Extremely Thick Steel Material for Flange Having Excellent Strength and Low Temperature Impact Toughness, and Manufacturing Method for Same

The present disclosure relates to steel that may be used for wind power generation towers, systems and the like, and a method of manufacturing the same, and more specifically, to an extremely thick steel material for a flange having excellent strength and low-temperature impact toughness, and a method of manufacturing the same.

Wind power generators are gaining attention as an eco-friendly means of generating electricity, and include components such as tower flanges, bearings, main shafts and the like. Thereamong, tower flanges are joint components necessary for connecting towers, and usually 5 to 7 flanges are used for one tower, and are also installed in the sea or in extreme temperature regions, and thus, high durability is required. In particular, in response to the demand for large-scale energy production and high efficiency, wind towers are also increasing in size, and accordingly, the steel used is also continuously required to be high-strength, high-toughness, and thick. As the thickness of the material increases, total deformation decreases, and thus the microstructure becomes larger, and the material tends to deteriorate due to defects in the material such as inclusions, segregation or the like. Therefore, to improve the internal and external soundness of the steel, the trend is to reduce the concentration of impurities such as non-metallic inclusions, segregation or the like, or to control cracks, pores and the like on the surface and inside the material to the extreme.

In particular, in the case of extremely thick materials exceeding 200 mmt in thickness, since the deformation of the center of the material is not large, if the unsolidified shrinkage pores that occur during continuous casting or casting are not sufficiently compressed during the forging process, they remain in the form of residual pores at the center of the flange.

These residual pores act as crack initiation points when the structure is subjected to axial stress in the thickness direction, and may eventually cause damage to the entire equipment in the form of lamellar tearing. Therefore, a process is necessary to sufficiently compress the center pores so that there are no residual pores before piercing (hole drilling forging) with small deformation and ring forging (product forming).

Patent document 1 related thereto is a technology for applying high reduction ratio in a thick plate rough rolling process. In detail, there may be used a technology for determining a thickness-specific limit reduction ratio at which thickness-specific plate bite occurs from a pass-specific reduction ratio set to be close to the design allowance (load and torque) of a rolling mill, a technology for distributing the reduction ratio by adjusting the index of a thickness ratio for each pass to secure the target thickness of a roughing mill, and a technology for modifying the reduction ratio so that plate bite does not occur based on the thickness-specific limit reduction ratio, thereby providing a manufacturing method in which an average reduction ratio of approximately 27.5% in the final three passes of roughing milling based on 80 mmt may be applied. However, in the case of the rolling method above, the average reduction ratio of the entire thickness of a product is measured, and in the case of extremely thick materials with a maximum thickness of 200 mmt or more, it is technically difficult to apply high strain to the center where residual pores exist.

One of other methods of manufacturing extremely thick materials is to utilize a forging machine with a higher effective strain per pass than a rolling mill. Patent Document 2 provides a method of manufacturing a thick-walled, high-toughness and high-strength material, using a slab comprising, in mass %, C: 0.08 to 0.20%, Si: 0.40% or less, Mn: 0.5 to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1 to 5.0%, Al: 0.010 to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, and satisfying the relationships of Formulas (1) and (2), the remainder being Fe and unavoidable impurities, in which hot forging is performed with a cumulative reduction amount of 25% or more, heating is performed at a temperature of Ac3 point or higher and 1200° C. or lower, hot rolling is performed with a cumulative reduction amount of 40% or more, and rapid cooling from a temperature of Ar3 point or higher to a low temperature of 350° C. or lower or Ar3 point or lower is performed, and a tempering heat treatment process is performed at a temperature of 450 to 700° C., thereby manufacturing the thick-walled, high-toughness and high-strength material with a plate thickness of 100 mmt or more, a yield strength of 620 MPa or more, and an absorbed energy of 70 J or more when evaluated for low-temperature impact toughness at −40° C.

However, the manufacturing method may cause surface defects due to localized strain concentration in the case in which the cumulative reduction amount is too high, and in particular, in the case in which a surface or subsurface defect exists in the cast state before forging, the defect may propagate during the forging process, further deteriorating the surface quality in the product state after rolling. In addition, in the case in which the forging reduction amount per pass is insufficient, it is difficult to sufficiently pressurize the pores remaining in the center even if the cumulative reduction amount is high, and the rolling process is also not suitable for controlling the central pores and structure of extremely thick materials because the effective deformation amount in the center is small compared to the surface deformation.

Meanwhile, Patent Document 3 discloses that a thick-walled high-strength steel plate having 100 mmt or more and a yield strength of 620 MPa or more may be manufactured through a process of heating a material provided with a predetermined alloy composition to 1200-1350° C., performing hot forging with a cumulative reduction amount of 25% or more, heating to a temperature of Ac3 point or higher and 1200° C. or lower, performing hot rolling with a cumulative reduction amount of 40% or more, reheating to a temperature of Ac3 point or higher and 1050° C. or lower, rapidly cooling from a temperature of Ac3 point or higher to a low temperature of 350° C. or lower or Ar3 point or lower, and performing tempering at a temperature of 450° C. to 700° C.

However, in the case of the ultra-high strength steel plate described above, the carbon equivalent (Ceq) and hardenability index (DI) are high, and thus in addition to being vulnerable to surface cracks during casting, in the case of flange steel manufactured by normalizing heat treatment, the corresponding process conditions cannot be easily applied. In addition, in the case in which the carbon equivalent (Ceq) and hardenability index (DI) are high, cracks easily occur on the surface layer of the cast due to the formation of surface hard tissue during the second cooling process of steelmaking, and the cracks may propagate during the forging process, which may deteriorate the surface quality of the final product.

