KR102011357B1 - Pressure vessel, and the method of manufacturing the same - Google Patents

Abstract

The present invention relates to a pressure vessel and a method of manufacturing the same.
More specifically, one embodiment of the present invention, C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 1.5% by weight based on 100% by weight of the total pressure vessel %, P: 0.03% or less (does not contain 0%), S: 0.015% or less (does not contain 0%), Al: 0.05% or less (does not contain 0%), N: 0.01% by weight or less (not including 0% by weight), the balance being a pressure vessel containing Fe and other unavoidable impurities, wherein the pressure vessel has a tensile strength of 440 to 650 MPa.

Description

PRESSURE VESSEL, AND THE METHOD OF MANUFACTURING THE SAME

One embodiment of the present invention relates to a pressure vessel and a method of manufacturing the same. More specifically, it relates to a pressure vessel steel plate excellent in strength and ductility and a method of manufacturing the same.

For pressure vessels such as LPG gas vessels, the bursting pressure in the bursting test by hydraulic pressure after the vessel is manufactured is important. Therefore, the strength of the hot rolled steel sheet used at this time is an important variable. Thus, in order to obtain high strength, the hot rolled steel sheet used for manufacturing a container is generally manufactured by adding C, Si, Mn, Ti, Nb, Mo, and V to a high purity steel which minimizes impurities in the steel.

Conventionally, in order to manufacture high strength hot rolled steel sheet, Ti, Nb, V, Mo, etc. may be added to prepare hot rolled steel sheet by utilizing precipitation strengthening of these elements, or by adding a large amount of Cr or Mn to secure strength, or Methods of strengthening impact strength and tensile properties by tempering annealing of Mn and Cr addition steels are known.

However, safety regulations have been strengthened recently due to the explosion of containers, and under the enhanced safety regulations, not only the burst pressure in the burst test but also the criteria for the volume expansion rate of the containers have been created. Therefore, in order to satisfy the safety regulations of the reinforced pressure vessel, the situation is required not only steel but also hot-rolled steel sheet having excellent ductility. The prior art presented above focuses only on high strength through solid solution strengthening by alloy components such as C, Si, Mn, Cr, Mo, and W and precipitation strengthening by alloy components such as Ti, Nb, and Mo. There is a limit to the ductility improvement for securing the volume expansion rate of the container.

In addition, important factors affecting the material of the steel sheet in the hot rolling process include final delivery temperature (FDT), coiling temperature (CT) and run out table (Run Out). The cooling pattern for adjusting the winding temperature (CT) in the table, ROT) can be a factor.

In particular, the hot rolled steel sheet may pass through the run-out table ROT to have a final phase through a phase transformation process due to forced cooling. Since the phase transformation process is influenced by temperature and time, even if the same filament rolling finish temperature (FDT) and the winding temperature (CT) has a different phase by the cooling pattern affects the material. Conventional hot rolling has a problem in that ductility is poor because it cannot form an equiaxed ferrite by continuously cooling to the winding temperature after finishing rolling.

In addition, the pressure vessel for LPG gas can be produced into a pressure vessel steel sheet through the manufacturing process of the pickling, forming, heat treatment and coating of the hot rolled steel sheet. At this time, even when using a hot-rolled steel sheet excellent in strength and ductility, it is possible to minimize the volume expansion rate by relieving stress such as work hardening occurred in the forming step through the appropriate heat treatment.

One embodiment of the present invention to provide a pressure vessel and its manufacturing method.

Pressure vessel according to an embodiment of the present invention, C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 1.5 with respect to 100% by weight of the total pressure vessel % By weight, P: 0.03% by weight or less (0% by weight), S: 0.015% by weight (does not contain 0% by weight), Al: 0.05% by weight or less (does not contain 0% by weight) , N: 0.01% by weight or less (does not include 0% by weight), the balance includes Fe and other unavoidable impurities, the tensile strength of the pressure vessel may be 440 to 650 MPa.

The other unavoidable impurities include Cr: 0.3% by weight or less (does not include 0% by weight), Ni: 0.3% by weight or less (does not contain 0% by weight), Mo: 0.2% by weight or less (does not include 0% by weight) Cu), 0.1 wt% or less (not including 0 wt%).

The pressure vessel is composed of a pearlite and a ferrite microstructure, the volume fraction of the ferrite tissue is 75% to 85% with respect to the total 100% by weight of the microstructure, the aspect ratio of the ferrite microstructure is 0.7 To 1.3.

In addition, with respect to the total percentage of the microstructure 100%, the volume fraction of the pearlite tissue may be 15% to 25%.

In addition, the volume expansion ratio of the pressure vessel may be 20 to 50%, the yield strength of the pressure vessel may be 295 to 520 MPa.

Pressure vessel manufacturing method according to another embodiment of the present invention, C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 1.5 with respect to 100% by weight % By weight, P: 0.03% by weight or less (0% by weight), S: 0.015% by weight (does not contain 0% by weight), Al: 0.05% by weight or less (does not contain 0% by weight) N: 0.01% by weight or less (does not include 0% by weight), the remainder comprising preparing a slab comprising Fe and other unavoidable impurities; Reheating the slab at 1100 to 1300 ° C .; Rough rolling the reheated slab; Finishing rolling the rough rolled slab; Cooling the finished rolled sheet; Winding the cooled sheet to obtain a hot rolled steel sheet; Pickling the obtained hot rolled steel sheet; And forming the pickled steel sheet to obtain a pressure vessel. It may include.

In addition, by molding the pickled steel sheet to obtain a pressure vessel; Thereafter, heat-treating the pressure vessel; Further comprising, heat-treating the formed pressure vessel steel plate; Annealing in the temperature range of 600 ℃ to 700 ℃; Or normalizing in a temperature range of 800 ° C. to 900 ° C .; It may include.

More specifically, the step of annealing in the temperature range of 600 ℃ to 700 ℃; May be performed for 10 to 30 minutes.

In addition, the step of normalizing in the temperature range of 800 ℃ to 900 ℃; May be performed for 1 to 5 minutes.

