JP5375933B2 - Thick steel plate with excellent brittle crack propagation stop properties - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thick steel plate which is excellent in characteristic of brittle crack propagation arrest. <P>SOLUTION: The thick steel plate has a microstrutcure which includes a ferrite phase as a main phase and one or more kinds of pearlite phase, bainitic phase and martensitic phase as a second phase. The ferrite phase with an average particle diameter of 3 &mu;m or less is formed in at least an area of 10-20% in plate thickness from the front and rear surfaces in the direction of the thick steel plate, and its area ratio to the total quantity of ferrite in the area is 30% or more. Also, it is formed in an area of less than 10% in plate thickness from the front and rear surfaces in the direction of the thick steel plate, and its area ratio to the total quantity of ferrite in the area is less than 30%. Thus, the characteristic in brittle crack propagation arrest can be greatly improved in the thick steel plate. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a thick steel plate suitable for use in large structures such as ships, offshore structures, low-temperature storage tanks, line pipes, buildings, civil engineering structures, and the like, and more particularly to improvement of brittle crack propagation stopping characteristics of the thick steel plates. . The “thick steel plate” here refers to a steel plate having a thickness of 6 mm or more.

  Conventionally, large structures such as ships, offshore structures, low-temperature storage tanks, line pipes, buildings, and civil engineering structures have been strongly required to prevent the occurrence of brittle fracture from the viewpoint of safety. For this reason, it is requested | required that the steel materials used for a large structure should be excellent in low temperature toughness. In particular, even when a crack occurs in a structure due to an unexpected accident or the like, a brittle crack propagation stop characteristic, so-called arrest characteristic, may be required from the viewpoint of preventing breakage. For example, in the case of a ship, it has been demanded that the steel material to be used has more excellent brittle crack propagation stopping characteristics from the viewpoint of ensuring safety when the ship is reused after a collision accident.

  As a steel material with excellent low temperature toughness, 9% Ni steel with an increased Ni content compared to general steel materials has been known, and has already been used on a commercial scale for liquefied natural gas (LNG) tanks. ing. However, since a large amount of Ni causes a material cost increase, there is a problem that it is difficult to apply it except for cryogenic use such as for LNG storage tanks. For this reason, there has been a demand for a low-temperature toughness improvement measure that can improve low-temperature toughness without containing a large amount of alloy elements.

  As such a low temperature toughness improvement measure, there is a thermo-mechanical control process (TMCP) method in which controlled rolling and controlled cooling are combined. In this method, (1) recrystallization of austenite (γ) is repeated, and austenite is refined. (2) The cumulative rolling reduction in the non-recrystallization zone rolling of austenite is increased, and the expansion of austenite grains is increased. Introducing a large number of deformation bands and increasing the number of ferrite nucleation sites during the subsequent ferrite transformation to refine the ferrite (α). (3) γ / α conversion ratio by controlled cooling after rolling. To reduce the size of ferrite and introduce fine bainite. Steel materials to which this TMCP method is applied have been widely used for so-called cold districts that do not reach extremely low temperatures such as LNG.

However, this TMCP method can remarkably improve the brittle crack propagation stop property of thin steel materials with a plate thickness of about 40 mm or less. There is a problem that the degree is small.
In addition, the TMCP method is known in which a texture is developed by applying a reduction to ferrite transformed from austenite. According to this method, separation due to the texture occurs on the fracture surface of the steel material, and the stress at the brittle crack tip (notch tip) is relieved, so the resistance to the propagation of the brittle fracture crack is increased, and the steel material It is possible to improve the brittle crack propagation stop characteristics. However, with this method, when the steel plate becomes thicker, it becomes difficult to sufficiently exhibit such TMCP effect, and when excessive reduction is applied at a temperature below the Ar ₃ transformation point, the toughness is increased. There is a problem of deterioration.

