KR20200142969A - Steel material and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a steel material which comprises: 0.03-0.10 wt% of carbon (C); 0.05-0.40 wt% of silicon (Si); 1.10-1.70 wt% of manganese (Mn); over 0 wt% and equal to or below 0.012 wt% of phosphorus (P); over 0 wt% and equal to or below 0.006 wt% of sulfur (S); 0.015-0.055 wt% of soluble aluminum (S_Al); 0.10-0.40 wt% of copper (Cu); 0.3-1.0 wt% of nickel (Ni); over 0 wt% and equal to or below 0.1 wt% of chrome (Cr); 0.01-0.15 wt% of molybdenum (Mo); 0.01-0.04 wt% of niobium (Nb); 0.007-0.025 wt% of titanium (Ti); 0.0005-0.002 wt% of boron (B); 0.006-0.010 wt% of nitrogen (N); and residual Fe and other unavoidable impurities. The present invention aims to provide the steel material and the method for manufacturing the same, which are able to be applied to large vessels and marine structures in various environments.

Description

Steel material and its manufacturing method {STEEL MATERIAL AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a steel material and a method of manufacturing the same, and more particularly, to a steel material having excellent resistance to welding cracking of an FCAW weld, a method of manufacturing the same, and a thick steel sheet manufactured using the same.

There is an increasing demand for steel materials that can be applied to large ships operating in cold environments and offshore structures operating in various fatigue environments. Existing steels can increase the fraction of brittle bainitic ferrite and granular bainite in the welding heat affected zone (HAZ) due to their high carbon equivalent, so ships and marines operating in polar regions requiring low heat input welding crack resistance. There is a problem that is not suitable for use in structures. In addition, there is a problem in that the influence of various alloying elements on the weld crack resistance of the FCAW weld is not considered because the existing steel is focused on reducing the yield ratio to improve the seismic resistance.

The related prior art is Korean Patent No. 1019887680000.

The technical problem to be achieved by the present invention is a steel material having excellent resistance to low heat input welding cracking and a manufacturing method thereof, specifically, a steel material that can be applied to large ships operating in cold environments and offshore structures operating in various fatigue environments, and a manufacturing method thereof. Is to provide.

The steel material according to an embodiment of the present invention for achieving the above object is carbon (C): 0.03 to 0.10 wt%, silicon (Si): 0.05 to 0.40 wt%, manganese (Mn): 1.10 to 1.70 wt%, phosphorus (P): more than 0 0.012% by weight, sulfur (S): more than 0 0.006% by weight, soluble aluminum (S_Al): 0.015 to 0.055% by weight, copper (Cu): 0.10 to 0.40% by weight, nickel (Ni) : 0.3 to 1.0 wt%, chromium (Cr): greater than 0 and 0.1 wt% or less, molybdenum (Mo): 0.01 to 0.15 wt%, niobium (Nb): 0.01 to 0.04 wt%, titanium (Ti): 0.007 to 0.025 wt. %, boron (B): 0.0005 ~ 0.002% by weight, nitrogen (N): 0.006 ~ 0.010% by weight and the rest of iron (Fe) and other inevitable impurities, but yield strength (YS): 500MPa or more and tensile strength (TS ): It is characterized in that 610 MPa or more.

In the steel material, a ratio of titanium (Ti) and nitrogen (N) may be 2.4 to 3.0.

The steel may have a carbon equivalent (Ceq) of 0.4 or less and a weld crack susceptibility index (Pcm) of 0.18 or less.

The steel material may include bainitic ferrite, granular ferrite, and acicular ferrite as a microstructure of the welding heat affected zone.

The steel material may have a hardness of less than or equal to 280 HV at room temperature without preheating.