At this time, the other unavoidable impurities include: Cr: 0.3% by weight or less (does not include 0% by weight), Ni: 0.3% by weight or less (does not contain 0% by weight), Mo: 0.2% by weight or less (0% by weight) Cu): 0.1% by weight or less (not including 0% by weight) may be included.

Finishing rolling the rough rolled slab; In, the finish rolling may be carried out in the temperature range of A ₁ -20 ℃ to A ₁ +20 ℃.

A ₁ = 932.1-392.8 [C] -61.9 [Mn] +43.9 [Si] +420.6 [P] +220.0 [Al] -15.5 [Cr] -15.2 [Ni] +31.5 [Mo] -20 [Cu]- --------------------- Formula (1)

However, [C], [Mn], [Si], [P], [Al], [Cr], [Ni], [Mo], and [Cu] mean weight percent of each component content.

Finishing rolling the rough rolled slab; By the finishing rolled slab may have an austenite microstructure of 10 to 40 ㎛ size.

Cooling the finished rolled sheet; The first cooling step of cooling the finish-rolled sheet at a rate of 40 ℃ / s to 60 ℃ / s; And a second cooling step of cooling the first cooled plate at a rate of 1 ° C./s to 8 ° C./s. It may include.

A first cooling step of cooling the finished rolled sheet at a rate of 40 ° C./s to 60 ° C./s; Is, A can be cooled to -20 ℃ to ₂ A ₂ + 20 ℃ temperature range.

A ₂ = 740.1-35.4 [C] -64.5 [Mn] +29.1 [Si] +16.9 [Cr] -16.9 [Ni] ----------- Equation (2)

However, [C], [Mn], [Si], [Cr] and [Ni] mean weight percent of each component content.

A second cooling step of cooling the first cooled plate at a rate of 1 ° C./s to 8 ° C./s; By, the primary cooled plate can be cooled for 1 second to 8 seconds.

Cooling the finished rolled sheet; By this, an equiaxed ferrite structure can be formed.

Cooling the finished rolled sheet; By, the plate may have 75% to 85% equiaxed ferrite tissue and 15% to 25% austenite tissue, relative to 100% by weight of the total microstructure.

Winding the cooled sheet to obtain a hot rolled steel sheet; By, the plate may be wound by cooling to a temperature range of A ₃ -20 ℃ to A ₃ +20 ℃.

A ₃ = 693.4-444.5 [C] -80.5 [Mn] -35.0 [Si] -76.0 [Cr] -35.0 [Ni] -85.7 [Mo]-(3)

However, [C], [Mn], [Si], [Cr], [Ni], and [Mo] mean weight percent of each component content.

Winding the cooled sheet to obtain a hot rolled steel sheet; In, the plate may be wound by cooling at a rate of 40 ℃ / s to 60 ℃ / s.

Winding the cooled sheet to obtain a hot rolled steel sheet; By this, a pearlite structure can be further formed.

The yield strength of the hot rolled steel sheet may be 295 to 520 MPa, and the (yield strength X elongation) value of the hot rolled steel sheet may be 11,500 MPa ·% or more and 17,500 MPa ·% or less.

Reheating the slab at 1100 to 1300 ° C .; In, the slab can be reheated for 100 to 400 minutes.

Molding the pickled steel sheet to obtain a pressure vessel; In, the pickled steel sheet may be formed by a method by blanking, drawing, tube, welding, or a combination thereof. However, it is not limited thereto.

The pressure vessel is composed of pearlite and ferrite microstructure, the volume fraction of the ferrite tissue may be 75% to 85% relative to the total 100% by weight of the microstructure.

The volume fraction of the pearlite tissue may be 15% to 25% based on 100% of the total microstructure.

The tensile strength of the pressure vessel may be 440 to 650 MPa, the volume expansion rate of the pressure vessel may be 20 to 50%.

According to one embodiment of the present invention, it is possible to provide a pressure vessel having excellent burst resistance and a method of manufacturing the same through appropriate heat treatment.

1 is a graph showing values derived from yield strength and elongation and (yield strength x elongation) of hot-rolled steel sheets according to the invention and comparative examples.
Figure 2 shows the cooling pattern of the hot rolled steel sheet according to the invention example.
3 is a graph showing the tensile strength and the volume expansion rate of the pressure vessel after the pickling, forming, and heat treatment step of the hot rolled steel sheet according to the invention and comparative examples.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be embodied in various different forms, and the present embodiments merely make the disclosure of the present invention complete, and are common in the art to which the present invention pertains. It is provided to fully inform those skilled in the art of the scope of the invention, which is to be defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

Thus, in some embodiments, well known techniques are not described in detail in order to avoid obscuring the present invention. Unless otherwise defined, all terms used in the present specification (including technical and scientific terms) may be used as meanings that can be commonly understood by those skilled in the art. When any part of the specification is to "include" any component, this means that it may further include other components, except to exclude other components unless otherwise stated. In addition, singular forms also include the plural unless specifically stated otherwise in the text.

Pressure vessel according to an embodiment of the present invention, C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 1.5% by weight relative to the total 100% by weight , P: 0.03% or less (does not contain 0% by weight), S: 0.015% or less (does not contain 0% by weight), Al: 0.05% or less (does not contain 0% by weight), N 0.01 wt% or less (not including 0 wt%), the balance may include Fe and other unavoidable impurities.

Hereinafter, the reason for limiting the composition range of the present invention is as follows.

Carbon (C) plays the most economical and effective role in reinforcing steel. When the content of C is less than 0.1% by weight, pearlite is insufficient to produce desired strength. On the other hand, when the content of C exceeds 0.2% by weight, there is a problem in that ductility is lowered due to excessive strength increase. Therefore, C may contain 0.1 to 0.2% by weight.

Silicon (Si) deoxidizes molten steel and has a solid solution effect. When the content of Si exceeds 0.5% by weight, a red scale formed by Si is formed on the surface of the steel sheet during hot rolling, so that the surface quality of the steel sheet may be very bad and the weldability may be reduced. Therefore, the silicon should be included essentially, but may be included in less than 0.5% by weight.

