JP2007291511A - High-tensile strength thick steel plate having excellent toughness and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-tensile strength thick steel plate having excellent toughness and fatigue crack propagation resistance in the plate thickness direction, and to provide its production method. <P>SOLUTION: A steel stock having a composition comprising, by mass, 0.03 to 0.3% C, 0.03 to 1.5% Si, 0.1 to 3% Mn, ≤0.1% Al and ≤0.01% N, and the balance Fe with inevitable impurities, and having a structure composed of a ferrite phase and a pearlite phase with the average grain size of ≥40 μm is heated to a temperature in the temperature range of (an Ac<SB>1</SB>transformation point+30°C) to (an Ac<SB>1</SB>transformation point+100°C), is thereafter subjected to low rolling reduction-multipass rolling at a cumulative draft of ≥80% and a rolling finishing temperature of ≥550°C, and is then air-cooled. In this way, the thick steel plate having a structure where a ferrite phase including a fine ferrite phase with the average grain size of ≤3 μm by ≥30 area% is the main phase, the second phase is an elongated pearlite phase, and the average minor axis size is ≤5 μm, and having excellent toughness and fatigue crack propagation resistance in the plate thickness direction is obtained. Further, the thick steel plate may comprise one or more kinds selected from Nb, V and Ti, and one or more kinds selected from Cu, Ni, Cr and Mo. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-tensile steel plate suitable for use in welded steel structures such as ships, offshore structures, bridges, buildings, tanks, etc., and particularly to toughness, and further to improving fatigue crack propagation resistance in the thickness direction. . As used herein, the “high-tensile thick steel plate” refers to a steel plate having a thickness of 10 mm or more and a tensile strength of 500 MPa or more.

  Conventionally, various studies have been conducted on methods for improving strength and toughness with the aim of a steel plate having high strength and high toughness. In order to improve both strength and toughness, it is known that refinement of crystal grains is effective. One of the methods for refining crystal grains is industrial heat treatment (TMCP: Thermo-Mechanical Control Process). For example, in a structure mainly composed of ferrite, refinement of crystal grains up to an average grain size of about 5 μm is achieved relatively easily by applying TMCP.

Recently, a method for further refinement of crystal grains has been pursued for the purpose of further improving strength and toughness. Various methods have been proposed in which the ferrite crystal grain size can be reduced to about 1 μm. When these are roughly divided,
(1) A method utilizing the transformation from austenite (γ) to ferrite (α),
(2) The method can be classified into methods using recrystallization of ferrite (α).

  The method of using the transformation from γ to α in (1) is a fine ferrite of around 2 μm or less after the γ → α transformation by applying large rolling in the temperature range from the metastable γ region to the (γ + α) two-phase region. This is a method for obtaining a structure composed of grains and second phase grains. On the other hand, in the method (2) using α recrystallization (hereinafter also referred to as “ferrite continuous recrystallization method”), the processing is performed in the α temperature range, and α continuous recrystallization is used. It is a method of trying to obtain the structure | tissue which consists of this fine ferrite grain. In this ferrite continuous recrystallization method, since the cumulative reduction effect can be used, small reduction multi-pass rolling becomes possible. Further, since the transformation is not used, there is an advantage that the structure is not uneven due to cooling. Small-pass multipass rolling is considered to be most suitable for stable production of thick steel plates.

  In this continuous recrystallization of ferrite, unlike the conventional discontinuous recrystallization by nucleation / growth of new grains, the subgrains surrounded by the low-angle grain boundaries caused by the recovery become larger with increasing strain, This is due to the mechanism of grains surrounded by large-angle grain boundaries. For this reason, the ferrite grain size is uniquely determined by the Zener-Hollomon parameter, which is a function of temperature and strain rate, and the continuous recrystallization is promoted as the strain is applied, and the region where ultrafine grains are formed increases. Become.

  Many proposals have already been made for a method for producing a steel material in which crystal grains are refined using such a continuous ferrite recrystallization method. For example, in Patent Document 1, a steel slab containing C: 0.03-0.45%, Si: 0.01-0.50%, Mn: 0.02-5.0%, Al: 0.001-0.1% is cooled to room temperature after casting and then reheated. Or after pre-casting without cooling after casting or without preliminary hot-working, after cooling to a temperature range from 600 ° C to room temperature, then 700-500 ° C Heating to a temperature range of 1 and processing for 2 passes or more with the time between passes within 20 s within that temperature range is performed under conditions of strain rate 0.1-20 / s and total strain amount 0.8-5, and allowed to cool. A method for producing high-tensile steel having excellent toughness has been proposed. According to the technique described in Patent Document 1, the crystal grains can be refined to 1 μm or less by dynamic recrystallization of ferrite, and a high-tensile steel material excellent in toughness can be provided at low cost.

  Patent Document 2 discloses that steel containing C: 0.05 to 0.30 mass% is rolled with a cumulative strain of 75% or more and a final 10% or more at a temperature of 650 ° C. or less, and a ferrite having a grain size of 2.5 μm or less. There has been proposed a manufacturing method of high-strength ultra-fine structure steel having a structure made of granular carbides and having a ratio of the volume ratio (%) to the diameter (μm) of the granular carbides of 8 or more. According to the technique described in Patent Document 2, it has high strength of 650 MPa or more in tensile strength, and can suppress a decrease in uniform elongation unique to ultrafine particles, and can provide strength / uniform elongation balance, strength / toughness. It is said that a steel plate with excellent balance can be obtained.