A method of manufacturing a steel according to an embodiment of the present invention for achieving the above object is (a) carbon (C): 0.03 to 0.10 wt%, silicon (Si): 0.05 to 0.40 wt%, manganese (Mn): 1.10 ~ 1.70% by weight, phosphorus (P): more than 0 and less than 0.012% by weight, sulfur (S): more than 0 and less than 0.006% by weight, soluble aluminum (S_Al): 0.015 ~ 0.055% by weight, copper (Cu): 0.10 ~ 0.40 weight %, nickel (Ni): 0.3 to 1.0 wt%, chromium (Cr): more than 0 and 0.1 wt% or less, molybdenum (Mo): 0.01 to 0.15 wt%, niobium (Nb): 0.01 to 0.04 wt%, titanium (Ti ): 0.007 to 0.025% by weight, boron (B): 0.0005 to 0.002% by weight, nitrogen (N): 0.006 to 0.010% by weight, and the remaining iron (Fe) and other unavoidable impurities; (b) heating the steel material to 900 ~ 1200 ℃ before hot rolling; (c) hot rolling the steel material under the condition that the hot rolling end temperature is 750 to 900°C; And (d) cooling the hot-rolled steel material under conditions of a cooling end temperature of 400 to 600°C. Includes.

In the method of manufacturing the steel material, in step (d), the cooling start temperature may be 750°C or higher and the cooling rate may be 10 to 25°C/sec.

According to an embodiment of the present invention, a steel material that can be applied to a large ship operated in a cold environment and an offshore structure operated in various fatigue environments, and a method of manufacturing the same can be implemented. Of course, the scope of the present invention is not limited by these effects.

1 is a flow chart schematically showing a method of manufacturing a steel according to an embodiment of the present invention.
2 is a view showing the results of the highest hardness test of the heat-affected zone of the steel according to the experimental example of the present invention.
3 is a photograph of the microstructure of the test material of the highest heat affected zone of the steel according to the experimental example of the present invention.
4 is a view showing the results of a welding crack resistance test of a steel material according to an experimental example of the present invention.

A steel material according to an embodiment of the present invention and a manufacturing method thereof will be described in detail. Terms to be described later are terms appropriately selected in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout the present specification.

In order to compensate for the disadvantage of forming a weak structure in the weld zone due to the high carbon equivalent (Ceq), which is a problem of the prior art, this patent aims to reduce the fraction of bainitic ferrite and granular bainite by limiting the carbon content. The composition range is as follows.

Steel

Steel according to an embodiment of the present invention is carbon (C): 0.03 to 0.10 wt%, silicon (Si): 0.05 to 0.40 wt%, manganese (Mn): 1.10 to 1.70 wt%, phosphorus (P): greater than 0 0.012 wt% or less, sulfur (S): greater than 0 and 0.006 wt% or less, soluble aluminum (S_Al): 0.015 to 0.055 wt%, copper (Cu): 0.10 to 0.40 wt%, nickel (Ni): 0.3 to 1.0 wt% , Chromium (Cr): greater than 0 and less than 0.1 wt%, Molybdenum (Mo): 0.01 to 0.15 wt%, Niobium (Nb): 0.01 to 0.04 wt%, Titanium (Ti): 0.007 to 0.025 wt%, Boron (B) : 0.0005 to 0.002% by weight, nitrogen (N): 0.006 to 0.010% by weight and the rest of iron (Fe) and other inevitable impurities.

Hereinafter, the role and content of each component included in the steel according to an embodiment of the present invention will be described.

Carbon (C)

Carbon (C) is added to secure strength. The carbon (C) is preferably added in a content ratio of 0.03 to 0.10% by weight of the total weight of the steel according to the present invention, and more specifically, it may be added in a content ratio of 0.03 to 0.06% by weight of the total weight of the steel. If the content of carbon (C) is less than 0.03% by weight, it may be difficult to secure sufficient strength. Conversely, when the content of carbon (C) exceeds 0.10% by weight, it may cause a decrease in toughness.

Silicon (Si)

Silicon (Si) acts as a deoxidizer in steel and contributes to securing strength. The silicon (Si) is preferably added in a content ratio of 0.05 to 0.40% by weight of the total weight of the steel according to the present invention, and more specifically, may be added in a content ratio of 0.05 to 0.02% by weight of the total weight of the steel material. When the content of silicon (Si) is less than 0.05% by weight, the additive effect cannot be properly exhibited. Conversely, when the content of silicon (Si) exceeds 0.40% by weight, there is a problem that toughness and weldability of the steel material are deteriorated.

Manganese (Mn)

Manganese (Mn) is an element useful for improving strength without deteriorating toughness. The manganese (Mn) is preferably added in a content ratio of 1.10 to 1.70% by weight of the total weight of the steel according to the present invention. If the content of manganese (Mn) is less than 1.10% by weight, the addition effect cannot be properly exhibited. Conversely, when the content of manganese (Mn) exceeds 1.70% by weight, there is a problem of increasing the sensitivity to temper embrittlement.

