture. While the impact energy value is inferior, when the yield strength is less than the fracture strength, the steel is deformed to be ductile and hardened and absorbs the impact energy as it is hardened, but when the yield strength reaches the fracture strength, it is changed to brittle fracture. In other words, as the difference between the yield strength and the fracture strength increases, the amount of deformation of the steel to ductility increases, so that the impact energy absorbed increases. Therefore, when cold deformation of steel for the production of steel pipes or other complex structures, the yield strength of the steel increases as the deformation continues, resulting in a decrease in impact toughness due to a small difference between the fracture strength.

Thus, in order to prevent the deterioration of toughness due to cold deformation, conventionally, in order to suppress the increase in strength due to aging after deformation, the amount of carbon (C) or nitrogen (N) dissolved in steel is minimized or precipitated. Addition of elements (ex, titanium (Ti), vanadium (V), etc.) in a minimum amount or more, by applying SR (Stress Relief) heat treatment after cold deformation, the potential generated in the steel is reduced and increased by work hardening It is proposed to lower the yield strength, and to add elements (ex, nickel (Ni), etc.) to lower the stacking fault energy to increase the ductility of the steel at low temperatures, thereby facilitating the movement of dislocations. It is applied.

However, as structures and the like continue to grow in size and complexity, the amount of cold deformation required for steel materials increases, and the temperature of the use environment is also reduced to the level of the Arctic Ocean. There is a problem that it is difficult to effectively prevent degradation.

(Non-Patent Document 1) Effect of Ti on Deformation Aging of Low Carbon Steel Wires (Ochiaikuo, Obahiroshi, Iron and Steel 75th Year (1989) No. 4, P. 642 ~)

(Non-Patent Document 2) The effect of processing variables on the mechanical properties and strain ageing of high-strength low-alloy V and VN steels (VK Heikkinen and JD Boyd, CANADIAN METALLURGICAL QUARTERLY Volume 15 Number 3 (1976), P. 219 To)

One aspect of the present invention, as well as ensuring high strength and high toughness, can minimize the increase in strength due to cold deformation to provide a steel material and a method of manufacturing the same can be suitably applied as a material for pressure vessels, offshore structures, etc. It is.

One aspect of the present invention, in weight%, carbon (C): 0.04 ~ 0.14%, silicon (Si): 0.05 ~ 0.60%, manganese (Mn): 0.6 ~ 1.8%, soluble aluminum (Sol.Al): 0.005 ~ 0.06%, niobium (Nb): 0.005 to 0.05%, vanadium (V): 0.01% or less (excluding 0%), titanium (Ti): 0.001 to 0.015%, copper (Cu): 0.01 to 0.4%, nickel (Ni): 0.01-0.6%, Chromium (Cr): 0.01-0.2%, Molybdenum (Mo): 0.001-0.3%, Calcium (Ca): 0.0002-0.0040%, Nitrogen (N): 0.001-0.006%, Phosphorus (P): 0.02% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%), residual Fe and other unavoidable impurities,

High-strength steel material containing fine mixed ferrite, pearlite, bainite and MA (Martensite-Austenitic composite phase) as a microstructure, and having excellent low-temperature strain aging impact properties with a fraction of 3.5% or less (excluding 0%) of the MA phase. To provide.

Another aspect of the invention, the step of reheating the steel slab that satisfies the above-described component composition in the temperature range of 1080 ~ 1250 ℃; Manufacturing the hot-rolled steel sheet by rolling the reheated slab to a rolling end temperature of 780 ° C. or more; Cooling the hot rolled steel sheet by air cooling or water cooling; And it provides a method of manufacturing a high strength steel excellent in low temperature strain aging impact characteristics comprising the step of normalizing heat treatment in the temperature range of 850 ~ 960 ℃ after the cooling.

Advantageous Effects According to the present invention, it is possible to provide heat-treated steel which is not only excellent in deformation aging impact characteristics at low temperature but also provided with high strength at the same time, and the steel is suitable as a material for pressure vessels, offshore structures, etc., which are becoming larger and more complex. Can be applied.

1 is a graph showing the lower yield strength and tensile strength in the tensile curve of the steel according to an aspect of the present invention.

