JP5712484B2 - A steel material excellent in ductile crack initiation characteristics of a weld heat-affected zone and a base metal zone, and a method for producing the same. - Google Patents

Description

  The present invention relates to a steel material suitable for use in welded structures such as pipelines, bridges, and buildings where structural safety is required, and to a method for manufacturing the same, and more particularly, to a ductile heat resistant part and a base metal part. It relates to a material having excellent crack initiation characteristics. Specifically, the welded heat-affected zone and the base metal are excellent in ductile crack generation characteristics, and tensile strength: strength of 490 MPa or more in TS, ductility of Charpy impact test (conforming to JIS Z 2242) / Brittle fracture surface transition temperature: For structural steel materials having high toughness with vTrs of 0 ° C. or lower.

  When welded structures such as pipelines, bridges, and buildings are exposed to large external loads such as earthquakes, ductile cracks are generated at stress-concentrated parts such as weld toes and the generated ductile cracks. It is known that there is a case where a brittle fracture occurs and the structure is damaged or broken.

  In order to avoid such damage and breakage of the welded structure, it is important that the steel materials constituting them have excellent ductile crack generation characteristics.

  Patent Document 1 discloses that the microstructure of a steel surface portion has a ferrite fraction of 10 to 40%, a bainite fraction of 50% or more, and an average particle size of 5 μm or less. A high-strength steel material having excellent generation characteristics is described.

  In Patent Document 2, the microstructure is a steel plate substantially composed of a ferrite structure, a pearlite structure, and a bainite structure, and when divided into three layers of both surface portions of the steel plate and the central portion in the plate thickness direction, Steel sheets each having a specific microstructure and excellent arrest properties and ductile crack fracture properties are described.

    Both surface portions of the steel sheet have a ferrite structure having a ferrite grain having a circle equivalent particle diameter of 7 μm or less and an aspect ratio of 2 to 4 over 5% or more of the plate thickness, and the bainite fraction of the part. Is composed of a layer having a thickness of 5 to 25% or less, and the center portion in the thickness direction of the steel sheet has ferrite grains having a circle-equivalent average grain size of 4 to 10 μm and an aspect ratio of 2 or less over 50% or more of the thickness And it is comprised by the layer whose bainite fraction of the said part is 10% or less.

  That is, the technique of Patent Document 2 is a steel sheet in which three layers having ferrite / pearlite structures composed of ferrite grains having different aspect ratios exist from the surface of the steel sheet in the thickness direction, and further the soft phase. A bainite structure as a hard phase is appropriately dispersed in the ferrite pearlite structure.

  Of these, both surface portions of the steel plate are actively formed with processed ferrite grains having a large aspect ratio, and the arrest characteristics are enhanced by appropriately dispersing the bainite structure, while the central portion of the steel plate is uniform, etc. While controlling the axial ferrite grain structure and suppressing the bainite structure, it is important to improve the elongation properties important for ductile fracture at room temperature. This is a technology that satisfies both conflicting characteristics of "arrest characteristics" and "ductile fracture characteristics" by controlling.

  In addition, the technique of Patent Document 3 is a technique for forming processed ferrite grains on the steel sheet surface layer of ferritic pearlite steel and making the microstructure inside the steel sheet uniform and equiaxed ferrite grains as in the technique of Patent Document 2. It is.

  That is, Patent Document 3 describes a method for producing a thick steel plate having excellent arrest characteristics and ductile fracture characteristics in which rolling conditions are strictly controlled and a steel sheet surface layer portion has a specific microstructure.

Specifically, when the thickness in the middle of rolling is t, the non-recrystallization temperature of Ar ₃ transformation point or more and 900 ° C. or less with respect to the surface layer region of 0.05 t or more and 0.15 t or less from both surfaces in the plate thickness direction. In the region, an equivalent plastic strain ε satisfying ε ≧ 0.5 is applied.

Thereafter, the temperature of the inner region on the core side from the thickness t / 4 position from both surfaces within the time when the residual accumulated equivalent plastic strain amount εr of the surface layer region satisfies εr ≧ 0.5 is set to the Ar ₃ transformation point. While maintaining the above, the surface layer region is cooled to a temperature range of 450 to 650 ° C. at a cooling rate of 2 to 15 ° C./s, and then rolling is resumed.

In the restarted rolling, a residual cumulative equivalent plastic strain εr of 0.35 ≦ εr <0.55 is applied to the internal region, and the rolling is completed at the Ar ₃ transformation point or higher, and the processing heat generation and internal manifestation are performed. The surface region is reheated to the Ar ₃ transformation point or less by heat, and then cooled so that the average cooling rate becomes 1 to 10 ° C./s.

