JP2001020034A - Non-heattreated type steel for low temperature use excellent in large heat input weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide non-heat treated type high tensile strength steel having excellent weld zone toughness even in large heat input welding of >=200 kJ/cm heat input. SOLUTION: This steel contains, by weight, <=0.02% C, <=0.50% Si, 0.5 to 2.0% Mn, 0.005 to 0.10% Al, 0.010 to 0.10% Nb, 0.3 to 3.0% Ni, 0.0003 to 0.0040% B and <0.0050% N, and the ratio between the B content and the N content, i.e., B/N is also controlled to the range of 0.3 to 1.0. The steel may moreover contain one or >= two kinds among 0.05 to 0.70% Cu, 0.10 to 0.60% Cr, 0.10 to 0.50% Mo and 0.005 to 0.05% Ti and may further contain 0.0005 to 0.0100% Ca.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a low-temperature steel material suitable for a liquefied gas storage tank, and particularly to a large-sized steel material having good weld toughness even when subjected to large heat input welding of 200 kJ / cm or more. The present invention relates to a non-heat treated low temperature steel excellent in heat input weldability.

[0002]

2. Description of the Related Art In recent years, use of liquefied gas, which is cleaner energy, has been increasing in consideration of the environment. For this reason, there has been a demand for the development of a low-temperature steel material suitable for use as a member for such a gas carrier and storage tank. In addition, from the viewpoint of ensuring safety, welding with limited heat input has been performed on this type of low-temperature steel material from the viewpoint of ensuring safety. However, recently, from the viewpoint of economy, welding construction by large heat input welding has been directed, and even when large heat input welding is performed, a non-heat treated low temperature steel material having excellent weld toughness has been demanded. I have.

In general, the toughness of a weld is determined by the heat-affected zone of the base metal,
In particular, it is determined by the toughness of the bond. Since the bond portion is heated to a high temperature just below the melting point, the crystal grains become the coarsest, so that the subsequent cooling produces a fragile martensite structure and an upper bainite structure, which lowers the notch toughness. In particular, in so-called large heat input welding such as electrogas welding and submerged arc welding, this tendency is remarkably exhibited.

[0004] As methods for preventing such toughness deterioration of the welded portion, there are roughly three methods: (1) suppression of coarsening of austenite grains using inclusions and precipitates (2) control of B / N (3) Toughening of the post-transformation structure is considered.

[0005] As an example of the above (1), for example,
No. 184663 discloses Ti nitrides and rare earth elements (RE
A technique has been disclosed in which sufficient low-temperature toughness of a welded portion can be obtained even when welding with a heat input of 100 kJ / cm or more by effectively finely dispersing the sulfated oxide of M). As an example of the above (2), for example, Japanese Patent Publication No. 55-31820
In the publication, REM and B coexist, and B content and N
A large heat input welding steel in which the content ratio, B / N is controlled to 0.3 to 1.0, has been proposed, and as a result, the structure of the single-layer weld bond having a heat input of 60 kJ / cm or more becomes a fine ferrite + pearlite structure. It is said that the notch toughness of the weld is improved.

Japanese Patent Application Laid-Open No. 52-41111 discloses (B
(%) - 0.77N (%) ) × 10 3 to -2.2 tone excellent high heat input weld toughness for limiting the 1.0 quality type low alloy high strength steel have been proposed. In the technique described in Japanese Patent Application Laid-Open No. 52-41111, it is considered that B can be used as solid solution B to secure sufficient quenchability, thereby making the structure of the heat-affected zone lower bainite and obtaining excellent toughness. I have.

As an example of the above (3), for example, Japanese Patent Publication No. 59-11658 discloses that C is set to 0.03% or less and the amount of island-like martensite in the HAZ portion of the large heat input welded joint is set to a certain amount or less. In addition, a technique has been disclosed in which a steel having excellent low-temperature toughness and capable of high-efficiency welding can be manufactured by adding Ni that is particularly effective for low-temperature toughness.

