KR101767771B1 - The steel sheet for welding structure having excellent heat affected zone toughness and method for manufacturing the same - Google Patents
The steel sheet for welding structure having excellent heat affected zone toughness and method for manufacturing the same Download PDFInfo
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- KR101767771B1 KR101767771B1 KR1020150183887A KR20150183887A KR101767771B1 KR 101767771 B1 KR101767771 B1 KR 101767771B1 KR 1020150183887 A KR1020150183887 A KR 1020150183887A KR 20150183887 A KR20150183887 A KR 20150183887A KR 101767771 B1 KR101767771 B1 KR 101767771B1
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 87
- 239000010959 steel Substances 0.000 title claims abstract description 87
- 238000003466 welding Methods 0.000 title claims description 18
- 238000000034 method Methods 0.000 title claims description 15
- 238000004519 manufacturing process Methods 0.000 title description 16
- 239000011572 manganese Substances 0.000 claims description 35
- 238000005096 rolling process Methods 0.000 claims description 30
- 239000010936 titanium Substances 0.000 claims description 28
- 238000001816 cooling Methods 0.000 claims description 24
- 229910000859 α-Fe Inorganic materials 0.000 claims description 21
- 229910001563 bainite Inorganic materials 0.000 claims description 16
- 229910052799 carbon Inorganic materials 0.000 claims description 11
- 229910052748 manganese Inorganic materials 0.000 claims description 11
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 10
- 229910052757 nitrogen Inorganic materials 0.000 claims description 8
- 230000009467 reduction Effects 0.000 claims description 8
- 229910052719 titanium Inorganic materials 0.000 claims description 8
- 239000012535 impurity Substances 0.000 claims description 7
- 230000001186 cumulative effect Effects 0.000 claims description 6
- 238000003303 reheating Methods 0.000 claims description 6
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 5
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 5
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 claims description 5
- 229910052710 silicon Inorganic materials 0.000 claims description 5
- 239000010703 silicon Substances 0.000 claims description 5
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 claims description 4
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 claims description 3
- 229910052717 sulfur Inorganic materials 0.000 claims description 3
- 239000011593 sulfur Substances 0.000 claims description 3
- 229910000746 Structural steel Inorganic materials 0.000 claims description 2
- 230000000052 comparative effect Effects 0.000 description 21
- 239000000203 mixture Substances 0.000 description 11
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 7
- 230000015572 biosynthetic process Effects 0.000 description 6
- 238000005098 hot rolling Methods 0.000 description 6
- 229910001566 austenite Inorganic materials 0.000 description 5
- 238000009628 steelmaking Methods 0.000 description 5
- 230000006872 improvement Effects 0.000 description 4
- 239000010955 niobium Substances 0.000 description 4
- 238000009749 continuous casting Methods 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000005336 cracking Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 238000001953 recrystallisation Methods 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 230000009466 transformation Effects 0.000 description 2
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 230000003749 cleanliness Effects 0.000 description 1
- 239000000470 constituent Substances 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000002542 deteriorative effect Effects 0.000 description 1
- 230000014509 gene expression Effects 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000009863 impact test Methods 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 230000005764 inhibitory process Effects 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 229910052698 phosphorus Inorganic materials 0.000 description 1
- 239000011574 phosphorus Substances 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000001556 precipitation Methods 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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Classifications
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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
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/42—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for step-by-step or planetary rolling
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B37/00—Control devices or methods specially adapted for metal-rolling mills or the work produced thereby
- B21B37/74—Temperature control, e.g. by cooling or heating the rolls or the product
-
- 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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- 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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- 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
-
- 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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- 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
Abstract
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for use in a ship, an offshore structure, an architectural structure, a line pipe, a pressure vessel, and the like.
Description
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for use in a ship, an offshore structure, an architectural structure, a line pipe, a pressure vessel, and the like.
Since the structural steel sheet used for constructing structures such as ships, offshore structures, and architectural structures is required to be welded during the manufacture of the above-described structure, it is necessary to secure the toughness in the weld heat affected zone It is important.