  Moreover, all of the techniques of Patent Documents 1 to 3 form fine subgrains in austenite by rolling in an austenite non-recrystallized region (fine graining temperature region) or rolling at a rolling finishing temperature Ar3 or higher. And a technique for refining the structure after transformation.

JP 2008-202119 A JP 2000-328177 A JP 2003-221619 A

  However, in the techniques of Patent Documents 1 to 3, there is a concern that the effect of ductile crack generation is lost when the surface layer structure is changed to a weld heat affected zone by welding or the like.

  Moreover, the temperature set with the rolling process in the fine grain temperature range as described in the scale breaker used for the process of the slab surface extracted from the heating furnace as described in the Example of patent document 1, or the example of patent document 2 The rolling process in two zones, that is, the rolling process in two stages, and the diversified rolling and temperature control for creating the structure of the surface layer and the structure in the steel plate as in Patent Document 3, all involve complicated manufacturing processes.

  Therefore, in view of the problems of the prior art, the present invention aims to provide a steel material having excellent ductile crack generation characteristics in a welding heat-affected zone and a base metal, and a method for producing the same, by a simple method. To do.

  In order to achieve the above-mentioned object, the present inventors have conducted intensive research on a base material structure excellent in ductile crack generation characteristics of a weld heat-affected zone, and presenting an average structure in the thickness direction of a steel sheet. When the base metal structure is a ferrite and a hard phase that define the area fraction of the hard phase and the average aspect ratio of ferrite at the 1/4 position of the thickness, the welded heat-affected zone has excellent ductile cracking characteristics. In addition, the present inventors have found that such a steel material is also excellent in the ductile crack generation characteristics of the base material portion, and further, manufacturing conditions for a steel sheet having the microstructure.

The present invention has been completed on the basis of such findings, and has been completed.
(1) By mass%, C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.1 to 2.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, having a composition comprising the balance Fe and inevitable impurities, and the microstructure at the 1/4 position of the plate thickness is ferrite. A steel material having excellent ductile cracking characteristics, comprising a hard phase, having an area fraction of the hard phase of 50 to 90%, and an average aspect ratio of the ferrite of 1.5 or more.
(2) In addition to the component composition, Cu: 0.01-2%, Ni: 0.01-5%, Cr: 0.01-3%, Mo: 0.01-2%, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, REM: 0.1% or less The steel material having excellent ductile crack generation characteristics as described in (1), wherein the steel material contains one or more kinds.
(3) In (1) or (2), the structure of the steel sheet surface is composed of ferrite and a hard phase, the area ratio of the ferrite exceeds 40%, and the average aspect ratio of the ferrite grain size is 2 Steel material with excellent ductile cracking characteristics that exceeds.
(4) A steel material excellent in ductile crack initiation characteristics of a weld heat-affected zone having a component composition according to (1) or (2) and a microstructure at a 1/4 position of the plate thickness according to (1).
(5) Furthermore, the steel material excellent in the ductile crack generation characteristic of the welding heat affected zone as described in (4) provided with the structure of the steel plate surface as described in (3).
(6) A steel material excellent in ductile crack initiation characteristics of the weld heat-affected zone and the base metal having the component composition described in (1) or (2) and the microstructure at the 1/4 position of the plate thickness described in (1) .
(7) Further, a steel material having the structure of the steel sheet surface described in (3) and excellent in the ductile crack generation characteristics of the weld heat affected zone and the base material in terms of ductile crack generation characteristics.
(8) The steel material having the component described in (1) or (2) is reheated to 1000 ° C. or higher, the rolling reduction in the temperature range of 900 ° C. or higher is 50% or higher, and the rolling finish temperature is Ar ₃ points to After rolling to Ar ₃ -50 ° C., water cooling is started at Ar ₃ -10 ° C. to Ar ₃ -70 ° C., and water cooling is ended at 500 ° C. or less. Excellent steel manufacturing method.
(9) The method for producing a steel material having excellent ductile crack generation characteristics according to (8), further comprising performing a tempering treatment at a temperature less than _one point of the maximum heating temperature Ac after water cooling.
(10) The steel material having the component described in (1) or (2) is reheated to 1000 ° C. or higher, the rolling reduction in the temperature range of 900 ° C. or higher is 50% or higher, and the rolling finishing temperature is Ar ₃ points to After rolling to Ar ₃ -50 ° C, water cooling is started at Ar ₃ -10 ° C to Ar ₃ -70 ° C, and water cooling is ended at 500 ° C or less, and the ductility of the weld heat affected zone is characterized by A method of manufacturing steel with excellent crack initiation characteristics.
(11) After the water cooling, the tempering treatment is further performed at a maximum heating temperature Ac of less than ₁ point, and the steel material having excellent ductile crack initiation characteristics of the weld heat affected zone according to (10) Production method.
(12) The steel material having the component described in (1) or (2) is reheated to 1000 ° C. or higher, the rolling reduction in the temperature range of 900 ° C. or higher is 50% or higher, and the rolling finish temperature is Ar ₃ points to Welding heat-affected zone and base material characterized by starting water cooling at Ar ₃ −10 ° C. to Ar ₃ −70 ° C. and ending water cooling at 500 ° C. or lower after rolling to Ar ₃ −50 ° C. A method for producing a steel material with excellent ductile crack initiation characteristics.
(13) After performing water cooling, the tempering treatment is further performed at a maximum heating temperature Ac of less than ₁ point, and the weld heat-affected zone and the base metal are excellent in ductile crack generation characteristics as described in (12) Steel manufacturing method.