The inventors have continued to increase the cold deformation amount of steels used as materials for pressure vessels, offshore structures, etc., while deeply developing steels having high strength and high toughness while preventing toughness of steels from deformation aging. As a result of the study, it was confirmed that the steel material having a microstructure advantageous for securing the above-described physical properties can be provided from the optimization of the steel component composition and manufacturing conditions, and thus, the present invention has been completed.

In particular, the steel material of the present invention is to minimize the MA phase (martensite-austenite composite phase) in the range of securing the toughness of the steel by optimizing the content of the elements affecting the formation of the MA phase in the steel composition toughness by strain aging The fall can be effectively prevented.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In accordance with an aspect of the present invention, the high-strength steel having excellent low-temperature strain aging impact characteristics in weight%, carbon (C): 0.04 ~ 0.14%, silicon (Si): 0.05 ~ 0.60%, manganese (Mn): 0.6 ~ 1.8 %, Soluble Aluminum (Sol.Al): 0.005-0.06%, Niobium (Nb): 0.005-0.05%, Vanadium (V): 0.01% or less (excluding 0%), Titanium (Ti): 0.001-0.015%, Copper (Cu): 0.01-0.4%, Nickel (Ni): 0.01-0.6%, Chromium (Cr): 0.01-0.2%, Molybdenum (Mo): 0.001-0.3%, Calcium (Ca): 0.0002-0.0040%, Nitrogen (N): 0.001 to 0.006%, phosphorus (P): 0.02% or less (excluding 0%), sulfur (S): 0.003% or less (excluding 0%) preferably.

Hereinafter, the reason for controlling the alloy component of the high strength steel provided by the present invention as described above will be described in detail. At this time, unless otherwise specified, the content of each component means weight%.

C: 0.04-0.14%

Carbon (C) is an advantageous element for securing the strength of steel, and is a major element for securing tensile strength by being present as carbon and nitride in combination with pearlite, niobium (Nb), nitrogen (N), and the like. If the C content is less than 0.04%, the tensile strength on the matrix may be lowered, which is undesirable. On the other hand, if the content exceeds 0.14%, the pearlite may be excessively formed to deteriorate the strain aging impact characteristics at low temperatures. There is a risk of making.

Therefore, the content of C in the present invention is preferably limited to 0.04 ~ 0.14%.

Si: 0.05 ~ 0.60%

Silicon (Si) is an element added for the purpose of strengthening solid solution with deoxidation and desulfurization of steel, and is preferably added at 0.05% or more to secure yield strength and tensile strength. However, if the content exceeds 0.60%, the weldability and low temperature impact characteristics are deteriorated, and the steel surface is easily oxidized, so that an oxide film may be severely formed, which is not preferable.

Therefore, the content of Si in the present invention is preferably limited to 0.05 ~ 0.60%.

Mn: 0.6 ~ 1.8%

Manganese (Mn) is preferably added at least 0.6% because the strength increase effect by solid solution strengthening. However, when the content of Mn becomes excessive, segregation becomes severe at the center of the thickness direction of the steel sheet, and at the same time, it promotes the formation of MnS, which is a nonmetallic inclusion, with the segregated S. The MnS inclusions generated in the center portion are stretched by rolling, and as a result, there is a problem of significantly inhibiting low-temperature toughness and resistance to lamella tear, so it is preferable to control the Mn content to 1.8% or less.

Therefore, the content of Mn in the present invention is preferably limited to 0.6 ~ 1.8%.

Sol.Al: 0.005 ~ 0.06%

Soluble aluminum (Sol.Al) is used as a strong deoxidizer in the steelmaking process together with Si, and it is preferable to add at least 0.005% at the time of single or complex deoxidation. However, if the content exceeds 0.06%, the above-mentioned effect is saturated, and the fraction of Al ₂ O ₃ in the oxidative inclusions produced by the deoxidation of the deoxidation increases more than necessary and its size is coarse to remove during refining. It is not easy, which is undesirable since it will eventually greatly reduce the low temperature toughness.

Therefore, the content of Sol.Al in the present invention is preferably limited to 0.005 ~ 0.06%.

Nb: 0.005-0.05%

Niobium (Nb) is dissolved in austenite during slab reheating to increase the hardenability of austenite, and precipitates as fine carbon nitride (Nb, Ti) (C, N) during hot rolling to suppress recrystallization during rolling or cooling. The effect of finely forming the final microstructure is great. In addition, as the amount of Nb added increases the strength by promoting bainite or MA formation, but when the content exceeds 0.05%, excessive MA formation and coarse precipitates are formed in the center of the thickness direction. Since it becomes easy and there exists a problem to inhibit the low-temperature toughness of the center part of steel materials, it is unpreferable.