The weldability of the steel plate shows the resistance to low-temperature cracks mainly occurring after a certain period of time after welding, and can be improved by lowering the cracking sensitivity index Pcm of the steel plate.
On the other hand, the toughness of the welded heat affected zone is influenced by a more complicated mechanism than the aforementioned weldability, and techniques for securing the toughness of the welded heat affected zone have been proposed.
In particular, techniques for securing the toughness of a coarse-grained heat affected zone (CGHAZ), which is known to be the most vulnerable of the heat affected zone, have been proposed.
For example, Patent Document 1 proposes a method of distributing oxide particles widely in the steel using Ca and Mg so as to suppress the coarsening of austenite by welding heat, thereby securing toughness of the weld heat affected zone.
Patent Document 2 discloses a method for producing a high-strength steel sheet excellent in weld heat-affected portion toughness at low cost by increasing the Mn content to secure the weld heat-affected portion toughness by the formation of ferrite within the steel by MnS, , The TiN is appropriately precipitated to improve the toughness.
However, the above-mentioned Patent Document 1 does not know whether it is possible to secure excellent toughness even in the welding of small heat (low heat input) of about 5 kJ / mm as a technique suitable for large heat input welding of 20 kJ / mm or more, It is inevitable that a complicated steelmaking process must be performed to increase the production time and manufacturing cost.
Patent Document 2 relates to a method for securing toughness at an amount of heat input of about 10 kJ / mm or so, and it is not known whether it is possible to secure excellent toughness in small heat input (low heat input) and high heat input welding. Particularly, when the contents of C and Mn are high at the same time, segregation is promoted by the interaction of the two elements, and the upper bainite and MA structure are formed during welding, and the toughness may be lowered.
An aspect of the present invention is to provide a steel sheet for welded structure having excellent toughness of a welded heat affected zone, particularly CGHAZ, in a wide heat input range of 5 to 30 kJ / mm by optimization of steel component composition and manufacturing conditions, and a method for manufacturing the same .
An aspect of the present invention is a method for manufacturing a semiconductor device, comprising: 0.03 to 0.1% of carbon (C), 0.05 to 0.4% of silicon (Si), 1.0 to 2.0% of manganese (Mn) (N): 0.005 to 0.03%, titanium (Ti): 0.005 to 0.015%, nitrogen (N): 0.002 to 0.006%, the balance Fe and unavoidable impurities,
Wherein a content ratio (Mn / C) of Mn and C is 15 or more, a content ratio (Ti / N) of Ti and N satisfies 1 to 3,
Provided is a steel sheet for welded structure excellent in weld heat affected zone toughness having a sum of area percentages of upper bainite and intergranular ferrite in a coarse-grained heat affected zone (CGHAZ) during welding.
According to another aspect of the present invention, there is provided a method for manufacturing a steel slab, comprising the steps of: reheating a steel slab satisfying the above-described composition and composition relationship to 1000 to 1150 占 폚; Firstly rolling the reheated slab at a temperature of 900 캜 or higher; After the primary rolling, secondary rolling to a cumulative rolling reduction of 40% or more at a temperature of 900 ° C or less; And cooling the steel sheet at a cooling rate of 4 to 50 DEG C / s to 300 to 600 DEG C after starting the cooling at a temperature higher than Ar3 after the secondary rolling, thereby providing a method for manufacturing a steel sheet for welded structure do.
According to the present invention, it is possible to provide a steel sheet for welded structure excellent in weld heat resistance toughness in a wide heat input range of 5 to 30 kJ / mm.
FIG. 1 is a graph showing the results of CGHAZ impact toughness measurement according to an Mn / C content ratio according to an embodiment of the present invention, according to the heat input.
FIG. 2 shows CGHAZ microstructure according to an Mn / C content ratio according to an embodiment of the present invention (corresponding to inventive steels 3 and 5 and comparative steels 1 and 2).