Japanese Patent Application Laid-Open No. 56-150157 discloses that C is
A low-temperature steel excellent in weld toughness having a strength of 40 to 70 kgf / mm ² in tensile strength utilizing the quenching effect of B and making use of the quenching effect of B is proposed. Also, JP-A-59-5363
No. 53 proposes an ultra-thin low-temperature steel having excellent weld toughness in which C is made 0.03% or less, B is added, and Ni is added in an amount of 2.0 to 4.0%.

[0009]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open
As in the technique described in US Pat.
Precipitates such as nitrides and sulfates of rare earth elements (REMs) are effectively and finely dispersed to prevent coarsening of austenite grains during large heat input welding and to prevent deterioration of weld toughness. There were the following two problems.

One is that when adding REM, REM
The problem is that REM inclusions such as sulfated oxides tend to cause agglomeration and coarsening, and serve as a starting point for fracture, deteriorating toughness. Another problem is that since a part of TiN is redissolved at a high temperature, solute N increases in the heat-affected zone, and the toughness decreases. Further, in the technique described in Japanese Patent Publication No. 55-31820, REM is contained, B / N is controlled, and the weld bond is made into a fine ferrite + pearlite structure to improve the toughness of the weld bond. However, there is a problem that it is impossible to obtain a fine ferrite + pearlite structure by large heat input welding at a heat input of 200 kJ / cm or more in which the austenite grains become coarser, and the toughness of the weld is deteriorated.

According to the technology described in Japanese Patent Application Laid-Open No. 52-41111, B is precipitated as BN at the time of a large heat input welding of 200 kJ / cm or more at a slower cooling speed, and the quenching effect of solid solution B is sufficient. There was a problem that it could not be demonstrated. In addition, this technology is disadvantageous in terms of economy because it performs a refining process. Further, the technique described in Japanese Patent Publication No. 11658/1984 has a problem that a large amount of expensive Ni is added, which increases the production cost and is disadvantageous economically.

Further, in the techniques described in JP-A-56-150157 and JP-A-59-53653, by setting C to 0.03% or less, very excellent welding can be achieved at a heat input of 50 kJ / cm. It is said that welded toughness can be obtained, but in recent years there has been a demand for a steel material that can be welded with even higher heat input. The present invention advantageously solves the above-mentioned problems of the prior art and is a hot-rolled non-heat treated low-temperature steel material that does not require quenching and tempering, and has a large heat input of 200 kJ / cm or more. It is an object of the present invention to propose a non-heat treated low temperature steel material having excellent heat input weldability having excellent toughness even in a heat welded joint portion.

[0013]

Means for Solving the Problems The present inventors have achieved the above-mentioned object without using the means for suppressing austenite grain coarsening utilizing inclusions and precipitates, and further, the structure of the weld heat affected zone. A method for improving the toughness of a large heat input welded joint without directing a ferrite + pearlite structure was intensively studied. As a result, the present inventors
The effect of alloying elements such as N and B on the toughness of a large heat input welded joint with a heat input of 200 kJ / cm or more was studied in a system with the amount reduced as much as possible. As a result, it has been found that there is an optimum range of B / N with respect to the toughness of the large heat input welded joint having a bainite structure. Furthermore, the present inventors set the B / N to an appropriate range and set the ultra-low carbon of 0.02 wt% or less to obtain a uniform ultra-low carbon bainite structure in the base material and the welded portion, and added a large amount of Ni It has been found that it is possible to prevent a decrease in toughness of a large heat input welded joint portion without the need for a steel sheet.

Further, the present inventors have found that, by adding an appropriate amount of Mn and Nb, it has a tensile strength of 490 MPa or more and is capable of performing a large heat input welding with a heat input of 200 kJ / cm or more. It has been found that low temperature steel can be produced as hot rolled.
First, the experimental results on which the present invention is based will be described. 0.2 wt% Si-1.4 wt% Mn-1.0 wt% Ni-0.025 wt%
Thick steel plates (20 mm thick) were produced in which Nb was a basic component, C was changed at 0.05 wt% or less, and B and N contents were changed. A reproducible heat cycle test specimen was collected from these steel sheets, subjected to a heat cycle equivalent to the bond of an electrogas arc welded joint with a heat input of 200 kJ / cm, and then a JIS No. 4 impact test specimen was collected and subjected to Charpy absorption at a test temperature of -50 ° C. Energy values were determined.