Therefore, the content of Nb in the present invention is preferably limited to 0.005 ~ 0.05%, more preferably 0.02% or more, even more advantageously limited to 0.022% or more.

V: 0.01% or less (except 0%)

Vanadium (V) is almost completely reused upon slab reheating, and thus hardly increases the strength due to precipitation or solid solution in the state after rolling and normalizing heat treatment. In addition, since V has a problem of causing a cost increase when a large amount of V is added as an expensive element, it is preferable to add V to 0.01% or less.

Ti: 0.001-0.015%

Titanium (Ti) exists as a hexagonal precipitate mainly in the form of TiN at high temperature, or forms carbon / nitride (Nb, Ti) (C, N) precipitates such as Nb to suppress grain growth of the weld heat affected zone. have. To this end, it is preferable to add Ti to 0.001% or more, but when the content is excessively greater than 0.015%, coarse TiN is formed at the center of the steel thickness direction, which acts as an initiation point of fracture cracking and thus the strain aging impact characteristics. There is a problem of greatly reducing the problem.

Therefore, the content of Ti in the present invention is preferably limited to 0.001 ~ 0.015%.

Cu: 0.01 ~ 0.4%

Copper (Cu) is an element that can greatly improve the strength by solid solution and precipitation, and does not significantly deteriorate the strain aging impact characteristics, but if excessively added, it causes cracks on the steel surface and is an expensive element. In consideration of this, it is preferable to limit the content to 0.01 ~ 0.4%.

Ni: 0.01 ~ 0.6%

Nickel (Ni) has little effect of increasing strength, but is effective in improving the strain aging impact characteristics at low temperatures, and particularly, in the case of adding Cu, suppresses surface cracks due to selective oxidation generated during reheating of the slab. To this end, it is preferable to add Ni to 0.01% or more, but it is preferable to limit the content to 0.6% or less in consideration of economical efficiency as an expensive element.

Cr: 0.01 ~ 0.2%

Chromium (Cr) has a small effect of increasing yield strength and tensile strength due to solid solution, but has an effect of preventing a drop in strength by slowing down the decomposition rate of cementite during tempering or heat treatment after welding. To this end, it is preferable to add Cr in an amount of 0.01% or more, but if the content exceeds 0.2%, not only the manufacturing cost increases but also the problem of inhibiting low temperature toughness of the weld heat affected zone is not preferable.

Mo: 0.001-0.3%

Molybdenum (Mo) has the effect of delaying transformation in the cooling process after heat treatment and consequently increasing the strength, and also effective in preventing the strength drop during heat treatment after tempering or welding like Cr, and grain boundary segregation of impurities such as P It is effective in preventing the fall of toughness by. For this purpose, it is preferable to add more than 0.001%, but it is also preferable to limit the content to 0.3% or less because it is economically disadvantageous when excessively added as an expensive element.

Ca: 0.0002-0.0040%

When calcium (Ca) is added after Al deoxidation, the MnS is combined with S present in MnS to suppress MnS formation, and at the same time, spherical CaS is formed to suppress cracking at the center of steel. Therefore, in order to sufficiently form S added with CaS in this invention, it is preferable to add in 0.0002% or more. However, if the content exceeds 0.0040%, CaS is formed and remaining Ca combines with O to produce coarse oxidative inclusions, which are stretched and fractured in rolling and act as crack initiation points.

Therefore, the content of Ca in the present invention is preferably limited to 0.0002 ~ 0.0040%.

N: 0.001-0.006%

Nitrogen (N) combines with added Nb, Ti, Al and the like to form a precipitate to refine the crystal grains of the steel to improve the strength and toughness of the base metal, but if the content is excessive, N remaining atoms after forming the precipitate It is known as the most representative element that exists in the state and causes aging after cold deformation, thereby reducing low-temperature toughness. In addition, there is a problem of promoting surface cracks due to embrittlement at high temperatures during slab production by continuous casting.

Therefore, in consideration of this, in the present invention, it is preferable to limit the content of N to 0.001 ~ 0.006%.