For such a problem, for example, Patent Document 1 proposes a method for manufacturing a steel sheet having excellent brittle crack propagation stopping characteristics and low temperature toughness. The technique described in Patent Document 1 is such that the regions of the upper and lower surface layers corresponding to 2 to 33% of the thickness of the structural steel material including C: 0.01 to 0.30% are moved from the temperature above the Ar ₃ transformation point to 2 ° C. After cooling at a cooling rate of at least / s and cooling to below the Ar ₃ transformation point, the cooling is stopped and the process of reheating is performed at least once, and the finish is completed until the reheating after the last cooling is completed. subjected to rolling, less than the area of the upper and lower surface layer portion after the end of rolling finish Ac ₃ transformation point, Ac ₃ transformation point or more, the manufacturing method of the steel sheet for recuperation in any of Ac ₃ transformation point and the upper and lower temperature range . As a result, rolling is performed during temperature rise after one or more reverse transformations, and the structure of the steel sheet surface layer portion is refined by repeated transformation and processing recrystallization, so that the shearing effect of the steel sheet surface layer portion is increased and brittle. It is said that the crack propagation stop property is improved and the low temperature toughness at the center of the plate thickness is improved.

In Patent Document 2, a steel slab having a temperature not lower than the Ac ₃ transformation point is rapidly cooled to a temperature below the Ar ₃ transformation point at a cooling rate of 2 ° C./s or more in a surface layer region of 2% or more in the thickness direction from the surface. After that, the surface region starts rolling from a temperature equal to or higher than the Ar ₁ transformation point and finishes rolling until reaching the maximum recuperation temperature in the range of Ac ₃ transformation point to (Ac ₃ transformation point + 60 ° C.). A method for producing a structural steel sheet having excellent arrest performance that cools to the Ar ₁ transformation point at a cooling rate of 1 ° C./s or more has been proposed. According to the technique described in Patent Document 2, a surface layer structure in which a ferrite structure or a bainite structure having an average equivalent circle diameter of 3 μm or less occupies 30% or more from the front and back surfaces of the steel sheet to a range of at least 2% of the plate thickness. It is said that a structural steel plate having excellent arrest performance is obtained in which the texture colonies having the same crystal orientation of the surface layer structure have an aspect ratio of 4 or more.

In addition, Patent Document 3 discloses that at least 0.3 cumulative cumulative plastic strain is applied to the steel sheet surface layer at a temperature equal to or higher than the Ar ₃ transformation point before finishing rolling, and then 2 ° C./s or higher up to a temperature range of 650 ° C. or lower. Rolling is started immediately after cooling at a speed of, and finish rolling is performed while the surface layer is reheated to a temperature below the Ar ₃ transformation point by the internal latent heat and processing heat of the steel plate, and a method for producing a steel plate with excellent arrest properties Has been proposed. In the technique described in Patent Document 3, the finish rolling is rolling with a maximum reduction ratio per pass of 12% or less and a cumulative reduction ratio of 30% or more. Thereby, the local recrystallization phenomenon is suppressed and the particle size variation is suppressed. According to the technique described in Patent Document 3, the surface layer region at least 5% of the plate thickness from the surface has an equivalent circle average particle size of 5 μm or less, an aspect ratio of 2 or more, and a standard deviation of particle size variation of 3 μm or less. It is said that a steel sheet having a structure mainly composed of ferrite and pearlite and having excellent arrest properties can be obtained.

  In any of the techniques described in Patent Documents 1 to 3, after cooling only the steel plate surface layer part, the steel sheet is rolled while being reheated to improve the brittle crack propagation stopping property with the surface layer part being particularly refined. This is a technique for obtaining a suitable structure and improving the brittle crack propagation stopping characteristics of a steel sheet. However, in order to apply these technologies to the actual production scale, there is a problem that the temperature control of heating / cooling and recuperation is not easy. In addition, all of the techniques described in Patent Documents 1 to 3 refine the structure by utilizing the processing recrystallization of ferrite, but the ferrite formed by the processing recrystallization has a structure in which growth is likely to occur. In this case, there is a problem that the structure and characteristics are liable to be uneven due to slight fluctuations in thermal history.

