JP2006257497A - Method for producing low yield ratio steel material for low temperature, excellent in toughness in welded part - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a low yield ratio steel material for low temperature, excellent in the toughness in a welded part. <P>SOLUTION: After molten steel is pre-deoxidized with Al, the deoxidation with Si and Mn is performed and REM is added, and soluble oxygen content is adjusted to 0.0010-0.0050% and this molten steel contains 0.05-0.12% C, ≤0.50% Si, 0.8-1.8% Mn, ≤0.02% P, 0.0005-0.0060% S, 0.0030-0.0200% REM and if necessary, after adding B, Nb, V, Cu, Ni, Cr, Mo or W, the steel blank is made to contain ≤0.004% each of Al and Ti. Further, after heating this steel blank to 1050-1200°C, hot-rolling of ≥30% accumulative rolling reduction ratio at ≥950°C of the temperature zone and ≥30% accumulative rolling reduction ratio at <900°C of the temperature zone, is applied and thereafter, the front-step cooling at 750-600°C cooling stop temperature, is performed at <10°C/s cooling speed and the rear-step cooling to <600°C from the front-step cooling stop temperature, is performed at ≥10°C/s cooling speed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a low-yield ratio steel material for low temperature, and more particularly to a material suitable for a multipurpose tank in which liquefied ammonia and liquefied propane gas are mixedly loaded.

  A steel material used for a tank for liquefied ammonia is required to have a low YS of 400 MPa or less to prevent stress corrosion cracking (SCC) and a TS of 530 MPa or more to reduce the total weight of the steel material. In the case of a tank in which liquefied propane gas is mixed with liquefied ammonia, the steel material used is required to have further excellent low temperature toughness.

  In Patent Document 1, a low C—Ni—Nb—Ti based low Pcm steel is hot-rolled, reheated to 750 to 870 ° C. and quenched, and tempered at a temperature equal to or lower than the Ac1 transformation point. Discloses a method for producing a low yield ratio high strength steel excellent in weldability and low temperature toughness.

  In Patent Document 2, a low C—Ni—Nb—Ti-based low Pcm steel is directly quenched after rolling at a cumulative reduction of 30% or more in the austenite non-recrystallization temperature range and a rolling end temperature of 800 ° C. or more. Further, a method for producing a low yield ratio high strength steel excellent in weldability and low temperature toughness by reheating and quenching at 750 to 870 degrees and tempering at a temperature below the Ac1 transformation point, direct quenching and reheating quenching and tempering. It is disclosed.

  In Patent Document 3, a low C—Ni—Nb—Ti-based low Pcm steel is rolled at 680 ° C. after rolling at a rolling reduction temperature of 720 ° C. or more at a cumulative reduction amount of 30% or more in the austenite non-recrystallization temperature range. Disclosed is a method for producing a low-yield ratio high-strength steel excellent in weldability and low-temperature toughness by combining water-cooling at 150 to 350 ° C. Yes.

  The technique described in Patent Document 3 prevents transformation precipitation of coarse pro-eutectoid ferrite by setting the cooling start temperature to 680 ° C. or higher, draws a curve with a round load-elongation curve, and yield point is It is characterized by achieving a low yield ratio.

  In addition, as a means of reducing the yield ratio, after hot rolling, ferrite is first precipitated at the time of air cooling, quenched from the ferrite-austenite two-phase region, and transformed into the pro-eutectoid soft ferrite phase. There are used means for simultaneously achieving high tension and low yield ratio by a two-phase region quenching and tempering process in which a structure composed of phases is formed, or two-phase regions are quenched after heating and further tempered.

  On the other hand, since a large heat input welding method is applied to reduce the construction cost at the time of tank construction, various techniques have been proposed as measures against toughness of the weld heat affected zone. For example, TiN is finely dispersed in steel to suppress the coarsening of austenite, or a technique such as utilization as a transformation nucleus of ferrite has been put into practical use.

  In Patent Documents 4 and 5, rare earth elements (REM) are added together with Ti and finely dispersed in steel to prevent the growth of austenite grains and improve the toughness of the weld heat affected zone. It is disclosed.

