JP2002030380A - High tensile strength steel excellent in welded joint toughness and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high tensile strength steel having excellent toughness in the heat affected zone by welding of heat input in a wide range from small- medium heat input to large heat input and to provide its production method. SOLUTION: This high tensile strength steel has a composition containing, by weight, <=0.05% C, 0.01 to 0.5% Si, 0.2 to 2.0% Mn, 0.001 to 0.03% Ti, 0.0005 to 0.010% Mg, <=0.01% Al, <=0.02% P and <=0.010% S and the balance Fe with inevitable impurities, in which the ratio of Ti to N satisfies 2.0<=Ti/N<=3.5, and in which Ti-Mg containing oxide and one or more kinds of TiN precipitated with the oxide as the nucleus have a grain size of 0.005 to 0.2 μm and the dispersedly precipitated by 1.0×105 to 1.0×107 pieces/mm2, and in the method for producing the high tensile strength steel, steel having the above composition is heated to a temperature of the Ac3 point to 1,300 deg.C, is hot-rolled at Ac1+50 deg.C and is subsequently subjected to air cooling or accelerated cooling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to shipbuilding, construction, civil engineering,
The present invention relates to a high-strength steel for a welded structure having excellent second-hand toughness used in fields such as bridges, construction machinery, industrial machinery, pressure vessels, marine structures, and line pipes. In particular, T
This is effective for steel having a base material strength of S570 MPa or more, and the steel may be any of a thick plate, a thin plate, a steel pipe, a shaped steel, a section steel, and a steel bar.

[0002]

2. Description of the Related Art From the viewpoint of preventing brittle fracture of welded structures such as ships, buildings, bridges, etc., many studies have been reported on the improvement of the weld heat affected zone (hereinafter abbreviated as HAZ) toughness of steel materials used. Furthermore, in recent years, demands for large heat input welding (up to about 250 kJ / cm) have been increasing in order to improve welding work efficiency.

[0003] The decrease in HAZ toughness of a large heat input welded joint is as follows.
It is known that the coarsening of the crystal grains of the microstructure has an influence, and it is effective to suppress the coarsening of the crystal grains (particularly, old austenite grains: hereinafter, abbreviated as old γ grains) to improve toughness. . For example, Japanese Patent Publication No. 55-26164 and Japanese Patent Laid-Open Publication No.
As described in JP-A-314, fine TiN
Also, there is disclosed a measure for dispersing ZrN or the like in steel finely to prevent coarsening of old γ grains by utilizing their pinning effect.

[0004] However, such a nitride does not melt in welding with a relatively low heat input and retains the effect of pinning, contributing to the refinement of old γ grains, but the residence time at a high temperature of 1400 ° C or more is high. However, there is a problem that the fine nitride that contributes to the pinning of the old γ grains easily dissolves and disappears with the large heat input welding heat having a long heat input.

On the other hand, in recent years, for the purpose of further improving the HAZ toughness, a technique using an oxide generated in molten steel has been disclosed. For example, as described in JP-A-59-190313, production of a steel material having excellent weldability characterized by deoxidizing molten steel with Ti or a Ti alloy and then adding Al, Mg and the like. A method is disclosed.
This is due to the effect that the Ti oxide acts as a transformation nucleus of ferrite and increases the ferrite fraction. Conventionally, the improvement of the toughness of the HAZ portion by a method different from the pinning effect by a precipitate such as a nitride is considered. planned. afterwards,
As similar inventions, JP-A-61-79745, JP-A-5-43977 and the like disclose various inventions such as devising to increase the number of oxides as intragranular transformation nuclei.

[0006] In particular, as described in JP-A-59-190313, the gist of these inventions is "γ →
Ferrite nucleation during α transformation, that is, fine and uniform dispersion of Ti-containing oxide that can be used to refine the ferrite structure.
・ ”Means that the pinning effect is not intended by the nitride or the like as described above.
By promoting ferrite transformation at the time of α transformation, formation of a coarse embrittlement structure is suppressed, and the structure is refined. All of these toughness improvement methods, in order to promote ferrite in the grain in a coarse structure,
An oxide is used as a transformation nucleus.