  According to the present invention, for example, even if a large deformation due to an earthquake or the like occurs in a steel structure, it is possible to suppress the occurrence of a ductile crack from a stress concentrated portion such as a weld toe, and prevent the steel structure from collapsing or breaking. Steel products that can suppress the occurrence of ductile cracks in the weld heat-affected zone and the base metal can be easily and stably mass-produced, and have remarkable industrial effects.

It is a figure which shows the ductile crack generation test method of a welding heat affected zone. It is a figure which shows the influence of the area fraction of a hard phase and the average aspect-ratio of a ferrite which has on the ductile crack generation | occurrence | production of 1400 degreeC reproduction thermal cycle material. It is a figure which shows the ductile crack generation test method of a base material part. It is a figure which shows the influence of the area fraction of a hard phase, and the average aspect-ratio of a ferrite which acts on the ductile crack generation | occurrence | production of a base material part.

  In the present invention, the component composition and the microstructure are defined. Unless otherwise specified in the description of the component composition, the mass% is simply represented by%.

[Ingredient composition]
C: 0.02-0.2%
C is an element having an action of increasing the strength of steel, and in the present invention, contributes particularly to the formation of a hard phase. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, when it contains exceeding 0.2%, ductility and bending workability will be reduced, and weldability will fall. For this reason, C was limited to 0.02 to 0.2% of range. More preferably, it is 0.02 to 0.18%.

Si: 0.01 to 0.5%
Si acts as a deoxidizer and has the effect of improving the strength of the steel by solid solution. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, the content exceeding 0.5% reduces toughness and weldability. For this reason, Si was limited to the range of 0.01 to 0.5%. More preferably, it is 0.01 to 0.4%.

Mn: 0.1 to 2.5%
Mn has the effect of increasing the strength of the steel and improving the toughness through improving the hardenability. In order to obtain such an effect, the content of 0.1% or more is required. On the other hand, the content exceeding 2.5% lowers the weldability. For this reason, Mn was limited to the range of 0.1 to 2.5%. More preferably, it is 0.5 to 2.0%.

P: 0.05% or less Since P leads to deterioration of toughness, it is preferable to reduce it as much as possible, but 0.05% is acceptable. For this reason, P was limited to 0.05% or less.
More preferably, it is 0.04% or less.

S: 0.05% or less S is present as an inclusion in steel and deteriorates ductility and toughness. Therefore, it is desirable to reduce it as much as possible, but 0.05% is acceptable. For this reason, S was limited to 0.05% or less. More preferably, it is 0.04% or less.
Al: 0.1% or less Al is an element that acts as a deoxidizer and contributes to refinement of crystal grains, but excessive content exceeding 0.1% leads to a decrease in toughness. For this reason, Al was limited to 0.1% or less. More preferably, it is 0.05% or less.

N: 0.01% or less N is an element that increases the strength of the steel by solid solution strengthening in the same manner as C. However, since excessive inclusion leads to a decrease in toughness, N is limited to 0.01% or less. More preferably, it is 0.005% or less.

  Although the above-mentioned components are basic components, according to the present invention, Cu: 0.01-2%, Ni: 0.01-5%, Cr: 0.01-3%, Mo: 0.01-2%, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, REM: 0 One or two or more selected from 1% or less may be contained.

