JP2011074447A - High strength steel excellent in toughness in high heat input weld heat-affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel which is excellent in toughness in a weld heat-affected zone even for high heat input welding having a welding heat input of >300 kJ/cm and which has a sheet thickness of ≥40 mm and a yield strength of >460 N/mm<SP>2</SP>. <P>SOLUTION: The steel has a steel composition containing, by mass, 0.03 to 0.09% C, 0.02 to 0.15% Si, 1.5 to 2.5% Mn, 0.005 to 0.06% Al, ≤0.015% P, 0.0005 to 0.0050% S, 0.005 to 0.025% Nb, 0.005 to 0.02% Ti, 0.0040 to 0.0070% N, 0.0005 to 0.0030% Ca, 0.0005 to 0.0025% B and 0.38 to 0.45% Ceq (IIW), and if required, further containing one or more selected from among V, Ni, Cu, Cr, Mo and W, and the balance Fe with inevitable impurities, and the steel contains 50 to 1,000 pieces/mm<SP>2</SP>of acid sulfide grains having a diameter of the equivalent circle of 0.5 to 3 μm and containing at least Ca, Al, Mn, O and S and elements other than O in specified quantity. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a high-strength steel having a yield strength exceeding 460 N / mm ² and a plate thickness of 40 mm or more, which is used in various structures such as shipbuilding, construction, and civil engineering, and particularly has a large welding heat input exceeding 300 kJ / cm. It relates to a material having excellent toughness in the heat affected zone even in heat input welding.

With increasing strength and thickness of steel materials, high heat input welding with excellent production efficiency such as submerged arc welding, electrogas welding, and electroslag welding has been increasingly applied to welding construction.

Since the toughness of heat-affected zone welded with high heat input in steel materials decreases, various steels for high heat input welding have been proposed, and TiN is finely dispersed in the steel, resulting in coarsening of austenite grains in the weld heat-affected zone. A technique for suppressing the above-described problem and utilizing it as a ferrite transformation nucleus in a weld heat-affected zone has been put into practical use.

  Moreover, Ti oxide (oxycide) is dispersed in the weld heat affected zone (Patent Document 1), and Ca is added to improve the toughness of the weld heat affected zone by controlling the form of sulfide (sulfide). (Patent Document 2) has been proposed.

However, when TiN is mainly used, a region heated to a temperature region where TiN dissolves in the heat affected zone is not effective, and the ground structure becomes brittle due to solute Ti and solute N. There was a problem that the toughness was significantly reduced.

Further, the technology using Ti oxide has a problem that it is difficult to disperse the oxide uniformly and finely. On the other hand, various studies have been made to improve the dispersibility by a method such as compounding oxides, but in large heat input welding where the heat input exceeds 300 kJ / cm,
It was difficult to sufficiently suppress the growth of austenite grains in the weld heat affected zone.

On the other hand, in Patent Document 3, Ca-based nonmetallic inclusions that promote ferrite transformation in the weld heat affected zone are dispersed in steel by appropriately controlling the Ca, O, and S contents, thereby improving toughness. Is disclosed.

Patent Document 4 proposes a technique that defines the chemical composition of the steel sheet and the particle size and number density of the composite oxide contained therein. In particular, in order to use the composite oxide as a precipitation site for Ti (Nb) nitride and B nitride which are the nuclei of intragranular ferrite, a composite oxide having an equivalent circle diameter of 0.005 to 0.5 μm is used in an amount of 100 to 3000. It is said that it is effective to contain pieces / mm ² .

Further, Patent Document 5 proposes a technique that regulates the particle size, number density, and composition of the composite oxide in order to suppress the grain growth of heated austenite. That is, an oxide particle having a circle equivalent diameter of 0.005 to 2 μm is contained in a number per unit area of 100 to 5000 / mm ² , and the element contains at least Ca, Al, O and excludes O. The ratio is Ca: 5% or more and Al: 5% or more.

  However, it has been found that the above technique has insufficient HAZ toughness when steel materials with higher strength and Ceq exceeding 0.38 are targeted.