However, the demand for high-strength steel is increasing due to the increase in the size and weight of the welded structure, and the amount of alloying elements added tends to increase. In that case, due to an increase in hardenability in the HAZ, the structure of the HAZ becomes martensite or bainite, and conventional measures for improving the HAZ toughness using ferrite transformation are becoming ineffective.

[0008] Conventionally, since the strength of the base material is easily secured, C
In general, steel grades with a content exceeding 0.05% have been used, and in such steel grades, the strength and toughness are adjusted by quenching / tempering or working heat treatment (TMCP), but avoidance of preheating during welding In view of improvement in HAZ toughness, reduction of C has become an extremely effective and indispensable technique. For example, in the ultra-low C steel described in Japanese Patent Application Laid-Open No. H8-144019 and the like, a technique of forming a bainite single-phase structure at a wide range of cooling rate and suppressing material variation is disclosed. For bainite steels whose structure change is not sensitive, the joint structure becomes bainitic instead of ferrite or martensite, and the HAZ using the conventional ferrite transformation is used.
No measures to improve toughness are available.

In view of the above, in order to improve the base metal toughness and the HAZ toughness of a steel material mainly composed of a low C bainitic structure, it is necessary to reduce the former γ grains during large heat input welding. It is desired to develop a technology capable of finely dispersing oxide / nitride particles and the like that are hardly dissolved even at a high temperature in a steel so that a pinning effect can be expected.

As a method for introducing oxides, there is a method of adding a deoxidizing element such as Ti alone in the process of smelting steel. However, in many cases, agglomeration and coalescence of oxides occur during the holding of molten steel and coarse particles are formed. A large oxide is generated, which impairs the cleanliness of the steel and lowers the toughness. Therefore, various attempts have been made to reduce the size of these oxides, such as a complex deoxidation method, as described in the previous example. However, according to the conventionally known method, it is not possible to disperse a fine oxide enough to prevent coarsening of crystal grains during large heat input welding heat.

[0011]

Therefore, the present inventors have improved the conventional composite deoxidation method for steel mainly composed of a bainitic structure having an extremely low C content, and have oxidized the steel more than before. It is an object of the present invention to establish a technique for manufacturing a high-strength steel excellent in toughness of a base material and HAZ toughness of large heat input welding by dispersing a material or nitride finely and uniformly.

[0012]

The gist of the present invention is as follows. C: 0.05% or less by weight%, Si: 0.
01-0.5%, Mn: 0.2-2.0%, Ti: 0.
001-0.03%, Mg: 0.0005-0.010
%, Al: 0.01% or less, P: 0.02% or less, S:
0.010% or less and the ratio of Ti to N is 2.0
≦ Ti / N ≦ 3.5, Ca: 0.006% or less,
REM: 0.02% or less, Cu: 0.05 to 1.5%,
Ni: 0.05 to 2.0%, Cr: 0.01 to 1.0
%, Mo: 0.01 to 0.5%, Nb: 0.001 to
0.15%, V: 0.001 to 0.1%, B: 0.00
Containing 1 to 0.0050% of one or more kinds,
The balance consists of Fe and unavoidable impurities, Ti, Mg
-Containing oxides and Ti precipitated with the oxides as nuclei
One or two or more of N have a particle diameter of 0.005 to 0.5.
1.0 × 10 ^{5 to} 1.0 × 10 ⁷ pieces / size of 2 μm /
This is a high-strength steel having excellent weld joint toughness, characterized in that mm ² is dispersed and precipitated. The manufacturing method is such that the steel ingot and the slab having the above composition are reheated to a temperature of Ac 3 point to 1300 ° C., hot rolling is completed at Ac 1 point + 50 ° C. or higher, and after rolling, air cooling, accelerated cooling, or 450 ° C. More than 700
After accelerated cooling to a predetermined temperature of less than ° C., the temperature range is air-cooled or maintained in the temperature range for a certain time.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Mg has been known as a strong deoxidizing agent and a desulfurizing agent to improve HAZ toughness by improving the cleanliness of steel. Further, as a technique for improving the HAZ toughness by controlling the dispersion of oxides, JP-A-59-19
No. 0313 discloses a technique of composite addition in which Mg is added after Ti addition. However, the purpose of the technique, as quoted above, is to promote the increase of Ti oxide, which is a transformation nucleus of intragranular ferrite, by adding Mg, and to disperse and precipitate the oxide more finely, and crystallize by pinning. It does not achieve grain refinement.