Generally, the toughness of the weld heat affected zone decreases due to crystal grain coarsening, upper bainite formation, MA structure formation, grain boundary embrittlement, precipitation hardening, etc. In the case of welding with a low heat input of about 5 kJ / mm, Toughness is deteriorated. When welding is performed at a high heat input of about 30 kJ / mm, grain boundary ferrite is a main factor of toughness degradation.
Accordingly, the present inventors have intensively studied a method for securing excellent CGHAZ (coarse-grained heat affected zone) toughness in a wide heat input range of 5 to 30 kJ / mm.
Particularly, in addition to the upper bainite which causes the decrease in toughness when the low heat input is applied, a method of reducing the fraction of the intergranular ferrite which causes the decrease in toughness upon application of high heat input was searched.
As a result, it was confirmed that the control of the Mn and C content ratio (Mn / C) influences the above-mentioned structure fraction and that the inhibition of austenite growth before the formation of the final microstructure is advantageous for securing the toughness of the weld heat- The present invention has been completed.
Hereinafter, the present invention will be described in detail.
A steel sheet for welded structure excellent in weld heat-affected portion toughness according to one aspect of the present invention comprises 0.03 to 0.1% of carbon (C), 0.05 to 0.4% of silicon (Si), 1.0 to 0.4% of manganese (Mn) (Al): 0.005 to 0.03%, Nb: 0.005 to 0.03%, and Ti (Ti): 0.005 to 0.015% , And nitrogen (N): 0.002 to 0.006%.
Hereinafter, the reason for controlling the steel material composition of the present invention as described above will be described in detail. At this time, the content of each component means weight% unless otherwise specified.
C: 0.03 to 0.1%
Carbon (C) is an element which is very useful for securing strength of steel but increases the hardness of the microstructure constituting the weld heat affected zone and greatly reduces the weld heat affected zone toughness by formation of M-A structure.
In the present invention, when the content of C is less than 0.03%, it is difficult to secure the strength of the steel, so that it is inevitable to inject the element capable of improving the hardenability of the steel. On the other hand, if the content exceeds 0.1%, there is a problem that it is difficult to secure the toughness of the welding heat affected zone.
Therefore, in the present invention, the content of C is preferably limited to 0.03 to 0.1%.
Si: 0.05 to 0.4%
Silicon (Si) contributes to securing the hardenability of steel and contributes to deoxidation in steelmaking, and is preferably added in an amount of 0.05% or more. However, if the content is excessively high, the MA fraction increases and the toughness of the welded heat affected zone is lowered. Therefore, the upper limit is preferably limited to 0.4%.
Mn: 1.0 to 2.0%
Manganese (Mn) is a hardenable element that contributes to the improvement of steel strength and is less affected by the toughness deterioration of the weld heat affected zone. It is added at a content of 1.0% or more considering both the strength of the steel and the toughness .
However, when the content exceeds 2.0%, segregation is promoted at the center of the thickness, thereby deteriorating the toughness of the weld heat affected zone.
Therefore, in the present invention, the content of Mn is preferably limited to 1.0 to 2.0%.
P: not more than 0.02%
Phosphorus (P) is an impurity which is inevitably incorporated into steel during steelmaking. It is advantageous that the content is low, but the cost of the steelmaking process increases in order to reduce the content of P to the maximum. It is preferable to limit the content to 0.02% or less in consideration of the toughness of the weld heat affected zone.
S: not more than 0.005%
Sulfur (S) is an impurity which is inevitably incorporated into the steel. The lower the content of S, the better. However, considering the toughness of the weld heat affected zone in the present invention and the load of the steelmaking process, Limitations are desirable.
Al: 0.01 to 0.05%
Aluminum (Al) is an essential element for steel deoxidation and should be added in an amount of 0.01% or more in order to ensure the cleanliness of the steel. If the content is excessive, the proportion of MA structure is increased similarly to Si, , And the content thereof is preferably limited to 0.05% or less.