Manganese (Mn), like Si, is an effective element in solid solution strengthening of steel. In one embodiment of the present invention may be included in more than 0.5% by weight to exhibit the effect. However, when the content of Mn exceeds 1.5% by weight, the segregation part is greatly developed at the center of thickness during slab casting in the playing process, thereby reducing the weldability and formability of the final product. Therefore, the content of Mn may include 0.5 to 1.5% by weight.

Phosphorus (P) is an impurity element, and if the content thereof exceeds 0.03% by weight, the weldability may decrease and the risk of brittleness of the steel may increase. Therefore, it is essential to include, but may include less than 0.03% by weight.

Sulfur (S) is inevitably contained in the same way as the phosphorus (P) element, and may combine with Mn to form a non-metallic inclusion. As a result, the toughness of the steel can be greatly reduced, and it is necessary to suppress the content thereof as much as possible. In theory, the sulfur content is advantageously limited to 0% by weight, but inevitably contained in the manufacturing process. Therefore, although it may be included inevitably, sulfur (S) element according to an embodiment of the present invention may include 0.015% by weight or less.

Aluminum (Al) is an element added for deoxidation, and may be added in excess of 0.05% by weight as a deoxidizer in the steelmaking process. However, when the addition amount is too large, clogging of the nozzle may occur during the play, the productivity may be reduced. Therefore, it should be included essentially, but may be included in less than 0.05% by weight.

Nitrogen (N) contributes to the hardness of the steel, but has a disadvantage of difficult control. Therefore, if the content exceeds 0.01% by weight, the risk of brittleness may be increased. Thus, it should be included essentially, but may be included in less than 0.01% by weight.

More specifically, the other unavoidable impurities include Cr: 0.3% by weight or less (does not include 0% by weight), Ni: 0.3% by weight or less (does not contain 0% by weight), Mo: 0.2% by weight or less (0 Weight percent), Cu: 0.1 weight percent or less (not including 0 weight percent). However, the other impurities are not limited thereto, and other conventional impurities may be further included.

More specifically, chromium (Cr) serves to harden steel and retard bainite phase transformation upon cooling, thereby making it easier to obtain ferrite of equiaxed crystals. However, when the content of chromium exceeds 0.3% by weight, the ferrite transformation is excessively delayed, so that the desired ferrite fraction cannot be obtained, and the elongation may be reduced. In addition, the elemental chromium does not include 0% by weight since steel may be inevitably included in the process step.

Nickel (Ni) serves to improve the strength and toughness of the base material at the same time. However, since it is an expensive element, if it exceeds 0.3% by weight, the economic efficiency may be lowered, which may cause a problem of deterioration of weldability. In addition, the nickel element does not include 0% by weight since it may be inevitably included in the process step when manufacturing steel.

Molybdenum (Mo) plays a role in improving the impact toughness by strengthening the yield strength through strengthening the employment and grain boundaries. However, since it is an expensive element, if it exceeds 0.2%, not only the manufacturing cost may be increased but also the weldability may be inferior. In addition, the molybdenum element does not contain 0% by weight since it may be inevitably included in the process step in the steel production.

Copper (Cu) serves to increase the strength by promoting the fine precipitates. However, when it exceeds 0.1 weight%, hot and normal temperature workability may deteriorate. In addition, the copper element does not include 0% by weight since it may be inevitably included in the process step in the steel production.

In addition, the steel sheet satisfying the component and composition range may be composed of pearlite and ferrite microstructure, with respect to the total 100% by microstructure of the steel sheet, the volume fraction of the ferrite structure may be 75% to 85%. . In addition, with respect to 100% by weight of the total microstructure of the steel sheet, the volume fraction of the pearlite structure may be 15 to 25%.

As described above, when the ferrite microstructure fraction is 75 to 85%, the pressure vessel steel sheet may have a yield strength in the range of 295 to 520 MPa. However, when the fraction of the ferrite microstructure is less than 75%, the second phase including pearlite may be inferior to the tensile strength to be described later than 25%. In contrast, when the fraction of the ferrite microstructure exceeds 85%, the yield strength may be inferior to less than 15% of the second phase including pearlite.

In addition, the yield strength is a value that is a standard of strength in the tensile test of the material, and means the stress at the start of plastic deformation. More specifically, the yield strength may be measured by a 0.2% offset method based on the upper yield point when yield point stretching occurs, and when there is no yield point stretching. Hereinafter, the definition and the measuring method of yield strength mentioned in the present specification are the same.

The tensile test for measuring the yield strength was tested using ZWICK Z600 Load cell and BTC-EXMULTI.003 Type, strain rate was 5mm / min.

In addition, the tensile strength of the pressure vessel may be 440 to 560MPa, the volume expansion coefficient of the pressure vessel may be 20 to 50%. This is a physical property required for safety when using the pressure vessel, more specifically, it can provide a pressure vessel excellent in burst resistance when the tensile strength and the volume expansion ratio of the above range.

The tensile strength means the value obtained by dividing the maximum load until the material breaks in the tensile test by the original cross-sectional area of the parallel part of the test piece, and the tensile strength is measured in the same test equipment and test conditions used in the above-described yield strength measurement. It was.

In addition, the volume expansion ratio refers to a percentage after dividing the volume of the container by the volume of the original container by expanding the pressure vessel using water pressure.

In addition, physical properties such as tensile strength and volume expansion rate may be obtained according to heat treatment conditions in a pressure vessel manufacturing method according to an embodiment of the present invention described later.

In addition, an aspect ratio of the ferrite microstructure may be 0.7 to 1.3. An isometric ratio means an aspect ratio which is a ratio of a width | variety and a vertical ratio, and the isometric rate of the said microstructure can be known through the electronic retrospective refraction (EBSD) analysis of the said steel material.

More specifically, 10 times the electronic gray retrospective refraction (EBSD) at a random position at a magnification of 500 times, and using the Grain Shape Aspect Ratio program provided by the TSL OIM Analysis 6.0 software as a standard The average value was taken.