Further, in Patent Document 3, at least two of the cumulative rolling strains ε _T , ε _W , ε _L in the plate thickness direction, the plate width direction, and the plate longitudinal direction in the temperature range of 350 to 750 ° C. There has been proposed a method for producing a thick steel plate having an ultrafine grain structure in which multi-directional rolling warm multi-pass rolling is performed in which the total cumulative rolling strain ε _T + ε _W + ε _L is 1.8 or more. According to the technique described in Patent Document 3, continuous recrystallization of ferrite is promoted by performing warm rolling in multiple directions, and a grain size of 1 μm surrounded by a large angle grain boundary without adding an alloy element. A thick steel plate having the following ultrafine grains can be manufactured, and a thick steel plate having high strength and high toughness is obtained.

Non-Patent Document 1 describes the characteristics of an ultrafine grained steel sheet manufactured using a ferrite continuous recrystallization method, and the ductile-brittle transition temperature (vTrs) is notable in the ultrafine grained steel sheet. It has been shown to be cold.
In addition, a method has been proposed in which ultra-fine grains are ensured and high toughness is obtained by using two-phase rolling in which the rolling temperature is slightly increased from warm rolling. For example, Patent Document 4 contains an appropriate amount of C, Si, Mn, P, S, Al, and N, and the structure is martensite or bainite single phase or a mixed structure thereof, or the ferrite ratio is less than 20%. , Ferrite and martensite, bainite, pearlite, or a mixed structure, or a ferrite ratio of 20% or more and an average grain size of ferrite of 20 μm or less. A ferrite single-phase structure or ferrite and martens A steel having a mixed structure composed of one or more of sites, bainite and pearlite is heated to a temperature of Ac ₃ transformation point to (Ac ₃ transformation point −100 ° C.), and the temperature of the steel during rolling In the range of (Ac ₁ transformation point −50 ° C.) to (Ac ₃ transformation point −50 ° C.) and rolling with a cumulative rolling reduction of 50 to 90%, a method for producing high strength steel excellent in toughness and fatigue strength Is proposed There. The steel sheet manufactured by the technique described in Patent Document 4 has an ultrafine grain structure with an average ferrite grain size of about 1 to 3 μm, a tensile strength of 490 MPa or more, excellent toughness, and further brittleness. It is said to have crack propagation stop properties and at the same time good weld joint fatigue properties.

On the other hand, steel materials used in structures such as ships, offshore structures, bridges, buildings and tanks have excellent mechanical properties such as strength and toughness and weldability, as well as ensuring structural safety. Therefore, it is required to have excellent fatigue resistance. The fatigue characteristics are evaluated by fatigue crack generation characteristics and fatigue crack propagation characteristics.
In the case of a welded structure, the weld toe portion tends to be a stress concentration portion, and a tensile residual stress due to welding also acts and often becomes a source of fatigue cracks. As measures to prevent this, it is known to improve the shape of the toe portion by tanning welding or the like to alleviate stress concentration, or to introduce compressive residual stress by shot peening. However, there are a large number of weld toes in the welded structure, and improving the shape and residual stress state for each toe requires a lot of labor and time, leading to an increase in construction costs. These methods are unsuitable for implementation on an industrial scale. Therefore, improvement of the fatigue resistance characteristics of the welded structure is often achieved by improving the fatigue crack propagation characteristics of the steel material used.

For example, Patent Document 5 includes C: 0.015 to 0.20%, Si: 0.40 to 2.0%, Mn: 0.1 to 2.0%, P and S are adjusted to appropriate amounts, and the balance is Fe and unavoidable impurities. Using micro-segregation, ferrite area ratio: 80% or more, thickness: 30-100μm layer and martensite area ratio: 20% or more, thickness: 10-30μm layer in the plate thickness direction A thick steel plate having excellent fatigue crack propagation characteristics in the thickness direction, which is alternately arranged in layers, is described.
Japanese Patent No. 3383148 Japanese Patent Laid-Open No. 2001-240935 JP 2003-253332 A JP 2002-363644 A JP-A-8-73980 Iron and steel, vol.89 (2003), No.7, p.765

The toughness of steel is evaluated not only by the ductile-brittle transition temperature in the Charpy impact test but also by the absorbed energy in the Charpy impact test. In fact, in steel sheets for welded structures used in the shipbuilding and construction fields, the absorbed energy value at a predetermined temperature in the Charpy impact test is usually specified as a standard value.
When steel manufactured by the technique described in Patent Literatures 1 to 3 using the continuous recrystallization method of ferrite is evaluated at the ductile-brittle transition temperature, the ductile-brittle transition temperature in the Charpy impact test becomes significantly low. Can be said to have excellent toughness. However, as shown in Non-Patent Document 1, in the ultrafine-grained steel plate manufactured using the ferrite continuous recrystallization method, the ductile-brittle transition temperature in the Charpy impact test is remarkably low, The absorbed energy shows only a very low value of about 100 J at room temperature. When the toughness is evaluated by the absorbed energy value, it is difficult to say that the ultrafine-grained steel sheet manufactured using the ferrite continuous recrystallization method has excellent toughness.