In order to solve such a problem, for example, in Patent Document 4, in order to stably form a subgrain structure formed in ferrite crystal grains as well as refinement of ferrite crystal grains, (a) Ar ₃ points or more A step of temporarily suspending rolling at a temperature of (b) Ae ₃ point −air cooling to a temperature range of −20 ° C. or lower and staying in that temperature range for 60 seconds or more; and (c) Ar ₃ point or less Ar ₃ point− Plastic deformation, comprising a step of rolling at a cumulative reduction ratio of 50% or higher in a temperature range of 100 ° C or higher, and (d) a step of controlled cooling to a temperature range of 650 ° C or lower immediately after a cooling rate of 2 ° C / s or higher. A method of manufacturing a thick steel plate having excellent brittle crack propagation stopping characteristics after receiving the steel sheet is disclosed. According to the technique described in Patent Document 4, a structure including ferrite as a main structure, a subgrain having a maximum diameter of 5 μm or less in a ferrite crystal grain having a flatness ratio of 2 or more and a minor axis diameter of 5 μm or less. It is said that a thick steel plate occupying a portion of 5% or more of the plate thickness is obtained.

JP-A-4-141517 JP-A-5-271863 JP 2002-256375 A Japanese Patent No. 3467767

  According to the technique described in Patent Document 4, a thick steel plate having brittle crack propagation stop characteristics can be manufactured without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer. However, in the technique described in Patent Document 4, it is necessary to interrupt the rolling of the steel slab and make it stay in a predetermined temperature range, so that there is a problem that a reduction in rolling efficiency is expected. In addition, even if the maximum diameter of the subgrain is 5 μm or less, if the short axis diameter of the flat ferrite grains exceeds 5 μm, improvement in brittle crack propagation stop property cannot be expected, so in the technique described in Patent Document 4, There is also a problem that it is difficult to stably obtain a structure having excellent brittle crack propagation stopping characteristics.

As described above, the conventional technology intended to improve the brittle crack propagation stop property by forming a fine ferrite structure in the outermost layer of the steel material as described above has excellent brittle crack propagation stop property on an industrial scale. It is considered difficult to obtain a tissue having
Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a thick steel plate having excellent brittle crack propagation stop characteristics. In addition, the present invention does not require complicated temperature control such as cooling and recuperation of the steel sheet surface layer, and is an industrially simple process that is more stable than before and has excellent brittle crack propagation stopping characteristics. It is another object of the present invention to provide a method for producing a thick steel plate that can produce a steel plate and has excellent brittle crack propagation stopping characteristics.

  In order to achieve the above-described problems, the present inventors have made extensive studies on the influence of the microstructure on the brittle crack propagation stopping characteristics. As a result, the structure of a predetermined region near the front and back surfaces of the steel sheet is made a structure in which ultrafine ferrite having an average grain size of 3 μm or less accounts for 30% or more in terms of the area ratio with respect to the total amount of ferrite in the region, thereby stopping brittle crack propagation It has been found that the characteristics are remarkably improved. In addition, the inventors of the present invention found that a thick steel plate having the above structure is heated to a two-phase temperature range on a steel piece having a coarse ferrite + pearlite structure, and the average rolling reduction per pass is 10% or less. By applying multi-pass rolling with a rolling reduction of 70% or more and a rolling end temperature of 550 ° C. or more, strain can be concentrated in a predetermined region near the front and back surfaces of the steel sheet, whereby continuous recrystallization of ferrite occurs in the predetermined region. It has been found that a thick steel plate having the above-described structure can be produced by an accelerated and industrially very simple process.

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 thick steel sheet having a structure including at least one of a pearlite phase, a bainite phase, and a martensite phase as a second phase, the ferrite phase being a main phase, and the ferrite phase having an average particle size of 3 μm or less Including at least 30% of the ferrite phase in an area in the range of 10 to 20% of the plate thickness from the front and back surfaces in the plate thickness direction of the thick steel plate, the area ratio with respect to the total amount of ferrite in the region, A steel plate having excellent brittle crack propagation stopping characteristics, characterized in that the area ratio is less than 30% in the region of less than 10% of the plate thickness from the front and back surfaces and the total amount of ferrite in the region.