JP 57-51243 A JP 60-204863 A Japanese Patent No. 3546308 JP 2005-307261 A JP 2007-277642 A

Steel materials used in conjunction with the increase in size of welded structures have become thicker and thicker, and thick steel plates exceeding 40 mm have been adopted in various fields. For example, recently, the application of high heat input welding to high strength steel having a yield strength exceeding 460 N / mm ² class has increased, and high heat input welding to a higher strength steel material has begun to be studied. As the base metal strength increases, the amount of alloying elements in the steel material increases, so the microstructure of the high heat input welding heat-affected zone becomes a mixed structure of ferrite and bainite, and it is difficult to improve toughness in the above-mentioned patent documents.

Therefore, the present invention is a high strength thick steel material with a yield strength exceeding 460 N / mm ² and a plate thickness of 40 mm or more, and the toughness of the weld heat affected zone even in high heat input welding with a heat input of welding exceeding 300 kJ / cm. The purpose is to provide a product that excels.

In order to improve toughness by making the high heat input welding heat-affected zone of high-strength thick-walled steel with a yield strength exceeding 460 N / mm ² into a fine microstructure, the inventors have made a dispersion to refine the prior austenite grain size. The pinning particles were studied earnestly and the following knowledge was obtained.

1. Since most of the TiN as dispersed pinning particles dissolves in the vicinity of the weld bond, it is necessary to use a stable oxysulfide together with TiN at a high temperature in order to exert a pinning effect throughout the weld heat affected zone. It is very effective for enhancing the pinning effect. That is, TiN does not dissolve and acts effectively except in the vicinity of the bond portion, and high-temperature stable oxysulfide acts effectively in the vicinity of the bond portion.
2. The pinning effect of the dispersed pinning particles increases as the volume ratio increases and the individual particle diameter increases, and it is important for the particles to contain Ca and Mn at a certain ratio or more at the same time to increase the particle diameter.
3. When the volume fraction of dispersed pinning particles is constant, the smaller the average diameter, the larger the number of particles and the greater the pinning effect. However, the austenite grains in the vicinity of the high heat input weld bond are coarse grains, and in order to obtain a pinning effect on such coarse austenite grains, it is necessary to increase the particle size of the dispersed pinning particles accordingly. It is.
4). As the number of dispersed pinning particles is larger, the pinning effect is larger. However, if the number is too large, adverse effects such as a decrease in Charpy absorbed energy are caused.

The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1. Steel composition by mass% C: 0.03-0.09%
Si: 0.02-0.15%
Mn: 1.5 to 2.5%
Al: 0.005-0.06%
P: 0.015% or less S: 0.0005 to 0.0050%
Nb: 0.005 to 0.025%
Ti, 0.005-0.02%
N: 0.0040 to 0.0070%
Ca: 0.0005 to 0.0030%
B: 0.0005 to 0.0025%
Ceq (IIW) is 0.38% or more, 0.45% or less, the balance Fe and unavoidable impurities,
The steel contains 50 to 1000 oxysulfide particles having an equivalent circle diameter of 0.5 to 3 μm / mm ^2, and the composition of the oxysulfide particles includes at least Ca, Al, Mn, O, and S. Excellent toughness of high heat input welding heat-affected zone characterized in that the elements excluding the elements include Ca: 5% or more, Al: 5% or more, Mn: 3% or more, S: 5% or more High strength steel.
2. As a steel composition, V: 0.04% or less Ni: 0.4% or less Cu: 1.0% or less Cr: 0.7% or less Mo: 0.7% or less W: 0.5% or less as a steel composition The high strength steel excellent in the toughness of the high heat input welding heat-affected zone according to 1, which contains one or more of the above.

According to the present invention, a high strength thick steel material having a yield strength exceeding 460 N / mm ² and a plate thickness of 40 mm or more, which is suitable for various structures such as shipbuilding, construction, and civil engineering, has a heat input of 300 kJ / Even with high heat input welding exceeding cm, a material having excellent toughness of the heat affected zone can be obtained, which is extremely useful industrially.

