BRPI0607524A2 - excellent hardness action in the heat affected zone of welding - Google Patents

Abstract

EXCELLENT HARDNESS STEEL IN THE AREA AFFECTED BY THE HEAT OF WELDING. The present invention relates to an excellent toughness steel in a heat affected zone of welding characterized in that it contains by weight% O: 0.02 to 0.06% Si: 0.05 to 0.30% , Mn: 1.7 to 2.7%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, O: 0.0010 to 0.0045%, and N: 0.0020 0.0060% and comprising an iron balance and unavoidable impurities, having a quantity of impurities intermixtures limited to A: 0.004% or less, Nb: 0.003% or less, and V: 0.030% or less, and having a CeH represented by formula (A) of 0.04 or less: CeH = C + 1 / 4Si-1 / 24Mn + 1 / 48Cu + 1 / 32Ni + 1 / 0.4Nb + 1 / 2V ... (A) where C, Si, Mn, Ou, Ni, Nb, and V represent compositions in steel (mass%).

Description

Report of the Invention Patent for "EXCELLENT HARDNESS STEEL IN THE AREA AFFECTED BY THE HEAT OF WELDING".

Technical Field

The present invention relates to an excellent steel in toughness in the weld heat affected zone (HAZ) in small weld heat inlets to medium weld heat inlets and a method of their production.

Background Art

The HAZ toughness of a low alloy steel is governed by several factors such as (1) crystal grain size, (2) the dispersion state of hard phases such as high carbon martensite (M *), upper bainite (Bu). ), and side plate ferrite (FSP), (3) the hardening state of precipitation, (4) the presence of any intergranular brittleness,

and (5) the micro segregation of the elements. These factors are known to have a great effect on toughness. Many technologies are being marketed to improve HAZ toughness.

It is safe to say that such toughness inhibiting factors are caused by the additive elements. Reducing the alloy element content increases the toughness. However, greater strength is being neglected in structural steels. Therefore, the addition of alloying elements is required. That is, the demands on strength and toughness are contradictory from the point of view of alloying element content. A toughening technology that does not depend on the alloying elements has been sought.

As a particularly excellent technology, it is known to use a range that does not include substantially any Al to make the microstructure thinner and in addition correctly balance the Ti, O, and N elements to suppress TiC precipitation and reduce precipitation hardening and thus improve precipitation. toughness (Japanese Patent Application (a) No. 5-247531). In this case, the toughness of the heat-affected zone of the weld is determined by the balance of microstructure effects and hardened layer effects that includes M *. In the prior art, this has been resolved by improving the toughness of the base material matrix by NI or the like. However, the addition of large amounts of Cu, Ni, and other expensive alloying elements necessary for the realization of this technology causes an increase in production costs. This becomes an obstacle in the production of high strength steel, excellent ma CTOD property.

The steel point according to this invention not substantially including any Al and Nb is made use of in the present invention as well. However, in this invention the C content is high, so the problem of falling tenacity when increasing Mn remains unsolved. There was also a concern that impurities Nb and V had a detrimental effect on toughness.

In addition, Japanese Patent Application (A) No. 2003-147484 follows the thinking of Japanese Patent Application (A) Ne 5-247531 and, while making use of Ti oxides, adds Nb and increases Mn content. This causes the starting temperature of the austenite-ferrite transformation to fall to thereby suppress hard phase formation and at the same time obtain a suitable microstructure to thereby satisfy the -10 ° C CTOD property. However, the invention of this Japanese Patent Application (A) No. 2003 -147484 does not sufficiently satisfy the required CTOD property of welded joints at the hardest level of -40 ° C or less.

The present invention provides technology that produces high-strength steel that is excellent in toughness for welding multiple layers of small or medium heat inputs. The steel produced by the present invention is extremely good at the CTOD property of small and medium inlet multi-layer weld welds between the toughness levels of the heat affected zones of the weld. The essence of the present invention is as follows:

(1) An excellent toughness steel in an area affected by welding heat, characterized in that it contains by weight% C: 0.02 to 0.06%, Si: 0.05 to 0.30%, Mn: 1.7 2.7%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, O: 0.0010 to 0.0045%, and N: 0.0020 to 0.0060% and comprising a balance of iron and unavoidable impurities, having an amount of impurity intermixture limited to Al: 0.004% or less, Nb: 0.003% or less, and V: 0.030% or less, and having a CeH represented by formula (A ) in the range 0,04 or less:

CeH = C + 1 / 4Si-1 / 24Mn + 1 / 48Cu + 1 / 32Ni + 1 / 0.4Nb + 1 / 2V ... (A) where C, Si, Mn, Cu, Ni, Nb, and V represent notion compositions (mass%).

