JPH03264614A - Manufacture of steel for high heat input welding having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To stably maintain high toughness at low temp. even after high heat input welding by successively subjecting steel having specified Mn, Ni and Cu contents, specified relation among the contents and a specified ratio of Ti to N and contg. a very small amt. of added B and Nb to continuous casting, controlled cooling, heating and rolling. CONSTITUTION:Steel consisting of, by weight, 0.02-0.15% C, <=0.3% Si, 0.5-2.0% Mn, 0.2-1.5% Ni, 0.2-1.5% Cu (Mn/6+(Cu+Ni)/15=0.28-0.40%), 0.0020-0.010% N, B and/or Nb satisfying BX10,000+NbX1,000=4-10, Ti satisfying 2.0-4.0 ratio of Ti to N, 0.005-0.1% Al, 0.003-0.008% S and the balance Fe with inevitable impurities is continuously cast and cooled at <=5.0 deg.C/min cooling rate in the temp. range of 1,000-600 deg.C. The steel is then heated to <=1,150 deg.C and rolled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a steel material for high heat input with excellent low-temperature toughness.

(Conventional technology) In recent years, due to increasing energy demand, oil in the ocean,
The development of natural gas, etc. is being carried out vigorously. Particularly recently, huge offshore structures have been constructed in cold regions such as the North Sea and the Arctic Ocean in search of richer oil resources.

Such offshore structures are exposed to low temperatures of -30°C or lower and are operated under complex load stress conditions due to the influence of waves, etc., so the steel materials used therein are Excellent brittle fracture properties are required.

In particular, the toughness of the weld heat-affected zone, which has a lower toughness than the base metal, directly affects the safety of the structure, so it is evaluated by impact tests, etc. Impact values may be required.

In addition, increasing the size of structures increases construction costs, so it is necessary to use high-strength steel materials, such as those with a yield point of 36 kg.
By using steel materials with f/mnt or higher, it is possible to reduce the weight of the superstructure and to reduce welding costs by adopting a high heat input welding method.

As a method for manufacturing this steel material, for example, JP-A-83-1
As disclosed in Japanese Patent No. 03021, there is production by controlled rolling with limited component elements and accelerated cooling method. Such conventional technology certainly provides steel materials with excellent toughness in the weld heat affected zone under normal welding heat input (50 kJ/cm or less), but the effect is not expected in high heat input welding. Can not.

Examples of techniques to improve the toughness of the weld heat affected zone include:
JP-A-60-245788 and JP-A-80-1
As described in Japanese Patent No. 521328, a technique has been proposed in which the toughness of the weld heat affected zone is improved by generating intragranular ferrite using oxides as ferrite transformation nuclei.

However, these steels have the drawback that it is difficult to uniformly disperse oxides during the casting process, and stable toughness of the weld heat affected zone cannot be ensured.

(Issues to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing a steel plate for marine structures that is used in cold regions and polar regions and has high strength and excellent weld heat affected zone toughness. be.

(Means for Solving the Problems) The present invention has been made to solve the above problems, and the gist thereof is as follows: C: 0.02 as weight %
~0.15%, Si: 0.3% or less, Mn: 0.5-2
．． 0%, Ni: o, 2-1.5%, Cu: 0.2-1
．． 5%, however, Mn, Nf, and Cu are Mn/6+(
Cu 10N+)/15-0.28~0.40%, N: 0.0020~0.010%, BX10
The value of the formula 000+Nl)xtooo is 4 to 10
One or two types of B, Nb such that the ratio of TI and N (TJ/N) is 2.0 to 4.0, Ti, An)
:0.005~0.1%, s:0.003~0.00
8%, the balance being Fe and unavoidable impurities, is continuously cast, and the subsequent cooling rate is 1000°C to 600°C.
A method for producing a high heat input welding steel with excellent low-temperature toughness, characterized in that the steel is cooled at an average cooling rate of 5.0°C/min or less in the range up to 5.0°C, and then heated to 1150°C or less before rolling. It is related to.

(Function) Based on many experimental facts related to improving the toughness of the weld heat-affected zone (hereinafter referred to as HAZ), the present inventor found that: ■ Intragranular ferrite generated during the cooling process during welding is a composite of TiN and MnS. It is generated from precipitates (hereinafter referred to as TiNMn5 precipitates) and improves the toughness of the HAZ. (2) It was found that the size of TiN-Mn5 precipitates that contribute to improving the toughness of HAZ is 0.4 or more. In order to accomplish this, they discovered a method of coagulating MnS at high temperatures.

