JP2930772B2 - High manganese ultra-high strength steel with excellent toughness of weld heat affected zone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention has a tensile strength of 100 kg.
f / mm ² or more and welding heat input is 40 kJ / cm to 2
Even in welding of a wide range of heat input from medium heat input of about 00 kJ / cm to large heat input welding, it relates to a high manganese ultra-high tensile steel with excellent toughness in the weld heat affected zone having good low temperature toughness in the weld heat affected zone. Things.

[0002]

2. Description of the Related Art In recent years, requirements for materials of large structures, such as marine structures, shipbuilding, and storage tanks, have become increasingly strict in terms of ensuring safety. In particular, it is desired to improve the low-temperature toughness of the heat-affected zone of the weld, which tends to deteriorate compared to the base metal. Generally, when performing automatic welding with large heat input such as submerged arc welding or electroslag welding to steel materials,
When the austenite crystal grains in the weld heat affected zone (hereinafter, referred to as HAZ) become coarse, the HAZ structure becomes coarse and the HAZ toughness is significantly reduced. In order to improve the HAZ toughness, it is necessary to reduce the size of the HAZ, particularly the HAZ structure near the fusion portion (fusion line, hereinafter referred to as FL) exposed to high temperatures. Conventionally, various types of H
An AZ structure refinement method has been proposed. For example, Showa 5
In “Iron and Steel”, Vol. 65, No. 8, page 1232, issued in June 2004, the austenite grains of HAZ are refined by finely precipitating TiN, and large heat input of 50 kgf / mm ² class high strength steel is performed. A technique for improving the HAZ toughness during welding is disclosed. More recently, the formation of intragranular ferrites without resorting to austenite refinement has led to the
Techniques for miniaturizing the AZ structure have been developed. Ti oxide is effective as a nucleus for forming intragranular ferrite,
Oxide does not dissolve even when exposed to high temperature, FL
In the immediate vicinity, it acts as a core of intragranular ferrite, and can refine the structure. Compared with steel using TiN or the like, H
It is disclosed, for example, in Japanese Patent Application Laid-Open No. 61-117245 that the AZ toughness can be significantly improved.

However, any of the above HAZ toughness improving techniques can be applied only to a component region in which ferrite is formed even in a part of the HAZ structure, and is at most 60 kgf / mm in view of the tensile strength of the base material.
Up to level ² . Further, in steel having a higher strength level, a microstructure refining technique similar to the above-mentioned technique was not recognized, and almost the only method of improving the HAZ toughness was to add a large amount of Ni. That is, a large amount of Ni, such as 5 to 6%, and even about 9%
In the case of steel containing, the HA of a base metal and a weld having a not so large welding heat input even at a very low temperature, generally -100 ° C or lower.
It is possible to guarantee Z toughness. However, even with such a high Ni steel, the HAZ toughness in the so-called large heat input welding with welding heat input of about 200 kJ / cm has not yet been easily assured. In addition, such a high Ni steel is inevitably very expensive compared to a normal steel material, so that it cannot be widely used for all structures, and is inexpensive and has excellent HAZ toughness even under various welding conditions. It is desired to establish a manufacturing technique for high-strength steels having high strength, particularly high-strength steels having a strength level at which ferrite generation is difficult.

[0004]