Phosphorus (P)

Phosphorus (P) is added to inhibit cementite formation and increase strength. However, phosphorus (P) may deteriorate weldability and cause final material deviation due to slab center segregation. Therefore, in the present invention, the content of phosphorus (P) is limited to a content ratio of 0.012% by weight or less of the total weight of the steel.

Sulfur (S)

Sulfur (S) impairs the toughness and weldability of the steel. In particular, the sulfur (S) deteriorates resistance to stress corrosion cracking by combining with manganese (Mn) to form MnS non-metallic inclusions, thereby causing cracks during processing of the steel. Therefore, in the present invention, the content of sulfur (S) is limited to a content ratio of 0.006% by weight or less of the total weight of the steel.

Soluble Aluminum (S_Al)

Soluble aluminum (S_Al) acts as a deoxidizing agent to remove oxygen from the steel. The soluble aluminum (S_Al) is preferably added in a content ratio of 0.015 to 0.055% by weight of the total weight of the steel according to the present invention. When the content of soluble aluminum (S_Al) is added in an amount of less than 0.015% by weight, the above deoxidation effect cannot be properly exhibited. On the contrary, when the content of soluble aluminum (S_Al) exceeds 0.055% by weight, it is difficult to play and thus productivity may be reduced.

Copper (Cu)

Copper (Cu) plays a role of improving the strength by contributing to solid solution strengthening. The copper (Cu) is preferably added in a content ratio of 0.10 to 0.40% by weight of the total weight of the steel according to the present invention. When the content of copper (Cu) is less than 0.10% by weight, the additive effect cannot be properly exhibited. Conversely, when the content of copper (Cu) exceeds 0.40% by weight, there is a problem in that the hot workability of the steel material is deteriorated and the sensitivity to stress relief cracking after welding is increased.

Nickel (Ni)

Nickel (Ni) is effective in improving toughness while improving hardenability. The nickel (Ni) is preferably added in a content ratio of 0.3 to 1.0% by weight of the total weight of the steel according to the present invention, and more specifically, it may be added in a content ratio of 0.4 to 0.8% by weight of the total weight of the steel. When the content of nickel (Ni) is less than 0.3% by weight, the effect of addition cannot be properly exhibited. Conversely, when the content of nickel (Ni) exceeds 1.0% by weight, cold workability of the steel material is deteriorated. In addition, the addition of excessive nickel (Ni) greatly increases the manufacturing cost of steel materials.

Chrome(Cr)

Chromium (Cr) is a ferrite stabilizing element and contributes to strength improvement. In addition, chromium (Cr) plays a role in improving the slab surface quality by expanding the δ ferrite region and shifting the hypo-peritectic region to the high carbon side. However, when the content of chromium (Cr) exceeds 0.1% by weight and is excessively added, there is a problem that the toughness of the heat affected zone (HAZ) is deteriorated. Therefore, chromium (Cr) is preferably added in a content ratio of 0.1% by weight or less of the total weight of the steel according to the present invention.

Molybdenum (Mo)

Molybdenum is very effective in increasing the strength of a material, and is an element that simultaneously secures high strength and high toughness by promoting the formation of acicular ferrite, a low-temperature transformation structure. To this end, the molybdenum should be added in an amount of 0.01% by weight or more, but it is an expensive element, and if it exceeds 0.15% by weight, weldability decreases, so the content of Mo is preferably limited to 0.01 to 0.15% by weight.

Niobium (Nb)

Niobium (Nb) combines with carbon (C) and nitrogen (N) at high temperature to form carbides or nitrides. Niobium-based carbides or nitrides suppress grain growth during rolling to refine grains, thereby improving the strength and low-temperature toughness of steel materials. Niobium (Nb) is preferably added in a content ratio of 0.01 to 0.04% by weight of the total weight of the steel according to the present invention. When the content of niobium (Nb) is less than 0.01% by weight, the effect of addition cannot be exhibited properly. On the contrary, if the content of niobium (Nb) exceeds 0.04% by weight, the weldability of the steel is degraded, and the strength and low-temperature toughness are no longer improved, but the impact toughness is rather deteriorated as it exists in a solid solution in ferrite. have.