  The reason why the ultrafine-grained steel manufactured using the ferrite continuous recrystallization method has a very low value of about 100 J at room temperature is not clear at present. However, (1) Separation occurs when the (100) plane, which is the cleavage plane of the BCC metal, is aligned in parallel with the plate surface, or (2) The continuous recrystallization of ferrite is partial and brittle, work hardening Possible reasons include the presence of ferrite. Non-Patent Document 1 shows that by performing multidirectional reduction, the accumulation of texture is relaxed, continuous recrystallization of ferrite is promoted, the occurrence of separation is reduced, and the absorbed energy is increased. Yes. However, there is a problem that it is difficult to adopt multidirectional reduction in a normal plate rolling process.

  In addition, the technique described in Patent Document 4 is a technique described in Patent Document 4, although the cumulative reduction amount is relatively small at 50% or more and is not a technique that intentionally uses the continuous recrystallization method of ferrite. The manufactured steel has high strength with a tensile strength of 460 MPa or more, ultrafine grains with a ferrite grain size of 1 to 3 μm, and a low ductility-brittle transition temperature of −120 ° C. or less. However, the high absorption energy value was not satisfied, and there was a problem in toughness when evaluated by the absorption energy value.

  Moreover, in the technique described in Patent Document 5, in order to distribute martensite in a layered manner using micro segregation, it is necessary to perform natural cooling to a specific temperature range in a two-phase region after rolling and then perform rapid cooling. There is. For this reason, even with natural cooling, it is difficult to achieve a uniform temperature in the thickness direction of the steel sheet during cooling, and the distribution of martensite is not uniform in the thickness direction due to non-uniform temperature distribution at the start of rapid cooling. It could be uniform and left a problem.

The present invention solves the above-mentioned problems of the prior art, exhibits high absorbed energy in the Charpy impact test, has excellent toughness, and further has excellent fatigue crack propagation stopping properties in the thickness direction, and such a steel plate. An object of the present invention is to provide a simple and stable production method. Here, “excellent toughness” means that the absorbed energy at a test temperature of 20 ° C. in a Charpy impact test is 180 J or more. Further, the term “excellent in fatigue crack propagation resistance in the thickness direction” as used herein means that the fatigue crack propagation rate in the thickness direction at ΔK = 15 MPa√m determined by a method in accordance with the provisions of ASTM E647 is 8.0. × 10- ⁹ m / cycle refers to the case of the following.

  In order to achieve the above-mentioned problems, the present inventors have earnestly studied the influence of the metal structure on the absorbed energy value in the Charpy impact test of the ultrafine-grained steel sheet obtained by using the ferrite continuous recrystallization method. Studied. The inventors first focused on the degree of promotion of continuous recrystallization of ferrite. And the Charpy absorbed energy value is remarkable by making the steel sheet structure into a structure in which a fine ferrite phase having an average grain size of 3 μm or less is contained in an area ratio of 30% or more with respect to the total amount of ferrite phase from the steel sheet surface layer to the central part. Found to increase.

  And it discovered newly that the above-mentioned steel plate structure was realizable by a very simple process industrially. That is, as a starting material, a steel slab having a coarse ferrite + pearlite structure is used, and the steel slab is heated within a specific temperature range of a two-phase temperature range to obtain a ferrite + austenite two-phase state with a high ferrite area ratio. The present inventors have found that a small reduction multi-pass rolling with a cumulative reduction ratio of 80% or more is performed to induce continuous recrystallization of ferrite, followed by air cooling.

  According to the study by the present inventors, in the above-described process, austenite having higher deformation resistance than ferrite is mixed in the structure of the steel slab. Therefore, compared with the case where austenite does not exist, strain concentrates more efficiently on the ferrite during rolling, and the continuous recrystallization of ferrite is significantly promoted. As a result, it is considered that the continuous recrystallization of ferrite is promoted even in the central portion of the plate thickness where distortion is difficult to enter, and in the central portion of the plate thickness, the fine ferrite phase is 30% or more in terms of the area ratio with respect to the total amount of the ferrite phase.

  Further, in the above-described process, the inventors of the present invention, in addition to the above-described ferrite phase, a stretched pearlite phase that is the second phase is a layered layer having an average minor axis diameter of 5 μm or less and extending parallel to the rolling surface. It was found that a pearlite phase can be obtained. As a result, it has been found that the progress of fatigue cracks in the plate thickness direction can be suppressed, and the fatigue crack propagation characteristics in the plate thickness direction can be remarkably improved.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) A high-strength thick steel plate having a structure in which a ferrite phase is a main phase and a second phase is a pearlite phase, and the ferrite phase is a fine ferrite phase having an average particle size of 3 μm or less. A high-tensile thick steel plate excellent in toughness and fatigue crack propagation resistance in the plate thickness direction, characterized in that the average minor axis diameter of the pearlite phase is 5 μm or less.