As a means for increasing the strength without using expensive alloy elements such as Ni, the use of Mn can be considered. Since Mn is an element that is inexpensive and can increase hardenability with a small amount of addition compared to Ni, the present inventors have attempted to maximize its effective use by inexpensive high strength and toughness. Considering one possibility of steel, as a result of various investigations, the tensile strength of the base material was 100 kgf / mm ² or more without using expensive alloying elements other than Mn as much as possible.
In addition, the present invention has led to the invention of a high manganese ultra-high tensile steel having excellent toughness of a weld heat affected zone. Usually, tensile strength 100
When a high strength of kgf / mm ² or more is to be obtained only by the addition of Mn, the addition amount inevitably requires several percent or more. In this case, the susceptibility to grain boundary fracture increases regardless of the base material or HAZ, Generally, toughness is greatly deteriorated due to grain boundary fracture, and when Mn is simply increased, it is very difficult to improve the toughness of the base material and HAZ even if the strength can be increased. The general relationship between HAZ toughness and structure is very well known in low alloy steels, and it is commonly known that HAZ toughness deteriorates most in the case of upper bainite structure. In order to suppress this toughness degradation, it is common to refine the ferrite structure typified by intragranular ferrite transformation on the low-strength side, and to form the lower bainite-based structure by optimizing hardenability on the high-strength side. This is an effective method. However, this finding is for a low alloy steel having a relatively small Mn content of about 1%, and it is not completely clear whether the conventional finding is applicable to a high Mn steel much exceeding 2%, for example. . Therefore, in order to improve the toughness of both the base material and the HAZ at a high Mn, which is the basis of the present invention, it is necessary to clarify the optimum structure and to find a means for obtaining it and a means for preventing grain boundary fracture at the same time. Becomes

[0005]

The present inventors have conducted detailed studies to overcome these problems, and as a result, have found that the base material has a tensile strength of 100 kg without adding a large amount of expensive Ni.
f / mm ² or more, not only the base material toughness but also the toughness of the weld heat-affected zone in a wide heat input range including large heat input welding. .06
%, Si: 0.01 to 1.0%, Mn: 6 to 15%,
P: 0.01% or less, S: 0.01% or less, Al: 0.
005 to 0.1%, B: 0.0003 to 0.010%,
N: 0.010% or less, and if necessary, N
i: 3.0% or less, Cu: 1.5% or less 1 or 2
The present invention has led to the invention of a high manganese ultra-high tensile strength steel containing a seed and the balance consisting of Fe and unavoidable impurities.

Hereinafter, the gist of the present invention will be described in detail based on experimental results. In the present invention, C, Mn, and B are the most important compositions. First, regarding the toughness of the weld heat affected zone, in the case of a conventional high-strength steel containing Cr, Mo, Ni, etc. for the purpose of low-temperature toughness, when the composition becomes a fine upper bainite or lower bainite and a martensitic structure. The best toughness is shown, for example, in "Iron and Steel", Vol. 65, No. 8, page 1223, issued in June 1979. However, without using these expensive alloy elements,
It has been found that when aiming to secure the strength toughness mainly with only Mn, good toughness cannot be obtained in the component range where bainite is present. That is, C is about 0.02%
Alternatively, 0.1%, Si is about 0.1%, and Mn is about 1-2.
Welding reproduction thermal cycle assuming large heat input welding using small vacuum melting steel of 0%, Al about 0.03%, N about 0.003%, B added or about 0.0010% Toughness (Maximum heating temperature: 1400 ° C, 800 to 500 ° C
As a result, the absorption energy in the Charpy test was 7 k for a component having Mn of 4 to 5% or less in which a bainite structure exists, as shown in FIG.
gfm become temperature (vTr _{7. 0)} when evaluating the toughness in, there is no range in which a good toughness is obtained, but rather Mn as found in B-added steel is more than about 6%, about 1
It has been found that good HAZ toughness can be obtained in a component region where a martensitic structure of 00% is obtained, and in order to improve HAZ toughness, it is necessary to reduce the amount of C with the optimization of the amount of Mn and to add B. . In addition, microscopically, the structure in the composition range of the steel of the present invention may contain body-centered tetragonal martensite, some retained austenite depending on the composition, and even a small amount of dense hexagonal ε martensite. However, the quantity is mainly tetragonal martensite. Furthermore, the steel of the present invention has very high hardenability, and even if the cooling rate greatly changes depending on the welding conditions, basically all of them have a martensitic structure, so that little change in toughness is observed. By the way, we have 800-500 ° C
The toughness under heat cycle conditions of 30 seconds and 320 seconds of cooling time was also investigated, and in all cases, results similar to those in FIG. 1 were obtained. For the same reason as for the base metal, the steel of the present invention usually has a martensitic structure stably within the range of an industrially obtainable cooling rate. Regardless of the quenching and tempering treatment, the change in strength and toughness is very small. Further, since the Mn content is as high as 6% or more, the C content is 0.01% or more.
Despite being as low as 0.06%, the tensile strength is 100%.
kgf / mm ² or more can be stably achieved.