Hereinafter, the invention will be described in detail based on the above findings.

FIG. 1 shows the effect of the number of TiN-Mn5 precipitates on the toughness after applying a welding thermal cycle equivalent to 200 kJ/am.

The chemical components of the samples used in this experiment are shown in Table 1.

Table 1 (νt%) This figure shows that as the number of TiN-MnS precipitates increases, the toughness of the HAZ improves.
It can be seen that the precipitates are significantly effective in improving HAZ toughness.

Furthermore, Figure 2 shows the relationship between the average cooling rate and the number of TiN-Mn5 precipitates in the temperature range of 1000 to 600°C after solidification when the same test material was used in an experiment. , 0° C./min or less, the number of precipitates can be significantly increased.

From the above experimental facts, by controlling the cooling rate after solidification, B, TiN-Mn5, which becomes the transformation nucleus of intragranular ferrite,
It has become clear that the toughness of HAZ can be improved by increasing precipitates.

In addition, since the TiN-Mn5 precipitates thus precipitated will easily melt when heated at a temperature of 1300°C or higher, it is preferable that the slab heating temperature before subsequent hot rolling is low, and desirably It should be heated to below 1150°C.

Next, the reasons for limiting the components in the present invention will be described.

C is a necessary element to ensure strength.Is it necessary to add 0.02% or more to ensure strength?
Addition of a large amount leads to a decrease in the toughness of the HAZ, so the upper limit is set to 0.15%.

St is an element that reduces HAZ toughness when added in a large amount, and the upper limit of B is 0.3%.

It is necessary to add 0.5% or more of Ml from the viewpoint of ensuring strength and controlling the microstructure of the HAZ part, which will be described later.
If added in a large amount, HAZ toughness decreases, so the upper limit is set at 2.0%.

Ni is effective in improving the strength and toughness of the base metal, and at the same time
Similar to Mn, HA can be produced by controlling the HAZ microstructure.
B is an element that improves Z toughness, and it is necessary to add it in an amount of 0.2% or more, but if it is added in an amount exceeding 1.5%, it will cause a decrease in HAZ toughness, so the range should be adjusted to 0.2 to 1. Limited to .5%.

Cu is an element that is effective in improving the strength of the base material, and at the same time, in the present invention, together with Mn and Ni, it is an element that controls the microstructure of the HAZ and improves the toughness. It has no effect, and if it exceeds 1.5%, it will actually cause HA.
In order to reduce Z toughness, the range is limited to 0.2 to 1.5%.

N is an important element that combines with TI to form nitrides, but it needs to be added in an amount of 0.0020% or more, but if it exists freely in steel, it will lead to a decrease in HAZ toughness, so the upper limit must be set. It shall be 0.010%.

In the present invention, B and Nb are added as elements that suppress the formation of coarse ferrite from prior austenite grain boundaries that reduce HAZ toughness when added in small amounts, and improve toughness.

FIG. 3 shows the influence of B and Nbff1 on the impact value when an impact test was conducted at -60° C. after a simulated thermal cycle equivalent to a heat input of 200 kJ/am was applied.

Table 2 shows the component system used in the experiment.

As can be seen from Figure 3, the impact value indicated by ○ is 6 kgf.
In order to obtain a high value of −m or higher, one or both of B and Nb should be added in weight% of B×10000+NbX
It is necessary to add so that the value of the formula 1000 is within the range of 4 to 10.

Ti is an essential element for the present invention, and combines with B and N to precipitate TiN, and serves as a precipitation nucleus for MnS. Therefore, in order to obtain optimal TiN, it is necessary to control the amounts of Ti and N. In other words, the weight ratio of T1 and N is 2.0
If it is less than this, excessive N will result in B and a decrease in HAZ toughness.
Conversely, when it exceeds 4.0, Tj becomes excessive, TiC precipitates, and the toughness of the base material decreases significantly.