Technology to ensure excellent weld heat affected zone toughness by dispersing Ti oxide, combining BN and oxide, or adding Ca or REM to control the form of sulfide. Has also been proposed.
JP-A-10-130721 Japanese Patent Laid-Open No. 10-168516 JP-A-11-293380 Japanese Patent Publication No. 03-053367 JP 60-184663 A

  However, tempering treatments such as reheating quenching and tempering, direct quenching-reheating quenching-tempering, and two-phase region heating quenching and tempering described in Patent Document 1, Patent Document 2 and the like are relatively stable. While the characteristics can be secured, the process is complicated, the production time is long and the production cost is high, and the non-refining treatment described in Patent Document 3 is difficult to produce stably.

In addition, when improving the toughness of the welded portion using TiN, the toughness is significantly lowered due to the embrittlement of the dough structure due to the solid solution Ti and the solid solution N in the bond portion heated to a temperature range where TiN dissolves. When Ti oxide is used, fine dispersion of the oxide cannot be performed sufficiently uniformly, and improvement of toughness may not be obtained.
An object of this invention is to provide the manufacturing method of the low yield ratio steel material for low temperatures which was non-tempered and was excellent in the low temperature toughness of a base material, and the weld part toughness.

In order to solve the above-mentioned problems, the present inventors have earnestly expressed various factors affecting the heat-affected zone toughness of the weld and the production conditions of the low yield ratio steel by non-tempering treatment, and obtained the following knowledge. It was.

  1 To increase the toughness of a welded portion, it is effective to finely disperse transformation nuclei for suppressing the coarsening of austenite grains in a region heated to a high temperature and for promoting ferrite transformation during cooling.

  2 In addition to adjusting the composition of particles such as oxides (oxycides) and sulfides (sulfides) in molten steel, the dispersed particles can be stably and uniformly controlled by controlling the form of dendrites formed during the solidification process. When finely dispersed, the HAZ toughness is significantly improved.

  After deoxidizing with 3 Si, Mn and adjusting the dissolved oxygen content of the molten steel before solidification to 0.0030 to 0.0120 mass%, when REM is added, dendrite morphology control is possible.

  4 After adjusting the amount of dissolved oxygen in the molten steel to a predetermined range, by adding REM, REM oxysulfide is crystallized at the solid-liquid interface, unidirectional growth of dendrites is suppressed, and equiaxed crystallization. A large amount of fine dispersed particles in which one or more of Si or Mn oxide, sulfide, and oxysulfide are combined are formed between the arms in a large amount and uniformly.

  5 This fine dispersed particle is formed in a large amount and uniformly, and is effective in preventing coarsening of austenite grains in the weld heat affected zone.

  6 If the cooling after rolling is two-stage cooling of the former stage and the latter stage and the respective cooling rates are adjusted, a mixed structure in which the second phase is dispersed in a ferrite ground having an appropriate grain size can be stably obtained. Due to the action of the sulfide as a ferrite transformation nucleus, the generation density of pro-eutectoid ferrite is increased, ferrite grains are finely generated, and the base material toughness at low temperature is improved.

The present invention has been made on the basis of the above findings and further studies, that is, the present invention
(1) Si and / or Mn is added to the molten steel for deoxidation, and the amount of dissolved oxygen is adjusted to 0.0030 to 0.0120 mass%, then REM is added,
The amount of dissolved oxygen is 0.0010 to 0.0050 mass%,
C: 0.05-0.12 mass%
Si: 0.50 mass% or less Mn: 0.8-1.8 mass%
P: 0.02% by mass or less S: 0.0005-0.0060% by mass or less REM: 0.0030-0.0200% by mass, and Al and Ti are limited to 0.004% by mass or less, respectively. Of molten steel,
Next, the molten steel is cast into a steel material, heated to 1050 to 1200 ° C., and the cumulative reduction in the temperature range of 950 ° C. or higher is 30% or higher, and the cumulative reduction in the temperature range of less than 900 ° C. is 30% or higher. After the hot rolling is completed, after the hot rolling is completed, the cooling at a cooling rate of less than 10 ° C./s is applied to the cooling stop temperature: 750 to 600 ° C. After that, the cooling is performed at a cooling temperature of less than 600 ° C. A method for producing a low yield ratio steel material for low temperature excellent in weld toughness, characterized by performing post-stage cooling to a cooling stop temperature at a cooling rate of 10 ° C / s or more.

