JP2931065B2 - Method for manufacturing ultra-high heat input welded structural steel sheet with excellent low-temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention provides a heat-affected zone including a bond (hereinafter referred to as HAZ) having a toughness even at a very large heat input welding of about 500 kJ / cm to 1000 kJ / cm. The present invention relates to a method for producing a steel sheet for welded structures excellent at 4.0 kgf-m or more at 20 ° C.

[Related Art] In recent years, in order to reduce welding costs, the use of ultra-high heat input welding has been studied for steel sheets for welded structures, and even in this case, a steel sheet having excellent HAZ toughness is desired.

Conventionally, in the field of large heat input welding, proposals of a steel sheet having good HAZ toughness and a method of manufacturing the same include, for example, Japanese Patent Publication No. 55-26164 and Japanese Patent Application Laid-Open No. 63-103051.

The proposal of Japanese Patent Publication No. 55-26164 proposes a method for welding a heat input having a heat input of 320 kJ / cm or less (electroslag welding) or less as a large heat input welding method.
The feature is that the austenitic grains of the HAZ are made smaller and the toughness of the HAZ is ensured by securing the HAZ in the steel.

However, this proposal is for welding with a heat input of 320 kJ / cm or less, and the guaranteed temperature of HAZ toughness is up to 0 ° C.

Also, the latter proposal of JP-A-63-103051 discloses that heat input is reduced.
It is intended for welding at 230 kJ / cm.
The HAZ toughness is improved by securing a required amount of fine TiN of 0.02 to 0.04 μm in the steel for the same reason as the proposal of Japanese Patent No. 26164.

On the other hand, super large heat input welding of about 500 kJ / cm to 1000 kJ / cm has already been made possible.

These ultra-high heat input welding methods are larger than the heat input amounts targeted in JP-B-55-26164 and JP-A-63-103051, and the residence time at 1350 ° C or higher at which TiN dissolves significantly increases. ing. Therefore, the bond part of ultra-high heat input welding,
In the HAZ portion, fine TiN dissolves and does not bring about an expected effect, making it difficult to secure HAZ toughness.

On the other hand, a welded structure constructed using the ultra-high heat input welding method is even larger than a welded structure constructed using the large heat input welding method, and the social impact of the safety of the structure is increased. Due to the size of the impact and the economic impact, HAZ toughness, including weld bonds, is more stringent and it is desirable to ensure this to lower temperatures.

[Problems to be Solved by the Invention] In response to this demand, the present invention provides an ultra-large heat input under a very large heat input welding environment of about 500 kJ / cm to 1000 kJ / cm, which is incomparable with the conventional heat input. The HAZ containing the bond is secured with TiN of a size that does not dissolve and disappear even under a long high-temperature residence time peculiar to welding, and a large amount of fine BN is precipitated using this TiN as a nucleus, and the HAZ toughness vE-20 is 4.0 kgf-m. An object of the present invention is to provide a method for manufacturing a steel plate for a welding structure as described above.

[Means for Solving the Problems] The first invention of the method for producing a steel plate for an ultra-high heat input welded structure having excellent low-temperature toughness according to the present invention is as follows: C: 0.03 to 0.18% by weight;
i: 0.1-1.0%, Mn: 0.5-1.8%, Al: 0.005-0.060%, T
i: 0.005 to 0.025%, B: 0.0001 to 0.0020%, EN =
In the formula N-0.292Ti-1.295B, 0 <EN <0.0020
Cast steel containing N satisfying the following, and the balance consisting of Fe and unavoidable impurities, and cooling at a cooling rate of 5 ° C./min or less in a solidification process up to 1100 ° C., and then a temperature of Ac ₃ point to 1200 ° C. And rolling the slab.

In the second invention, C: 0.03 to 0.18% by weight,
i: 0.1-1.0%, Mn: 0.5-1.8%, Al: 0.005-0.060%, T
i: 0.005 to 0.025%, B: 0.0001 to 0.0020%, EN =
In the formula N-0.292Ti-1.295B, 0 <EN <0.0020
After reheating a steel slab containing N that satisfies the following and the balance is made up of Fe and unavoidable impurities to 1200 to 1300 ° C, the cooling rate to 1100 ° C is set to 5 ° C / min or less, and after cooling, the Ac ₃ point 120
The slab is rolled at a temperature of 0 ° C.