Cu: 0.01-2%
Cu is an element having an action of increasing the strength of steel through an increase in hardenability and solid solution. In order to ensure such an effect, a content of 0.01% or more is required. On the other hand, if the content exceeds 2%, weldability deteriorates and flaws are likely to occur during the production of the steel material. For this reason, when adding, it is set as 0.01 to 2% of range. More preferably, it is 0.01 to 1%.

Ni: 0.01 to 5%
Ni contributes to improvement of low temperature toughness, increase of hardenability, and prevention of hot brittleness of Cu when Cu is contained, so it is added as necessary. Although such an effect is recognized by addition of 0.01% or more, addition of 5% or more causes a reduction in steel material cost and weldability. For this reason, when adding, it is set as 0.01 to 5% of range. More preferably, it is 0.01 to 4.5%.

Cr: 0.01 to 3%
Cr is added as necessary in order to increase the strength of the steel material by improving the hardenability and increasing the temper softening resistance. Such an effect is recognized when the content is 0.01% or more. On the other hand, addition exceeding 3% lowers weldability and toughness. For this reason, when adding, it is set as 0.01 to 3% of range. More preferably, it is 0.01 to 2.5% of range.

Mo: 0.01-2%
Mo is added as necessary in order to increase the strength of the steel material by improving the hardenability and increasing the temper softening resistance. Such an effect is recognized when the content is 0.01% or more. On the other hand, addition exceeding 2% lowers weldability and toughness. For this reason, when adding, it is set as 0.01 to 2% of range. More preferably, it is 0.01 to 1% of range.

Nb: 0.1% or less Nb is an element that precipitates as carbide or carbonitride during tempering and increases the strength of steel through precipitation strengthening. Nb also has the effect of improving the toughness by refining austenite grains during rolling. In order to obtain the effect, 0.001% or more is preferable. However, the content exceeding 0.1% lowers the toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is 0.05% or less.

V: 0.1% or less V is an element that precipitates as carbide or carbonitride during tempering and increases the strength of steel through precipitation strengthening. Moreover, it has the effect of refining austenite grains during rolling and improving toughness. In order to obtain the effect, 0.001% or more is preferable. However, the content exceeding 0.1% lowers the toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is 0.05% or less.

Ti: 0.1% or less Since Ti has the effect of refining austenite and improving toughness in the weld heat affected zone, it is added as necessary. In order to obtain the effect, 0.001% or more is preferable. However, addition exceeding 0.1% lowers toughness and leads to an increase in steel material cost. For this reason, when adding, it is made into 0.1% or less. More preferably, it is made 0.05% or less.

B: 0.01% or less B is added as necessary because it has the effect of improving the hardenability and increasing the strength of steel when contained in a small amount. In order to acquire the effect, 0.0001% or more is preferable. However, addition of 0.01% or less reduces weldability. For this reason, when adding, it is 0.01% or less. More preferably, it is 0.005% or less.

Ca: 0.01% or less Ca is added as necessary to improve the toughness of the base metal by controlling the form of the CaS inclusions and further to improve the toughness of the weld heat affected zone. In order to acquire the effect, 0.0001% or more is preferable. However, addition over 0.01% reduces toughness due to an increase in CaS inclusions. For this reason, when adding, it is 0.01% or less. More preferably, it is 0.009% or less.

REM: 0.1% or less REM is an element that improves the toughness of the weld heat affected zone, and is added as necessary. In order to acquire the effect, 0.0001% or more is preferable. However, addition exceeding 0.1% causes a decrease in toughness. For this reason, when adding, it is made into 0.1% or less. More preferably, it is made 0.05% or less.

  Note that REM is a generic name for rare earth elements such as Y and Ce, and the amount added here means the total amount of these rare earth elements.

[Microstructure]
In the steel material according to the present invention, the structure at a 1/4 position of the plate thickness is composed of ferrite and a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite grain size is 1.5. It has the above microstructure. If the area fraction of the hard phase is less than 50% or more than 90%, or the average aspect ratio of the ferrite grain size is less than 1.5, ductile cracks may occur.

  The upper limit value of the average aspect ratio of the ferrite grain size is not particularly required, but is set to 5 or less from the capability of the rolling mill. The area fraction of the hard phase is more preferably 52 to 90%, and the average aspect ratio of the ferrite particle diameter is more preferably 1.6 or more. More preferably, it is 1.7 or more.