  (2) Prior to deoxidation performed by adding Si and / or Mn, preliminary deoxidation by adding Al is performed, and the amount of dissolved oxygen before deoxidation is adjusted to 0.0080 to 0.0170 mass%. The manufacturing method of the low yield ratio steel material for low temperature excellent in the weld part toughness as described in (1) characterized by the above-mentioned.

(3) As a composition of molten steel,
B: 0.0003-0.0025 mass%
Nb: 0.05% by mass or less V: 0.2% by mass or less Cu: 1.0% by mass or less Ni: 1.5% by mass or less Cr: 0.7% by mass or less Mo: 0.7% by mass or less W: One or two or more types selected from 1.5% by mass or less are contained, (1) or (2), the method for producing a low temperature low yield ratio steel material excellent in weld zone toughness.

  According to the present invention, the YS (yield strength) of the base material is 440 MPa or less, the TS (tensile strength) is 530 MPa or more, the base material toughness is excellent, and the high heat input weld affected zone toughness is also low. The steel for yield ratio low temperature can be manufactured stably and at a low manufacturing cost, greatly contributes to the enlargement of a welded structure such as a multipurpose tank in which liquefied ammonia and liquefied propane gas are mixed, and is extremely useful industrially.

  In the present invention, the composition of the molten steel and the heating / rolling / cooling conditions of the steel are defined.

[Component composition of molten steel]
In the present invention, molten steel is melted by a conventional method using a known apparatus such as a converter, electric furnace, vacuum melting furnace, and the amount of dissolved oxygen is 0.0030 to 0.0120 mass by deoxidation treatment or degassing process. After adjusting to%, REM is added so that the dissolved oxygen amount is 0.0010 to 0.0050 mass%.

  In the present invention, deoxidation treatment is performed by adding Si and / or Mn, but it is preferable to perform preliminary deoxidation by adding Al before that to adjust the dissolved oxygen amount to 0.0080 to 0.0170%.

  When pre-deoxidization is performed by adding Al, if the amount of Al remaining in the molten steel exceeds 0.004% by mass, it becomes difficult to form a REM-based oxide.

  In the present invention, the amount of dissolved oxygen before REM addition is set to 0.0030 to 0.0120 mass%. As a result, one or more types of REM sulfide, REM oxide, and REM oxysulfide having an average particle size of 10 μm or less, preferably 1 μm or more crystallize at the solid-liquid interface during the solidification process, thereby suppressing unidirectional growth of dendrites. The equiaxed crystallization is achieved, the secondary dendrite arm interval is reduced, and the Mn inclusions (dispersed particles) produced by the subsequent secondary deoxidation are refined.

In order to suppress the unidirectional growth of dendrites, these particles are preferably dispersed at a particle number density of 20 particles / mm ² or more. When the particle number density is less than 20 particles / mm ² , equiaxed crystallization of the solidified structure occurs. It becomes difficult.

  If the amount of dissolved oxygen before REM addition is less than 0.0030% by mass, it becomes difficult to form a desired REM oxysulfide, and the above-described effects cannot be obtained.

  On the other hand, if the amount of dissolved oxygen before the addition of REM exceeds 0.0120% by mass, only the REM becomes an oxide, and it becomes difficult to form the desired REM sulfide or REM oxysulfide, and unidirectional growth of dendrites The ability to suppress decreases, and it becomes difficult to make the secondary dendrite arm interval fine.

  The amount of dissolved oxygen after REM addition is 0.0010 to 0.0050 mass%. Upon addition of REM, it is preferable to add S so that sulfides, oxides, and oxysulfides are formed and the amount of dissolved oxygen after the addition becomes a desired 0.0010 to 0.0050 mass%.

