JPH02250917A - Production of steel for large heat input welding excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a steel for large heat input welding having high hardness and excellent in toughness in a weld heat-affected zone when used in a cold district, etc., by subjecting a continuously cast slab with a specific composition to cooling under specific conditions and then to reheating and hot rolling. CONSTITUTION:As a steel for large heat input welding for use in marine structure having high strength and excellent in toughness in a weld heat-affected zone when used in a cold district, such as the polar regions, a steel which has a composition (containing, by wt.%: 0.02 to 0.3 C, <0.3 Si, 0.50 to 2.50 Mn, 0.2 to 4.5 Ni, 0.003 to 0.015 Nb, 0.2 to 2.0 Cu, <0.01 N, Ti in an amount in the range where the weight ratio of Ti to N is regulated to 2.0 to 4.0, 0.005 to 0.1 Al, and 0.003-0.008 S or further containing, as elements for improving the strength of the steel, one or >=2 kinds among 0.1 to 1.0 Cr, 0.01 to 0.2 V, and 0.1 to 1.0 Mo) is continuously cast. The resulting continuously cast slab is cooled through a temp. region from 1000 to 600 deg.C at <=5.0 deg.C/min average cooling rate and then reheated up to <=1150 deg.C to undergo hot rolling, by which the steel stock for a cold district can be produced.

Description

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

(Conventional technology) Due to the recent increase in energy demand, oil in the ocean,
2. Description of the Related Art The development of natural gas and other resources is being actively carried out, and in particular, in search of richer oil resources, huge offshore structures have recently been constructed in cold regions such as the North Sea and the Arctic Ocean.

Such offshore structures are exposed to low temperatures of -30°C or lower and are operated under complex loading and stress conditions due to the effects of waves, etc., so the steel materials used in them must be Brittle fracture properties are required.

In particular, the toughness of the weld heat affected zone, which is lower in toughness than the base metal, directly affects the safety of the structure, so it is evaluated by impact tests, etc.
In some cases, a higher impact value is required.

In addition, as structures become larger, construction costs increase, so it is necessary to use high-strength steel, such as steel with a yield point of 36k.
Efforts are being made to reduce the weight of superstructures by using steel materials with a rating of g/d or higher, and to reduce welding costs by adopting high heat input welding methods.

As a method for manufacturing such steel materials, for example, as described in Japanese Patent Laid-Open No. 68-103021, controlled rolling with limited constituent elements and accelerated cooling are known. Such conventional technology uses normal welding heat input (50k
J/cm or less) does provide a steel material with excellent toughness in the weld heat affected zone, but this effect cannot be expected in high heat input welding.

Examples of techniques to improve the toughness of the weld heat affected zone include:
JP-A No. 80-245788 and JP-A No. 60-1
As described in Japanese Patent No. 52828, a technique has been proposed in which the toughness of a weld heat affected zone is improved by generating intragranular phyllite 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.

(Issue 8 to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing a steel material for marine structures having high strength and excellent weld heat-affected zone toughness, which is used in the above-mentioned cold regions and polar regions. It is something.

(Means for Solving the Problems) The present invention has been made to solve the above problems, and its gist is as follows: (1) As ffi amount %, C:
0.02~0. (%, S 1: 0.3% or less, Mn:
D, 50-2.50%, S: 0.003-o, oos
%, AJ: 0.005-0.1%, Nb: 0.003
~0.015%, Cu:o, 2-2.0%, Nl:0.
2 to 4.5%, N: 0.01% or less and T with a Ti to N ratio (TI/N) of 2.0 to 4.0.
(2) furthermore, V: 0.2% or less, Mo: 1
．． Steel containing one or more of the strength-improving element group consisting of 0% or less and 1.0% or less of Cr, with the balance consisting of Fe and unavoidable impurities is cast using a continuous casting machine, and then cooled. It is characterized by performing cooling such that the average cooling rate is 5.0°C/ml or less within the range of 1000"c to eoo'c, and then heating to 1150"C or less before rolling. This invention relates to a method for producing low-temperature steel with high heat input weldability.

(Function) Based on many experimental facts, the present inventors have found that: ■ Intragranular fluorites generated during the cooling process during welding are not only oxides;
This TiN-MnS composite precipitate is generated even from composite precipitates of TLN and MnS (hereinafter referred to as TlN-Mn5 precipitates) and improves the toughness of the weld heat-affected zone. It was discovered that it is possible to precipitate by controlling the cooling rate of .