(2) In (1), the high-tensile steel plate is in mass%, C: 0.03-0.3%, Si: 0.03-1.5%, Mn: 0.1-3%, Al: 0.1% or less, N: 0.01% A high-tensile thick steel plate comprising: a balance of Fe and inevitable impurities.
(3) In (2), in addition to the above composition, in addition to mass, one or two selected from Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, Ti: 0.001 to 0.1% A high-tensile thick steel plate characterized by having a composition containing the above.

(4) In (2) or (3), in addition to the above composition, Cu: 0.01-3%, Ni: 0.01-3%, Cr: 0.01-3%, Mo: 0.01-1% A high-strength thick steel plate having a composition containing one or more selected from among the above.
(5) By mass%, C: 0.03-0.3%, Si: 0.03-1.5%, Mn: 0.1-3%, Al: 0.1% or less, N: 0.01% or less, the balance being Fe and inevitable impurities And a steel material having a structure composed of a ferrite phase and a pearlite phase having an average particle size of 40 μm or more at a temperature range of (Ac ₁ transformation point + 30 ° C.) to (Ac ₁ transformation point + 100 ° C.). After heating, the steel material is subjected to small rolling multi-pass rolling with a cumulative rolling reduction of 80% or more and a rolling finish temperature of 550 ° C. or more, and then toughness and fatigue resistance in the thickness direction characterized by air cooling A method for producing high-tensile thick steel plates with excellent crack propagation characteristics.

(6) In (5), the steel material is selected from Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, and Ti: 0.001 to 0.1% in mass% in addition to the composition. A method for producing a high-tensile thick steel plate having a composition containing one or more types.
(7) In (5) or (6), in addition to the above composition, the steel material further includes, in mass%, Cu: 0.01 to 3%, Ni: 0.01 to 3%, Cr: 0.01 to 3%, Mo : A method for producing a high-strength thick steel plate having a composition containing one or more selected from 0.01 to 1%.

  According to the present invention, in a Charpy impact test, a high-strength thick steel plate that exhibits a high absorbed energy value, excellent toughness, and excellent fatigue crack propagation characteristics in the thickness direction can be industrially easily and stably produced. It becomes possible to manufacture, and there is a remarkable effect in the industry.

First, the reasons for limiting the composition of the steel plate of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.03-0.3%
C is an element having an action of promoting continuous recrystallization of ferrite through formation of cementite, and in order to obtain such an effect, the content of 0.03% or more is required. On the other hand, if the content exceeds 0.3%, the weldability decreases. For this reason, C was limited to the range of 0.03-0.3%. In addition, Preferably it is 0.05 to 0.15%.

Si: 0.03-1.5%
Si is an effective element that acts as a deoxidizer and increases the strength of steel by solid solution strengthening. Such an effect is recognized when the content is 0.03% or more. On the other hand, if the content exceeds 1.5%, the surface properties are impaired and the toughness is extremely lowered. For this reason, Si was limited to the range of 0.03-1.5%. In addition, Preferably it is 0.03-0.5%.

Mn: 0.1-3%
Mn acts as a strengthening element in steel. Such an effect is recognized when the content is 0.1% or more. On the other hand, a large content exceeding 3% lowers the weldability and causes an increase in material cost. For this reason, Mn was limited to the range of 0.1 to 3%. In addition, Preferably it is 0.1 to 1.5%.

Al: 0.1% or less
Al is an element that acts as a deoxidizer, but in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, the content exceeding 0.1% increases the amount of inclusions and also reduces toughness. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.06% or less.

N: 0.01% or less N is an element that combines with Al in steel to form AlN and contributes to strengthening of the steel through refinement of crystal grains during rolling. To obtain such an effect Is preferably contained in an amount of 0.002% or more. On the other hand, the content exceeding 0.01% lowers the toughness. For this reason, N was limited to 0.01% or less. In addition, Preferably, it is 0.002 to 0.005%.

The above-mentioned components are basic components. In the present invention, in addition to the basic components, Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, Ti: 0.001 to 0.1%, or one or two selected One or more selected from Cu: 0.01 to 3%, Ni: 0.01 to 3%, Cr: 0.01 to 3%, Mo: 0.01 to 1% be able to.
One or more selected from Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, Ti: 0.001 to 0.1%
Nb, V, and Ti are all elements that have the effect of forming nitrides, carbides, or carbonitrides, refining crystal grains, and strengthening steel. It can contain more than seeds. In order to obtain such an effect, it is desirable to contain Nb, V, and Ti in an amount of 0.001% or more. On the other hand, when it contains more than Nb: 0.05%, V: 0.1%, Ti: 0.1%, a slab will be cracked and the manufacturing cost will rise. For this reason, it is preferable to limit to the ranges of Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, and Ti: 0.001 to 0.1%, respectively.

One or more selected from Cu: 0.01-3%, Ni: 0.01-3%, Cr: 0.01-3%, Mo: 0.01-1%
Cu, Ni, Cr, and Mo are all elements that increase the hardenability of steel and contribute directly to strength improvement, as well as toughness, high-temperature strength, and weather resistance. It can contain seeds or two or more. Such an effect becomes remarkable when Cu, Ni, Cr, and Mo are each contained in an amount of 0.01% or more, but Cu: 3%, Ni: 3%, Cr: 3%, and Mo: 1% are excessive. Inclusion reduces toughness and weldability. For this reason, it is preferable to limit Cu to 0.01 to 3%, Ni to 0.01 to 3%, Cr to 0.01 to 3%, and Mo to 0.01 to 1%, respectively.