  In the solidification process, any of the REM sulfide particles, the REM oxide particles, and the REM oxysulfide particles is easily crystallized at the solid-liquid interface, thereby suppressing the unidirectional growth of dendrites.

  In addition, as a secondary deoxidation product, particles having an average particle size of 1 μm or less and a composite of one or more of Mn oxide, Mn sulfide, and Mn oxysulfide are obtained. -Suppresses the growth of stenite grains and improves HAZ toughness.

When the Mn-based composite particles are dispersed at a particle density of 1 × 10 ⁵ particles / mm ² or more, the pinning effect of austenite grains becomes remarkable in the high temperature retention region of the super large heat input welding HAZ, which is preferable.

  The average particle size of dispersed particles and the number density per unit area were determined by polishing the C cross section perpendicular to the rolling direction of a test piece taken from a steel plate and exposing the dispersed particles by electrolytic corrosion. Observe by. Observation is performed by taking 10 fields of view at a magnification of 5000 times and obtaining it with an image analyzer.

  When the amount of dissolved oxygen after REM addition is less than 0.0010% by mass, the dendrite arm interval becomes large, and fine dispersion of Mn-based composite composite particles that prevent austenite grains from coarsening as secondary deoxidation products Cannot be obtained, and the austenite grains cannot be sufficiently coarsened.

  On the other hand, when the amount of dissolved oxygen after REM addition exceeds 0.0050% by mass, the Mn oxide becomes coarse, and it becomes difficult to obtain fine Mn-based composite particles effective for preventing the coarsening of austenite grains. .

  In this invention, REM is added, the amount of dissolved oxygen is adjusted to 0.0010-0.0050 mass%, and the composition of molten steel is adjusted to the following compositions.

C: 0.05-0.12 mass%
C is an element that increases the strength of steel, and is added in an amount of 0.05% by mass or more in order to obtain the strength necessary for a thick high-strength steel plate. On the other hand, if added over 0.12% by mass, the toughness of the welded portion and the resistance to weld cracking are reduced, so 0.05 to 0.12% by mass.

Si: 0.50% by mass or less Si acts as a deoxidizer, but if it exceeds 0.5% by mass, the base material toughness deteriorates, and island martensite is generated in the weld heat affected zone. Is not more than 0.50% by mass, preferably 0.05 to 0.50% by mass.

Mn: 0.8 to 1.8% by mass
Mn acts as a deoxidizer and forms fine particles of oxides, sulfides, or oxysulfides as a secondary deoxidation product, resulting in coarse HAZ austenite grains. Suppression is made and HAZ toughness is improved.

  Further, in order to increase the strength of the steel by solid solution strengthening, 0.8% by mass or more is added. On the other hand, if added over 1.8% by mass, the toughness of the welded portion is remarkably lowered, so the content is made 0.8 to 1.8% by mass.

P: 0.02% by mass or less P is present in the steel as an inevitable impurity, and deteriorates toughness. Therefore, the refining cost is reduced within an allowable range. If it exceeds 0.02% by mass, the toughness of the base material and HAZ is lowered, so the content is made 0.02% by mass or less.

S: 0.0005-0.0060 mass%
When REM is added, S combines with REM and crystallizes in the solid-liquid interface as REM sulfide (sulfide) or REM oxysulfide (oxysulfide), and grows in one direction in dendrites. Is suppressed, the dendrite is equiaxed and the dendrite secondary arm interval is refined.

  Further, it binds to Mn as a secondary deoxidation product and finely crystallizes as Mn sulfide and acid S sulfide, thereby preventing coarsening of austenite grains of HAZ.

  When S is less than 0.0005% by mass, REM crystallizes out as an oxide at the molten steel stage, and the above-mentioned effects cannot be obtained. On the other hand, when it exceeds 0.0060% by mass, coarse MnS is formed and toughness is remarkable. Therefore, the content is made 0.0005 to 0.0060 mass%.