In the third invention, C: 0.03 to 0.18% by weight, S:
i: 0.1-1.0%, Mn: 0.5-1.8%, Al: 0.005-0.060%, T
i: 0.005 to 0.025%, B: 0.0001 to 0.0020%, and one or more of Ni ≦ 0.9%, Cu ≦ 0.5%, Nb ≦ 0.014%
Containing N or more, and in the formula of EN = N−0.292Ti−1.295B, containing N satisfying 0 <EN <0.0020, and the balance being Fe
And a steel consisting of unavoidable impurities, and cooled at a cooling rate of 5 ° C./min or less in the solidification process up to 1100 ° C.
_The slab is rolled at a temperature of _{3 to} 1200 ° C.

Further, the fourth invention is that, by weight%, C: 0.03 to 0.18%,
Si: 0.1-1.0%, Mn: 0.5-1.8%, Al: 0.005-0.060%, T
i: 0.005 to 0.025%, B: 0.0001 to 0.0020%, and one or more of Ni ≦ 0.9%, Cu ≦ 0.5%, Nb ≦ 0.014%
Containing N or more, and in the formula of EN = N−0.292Ti−1.295B, containing N satisfying 0 <EN <0.0020, and the balance being Fe
After reheating the slab consisting of unavoidable impurities to 1200 to 1300 ° C, the cooling rate to 1100 ° C is reduced to 5 ° C / min or less, and then the slab is rolled at a temperature of Ac ₃ points to 1200 ° C. It is characterized by the following.

The reasons for limiting the components of the steel sheet of the present invention and the amounts of the components will be described below.

C is added to ensure the strength of the base material, and the upper limit is determined to prevent TiC and / or BC from reacting with Ti and B added separately to prevent the toughness of HAZ from deteriorating.

Si is added to deoxidize steel and ensure base metal strength, and Mn is added to ensure base metal strength, but both have upper limits to prevent HAZ toughness degradation.

Al is added to refine the steel by Al nitride, and also to make the steel crystal orientation and recrystallization effective by solid solution and precipitation in the rolling process. However, when the addition amount is small, there is no effect, and when it is excessive, the toughness of the steel is deteriorated.

Ti is added to secure TiN of a size that does not melt in the weld bond, promotes re-precipitation of BN using this as a nucleus, and secures HAZ toughness by refining the HAZ structure. From the amount of N that can be fixed and determined from the prevention of
Is specified in order to prevent the generation of TiC due to excessive amount and causing deterioration of HAZ toughness.

As a result, the HAZ crystal grains are refined, the refinement of the HAZ structure is promoted, and the toughness of the HAZ including the bond is remarkably improved. Therefore, the upper limit is that the solid solution N due to the melting of AlN in the HAZ containing the bond becomes excessive with respect to the upper limit of B determined from the formation of BC, causing excess N, leaving the solid solution N in the HAZ containing the bond. It is determined to prevent the toughness from deteriorating.

B is added to secure the bond toughness by refining the HAZ structure after ultra-high heat input welding and fixing solid solution N. The upper limit is to improve the hardenability of HAZ by the formation of BC during reprecipitation.
It is specified to prevent the toughness from deteriorating.

N is the lower limit of the amount of N necessary to secure TiN of a size that does not melt even at a high temperature and a large heat capacity of an ultra-high heat input welding bond of about 500 to 1000 kJ / cm, and the upper limit suppresses the formation of BC as described above. From the upper limit of B to determine the upper limit and lower limit according to the amount of Ti and B in steel, to prevent N which could not be fixed as BN finally form a solid solution and degrade the toughness of the bond. ing.

S is desirably as small as possible to prevent MnS from forming inclusions in the steel and deteriorating the base metal toughness.

Also, there is no problem in adding one or more of Ni, Cr, Mo, Cu, Nb, and V according to the respective effects as commonly used in the art.

Ni should be added to improve the toughness of the base material and HAZ, and the upper limit should be set to increase the hardenability and prevent the HAZ structure from becoming bainite.Cr, Mo, Cu improve the strength of the base material. To add
It is necessary to consider the upper limit to prevent HAZ from hardening.

Nb and V form carbides and nitrides and promote the refinement of crystal grains in the base metal structure, and are added to increase strength and toughness, but care must be taken to the upper limit to prevent deterioration of HAZ toughness It is.

[Action] FIG. 1 is a diagram showing the ultra-high heat input welding of a steel sheet for an ultra-high heat input welding structure having excellent low-temperature toughness obtained by the production method of the present invention, and one side 1
4 shows an example of a measurement result of a heat cycle in the vicinity of a weld bond part in a large layer heat input welding method.

From the figure, it was found that the residence time of exposure to a high temperature of 1400 ° C. or more in the ultra-high heat input welding method was significantly longer than that of single-sided, single-layer welding.