The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, P: 0.04% or less and S: 0.02% or less are acceptable. The content exceeding P: 0.04% and S: 0.02% respectively reduces toughness.
As long as the effects of the present invention are not impaired, elements such as B, REM, Zr, Ca, and Mg may be contained in a trace amount (about 0.01% or less) in addition to the above-described components.

Further, the thick steel plate of the present invention preferably has the above-described composition, and further has a structure that is a pearlite phase in which the ferrite phase is the main phase and the second phase is extended throughout the entire plate thickness direction. The ferrite phase as the main phase is preferably 70% or more in terms of the area ratio relative to the total amount of the structure.
In the thick steel plate of the present invention, the ferrite phase as the main phase is a phase containing 30% or more of a fine ferrite phase having an average particle size of 3 μm or less over the entire area in the plate thickness direction with respect to the total amount of the ferrite phase. By setting the average particle size of the fine ferrite phase to 3 μm or less, the ductile-brittle transition temperature can be remarkably lowered. When the average grain size of the fine ferrite phase exceeds 3 μm, it becomes difficult to secure desired strength and toughness that are basic characteristics of the thick steel plate. In addition, if the fine ferrite phase is less than 30% in terms of the area ratio with respect to the total amount of ferrite phase, it becomes difficult to ensure the desired strength and toughness, which are the basic characteristics of thick steel plates, and in particular, expanded ferrite, that is, work-hardened ferrite As a result, the absorbed energy value in the Charpy impact test decreases.

In addition, the thick steel plate of the present invention has a structure in which the second phase other than the ferrite phase as the main phase has a pearlite phase extending parallel to the rolling surface. The pearlite phase as the second phase is preferably 30% or less in terms of the area ratio relative to the total amount of the structure from the viewpoint of improving toughness. More preferably, it is 20% or less.
In addition to making the ferrite phase that is the main phase the fine ferrite phase described above, in the thick steel plate of the present invention, the pearlite phase that extends in parallel with the rolling surface that is the second phase has an average diameter in the minor axis direction ( The pearlite phase has an average minor axis diameter of 5 μm or less. As the second phase, by making the structure in which the pearlite phase extending parallel to the rolling surface with an average minor axis diameter of 5 μm or less is dispersed, the propagation of fatigue cracks in the thickness direction is suppressed, and the fatigue crack propagation characteristics Will improve. In addition, it is not necessary to specifically limit the diameter in the extended direction (major axis direction). When the average minor axis diameter of the pearlite phase exceeds 5 μm, the reduction in the fatigue crack propagation rate in the thickness direction is reduced, and a remarkable improvement in the fatigue crack propagation resistance in the thickness direction is not recognized. For this reason, the average minor axis diameter of the extended pearlite phase, which is the second phase, is limited to 5 μm or less.

Below, the preferable manufacturing method of this invention high-tensile thick steel plate which has an above-described composition and structure | tissue is demonstrated.
The steel material used in the present invention is a steel material having the above-described composition and further having a coarse ferrite + pearlite structure. The reasons for limiting the structure of the steel material used in the present invention are as follows.

  When the structure of the steel material before rolling (the structure before rolling) is a coarse ferrite + pearlite structure, when the steel material is heated to a two-phase temperature range during rolling, the pearlite region is reversely transformed to become coarse austenite to some extent. Due to the influence of this coarse austenite, if the multi-pass rolling under small pressure in the two-phase temperature range is subsequently performed, the strain due to rolling (rolling strain) can be effectively distributed to the ferrite grains, and the continuous recrystallization of ferrite It is promoted and a structure having a ferrite phase containing an ultrafine ferrite phase having an average particle diameter of 3 μm or less as a main phase can be formed.

  This is remarkable when the steel material has a coarse ferrite + pearlite structure having an average particle diameter of 40 μm or more. When the ferrite grain size of the steel material is 40 μm or more, the size of the pearlite also becomes coarse accordingly, and reversely transforms to some coarse austenite during heating in the two-phase temperature range. In ferrite + pearlite structure with an average particle size of less than 40 μm, continuous recrystallization of the ferrite as described above cannot be promoted even if the two-phase temperature rolling is performed thereafter, and ferrite containing a large amount of fine ferrite phase A structure whose main phase is a phase cannot be formed. For this reason, the average grain size of ferrite in the structure before rolling of the steel material was limited to 40 μm or more.

  On the other hand, when the structure before rolling of the steel material is a structure mainly composed of bainite or martensite, when the steel material is heated to a two-phase temperature range, fine austenite precipitates with finely dispersed cementite as nucleation sites. At the same time, bainite and martensite are tempered. When rolling in a two-phase temperature range is applied to a structure containing such fine austenite, the rolling strain cannot be effectively distributed to the ferrite phase, and the continuous recrystallization of ferrite is not promoted. No ferrite phase can be formed.