Thus, when the aspect ratio of the ferrite microstructure is 0.7 to 1.3, the material anisotropy may be reduced to increase the elongation. In particular, it may be advantageous to drawability, which is one of two-part LPG gas container manufacturing process. On the other hand, if the aspect ratio of the ferrite microstructure is less than 0.7 or more than 1.3, there is a risk of cracking when forming a gas container due to inferior drawing property, and even if the crack does not occur during drawing, the rupture resistance of the final product is inferior. There is concern.

Pressure vessel manufacturing method according to another embodiment of the present invention, C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 100% by weight 1.5% by weight, P: 0.03% by weight or less (not including 0% by weight), S: 0.015% by weight or less (does not contain 0% by weight), Al: 0.05% by weight or less (does not contain 0% by weight) ), N: 0.01% by weight or less (not including 0% by weight), the remainder being prepared by the slab containing Fe and other unavoidable impurities; Reheating the slab at 1100 to 1300 ° C .; Rough rolling the reheated slab; Finishing rolling the rough rolled slab; Cooling the finished rolled sheet; And winding the cooled sheet to obtain a hot rolled steel sheet. Pickling the obtained hot rolled steel sheet; And forming the pickled steel sheet to obtain a pressure vessel. It may include.

However, molding the pickled steel sheet to obtain a pressure vessel; Thereafter, heat-treating the pressure vessel; It may further include.

More specifically, heat-treating the pressure vessel; Annealing in the temperature range of 600 ℃ to 700 ℃; Or normalizing in a temperature range of 800 ° C. to 900 ° C .; It may include. More specifically, annealing in a temperature range of 600 ° C. to 700 ° C .; May be performed for 10 to 30 minutes, normalizing in a temperature range of 800 ° C. to 900 ° C .; May be performed for 1 to 5 minutes.

More specifically, the annealing is Ac ₃ Heating below the temperature means normalizing means heating above the Ac ₃ temperature. All of the heat treatment methods are intended to sufficiently solve the work hardening occurred during forming the pressure vessel steel sheet.

More specifically, annealing in a temperature range of 600 ° C. to 700 ° C .; In the case of heat treatment below 600 ℃ may not be able to solve the work hardening even if heated for a long time, when the heat treatment exceeds 700 ℃ may not be able to obtain a pressure vessel of the desired strength due to grain growth. In addition, if the heat treatment for less than 10 minutes does not sufficiently solve the stress, it may not be able to obtain a pressure vessel of the desired volume expansion ratio, if the heat treatment for more than 30 minutes the stress is excessively resolved to achieve the desired strength May not be achieved. Therefore, when the heat treatment within the temperature and time range, it is possible to appropriately solve the stress generated by the molding.

In addition, the step of normalizing in the temperature range of 800 ℃ to 900 ℃; By means of this, the ferrite and pearlite mixed structure can be inversely transformed into austenite, and then a pressure vessel of the ferrite and pearlite mixed structure from which stress is removed by cooling can be obtained. At this time, if the heat treatment to less than 800 ℃ does not generate sufficient reverse transformation may have a limit in securing the volume expansion rate, when heat treatment above 900 ℃ may not be able to obtain a steel sheet of the desired strength due to grain growth. . In addition, if the heat treatment for less than 1 minute, the reverse transformation generation time is insufficient, the stress may not be sufficiently resolved, if the heat treatment for more than 5 minutes may not be obtained a steel sheet of the desired strength due to grain growth. Incidentally, the Ac ₃ temperature is disclosed in the Fe-C state diagram of the material engineering textbook, which is obvious to those skilled in the art, and thus a detailed definition thereof will be omitted.

First, C: 0.1-0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5-1.5% by weight, P: 0.03% by weight or less (0% by weight) based on 100% by weight of the total Not including%), S: 0.015% by weight or less (does not contain 0% by weight), Al: 0.05% by weight or less (does not contain 0% by weight), N: 0.01% by weight or less (0% by weight) The balance may prepare a slab containing Fe and other unavoidable impurities.

More specifically, the other unavoidable impurities include: Cr: 0.3% by weight or less (does not include 0% by weight), Ni: 0.3% by weight or less (does not contain 0% by weight), Mo: 0.2% by weight or less (0% by weight) %), Cu: 0.1 wt% or less (not including 0 wt%), but is not limited thereto.

Since the components and the composition range of the slab are the same as the components and the composition range of the hot rolled steel sheet described above, the reason for limiting the components is omitted.

Thereafter, reheating the slab at 1100 to 1300 ℃; Can be carried out. More specifically, the slab may be reheated for 100 to 400 minutes.

When reheating at 1100 ℃ or more as described above, it is possible to ensure the temperature of the slab to reduce the rolling load. However, when reheated at an excessively high temperature, the final microstructure may be coarse due to partial coarsening through abnormal growth of austenite grains. As a result, the grains may not be homogeneous.

Thereafter, rough rolling the reheated slab; Can be carried out.

More specifically, said rough rolling means the intermediate process of rolling performed before finishing rolling, and the meaning of the rough rolling mentioned in this specification is the same.

Thus, rough rolling may be performed such that the thickness of the roughly rolled slab is 10% to 25% of the thickness of the reheated slab. However, the rough rolling finish temperature may be carried out at a sufficiently high temperature so that the finish rolling temperature can be secured later. This can be easily understood by those skilled in the art, the detailed description thereof will be omitted.

Thereafter, finishing rolling the rough rolled slab; Can be carried out.

Finishing rolling the rough rolled slab; In, the finish rolling may be carried out in the temperature range of A ₁ -20 ℃ to A ₁ +20 ℃. The A ₁ is the same as the following formula (1), A ₁ -20 ℃ to A ₁ +20 ℃ temperature range is defined herein as the finish rolling temperature.