Titanium (Ti)

Titanium (Ti) has the effect of improving the toughness and strength of the hot-rolled steel by creating Ti (C, N) precipitates with high high temperature stability, thereby preventing the growth of austenite grains during welding and minimizing the structure of the welded portion. (Ti) is preferably added in a content ratio of 0.007 to 0.025% by weight of the total weight of the steel according to the present invention. If the content of titanium (Ti) is less than 0.007% by weight, there is a problem that age hardening occurs due to the remaining solid solution carbon and solid solution nitrogen without precipitation. Conversely, when the content of titanium (Ti) exceeds 0.025% by weight, there is a problem of reducing the low-temperature impact characteristics of the steel by generating coarse precipitates, and increasing the manufacturing cost without any further addition effect.

Boron (B)

Boron (B) is an element that is dissolved in steel to improve the hardenability of steel, thereby increasing the strength. In addition, when cooled after welding, it is dissolved in the austenite grain boundary to suppress the formation of grain boundary ferrite, and by combining with N to form nitride, austenite It plays a role of improving the toughness of the heat affected zone (HAZ) by suppressing the growth of night crystal grains. In order to obtain the above-described effect, the content of B needs to be added in an amount of 0.0005% by weight or more, and more specifically, 0.001% by weight or more, but if it is added too much, the hardenability is greatly increased and thus the welding heat affected zone (HAZ) Since the bainitic ferrite fraction of is increased and the weldability is lowered, and the toughness of the weld heat affected zone (HAZ) is deteriorated, it is preferable to limit the upper limit to 0.002% by weight.

Nitrogen (N)

Nitrogen (N) is a component contained in the steelmaking process, and is an important element that reacts with elements such as Ti, B, Al, and V to form nitride. To obtain this effect, the content of N is added in an amount of 0.006% by weight or more. Need to be. However, if the N content is too excessive, it may rather impair the toughness of the weld heat-affected zone, and may cause cracks on the surface during the playing process, so it is preferable to limit the upper limit to 0.010% by weight.

On the other hand, in this embodiment, by adding 0.007 to 0.025% by weight of the titanium (Ti) fraction and 0.006 to 0.010% by weight of the nitrogen (N) fraction to induce fine TiN precipitation in the HAZ portion, and as a result, the crystal grains of the HAZ portion can be refined. have. More specifically, the addition amount of Ti and N can be controlled so that the stoichiometry coupling ratio of Ti/N is 2.4 to 3.0.

As described above, the steel material according to an embodiment of the present invention having an alloying element composition may have a carbon equivalent (Ceq) of 0.4 or less and a weld crack susceptibility index (Pcm) of 0.18 or less. When the carbon equivalent (Ceq) and the weld crack susceptibility index (Pcm) are high, it is effective to improve the strength of the base material, but it is preferable to control the upper limit to 0.40 or less and 0.18 or less, respectively, since it lowers the weld crack resistance proposed in the present invention. Do.

The steel material according to an embodiment of the present invention having the above-described alloying element composition may have a yield strength (YS) of 500 MPa or more and a tensile strength (TS) of 610 MPa or more. The chemical composition proposed in the present invention has a significantly lower carbon equivalent (Ceq) compared to the chemical composition of conventional steel for ships and offshore structures having a yield strength of 500 MPa, which may cause a decrease in the strength of the base metal, so that boron (B) is 0.0005 to 0.002. It can be added in weight percent. When boron is added in an amount of 0.002% by weight or more, the hardenability is greatly increased and the bainitic ferrite fraction of the HAZ part is increased, so that excessive addition is not allowed.

Meanwhile, in the steel material, the hardness of the welding heat-affected portion may be 280 HV or less at room temperature without preheating. The steel material may include bainitic ferrite, granular ferrite, and acicular ferrite as a microstructure of the welding heat affected zone.

Hereinafter, a method of manufacturing a steel material according to an embodiment of the present invention having the above-described alloy element composition will be described.

Steel manufacturing method

1 is a flow chart schematically showing a method of manufacturing a steel according to an embodiment of the present invention.