P: 0.02% or less (except 0%)

Phosphorus (P) has the effect of increasing the strength when added, but in the heat-treated steel of the present invention, it is preferable to manage as low as possible as it is an element that significantly deteriorates low-temperature toughness due to grain boundary segregation as compared to the strength increase effect. However, since excessive cost is required to remove excessively P in the steelmaking process, it is preferable to limit it to a range that does not affect physical properties, that is, 0.02% or less.

S: 0.003% or less (except 0%)

Sulfur (S) is a representative factor that inhibits low-temperature toughness mainly by forming MnS inclusions in the thickness direction of the steel sheet by combining with Mn. Therefore, in order to secure the strain aging impact characteristics at low temperature, it is preferable to manage the content of S as low as possible, but it takes considerable cost to remove such S excessively, so that it does not affect the physical properties, that is, 0.003 Preferably limited to% or less.

The remaining component of the present invention is iron (Fe). However, in the usual steel manufacturing process, impurities which are not intended from raw materials or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art of ordinary steel manufacturing, not all of them are specifically mentioned herein.

The high-strength steel of the present invention that satisfies the above-described alloy component composition preferably includes a mixed structure of ferrite, pearlite, bainite, and MA (martensite-austenite composite phase) as a microstructure.

Among the structures, ferrite is the most important structure that enables the soft deformation of the steel, and it is preferable to include such ferrite as a main phase and to finely control the average size to 15 μm or less. As such, by making the ferrite grains fine, it is possible to increase the grain boundary to suppress the propagation of cracks, improve the basic toughness of the steel, and minimize the increase in strength due to the effect of lowering the work hardening rate during cold deformation. The strain aging impact characteristics can also be improved at the same time.

Hard phases including the pearlite, bainite, MA, etc., except for the ferrite, are advantageous for increasing the tensile strength of the steel to secure high strength, but due to the high hardness, the hardening phase becomes a starting point of failure or a propagation path and thus has a deformation aging impact characteristic. There is a problem to inhibit. Therefore, it is desirable to control the fraction, and it is preferable to limit the sum of the fractions of the hard phases to 18% or less (excluding 0%).

In particular, since the MA phase has the highest strength and transforms into brittle martensite by deformation, it is the factor that most significantly inhibits low-temperature toughness. Therefore, it is preferable to limit the fraction of MA phase to 3.5% or less (except 0%), and more preferably to 1.0 to 3.5%.

On the other hand, the high-strength steel of the present invention having the microstructure as described above includes carbon and nitride produced by Nb, Ti, Al, etc. of the added elements, the carbon nitride is grain growth during the rolling, cooling, heat treatment process Suppresses and plays an important role to make finer. In order to maximize the effect, it is preferable to include carbon-nitride having an average size of 300 nm or less in a weight ratio of 0.01% or more, preferably 0.01 to 0.06%.

Hereinafter, a method of manufacturing a high strength steel having excellent low temperature strain aging impact characteristics, which is another aspect of the present invention, will be described in detail.

First, to produce a steel slab that satisfies the above-described alloy composition, and then to obtain a steel material that satisfies the target microstructure, carbide conditions, and the like in the present invention by using the hot rolling (control rolling), cooling and normalizing heat treatment process It is preferable to carry out.

Prior to this, it is preferable to go through the process of reheating the steel slab produced.

At this time, the reheating temperature is preferably controlled to 1080 ~ 1250 ℃, if the reheating temperature is less than 1080 ℃ it becomes difficult to re-use the carbide generated in the slab during the performance. Therefore, it is preferable to carry out above the temperature at which Nb added in the present invention can be re-used at least 50%. However, when the temperature exceeds 1250 ℃, the austenite grain size is too coarse, there is a problem that the mechanical properties such as strength and toughness of the final steel is greatly reduced.

Therefore, the reheating temperature in the present invention is preferably limited to 1080 ~ 1250 ℃.

It is preferable to finish-roll the reheated steel slab as described above to produce a hot rolled steel sheet. At this time, it is preferable that the finishing rolling process is controlled rolling, and it is preferable to control the rolling end temperature to 780 ° C or higher.