Thus, when the structure of the steel material before rolling (the structure before rolling) is not a coarse ferrite + pearlite structure, a ferrite phase containing a large amount of fine ferrite phase even if it is subsequently subjected to two-phase temperature rolling. It is difficult to obtain a structure whose main phase is.
The manufacturing method of the steel material having the above composition and structure is not particularly limited, and any known method can be applied. It is preferable that the molten steel having the above composition is melted by a normal melting method to obtain a steel material (slab) having a predetermined size and shape by a normal casting method. In order to secure the above-described structure, it is preferable to perform casting in an austenite recrystallization region as cast or after casting, and then air cooling. As a result, a coarse structure with a ferrite + pearlite structure and ferrite grains of 40 μm or more can be obtained.

Next, the steel material having the above composition and structure is heated to a temperature in the temperature range of (Ac ₁ transformation point + 30 ° C.) to (Ac ₁ transformation point + 100 ° C.). The steel material is heated from a temperature below the Ac ₁ transformation point, and the heating rate is not particularly limited. When the heating temperature is less than (Ac ₁ transformation point + 30 ° C.), the area ratio of austenite formed during heating is as low as 5% or less, and strain cannot be effectively introduced into the ferrite by subsequent rolling. Continuous recrystallization is not promoted and a fine ferrite phase cannot be formed. On the other hand, when the heating temperature exceeds (Ac ₁ transformation point + 100 ° C) and becomes high, the area ratio of austenite formed during heating exceeds 50%, and the amount of pearlite generated after cooling increases, conversely toughness Incurs a decline. For this reason, the heating temperature of the steel material was limited to a temperature in the temperature range of (Ac ₁ transformation point + 30 ° C.) to (Ac ₁ transformation point + 100 ° C.).

Next, the steel material heated under the above-described conditions is subjected to small reduction multi-pass rolling with a cumulative reduction rate of 80% or more and a rolling end temperature of 550 ° C. or more to obtain a thick steel plate.
By subjecting the steel material heated under the above conditions to multi-pass rolling with a small rolling reduction of 80% or more, continuous recrystallization of ferrite occurs, and the average grain size of the ferrite crystal grains is 3 μm or less. Miniaturization can be achieved over the entire thickness direction. Thereby, the toughening of a thick steel plate becomes possible. Cumulative rolling reduction: If less than 80%, continuous recrystallization of ferrite is not sufficiently promoted, and the fine ferrite phase cannot be increased to 30% or more in terms of the area ratio with respect to the total amount of ferrite phase, improving toughness, especially high absorbed energy value Cannot be secured.

  Even if the small-pass multipass rolling is started at a temperature in the two-phase temperature range, the rolling temperature decreases due to heat removal from the steel plate during the multipass rolling process. Since the grain size of the continuously recrystallized ferrite becomes finer as the rolling temperature becomes lower, from the viewpoint of improving strength and toughness, even if the rolling temperature is lowered, it does not become a big problem. However, when the rolling temperature falls below 550 ° C, the load on the plate manufacturing equipment increases and the continuous recrystallization of ferrite is less likely to occur, increasing the area of ferrite that is simply expanded by processing. Therefore, it becomes difficult to improve toughness, particularly to secure a high absorbed energy value. For this reason, the rolling end temperature is limited to a temperature of 550 ° C. or higher.

The “small rolling multi-pass rolling” here refers to rolling in which a rolling pass having an average rolling reduction of 5 to 15% is performed a plurality of times to obtain a predetermined cumulative rolling reduction. If the rolling reduction per pass is less than 5%, the heat removal during rolling increases, and continuous recrystallization of ferrite is difficult to occur. Moreover, if the rolling reduction per pass exceeds 15%, the rolling load becomes excessive.
It should be noted that the strain rate in each rolling pass is not particularly limited, and it is sufficient if it is about 5 to 30 / s, which is a strain rate in a normal thick plate manufacturing facility. In order to cause continuous recrystallization of ferrite, it is preferable to set the time between the rolling passes in the above temperature range to 10 s or less.

  After finishing the above-described multi-pass rolling, the thick steel plate is air-cooled. Thereby, the structure of a thick steel plate can be made into the structure | tissue which is the pearlite phase which made the ferrite phase containing fine ferrite the main phase and extended in parallel with the rolling surface over the plate | board thickness direction whole region. Furthermore, if rolling is performed under the above-described conditions, the ferrite phase becomes a fine ferrite having an average particle diameter of 3 μm or less, and the pearlite phase, which is the second phase, extends in parallel with the rolling surface, and the elongation direction ( The diameter in the minor axis direction (minor axis diameter) orthogonal to the major axis direction can be 5 μm or less on average.

  Molten steel having the composition shown in Table 1 was melted and cast into an ingot (150 kg). These ingots were then rolled into a steel material (plate thickness 120 mm). Table 2 shows the structure of the obtained steel material. In the production of the steel material, the rolling condition was controlled, the austenite grain size was adjusted, and the ferrite average grain size after γ → α transformation was adjusted in the range of 20-100 μm. In addition, in some steel materials, it was water-cooled after the partial rolling, and the structure was a bainite structure.

  Next, the steel material was heated to the heating temperature shown in Table 2, and then subjected to multi-pass rolling under small conditions under the conditions shown in Table 2 using experimental rolling equipment, air-cooled after the end of rolling, and the plate thickness shown in Table 2 The thick steel plate. As a reference, the area ratio of austenite at the time of heating in small-pass multipass rolling is also shown in Table 2. The austenite area ratio at the time of heating was determined by preparing the same steel material separately from that for rolling, and cooling the steel material from the heated state and observing the assemblage.