(2) An excellent toughness steel in a zone affected by the welding heat as presented in item (1), characterized by the fact that CeH is in the range of 0.01 or less.

(3) An excellent toughness steel in an area affected by welding heat as set forth in items (1) and (2), characterized in that it also contains by weight% one or two types of Cu: 0.25% or less and Ni: 0.50% or less.

(4) A method of producing steel which is excellent in toughness in a heat-affected zone characterized by heating a plate that satisfies the steel ingredients and CeH of (1) or (g) to a temperature of 1100 ° C or less. , and then treating it by the thermo-mechanical control process.

(5) A method of producing steel which is excellent in toughness of a heat affected zone of welding characterized by heating a plate satisfying the steel ingredients and CeH from (3) to a temperature of 1100 ° C or less, and then treating it. o by the thermomechanical control process.

Brief Description of the Drawings

Figure 1 is a view showing the relationship of a cooling time from 800 to 500 ° C and a fraction of M *.

Figure 2 is a view showing the relationship of CeH and CTOD properties.

Best Mode for Carrying Out the Invention

According to the present inventors' research, the HAZ CTOD property at the time of small or medium heat desoldering (1.5 to 6.0 kJ / mm with a plate thickness of 50 mm) (property CTOD at a temperature of -40 ° C or below) is governed by the nature of extremely local regions. Controlling the microstructure of this portion and reducing the fragility elements are important. In other words, the CTOD property is not the average property of the materi-al, but is governed by local fragility zones. If there are regions that promote brittleness, even in only part of the steel material, the CTOD property of the steel plate will be significantly impaired.

Specifically, the local regions that have the greatest effect on CTOD property are M *, sideplate ferrite (FSP) and other hard phases. To suppress the formation of this type of hard phase, in the past it has been necessary to keep the hardening capacity of steel low. This has become an inhibitor of high strength.

The present invention is characterized by the following discoveries and their configuration in a high tensile steel HAZ. Specifically,

1) In a HAZ with small or medium welding heat input, generally the cooling time after welding is within 60 seconds. The inventors have found that under such cooling conditions, if the C content is sufficiently low, other fragilizing elements are adequately controlled, even if Mn is added up to 27%, M * which has a negative effect on toughness. It is more formed. Fig. 1 shows the fraction M * when changing the amount of Mn from 1.7% to 2.7% with 0.05% C, 0.15% Si. It is understood that even if the Mn content changes If the cooling time from 800 to 500 ° C is within 60 seconds or so, the fraction M * becomes very small. As a result, it becomes possible to increase the Mn content for which addition in a large quantity was thought to be impossible in the past due to deterioration in toughness.

2) The inventors found that the steel ingredients could be made suitable into a less aluminum based steel.

3) The inventors have eliminated the unexpected factors that reduce toughness by limiting Al, Nb and V present as notion impurities to certain limits or less.

That is, employing less Al-based steel makes it possible to confidently form TiO and effectively improve toughness.

By combining these three points, it has become possible to achieve good CTOD property under difficult temperature conditions of -20 ° C or less at a small to medium welding heat input at HAZ that cannot be achieved so far.

Even when very little M * is formed, control of embrittlement elements C, Si, Cu, Ni, Nb, V and the like is essential. Specifically, it is essential to control the CeH value of C + 1 / 4Si-1 / 24Mn + 1 / 48Cu + 1 / 32Ni + 1 / 0.4Nb + 1 / 2V to a predetermined range.

Figure 2 shows the results when 20 kg of the ingredients are produced by 0.05% C - 0.15% Si - 1.7 to 2.7% Mn vacuum melting by rolling it to steel plate. by giving him a heat history of a real joint welded three times by a simulated cyclothermal equipment, and then running a CTOD test.

T0c 0.1 (670.9 CeH-67.6) is the temperature when the lowest three values of the CTOD tests at different test temperatures is 0.1 mm. There is a clear tendency for T0c 0.1 (CTOD property) to become linearly substantially excellent as CeH falls. If CeH drops to around 0.01, Tôc 0.1 is found to reach -60 ° C.