Relationship between Charpy absorbed energy (vE _-50 ) at 50 ° C. and B / N (C: 0.01 to 0.02 wt%)
Is shown in FIG. 1 and the relationship with the C content is shown in FIG. Note that FIG.
Is for steel sheets having a B / N in the range of 0.4 to 0.8. From FIG. 1, it can be seen that the vE- ₅₀ equivalent to the bond portion of the electrogas arc welding with a heat input of 200 kJ / cm has a high toughness of 41 J or more only when the B / N is 0.3 to 1.0. From FIG. 2, it can be seen that by setting the C content to 0.02 wt% or less, the vE _-50 corresponding to the bond portion of the electrogas arc welding with a heat input of 200 kJ / cm becomes 41 J or more, and high toughness is obtained. These results are summarized and shown in FIG. 3 in the relationship between B / N and C amount. FIG. 3 shows that excellent weld toughness (vE _-50 ) can be obtained when the B / N is 0.3 to 1.0 and the C content is 0.02 wt% or less.

The present invention has been made based on the above findings. That is, in the present invention, C: 0.02 wt% or less, Si: 0.50 wt% or less, Mn: 0.5 to 2.0 wt%, Al: 0.005 wt%
~ 0.10wt%, Nb: 0.010 ~ 0.10wt%, Ni: 0.3 ~ 3.0
wt%, B: 0.0003 to 0.0040 wt%, N: less than 0.0050 wt%, and the ratio of B content to N content, B / N is 0.3
A non-heat treated low temperature steel excellent in large heat input weldability, characterized by having a composition of the balance of Fe and unavoidable impurities. Further, in the present invention, in addition to the above composition, Cu: 0.05 to 0.70 wt%, Cr: 0.10 to
0.60wt%, Mo: 0.10 ~ 0.50wt%, Ti: 0.005 ~ 0.05wt
%, One or more of them may be used. Further, in the present invention, in addition to each of the above compositions,
Ca: 0.0005 to 0.0100 wt% may be contained.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the chemical components of the steel material of the present invention will be described. C: 0.02 wt% or less C is an important element that governs the structure of the weld heat affected zone. In the present invention, the island that eliminates the formation of the pearlite phase in the equilibrium state and deteriorates the toughness even in the weld heat affected zone. C is 0.02wt% to suppress the formation of martensite
The following extremely low carbon was used.

Si: 0.50 wt% or less Si acts as a deoxidizing element at the time of refining and is an indispensable element. However, if it exceeds 0.50 wt%, the base material toughness is significantly deteriorated. For this reason, Si was limited to 0.50 wt% or less. Mn: 0.5 to 2.0 wt% Mn is an element that greatly affects the continuous cooling transformation behavior of steel materials in an extremely low carbon region, and has a granular bainitic ferrite structure (α _B ) that is rich in toughness in the weld heat affected zone. In order to obtain it, a content of 0.5 wt% or more is required. But 2.0
If the content exceeds wt%, bainitic
Produces a ferrite structure (α ° _B ). For this reason, Mn
It was limited to the range of 0.5 to 2.0 wt%.

Al: 0.005 to 0.10 wt% Al acts as a deoxidizing agent, and the present invention requires a content of 0.005 wt% or more. On the other hand, if the content exceeds 0.10 wt%, oxide inclusions increase in the steel, which causes an increase in surface defects and a decrease in base material toughness. For this reason, Al
Was limited to the range of 0.005 to 0.10 wt%.