The present inventors have focused on the action of Mg as a strong deoxidizing agent and desulfurizing agent, and have achieved high strength and high toughness by utilizing the property that aggregation and coarsening are less likely to occur than Al. The order of addition of the deoxidizing agent in the steel making process is high even for a Ti-added high-strength steel mainly composed of a microstructure (hereinafter abbreviated as a bainitic structure) such as tempered martensite, bainite, bainitic ferrite, etc. By controlling the amount, it is thought that there is room for expecting fine dispersion precipitation of oxides and nitrides.

[0015] The present inventors systematically compare the state of oxides and nitrides when Mg is added to molten steel that has been weakly deoxidized by adding Ti to a steel material mainly composed of a low C bainitic structure. I checked. As a result, by adding Mg under certain conditions after Ti deoxidation, TiO, MgO or (Ti, Mg)
The amount of the O-TiN composite particles is remarkably increased, and in the steel material mainly composed of bainitic structure, the old γ grains are refined by the pinning effect, and as a result, the bainite structure having the same orientation as the fracture surface unit of the brittle fracture. It has been clarified that the unit (packet) can be miniaturized, the base material toughness is improved, and the HAZ toughness is also improved.

Observation of the steel material in which the crystal grains were refined by an electron microscope revealed that a large number of MgO particles or MgO-TiN composite particles of 0.2 μm or less were present. Furthermore, from observation with an ultra-high resolution electron microscope,
It has been clarified that a favorable equilibrium relationship is established between the TiN particles and the crystal orientation. This means that MgO becomes Ti
It is presumed that it acts as a preferential precipitation site for N, and it is considered that the presence of many such precipitation sites increases the pinning effect of crystal grains by TiN and sulfide precipitated based on them.

Furthermore, in the case of large heat input welding at a high temperature and a long residence time, TiN particles are dissolved, and the pinning effect of the former γ grains is lost by itself, but in the present invention, a large number of TiO, The presence of the MgO and MgO-TiN particles enables the pinning effect to be more excellent than that of conventional steel even at a high temperature even when the TiN particles are dissolved, because the fine TiO and MgO particles still exist.

That is, the feature of the present invention is that MgO is finely dispersed in steel and crystallographically excellent equilibrium with TiN as compared with conventional steel in which crystal grains are pinned using nitrides such as TiN. By utilizing the relationship to provide the precipitation nuclei of TiN, to increase the number of TiN, and to dissolve the TiN, even in a high temperature region where the effect of improving the toughness has been lost, the effect of MgO alone, It is to be able to exhibit an unprecedented excellent effect of crystal grain refinement.

In the present invention, the size of each of the Ti and Mg-containing oxides and the composite precipitates with TiN deposited using the oxides as nuclei is limited to 0.005 to 0.2 μm.
This is because a precipitate having a particle diameter of less than 0.005 μm cannot provide a sufficient pinning effect effective for refining old γ grains, and a precipitate having a particle diameter of more than 0.2 μm is brittle. This is because it becomes a starting point of fracture and causes a decrease in toughness. Further, when the average particle spacing is more than 5 μm, the number of dispersed particles decreases, and the pinning force of the old γ grains as a whole decreases, and a sufficient grain-reducing effect cannot be obtained. Are dispersed at 5 μm or less.

The method of measuring the number of grains to prepare a extracted replica from steel, at least 1000 .mu.m ² or more the number of particles bigger than 0.001~0.2μm a transmission electron microscope with a characteristic X-ray detector Measure the area of
Convert to the number per unit area. For example, when one field of view is observed at 100,000 × 80 mm at a magnification of 20,000 times, the observation area per one field of view is 20 μm ² , so that at least 50 fields of view are observed. 0.0 at this time
If the number of particles having a size of from 0.05 to 0.2 μm is 200, the number of particles can be converted to 2 × 10 ⁵ per 1 mm ² .

