JP4673788B2 - Steel excellent in toughness of weld heat-affected zone and method for producing the same - Google Patents

Description

  The present invention relates to a steel excellent in toughness (HAZ toughness) of a weld heat affected zone (HAZ portion) from small heat input welding to large heat input welding, and a method for producing the same. In particular, this steel can be used for welded structures such as building structures, bridges, offshore structures, pressure vessels, shipbuilding, line pipes and the like.

  In a welded structure, a large range of welding inputs are often applied by arc welding or the like, for example, large heat input welding and small heat input welding are simultaneously applied to the same member. High-strength steel plates are used for welded structures, but high-strength steel plates contain a large amount of alloying components to ensure the strength of the base metal, so that HAZ with a fast cooling rate is cured under small heat input welding conditions. Therefore, there is a problem that weld cracks (cold cracks) are likely to occur. In order to improve cold cracking, low alloy steels with limited addition of alloy components have been developed. However, the low alloy steel has a problem in the HAZ toughness particularly in the high heat input welding.

  The HAZ (Heat Affected Zone) toughness of low alloy steels includes (1) grain size, (2) high carbon martensite (M "), upper bainite (Bu) and ferrite side plate (FSP). It is governed by various factors such as the dispersion state of the hardened phase, (3) precipitation hardening state, (4) presence / absence of grain boundary embrittlement, and (5) elemental microsegregation.

  These factors are known to have a great influence on toughness, and many techniques have been put to practical use in order to improve HAZ toughness.

  For example, there is a technique for improving the HAZ toughness by suppressing the growth of austenite grains in the HAZ using the pinning by TiN. In this technique, in HAZ which has received a large heat input, ultrafine TiN is dissolved and formed as TiC by the subsequent welding heat, so austenite grain growth cannot be suppressed and HAZ toughness is deteriorated. In addition, since TiN that is somewhat large is further coarsened, there is a problem that it is difficult to sufficiently exert the toughness improvement effect by TiN, and TiN particles are stabilized by actively containing Nb in TiN. And an invention that makes the toughness from large heat input to small heat input excellent (see, for example, Patent Document 1).

  In addition, as a particularly excellent technique, the microstructure is refined with Ti oxide, and in addition to this, the balance of Ti, O, and N is −0.020% ≦ Ti-20O-3.4N ≧ −0.010%. There has been known an invention that is optimized so as to be satisfied, suppresses precipitation of TiC, reduces precipitation hardening, and improves HAZ toughness (see, for example, Patent Document 2).

  The toughness of the heat affected zone is greatly affected by the microstructure described above and the hardened layer containing high carbon martensite (M ″), and the conventional technology can be solved by toughening the base metal with Ni or the like. However, the addition of expensive alloy elements such as Ni has caused an increase in production cost, which has been an obstacle to producing steel having excellent toughness in the weld heat affected zone.

Japanese Patent Application No. 2004-2108010 Japanese Patent Laid-Open No. 5-247531

  An object of the present invention is to provide a high-strength, inexpensive steel excellent in toughness of a weld heat-affected zone in multi-layer welding with a small to large heat input, and a method for producing the same.

  Based on the knowledge that the control of the microstructure is important for the toughness of the weld heat-affected zone (HAZ) of small heat input to large heat input welding, the present inventor has been able to control the microstructure of the weld heat-affected zone. Focusing on Ca as an effective component element, intensive studies were conducted on the influence of the amount of Ca in steel on the weld heat affected zone. As a result, it has been found that when Ca is added to steel in a certain amount or more, a fine oxide mainly composed of Ca becomes a nucleus and the microstructure of the weld heat affected zone is refined, and the toughness of the HAZ portion can be improved. Was completed.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%
Steel with excellent toughness in the heat affected zone of welding, characterized in that the balance is made of iron and inevitable impurities.

(2) In mass%,
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
It contains, in addition,
V : 0.040% or less,
Contain,
A steel with excellent toughness in the heat affected zone of welding, characterized in that the balance is made of iron and inevitable impurities.

(3) In mass%,
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
Containing
The welding heat-affected zone is characterized in that a steel that is essentially free of A1 and the balance of which is iron and inevitable impurities is made into a slab by a continuous casting method, and then subjected to thermomechanical treatment after reheating at 900 to 1150 ° C. A method of manufacturing steel with excellent toughness.