In addition, the average rolling reduction per pass of the multi-roll rolling under small reduction was 11%, the number of rolling passes was 8 to 20 passes, the time between passes was about 8 s, and the strain rate was about 10 / s.
The transformation point (Ac ₁ , Ac ₃ ) of each steel material was calculated from the following equation.
Ac ₁ (° C.) = 751−26.6C + 17.6Si−11.6Mn−22.9Cu−23Ni + 24.1Cr + 22.5Mo−39.7V−5.7Ti + 233Nb−169sol.Al−895B
Ac ₃ (° C.) = 937−476.5C + 56Si−19.7Mn−16.3Cu−26.6Ni−4.9Cr + 38.1Mo
+ 124.8V + 136.3Ti-19Nb + 198 sol.Al + 3315B
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B: Element content (mass%))
The obtained thick steel plate was subjected to a structure observation, a tensile test, an impact test, and a fatigue crack propagation test to evaluate the structure, strength, toughness, and fatigue crack propagation resistance in the thickness direction.

  For structure observation, a specimen for structure observation is collected from the obtained thick steel plate, and the region from the surface in the rolling direction to the center of the plate thickness is imaged using a scanning electron microscope (magnification: 2000 times), Using an image analyzer, the average crystal grain size and area ratio of the ferrite phase, the area ratio of the fine ferrite phase having an average grain diameter of 3 μm or less to the total amount of the ferrite phase, and the type and area of the second phase The rate was measured. When the second phase was a pearlite phase, the minor axis diameter was measured and the average minor axis diameter was calculated. The average crystal grain size of the ferrite phase is determined by measuring the area of each ferrite grain using an image analysis device, calculating the equivalent circle diameter from the area as the grain diameter of the crystal grain, and calculating the average value of the steel sheet. The average grain size of the ferrite phase was used. In addition, the average minor axis diameter of the pearlite phase is determined by measuring the minor axis diameter of each pearlite phase using an image analyzer, arithmetically averaging the minor axis diameter of each pearlite phase, and calculating the average value of the average value of the steel sheet. The short axis diameter was used. Note that the number of fields to be measured was three or more.

In the tensile test, a JIS 14A test piece (6 mmφ: diameter of the parallel part) was sampled so that the tensile direction was the rolling direction (sheet length direction) from the center of the thickness of the obtained thick steel plate. A tensile test was carried out in accordance with the regulations, and yield strength YS and tensile strength TS were determined.
Impact test in the rolling direction from the center of plate thickness of the obtained steel plate, taken V-notch test piece, Charpy impact test was performed in conformity with the provisions of JIS Z 2242, it absorbed energy vE ₂₀ at 20 ° C. (J) and the fracture surface transition temperature vTrs (° C.) were determined. The absorbed energy vE ₂₀ (J) at 20 ° C. was the average value of three test pieces.

  In the fatigue crack propagation test, specimens with the dimensions shown in Fig. 1 were collected from the obtained thick steel plate, and subjected to a fatigue crack propagation test in accordance with ASTM E647 in the room temperature atmosphere. The fatigue crack propagation rate was measured. In addition, the fatigue precrack was introduce | transduced into the test piece beforehand. The test conditions were a frequency of 30 Hz, a stress ratio of 0.1, and a fatigue crack propagation rate (m / cycle) when the stress intensity factor range ΔK was ΔK = 15 MPa√m.

  The obtained results are shown in Table 3.

All examples of the present invention have a target tensile strength: 500 MPa or more and a target toughness of ₂₀ JC Charpy impact test absorbed energy vE20 of 180 J or more, and a high tensile thickness with excellent toughness. It is a steel plate. Note that the fracture surface transition temperature vTrs of each of the inventive examples was lower than −100 ° C. On the other hand, in comparative examples that are out of the scope of the present invention, the strength is insufficient, the toughness is insufficient, or both the strength and the toughness are lowered.

Further, examples present invention both includes a fatigue crack growth rate in the thickness direction 8.0 × 10- ⁹ m / cycle or less, and has an excellent thickness direction of the fatigue crack propagation characteristics. Meanwhile, in the comparative example outside the scope of the present invention, the fatigue crack growth rate in the thickness direction is greater than the 8.0 × 10- ⁹ m / cycle, fatigue crack propagation properties in the thickness direction is reduced.
In the comparative examples (steel plates No. 1 and No. 2) in which the heating temperature of the steel material is outside the range of the present invention, the area ratio of austenite at the start of rolling is as low as 0% and 4%, so the cumulative reduction rate is 90%. Despite being subjected to multi-pass rolling, the continuous recrystallization of ferrite is not sufficiently induced, the area ratio of the fine ferrite phase in the center is less than 30%, and the absorbed energy value vE ₂₀ decreases. further the thickness direction of the fatigue crack growth rate exceeds 8.0 × 10- ⁹ m / cycle. Moreover, in the comparative example (steel plate No. 5) in which the heating temperature of the steel material is out of the range of the present invention, the austenite area ratio at the start of rolling is as high as 65%, the ferrite phase transformed from austenite increases, the area ratio of the ferrite phase is decreased to less than 30% in the surface layer portion and the central portion, reduces the absorbed energy value vE _20, further fatigue crack growth rate in the thickness direction exceeds the 8.0 × 10- ⁹ m / cycle Yes.