A ₁ = 932.1-392.8 [C] -61.9 [Mn] +43.9 [Si] +420.6 [P] +220.0 [Al] -15.5 [Cr] -15.2 [Ni] +31.5 [Mo] -20 [Cu]- --------------------- Formula (1)

However, [C], [Mn], [Si], [P], [Al], [Cr], [Ni], [Mo], and [Cu] mean weight percent of each component content.

In order to obtain a hot-rolled steel sheet excellent in strength and ductility at the same time, it means that it is necessary to control the finish rolling temperature according to the change of the alloying components using the above formula (1). More specifically, when the finish rolling temperature exceeds A ₁ + 20 ° C., the austenitic grains of the slab may be coarsened to lower the yield strength, thereby obtaining a desired (yield strength x elongation) value. It may not be possible. In addition, when the finish rolling temperature is less than A ₁ -20 ℃, ductility may be reduced due to the generation of the mixed structure due to the abnormal reverse rolling, the rolling load during hot rolling may increase significantly and productivity may be lowered. In addition, the mixed structure means that crystal grains having different particle sizes are mixed.

Thus, when finishing hot rolling in the temperature range, the finished rolled slab can obtain a fine austenite structure of 10 to 40 ㎛ of the desired size.

Then cooling the finished rolled sheet; Can be carried out. Cooling the finished rolled sheet; The first cooling step of cooling the finish-rolled sheet at a rate of 40 ℃ / s to 60 ℃ / s; And a second cooling step of cooling the first cooled plate at a rate of 1 ° C./s to 8 ° C./s. It may include.

More specifically, the first step of cooling the finish-rolled plate at a rate of 40 ℃ / s to 60 ℃ / s; Is, A can be cooled to -20 ℃ to ₂ A ₂ + 20 ℃ temperature range. In addition, A ₂ is the same as the following formula (2), A ₂ -20 ℃ to A ₂ +20 ℃ temperature range is defined herein as the intermediate step temperature.

A ₂ = 740.1-35.4 [C] -64.5 [Mn] +29.1 [Si] +16.9 [Cr] -16.9 [Ni] ----------- Equation (2)

(However, [C], [Mn], [Si], [Cr] and [Ni] refer to the weight percent of each component content)

More specifically, the first step of cooling the finish-rolled plate to the temperature range of A ₂ -20 ℃ to A ₂ +20 ℃; By the finish-rolled plate may be cooled to the temperature range of A ₂ -20 ° C to A ₂ +20 ° C at a rate of 40 ° C / s to 60 ° C / s.

Thereafter, the second cooling step of cooling the first cooled plate at a rate of 1 ℃ / s to 8 ℃ / s; By, the primary cooled plate can be cooled for 1 second to 8 seconds.

This means that in order to obtain a hot-rolled steel sheet having excellent strength and ductility at the same time, it is necessary to control the intermediate stage temperature according to the change of the alloying components using the above formula (2).

When manufacturing a hot rolled steel sheet by the conventional continuous cooling method, since the time to reach winding temperature from the finish rolling temperature is very short, sufficient production of an equiaxed ferrite microstructure cannot be expected. As a result, needle-like ferrite is formed and the ductility is inferior.

On the other hand, by controlling the intermediate step temperature range and cooling conditions through the alloy component and the formula (2) according to an embodiment of the present invention, it is possible to promote the formation of equiaxed ferrite to improve the ductility. The equiaxed ferrite refers to a ferrite structure having an aspect ratio of the ferrite microstructure of 0.7 to 1.3, and ductility may be improved when the equiaxed ferrite fraction is increased due to the formation of the structure.

However, when the intermediate stage temperature is excessively high and exceeds A ₂ + 20 ° C., an equiaxed ferrite is formed, but yield strength may be lowered by ferrite grain growth. On the other hand, when the intermediate stage temperature is less than A ₂ -20 ° C, ductility may decrease due to the formation of acicular ferrite. Therefore, the value of (yield strength x elongation) is less than 11,500 MPa ·%, which may not satisfy the range according to one embodiment of the present invention.

In this case, the elongation means a rate at which the material is stretched. More specifically, the elongation measurement can measure the elongation at the time of breaking, if the tensile specimen is broken. In addition, when the tensile test piece is not broken, the elongation can be measured at 80% dropped strength of the tensile strength applied above. The elongation measurement method was measured in the same test equipment and test conditions used in the above-mentioned yield strength measurement, and the yield strength and its measuring method are omitted since they are described above.

Therefore, when controlling and cooling the intermediate step temperature in the A ₂ -20 ℃ to A ₂ +20 ℃ temperature range, it is possible to obtain a plate formed with an equiaxed ferrite structure. In addition, it can have a 75% to 85% equiaxed ferrite structure and 15% to 25% austenite structure to the 100% by weight of the total microstructure of the cooled plate.

Finally winding the cooled sheet to obtain a hot rolled steel sheet; Can be carried out. More specifically, winding the cooled plate to obtain a hot rolled steel sheet; By, the plate can be wound by cooling to a temperature range of A ₃ -20 ℃ to A ₃ +20 ℃. The A ₃ is the same as the following formula (3), the A ₃ -20 ℃ to A ₃ +20 ℃ temperature range is defined herein as the winding temperature.

A ₃ = 693.4-444.5 [C] -80.5 [Mn] -35.0 [Si] -76.0 [Cr] -35.0 [Ni] -85.7 [Mo]-(3)

However, [C], [Mn], [Si], [Cr], [Ni], and [Mo] mean weight percent of each component content.

Winding the cooled sheet to obtain a hot rolled steel sheet; In, the plate may be wound by cooling to a temperature range of A ₃ -20 ° C to A ₃ +20 ° C at a rate of 40 ° C / s to 60 ° C / s.

In addition, it may be carried out for 1 to 50000 minutes. More specifically, the winding may be carried out for 1 minute to several hundred hours. More specifically, the winding may be carried out for 1 minute to several ten hours. After coiling, the coil can be followed by a subsequent operation when the temperature is close to room temperature. As described above, the coiling may take a long time.