Nb: 0.010 to 0.10 wt% Nb, like Mn, is an element that greatly affects the continuous cooling transformation behavior of steel materials in an extremely low carbon region. An important element for obtaining a ferrite structure (α _B ).
Requires a content of at least t%. However, when the content exceeds 0.10 wt%, a baitic ferrite structure (α ° _B ) having low toughness is generated. Therefore, Nb is 0.010 to 0.10
Limited to the wt% range.

Ni: 0.3 to 3.0 wt% Ni is an element that is extremely effective in increasing the strength and improving the toughness of the heat affected zone, and is an essential element in low-temperature steel. However, when the content is less than 0.3 wt%, such an effect is hardly recognized, and therefore, the lower limit of the Ni content is set. On the other hand, Ni is an expensive element, and even if contained in excess of 3.0 wt%, the effect of improving toughness tends to be saturated, and an effect commensurate with the content cannot be expected. Therefore, in the present invention, 3.0wt
% Is the upper limit of the Ni content.

B: 0.0003 to 0.0040 wt% B is an element that improves the hardenability, and is an important element in ultra-low carbon steel for forming a uniform bainite structure and increasing the strength. In order to make the structure of the weld heat affected zone a granular bainitic ferrite structure (α _B ) rich in toughness, B needs to be contained in an amount of 0.0003 wt% or more. However, even if the content exceeds 0.0040 wt%, the strength increasing effect tends to be saturated, and the toughness is rather deteriorated due to a large amount of solid solution B. For this reason, B
It was limited to the range of 0.0003 to 0.0040 wt%.

N: less than 0.0050 wt% N is an element which is always present in steel and forms a solid solution or precipitates as a nitride to increase the strength. Deteriorate. Therefore, in order to reduce solid solution N and prevent toughness deterioration of a large heat input welded joint having a heat input of 200 kJ / cm or more, N is set to 0.00 in the present invention.
Limited to less than 50% wt. When N is contained in an amount of 0.0050 wt% or more, the toughness of a large heat input welded joint having a heat input of 200 kJ / cm or more is significantly deteriorated.

B / N: 0.3 to 1.0 It is important to reduce the solute N which adversely affects toughness in order to prevent the toughness deterioration of the heat affected zone by welding, and in the present invention, it becomes a solute N source. In order to reduce the N content and reprecipitate the nitride once dissolved, B having a high diffusion rate is effectively used. Therefore, B / N is set to 0.3 to 1.
0 is an appropriate range. If B / N is less than 0.3, solid solution N becomes excessive, while if B / N exceeds 1.0, solid solution B becomes excessive and the toughness deteriorates.

Cu: 0.05-0.70 wt%, Cr: 0.10-0.60 wt%
%, Mo: 0.10 to 0.50 wt%, Ti: 0.005 to 0.05 wt%, one or more of them Cu, Cr, Mo, and Ti all act to increase the strength of ultra-low carbon steel. have. In the present invention, one or more of Cu, Cr, and Mo can be contained as necessary, in addition to the above-described components. These elements increase the transformation strain that occurs during the formation of the granular bainitic ferrite structure (α _B ), which is the bainitic structure of ultra-low carbon steel. As the transformation strain increases and the dislocation density increases, the strength of the steel increases. Such an effect is 0.05% for Cu.
wt% or more, Cr 0.10wt% or more, Mo 0.10wt% or more, Ti
It is recognized at a content of 0.005 wt% or more. On the other hand, Cu is 0.70wt
%, Cr is 0.60wt%, Mo is 0.50wt%, Ti is 0.05wt%
If the content exceeds the above range, no further increase in strength can be expected, and conversely, toughness is deteriorated. Therefore, 0.05 to 0.70 wt% of Cu, Cr
Is 0.10 ~ 0.60wt%, Mo is 0.10 ~ 0.50wt%, Ti is 0.005 ~
It is preferable to limit the range to 0.05 wt%.