In the present invention, the component composition and the form and distribution of inclusions in the steel are defined.
[Component Composition] In the following description, “%” means “mass%”.
C: 0.03-0.09%
C is an element that improves the strength of the steel, and in order to ensure the strength, it is necessary to contain 0.03% or more, but if it exceeds 0.09%, not only the weldability is deteriorated, but also toughness. Since there is an adverse effect, it was specified in the range of 0.03 to 0.09%. In addition, Preferably it is 0.04 to 0.08%.

Si: 0.02-0.15%
Si is effective as a deoxidizing element and as a steel strengthening element, but if the content is less than 0.02%, the effect cannot be obtained. On the other hand, if it exceeds 0.15%, not only the surface properties of the steel are impaired, but also the toughness is extremely deteriorated. Therefore, the addition amount is set to 0.02% or more and 0.15% or less.

Mn: 1.5 to 2.5%
Mn is an important alloying element in the present invention. Although added as a strengthening element, the effect is not sufficient when it is less than 1.5%. Further, if the content is less than 1.5%, it may be difficult to contain a predetermined amount of Mn in the oxysulfide that becomes dispersed pinning particles. On the other hand, if it exceeds 2.5%, the weldability deteriorates and the steel material cost also rises.

Al: 0.005-0.06%
Al acts as a deoxidizer, and for this purpose, a content of 0.005% or more is required. However, if it exceeds 0.06%, the toughness is lowered and, when welded, weld metal Reduce the toughness of the part. For this reason, it is set as 0.005 to 0.06%. In addition, Preferably, it is 0.02 to 0.05%.

P: 0.015% or less, S: 0.0005 to 0.0050%
If P is contained in an amount exceeding 0.015%, the toughness of the welded portion is deteriorated. S is required to contain 0.0005% or more in steel as a component of dispersed pinning particles. On the other hand, if the content exceeds 0.0050%, the toughness of the base metal and the welded portion is deteriorated, so the content is made 0.0005 to 0.0050%.

Nb: 0.005 to 0.025%
It is preferable to apply controlled rolling to a high strength thick steel material having a yield strength exceeding 460 N / mm ² and a plate thickness of 40 mm or more according to the present invention, and Nb is an indispensable element that effectively acts on strengthening of steel. It is made to contain 0.005% or more. However, if the content exceeds 0.025%, the toughness of the high heat input welding heat-affected zone is lowered by precipitation hardening, so 0.005 to 0.025%.

Ti: 0.005 to 0.02%,
Ti precipitates as TiN during solidification of the molten steel and subsequent cooling of the slab, and serves as dispersed pinning particles in the weld heat affected zone, effectively acting to suppress coarsening of austenite and contributing to high toughness. If it is less than 0.005%, the effect is small, and if it exceeds 0.02%, the expected effect cannot be obtained due to the coarsening of TiN particles, so 0.005 to 0.02%.

N: 0.0040 to 0.0070%
N secures the necessary amount of TiN, which is dispersed pinning particles, so that its content is specified. If the amount is less than 0.0040%, a sufficient TiN amount cannot be obtained. If the amount exceeds 0.0070%, the amount of solid solution N increases in the region near the weld bond where TiN dissolves, and the toughness is remarkably lowered. 0040 to 0.0070%.

Ca: 0.0005 to 0.0030%
Ca is an essential element as a constituent element of oxysulfide. In order to exhibit such an effect, at least 0.0005% is contained. On the other hand, even if the content exceeds 0.0030%, the effect is saturated, so the content is made 0.0005 to 0.0030%.

B: 0.0005 to 0.0025%
B fixes N which is released by dissolution of TiN in the welding heat-affected zone as BN, and suppresses a decrease in the toughness of the welding heat-affected zone. Moreover, BN becomes a ferrite formation nucleus and contributes to the improvement of the weld heat affected zone toughness by refining the weld heat affected zone structure and suppressing the formation of MA (Martensite-Austenite constituent). Furthermore, it improves hardenability and contributes effectively to securing the strength of the base material. Those effects are exhibited by addition of 0.0005% or more, and even if 0.0025% or more is added, the effect is saturated, so 0.0005 to 0.0025%.