  In a two-phase mixed structure consisting of ferrite and hard phase, the yield ratio of the base metal decreases, and strain concentration in the stress concentrating part is relaxed even after reproducible thermal cycling that simulates the base metal or the weld heat affected zone. Is done. Such an effect cannot be obtained when the ferrite single phase or the hard phase single phase is used.

  In the steel material according to the present invention, the structure of the steel plate surface (1 mm from the plate surface) is composed of ferrite and a hard phase, and the area ratio of ferrite is more than 40%, more preferably 50% or more. Further, the average aspect ratio of the ferrite grain size exceeds 2. When the area ratio of the ferrite is less than 40% or the average aspect ratio of the ferrite grain size is 2 or less, the ductile crack resistance in the weld heat affected zone is inferior.

  In the present invention, the hard phase is bainite, martensite, or a bainite / martensite mixed structure, and includes an island-like martensite (MA) having an area fraction of 5% or less.

    FIG. 2 shows the results of investigating ductile crack initiation characteristics using a reproducible thermal cycle test piece (maximum heating temperature 1400 ° C.) of a welded portion. As shown in FIG. In the case where the area fraction is 50 to 90% and the average aspect ratio of ferrite is 1.5 or more, the occurrence of ductile cracks is not observed after the reproducible thermal cycle.

    The results shown in FIG. 2 are as follows. Steel having a composition within the scope of the present invention was produced by various manufacturing methods and the microstructure was changed, and the thickness was 12 mm (= plate thickness direction) × 12 mm width × 200 length. The test piece was taken from the center of 1/4 of the plate thickness (the plate thickness of 25 mm or less is 1/2 center of the plate thickness), and the reproducible heat cycle of the welded portion (time to reach the maximum heating temperature: 6 s) A cooling rate from the maximum heating temperature to room temperature: 40 ° C./s) was applied to obtain a test material.

  FIG. 1 shows the test piece shape and test conditions. The edge of the specimen (test piece 1) to which the reproducible heat cycle was applied was introduced with a one-side through notch in the thickness direction of 3 mm in the center of the reproducible heat cycle part 2, restrained with a clamp 5, and a knife edge screwed After the tensile loading (arrow 6) to 0.6 mm with the displacement of the clip gauge 3 between 4, unloading, grinding to the center of the test piece width and mirror polishing to evaluate the presence or absence of cracks at the notch tip did. The case where the ductile crack from the notch bottom was 50 μm or more was defined as crack initiation.

  The results shown in FIG. 2 show that the yield ratio (0.2% proof stress / tensile strength) is reduced even in the structure after the reproducible thermal cycle by making the base material a composite structure of ferrite and hard phase, and the tip of the notch This is thought to be caused by a decrease in the degree of strain concentration.

  In addition, such excellent characteristics were commonly recognized even in the base material portion to which no reproducible thermal cycle was given. This is also considered to be because the strain concentration at the tip of the notch due to the decrease in yield ratio was relaxed.

  That is, FIG. 4 shows the result of investigating the influence of the microstructure of the base material portion on the ductile crack initiation characteristics. As shown in FIG. 4, the area fraction of the hard phase of the base material is 50 to 90. %, And when the average aspect ratio of the ferrite is 1.5 or more, ductile cracks are not observed.

  The result of the base material part shown in FIG. 4 is that a steel having a composition within the range of the present invention was produced by various manufacturing methods and the microstructure was changed, and the thickness was 12 mm (= plate thickness direction) × 12 mm width. A test piece having a length of × 200 was taken from the center of 1/4 of the plate thickness (plate thickness of 25 mm or less is 1/2 center of the plate thickness) (FIG. 3).

  FIG. 3 shows the test piece shape and test conditions. A test piece (test piece 1) with a one-side through notch introduced in the thickness direction of 3mm in the center is restrained by a clamp 5, and the displacement of the clip gauge 3 between screwed knife edges 4 is 0.8mm. After tensile loading (arrow 6), the unloading was performed, and the test piece was ground and mirror-polished to the center of the width of the specimen to evaluate the presence or absence of cracks at the notch tip. The case where the ductile crack from the notch bottom was 50 μm or more was defined as crack initiation.

  The results shown in FIG. 4 show that the yield ratio (0.2% proof stress / tensile strength) is reduced and the degree of strain concentration at the notch tip is reduced by making the base material a composite structure of ferrite and hard phase. This is probably due to this.