Nb: 0.005 to 0.03%
Niobium (Nb) contributes to the improvement of the strength by increasing the hardenability of the steel, and contributes to the improvement of the toughness of the base material by making the structure finer during the production of the steel sheet. However, if the content is excessively high, the hardening ability of the steel excessively increases, thereby increasing the fraction of the upper bainite and lowering the toughness of the weld heat affected zone. Therefore, the content is preferably limited to 0.03% or less.
Ti: 0.005 to 0.015%
Titanium (Ti) bonds to N to form TiN which exhibits high thermal stability at high temperature, and is an element effective for improving weld heat resistance toughness by inhibiting coarsening of austenite grains by the formation of TiN.
In order to obtain the above-mentioned effect, it is preferable that Ti is contained in an amount of 0.005% or more. If the content is more than 0.015%, the effect of suppressing grain coarsening is decreased due to precipitation of coarse TiN, There is a concern.
Therefore, the content of Ti in the present invention is preferably limited to 0.005 to 0.015%.
N: 0.002 to 0.006%
Nitrogen (N) combines with Ti to form TiN, thereby contributing to improvement in toughness of the weld heat affected zone, so that it is preferably added in an amount of 0.002% or more. However, if it is excessively excessive, surface cracking may occur in the slab during continuous casting, so it is preferable to limit the upper limit to 0.006%.
The remainder of the present invention is iron (Fe). However, impurities which are not intended from the raw material or the surrounding environment in the course of ordinary production can be inevitably incorporated, so that this can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.
On the other hand, the steel sheet of the present invention satisfying the above-mentioned composition has a content ratio (Mn / C) of Mn and C of not less than 15 and a content ratio Ti / N of 1 to 3 .
Mn / C: 15 or more
In the present invention, the content ratio of Mn and C (Mn (% by weight) / C (% by weight)) is the ratio of the interfacial ferrite that causes the lowering of toughness at the time of high- It is an important factor to control.
If the Mn / C value is less than 15, the sum of the area fraction of the upper bainite and intergranular ferrite in the coarse-grained heat affected zone (CGHAZ) exceeds 15%, and the target welding heat There is a problem that the impact resistance can not be secured.
Ti / N: 1 to 3
It is preferable to form a TiN precipitate in order to suppress the coarsening of the austenite particles when the weld heat affected zone is exposed to a high temperature by welding.
Therefore, the content ratio of Ti and N (Ti (% by weight) / N (% by weight)) is limited to 3 or less and the content ratio is 1 , There is a fear that the toughness of the welded heat affected zone is lowered by the free N that is not bonded with Ti, and surface cracks may occur on the slab.
Therefore, in the present invention, it is preferable to limit the Ti / N content ratio (Ti / N) to 1 to 3.
As described above, the steel sheet of the present invention, which satisfies all of the above-described steel component composition and component relationship, not only can obtain a weld heat affected zone having needle-shaped ferrite at the time of welding but also has a coarse-grained heat affected zone The sum of the area fraction of the upper bainite and the intergranular ferrite in the CGHAZ can be controlled to 15% or less.
As a result, the steel sheet of the present invention has a Charpy impact energy of 100 J or more at 0 캜 when welded at an input heat quantity of 5 to 30 kJ / mm, and a weld heat affected zone having excellent toughness in a wide heat receiving range can be obtained.
The production method of the present invention roughly provides a method for producing a steel slab which satisfies the above-mentioned component system and constituent relationships by heating and homogenizing the steel slab, followed by hot rolling and cooling. Hereinafter, detailed conditions for each step will be described.
Slab reheating temperature: 1000 ~ 1150 ℃
First, it is preferable to prepare a steel slab satisfying the above-described composition and composition relationships through a continuous casting method, and then reheat the steel slab.
If the reheating temperature is higher than 1150 占 폚, the TiN finely precipitated by the continuous casting may be coarsened to deteriorate the toughness of the weld heat affected zone, On the other hand, if the reheating temperature is too low to be less than 1000 占 폚, the rolling load during the subsequent hot rolling increases and the rolling pass increases, which is economically disadvantageous.