That is, by satisfying the steel requirements of the present invention by controlling the CeH, the desired CTOD property can be obtained. With the steel of the present invention, control of the CeH value as required by the CTOD is one of the features of the invention. . In addition to the CeH value control, grinding of the grades of the other alloying elements is required to produce the steel supplied with both high strength and superior CTOD property. Below, the limitation ranges and reasons will be explained.

C has to be 0.02% or more to get resistance, but

If above 0.06%, it degrades the toughness of the welding HAZ and does not allow the satisfaction of a good CTOD property, so 0.06% is the upper limit.

Si inhibits the toughness of HAZ, so a smaller amount is preferable in order to obtain good toughness of HAZ. However, as conventional steel, no Al is added, so the addition of 0.05% or more is required for deoxidation. However, if the content is above 0.30%, the HAZ toughness is impaired, so 0.30% is made the upper limit.

Mn is an inexpensive element with a large microstructure rectifying effect and lower CeH, so its addition does not impair the small and medium heat input HAZ, so it is desirable to make its high content to achieve high strength. . However, if above 2.7%, it promotes segregation of plaque and facilitates the formation of Bu detrimental to toughness, so the content has been made up to an upper limit of 2.7%. In addition, if it is less than 1.7 %, the effect is small, so the lower limit was made 1.7%. Note that from a tenacity standpoint, above 2.0% is more preferable.

P and S should both be small in number of views of the base material toughness and HAZ toughness, but limits for their reduction in industrial production. 0.015% and 0.010%, preferably 0.008% and 0.003%, were therefore made their upper limits.

Al is not deliberately added in the present invention, but inclusions such as impurities in steel are inevitable. These form Al oxides that inhibit the formation of Ti oxides, so a lower content is desirable, but there are limits to their reduction in industrial production. 0.004% is therefore the upper limit.

Ti forms Ti oxide and makes the microstructure thinner, so it greatly contributes to improved toughness, but if the content is too high, it forms TiC. This degrades the toughness in HAZ, so 0.005 to 0.015% is a suitable range.

O (oxygen) is required for the formation of a large amount of Ti oxide. If less than 0.0010%, the effect is small, while if above 0.0045%, it forms crude Ti oxide to degrade pronoun. Regarding toughness, then the content range was made 0.0010 to 0.0045%.

N is required to form fine Ti nitrides and improve base material inactivity and toughness in HAZ, but if it is less than 0.002% the effect is small while above 0.006% surface defects are formed at the time of production. bar, so the upper limit was made 0.006%.

In addition, Nb and V are inherently embrittlement elements. As shown by the large coefficient in formula (A), its presence causes CeH to grow greatly and makes HAZ tenacity to drop noticeably, so these are not deliberately added in the present invention. Even when included as impurities in steel to ensure attenuation, Nb must be limited to 0.003% or less. In addition, the V must be limited to 0.030% or less, preferably 0.020% or less.

Cu and Ni result in slight deterioration of HAZ toughness due to their addition, have the effect of increasing the strength of the base material, and are effective for further property improvement, but increase production costs, so the upper limits of the contents when added were made: Cu: 0.25% and Ni: 0.50%.

Even if we limit the ingredients in steel of the above form, but if they form a suitable structure by a suitable production method, the desired effects cannot be exhibited. Because of this, production conditions also have to be considered.

The steel of the present invention is preferably industrially produced by continuous casting. The reasons are that the solidification cooling rate of cast steel is fast and it is possible to form Ti oxidesfines and Ti nitrides in large quantities in the plate. When selaminating the plate, the reheat temperature has to be made at 1100 ° C or less. If the reheat temperature exceeds 1100 ° C, Ti nitrides become coarser, the base material toughness decreases, and the effect of improved toughness on HAZ cannot be expected.

Next, the production method after reheating requires treatment by the thermomechanical control process. The reason is that even higher toughness in HAZ is obtained, if the toughness of the base material is lower, the steel product is insufficient. As treatment methods by thermomechanical control process, 1) controlled lamination, 2) accelerated cooling-lamination control, 3) direct tempering-after tempering, etc. may be mentioned, but the preferred methods are controlled lamination-accelerated cooling and direct cooling-tempering after lamination.