REM: 0.0030 to 0.0200 mass%
REM combines with S and / or O during the solidification process of molten steel, and forms one or more of sulfide (sulfide), oxide (oxycide), and oxysulfide (oxysulfide) at the solid-liquid phase interface. It has the effect of crystallizing and suppressing the unidirectional growth of the dendrite, making the dendrite equiaxed, and reducing the dendrite secondary arm interval.

  In order to obtain such an effect, 0.0030% by mass or more is contained. On the other hand, if the content exceeds 0.0200% by mass, coarse REM compounds increase and the base material toughness deteriorates. -0.0200 mass%.

Al: 0.004% by mass or less Al is a strong deoxidizing element, which combines with oxygen in molten steel to form alumina (Al ₂ O ₃ ) and reduce dissolved oxygen. Oxysulfide), or Mn oxide (oxycide) and oxysulfide (oxysulfide) as secondary deoxidation products are inhibited, dendrite morphology control and secondary deoxidation product fineness Adversely affects dispersion.

  Therefore, in this invention, it is set as Si and Mn deoxidation instead of Al deoxidation, and Al content is restrict | limited to 0.004 mass% or less. In addition, as long as it restrict | limits to this range, it does not matter to use Al as preliminary deoxidation.

Ti: 0.004 mass% or less
Ti, like Al, has a stronger deoxidizing power than Si and Mn. Therefore, Ti is preferably reduced as much as possible without adversely affecting the fine dispersion of the secondary deoxidation product. The following.

  The above is the basic component composition of the present invention, but it is preferable to contain one or more of Nb, V, Cu, Ni, Cr, and Mo in order to increase the strength of the base material and the welded joint.

Nb: 0.05% by mass or less Nb improves the strength and toughness of the base material and increases the joint strength. Such an effect becomes remarkable when the content is 0.01% by mass or more. However, if the content exceeds 0.05% by mass, the HAZ toughness is lowered. . In addition, Preferably it is 0.01-0.05 mass%.

V: 0.2 mass% or less V improves the strength of the base material. Such an effect becomes remarkable when the content is 0.02% by mass or more. However, if the content exceeds 0.2% by mass, the HAZ toughness is lowered. . In addition, Preferably it is 0.02-0.2 mass%.

Cu: 1.0 mass% or less Cu increases the strength similarly to Ni. Such an effect becomes remarkable when the content is 0.2% by mass or more. However, if the content exceeds 1.0% by mass, the HAZ toughness is lowered. . In addition, Preferably it is 0.2-1.0 mass%.

Ni: 1.5% by mass or less Ni increases the strength while maintaining the toughness of the base material. Such an effect becomes remarkable when the content is 0.2% by mass or more, but is an expensive element. In addition, Preferably it is 0.2-1.5 mass%.

Cr: 0.7% by mass or less, Mo: 0.7% by mass or less, W: 1.5% by mass or less Cr, Mo, and W are all effective for increasing the strength of the base material. Such an effect becomes remarkable when Cr: 0.2% by mass or more, Mo: 0.1% by mass or more, and W: 0.2% by mass or more, but Cr: 0.7% by mass, Mo: 0 When it exceeds 0.7% by mass and W: 1.5% by mass, the toughness is reduced. Therefore, when added, Cr: 0.7% by mass or less, Mo: 0.7% by mass or less, W: 1.5% by mass % Or less. In addition, Preferably, it is set as Cr: 0.2-0.7 mass%, Mo: 0.1-0.7 mass%, W: 0.2-1.5 mass%.

B: 0.0003-0.0025 mass%
B improves hardenability and increases the strength of the steel. If such an effect is less than 0.0003 mass%, on the other hand, if it exceeds 0.0025 mass%, the hardenability is remarkably increased and the toughness of the base material is lowered. 0.0003-0.0025 mass%.

  The remaining molten steel other than the components described above is Fe and unavoidable impurities, and N: 0.0040% by mass or less is allowed as an unavoidable impurity.

  The molten steel adjusted to the above-described composition is cast into a steel material (slab). The casting method is not particularly limited, but in order to control the size and form of the dispersed particles within a desired range, it is preferable to use a continuous casting method in which the casting speed and the cooling rate can be controlled in the solidification stage.