(2) In (1), by mass%, C: 0.03-0.3%, Si: 0.03-0.5%, Mn: 0.1-2.0%, Al: 0.1% or less, N: 0.01% or less, the balance being A thick steel plate having a composition comprising Fe and inevitable impurities.
(3) In (2), in addition to the above composition, in addition to mass, one or two selected from Ti: 0.001 to 0.02%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.1% A thick steel plate characterized by having a composition containing the above.

(4) In (2) or (3), in addition to the above composition, Cu: 0.01-0.2%, Ni: 0.01-0.1%, Cr: 0.01-2.0%, Mo: 0.01-1.0% A thick steel plate characterized by having a composition containing one or more selected from among the above.

  According to the present invention, it is possible to easily and inexpensively manufacture a thick steel plate having excellent brittle crack propagation stopping characteristics, and achieve a remarkable industrial effect. Further, according to the present invention, a predetermined region in the vicinity of the front and back surfaces of the steel sheet is obtained by multi-pass rolling which is an industrially simple process without requiring complicated temperature control such as cooling and recuperation of the steel sheet surface layer. Can promote continuous recrystallization of ferrite, can stably form ultrafine ferrite with an average grain size of 3 μm or less, and is more stable, easier and less expensive than conventional, and has excellent brittle crack propagation stopping characteristics There is also an effect that a thick steel plate can be manufactured.

The reason for limiting the structure of the thick steel plate of the present invention will be described.
In the thick steel plate of the present invention, an ultrafine ferrite phase having an average grain size of 3 μm or less in the region of 10 to 20% of the plate thickness from the front and back surfaces at least in the plate thickness direction with respect to the total amount of ferrite in the region. The organization should include 30% or more in area ratio. When the ferrite phase includes the ultrafine ferrite phase, the ability to absorb the propagation energy of the brittle crack increases, so the resistance to the propagation of the brittle crack increases, and the ability to prevent the propagation of the brittle crack is improved. When the ferrite phase becomes coarser with an average particle size exceeding 3 μm, brittle fracture is likely to occur starting from the coarse ferrite phase, and the brittle crack propagation stopping property is deteriorated. For this reason, the average particle diameter of the contained fine ferrite phase was limited to 3 μm or less.

  If the amount of the ultrafine ferrite phase contained in the region of 10 to 20% of the plate thickness from the front and back surfaces is less than 30% in terms of the area ratio relative to the total amount of ferrite in the region, the extended ferrite other than the ultrafine ferrite phase The phase ratio becomes high, and the toughness is greatly lowered due to the influence of work-hardened ferrite, and it is difficult to secure desired brittle crack propagation stop characteristics. For this reason, the amount of the ultrafine ferrite phase contained in the region of 10 to 20% of the plate thickness from the front and back surfaces is limited to 30% or more in terms of the area ratio with respect to the total amount of ferrite in the region. In addition, Preferably it is 50% or more.

  The reason why the region containing 30% or more of the ultrafine ferrite phase is at least 10 to 20% of the plate thickness from the front and back surfaces is as follows. When the ultrafine ferrite phase exists only in the region of less than 10% of the plate thickness from the front and back surfaces, the energy absorption due to ductile fracture of the ultrafine ferrite phase is small, so that the propagation of brittle cracks should be sufficiently prevented. The desired brittle crack propagation stop characteristic cannot be ensured. On the other hand, it is preferable that the ultrafine ferrite phase exists in the region exceeding 10% of the plate thickness from the front and back surfaces to the central portion of the plate thickness, which can sufficiently prevent the propagation of brittle cracks. In this manufacturing method, a desired structure is obtained in an area of at least 20%. For this reason, a region containing 30% or more of the ultrafine ferrite phase is defined as a region having a thickness of 10 to 20% from at least the front and back surfaces. Needless to say, an ultrafine ferrite phase may also be included in the region from the front and back surfaces of less than 10% or more than 20% of the plate thickness.