  Also, by increasing the average aspect ratio of ferrite, that is, by developing a specific texture, the slip surface was greatly inclined with respect to the crack initiation direction as it was as the base metal and even after reproducible thermal cycling. Is also one of the factors. The aspect ratio refers to the ferrite particle diameter in the rolling direction (long diameter) / plate thickness direction (short diameter) in the cross section parallel to the rolling direction.

  In addition, the same result as FIG. 2 was obtained also when the maximum heating temperature of the reproduction | regeneration thermal cycle was 760 degreeC, 900 degreeC, and 1200 degreeC.

  The steel material according to the present invention can be obtained by sequentially subjecting the steel material having the above components to a hot rolling process, a water cooling process, or a tempering process.

In the hot rolling, re-heating is performed to 1000 ° C. or higher, and rolling is performed at a temperature range of 900 ° C. or higher with a reduction rate of 50% or higher and a rolling finishing temperature of Ar ₃ points to Ar ₃ −50 ° C. A more preferable rolling finishing temperature is less than Ar ₃ points to Ar ₃ -40 ° C. By making it into this rolling finishing temperature range, processing strain can be applied to the ferrite generated during rolling, and the aspect ratio of the ferrite can be increased. When the reheating temperature is lower than 1000 ° C., hot rolling that imparts a desired cumulative reduction ratio to the steel material cannot be performed.

Further, when the cumulative rolling reduction at 900 ° C. or higher is less than 50%, desired strength and toughness cannot be ensured. When the rolling finishing temperature exceeds the Ar ₃ point, the ferrite aspect ratio is not 1.5 or more. If finish rolling temperature falls below the Ar 3 _-50 ° C., the fraction of hard phase obtained by the subsequent water cooling is not more than 50%.

In the water cooling step, immediately after hot rolling, water cooling is started at Ar ₃ -10 ° C to Ar ₃ -70 ° C, and the water cooling is ended at 500 ° C or less. When the water cooling start temperature exceeds Ar ₃ −10 ° C., it becomes a ferrite (hard phase exceeding 90% in area fraction) with an area fraction below 10%.

Also, when the water cooling start temperature is lower than Ar ₃ -70 ° C. or immediately after hot rolling (within 300 seconds), when water cooling is not started, ferrite with an area fraction exceeding 50% (in area fraction) In the present invention, pearlite that is not desired to precipitate is precipitated, and the desired characteristics cannot be satisfied.

After performing the above-described cooling, a tempering treatment can be further performed at less than Ac ₁ point. By performing the tempering treatment, the toughness and ductility can be improved and adjusted to the desired strength and toughness. When the tempering temperature exceeds ₁ point of Ac, a large amount of island martensite is generated, and the toughness is lowered.

Ar ₃ points and Ac ₁ point can be calculated by the following formula based on the content (% by mass) of each component.

Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo
Ac ₁ (° C.) = 723-14Mn + 22Si-14.4Ni + 13.3Cr
Hereinafter, the present invention will be described in more detail based on examples.

[Example 1]
The steel materials having the components shown in Table 1 were hot-rolled under the conditions shown in Table 2 to obtain steel plates having a thickness of 12 to 100 mm.

  The obtained steel sheet was subjected to a structure observation, a tensile test, a toughness test, a ductile crack generation test after a reproduction thermal cycle, and a ductile crack generation test of the base material. The test method was as follows (1) to (5).

(1) Microstructure observation From the obtained steel plate, a test piece was taken in a cross section parallel to the rolling direction, and after mirror polishing and night etching, the microstructure was observed at 1/4 position of the plate thickness and 1 mm below the surface. Each observation was performed with 20 visual fields. The area fraction was obtained by binarizing ferrite and a hard phase and multiplying by 200 times. The average aspect ratio of the ferrite is determined by determining the length in the rolling direction and the length in the plate thickness direction of each ferrite in the field of view at a magnification × 400, and calculating the length in the rolling direction / the length in the plate thickness direction. Obtained as an average value.

(2) Tensile test JIS No. 5 test piece of full thickness was sampled from the obtained steel sheet so that the tensile direction was perpendicular to the rolling direction of the steel sheet in accordance with the provisions of JIS Z 2201 (1998). . The tensile test was performed according to JIS Z 2241 (1998), and 0.2% proof stress (σ _0.2 ) and tensile strength (TS) were determined to evaluate static tensile properties.

(3) Toughness test From the obtained steel sheet, a V-notch test piece was taken so that the longitudinal direction was parallel to the rolling direction in accordance with the provisions of JIS Z 2242 (2005), and a ductile / brittle fracture surface transition was performed. The temperature was determined and the toughness was evaluated. The test specimens were sampled so that they were centered on 1/4 position of the plate thickness when the plate thickness was 20 mm or more, and 1/2 position of the plate thickness when the plate thickness was less than 20 mm.