Ceq (IIW): 0.38 to 0.45%
Ceq (IIW) is set to 0.38 or more in order to secure a yield strength exceeding 460 N / mm ^{2 at} a plate thickness of 40 mm or more. On the other hand, if it exceeds 0.45, the production amount of MA increases in the weld heat affected zone and the toughness decreases, so the content is made 0.38 to 0.45%. In addition, Ceq (IIW) = C + Mn / 6 + (Cu + Ni) / 15 + (V + Mo + Cr) / 5, each element symbol is content (mass%).

  The above is the basic component composition of the present invention, but in order to further improve the characteristics, it is possible to contain one or more of V, Ni, Cu, Cr, Mo, W.

V, Ni, Cu, Cr, Mo, W
V, Ni, Cu, Cr, Mo, and W are all elements that enhance the hardenability of steel. While it contributes directly to the strength increase after rolling and can be added to improve the functions such as toughness, high-temperature strength, or weather resistance, excessive addition deteriorates the toughness and weldability. : 0.04%, Ni: 0.4%, Cu: 1.0%, Cr: 0.7%, Mo: 0.7%, W: 0.5% are preferable. On the other hand, since the effect cannot be obtained when the content of these elements is less than 0.01%, when added, the content is preferably 0.01% or more.
[Form and distribution of inclusions in steel]
Oxysulfide particles having an equivalent circle diameter of 0.5 to 3 μm are contained in the steel in a number per unit area of 50 to 1000 particles / mm ² . In the present invention, in order to suppress the growth of austenite grains in the structure near the high heat input weld bond, oxysulfide particles are used as dispersed pinning particles in combination with TiN, and the composition, particle diameter, and number are specified. To do.

  The composition of the oxysulfide particles includes at least Ca, Al, Mn, O, and S, and elements excluding O are in a mass ratio, Ca: 5% or more, Al: 5% or more, Mn: 3% or more, S: Including 5% or more.

  In order to make the oxysulfide particles large enough to obtain the pinning effect described later, the composition of the particles contains both Ca and Mn, and the oxides of Ca and Mn are sulfided with the Al oxide produced by Al deoxidation as the nucleus. The product is deposited in a complex manner.

  The composition of the oxysulfide particles has a tendency to increase in particle diameter from various dissolution experiments with addition of Mn, Al and Ca and an evaluation test of the pinning effect. Ca: 5% or more, Al: 5% or more, Mn : 3% or more and S: 5% or more.

  The pinning particle size needs to be defined according to the austenite particle size in the vicinity of the high heat input weld bond, which becomes the ground structure, and in order to suppress the growth of austenite particles with a particle size of about 200 μm at high temperatures, The particle diameter of the particles is 0.5-3 μm in terms of equivalent circle diameter. When the thickness is smaller than 0.5 μm, the pinning effect cannot be obtained, and when it exceeds 3 μm, cracks are generated from the periphery, and toughness and ductility are lowered.

Further, if the number of oxysulfide particles is less than 50 / mm ² , the pinning effect cannot be obtained. On the other hand, if it exceeds 1000 / mm ² , the pinning effect is saturated and the toughness and ductility are reduced. Therefore, it is set to 50 to 1000 / mm ² .

  Needless to say, in order to make the oxysulfide particles have a predetermined composition, size, and number, it is necessary to set the steel components within the component ranges according to the present invention, but a preferable production method will be described. Industrially, refining such as component adjustment and vacuum degassing is performed in a converter to perform Al deoxidation, but it is desirable to reduce the amount of dissolved oxygen before addition of Al to 100 ppm or less. If it exceeds 100 ppm, Al tends to be insufficient in the composition of the oxide particles. The casting is preferably continuous casting, and the solidification rate is preferably in the range of 0.05 ° C to 50 ° C / s. If the solidification rate is slow, the particle size is large and the number may be small. Further, if the solidification rate is high, the particle diameter becomes too fine and the desired pinning effect may not be obtained.

  In the present invention, since the above-described oxysulfide particles are used in combination with TiN as dispersed pinning particles, the effect of suppressing grain growth can be obtained even if most of TiN is dissolved in the vicinity of the high heat input weld bond.