In order to obtain a hot-rolled steel sheet excellent in strength and ductility at the same time, it means that the winding temperature according to the change of the alloying components should be controlled as in Formula (3) to form fine pearlite structures.

Thus, when the coiling temperature exceeds A ₃ + 20 ° C., coarse pearlite may be formed to lower the yield strength, and a desired (yield strength x elongation) value may not be obtained. On the other hand, when the coiling temperature is less than A ₃ -20 ℃, ductility may be inferior. More specifically, when the coiling temperature is less than A ₃ -20 ° C, fine pearlite is formed and the yield strength increases, but the ductility is inferior, and the desired (yield strength x elongation) value may not be obtained. Therefore, when the winding temperature is controlled and cooled in the above temperature range, a hot rolled steel sheet having a pearlite structure can be obtained.

2 is a graph showing a cooling pattern after the finish rolling of a hot rolled steel sheet according to an embodiment of the present invention.

More specifically, finish rolling temperature (A ₁ -20 ° C to A ₁ +20 ° C), intermediate stage temperature (A ₂ -20 ° C to A ₂ +20 ° C), and winding temperature (A ₃ -20 ° C to A ₃ + 20 ° C.) is controlled according to the alloy component to show temperature change such as cooling rate in stages. Therefore, the above-described temperature conditions and cooling rate can be confirmed through FIG.

The hot rolled steel sheet manufactured by the above-described method may be composed of pearlite and ferrite microstructure. In addition, with respect to the total percentage of the microstructure 100%, the volume fraction of the ferrite tissue may be 75% to 85%, the volume fraction of the pearlite tissue may be 15% to 25%. At this time, the equiaxed crystal grains of the ferrite grains may be 0.7 to 1.3.

The yield strength of the hot rolled steel sheet having the microstructure may be 295 to 520 MPa. In addition, the (yield strength X elongation) value, which is a combination of yield strength and elongation of the hot rolled steel sheet, may be 11,500 MPa ·% or more.

Thereafter, pickling the hot rolled steel sheet; Can be further performed.

More specifically, pickling the hot rolled steel sheet; Since all pickling processes used by those skilled in the art are applicable, detailed description thereof will be omitted.

In addition, molding the pickled steel sheet to obtain a pressure vessel; Can be carried out. More specifically, the pickled steel sheet may be formed by a method by blanking, drawing, tube, welding, or a combination thereof. However, the present invention is not limited thereto, and any person skilled in the art may form a steel sheet. In addition, since the said method is a well-known steel plate shaping | molding method normally, the detailed description is abbreviate | omitted.

The pressure vessel manufactured by the above-described method may be composed of pearlite and ferrite microstructure. In addition, with respect to the total percentage of the microstructure 100%, the volume fraction of the ferrite tissue may be 75% to 85%, the volume fraction of the pearlite tissue may be 15% to 25%. At this time, the equiaxed crystal grains of the ferrite grains may be 0.7 to 1.3.

In addition, the tensile strength of the pressure vessel produced by the method according to an embodiment of the present invention may be 440 to 650 MPa, the volume expansion ratio may be 20 to 50%. Due to the above characteristics, it is possible to provide a pressure vessel having excellent burst resistance.

Hereinafter, the embodiment will be described in detail. However, the following examples are merely to illustrate the invention, but the content of the present invention is not limited by the following examples.

Example

Steel grade no. C Mn Si P Al S N Cr Ni Mo Cu Inventive Steel 1 0.15 0.70 0.01 0.01 0.025 0.004 0.005 0.015 0.01 0.001 0.01 Inventive Steel 2 0.16 1.00 0.01 0.01 0.035 0.004 0.005 0.015 0.01 0.001 0.01 Comparative Steel 1 0.07 0.80 0.01 0.01 0.025 0.004 0.005 0.015 0.01 0.001 0.01 Comparative Steel 2 0.23 0.80 0.01 0.01 0.035 0.004 0.005 0.015 0.01 0.001 0.01 Comparative Steel 3 0.15 0.40 0.01 0.01 0.025 0.004 0.005 0.015 0.01 0.001 0.01 Comparative Steel 4 0.16 1.70 0.01 0.01 0.035 0.004 0.005 0.015 0.01 0.001 0.01

Steel grade no. Equation 1
(A1) Equation 2
(A2) Equation 3
(A3) Inventive Steel 1 839.4 690.1 568.4 Inventive Steel 2 819.1 670.4 539.8 Comparative Steel 1 864.6 686.5 595.9 Comparative Steel 2 804.0 680.8 524.8 Comparative Steel 3 858.0 709.4 592.6 Comparative Steel 4 775.7 625.3 483.5

Slab components of the inventive steel and the comparative steel prepared by way of example in one embodiment of the present invention are as described in Table 1 and Table 2.

Thereafter, the slabs including the components of Tables 1 and 2 were reheated at 1150 ° C. for 200 minutes.

After rough-rolling the reheated slab, finish rolling was performed at the finish rolling temperature (A ₁ -20 ° C to A ₁ + 20 ° C) shown in Table 3 below.

The finished rolled plate was first cooled at a rate of 50 ° C./s to the intermediate temperature (A ₂ -20 ° C. to A ₂ + 20 ° C.) described in Table 3 below. After reaching the intermediate temperature, it was secondarily cooled at a rate of 5 ° C./s for 5 seconds.

The cooled plate was cooled and wound at a rate of 50 ° C./s to the winding temperature (A ₃ −20 ° C. to A ₃ + 20 ° C.) shown in Table 3 to obtain a hot rolled steel sheet.