  The size of the dispersed particles is mainly determined by the amount of dissolved oxygen and the amount of Mn and S. However, since the casting speed is also affected, the casting method is preferably a continuous casting method rather than the ingot forming method.

[Heating / rolling-cooling conditions]
A steel material (slab) is rolled under the following conditions to obtain a low-yield steel material for low temperature use having a desired thickness.

1 Slab heating temperature The steel material is heated to 1050 ° C. or higher in order to crimp a casting defect. On the other hand, when it exceeds 1200 ° C., the austenite grain size becomes coarse and the toughness of the base material deteriorates, so that it is heated to 1050 to 1200 ° C.

2 Rolling conditions The cumulative rolling reduction at 950 ° C or higher is 30% or higher, and the cumulative rolling reduction at 900 ° C or lower is 30% or higher. Since the austenite grains are recrystallized by rolling at 950 ° C. or higher, the structure can be refined. If the cumulative rolling reduction is less than 30%, abnormally large grains remain during heating, which adversely affects the base material toughness. Therefore, it is 30% or more.

If the temperature is less than 900 ° C., the austenite is recrystallized by rolling or a defect such as a deformation band is introduced into the austenite grains without recrystallization, thereby increasing the number of ferrite transformation generation sites and reducing the structure. Refinement and base material toughness are improved.
In order to obtain such an effect, the cumulative rolling reduction at less than 900 ° C. is set to 30% or more.

  The rolling end temperature of the hot rolling is not particularly specified, but if it is less than 720 ° C., the proeutectoid ferrite is processed and the yield strength and yield ratio are increased, so that it is preferably 720 ° C. or higher.

3 Cooling condition Cooling is a two-stage cooling of the former stage cooling and the latter stage cooling. In order to stably obtain a mixed structure in which the second phase is dispersed in a ferrite ground having an appropriate particle size, the cooling rate of the subsequent cooling is set to be larger than that of the preceding cooling.

  The pre-stage cooling starts immediately after the hot rolling at less than 10 ° C./s, and the cooling end temperature is set to less than 750 ° C. and 600 ° C. or more. When the cooling rate is 10 ° C./s or more, the soft ferrite fraction decreases and desired characteristics cannot be obtained. When the cooling end temperature is 750 ° C. or higher or lower than 600 ° C., the ferrite fraction increases and the base metal strength decreases.

  In the latter-stage cooling, after the first-stage cooling is stopped, the cooling is performed at a cooling rate of 10 ° C./s or more to less than 600 ° C. When the cooling rate is less than 10 ° C., the hard bainite fraction decreases and the strength decreases. When the cooling stop temperature is 600 ° C. or higher, the yield ratio is reduced and the strength is saturated. It is preferable to use air cooling after the latter stage cooling is stopped.

  The steel plate thus obtained contains 0.0070% by mass or less of O as an unavoidable impurity. When O exceeds 0.0070 mass%, the oxide in steel will increase and cleanliness will deteriorate. In order to disperse the REM oxide, oxysulfide, Mn oxide, and oxysulfide in a required amount or more, the content is preferably 0.0015% by mass or more.

  Molten steel having the composition shown in Table 1 was melted in a converter and subjected to RH degassing treatment, and then made into a steel material (215 to 310 mm thick) by a continuous casting method. During the melting, deoxidation treatment was performed to adjust the amount of dissolved oxygen immediately before REM addition, and for some molten steel (steel No. 7), preliminary deoxidation was performed by addition of Al.

  The amount of dissolved oxygen after REM addition was adjusted by the amount of REM and S added. In Table 1, steel no. 1 to 7 are within the scope of the present invention. 8-24 are outside the scope of the present invention.

  The obtained steel material was made into a thick steel plate using the heating, rolling, and cooling conditions shown in Table 2, and the base metal structure, tensile properties, and toughness were investigated.

1 Base Material Structure The type of dispersed particles, average particle size, and particle number density were investigated by the method described above. After obtaining the average value for each visual field, these values were obtained for the entire visual field and used as a steel plate value. The type of dispersed particles was determined by an EDX apparatus of a scanning electron microscope.