Next, TiO, MgO, (Mg, T
i) The existence ratio of -TiN is measured, and the existence ratio is multiplied by the number of particles obtained in advance to obtain TiO, MgO, (M
g, Ti) Obtain the number of O-TiN.

The method of adding Ti and Mg used in the present steel is as follows. First, after adding Si and Mn, first, Ti is added to adjust the amount of oxygen in the molten steel, and then Mg is added. Ti is added first in order to reduce the amount of oxygen in the molten steel and to reduce the previously formed Ti oxide with Mg. Optimal Mg
The amount of addition depends on the amount of oxygen present in the molten steel after the addition of Ti, but in experiments, the oxygen concentration at that time depends on the amount of Ti added, and if the amounts of Ti and Mg are controlled within an appropriate range. good. Regarding the method of adding Mg, when melted under normal atmospheric pressure, it is desirable to add an alloy having a relatively high specific gravity as an alloy composed of Mg, Ni, and Si.

The reasons for limiting the components of the present invention are described below. All percentages in the following text are percentages by weight. C: C enhances the strength of the base metal of the steel, but from the viewpoint of weldability, it is desirable that C is low to suppress the hardening of the weld heat affected zone. Further, by setting the low C of 0.05% or less, the formation of a hard phase such as martensite or pearlite is suppressed,
Since stable strength and toughness can be obtained by mainly using a bainitic structure, the upper limit is set to 0.05%.

Si: Si is an element necessary not only for strengthening steel materials but also as a deoxidizing element in steel making.
The above addition is necessary, but if it exceeds 0.5%, the formation of island-like martensite is promoted and the HAZ toughness is remarkably reduced. Therefore, the range of Si is set to 0.01% to 0.5%.

Mn: Mn is an element necessary for securing the strength and toughness of the base material. If it exceeds 2.0%, it significantly impairs the HAZ toughness. Since it is difficult to secure the strength, the range is 0.2 to 2.0%.
And

Al: Al is usually added as a deoxidizer, but in the present invention, if added in an amount exceeding 0.01%, the effect of the addition of Mg is impaired. 01% or less.

Ti: Ti is an element which can be expected to have an effect of refining crystal grains as a deoxidizing material and further as an element forming oxides and nitrides. In order to achieve this, it is necessary to set the upper limit to 0.03%.
% Or more is required. Therefore, the range of Ti is set to 0.001 to 0.03%.

Mg: Mg is mainly added as a deoxidizer, but if it exceeds 0.010%, the toughness of the base material and the HAZ decreases due to the formation of coarse oxides. However, if less than 0.0005% is added, the aforementioned HAZ
Since it does not work as pinning particles for refining old γ grains required for improvement of toughness, its addition is 0.0005 to 0.5.
010%.

P: P is an element that affects the toughness of steel, and if it is contained in excess of 0.02%, the base material of steel and H
Since the toughness of AZ is significantly impaired, the upper limit of the content is set to 0.02%.

S: If S exceeds 0.010%, the toughness of the steel base material and HAZ is significantly impaired.
The upper limit was set to 0.010%. Although the range of N is not described in the present invention, the ratio with Ti (Ti / N) is limited to 2.0 or more and 3.5 or less in consideration of TiN precipitation.

Ca: Ca is effective for improving the HAZ toughness by controlling the form of the sulfide-based inclusions.
%, The upper limit is 0.006% because inclusions in steel are formed to reduce the properties of steel.

REM: REM becomes oxysulfide and suppresses grain growth of austenite grains and contributes to improvement of HAZ toughness. However, if added in excess of 0.02%, the cleanliness of steel is impaired. The upper limit was made 0.02%.

Cu: Cu is an element that can increase the strength without lowering the toughness.
Addition of 0.05% or more is necessary. If it exceeds 1.5%, hot cracks are likely to occur at the time of heating the slab or welding.
Therefore, the content is set to 0.05 to 1.5%.