  In the thick steel plate of the present invention, the amount of the ferrite phase having an average grain size of 3 μm or less in the region less than 10% of the plate thickness from the front and back surfaces in the plate thickness direction, that is, the area ratio relative to the total amount of ferrite in the region. Less than 30%. This is because, according to FEM analysis, the distribution of equivalent strain in the thickness direction due to thick plate rolling has a peak in the vicinity of 15% of the plate thickness from the front and back surfaces, and 10 to 20% of the plate thickness from the front and back surfaces. Compared to the region, the amount of strain applied to the outermost surface layer, which is a region of less than 10% of the plate thickness from the front and back surfaces, is small, and it is considered that the generation of the ultrafine ferrite phase in this region is reduced.

Below, the reason for limitation of the preferable composition of this invention thick steel plate is demonstrated. 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 0.03-0.2%.

Si: 0.03-0.5%
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 0.5%, the surface properties are impaired and the toughness is extremely lowered. For this reason, Si was limited to the range of 0.03-0.5%. In addition, Preferably it is 0.03-0.35%.

Mn: 0.1-2.0%
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 2.0% lowers the weldability and causes an increase in material cost. For this reason, Mn was limited to the range of 0.1 to 2.0%. 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 0.001% 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.
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 0.001% 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 the present invention, in addition to the basic composition, the above-described components are further selected from Ti: 0.001 to 0.02%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.1%. Or two or more, and / or one or more selected from Cu: 0.01 to 0.2%, Ni: 0.01 to 0.1%, Cr: 0.01 to 2.0%, Mo: 0.01 to 1.0%, It can be selected and contained as necessary.
Ti: 0.001 to 0.02%, Nb: 0.001 to 0.05%, V: one or more selected from 0.001 to 0.1%,
Ti, Nb, and V are all contained in small amounts, and form nitrides, carbides, or carbonitrides, refine the crystal grains, and strengthen the steel. Select as needed. 1 type or 2 types or more can be contained. 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 Ti is contained in a large amount exceeding 0.02%, Nb: 0.05%, and V: 0.1%, the slab is cracked and the manufacturing cost is increased. For this reason, it is preferable to limit to Ti: 0.001 to 0.02%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.1%, respectively.

One or more selected from Cu: 0.01-0.2%, Ni: 0.01-0.1%, Cr: 0.01-2.0%, Mo: 0.01-1.0%
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 effects become remarkable when Cu, Ni, Cr, and Mo are each contained in an amount of 0.01% or more, but excessive amounts exceeding Cu: 0.2%, Ni: 0.1%, Cr: 2.0%, Mo: 1.0%, respectively. Inclusion reduces toughness and weldability. For this reason, it is preferable to limit Cu to 0.01 to 0.2%, Ni to 0.01 to 0.1%, Cr to 0.01 to 2.0%, and Mo to 0.01 to 1.0%, respectively.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities are P: 0.04% or less and S: 0.02% or less, respectively. If the content exceeds P: 0.04% and S: 0.02%, the toughness is lowered. Therefore, it is desirable to adjust P: 0.04% or less and S: 0.02% or less.
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.

Below, the preferable manufacturing method of this invention thick steel plate which has an above described composition and structure | tissue is demonstrated.
In the present invention, the steel slab is heated and subjected to thick plate rolling to obtain a thick steel plate.
The steel slab used in the present invention is a steel slab having the above-described composition and having a relatively coarse ferrite + pearlite structure. The reasons for limiting the structure of the steel slab used in the present invention are as follows.

  By making the structure before rolling of the slab (pre-rolling structure) a coarse ferrite + pearlite structure, when the steel sheet is heated to a two-phase temperature range during the thick plate rolling, the pearlite region undergoes reverse transformation and a certain degree of coarse austenite Become. Due to the influence of this coarse austenite, if rolling is performed under small pressure in the two-phase temperature range, the strain (rolling strain) due to rolling can be effectively distributed to the ferrite grains, especially near the surface layer. Ferrite, which contains 30% or more of an ultrafine ferrite phase with an average grain size of 3 μm or less in an area of 10 to 20% of the plate thickness from at least the front and back surfaces in an area ratio with respect to the entire area. A structure having a phase as a main phase can be formed.