S is an element necessary for precipitation of Mn. Figure 4 shows a steel sheet with a thickness of 32m+n within the composition range shown in Table 2, which was actually 3-electrode latent arc welding with a heat input of 200kJ/am at -60°C.
The influence of the amount of S on the impact value of is shown. As can be seen from this chart, if less than 0.003% is added, the amount of precipitation is insufficient (B), the expected improvement in toughness cannot be obtained, and 0.003% is added.
When added in excess of 0.008%, a large amount of MnS precipitates,
0.003 to 0.00 to actually inhibit toughness.
It is limited to a range of 8%, preferably 0.003 to 0.00%.
It should be added in the range of 0.005%.

AΩ is an element necessary for deoxidation, and is 0.005%
Although it is necessary to add more than 0.1%, the upper limit is set at 0.1% since adding a large amount impairs toughness.

As a result of repeated experiments with the aim of further improving the HAZ toughness during high heat input welding within the above component range, the present inventors found that Ml
By effectively using elements with high hardenability such as , Cu, and Nj, it is possible to suppress the formation of coarse ferrite and upper bainite that are generated from prior austenite grain boundaries in the HAZ microstructure, which are the main causes of toughness reduction. It was discovered that HAZ toughness can be dramatically increased.

Figure 5 shows that steel having the composition range shown in Table 2 is used as a test material with a thickness of 32 mm, and is welded by single-sided latent arc welding with a heat input of 200 kJ/cm. The vertical axis 1 shows the average value of the impact value when the test was conducted, and the horizontal axis shows Mn /6+ (Cu +
2 is a chart showing values of the formula Ni)/15.

In addition, the results shown with ○ are the amount of S or 0.003% by weight.
and (containing 1,004%, T i N −M n
When S precipitates are finely dispersed, ・ means S is 0
．． 001% or less, T i N −M n
The results are shown when almost no S precipitates are generated.

As you can see from this diagram, T i N −M n S
For steel with dispersed precipitates (○), the value of the above formula is 0.25
As the impact value increases from 0.33 to 0.33, the impact value significantly improves, and when it exceeds 0.33, the toughness tends to decrease.

This improvement in toughness is due to the observation of the microstructure of Mn, C
u, As the hardenability of steel increases with the addition of NI,
Coarse ferrite generated from grain boundaries can be suppressed even at slow cooling rates during high heat input welding, and at the same time, TiN-Mn can be added to the grains.
5. Fine intragranular ferrite is generated with the precipitates as nuclei, and H
This is thought to be due to the fact that the microstructure of AZ becomes significantly finer.

2 However, if the value of the above formula exceeds 0.33, the hardenability increases too much, and upper bainite and island-like martensite #I textures that inhibit toughness are generated in the prior austenite grains during cooling. As a result, toughness decreases.

On the other hand, steel (・) in which TiN-Mn5 precipitates are not dispersed exhibits low toughness regardless of the value of the above equation.
, the difference from the steel of the present invention is obvious.

From the above findings, it is possible to finely disperse TiN-Mn5 precipitates according to the present invention, and to disperse Mn/6 + (Cu + Ni
)/15, HAZ toughness during high heat input welding can be improved. Note that the range is set to 0.28 to 0.40, considering that the industrially required impact value is approximately 3.5 kgf-m.

Steel having the above-mentioned components is melted in an electric furnace or a converter, and then cast in a continuous casting machine. Cooling rate during solidification at this time: 5.09C/mi in the temperature range of 000 to 600℃
Cooling is performed such that the temperature is below n.

In order to improve HAZ toughness, it is necessary to ensure a certain number density of TiN-3 4 MnS precipitates, and for this purpose it is necessary to finely disperse TiN, which serves as precipitation nuclei of MnS.

In other words, from conventional knowledge, it is known that the faster the cooling rate during solidification, the finer the TiN will be dispersed, and the continuous casting method, which has a faster cooling rate during solidification than casting by the agglomeration/blowing method. Adopt.

MnS is precipitated on the TiN thus precipitated in a temperature range of 1000° C. or less. However, for the formation of TiN-Mn5 precipitates that are effective in improving HAZ toughness, M
Since nS is insufficiently precipitated, it does not act as transformation nuclei for intragranular ferrite generated during cooling during welding, and no improvement in HAZ toughness can be expected.

Note that the slower the cooling rate, the better, but its upper limit is limited by the performance of the continuous casting machine.

After that, reheating is performed for hot rolling, but the temperature at that time is to ensure the strength and toughness of the base material and to change the morphology of the TiN-MnS precipitates generated by the heat treatment described above. In order to prevent this, it is necessary to keep the temperature below 1150°C.