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

Figure 1 shows the number of precipitates and heat input of TlN-Mn5 at 100k.
This is the change in toughness after applying a welding thermal cycle equivalent to J/cm.

The chemical components of the sample at this time were as follows.

Table 1 From this figure, it can be seen that the toughness after the welding thermal cycle improves as the TiN-MnS precipitates increase.

Furthermore, Fig. 2 shows the relationship between the average cooling rate and the number of TiN-Mn5 composite precipitates in the range of 1000 to 600°C after solidification when the same test material was used for the experiment. It can be seen that the number of precipitates can be significantly increased by reducing the temperature to 5.0° C./S or less.

From the above experimental facts, it is clear that by controlling the cooling rate after solidification, TiN-MnS, which becomes the transformation nucleus of intragranular ferrite, can be
It is possible to increase the number of composite precipitates and improve the toughness of the weld heat affected zone. It became clear.

In addition, since the TiN-MnS composite precipitate precipitated in this way easily dissolves when heated at a temperature of 1300 ° C. or higher, it is preferable that the slab heating temperature before subsequent hot rolling is low. should be heated to below 1150°C.

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

C is an element necessary to ensure strength, and in order to ensure strength, it is necessary to add 0.02% or more, but adding a large amount leads to a decrease in the toughness of the weld heat affected zone, so it is The upper limit is set to 0.3%.

Since adding a large amount of Sl inhibits the toughness of the weld heat affected zone, the upper limit is set at 0.3%.

Mn needs to be added in an amount of 0.5% or more to ensure strength, but since adding a large amount causes a decrease in toughness, the upper limit is set at 2.5%.

Ni is an effective element for toughness and hardenability, and at the same time Cu
It is also effective in reducing hot cracking, which is a problem when added.
If less than 0.2% is added, the effect is not recognized, and if added in a large amount, Ni is expensive, so the content is limited to 4.5% or less.

N is an important element that combines with Ti to form precipitates, but if it exists freely in steel, it causes a decrease in the toughness of the weld heat affected zone, so its upper limit is set to o, oto%.

TI is an essential element for the steel of the present invention, combines with N to precipitate TiN, and acts as a precipitation nucleus for MnS. Therefore, in order to obtain optimal TiN, it is necessary to control the amounts of Tl and N. In other words, the weight ratio of Ti and N is 2.0.
If it is less than 4.0, there will be an excess of N, leading to a decrease in the toughness of the weld heat-affected zone, and if the TI/N exceeds 4.0, there will be an excess of Ti, causing TiC to precipitate, resulting in a decrease in the toughness of the base metal.

Nb is an element necessary to ensure the strength and toughness of the base metal, and if it is added less than 0.003%, the effect of suppressing recrystallization is lost and the toughness of the base metal decreases, and on the contrary, if it exceeds 0.015% Since addition causes a decrease in the toughness of the weld heat affected zone, it is limited to the above range.

Cu is an element that is effective in increasing strength, but if it is less than 0.2%, it has no effect, and if it exceeds 2.0%, it will cause cracks during hot working and inhibit weldability. .2
-2.0%.

S is an important element for the precipitation of MnS, and as shown in Figure 3, if it is added less than 0.008%, the precipitation will be insufficient, and if it is added more than 0.008%, it will cause MnS precipitation.
Since a large amount of S precipitates and actually impairs toughness,
．． It is limited to the range of 0.008% to 0.008%. in this case,
More preferable effects can be obtained within the range of 0.008 to 0.005%.

The base composition of the steel in Figure 3 is 0.05C-0.11S 1
-1.57Mn-0,005P -0,30Cu -0
,3ONi −0,01ONb −0,008TI −
It is 0,003 ON.

AJ 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 significantly reduces the toughness.

In the present invention, in addition to the above basic component system, Cr.

One or more types of V and Mo are added. These components have a uniform effect of improving the strength of steel, and in order to ensure the desired effect, the lower limits of content should be set to 0.1% for Cr, 0.01% for V, and 0.01% for Mo: 0.1
It needs to be %. However, Cr: 1.0%
, V: 0.2%, Mo: 1.0%, the weldability and base metal toughness will be reduced, so the limits are set as above.

After melting steel having the above-mentioned components in an electric furnace or converter and casting it in a continuous casting machine, the cooling rate after solidification is 1000-
Cooling is performed such that the temperature is below 5.0"C,'+In in a temperature range of 800°C.

In order to improve the toughness of the weld heat affected zone, T i N
- For companies that need to ensure the number density of MnS composite precipitates, it is necessary to finely disperse TiN precipitates that serve as precipitation nuclei of MnS.