Therefore, a method of performing forging to improve the internal soundness of the final product by compressing the central pore has been proposed, but no practical method has been presented to secure both the appropriate material and excellent surface quality of the steel for a flange.

An aspect of the present disclosure is to provide an extremely thick steel material for a flange having excellent strength and low-temperature impact toughness and a method of manufacturing the same.

The subject matter of the present disclosure is not limited to the above-described contents. Those skilled in the art will have no difficulty in understanding the additional subjects of the present disclosure from the overall content of this specification.

According to an aspect of the present disclosure, an extremely thick steel material for a flange includes,

In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (in weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel, respectively, and 0 is substituted if these components are not added intentionally.

In addition, the steel may have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value of −50° C. Charpy impact test of 50 J or more.

A maximum surface crack depth of the steel may be 0.1 mm or less (including 0).

According to an aspect of the present disclosure, a method of manufacturing an extremely thick steel material for a flange includes,

In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in steel, respectively, and 0 is substituted if these components are not intentionally added.

The slab may be manufactured using one of a continuous casting process, a semi-continuous casting process, and an ingot casting process.

It is preferable that after manufacturing the slab, a prior austenite grain size of a surface layer of the slab before forging is 1000 μm or less, and a microstructure of the surface layer of the slab before forging is composed of a composite structure of polygonal ferrite of 15% or more and residual bainite.

A size of a forging surface punched during the first upsetting may be 1000-1200 mm×1800-2000 mm when being initially 700 mm×1800 mm.

In the case of the bloom forging, when forging is completed, a size of a forging surface may be 1450-1850 mm×2100-2500 mm when being initially 1000-1200 mm×1800-2000 mm.

When the round forging and the second upsetting are completed, a size of a product may be 1450-1850Ø×1300-1700 mm.

When the third upsetting is completed, a size of a product may be 2300-2800Ø×400-800 mm.

The flange made of the steel may have a maximum thickness of 200 to 500 mm, an inner diameter of 4000 to 7000 mm, and an outer diameter of 5000 to 8000 mm.

It is preferable that during the normalizing heat treatment, a heat treatment is performed such that an LMP defined by the following relational expression 2 satisfies 20 to 33.

In the relational expression 2, T is Kelvin reference temperature, t is time, and an exponent of log is 10.

The method may further include an operation of performing a post-weld heat treatment, a stress relieving heat treatment, or a tempering heat treatment, when welding is performed on the steel after the normalizing heat treatment.

It is preferable that the post-weld heat treatment is performed in a range where a value defined by the following relational expression 2 is LMP 19.3 or less.

In the relational expression 2, T is Kelvin reference temperature, t is time, and an exponent of log is 10.

According to an aspect of the present disclosure, a

method of manufacturing an extremely thick steel material for a flange includes,

In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] represent contents (weight %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in steel, respectively, and 0 is substituted if these components are not intentionally added.

In the normalizing heat treatment, it is preferable that a heat treatment is performed so that an LMP defined by the following relational expression 2 satisfies 20 to 33.

In the relational expression 2, T is Kelvin reference temperature, t is time, and a exponent of log is 10.

The present disclosure having the above-described configuration may effectively provide an extremely thick steel material that may be used for flanges, having excellent low-temperature impact toughness as well as strength, by compressing the central pore of the steel material by optimizing a forging process and thereby improving the internal soundness of a final product.

The present disclosure relates to an extremely thick steel for flanges having excellent strength and low-temperature impact toughness and a method of manufacturing a product. Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These implementation examples are provided to further detail the present disclosure to a person having ordinary knowledge in the technical field to which the present disclosure belongs.

Hereinafter, extremely thick steel for a flange of the present disclosure will be described in more detail.

An extra heavy steel material for a flange of the present disclosure includes, in wt %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, the remainder of Fe and other unavoidable impurities, the extremely thick steel material for a flange having a Ceq according to the relational expression 1 satisfying a range of 0.35 to 0.55, having a thickness of 200 to 500 mm, having a steel microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 μm or less, having a maximum size of cementite existing in a ferrite-ferrite and/or ferrite-pearlite grain boundary, being 5 μm or less, having a porosity of 0.1 mm/g or less in a central portion of a product, which is a region of ⅜t to ⅝t (where t represents the steel thickness (mm)) in the thickness direction from the steel surface, and having five or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1 μm, among the precipitates observed in the cross section of the steel.

Hereinafter, the alloy composition of the present disclosure will be described in more detail, and unless otherwise specifically indicated, the % and ppm described in relation to the alloy composition are based on weight.

Carbon (C) is the most important element for securing basic strength, and thus it needs to be contained in steel within an appropriate range, and to obtain this addition effect, 0.05% or more of carbon (C) may be added. Preferably, 0.10% or more of carbon (C) may be added. On the other hand, if the content of carbon (C) exceeds a certain level, the fraction of pearlite increases during normalizing heat treatment, which may excessively exceed the strength and hardness of the base material, resulting in surface cracks during a forging process and deterioration of the low-temperature impact toughness and lamellar tearing resistance characteristics in the final product. Therefore, the present disclosure may limit the carbon (C) content to 0.20%, and the upper limit of the more desirable carbon (C) content may be 0.18%.

Silicon (Si) is a substitutional element that

improves the strength of steel through solid solution strengthening and has a strong deoxidation effect, and thus is an essential element for manufacturing clean steel. Therefore, silicon (Si) may be added at 0.05% or more, and more preferably, may be added at 0.20% or more. On the other hand, if silicon (Si) is added in a large amount, a Martensite-Austenite (MA) phase is generated and the strength of the ferrite matrix excessively increases, which may deteriorate the surface quality of the ultra-thick product, and thus the upper limit of the content may be limited to 0.50%. A more preferable upper limit of the silicon (Si) content may be 0.40%.