Example No. Steel grade Equation 1 (A1) Equation 2 (A2) Equation 3 (A3) Finish rolling
Temperature (℃) Middle steps
Temperature (℃) Winding temperature
(℃) Inventive Example 1 Inventive Steel 1 839.4 690.1 568.4 840 680 560 Inventive Example 2 Inventive Steel 2 819.1 670.4 539.8 830 680 550 Comparative Example 1 Comparative Steel 1 864.6 686.5 595.9 850 690 590 Comparative Example 2 Comparative Steel 2 804.0 680.8 524.8 820 680 540 Comparative Example 3 Comparative Steel 3 858.0 709.4 592.6 850 700 580 Comparative Example 4 Comparative Steel 4 775.7 625.3 483.5 820 690 540 Comparative Example 5 Inventive Steel 1 839.4 690.1 568.4 880 680 560 Comparative Example 6 Inventive Steel 1 839.4 690.1 568.4 840 720 560 Comparative Example 7 Inventive Steel 2 819.1 670.4 539.8 830 620 550 Comparative Example 8 Inventive Steel 2 819.1 670.4 539.8 830 680 610 Comparative Example 9 Inventive Steel 1 839.4 690.1 568.4 840 680 530 Comparative Example 10 Inventive Steel 1 839.4 690.1 568.4 870 continuity 580 Comparative Example 11 Inventive Steel 2 819.1 670.4 539.8 860 continuity 580

Example No. Perlite Microstructure Volume Fraction
(%) ferrite
Microstructure
Grain size
(Μm) Isometric Rate of Ferrite Grains surrender
burglar
(MPa) Seal
burglar
(MPa) Elongation
(%) Yield strength
X
Elongation (MPa%) Inventive Example 1 17 10.8 1.18 350 474 38 13300 Inventive Example 2 24 14.1 0.85 432 568 32 13824 Comparative Example 1 2 8.1 0.91 280 434 39.5 11060 Comparative Example 2 55 9.4 0.76 462 601 20.1 9286 Comparative Example 3 10 9.2 1.21 295 444 37.3 11004 Comparative Example 4 23 9.3 0.65 438 589 24.2 10600 Comparative Example 5 16 12.1 0.77 309 467 35.3 10908 Comparative Example 6 19 11.8 1.67 320 453 34.1 10912 Comparative Example 7 22 7.5 0.56 440 575 25.4 11176 Comparative Example 8 25 15.1 1.01 406 558 27.3 11084 Comparative Example 9 10 10.6 0.77 327 466 33.7 11020 Comparative Example 10 11 10.4 1.98 350 490 30.7 10745 Comparative Example 11 21 13.9 2.11 440 580 23.1 10164

Using a slab having the components and composition ranges of Table 1 and Table 2, hot-rolled steel sheet was manufactured under the conditions of finish rolling temperature, intermediate temperature, and winding temperature of Table 3. As a result, the microstructure, tensile properties and the like of the final hot rolled steel sheet in Table 4 were evaluated and disclosed.

In addition, the tensile strength of Table 4 means the value obtained by dividing the maximum load until the material breaks in the tensile test by the original cross-sectional area of the parallel portion of the test piece, the tensile strength is a test device used to measure the yield strength and elongation described above And the same in the test conditions. Other methods and apparatuses for measuring yield strength and elongation are the same as described above, and thus will be omitted.

More specifically, the comparative steels 1 and 2 of Table 1 and Table 2 were different from the carbon content according to one embodiment of the present invention. More specifically, in Comparative Example 1 using Comparative Steel 1 containing less than 0.1% by weight of the carbon component, it can be confirmed from Table 4 that the pearlite fraction is also very low due to the lack of solid solution strengthening effect by carbon. Therefore, as disclosed in Table 4 above, it can be seen that the value of (yield strength x elongation) is poor.

On the other hand, in Comparative Example 2 using Comparative Steel 2 containing more than 0.2% by weight of carbon component, it can be seen that the elongation is inferior due to excessive generation of pearlite microstructure. Therefore, it was confirmed that the value of (yield strength x elongation) is also inferior.

In addition, Comparative Steels 3 and 4 of Tables 1 and 2 were different from the manganese content according to the embodiment of the present invention. More specifically, in the case of Comparative Steel 3 containing less than 0.5% by weight manganese component, it can be confirmed in Comparative Example 3 of Table 4 that the pearlite fraction is low due to the lack of solid solution strengthening by manganese. When the pearlite fraction is low, the yield strength may be lowered, so it can be seen from Table 4 that the (yield strength x elongation) value of Comparative Example 3 is poor.

On the other hand, in the case of Comparative Example 4 using Comparative Steel 4 containing more than 1.5% by weight manganese component, the actual rolling was impossible because the finish rolling temperature derived from Equation (1) is too low. As a result, it can be confirmed that the desired (yield strength x elongation) value cannot be obtained.

In addition, in the case of the comparative example 5, it turns out that it exceeds the range of the finish rolling temperature derived from Formula (1). More specifically, it can be seen that when exceeding the finish rolling temperature range derived from Equation (1) as in Comparative Example 5, the yield strength (elongation strength x elongation) value is detrimental as the yield strength decreases.

On the other hand, in Comparative Examples 6 and 7, it can be seen that the intermediate step temperature range derived from Equation (2) is outside the range according to one embodiment of the present invention.

Thus, in Comparative Examples 6 and 7, it can be seen that the value of (yield strength x elongation) is thermally deteriorated as the elongation falls.

In addition, in the case of Comparative Examples 8 and 9, it can be seen that the winding temperature range derived from Equation (3) is out of the range according to one embodiment of the present invention.

More specifically, it can be seen that the intensity is inferior in Comparative Example 8 that does not satisfy the winding temperature range derived from Equation (3), whereas in Comparative Example 9, the elongation is inferior. For this reason, it can be seen that Comparative Examples 8 and 9 are inferior in (yield strength x elongation) value.

In the case of Comparative Examples 10 and 11, the case was manufactured by a conventional manufacturing process, and shows the case of continuous cooling from the finish rolling temperature to the coiling temperature without passing through the intermediate step temperature.

As a result, in the case of Comparative Examples 10 and 11, it can be seen that the ductility is inferior because it does not satisfy the range of the ferrite isoaxial constant according to the embodiment of the present invention. In addition, it can be seen that the value of (yield strength x elongation) is inferior.

On the other hand, in the case of Inventive Examples 1 to 2, which satisfies both the hot rolling conditions derived from the composition range and the formula according to an embodiment of the present invention, it can be seen that the combination of yield strength and elongation is excellent.