[0007]

The above is the basic gist of the present invention. In order to achieve the desired characteristics of the present invention, it is necessary to limit the amount of each constituent element to an appropriate range as described below. First, C is an effective component for improving the strength. According to the results of detailed studies by the present inventors, in a high Mn steel as in the present invention, as the C content increases, the base material toughness, HAZ toughness deteriorates. The adverse effect of C is more pronounced in base metal toughness, with vT
r ₇ . ₀ increases by about 10-15 ° C. In the present invention, C is set to 0.01% to 0.06% as a range in which the strength of the base material can be secured and the toughness of the base material is not extremely deteriorated. Next, Si is an element effective in deoxidizing molten steel and is also effective in increasing the strength, but when added in a large amount, coarse oxides are easily generated, and the strength as in the present invention is high. Since high steel greatly impairs ductility and toughness, the range is 0.01 to 1.0%.
Mn is one of the most important constituent elements of the present invention. In the present invention, the lower limit is set to 6% as an amount required to substantially form a martensite single phase structure in a wide range of the cooling rate and to stably secure the base material strength and the HAZ toughness. When the amount of Mn is further increased, the HAZ toughness is improved.
On the other hand, when the content exceeds 15% as observed in the 02% CB added steel, the toughness starts to deteriorate. Similarly, the base material toughness tends to deteriorate when the Mn content exceeds 15%.
Since the deterioration amount is more remarkable than the HAZ toughness, the upper limit of the Mn content is set to 15% in the present invention. P
In order to promote grain boundary embrittlement and deteriorate the toughness of both the base material and the HAZ, it is preferable to reduce as much as possible. However, the allowable amount is set to 0.01% or less. S also forms MnS and segregates at grain boundaries to deteriorate ductility and toughness.
Although it is preferable to reduce as much as possible, the allowable amount is set to 0.01% or less. Al is effective as a deoxidizing element like Si, but when added excessively, it forms a coarse oxide and causes deterioration of ductility and toughness.
-0.1%. B is an element that is particularly important for suppressing grain boundary embrittlement in high Mn steel, and it is necessary to add 0.0003% or more in order to produce the effect. However, when the addition exceeds 0.010%, precipitates are easily formed, the effect of suppressing grain boundary embrittlement is lost, and toughness is deteriorated by the precipitates.
The range was 3 to 0.010%. N forms BN to form B
In order to reduce the effect of suppressing grain boundary embrittlement, it is preferable that the content is small, but as an acceptable range, the upper limit is set to 0.010.
%. The above is the reason for limiting each of the basic components of the steel of the present invention. For the purpose of improving the toughness of the base material and the HAZ, one or more of Ni and Cu can be contained as necessary. As the Ni content increases, both the base material toughness and the HAZ toughness improve as the transition temperature, but on the other hand, the shelf energy tends to decrease, and even if added over 3.0%, the toughness improving effect is obtained. Is saturated, so the upper limit is set at 3.0% in consideration of economy. The effect of Cu is qualitatively Ni
However, the addition of a large amount exceeding 1.5% makes the problem of slab cracking and precipitation embrittlement remarkable, so the upper limit was made 1.5%.

[0008]