Note that after steel production, even if we reheat to a temperature of transformation point Ar3 or higher for the purpose of dehydration etc. the characteristic features of the present invention are not impaired.

In addition, the above method is an example of a steelmaking method of the present invention. The steel production method of the present invention is not limited to the above method.

Examples

Coarse-thickness steel sheets of various steel-making ingredients were produced by the continuous converter casting process of bitolagross sheets. The strength of the base material was determined and the CTOD test of the welded joints was performed. Welding was performed by the submerged arc welding (SAW) method, commonly used for test welding, with a welding heat input of 4.5 to 5.0kJ / mm in slot K so that the melting line weld (FL) becomes perpendicular. The CTOD test was performed by a notched sheet of t (plate thickness) x 2t notched by the introduction of a fatigue fracture at the FL location. Table 1 shows examples of the present invention and comparative examples.

The steel plate produced by the present invention (Invention Steels 1 to 20) had yield strengths (YS) of 430 N / mm2 or more and had good breaking strength of CTOD values at -20 ° C, -40 ° C and -60 ° C, all 0.27 mm or more.

In contrast, Comparative Steels 21 through 26 had lower strengths and CTOD values than the steels of the invention, and did not have the necessary properties as steel plates used under dry environments. Comparative Steel 21 had Nb added, so the Nb content of the steel plate became high, so the CTOD value was low. Comparative steel 22 had a very high C content and also a very high CeH value, so the CTOD value was a low value. Comparative steels 23 and 24 had low CeH's, but the Al content was too high, the Tiforam oxides formed insufficiently, and the microstructure was not thinned sufficiently. Comparative steel 25 had a CeH of about the same value as the steel of the invention, but the C content was very low and the Ofoi content very high, so the strength of the base material was low and the CTOD value was low. Comparative steel 26 had an excessively high amount of Nb mixed as an impurity, so although CeH was low, the strength of the base material and the CTOD value were low.Table 1

<table> table see original document page 11 </column> </row> <table> <table> table see original document page 12 </column> </row> <table> Work heat treatment method:

CR: controlled lamination (lamination in optimum temperature region for strength and toughness)

ACC: Accelerated cooling (water cools below temperature range from 400 to 600 ° C after controlled lamination)

DQ: Direct cooling-quenching after laminationIndustrial Applicability

The steel produced by the present invention has high strength, has an extremely good CTOD property of the FL part where toughness degrades most of the welding time, and has superior toughness. Because of this, it has become possible to produce a high strength steel product that can be used in offshore structures, earthquake resistant buildings, and other harsh environments.

Claims (5)

1. Excellent toughness steel of an area affected by the welding gauge, characterized in that it contains by weight% C: 0.02 to 0.06%, Si: 0.05 to 0.30%, Mn: 1.7 to 2.7%, P: 0.015% or less, S: 0.010% or less, Ti: 0.005 to 0.015%, O: 0.0010 to 0.0045%, and N: 0.0020 to 0.0060% and comprising a balance of iron and unavoidable impurities, having an amount of impurity intermixture limited to Al: 0.004% or less, Nb: 0.003% or less, and V: 0.030% or less, and having a CeH represented by formula (A) in 0.04 or less: CeH = C + 1 / 4Si-1 / 24Mn + 1 / 48Cu + 1 / 32Ni + 1 / 0.4Nb + 1 / 2V ... (A) where C, Si, Mn, Cu, Ni, Nb, and V represent ingredients in steel (% by mass). Excellent steel in toughness of an area affected by the welding gauge as set forth in claim 1, characterized by the fact that CeH is in the range of 0.01 or less. Excellent toughness steel in an area affected by the weld gauge as set forth in claims 1 or 2, characterized in that it also contains, by weight%, one or two types of Cu: 0.25% or less and Ni: 0.50% or less. A method of producing excellent heat-affected zone toughness steel characterized by heating a plate that satisfies the steel compositions and the CeH as defined in claim 1 to a temperature of 1100 ° C or less, and then treating it by the process of thermomechanical control. 5. Production method of excellent steel in toughness of a heat-affected zone of welding characterized by heating a plate satisfying the steel ingredients and the CeH as defined in claim 3 to a temperature of 1100 ° C or less, and then treating it. by the thermomechanical control process.