  This is remarkable when the steel slab has a coarse ferrite + pearlite structure having an average particle diameter of 40 μm or more. When the average grain size of ferrite in the steel slab is 40 μm or more, the size of pearlite also becomes coarse accordingly and reversely transforms to some coarse austenite during heating in the two-phase temperature range. In the ferrite + pearlite structure with an average particle size of less than 40 μm, the continuous recrystallization of the ferrite as described above cannot be promoted even if the two-phase temperature rolling is performed thereafter, and at least 10 mm of the plate thickness from the front and back surfaces. In the region of ˜20%, it is impossible to form a structure having a ferrite phase containing a large amount of ultrafine ferrite phase as a main phase. For this reason, the average grain size of ferrite in the structure before rolling of the steel slab was limited to 40 μm or more.

  On the other hand, when the pre-rolled structure of the steel slab is a bainite or martensite-based structure, when the steel slab 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. If rolling in a two-phase temperature range is applied to a structure in which such fine austenite exists, rolling strain cannot be effectively distributed to the ferrite phase, and continuous recrystallization of ferrite is not promoted. A fine ferrite phase cannot be formed.

Thus, when the structure of the steel slab before rolling (the structure before rolling) is not a coarse ferrite + pearlite structure, at least 10 mm of the plate thickness from the front and back surfaces even if rolling is performed in the two-phase temperature range thereafter. It is difficult to obtain a structure whose main phase is a ferrite phase containing a large amount of ultrafine ferrite phase in a region of ˜20%.
The manufacturing method of the steel slab 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 piece 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. Thereby, a coarse structure having a ferrite grain of 40 μm or more in a ferrite + pearlite structure can be obtained.

Next, the steel slab having the composition and structure described above is heated to a temperature in the two-phase temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point. The steel slab is heated from a temperature below the Ac ₁ transformation point, and the heating rate is not particularly limited. When heated to a temperature in the two-phase temperature range and the steel slab has a two-phase structure of ferrite + austenite, rolling strain can be effectively introduced into the ferrite during rolling, and continuous recrystallization of ferrite can be promoted. it can. If the austenite fraction of the two-phase structure is as low as less than 5% in terms of area ratio, rolling strain cannot be effectively introduced into ferrite, and continuous recrystallization of ferrite cannot be promoted. On the other hand, if the austenite fraction exceeds 50% in terms of area ratio, the amount of second phase formed of pearlite, bainite, martensite, etc. increases depending on the cooling conditions after rolling, and the toughness decreases. For this reason, the heating temperature of the steel strip, of the temperature range of Ac ₁ transformation point to Ac ₃ transformation point, in particular the austenite fraction is 5-50% by area ratio, (Ac ₁ transformation point + 30 ° C. ) Or more (Ac ₃ transformation point −40 ° C.) or less.

The steel slab heated to the above heating temperature is then set to an average reduction rate of 10% or less per pass in the (γ + α) two-phase temperature range, a cumulative reduction rate of 70% or more, and a rolling end temperature of 550 ° C or more. Multipass rolling is applied to make a thick steel plate.
Here, since the heated steel slab is immediately rolled, the heating temperature of the steel slab and the rolling start temperature are substantially equal. In addition, the cumulative reduction ratio here refers to the total reduction ratio from the start of rolling to the end of rolling.

  In the present invention, the steel slab heated to the above temperature is subjected to multi-pass rolling with a rolling end temperature of 550 ° C. or higher. The lower the rolling temperature, the finer the recrystallized ferrite grains become. However, when the rolling end temperature is less than 550 ° C, the load on the rolling equipment increases and the continuous recrystallization of ferrite is less likely to occur. As a result, the ferrite just expanded is increased, leading to a decrease in toughness. For this reason, the rolling end temperature is limited to 550 ° C. or higher.