Therefore, the reheating is preferably performed at a temperature in the range of 1000 to 1150 ° C.
Then, the reheated slab is preferably subjected to hot rolling to obtain a post-steel plate. At this time, the hot rolling is preferably performed stepwise as described later.
Hot rolling
It is preferable that the reheated slab is subjected to primary rolling at a temperature of 900 ° C or higher and then secondary rolling is performed at a cumulative rolling reduction of 40% or higher at a temperature of 900 ° C or lower.
The primary rolling is a rolling process in a recrystallization zone, and the austenite grains can be made fine through the rolling process. Thereafter, a secondary ferrite rolling is performed in a non-recrystallization temperature region to form a ferrite transformation nucleus, A desired microstructure can be obtained.
The cumulative rolling reduction during secondary rolling is advantageous, but it is preferable to limit the lower limit to 40%, even though the cumulative rolling reduction at a cumulative rolling reduction of 40% or more is insufficient even if the rolling reduction is further increased.
After the hot rolling as described above, the final steel sheet can be obtained by water cooling.
Cooling: Start at Ar3 or higher, at a cooling rate of 4 to 50 ° C / s to 300 to 600 ° C
It is preferable to perform cooling after completion of the rolling according to the above description. At this time, it is preferable to start cooling at a ferrite transformation start temperature of Ar3 or higher.
At this time, Ar3 is calculated by a relation of [Ar3 (占 폚) = 910 - 310C - 80Mn], and the elements in the relational expressions refer to the weight content of the element.
If the cooling rate during cooling is less than 4 ° C / s or the cooling termination temperature exceeds 600 ° C On the other hand, when the cooling rate exceeds 50 DEG C / s or the cooling end temperature is lower than 300 DEG C, a large amount of low-temperature structure with high hardness is generated in a large amount, and the toughness of the steel sheet There is a problem that it becomes difficult to secure it.
The steel sheet produced according to the above is not only suitable as a steel for welded structure because its yield strength is not less than 400 MPa and its strength and toughness are uniform in the thickness direction and is also suitable for a welded structure in a wide heat input range of 5 to 30 kJ / The toughness can be ensured excellently.
Hereinafter, the present invention will be described in more detail by way of examples. It should be noted, however, that the embodiments described below are for illustrating and embodying the present invention, and not for limiting the scope of the present invention. And the scope of the present invention is determined by the matters described in the claims and the matters reasonably deduced therefrom.
( Example )
Steel slabs having the composition shown in the following Table 1 were reheated at 1000 to 1150 占 폚, and then steel slabs were rolled under the conditions shown in Table 2 below and then cooled to produce hot-rolled steel sheets (post-steel plates).
The specimens were then pulled perpendicularly to the rolling direction at 1/4 thickness (1/4 t (mm)) for each of the manufactured steel plates. Tensile tests were carried out at 5 kJ / mm, 10 kJ / mm, 30 kJ / The microstructure of the CGHAZ was observed in the weld heat affected zone after the submerged arc welding with the heat input of. Further, a Charpy V-notch impact test was conducted at 0 占 폚, and the results are shown in Table 3 below.
(mm)
Temperature (℃)
Temperature (℃)
Reduction rate (%)
Temperature (℃)
Temperature (℃)
(° C / s)
(MPa)
(MPa)
Fraction (%)
(J)
Fraction (%)
(J)
Fraction (%)
(J)
(In Table 3, 'UB + IF fraction' represents the sum of area fraction of upper bainite and intergranular ferrite in CGHAZ, 'UB' is upper bainite, and 'IF' means intergranular ferrite.)
As shown in Tables 1 to 3, inventive steels 1 to 6 having a steel component composition, a component relationship, and a manufacturing condition satisfied the present invention have high strength at a yield strength of 400 MPa or more and high bainite It is confirmed that the sum of the area fraction of intergranular ferrite is 15% or less and the impact toughness is excellent.