Moreover, in the comparative example (steel plate No. 6) in which the average grain size of the ferrite phase of the steel material is out of the range of the present invention, the austenite grain size in the heated state before rolling is out of the proper range, which is effective for ferrite. since the strain could not be concentrated in a continuous recrystallization ferrite is not promoted, reduces the absorbed energy value vE _20, further plate thickness direction of the fatigue crack growth rate exceeded 8.0 × 10- ⁹ m / cycle ing.

In the comparative example (steel plate No. 8) in which the structure of the steel material deviated from the scope of the present invention, the deformation resistance during rolling was high, so the strain was more likely to be biased in the surface layer portion, and in the surface layer portion, the long-term recrystallization of ferrite was Although promoted, continuous recrystallization of ferrite is not promoted at the center of the plate thickness, the area ratio of the fine ferrite phase decreases, the absorbed energy value vE ₂₀ decreases, and the fatigue crack propagation rate in the plate thickness direction also increases. It is greater than the 8.0 × 10- ⁹ m / cycle.

Moreover, in the comparative examples (steel plates No. 9 and No. 10) in which the cumulative rolling reduction in small rolling multipass rolling is out of the range of the present invention, continuous recrystallization of ferrite is sufficiently promoted by small rolling multipass rolling. not less fine ferrite area ratio, reduces the absorbed energy value vE _20, further the thickness direction of the fatigue crack growth rate exceeds 8.0 × 10- ⁹ m / cycle.
Further, in the comparative example (steel plate No. 12) in which the rolling end temperature of the multi-pass rolling under the small reduction is out of the range of the present invention, the fine ferrite area ratio is decreased and the elongated ferrite phase is increased. ₂₀ is lowered further the thickness direction of the fatigue crack growth rate exceeds 8.0 × 10- ⁹ m / cycle.

Moreover, in the comparative examples (steel plates No. 17, No. 18, No. 19) in which the heating temperature of the steel material is outside the scope of the present invention, the area ratio of austenite formed during heating exceeds 50%, and during cooling The amount of pearlite generated is increased, continuous recrystallization of ferrite is difficult to induce, the fine ferrite area ratio is low, the absorbed energy value vE ₂₀ is lowered, and the minor axis diameter of the pearlite phase is larger than 5 μm. become, fatigue crack growth rate in the thickness direction is greater than 8.0 × 10- ⁹ m / cycle.

It is explanatory drawing which shows the outline of the test piece dimension shape of the fatigue crack propagation test used in the Example.

Claims (7)

  A high-tensile thick steel plate having a structure in which a ferrite phase is a main phase and a second phase is a pearlite phase, and the ferrite phase has a fine ferrite phase with an average particle size of 3 μm or less as an area ratio with respect to the total amount of the ferrite phase. A high-tensile steel plate excellent in toughness and fatigue crack propagation resistance in the plate thickness direction, wherein the average minor axis diameter of the pearlite phase is 5 µm or less. The high-tensile thick steel plate is in mass%,
C: 0.03-0.3%, Si: 0.03-1.5%,
Mn: 0.1 to 3%, Al: 0.1% or less,
The high-tensile thick steel plate according to claim 1, wherein N: 0.01% or less, and the balance is composed of Fe and inevitable impurities.   In addition to the above composition, the composition further contains one or more selected from Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, and Ti: 0.001 to 0.1% by mass%. The high-tensile thick steel plate according to claim 2, wherein   In addition to the above-mentioned composition, one or more selected from Cu: 0.01-3%, Ni: 0.01-3%, Cr: 0.01-3%, Mo: 0.01-1% in mass% The high-tensile thick steel plate according to claim 2 or 3, characterized in that the composition contains % By mass
C: 0.03-0.3%, Si: 0.03-1.5%,
Mn: 0.1 to 3%, Al: 0.1% or less,
N: A steel material containing 0.01% or less, the balance being Fe and inevitable impurities, and a structure consisting of a ferrite phase and a pearlite phase with an average particle size of 40 μm or more, (Ac ₁ transformation point + 30 ° C.) ) To (Ac ₁ transformation point + 100 ° C.) After heating to a temperature in the temperature range, the steel material is subjected to small reduction multi-pass rolling with a cumulative reduction ratio of 80% or more and a rolling end temperature of 550 ° C. or more. Next, a method for producing a high-tensile steel plate excellent in toughness and fatigue crack propagation resistance in the thickness direction, characterized by air cooling.   In addition to the above composition, the steel material further contains one or more selected from the group consisting of Nb: 0.001 to 0.05%, V: 0.001 to 0.1%, and Ti: 0.001 to 0.1%. The method for producing a high-tensile thick steel plate according to claim 5, wherein:   The steel material is 1% selected from Cu: 0.01 to 3%, Ni: 0.01 to 3%, Cr: 0.01 to 3%, and Mo: 0.01 to 1%, in addition to the above composition, in mass%. It has the composition containing a seed | species or 2 or more types, The manufacturing method of the high-tensile thick steel plate of Claim 5 or 6 characterized by the above-mentioned.

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