FIG. 2 shows the residence time of 1400 ° C. or more at the bond portion during welding and the critical dimension (diameter obtained by converting the shape of the precipitate into a circle) of the TiN precipitate dissolved at that time.

The residence time corresponding to welding of 400 kJ / cm or less, which is proposed in Japanese Patent Publication No. 55-26164 or Japanese Patent Application Laid-Open No. 63-103051, is the residence time compared to the ultra-high heat input welding targeted by the present invention. Because of the short length, TiN precipitates of 0.05μm or less exist even after being affected by heat and can contribute to the refinement of the HAZ structure.However, in ultra-high heat input welding, TiN of less than 0.1μm will dissolve and disappear. Was found.

Furthermore, Fig. 3 shows the relationship between the HAZ toughness of the ultra-high heat input weld and the number of TiN precipitates having a diameter of 0.1 µm or more in the steel plate part subjected to the welding cycle for B-containing steel and B-free steel. It is.

From this figure, it can be seen that the HAZ toughness is poor for B-free steel regardless of the number of TiN precipitates, and for B-containing steel, EN = N-0.292Ti-1.295B in the case of 0 <EN <0.0020. It was found that HAZ toughness was ensured even under ultra-high heat input welding when TiN precipitates having a diameter of 0.1 μm or more remaining after the welding heat cycle were present in an amount of 3 × 10 ⁵ or more per 1 mm ² .

Therefore, in order to sufficiently secure a TiN precipitate having a diameter of 0.1 μm or more, the solidification cooling rate of steel having the chemical components shown in Table 1 is set to Ti
The effect on the size of N precipitates was studied.

 The results are shown in FIG. 4 and FIG.

As is clear from FIG. 4, at a cooling rate of 5 ° C./min or more, the diameter of the precipitated TiN becomes finer than 0.1 μm,
It has been found that it is necessary to solidify at a cooling rate of 5 ° C./min or less in order to secure deposited TiN of 0.1 μm or more.

From FIG. 5, the steel slab cooled at a cooling rate of more than 5 ° C./min during solidification cooling was reheated to 1200 ° C. to 1300 ° C.
/ Min without being affected by the cooling rate during solidification if the cooling in the following cooling rate, also during rolling, either the mom rolling, the heating temperature by the Ac ₃ point to 1200 ° C., diameter 0.1 It has been found that TiN having a thickness of μm or more can be secured up to the time of ultra-high heat input welding.

FIG. 6 shows that the solidification cooling rate was changed to 5 ° C. by changing various chemical components.
The results of investigating the effect of chemical components on the toughness of a super-high heat input weld using a steel sheet manufactured at a rate of not more than / min are shown.

From this figure, in the steel containing B, when N is in the range of EN <N−0.292Ti−1.295B in the range of 0 <EN <0.0020, the thermal cycle toughness at the peak temperature of 1400 ° C. equivalent to super large heat input welding is high. It became clear that the value could be secured.

An investigation of the relationship between these HAZ structures and TiN precipitates revealed that the mechanism of improvement in HAZ toughness due to TiN precipitates with a diameter of 0.1 μm or more was fine TiN precipitates as conventionally known.
Not the effect of preventing austenite coarsening due to precipitates,
It was found that coarse TiN precipitates remaining undissolved, and massive pro-eutectoid ferrite, which formed as a transformation nucleus a composite precipitate of BN deposited at a temperature just above the ferrite transformation using the TiN precipitate as a nucleus.

That is, the presence of this massive pro-eutectoid ferrite suppresses the formation of plate-like pro-eutectoid ferrite, and as a result, the ferrite which develops this plate-like pro-eutectoid ferrite as a generation site
Since the formation of side plates (hereinafter referred to as FSP) is suppressed and the formation of FSP which is harmful to toughness is suppressed, HAZ
It was found that the toughness was improved.

In the case of B-containing steel, when N is EN> 0.0020% in the equation of EN = N−0.292Ti−1.295B, it is difficult to secure toughness due to excess N. When EN <0%, Since the amount of N required for the precipitation of BN is not secured, the quenching property of the bond increases due to the solid solution B due to the inability to precipitate BN, and the bulk eutectoid ferrite is not generated, FSP develops, and the toughness is secured. It turned out to be difficult.

FIG. 7 (1) shows the positional relationship and the shape of the massive proeutectoid ferrite α1 in the steel sheet obtained by the production method of the present invention, which forms a fine HAZ structure and improves the HAZ toughness. FIG. 2 (2) schematically shows the position of the above-mentioned plate-like pro-eutectoid ferrite α2 and FSP and the upper bainite Bu on the conventional steel sheet, which form an inferior HAZ structure and deteriorate the HAZ toughness. It shows the outline of the relationship and the shape.