In the case of rolling in the usual rolling process, the end temperature of rolling is about 820 ~ 1000 ℃, but when it is lowered below 780 ℃, the hardenability is lowered in the region where Mn is not segregated during rolling, so that ferrite is formed during rolling. As is generated, C and solute are segregated into the residual austenite region and concentrated. Accordingly, the region where C or the like is concentrated during cooling after rolling is transformed into bainite, martensite, or MA phase to produce a strong layered structure composed of ferrite and hardened structure. The hardened structure of the layer where C and the like are concentrated has not only high hardness but also a large increase in the fraction of the MA phase. As described above, since the low-temperature toughness is reduced by increasing the hardened structure and the arrangement in the layered structure, it is preferable to control the rolling end temperature to 780 ° C or higher.

As described above, the hot rolled steel sheet obtained by control rolling may be cooled by air cooling or water cooling, and then normalized heat treatment at a predetermined temperature range may produce a steel material having target properties.

The normalizing heat treatment is preferably maintained in a temperature range of 850 ~ 960 ℃ for a predetermined time and then cooled in the air. If the normalizing heat treatment temperature is less than 850 ° C., re-use of cementite and MA in pearlite and bainite is difficult to re-use, so that the amount of dissolved C decreases, making it difficult to secure strength and finally remaining hardened phase remains coarse. Aging impact toughness is also greatly worsened. On the other hand, when the temperature exceeds 960 ℃ grain growth occurs there is a problem that inhibits the strain aging impact characteristics.

When the normalizing heat treatment is performed in the above temperature range, it is preferable to hold for {(1.3 × t) + (10 to 60)} minutes (where 't' means steel thickness (mm)). If it is less than this time, it becomes difficult to homogenize the tissue, and if it exceeds this time, there is a problem that productivity is inhibited.

The high-strength steel obtained as described above is not only excellent in strength and toughness, but also can effectively prevent a decrease in toughness due to strain aging during cold deformation. In particular, the yield ratio after heat treatment (YS (lower yield strength) / TS (tensile strength)) can be secured to 0.65 ~ 0.80.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only intended to illustrate 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 described in the claims and the matters reasonably inferred therefrom.

(Example)

The steel slab having the composition of the following Table 1 was subjected to reheating, hot rolling and normalizing heat treatment under the conditions shown in Table 2 below to produce a hot rolled steel sheet having a final thickness of 6 mm or more.

For each hot rolled steel sheet prepared, the microstructure fraction, size and carbon and nitride fraction and size were measured. In addition, the Charpy impact transition temperature was measured by aging at 250 ° C. for 1 hour after cold deformation 5% tensile strength, which can represent the strength (tensile strength and lower yield strength) and deformation aging impact characteristics of each hot rolled steel sheet. Shown in

The microstructure of each hot-rolled steel sheet is polished by mirror surface of steel sheet and then etched with Nital or Lepera according to the purpose, and a certain area of the specimen is 100 ~ 500 times magnified by optical or scanning electron microscope. After measuring the fraction of each phase from the measured image using an image analyzer (image analyzer). In order to obtain statistically significant values, the same specimens were repositioned and repeatedly measured, and their average values were obtained.

The fraction of fine carbon and nitride with an average size of 300 mm or less was measured by the extraction residue method.

Tensile characteristic values were measured from the nominal strain-nominal stress curves obtained by the normal tensile test, respectively, and the lower yield strength, tensile strength, and yield ratio (lower yield strength / tensile strength) were measured. , 5% and 8% were added in advance, and the stretched specimens were aged at 250 ° C. for 1 hour and then measured by Charpy V-notch impact test.

Steel grade	Ingredient composition (% by weight)
	C	Si	Mn	P	S	Sol.Al	Cu	Ni	Cr	Mo	Ti	Nb	V	N	Ca
One	0.067	0.36	1.54	0.008	0.0012	0.028	0.15	0.26	0.03	0.09	0.003	0.027	0.003	0.0028	0.0005
2	0.091	0.51	1.36	0.013	0.0018	0.035	0.02	0.03	0.12	0.04	0.001	0.035	0.001	0.0035	0.0012
3	0.031	0.35	1.63	0.007	0.0008	0.036	0.17	0.02	0.03	0.12	0.001	0.021	0.006	0.0036	0.0013
4	0.158	0.27	0.85	0.011	0.0021	0.022	0.04	0.13	0.05	0.07	0.002	0.012	0.003	0.0035	0.0018
5	0.103	0.45	1.45	0.015	0.0007	0.032	0.05	0.03	0.12	0.06	0.018	0.016	0.002	0.0022	0.0022
6	0.051	0.35	1.22	0.005	0.0013	0.021	0.11	0.02	0.03	0.05	0.001	0.002	0.006	0.0027	0.0012
7	0.075	0.49	1.34	0.005	0.0003	0.006	0.51	0.12	0.12	0.07	0.001	0.017	0.003	0.0039	0.0023