(4) Ductile crack generation test after reproducible thermal cycle From the obtained steel plate, the thickness is 1/4 center (plate thickness of 25 mm or less is 1/2 center of the plate thickness) and 12 mm thickness (= plate thickness direction) = T) A test piece having a width of 12 mm and a total length of 200 mm was collected. A reproducible heat cycle of the weld heat affected zone with a maximum heating temperature of 760 ° C., 900 ° C., 1200 ° C., 1400 ° C. was given to this test piece using a greeble tester (reach time to the maximum heating temperature: 6 s, Cooling rate from maximum heating temperature to room temperature: 40 ° C./s).

  Thereafter, as shown in FIG. 1, a one-side through notch was introduced in the thickness direction of 3 mm in the center of the reproduction heat cycle portion. The notch machining was performed by electric discharge machining and the notch tip radius was set to 0.1 mm.

  In the test, the left and right ends of the test piece were gripped with a restrained length of 50 mm, and a tensile load was applied. During the test, the displacement between the knife edges attached by screwing in the vicinity of the notch was measured with a clip gauge, and after the tension loading to 0.6 mm by the clip gauge displacement, the load was removed.

  Thereafter, the test was cut to the center of the width and mirror-polished, and the state of crack generation at the bottom of the notch was examined with a microscope of magnification x50. The definition of ductile crack initiation was defined as when the ductile crack extended from the notch bottom by 50 μm or more.

(5) Ductile crack initiation test of base material From the obtained steel plate, it is 12 mm thick (= plate thickness direction = t) at the center of 1/4 of the plate thickness (plate thickness of 25 mm or less is 1/2 center of plate thickness). ) A test piece having a width of 12 mm and a total length of 200 mm was collected.

  As shown in FIG. 3, a one-side through notch was introduced into the center of the test piece in the thickness direction of 3 mm in the obtained test piece. The notch machining was performed by electric discharge machining and the notch tip radius was set to 0.1 mm.

  In the test, the left and right ends of the test piece were gripped with a restrained length of 50 mm, and a tensile load was applied. During the test, the displacement between the knife edges attached by screwing in the vicinity of the notch was measured with a clip gauge, and after the tension loading to 0.8 mm by the clip gauge displacement, the load was removed.

  Thereafter, the test was cut to the center of the width and mirror-polished, and the state of crack generation at the bottom of the notch was examined with a microscope of magnification x50. The definition of ductile crack initiation was defined as when the ductile crack extended from the notch bottom by 50 μm or more.

  Table 3 shows the experimental results obtained for the test pieces to which the reproducible thermal cycle was applied. The components specified in the present invention, No. produced by the production method. 1-No. Each of the 10 steel plates has the prescribed structure of the present invention. And while having the outstanding intensity | strength and toughness, it turns out that it is excellent in the ductile crack generation characteristic of a welding heat affected zone.

  On the other hand, No. C is less than the lower limit of the range of the present invention. No. 11 steel plate (steel type K *) has low tensile strength. In addition, C. P, S. No. exceeding the upper limit of the scope of the present invention. Steel plate No. 12 (steel type L *) has low toughness and is inferior in ductile crack initiation characteristics in the weld heat affected zone.

  The reheating temperature of the slab is lower than that of the present invention, and the cumulative rolling reduction of 900 ° C. or more falls outside the scope of the present invention. No. 13 steel sheet has low toughness. The rolling finish temperature and the water cooling start temperature exceed the range of the present invention. The steel plate No. 14 does not form ferrite and does not have the structure defined by the present invention, and is inferior in the ductile crack initiation characteristics of the weld heat affected zone.

  The water cooling start temperature falls below the range of the present invention. No. 15 steel plate and water cooling stop temperature No. exceeding the range of the present invention. Steel plate No. 16 has a hard phase fraction and an average aspect ratio of ferrite that do not become the values prescribed in the present invention, both have low tensile strength, and are inferior in ductile crack initiation characteristics in the weld heat affected zone.

  No. whose tempering temperature exceeds the range of the present invention. Steel plate No. 17 has low toughness because a large amount of island-like martensite is generated, and is inferior in ductile crack initiation characteristics in the weld heat affected zone.