EXAMPLES Table 1 shows the chemical composition, base material strength toughness, HAZ toughness, and the like of the steel sheet and the comparative steel sheet manufactured in accordance with the present invention. Here, No. 1 to No. 12 is the steel of the present invention,
No. 13- No. 22 is a comparative steel. The steel of the present invention and the comparative steel were rolled into 20 mm steel plates. No. of the steel of the present invention. 1 to No. 8 and the comparative steel were further subjected to quenching and tempering after ductility, and then a characteristic investigation was conducted.
In addition, the steel of the present invention has no. 9-No. For No. 12, the characteristics were examined using test pieces taken from the as-rolled material.
All test pieces were taken from the center of the sheet thickness in a direction parallel to the rolling direction. The strength of the base metal was evaluated by the 0.2% proof stress and tensile strength of the round bar tensile test, while the base metal toughness was evaluated by the absorbed energy at −60 ° C. in the Charpy impact test. In addition, the HAZ toughness has a maximum heating temperature of 1400 ° C,
The cooling energy from 800 ° C. to 500 ° C. was evaluated by the absorbed energy at −60 ° C. in the Charpy impact test when a reproduction heat cycle of 160 seconds was applied. By the way, the heat cycle conditions are as follows.
This corresponds to the thermal history in FL at the time of submerged arc welding at 0 kJ / cm. As is clear from Table 1, 1
-No. Twelve inventive steels have a better HAZ than the comparative steels
It can be seen that it has toughness and shows sufficient absorbed energy in Charpy test even at a low temperature of -60 ° C to ensure the safety of the structure. The base material has a tensile strength of 100 kgf / m.
m ² or more and excellent toughness. That is, according to the present invention, it is apparent that a steel having very high strength, excellent base metal, and HAZ toughness can be obtained. On the other hand, No.
13- No. It is also evident from Table 1 that the comparative steel No. 22 does not satisfy the requirements of the present invention, and therefore, the base metal strength, toughness or HAZ toughness is inferior to the steel of the present invention. That is, the comparative steel No. No. 13 has a low base metal tensile strength and a low H
AZ toughness is also poor. No. In No. 14, although the Mn content was within the range of the present invention, deterioration of HAZ toughness was particularly remarkable because B was not added. In No. 15, both the base material toughness and the HAZ toughness are extremely low due to an excessive amount of Mn and no addition of B. No. 1
6, no. No. 17 contains B, but has an insufficient Mn content, so that the base material toughness or the HAZ toughness is not sufficient. In addition, No. 18, No. No. 19 has a high strength because of an excessive C content, but has very low base metal toughness and HAZ toughness. Comparative steel No. 20-No. No. 22 shows that any one of P, N and S is out of the range of the present invention,
The AZ toughness is greatly deteriorated as compared with the steel of the present invention. From the above examples, according to the present invention, the tensile strength is 100 kg.
It is evident that an ultra-high-strength steel having an HAZ toughness and an excellent base material that can withstand safe use even at f / mm ² or more and at a low temperature of about −60 ° C. is obtained.

[0009]

[Table 1A]

[0010]

[Table 1B]

[0011]

According to the present invention, an ultra-high tensile strength having very high tensile strength, excellent base metal toughness, and excellent HAZ toughness over a wide heat input range without containing a large amount of expensive alloying elements such as Ni. The use of steel according to the present invention makes it possible to produce a high-strength, highly safe welded structure even under severe use conditions, and the effect is extremely remarkable. is there.

[Brief description of the drawings]

FIG. 1 shows the maximum heating temperature of steel with different amounts of C, B, and Mn and the amount of Mn when a welding reproduction heat cycle in which the cooling time from 800 ° C. to 500 ° C. was 160 seconds was applied. FIG. 4 is a diagram showing the relationship between the characteristic and the Charpy characteristic.

Continuation of front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00 302 C22C 38/06

Claims (2)

(57) [Claims] C: 0.01 to 0.06% in weight% Si: 0.01 to 1.0% Mn: 6 to 15% P: 0.01% or less S: 0.01% or less Al: B: 0.005% to 0.1% B: 0.0003% to 0.010% N: 0.010% or less, with the balance being Fe and unavoidable impurities. High manganese ultra-high tensile steel. 2. C: 0.01 to 0.06% in weight% Si: 0.01 to 1.0% Mn: 6 to 15% P: 0.01% or less S: 0.01% or less Al: 0.005% to 0.1% B: 0.0003% to 0.010% N: 0.010% or less Ni: 3.0% or less Cu: 1.5% or less High manganese ultra-high tensile steel with excellent toughness in the heat-affected zone of the weld, characterized by containing a seed.