Steel grade	Product thickness (mm)	Reheating Temperature (℃)	Rolling end temperature (℃)	Normalizing Temperature (℃)	Normalizing time (min)	division
One	100.0	1165	994	912	149	Inventive Example 1
2	76.0	1181	928	906	120	Inventive Example 2
2	76.0	1181	912	906	127	Inventive Example 3
One	76.0	1222	762	923	45	Comparative Example 1
2	76.0	1023	890	913	125	Comparative Example 2
3	25.0	1181	935	909	87	Comparative Example 3
4	51.0	1146	990	893	94	Comparative Example 4
5	100.0	1175	954	918	150	Comparative Example 5
6	76.0	1175	891	908	110	Comparative Example 6
7	51.0	1175	945	919	84	Comparative Example 7

division	Microstructure					Mechanical properties
	F fraction (%)	F size (㎛)	Cured phase fraction (%)	MA fraction (%)	Carbon and nitride fraction (%)	Lower yield strength (MPa)	Tensile Strength (MPa)	Yield fee	5% strain aging DBTT temperature (℃)
Inventive Example 1	91.8	10.0	8.2	2.3	0.034	378	518	0.73	-61
Inventive Example 2	88.4	9.3	11.6	2.5	0.039	383	537	0.71	-59
Inventive Example 3	89.4	10.3	10.6	2.5	0.059	374	528	0.71	-54
Comparative Example 1	92.1	8.7	7.9	2.3	0.016	363	605	0.60	-34
Comparative Example 2	88.5	8.6	11.5	2.5	0.044	321	441	0.73	-51
Comparative Example 3	98.1	17.0	1.9	1.3	0.015	337	456	0.74	-77
Comparative Example 4	81.0	8.6	19.0	3.8	0.024	378	571	0.66	-32
Comparative Example 5	86.8	8.2	13.2	2.8	0.023	380	544	0.70	-28
Comparative Example 6	94.0	9.6	6.0	1.7	0.006	332	476	0.70	-66
Comparative Example 7	91.1	9.5	8.9	4.1	0.012	331	611	0.54	-31

('F fraction' in Table 3 means the ferrite fraction, 'F size' means the average size of the ferrite grains.

In addition, the said hardening phase fraction (%) is shown including the carbon-nitride fraction (%).)

As shown in Tables 1 to 3, the hot-rolled steel sheets of Inventive Examples 1 to 3 satisfying both the composition of the composition and the manufacturing conditions of the present invention are not only high strength, but also have excellent low-temperature toughness even after cold deformation, thereby increasing the size and complexity. Suitable for pressure vessels, offshore structures, etc.

On the other hand, although the steel composition satisfies the present invention, in the case of Comparative Example 1 in which the end temperature of rolling is too low during hot rolling after reheating, low-temperature toughness decreases by 5% due to the formation of a strong layered structure composed of ferrite and a hardened structure. The impact transition temperature after cold deformation was high as -34 ° C.

In addition, in Comparative Example 2 in which the reheating temperature was too low, the added Nb was not sufficiently reusable, and the reinforcement effect by phase transformation control or precipitation by Nb was remarkably low, so that the lower yield strength was less than 350 MPa and the tensile strength was less than 500 MPa.

On the other hand, Comparative Examples 3 to 7 is a case in which the manufacturing conditions satisfy the present invention, but the steel composition does not satisfy the present invention, it can be confirmed that the strength is low or the low-temperature toughness deteriorated.

Among these, Comparative Example 3 is a case where the content of C is not sufficient, ferrite grains are coarsened during rolling and heat treatment, and sufficient strength cannot be secured.

Comparative Example 4 is a case where the content of C is excessive, the hardening phase fraction is more than 18%, the yield ratio is lowered as the fraction of the MA phase is also significantly increased, and eventually the impact transition temperature after 5% cold deformation was high.

Comparative Example 5 is a case in which the content of Ti is excessive, in which excessively added Ti is formed as a coarse TiN precipitate compared to the added N, which acts as a starting point of the crack during impact after 5% cold deformation, thereby increasing the impact transition temperature. The result was obtained.