On the other hand, in the case of comparative steels 1 and 2 in which the content of C is excessive, the compositional relationship proposed in the present invention was not satisfied, so that the upper bainite and intergranular ferrite were excessively formed in the CGHAZ and the impact toughness was weakened.
Of these, in the case of the comparative steel 2 having the cooling termination temperature exceeding 600 캜, sufficient strength could not be secured.
Further, in the case of the comparative steel 3 which does not satisfy the compositional relationship of Mn / C and the cooling rate during cooling does not satisfy the present invention, not only the strength but also the upper bainite and intergranular ferrite in the CGHAZ are excessively formed, You can see that it is open.
On the other hand, in the comparative steels 4 and 5 which do not satisfy the Ti / N compositional relationship, the upper bainite and intergranular ferrite in the CGHAZ are formed at 15% or less in the heat input amounts of 5 kJ / mm and 10 kJ / mm, However, in the case of applying the heat of 30 kJ / mm, it can be confirmed that the impact toughness is increased by increasing the fraction of the upper bainite and grain boundary ferrite in the CGHAZ.
FIG. 1 is a graph showing the results of the CGHAZ impact toughness measurement according to the Mn / C content ratio according to the heat input. It can be seen that the CGHAZ impact toughness can be secured to 100 J or more only when the Mn / C content ratio is 15 or more.
FIG. 2 shows the observation of the CGHAZ microstructure according to the Mn / C content ratio. When Mn / C is less than 15 (corresponding to comparative steels 1 and 2), it is confirmed that the upper bainite and intergranular ferrite are excessively formed .
Claims (5)
Wherein a content ratio (Mn / C) of Mn and C is 15 or more, a content ratio (Ti / N) of Ti and N satisfies 1 to 3,
Welding at 5 ~ 30kJ / mm Welding with Good Coverage of welded heat-affected zone with less than 15% area fraction of upper bainite and intergranular ferrite in coarse-grained heat affected zone (CGHAZ) Structural steel plate.
Wherein the steel sheet has a weld heat affected zone including needle-shaped ferrite at the time of welding, and is excellent in toughness of the weld heat affected zone.
Wherein the steel sheet has a weld heat affected zone having a Charpy impact energy of at least 100 J at 0 캜 when welded.
Firstly rolling the reheated slab at a temperature of 900 캜 or higher;
After the primary rolling, secondary rolling to a cumulative rolling reduction of 40% or more at a temperature of 900 ° C or less; And
After the secondary rolling, cooling is started at Ar3 or higher and cooling is performed at a cooling rate of 4 to 50 ° C / s to 300 to 600 ° C
And the welded structure is welded to the welded structure.
Priority Applications (1)
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| KR1020150183887A KR101767771B1 (en) | 2015-12-22 | 2015-12-22 | The steel sheet for welding structure having excellent heat affected zone toughness and method for manufacturing the same |
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| KR102503447B1 (en) * | 2020-12-21 | 2023-02-28 | 현대제철 주식회사 | Steel for line pipe having excellent weldability and method of manufacturing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2002285239A (en) | 2001-03-27 | 2002-10-03 | Kawasaki Steel Corp | Method for producing non-heat-treated thick high tensile strength steel having excellent weld heat affected zone toughness |
| JP2012214884A (en) * | 2011-03-28 | 2012-11-08 | Jfe Steel Corp | Thick steel sheet having superior fatigue resistance properties in sheet thickness direction, and method for producing same |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP2002285239A (en) | 2001-03-27 | 2002-10-03 | Kawasaki Steel Corp | Method for producing non-heat-treated thick high tensile strength steel having excellent weld heat affected zone toughness |
| JP2012214884A (en) * | 2011-03-28 | 2012-11-08 | Jfe Steel Corp | Thick steel sheet having superior fatigue resistance properties in sheet thickness direction, and method for producing same |
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