(4) In mass%,
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
It contains, in addition,
V : 0.040% or less,
Containing the balance and a slab of steel continuous casting process of iron and inevitable impurities, then after reheating to a temperature of 900 to 1150 ° C., the toughness of the weld heat affected zone characterized by mechanical treatment excellent Steel manufacturing method.

  The steel (steel) manufactured according to the present invention has extremely good toughness in the weld heat-affected zone with small to large heat input, and the Charpy impact value of the weld heat-affected zone with the most deteriorated toughness during welding with small to large heat input. Extremely good and excellent HAZ toughness. As a result, it is possible to provide a high-strength steel that can be used in harsh environments such as tanks, earthquake-resistant buildings, and bridges.

  The present invention will be described in detail below.

  According to the study by the present inventors, the toughness of the weld heat-affected zone (HAZ) with a small to large heat input (5.0 to 20.0 kj / mm when the plate thickness is 32 mm) is controlled by the microstructure and the embrittlement element. It makes clear that reduction is important. The present inventors pay attention to the fact that Ca is effective in controlling the microstructure of the weld heat affected zone (HAZ) and reducing the embrittlement elements, and without adding A1 by vacuum melting in the laboratory, An experiment was conducted to change the Ca content, and it was found that the toughness of HAZ was significantly improved in the region where the Ca content was 30 ppm or more.

  Further, the improvement of HAZ toughness was considered to be an effect of fine ferrite (IGF) whose microstructure is formed from the grains of prior austenite. The main chemical components of the steel used in this experiment are shown in Table 1, the relationship between the Ca amount (%) and Charpy impact value (vTrs · ° C) in the reproducible thermal cycle is shown in Fig. 1, and the Ca amount (%) and IGF fraction. The relationship (%) is shown in FIG. As shown in FIG. 1, when the Ca content is 0.003% or more, the Charpy impact value (vTrs · ° C.) is improved, and as shown in FIG. 2, the microstructure is improved by increasing the Ca content (IGF fraction). The tendency to improve toughness was clear. In FIG. 1 and FIG. 2, in the reproducible thermal cycle test, the peak temperature (PT) is heated to 1400 ° C., cooled, and the required time from the transformation temperature of 800 to 500 ° C. is 54 seconds (this Is the case of SAW (submerged arc welding) of a steel plate having a thickness of 32 mm, which corresponds to small heat input welding at 5.0 kJ / mm), and the ● mark indicates a peak temperature (PT) of 1400 ° C. in a reproducible thermal cycle test. 192 seconds (this is a SAW (submerged arc welding) of a steel plate having a thickness of 32 mm and a high heat input welding at 20.0 kJ / mm) This corresponds to the condition).

  Until now, it has been known as a technique that is particularly excellent in improving the HAZ toughness to refine the microstructure with Ti oxide and improve the toughness. However, even in this technique, the microstructure is not sufficiently refined and the toughness is insufficient in high heat input welding. This is probably because the number of Ti oxides that become the core of the microstructural refinement is small, and the microstructure tends to become coarse in the case of high heat input welding.

  In contrast, in the steel of the present invention, when a certain amount or more of Ca is added to the steel, a fine oxide mainly composed of Ca is formed, which becomes a nucleus, which refines the microstructure together with the nucleus of the oxide of Ti, toughness The present inventors have found that improvement can be achieved and completed the present invention.

  The reason why the steel components and the content thereof (hereinafter referred to as “mass%”) are limited will be described below.

  Ca: The amount of Ca added to the steel is 0.0030% or more having a great effect on the refinement of the microstructure and the improvement of toughness. However, if it is 0.0050% or more, the oxide becomes coarse and the microstructure becomes finer. Toughness improvement is insufficient. For this reason, the appropriate addition range of Ca is 0.0030 to 0.0050%. By controlling the Ca content to 0.0030 to 0.0050%, the microstructure tends to be refined and the HAZ toughness tends to be improved. Steel with both HAZ toughness cannot be obtained.