Referring to Figure 1, a method of manufacturing a steel according to an embodiment of the present invention is (a) carbon (C): 0.03 to 0.10 wt%, silicon (Si): 0.05 to 0.40 wt%, manganese (Mn): 1.10 ~ 1.70% by weight, phosphorus (P): more than 0 and less than 0.012% by weight, sulfur (S): more than 0 and less than 0.006% by weight, soluble aluminum (S_Al): 0.015 ~ 0.055% by weight, copper (Cu): 0.10 ~ 0.40 weight %, nickel (Ni): 0.3 to 1.0 wt%, chromium (Cr): more than 0 and 0.1 wt% or less, molybdenum (Mo): 0.01 to 0.15 wt%, niobium (Nb): 0.01 to 0.04 wt%, titanium (Ti ): 0.007 to 0.025% by weight, boron (B): 0.0005 to 0.002% by weight, nitrogen (N): 0.006 to 0.010% by weight, and the remaining iron (Fe) and providing a steel made of other inevitable impurities (S100); (b) heating the steel material to 900 to 1200° C. before hot rolling (S200); (c) hot rolling the steel material under the condition that the hot rolling end temperature is 750 ~ 900°C (S300); And (d) cooling the hot-rolled steel material under conditions of a cooling end temperature of 400 to 600°C (S400). Includes. In the method of manufacturing the steel material, in step (d), the cooling start temperature may be 750°C or higher and the cooling rate may be 10 to 25°C/sec.

In order to manufacture the steel material proposed in this patent and the thick steel sheet using the same, the heating temperature before hot rolling is 900 ~ 1200℃, the hot rolling end temperature is 750 ~ 900℃, the cooling start temperature after hot rolling is over 750℃, and the cooling rate is 10 ~ 25℃ , Cooling end temperature 400 ~ 600℃ can be applied. If the hot rolling end temperature exceeds 900°C, the rate at which the austenite grains grow after recrystallization is high, and the final microstructure may change to a bainite-based structure, so the upper limit is set at 900°C. On the other hand, when the hot rolling end temperature is less than 750° C., rolling is terminated and cooled in the ideal region of austenite and cornerstone ferrite, resulting in non-uniformity of the final microstructure, which deteriorates the impact toughness of the base metal. Therefore, it starts cooling in the single phase area and its lower limit is set at 750℃. As for the cooling start temperature after the completion of hot rolling, if cooling is started in an ideal area, a coarse grainy ferrite and bainite mixed structure will result, resulting in deterioration of the impact toughness of the base material, so the lower limit is set at 750℃. The cooling rate is limited to 10 ~ 25 ℃ so that the final microstructure can secure the fraction of fine needle-shaped ferrite to 20% or more. When the cooling termination temperature exceeds 600°C, the bainite fraction is insufficient, and thus the yield strength of 500 MPa and the tensile strength of 610 MPa or more targeted by the present invention cannot be secured. Therefore, the upper limit is 600°C. On the other hand, if the cooling end temperature is less than 400℃, the lower limit of the surface and 1/4t portion of the bainite and MA phases (martensite and austenite phases) may be high and deteriorate the impact toughness of the base metal. do.

Experimental example

Hereinafter, a preferred experimental example is presented to aid the understanding of the present invention. However, the following experimental examples are only to aid understanding of the present invention, and the present invention is not limited by the following experimental examples.

1. Chemical composition and manufacturing conditions of specimen

In this experimental example, specimens having the alloying element composition (unit: wt%) of Table 1 are provided. Table 2 shows the hot rolling and cooling conditions in this experimental example.

division C Si Mn P S S_Al Cu Ni Example 1 0.056 0.124 1.463 0.0087 0.002 0.054 0.194 0.59 Comparative Example 1 0.0821 0.204 1.58 0.0087 0.0014 0.043 0.19 0.69 division Cr Mo Nb Ti B N Ceq Pcm Example 1 0.03 0.04 0.03 0.019 0.0013 0.0071 0.366 0.163 Comparative Example 1 0.07 0.05 0.032 0.018 - 0.0047 0.428 0.196

division Heating temperature before hot rolling Total reduction rate Total reduction rate of unresolved power Hot rolled
End temperature Cooling end temperature after hot rolling Cooling rate Example 1 1025℃ 84% 60% 850℃ 497℃ 17℃/s Comparative Example 1 1147℃ 87% 48% 754℃ 446℃ 14℃/s