Regarding rolling after heating, B was subjected to controlled rolling in order to improve the strength and toughness of mother H, and after controlled rolling, no change was caused to the equivalent TiN-Mn5 precipitates even when water-cooled. Therefore, currently known manufacturing methods can be selected and employed as appropriate.

(Example) The chemical components of the test materials are shown in Table 3.

Here, steel A to steel C are component systems that correspond to the present invention B,
Steel D-G is a steel that deviates from the invention.

In addition, Table 4 shows the manufacturing conditions of the sample materials, the base material, and the HAZ
The toughness values are also shown.

These steel plates are melted in a converter and cast into a continuous casting machine to a thickness of 24 mm.
After being cast to a size of 0 to 250 mm and a width of 1,300 to 1,600 mm, a steel plate with a thickness of 32 mm was manufactured through pretreatment and hot rolling for rolling. The HAZ toughness was evaluated by taking an impact test piece from 1/4 of the plate thickness after one-sided single-layer latent arc welding (heat input: 200 kJ/c+n) and performing a Charpy impact test.

From Table 4, the steel plate manufactured by the method of the present invention (board: A
1. Bl, CI) exhibits excellent toughness in both the base metal and HAZ.

On the other hand, board A2 is heated at 1000 to 600°C during casting.
The average cooling rate in the range of B is high, and the HAZ toughness is decreased. In A3, the slab heating temperature before rolling is high, and the toughness of the base material and HAZ toughness are decreased.

Also, like board A2, board B2 has a value of 1000 to 600.
The average cooling rate in the range of .degree. C. is significantly outside the range of the present invention, and therefore the HAZ toughness is low.

Furthermore, the component ranges of DI, El, Fl, and Gl deviate from the present invention. In other words, the board D1 is B
The value given by the formula x 1oooo ten Nbx 1000 is outside the scope of the present invention, B, the HAZ toughness is decreased, and the board E1 is Mn/6 + (Cu + Ni)/15
If the value given by the formula B exceeds the range of the present invention, the HAZ toughness is still low.

In addition, since the Ti/N ratio of the board F1 is outside the range of the present invention, the HAZ toughness is low, and the board G1 has a high S content, so the HAZ toughness is low.

(Effects of the Invention) As described above, according to the present invention, the low-temperature toughness of the HAZ is stabilized even by high heat input welding, and a high-level steel material can be obtained, so that the present invention is extremely useful industrially.

[Brief explanation of drawings]

Figure 1 is a chart showing the number of TiN-Mn5 precipitates contained in steel and changes in toughness after welding heat cycles, and Figure 2 is a graph showing the average cooling rate in the temperature range of 1000 to 600°C during solidification and TiN- Figure 3 is a diagram showing the relationship between the number of Mn5 precipitates, Figure 3 is a diagram showing the influence of the amount of B and Nb added on the toughness after 0 welding thermal cycles, and Figure 4 is a diagram showing the effect of the amount of B and Nb added on the toughness after a heat input of 200 kJ/(Jl). A chart showing the influence of the amount of S on the impact value of the HAZ after single-sided latent arc welding. Figure 5 shows the HAZ after single-sided latent arc welding with a heat input of 200 kJ/am.
Mn/6+(Cu +N+)/15 on the impact value of
Mn and Cu expressed by the formula. It is a chart showing the influence of Ni. 1

Claims (1)

[Claims] As weight percent, C: 0.02-0.15% Si: 0.3% or less Mn: 0.5-2.0% Ni: 0.2-1.5% Cu: 0. 2 to 1.5% However, Mn, Ni, and Cu are Mn/6+(Cu+Ni)/15=0.28 to 0.40
%N: 0.0020 to 0.010% One or two of B and Nb such that the value of the formula B x 10000 + Nb x 1000 is 4 to 10 in weight %, Ti and N in weight % Steel with a ratio (Ti/N) of 2.0 to 4.0 of Ti, Al: 0.005 to 0.1%, S: 0.003 to 0.008%, and the balance consisting of Fe and unavoidable impurities is continuously produced. It is characterized by casting, followed by cooling at an average cooling rate of 5.0°C/min or less in the range of 1000°C to 600°C, and then heating to 1150°C or less before rolling. A method for producing high heat input welding steel with excellent low-temperature toughness.