That is, from conventional knowledge, during solidification (1500 to 1200
It is known that the faster the cooling rate (°C) is, the finer the TiN precipitates are dispersed, so the continuous casting method, which has a faster cooling rate during solidification than the agglomeration blooming method, is used.

MnS precipitates on the TiN precipitates thus precipitated in a temperature range of 1000°C or less, but there are constraints on the generation of TiN-MnS composite precipitates that are effective in improving the toughness of the weld heat-affected zone. If the cooling rate exceeds 5.0°C/mln, the formation of appropriate TiN-MnS composite precipitates is insufficient, and they do not act as precipitation nuclei for intragranular ferrite that is transformed and generated during cooling during welding. Improvement in the toughness of the weld heat affected zone cannot 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, and the temperature at that time is 1150°C in order to ensure the strength and toughness of the base material and to not change the form of the TiN-MnS composite precipitate due to the heat treatment mentioned above. It is necessary to do the following.

Regarding rolling after heating, in order to improve the strength and toughness of the base material, controlled rolling is performed, and even if water cooling is performed after controlled rolling, no change will occur in the TiN-MnS composite precipitate. Therefore, currently known manufacturing methods can be appropriately selected and employed.

(Example) The chemical components of the test materials are shown in Table 2.

Here, Steel A to Steel G are component systems that fall under the present invention,
Steel H is a steel that deviates from the invention.

Table 3 also shows the manufacturing conditions of the test materials, the strength and toughness of the base metal, and the toughness of the welded part.

These steel plates were melted in a converter and then cast in a continuous casting machine to a thickness of 240 mm and a width of 1800 mm. Thereafter, it was reheated and hot rolled to form a 32 Dragon steel plate and subjected to testing.

The toughness of the weld heat-affected zone was evaluated by Charpy impact test after single-sided, single-layer latent arc welding (heat input: 200 kJ/c+n).

From Table 3, the steel plate manufactured according to the present invention (plate number 1.3
．． 5.6.7.8.10) shows excellent toughness in both the base metal and the weld heat affected zone.

On the other hand, plate numbers 2 and 4 are 1000 to 8 when cast.
The average cooling rate at 00°C is high, and the toughness of the weld heat affected zone is reduced. In plate No. 9, the slab heating temperature before rolling deviates from the present invention, and the toughness of the base material is reduced. Plate No. 11 has a component range that deviates from the present invention, but in this case, even if the manufacturing conditions are within the range of the present invention, the toughness of the weld heat affected zone is inevitably reduced.

(Effects of the present invention) As described above, according to the present invention, a steel material of high quality with stable low-temperature toughness of the weld heat-affected zone can be obtained even by high heat input welding, which is extremely useful industrially. It is.

[Brief explanation of drawings]

Figure 1 is a chart showing the TLN-MnS composite precipitate and the change in toughness after welding thermal cycles, and Figure 2 is a graph showing the toughness of the TLN-MnS composite precipitate after the welding thermal cycle.
Average cooling rate and TiN-Mn in the temperature range of 00°C
5 Diagram showing the number and relationship of composite precipitates, Figure 3 shows heat input 2
2 is a chart showing the relationship between the toughness of the weld heat affected zone and the amount of (S) when single-sided, single-layer latent arc welding is performed at 00 kJ/am. Agent Patent Attorney Tatsuo Chanoki Figure 1, Figure 2! J7# Rejection degree ("CAM clear") (S) x fO' (wt9)

Claims (1)

[Claims] 1. As weight%, C: 0.02-0.3% Si: 0.3% or less Mn: 0.50-2.50% Ni: 0.2-4.5% Nb : 0.003 to 0.015% Cu: 0.2 to 2.0% N: 0.01% or less by weight, so that the Ti to N ratio (Ti/N) is 2.0 to 4.0. A steel containing Ti, Al: 0.005-0.1%, S: 0.003-0.008% and the balance consisting of Fe and unavoidable impurities is continuously cast, and the subsequent cooling rate is set to 1000°C to 600°C. A steel for high heat input welding with excellent low-temperature toughness characterized by being cooled at an average cooling rate of 5.0°C/min or less in the range of Production method. 2. One or more of the strength improving element group consisting of Cr: 0.1-1.0% V: 0.01-0.2% Mo: 0.1-1.0% as weight% 2. The method for producing a high heat input welding steel having excellent low-temperature toughness according to claim 1, wherein the steel further contains Fe and inevitable impurities.