  C: C needs to be 0.07% or more in order to obtain high strength of the base metal and the welded portion. However, if it exceeds 0.10%, the toughness of the welded HAZ deteriorates and the toughness of the welded portion cannot be satisfied. The upper limit is 10%.

  Si: Si is preferable to be low in order to obtain good HAZ toughness, but in the steel according to the invention, A1 is not added, so 0.05% or more is necessary for deoxidation. However, if it exceeds 0.15%, high carbon martensite (M ″) is likely to be formed and the HAZ toughness is impaired, so 0.15% is made the upper limit.

Mn: Mn is an indispensable element for securing the strength and toughness of steel. It is highly effective in improving the strength of the base metal. However, if it exceeds 2.5%, the upper limit is set to 2.5% in order to facilitate formation of upper bainite (Bu) harmful to the HAZ toughness. Further, since the effect is small at less than 1.7%, the lower limit is set to 1.98% in accordance with the example .

  P, S: P and S are better both in terms of base material toughness and HAZ toughness, but are also unavoidably contained elements due to industrial production restrictions, 0.008% and 0.005%, respectively. Was the upper limit.

  Al: A1 is preferably less because it inhibits the formation of Ti oxide, but is an element that is inevitably contained from the manufacturing process of steel, has an industrial production limitation, and 0.004% is an allowable upper limit. is there. In the present invention, it is preferable not to contain Al substantially.

  Ti: Ti is an important element for improving the HAZ toughness. When there is little Al, it combines with O to form Ti oxide, which becomes the nucleus of intragranular transformation ferrite formation and refines the microstructure, HAZ toughness To improve. Further, Ti that does not bind to O binds to N and finely precipitates in the slab as TiN, is effective in reducing the coarsening of the γ grains during heating, and is effective for refining the rolling structure. TiN refines the weld heat-affected zone structure during welding and improves HAZ toughness. However, if it is too much, TiC is formed and the HAZ toughness is deteriorated, so 0.010 to 0.018% is in the proper range.

  O: O is an element necessary for Ti oxide generation and Ca oxide generation, but if it exceeds 0.0030%, coarse Ti oxide is generated and the toughness is extremely deteriorated, so the upper limit is set to 0.0030. %.

  N: N is an element necessary for Ti nitride generation. However, if it is less than 0.0030%, the effect is small, and if it exceeds 0.005%, surface flaws occur during the production of steel slabs, so the upper limit is made 0.0050%. .

  Furthermore, the purpose of adding Nb and V to the basic components is to improve the strength of the base material, but it is necessary to make the range not significantly harming the HAZ toughness. For this reason, the upper limit of each addition amount was set to Nb: 0.030% or less and V: 0.040% or less.

The addition of Cu, Ni, etc. is effective and effective in improving the strength of the base material without deteriorating the toughness of the HAZ, but was not included in the scope of the present invention because it increased the manufacturing cost. According to the present invention, an inexpensive steel excellent in HAZ toughness having a high strength (600 N / mm class ² ) property can be obtained without adding an expensive alloy element.

  Even if the components of the steel are limited as described above, the intended effect cannot be exhibited unless the production method is appropriate. For this reason, it is necessary to limit the manufacturing conditions. The steel production conditions will be described below.

  The steel of the present invention is industrially required to be produced by a continuous casting method. The reason is that the solidification cooling rate of the molten steel is fast, and a large amount of fine Ti oxide, Ca oxide and Ti nitride effective in improving the HAZ can be generated in the slab.

  In rolling the slab, the reheating temperature needs to be 1150 ° C. or less. This is because when the reheating temperature exceeds 1150 ° C., the Ti nitride becomes coarse, and the expectation of the toughness deterioration of the base material and the improvement effect of the HAZ toughness cannot be expected. The lower limit of the reheating temperature needs to be 900 ° C. or higher, which is a temperature at which rolling can be easily carried out industrially.

  Next, thermomechanical processing is essential as a manufacturing process after reheating. The reason is that even if excellent HAZ toughness is obtained, if the toughness of the base material is inferior, it is insufficient as a steel material.

  Examples of the heat treatment method include 1) controlled rolling, 2) controlled rolling-accelerated cooling, and 3) direct quenching-tempering after rolling. A preferred method is controlled rolling-accelerated cooling. In addition, even if this steel is manufactured and reheated to a temperature below the Ar3 transformation point for the purpose of dehydrogenation, the characteristics of the present invention are not impaired.