In Comparative Example 6, when the content of Nb was insufficient, strength retardation was inferior because the effect of reinforcing phase due to retardation of Nb was not expressed and the strength reinforcing effect was not generated.

Comparative Example 7 is a case in which the Cu content is excessive, such Cu increases the solubility of austenite C during cooling after normalizing heat treatment, thereby increasing the fraction of MA phase after the final transformation, thereby lowering the yield ratio and 5% cold The result was to increase the impact transition temperature after deformation.

Claims (10)

1. By weight%, carbon (C): 0.04 to 0.14%, silicon (Si): 0.05 to 0.60%, manganese (Mn): 0.6 to 1.8%, soluble aluminum (Sol.Al): 0.005 to 0.06%, niobium (Nb) ): 0.005 to 0.05%, vanadium (V): 0.01% or less (excluding 0%), titanium (Ti): 0.001 to 0.015%, copper (Cu): 0.01 to 0.4%, nickel (Ni): 0.01 to 0.6 %, Chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.001-0.3%, calcium (Ca): 0.0002-0.0040%, nitrogen (N): 0.001-0.006%, phosphorus (P): 0.02% or less (Excluding 0%), sulfur (S): 0.003% or less (excluding 0%), balance Fe and other unavoidable impurities,

   High-strength steel material containing fine mixed ferrite, pearlite, bainite and MA (Martensite-Austenitic composite phase) as a microstructure, and having excellent low-temperature strain aging impact properties with a fraction of 3.5% or less (excluding 0%) of the MA phase. .
2. The method of claim 1,

   The steel is a high-strength steel having excellent low-temperature strain aging impact properties comprising niobium (Nb) 0.02 ~ 0.05%.
3. The method of claim 1,

   The steel is a high-strength steel with excellent low-temperature strain aging impact characteristics of the total fraction of the phase except for ferrite is 18% or less (excluding 0%).
4. The method of claim 1,

   The steel is a high-strength steel excellent in low temperature strain aging impact characteristics of an average ferrite grain size of 15㎛ or less.
5. The method of claim 1,

   The steel is a high-strength steel, including carbon and nitride, and excellent in low-temperature strain aging impact characteristics containing 0.01% or more by weight ratio of carbon and nitride having an average size of 300nm or less.
6. The method of claim 1,

   The steel is a high strength steel with excellent low temperature strain aging impact characteristics yield ratio (YS (lower yield strength) / TS (tensile strength)) is 0.65 ~ 0.80.
7. By weight%, carbon (C): 0.04 to 0.14%, silicon (Si): 0.05 to 0.60%, manganese (Mn): 0.6 to 1.8%, soluble aluminum (Sol.Al): 0.005 to 0.06%, niobium (Nb) ): 0.005 to 0.05%, vanadium (V): 0.01% or less (excluding 0%), titanium (Ti): 0.001 to 0.015%, copper (Cu): 0.01 to 0.4%, nickel (Ni): 0.01 to 0.6 %, Chromium (Cr): 0.01-0.2%, molybdenum (Mo): 0.001-0.3%, calcium (Ca): 0.0002-0.0040%, nitrogen (N): 0.001-0.006%, phosphorus (P): 0.02% or less Reheating the steel slab comprising sulfur (S): 0.003% or less (excluding 0%), balance Fe and other unavoidable impurities in a temperature range of 1080-1250 ° C .;

   Manufacturing the hot-rolled steel sheet by rolling the reheated slab to a rolling end temperature of 780 ° C. or more;

   Cooling the hot rolled steel sheet by air cooling or water cooling; And

   Normalizing heat treatment of the hot-rolled steel sheet after the cooling in the temperature range of 850 ~ 960 ℃

   Method for producing a high strength steel excellent in low temperature strain aging impact characteristics comprising a.
8. The method of claim 7, wherein

   The steel slab is niobium (Nb) containing 0.02 ~ 0.05% of the high-strength strain aging impact characteristics of excellent strength of the steel material.
9. The method of claim 7, wherein

   The normalizing heat treatment is performed for {(1.3 x t) + (10 ~ 60)} minutes (where 't' means the thickness of the steel (mm)) is a method of producing a high strength steel having excellent low-temperature strain aging impact characteristics .
10. The method of claim 7, wherein

    The reheated steel slab is a method of producing a high strength steel excellent in low-temperature strain aging impact properties that 50% or more of the Nb is reusable.

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