From the above findings, the present inventors have found that the necessary amount of Ti and N
The size and amount of the TiN precipitates are adjusted as described above by preparing a steel containing B and B, and setting the cooling rate at the time of casting solidification or the cooling rate after re-melting and heating of TiN to 5 ° C / min or less. Then, secure a TiN precipitate that does not dissolve even with a high-temperature, high-heat weld bond formed by an ultra-high heat input with a heat input of 500 to 1000 kJ / cm, make it a precipitation site, and precipitate BN just above the ferrite transformation, It succeeded in improving the toughness of the HAZ including the bond during heat input welding.

Due to the effect of BN on promoting ferrite transformation, massive pro-eutectoid ferrite α1 is formed at any part of the grain boundary, and no plate-like pro-eutectoid ferrite α2 and FSP shown in FIG. 7 (2) are present. The object of the present invention was achieved by forming a fine HAZ structure composed of refined crystal grains surrounded by massive pro-eutectoid ferrite α1.

The present inventors have analyzed the facts obtained from these findings and found that Ti: 0.005 to 0.25%, B: 0.0001 to 0.0020% N: EN = N−0.292 Ti−1.295B, 0 <EN <0.0020 Using the steel prepared in the above, the cooling rate during its solidification,
Alternatively, if the cooling rate after reheating to a temperature at which TiN partially dissolves is regulated to 5 ° C./min or less, TiN precipitates having a diameter of 0.1 μm or more are secured at 3 × 10 ⁵ or more per 1 mm ^{2 of the} steel sheet, and 500 kJ / c
It was confirmed that in ultra-high heat input welding of about m to 1000 kJ / cm, a high-strength steel sheet for an ultra-high heat input welding structure showing a high value of HAZ toughness (vE-20) of 4.0 kgf-m or more was obtained.

[Example] The test steel was cast into a slab by continuous casting, subjected to controlled rolling and controlled cooling known per se to form a steel sheet, and each obtained steel sheet was subjected to welding under the conditions shown in 4 respectively.

 The results are shown below.

1. Composition of test steel Table 1 shows.

2. Steel plate manufacturing conditions Table 2 shows.

3. Mechanical properties of steel sheet Table 2 shows.

4. Welding conditions Table 2 shows.

5. HAZ toughness is shown in Table 2.

Steel numbers 1 to 12 shown in Table 2 show examples of the present invention. In this example, each toughness vE-20 shown in the super-heat input welding of 500 kJ / cm to 1000 kJ / cm is 4.1 to 20 in the weld bond (FL).
9.6.0-18.0 for HAZ, 1mm, 14.3-27.2 for HAZ, 3mm, HA
21.6-27.0 for Z, 5mm, yield point of steel plate is 32-48kgf
It was / mm ^2.

On the other hand, steel numbers 13 to 22 show comparative examples. The comparative examples of steel numbers 13 to 17 all have a cooling rate of 5 ° C. during solidification or after reheating.
/ Min or more, the number of TiN precipitates of 0.1 μm or more is small, steel No. 18 is excessive in Ti and lacks N, and steel No. 19 is N
In steel Nos. 20-21, N was insufficient and a sufficient number of TiN precipitates could not be obtained, and in steel No. 22, B was excessive.

The comparative examples of steel Nos. 13 to 22 have the toughness vE-20 in the ultra-high heat input welding of 500 kJ / cm to 1000 kJ / cm under the same welding conditions as the present invention, and the weld bond (FL) is 0.4 to 3.4, HAZ, 1 mm is 2.1 to 4.3 for vE-20, HAZ, 3 mm is 3.5 to 14.0
And did not reach the subject HAZ toughness of 4.0 kgf-m or more.

[Effect of the Invention] The ultra-large heat input welding structural steel sheet having excellent low-temperature toughness obtained by the production method of the present invention has a heat input of 500 kJ / cm to 1000 kJ / c.
Corresponding to the specific environmental conditions of high temperature and long residence time given to HAZ including the bond of ultra-high heat input welding of about m, the welding bond at the time of welding under the specific environmental condition of high temperature and long residence time By securing the required number of TiN precipitates that do not dissolve in the HAZ containing Ti and the required number, the synergistic action of TiN and BN is exhibited, thereby including the weld bond
HAZ toughness enables the production of ultra-high heat input welded structural steel sheets with excellent low-temperature toughness of 4.0 kgf-m or more.The effect of improving safety, reliability, and economic efficiency brought to the field is extremely high. large.