  Hereinafter, the present invention will be described in detail based on examples.

  Thick steel plates of various steel components were manufactured in the converter-continuous casting-thick plate process, and Charpy impact tests were performed on the base metal strength and reproducible HAZ. As steel production conditions, a slab produced by a continuous casting method was reheated, followed by controlled rolling, and heat treatment (ACC: accelerated cooling, DQ: quenching and tempering immediately after rolling) to obtain a thick steel plate. Table 2 shows the chemical composition (mass%) of the steel subjected to the test.

  The reproduced HAZ conditions are as follows: A condition (small heat input welding) is SAW (submerged arc welding) of a steel sheet having a thickness of 32 mm, and SAW welding condition is equivalent to 5.0 kJ / mm, and B condition (large heat input welding) is the plate thickness. The SAW welding conditions are equivalent to 20.0 kJ / mm in SAW (submerged arc welding) of a 32 mm steel plate. The Charpy impact test was conducted by collecting test pieces from 1/4 t of the plate thickness. Table 3 shows steel manufacturing conditions and test results.

The steel plates manufactured in the present invention (present invention steels: 1 to 6 ) have a high strength with a yield strength (YS) of 518 N / mm ² or more and a tensile strength (TS) of 610 N / mm ² . A high HAZ toughness with a Charpy impact value (vE ₀ ) of 96 J or more was exhibited under both the A and B conditions at the test temperature.

  On the other hand, the comparative steel is inadequate as a steel plate used in a severe environment because the base material strength and the impact value of the reproduced HAZ are poor. That is,

Since the comparative steel 13 had a small amount of C and Ca was not added, the base metal strength was low, and the impact value (vE ₀ ) of the reproduced HAZ B condition (high heat input welding) was as low as 11 J.

  Since the comparative steel 14 has too much C content, the base metal strength is sufficient, but the impact value under the B condition (high heat input welding) of the reproduced HAZ was 30 J, which was a bad result.

Since comparative steel 15 does not contain Ca, the strength of the base material is sufficient, but the impact value (vE ₀ ) under the B condition (high heat input welding) of the reproduced HAZ was 9J, which was a bad result.

Although the comparative steel 16 has the same main steel components as the invention steel, but the O material is sufficient, the strength of the base metal is sufficient, but the impact value (vE ₀ ) of the reproduced HAZ B condition (large heat input welding) ) Was a bad result of 36J.

  As is clear from the above test results, it has been confirmed that the steel of the present invention exhibits excellent HAZ toughness even by welding from small heat input to large heat input.

It is a figure which shows the relationship between Ca amount (%) of a reproduction | regeneration thermal cycle, and a Charpy impact value (vTrs * degreeC). It is a figure which shows the relationship between Ca amount (%) and IGF fraction (%).

Claims (4)

% By mass
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%
Steel with excellent toughness in the heat affected zone of welding, characterized in that the balance is made of iron and inevitable impurities. % By mass
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
It contains, in addition,
V : 0.040% or less,
Contain,
A steel with excellent toughness in the heat affected zone of welding, characterized in that the balance is made of iron and inevitable impurities. % By mass
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
Containing
Production of steel with excellent toughness of weld heat affected zone characterized in that steel consisting of iron and inevitable impurities as the remainder is made into a slab by a continuous casting method, then reheated to a temperature of 900 to 1150 ° C. and then heat-treated. Law. % By mass
C: 0.07 to 0.10%,
Si: 0.05 to 0.15%,
Mn: 1.98 to 2.5%,
P: 0.008% or less,
S: 0.005% or less,
A1: 0.004% or less,
Ti: 0.010 to 0.018%,
Ca: 0.0030 to 0.0050%
O: 0.0030% or less,
N: 0.0030 to 0.0050%,
It contains, in addition,
V : 0.040% or less,
Containing the balance and a slab of steel continuous casting process of iron and inevitable impurities, then after reheating to a temperature of 900 to 1150 ° C., the toughness of the weld heat affected zone characterized by mechanical treatment excellent Steel manufacturing method.

Images