  Table 4 shows the experimental results obtained for the base metal part. The components specified in the present invention, No. produced by the production method. 18-No. The 27 steel plates all have the prescribed structure of the present invention. And while having the outstanding intensity | strength and toughness, it is recognized that it is excellent in the ductile crack generating characteristic.

  On the other hand, No. C is less than the lower limit of the range of the present invention. 28 steel plates (steel type W *) have low tensile strength. In addition, C. P, S. No. exceeding the upper limit of the scope of the present invention. 29 steel plate (steel type X *) has low toughness. The reheating temperature of the slab is below the range of the present invention, and the cumulative rolling reduction of 900 ° C. or higher is not in the range of the present invention. The 30 steel plate has low toughness.

  The rolling finish temperature and the water cooling start temperature exceed the range of the present invention. The steel plate No. 31 does not form ferrite and does not have the structure defined by the present invention, and is inferior in ductile crack resistance.

  The water cooling start temperature falls below the range of the present invention. No. 32 steel plate and water cooling stop temperature No. exceeding the range of the present invention. The steel plate No. 33 has a hard phase fraction and an average aspect ratio of ferrite that are not specified in the present invention, and both have low tensile strength and inferior ductility cracking characteristics. No. with tempering temperature exceeding the value of the present invention. The steel plate No. 34 has low toughness because a large amount of island-like martensite is generated, and is inferior in ductile crack resistance.

Claims (8)

  In mass%, C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.1 to 2.5%, P: 0.05% or less, S: 0.05 %, Al: 0.1% or less, N: 0.01% or less, the composition comprising the balance Fe and unavoidable impurities, the microstructure at the 1/4 position of the plate thickness is composed of ferrite and hard phase A steel material having excellent ductile cracking characteristics, wherein the area fraction of the hard phase is 50 to 90% and the average aspect ratio of the ferrite is 1.5 or more.   2. The ductile crack generation according to claim 1, wherein the structure of the steel sheet surface is composed of ferrite and a hard phase, the area ratio of the ferrite exceeds 40%, and the average aspect ratio of the ferrite grain size exceeds 2. Steel material with excellent characteristics.   In addition to the component composition, Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1 by mass% % Or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.01% or less, Ca: 0.01% or less, REM: 0.1% or less Or steel material excellent in the ductile crack generating characteristic of Claim 2 characterized by containing 2 or more types.   The component composition according to claim 1 or 3 and the microstructure of the 1/4 thickness of the plate thickness according to claim 1, and the resistance to welding heat affected zone having the structure of the steel sheet surface according to claim 2. Steel with excellent ductile cracking characteristics.   The component composition according to claim 1 or 3 and the microstructure of the 1/4 thickness of the plate thickness according to claim 1, and the resistance to welding heat affected zone having the structure of the steel sheet surface according to claim 2. Steel with excellent ductile crack initiation characteristics and base material ductile crack initiation characteristics. A method for producing a steel material according to any one of claims 1 to 3,
The steel material was reheated to 1000 ° C. or higher, and after rolling at a rolling reduction temperature of Ar ₃ point to Ar ₃ −50 ° C. at a rolling reduction rate of 50% or higher in a temperature range of 900 ° C. or higher, Ar ₃ − Water cooling is started at 10 ° C. to Ar ₃ -70 ° C., water cooling is ended at 500 ° C. or less, and tempering is performed at a maximum heating temperature Ac of less than _one point. Steel manufacturing method. It is a manufacturing method of the steel materials according to claim 4,
The steel material was reheated to 1000 ° C. or higher, and after rolling at a rolling reduction temperature of Ar ₃ point to Ar ₃ −50 ° C. at a rolling reduction rate of 50% or higher in a temperature range of 900 ° C. or higher, Ar ₃ − Water cooling is started at 10 ° C. to Ar ₃ -70 ° C., water cooling is ended at 500 ° C. or less, and tempering is performed at a maximum heating temperature Ac of less than _one point. A method for manufacturing steel with excellent crack initiation characteristics. It is a manufacturing method of the steel materials according to claim 5,
The steel material was reheated to 1000 ° C. or higher, and after rolling at a rolling reduction temperature of Ar ₃ point to Ar ₃ −50 ° C. at a rolling reduction rate of 50% or higher in a temperature range of 900 ° C. or higher, Ar ₃ − Water cooling is started at 10 ° C. to Ar ₃ -70 ° C., water cooling is ended at 500 ° C. or lower, and tempering is performed at a maximum heating temperature Ac of less than _one point. A method for producing steel with excellent ductile crack initiation characteristics.

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