Ca: 0.0005 to 0.0100 wt% Ca acts as a sulfide forming element and has a function of fixing S which has an adverse effect on the toughness of the weld heat affected zone. In the present invention, it can be added as needed for the purpose of improving toughness. Such an effect is recognized when the content is 0.0005 wt% or more. However, when the content exceeds 0.0100 wt%, cluster-like inclusions are formed, which adversely affects toughness. For this reason, Ca is preferably limited to the range of 0.0005 to 0.0100 wt%.

The balance other than the above components is Fe and inevitable impurities. As unavoidable impurities, P: 0.020
% Or less, S: 0.005% or less is acceptable. The steel material of the present invention may be manufactured by a usual method. That is, for example, a molten steel having the above-described composition is smelted by a commonly known smelting method such as a converter, an electric furnace, and a vacuum smelting furnace. Cast into rolling steel material. Next, the steel material for rolling is reheated to a temperature of 1000 to 1300 ° C. or hot rolled without being reheated, the rolling is completed at 750 ° C. or more, and air cooling or accelerated cooling is performed. Is done.

[0028]

Next, the effects of the present invention will be described based on examples. After heating a steel ingot having the composition shown in Table 1 to 1150 ° C, applying a reduction of 50% or more in the unrecrystallized region, rolling is completed at 800 ° C or more, and accelerated cooling by water cooling is immediately performed to 600 ° C. Thereafter, it was air-cooled to obtain a steel plate having a thickness of 38 mm.

The obtained steel sheet was subjected to a tensile test of a base material,
A Charpy impact test was performed. Incidentally, the Charpy impact test to determine the energy transition temperature of the base material _(V T _E). In addition, these steel sheets were subjected to beveling in the shape shown in FIG. 4, and large heat input welded joints were produced by electrogas arc welding with a heat input of 200 kJ / cm. A Charpy impact test specimen (JIS No. 4 specimen) was sampled from the welded joint (bond part), and the Charpy absorbed energy (vE- ₅₀ ) at a test temperature of -50 ° C was determined. Was evaluated.

The results are shown in Table 2.

[0031]

[Table 1]

[0032]

[Table 2]

From Table 2, it can be seen that the present invention shows that the vE- ₅₀ of the weld bond portion of the electrogas arc welded joint having a heat input of 200 kJ / cm is
This is a low-temperature steel material with excellent large heat input weld toughness of 41J or more. In contrast, comparative examples out of the scope of the present invention, vE _{-5 0} of the weld bond portion and less than 41J, not only shows a low toughness.

[0034]

According to the present invention, even when a large heat input welding of a heat input of 200 kJ / cm or more is performed, good weld toughness can be obtained and the efficiency of welding can be significantly improved. It works.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between vE- ₅₀ and B / N of a reproduced weld bond portion.

FIG. 2 is a graph showing a relationship between vE- ₅₀ and C content of a reproduced weld bond portion.

FIG. 3 is a graph showing the relationship between B / N and C content on vE- ₅₀ of a reproduced weld bond portion.

FIG. 4 is an explanatory view showing a groove shape.

Continued on the front page (72) Inventor Bunmaru Kawabata 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. (Without address) Inside Mizushima Works, Kawasaki Steel Corporation

Claims (2)

[Claims] C: 0.02 wt% or less, Si: 0.50 wt% or less, Mn: 0.5 to 2.0 wt%, Al: 0.005 to 0.10 wt%, Nb: 0.010 to 0.10 wt%, Ni: 0.3 to 3.0 wt% , B: 0.0003 to 0.0040 wt%, N: less than 0.0050 wt%, and the ratio of B content to N content, B / N is 0.3
Non-heat treated low temperature steel excellent in large heat input weldability, characterized by having a composition consisting of the balance of Fe and unavoidable impurities. 2. In addition to the above composition, Cu: 0.05-0.70 wt%, Cr: 0.10-0.60 wt%, Mo: 0.10-0
0.50 wt%, Ti: 0.005 to 0.05 wt%, Ca: 0.0005 to 0.01
The non-heat-treated low-temperature steel material excellent in large heat input weldability according to claim 1, characterized in that it contains one or more of 00wt%.