  In the present invention, the cumulative rolling reduction in the (γ + α) two-phase temperature range in the above-described multi-pass rolling is set to 70% or more. If the cumulative rolling reduction is less than 70%, the rolling reduction is small and the continuous recrystallization of ferrite is not sufficiently promoted. Therefore, the average crystal grain size exceeds 3 μm or less in at least 10 to 20% of the plate thickness from the front and back surfaces. It becomes impossible to form a structure containing 30% or more of the fine ferrite phase in the area ratio with respect to the entire region, and the brittle crack propagation stop characteristic is deteriorated. For this reason, the cumulative rolling reduction in multi-pass rolling is limited to 70% or more.

  Moreover, in this invention, the rolling reduction per pass in the above-mentioned multipass rolling shall be 10% or less on average. As the rolling reduction increases, that is, the amount of dislocations introduced into the ferrite increases as the amount of strain increases, it is possible to promote continuous recrystallization of ferrite. By reducing the average rolling reduction per pass to 10% or less, the strain concentration in the vicinity of the surface layer of the thick steel plate, especially in the region of 10 to 20% of the plate thickness from the front and back surfaces, becomes significant. Continuous recrystallization is promoted, and the structure of the region can be made to have a structure containing an ultrafine ferrite phase with an average crystal grain size of 3 μm or less in an area ratio of 30% or more with respect to the entire region. Remarkably improved. On the other hand, when the rolling reduction per pass is greater than 10% on average, the rolling load increases and the load on the rolling equipment increases. For this reason, the rolling reduction per pass in multi-pass rolling is limited to 10% or less on average. In addition, Preferably it is 5 to 7%. When the rolling reduction per pass is less than 5% on average, the steel sheet temperature is significantly lowered due to heat removal, and the rolling efficiency is lowered.

Note that the average "rolling rate per pass" is 10% or less means that the value obtained by adding the rolling reduction rate in each pass from the start of rolling to the end of rolling and dividing by the total number of passes is 10% or less. Means.
The above-described rolling and the thick steel plate are cooled to room temperature after the end of rolling. Although the cooling conditions are not particularly limited, it is preferable to cool by air or water according to the target strength level.

Molten steel having the composition shown in Table 1 was melted using a converter, and a steel slab (steel piece: wall thickness: 210 mm) was obtained by a continuous casting method.
Table 2 shows the structure of the obtained steel slab.
Next, the steel slab was heated under the conditions shown in Table 2 and then subjected to multi-pass rolling under the conditions shown in Table 2 with hot rolling equipment. After the rolling, the steel slab was cooled under the conditions shown in Table 2 and shown in Table 2. A thick steel plate was used. For reference, the area ratio of austenite at the time of heating in multipass rolling under small pressure is also shown in Table 2. The austenite area ratio at the time of heating was obtained by preparing the same steel slab separately from that for rolling, and cooling the steel slab from the heated state and observing the structure.

The transformation points (Ac ₁ , Ac ₃ ) of each steel slab were calculated from the following equations.
Ac ₁ (° C) = 750.8-26.6C + 17.6Si-11.6Mn-22.9Cu-23Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti + 232.6Nb-169.4Al
Ac ₃ (° C) = 937.2−476.5C + 56Si−19.7Mn−16.3Cu−26.6Ni−4.9Cr + 38.1Mo
+ 124.8V + 136.3Ti-19.1Nb + 198.4Al
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al: element content (mass%))
The obtained thick steel plate was subjected to a structure observation and an NRL drop weight test to evaluate the brittle crack propagation stop characteristics.

  The structure observation is performed by taking a structure observation specimen from the obtained thick steel plate, and using a scanning electron microscope (magnification: 2000 times) for each 1 mm pitch area from the surface in the rolling direction to the center of the plate thickness. Using an image analyzer, the average crystal grain size of the unextended fine ferrite phase, the area ratio of the ferrite phase, and the total amount of the ferrite phase of the fine ferrite phase having an average grain size of 3 μm or less. The area ratio, the type of the second phase, and the area ratio were measured. The average crystal grain size of the fine ferrite phase that has not been extended was determined by measuring the area of the fine ferrite grains that had not been extended and using an image analyzer.