[Brief description of the drawings]

FIG. 1 shows an example of a heat cycle measurement result immediately adjacent to a weld bond portion in super-large heat input welding and single-sided, single-layer large heat input welding to which the present invention is applied. Fig. 2 shows the residence time at 1400 ° C or more at the bond during welding, and the critical dimension (diameter) of the TiN precipitate dissolved at that time.
It is shown. Fig. 3 shows the relationship between the HAZ toughness of the ultra-high heat input weld and the number of TiN precipitates having a diameter of 0.1 µm or more in the steel sheet part subjected to the welding heat cycle for B-containing steel and B-free steel. It is. FIG. 4 shows the effect of the cooling rate after cast solidification of steel on the size of TiN precipitates. FIG. 5 shows the relationship between the cooling rate after reheating of a steel slab reheated to a temperature at which TiN partially melts and the size of TiN precipitates. FIG. 6 shows the effect of the chemical components on the toughness of the weld joint with a very large heat input. FIG. 7 schematically shows the positional relationship in the structure and the features of the shape of massive pro-eutectoid ferrite, IFP and plate-like ferrite, FSP, Bu and the like. (1) shows a steel plate of the present invention, and (2) shows a conventional steel plate.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshiaki Hashi Oita, Oita, Oita, 1st section of Nishinosu Oita Works Inside Nippon Steel Corporation Oita Works (72) Inventor, Hidesato Mabuchi 1st section of Oaza, Oita, Oita, Oaza Nippon Steel Corporation Oita Works (56) References JP-A-2-236224 (JP, A) JP-A-61-106722 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB Name) C21D 8/02 B22D 11/00-11/22

Claims (4)

(57) [Claims] [Claim 1] C: 0.03 to 0.18% by weight% Si: 0.1 to 1.0% Mn: 0.5 to 1.8% Al: 0.005 to 0.060% Ti: 0.005 to 0.025% B: 0.0001 to 0.0020%, EN = In the formula N-0.292Ti-1.295B, a steel containing N satisfying 0 <EN <0.0020 and the balance being Fe and unavoidable impurities is cast, and the cooling rate during the solidification process up to 1100 ° C is 5 ° C. A method for producing a steel sheet for ultra-high heat input welding with excellent low-temperature toughness, characterized in that the steel slab is rolled at a temperature of _{3 to} 1200 ° C. after cooling at a temperature not higher than / min. 2. C: 0.03 to 0.18% by weight% Si: 0.1 to 1.0% Mn: 0.5 to 1.8% Al: 0.005 to 0.060% Ti: 0.005 to 0.025% B: 0.0001 to 0.0020%, EN = In the formula N-0.292Ti-1.295B, a steel slab containing N satisfying 0 <EN <0.0020 and the balance being Fe and unavoidable impurities is reheated to 1200 to 1300 ° C, and then heated to 1100 ° C. After cooling at a cooling rate of 5 ° C./min or less, Ac ₃ points to 120
A method for producing an ultra-high heat input welding steel sheet having excellent low-temperature toughness, characterized by rolling the slab at a temperature of 0 ° C. (3) C: 0.03-0.18% Si: 0.1-1.0% Mn: 0.5-1.8% Al: 0.005-0.060% Ti: 0.005-0.025% B: 0.0001-0.0020% by weight%, Ni ≦ 0.9%, Cu ≦ 0.5%, Nb ≦ 0.014
% Of one or more types, and EN = N-0.292Ti-1.295B In the formula: 0 <EN <0.0020 N, the balance being Fe and unavoidable impurities. Then, the cooling rate in the solidification process up to 1100 ° C. is cooled at 5 ° C./min or less, and then the slab is rolled at a temperature of from Ac ₃ point to 1200 ° C. Manufacturing method of steel plate for heat welding. 4. The composition contains C: 0.03 to 0.18% Si: 0.1 to 1.0% Mn: 0.5 to 1.8% Al: 0.005 to 0.060% Ti: 0.005 to 0.025% B: 0.0001 to 0.0020% by weight%, Ni ≦ 0.9%, Cu ≦ 0.5%, Nb ≦ 0.014
% Or more, and in the formula of EN = N-0.292Ti-1.295B, a steel slab containing N satisfying 0 <EN <0.0020 and the balance being Fe and unavoidable impurities. after re-heated to 1200 to 1300 ° C., after cooling the cooling rate to 1100 ° C. as 5 ° C. / min or less, Ac ₃ point to 120
A method for producing an ultra-high heat input welding steel sheet having excellent low-temperature toughness, characterized by rolling the slab at a temperature of 0 ° C.