2 Base material tensile properties JIS No. 4 tensile test specimens were collected from the C direction of 1/4 part of the plate thickness, and subjected to a tensile test in accordance with the provisions of JISZ2204. Yield point (YP), tensile strength (TS) Asked.

3 Base material toughness A JIS No. 4 impact test piece was sampled from the C direction of 1/4 part of the plate thickness, and subjected to a Charpy impact test in accordance with the provisions of JIS Z2242, to determine the fracture surface transition temperature (vTrs).

4 Welded HAZ toughness Reproduced heat cycle test of welded bond, JIS No. 4 test piece was prepared, Charpy impact test was conducted in accordance with JISZ2242, and absorbed energy at -40 ° C: vE-40 (J) The weld HAZ toughness was evaluated.

The above test results are also shown in Tables 2 and 3. Yield strength (YP): 440 N / mm ² or less, Tensile strength (TS): 530 N / mm ² , Charpy impact property: vTrs: −86 ° C. or less, Reproducible thermal cycle test of weld bond: Test temperature −40 ° C. 106J or more was regarded as the present invention.

All the steel materials (steel plates No. 1 to 7) according to the present invention have a yield strength (YP) of 440 N / mm ² or less, a tensile strength (TS) of 530 N / mm ² or more, and a yield ratio (YR) of It has a low low temperature toughness as low as 80% or less and Charpy impact property is also vTrs: -86 ° C or less.

  Furthermore, it is the Charpy absorbed energy of the weld bond part by the welding reproduction heat cycle test, and it is 106 J or more at a test temperature: −40 ° C., and is excellent in the toughness of the weld part.

  On the other hand, the steel materials of comparative examples (steel Nos. 8 to 31) out of the scope of the present invention are welded by the base material yield strength (YP), tensile strength (TS), Charpy impact characteristics, and weld reproduction thermal cycle test. At least one of the Charpy absorbed energy in the bond part was out of the scope of the present invention.

Claims (3)

After deoxidizing the molten steel by adding Si and / or Mn, the amount of dissolved oxygen is 0.0030 to 0.0120 mass%, and then adding REM,
The amount of dissolved oxygen is 0.0010 to 0.0050 mass%,
C: 0.05-0.12 mass%
Si: 0.50 mass% or less Mn: 0.8-1.8 mass%
P: 0.02% by mass or less S: 0.0005-0.0060% by mass or less REM: 0.0030-0.0200% by mass, and Al and Ti are limited to 0.004% by mass or less, respectively. Of molten steel,
Next, the molten steel is cast into a steel material, heated to 1050 to 1200 ° C., and the cumulative reduction in the temperature range of 950 ° C. or higher is 30% or higher, and the cumulative reduction in the temperature range of less than 900 ° C. is 30% or higher. After the hot rolling is completed, after the hot rolling is completed, the cooling at a cooling rate of less than 10 ° C./s is applied to the cooling stop temperature: 750 to 600 ° C. After that, the cooling is performed at a cooling temperature of less than 600 ° C. A method for producing a low yield ratio steel material for low temperature excellent in weld toughness, characterized by performing post-stage cooling to a cooling stop temperature at a cooling rate of 10 ° C / s or more. Before deoxidation performed by adding Si and / or Mn, preliminary deoxidation by adding Al is performed, and the amount of dissolved oxygen before deoxidation is adjusted to 0.0080 to 0.0170 mass%. The manufacturing method of the low yield ratio steel materials for low temperature excellent in the weld part toughness of Claim 1. As the composition of the molten steel,
B: 0.0003-0.0025 mass%
Nb: 0.05% by mass or less V: 0.2% by mass or less Cu: 1.0% by mass or less Ni: 1.5% by mass or less Cr: 0.7% by mass or less Mo: 0.7% by mass or less W: The manufacturing method of the low yield ratio steel material for low temperature excellent in the weld part toughness of Claim 1 or 2 characterized by containing 1 type, or 2 or more types chosen from 1.5 mass% or less.