In the NRL drop weight test, drop weight test specimens were collected from the obtained thick steel plate so that the length direction of the test pieces coincided with the rolling direction, and the drop weight test was conducted in accordance with ASTM E208 regulations. The temperature was determined. The test piece used was a P-3 test piece.
The obtained results are shown in Table 3.

  In all of the examples of the present invention, an ultrafine ferrite phase having an average crystal grain size of 3 μm or less in an area of 10 to 20% of the plate thickness from at least the front and rear surfaces includes an area ratio of 30% or more with respect to the total amount of ferrite in the area. It has a microstructure and exhibits excellent brittle crack propagation stopping properties with an NDT temperature of -80 ° C or lower. In the outermost layer (region from the front and back surfaces of less than 10% of the plate thickness), an ultrafine ferrite phase having an average crystal grain size of 3 μm or less is less than 30% in area ratio with respect to the total amount of ferrite in the region. The structure had few ferrite phases.

On the other hand, in the comparative example that is outside the scope of the present invention, the NDT temperature is −50 ° C. or higher, and the brittle crack propagation stopping property is deteriorated. Steel plates No.14, No.15, No.18, No.19, No.20 have a cumulative rolling reduction in the two-phase temperature range that is outside the scope of the present invention, and strain concentration near the surface layer is not sufficient. For this reason, the brittle crack propagation stop characteristic is deteriorated. Steel plates No. 16, No. 21, and No. 22 have high heating temperatures exceeding the Ac ₃ transformation point, so the amount of strain concentration near the surface layer is small and the amount of ultrafine ferrite phase produced is small. At least in the region of 10 to 20% of the plate thickness from the front and back surfaces, an ultrafine ferrite phase cannot form a structure containing 30% or more in terms of the area ratio with respect to the entire region, and the brittle crack propagation stopping property is deteriorated. .

Steel plate No. 17 has a heating temperature below the Ac ₁ transformation point and is outside the scope of the present invention, and despite continuous multi-pass rolling with a cumulative rolling reduction of 80%, continuous recrystallization of ferrite is sufficient. There is little formation of a fine ferrite phase at least in the region of 10 to 20% of the plate thickness from the front and back surfaces, the NDT temperature is as low as −40 ° C., and the brittle crack propagation stop property is not sufficient.
Steel plates No. 23 and No. 24 have an average reduction rate of 20% per pass, which is far from the range of the present invention, and the concentration of strain in the vicinity of the front and back surfaces is not significant. In the range of 10 to 20%, the formation of a fine ferrite phase is small, and the brittle crack propagation stopping property is deteriorated. Steel plates No.25 and No.26 are outside the scope of the present invention, with the rolling end temperature being less than 550 ° C., the expanded ferrite grains increase, the amount of ultrafine ferrite phase is small, the NDT temperature is high, and brittleness The crack propagation stop property is degraded.

Claims (4)

  A thick steel plate having a structure including a ferrite phase as a main phase and at least one of a pearlite phase, a bainite phase, and a martensite phase as a second phase, wherein the ferrite phase has a ferrite phase having an average particle size of 3 μm or less. Including at least 30% of the area ratio with respect to the total amount of ferrite in the region of 10 to 20% of the plate thickness from the front and back surfaces in the thickness direction of the thick steel plate, and in the thickness direction of the thick steel plate A thick steel plate excellent in brittle crack propagation stopping characteristics, characterized in that the area ratio relative to the total amount of ferrite in the region is less than 30% in a region less than 10% of the plate thickness. % By mass
C: 0.03-0.3%, Si: 0.03-0.5%,
Mn: 0.1 to 2.0%, Al: 0.1% or less,
The 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 the group consisting of Ti: 0.001 to 0.02%, Nb: 0.001 to 0.05%, and V: 0.001 to 0.1%. The thick steel plate according to claim 2.   In addition to the above-mentioned composition, in addition by mass, Cu: 0.01-0.2%, Ni: 0.01-0.1%, Cr: 0.01-2.0%, Mo: 0.01-1.0% The thick steel plate according to claim 2, wherein the steel plate has a composition containing