Referring to Table 1, the composition of Example 1 of this experimental example is carbon (C): 0.03 to 0.10 wt%, silicon (Si): 0.05 to 0.40 wt%, manganese (Mn): 1.10 to 1.70 wt%, phosphorus ( P): more than 0 and 0.012% by weight or less, sulfur (S): more than 0 and not more than 0.006% by weight, soluble aluminum (S_Al): 0.015 to 0.055% by weight, copper (Cu): 0.10 to 0.40% by weight, nickel (Ni): 0.3 to 1.0 wt%, chromium (Cr): greater than 0 and 0.1 wt% or less, molybdenum (Mo): 0.01 to 0.15 wt%, niobium (Nb): 0.01 to 0.04 wt%, titanium (Ti): 0.007 to 0.025 wt% , Boron (B): 0.0005 ~ 0.002% by weight, nitrogen (N): 0.006 ~ 0.010% by weight and the rest of the composition range of iron (Fe) is satisfied, the carbon equivalent (Ceq) is 0.4 or less, and the weld crack susceptibility index (Pcm ) Satisfies the range of 0.18 or less. On the contrary, the composition of Comparative Example 1 of this Experimental Example does not satisfy the range of boron (B): 0.0005 to 0.002 wt%, has a carbon equivalent (Ceq) of 0.4 or less, and a weld crack susceptibility index (Pcm) of 0.18 or less. Not satisfied

2. Property evaluation

Table 3 shows the results of evaluating the mechanical properties of the base metal for specimens to which the composition and process conditions disclosed in Tables 1 and 2 were applied.

division Yield strength (MPa) Tensile strength (MPa) Total elongation (%) Shock absorption energy (-40℃) Example 1 589 661 16 249/253/276 Comparative Example 1 585 696 20 320/285/260

Referring to Table 3, the yield strength and tensile strength are significantly greater than the target degree of the present invention (yield strength (YS): 500 MPa or more and tensile strength (TS): 610 MPa or more) and are stable in both Example 1 and Comparative Example 1. It can be confirmed that it is secured. In the case of the elongation, it can be seen that the value of the elongation is slightly lower than that of Comparative Example 1 due to the increase in the hardenability due to the addition of B in Example 1, but the elongation target of the present invention, which is 14% or more, is sufficiently satisfied. In the case of impact toughness, the steel of Example 1 has a relatively low upper shelf energy compared to the steel of Comparative Example 1, but the deviation is small, so it can be inferred that the steel material of Example 1 has a uniform structure as a whole compared to Comparative Example 1.

2 is a view showing the results of the highest hardness test of the heat-affected zone of the steel according to the experimental example of the present invention. The hardness test of the highest heat affected zone was conducted in accordance with the standard of KS B 0893.

Referring to FIG. 2, as can be seen from the results of the highest heat affected zone hardness test, the steel of Example 1 shows a low maximum hardness of 280 HV or less even at room temperature without preheating, but the steel of Comparative Example 1 applies a preheat of 100°C. When the hardness was reduced to 280HV or less, it can be seen that the degree of embrittlement of the HAZ portion of the steel of Example 1 was lower.

3 is a photograph of the microstructure of the test material of the highest heat affected zone of the steel according to the experimental example of the present invention. Specifically, after the hardness test of the highest heat-affected zone in Example 1 and Comparative Example 1, the results of observation with a scanning electron microscope of the cross section are shown. Table 4 shows the microstructure in the FL (Fusion LIne) region of the highest heat affected zone test material of the steel according to the experimental example of the present invention, and Table 5 shows the FL+ of the highest heat affected zone test material of the steel according to the experimental example of the present invention. It shows the microstructure in the 5mm area. The FL+5mm area may mean an area within 5mm from the Fusion Line. In Tables 4 and 5, the BF item refers to Bainitic Ferrite, the GB item refers to Granular Ferrite, AF refers to Acicular Ferrite, and PAGS item PAGS refers to the prior austenite grain size (Prior Austenite Grain Size).

3 and Table 4 and Table 5, the steel material of Example 1 has a lower fraction of bainitic ferrite in the FL (Fusion LIne) and FL+5 mm regions compared to the steel material of Comparative Example 1, and PASGS (former austenite grains). You can see that the size of the size) is finer.