Ni: Ni is an element effective for improving toughness and strength. In order to obtain the effect, addition of 0.05% or more is necessary.
Since the hardenability increases and the weldability decreases, the upper limit is set to 2.0%.

Cr: Cr is effectively added in an amount of 0.01% or more to increase the strength of the steel material by precipitation strengthening. However, if added in a large amount, the hardenability increases and the toughness decreases. Therefore, the upper limit is set to 1.0%.

Mo: When Mo is added in an amount of 0.01% or more, the hardenability is greatly improved, and at the same time, the formation of carbonitride is effective in increasing the strength. However, the addition of a large amount exceeding 0.5%, together with unnecessarily strengthening, facilitates the formation of island-like martensite and lowers the toughness, so that the range is 0.01 to 0.5%. .

Nb: Nb also forms a carbonitride and is an element effective in improving the strength. However, the addition of less than 0.001% has no effect, and the addition of more than 0.15%
On the other hand, in order to cause a decrease in toughness, the range is set to 0.001.
To 0.15% or less.

V: V is an element that forms carbonitrides and is effective in improving the strength. However, when V is added less than 0.001%, the effect is not obtained. To reduce the toughness, the range is 0.001 to
0.1% or less.

B: B generally increases the hardenability when dissolved, but also decreases the dissolved N as BN to form HAZ.
It is an element that improves toughness. Therefore, the upper limit is 0.005% in order to avoid the addition of 0.0001% or more that can utilize the effect and to avoid the excessive addition that causes the decrease in toughness.
0%. The steel material of the present invention exhibits excellent large heat input welding HAZ toughness by adjusting the components to the basic composition described above. However, from the viewpoint of the properties and productivity of the base material, the following manufacturing process is desirable.

That is, in the steel ingot or slab whose components have been adjusted to the above-mentioned basic composition, the structure is austenitized, the precipitate is dissolved, and the alloy elements are once dissolved to form a solid solution. After heating,
Hot rolling is completed in the austenitic region of Ac1 point + 50 ° C or higher. After rolling, air cooling or accelerated cooling is performed subsequently, or after rolling, accelerated cooling is performed at a constant rate to a predetermined temperature of 450 ° C. or more and less than 700 ° C., and the temperature range is air-cooled or held for a predetermined time in the temperature range. Is recommended in terms of base material properties and productivity. Regarding cooling after rolling, air cooling or accelerated cooling may be used since the steel of the present invention has a small dependence on the cooling rate and exhibits a uniform bainitic structure. In addition, 45
Higher strength by air cooling or holding for a certain period of time in a temperature range of 0 ° C. or more and less than 700 ° C. to alleviate processing strain, decompose island-like martensite, and strengthen precipitation by carbonitride precipitation of Nb and V.・ Toughness balance can be obtained.

[0041]

Next, an embodiment of the present invention will be described. Example 1 Steel ingots of various components were produced by laboratory melting (50 kg, 120 mm thick steel ingot). These ingots were rolled under various conditions into steel plates having a thickness of 13 to 30 mm, and various mechanical properties were investigated. Mechanical properties of steel sheet (Yield strength: Y
S, tensile strength: TS, absorbed energy at −20 ° C. in Charpy impact test: vE-20 and 50% brittle fracture transition temperature: vTrs) were examined in the direction perpendicular to the rolling. The HAZ toughness (absorbed energy at −20 ° C. in the Charpy impact test: vE-20) was measured using HA reproduced with a reproducible thermal cycler.
(Maximum heating temperature: 1400 ° C., 800 to 5
(Cooling time at 00 ° C. [Δt _800-500 ]: 237 seconds). Ti
The size and the number of O, MgO and TiN precipitated using the nuclei were examined by observing the replica sample with a transmission electron microscope.