  Preferred production conditions for the steel of the present invention include the following production steps. The molten steel having the above-described component composition is melted in a converter or the like, then made into a steel material (slab) by continuous casting or the like, reheated to 1000 to 1200 ° C., and then hot-rolled.

  When the reheating temperature is 1000 ° C. or lower, the rolling efficiency is lowered. On the other hand, when it is 1200 ° C. or higher, the austenite grains are coarsened and the toughness is lowered, and the oxidation loss becomes remarkable and the yield is lowered. Therefore, it shall be 1000-1200 degreeC.

  From the viewpoint of toughness, a preferable heating temperature range is 1050 to 1150 ° C, more preferably 1050 to 1100 ° C.

  Thereafter, the steel sheet is rolled to a desired thickness by hot rolling, but controlled rolling is preferably performed to ensure strength and toughness. The finishing temperature is preferably in the range of 900 to 650 ° C, more preferably in the range of 800 to 700 ° C. Furthermore, accelerated cooling is applied as appropriate after rolling according to the plate thickness. The effects of the present invention will be described below using examples.

  Molten steel with various compositions is melted in a converter, made into a steel material (slab: 280 mm thick) by a continuous casting method, reheated, hot-rolled to a thickness of 50 to 70 mm, subjected to accelerated cooling, and tested A steel plate was used. Table 1 shows the component composition of the test steel, and Table 2 shows the production conditions.

  About the obtained thick steel plate, a JIS14A test piece of Φ14 was taken from 1/4 part of the plate thickness so that the longitudinal direction of the test piece coincided with the width direction of the test piece, and a tensile test was conducted to obtain a yield point (YS), tensile Strength (TS) was measured. Further, a JIS No. 4 impact test piece was sampled from ¼ part of the plate thickness so that the longitudinal direction of the test piece coincided with the rolling direction, and a Charpy test was conducted to determine the fracture surface transition temperature (vTrs).

  Further, the cross section of the steel plate was polished to a mirror surface, and an area of 1 mm × 1 mm was observed with a scanning electron microscope, and the number of particles having an equivalent circle diameter of 0.5 μm or more was measured. EPMA (electron beam microanalyzer) analysis of all particles was performed, and the average composition excluding oxygen was calculated for those containing oxygen.

Furthermore, V groove was given to the test plate for joints extract | collected from each steel plate, and the high heat input welded joint was produced by electrogas arc welding (heat input 450kJ / cm). A JIS No. 4 impact test piece having a notch position as a bond portion was collected from each welded joint, and a Charpy impact test was performed at a test temperature of −40 ° C. to obtain absorbed energy (vE- ₄₀ , average value of 10 pieces).

Table 3 shows the test results of the base metal mechanical properties and the Charpy impact test results of the high heat input welded joint. The steels of the present invention (production Nos. 1 to 16) showed excellent toughness of the heat-affected zone where the vE- ₄₀ of the weld joint portion was 70 J or more.

Claims (2)

Steel composition by mass% C: 0.03-0.09%
Si: 0.02-0.15%
Mn: 1.5 to 2.5%
Al: 0.005-0.06%
P: 0.015% or less S: 0.0005 to 0.0050%
Nb: 0.005 to 0.025%
Ti: 0.005-0.02%
N: 0.0040 to 0.0070%
Ca: 0.0005 to 0.0030%
B: 0.0005 to 0.0025%
Ceq (IIW) is 0.38% or more, 0.45% or less, the balance Fe and unavoidable impurities,
The steel contains 50 to 1000 oxysulfide particles having an equivalent circle diameter of 0.5 to 3 μm / mm ^2, and the composition of the oxysulfide particles includes at least Ca, Al, Mn, O, and S. Excellent toughness of high heat input welding heat-affected zone characterized in that the elements excluding the elements include Ca: 5% or more, Al: 5% or more, Mn: 3% or more, S: 5% or more High strength steel. As a steel composition, V: 0.04% or less Ni: 0.4% or less Cu: 1.0% or less Cr: 0.7% or less Mo: 0.7% or less W: 0.5% or less as a steel composition The high-strength steel excellent in toughness of the high heat input welding heat-affected zone according to claim 1, comprising one or more of the following.