BF (%) GB (%) PAGS (μm) Comparative Example 1 80 20 110 Example 1 60 40 70

BF (%) GB (%) AF (%) PAGS (μm) Comparative Example 1 75 25 - 50 Example 1 20 75 5 25

4 is a view showing the results of a weld crack resistance test of a steel material according to an experimental example of the present invention. Table 6 shows the welding crack resistance test conditions.

Process Consumable Current
(A) Voltage
(V) Speed
(cm/min.) Heat Input
(kJ/cm) Note FCAW SC-81K2
(1.4Φ, HYUNDAI) 200 24 28 10.3 DIFFUSABLE [H] :3.74㎖/100g
(Just after open, '18.12)

Referring to FIG. 4, the results of welding crack resistance through the Y-Groove test method of the steel material of Example 1 are shown, and it can be expected that welding can proceed without preheating even at room temperature sufficiently with a crack generation rate of 0%.

In the above, the embodiments of the present invention have been described mainly, but various changes or modifications may be made at the level of those skilled in the art. Such changes and modifications can be said to belong to the present invention as long as they do not depart from the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims (7)

Carbon (C): 0.03 to 0.10% by weight, silicon (Si): 0.05 to 0.40% by weight, manganese (Mn): 1.10 to 1.70% by weight, phosphorus (P): more than 0 0.012% by weight or less, sulfur (S): More than 0 0.006% by weight or less, Soluble aluminum (S_Al): 0.015 to 0.055% by weight, copper (Cu): 0.10 to 0.40% by weight, nickel (Ni): 0.3 to 1.0% by weight, chromium (Cr): more than 0 0.1 weight % Or less, molybdenum (Mo): 0.01 to 0.15 wt%, niobium (Nb): 0.01 to 0.04 wt%, titanium (Ti): 0.007 to 0.025 wt%, boron (B): 0.0005 to 0.002 wt%, nitrogen (N ): 0.006 to 0.010% by weight and the rest of iron (Fe) and other inevitable impurities,
Yield strength (YS): 500 MPa or more and tensile strength (TS): 610 MPa or more,
Steel. The method of claim 1,
Characterized in that the ratio of titanium (Ti) and nitrogen (N) is 2.4 to 3.0,
Steel. The method of claim 1,
The steel has a carbon equivalent (Ceq) of 0.4 or less and a weld crack susceptibility index (Pcm) of 0.18 or less,
Steel. The method of claim 1,
The steel material is characterized in that it comprises bainitic ferrite (Bainitic Ferrite), granular ferrite (Granular Ferrite) and acicular ferrite (Acicular Ferrite) as a microstructure of the welding heat affected zone,
Steel. The method of claim 1,
The steel material is characterized in that the hardness of the welding heat-affected zone is 280 HV or less at room temperature without preheating,
Steel. (a) Carbon (C): 0.03 to 0.10% by weight, silicon (Si): 0.05 to 0.40% by weight, manganese (Mn): 1.10 to 1.70% by weight, phosphorus (P): more than 0 and not more than 0.012% by weight, sulfur ( S): greater than 0 and 0.006% by weight or less, soluble aluminum (S_Al): 0.015 to 0.055% by weight, copper (Cu): 0.10 to 0.40% by weight, nickel (Ni): 0.3 to 1.0% by weight, chromium (Cr): 0 More than 0.1% by weight, molybdenum (Mo): 0.01 to 0.15% by weight, niobium (Nb): 0.01 to 0.04% by weight, titanium (Ti): 0.007 to 0.025% by weight, boron (B): 0.0005 to 0.002% by weight, Nitrogen (N): providing a steel material consisting of 0.006 ~ 0.010% by weight and the remaining iron (Fe) and other inevitable impurities;
(b) heating the steel material to 900 to 1200° C. before hot rolling;
(c) hot rolling the steel material under the condition that the hot rolling end temperature is 750 to 900°C;
(d) cooling the hot-rolled steel material under conditions of a cooling end temperature of 400 to 600°C; Containing,
Steel manufacturing method. The method of claim 6,
The step (d) is characterized in that the cooling start temperature is 750 ℃ or higher and the cooling rate is 10 ~ 25 ℃ / sec,
Steel manufacturing method.

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