Examples are shown in Tables 1 and 2. The steel sheet manufactured according to the present invention has a base metal toughness having a Charpy impact absorption energy at −40 ° C. of more than 200 J, and a HAZ at −20 ° C. having a Charpy impact absorption energy of more than 150 J, and excellent base metal and HAZ toughness. Having. On the other hand, the comparative steel has a small number of chemical components or Ti, Mg oxide particles and the number of TiN particles precipitated by using them as nuclei,
The Charpy impact absorption energy of HAZ at -20 ° C is low. Steel K has a large C content, has a structure containing coarse and hard carbides, and has a low Charpy impact absorption energy of HAZ. Steel L and O have improper balance of Ti / N,
Due to the excess of Ti, TiN particles are also coarsened at the same time that carbides of Ti are generated, and the Charpy impact absorption energy of HAZ is low. Since steels M and N have large amounts of Si and Mn, quenching properties are improved, generation of hard layers (such as island martensite) is increased, and the Charpy impact absorption energy of HAZ is low. Steel P has a large amount of Al, Ti, Mg
The number of oxides is reduced, and the HAZ has low Charpy impact absorption energy without obtaining a sufficient pinning effect. Since the steel Q has a large amount of Mg, it forms coarse MgO particles and the number thereof is also reduced, so that the HAZ has a low Charpy impact absorption energy. Steel R has no added Mg, and therefore has low Charpy impact absorption energy of HAZ.

[0043]

[Table 1]

[0044]

[Table 2] (continuation of Table 1)

[0045]

The present invention is limited to the chemical components and the manufacturing method of the present invention, and by appropriately adding Ti and Mg, it is possible to prevent the toughness of the heat-affected zone of large heat input welding from being reduced, and to prevent the structure from brittle fracture. Can be greatly improved.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K032 AA00 AA01 AA02 AA04 AA08 AA11 AA14 AA15 AA16 AA19 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 AA40 BA01 CA01 CA02 CA03 CB02 CC03 CC04 CD05

Claims (5)

[Claims] C: 0.05% or less by weight% Si: 0.01 to 0.5% Mn: 0.2 to 2.0% Ti: 0.001 to 0.03% Mg: 0.0005 -0.010% Al: 0.01% or less P: 0.02% or less S: 0.010% or less, and the ratio of Ti to N is 2.0 ≦ Ti / N ≦ 3.
5, the balance being Fe and unavoidable impurities,
One or more of Ti and Mg-containing oxides and TiN precipitated with the oxides as nuclei have a particle diameter of 0.0
1.0 × 10 ⁵ to 1.0 × with a size of 05 to 0.2 μm
High strength steel with excellent weld joint toughness characterized in that 10 ⁷ particles / mm ² are dispersed and precipitated. 2. The high-strength steel with excellent toughness of a welded joint according to claim 1, which contains one or two of Ca: 0.006% or less and REM: 0.02% or less by weight%. 3. In% by weight Cu: 0.05 to 1.5% Ni: 0.05 to 2.0% Cr: 0.01 to 1.0% Mo: 0.01 to 0.5% Nb: The composition according to claim 1, wherein the composition contains at least one of 0.001 to 0.15% V: 0.001 to 0.1% B: 0.0001 to 0.0050%. High strength steel with excellent toughness of weld joint. 4. A steel ingot or a slab having the steel composition according to any one of claims 1 to 3, which is re-heated and then hot-rolled to produce a high-strength steel having excellent weld joint toughness. ~
A method for producing a high-strength steel having excellent toughness of a welded joint, comprising heating to a temperature of 1300 ° C., completing hot rolling at a temperature of Ac1 + 50 ° C. or higher, and air-cooling or accelerated cooling after the rolling. 5. A steel ingot or a slab having the steel composition according to any one of claims 1 to 3, which is reheated and then hot-rolled to produce a high-strength steel excellent in weld joint toughness. ~
Heat to a temperature of 1300 ° C., complete hot rolling at a temperature of Ac1 + 50 ° C. or more, and after rolling, accelerate and cool to a predetermined temperature of 450 ° C. or more and less than 700 ° C., or air-cool the temperature range, or A method for producing high-strength steel having excellent toughness in welded joints, characterized in that the high-strength steel is maintained for a certain period of time in a weld zone.