More specifically, FIG. 1 is a graph showing values derived from yield strength and elongation and (yield strength x elongation) of the invention and comparative examples.

More specifically, the portions indicated by the squares represent the values of Comparative Examples 1 to 11, and the portions indicated by the circles represent the values of Inventive Examples 1 and 2. Therefore, as disclosed in FIG. 1, it can be seen that the invention examples 1 and 2 are excellent in strength and ductility. More specifically, in the case of Inventive Examples 1 and 2, it can be seen that the (yield strength x elongation) value is included in the gray region according to 11500 MPa ·% or more.

In addition, the hot-rolled steel sheet according to Inventive Example 1, Comparative Example 1, and Comparative Example 2 in Table 3 was pickled, and then molded to prepare a pressure vessel.

Thereafter, the pressure vessel obtained above was evaluated by varying the heat treatment temperature and time conditions disclosed in Table 4 below to evaluate the volume expansion ratio and tensile strength of the pressure vessel.

Example No. Steel grade Heat treatment temperature (℃) Heat treatment time (minutes) Volume expansion rate (%) Tensile Strength After Heat Treatment (MPa) Remarks Inventive Example A1 Inventive Example 1 630 13 24.6 522 Annealing Comparative Example A1 Inventive Example 1 570 28 9.8 612 Annealing Comparative Example A2 Inventive Example 1 720 11 34.2 423 Annealing Comparative Example A3 Inventive Example 1 620 5 14.3 579 Annealing Comparative Example A4 Inventive Example 1 640 60 35.2 422 Annealing Comparative Example A5 Comparative Example 1 620 14 25.4 391 Annealing Comparative Example A6 Comparative Example 2 630 12 9.1 575 Annealing Inventive Example A2 Inventive Example 1 870 2 28.6 483 Normalizing Comparative Example A7 Inventive Example 1 780 4 7.8 622 Normalizing Comparative Example A8 Inventive Example 1 930 3 37.1 400 Normalizing Comparative Example A9 Inventive Example 1 860 0.5 10.7 607 Normalizing Comparative Example A10 Inventive Example 1 880 10 36.6 408 Normalizing Comparative Example A11 Comparative Example 1 880 2 29.9 383 Normalizing Comparative Example A12 Comparative Example 2 880 3 13.1 510 Normalizing

More specifically, the volume expansion ratio in Table 5 is expressed as a percentage after dividing the volume of the container by the volume of the original container by inflating the pressure vessel using water pressure. In addition, the tensile strength was measured in the same test equipment and test conditions used in the above-mentioned yield strength and elongation measurement by using the specimen collected from the heat-treated pressure vessel steel plate.

More specifically, in Table 5, Inventive Example A1 and Comparative Examples A1 to A6 were annealed at a temperature range of 600 to 700 ° C. More specifically, in the case of Comparative Example A1 having a low temperature and Comparative Example A3, which lacked time, it can be seen that the volume expansion ratio was less than 20% because the work hardening generated in the molding step was not sufficiently resolved.

On the other hand, in the case of Comparative Example A2 with excessively high temperature and Comparative Example A4 with long heat treatment time, it can be seen that a pressure vessel having a tensile strength of 440 MPa or more was not obtained by softening due to excessive heat treatment.

In addition, in the case of Comparative Examples A5 and A6, it can be confirmed that the heat treatment conditions are included in the range defined in the embodiment of the present invention. However, Comparative Examples A5 and A6 did not satisfy the range defined in the embodiment of the present invention when the steel sheet composition range of the above-described hot rolled steel sheet of Table 3. Through this, it can be seen that not only the heat treatment conditions of the pressure vessel but also the steel sheet having the desired tensile strength and the volume expansion rate cannot be obtained even when the components and composition range of the hot rolled steel sheet are not satisfied.

On the other hand, in the case of Inventive Example A1, which satisfies both the composition range and the heat treatment condition of the component according to the embodiment of the present invention, it can be seen that both the tensile strength and the volume expansion ratio after heat treatment are excellent.

More specifically, in the case of Inventive Example A2 and Comparative Examples A7 to A12, when the normalizing heat treatment is performed, Comparative Example A7 having a lower temperature than the range according to one embodiment of the present invention and Comparative Example A9 which lacked time. In this case, the work hardening occurred in the hot-rolled steel sheet forming step is not sufficiently solved, it can be seen that the volume expansion rate is less than 20%.

On the other hand, in the case of Comparative Example A8 with excessively high temperature and Comparative Example A10 with too long time, it was found that a pressure vessel having a tensile strength of 440 MPa or more was not obtained due to softening phenomenon due to excessive heat treatment.

In addition, in the case of Comparative Examples A11 and A12, it can be confirmed that the heat treatment conditions are included in the range defined in the embodiment of the present invention. However, Comparative Examples A11 and A12 did not satisfy the range defined in the embodiment of the present invention when the steel sheet component range of the above-described hot rolled steel sheet of Table 3. Through this, it can be seen that not only the heat treatment conditions of the pressure vessel but also the steel sheet having the desired tensile strength and the volume expansion rate cannot be obtained even when the components and composition range of the hot rolled steel sheet are not satisfied.

On the other hand, in the case of Inventive Example 2, which satisfies both the composition range and the heat treatment condition according to the embodiment of the present invention, it can be seen that both the tensile strength and the volume expansion coefficient after heat treatment are excellent. Therefore, in order to ensure safety when using a pressure vessel (LPG gas vessel), even after heat treatment, a tensile strength of 440 MPa or more and a volume expansion rate of 20% or more should be ensured. The embodiments according to one embodiment of the present invention satisfy all of them. It can be seen that.

Although embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may implement the present invention in other specific forms without changing the technical spirit or essential features thereof. I can understand that.

Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. .

Claims (19)

delete delete delete C: 0.1 to 0.2% by weight, Si: 0.5% by weight or less (not including 0% by weight), Mn: 0.5 to 1.5% by weight, P: 0.03% by weight or less (0% by weight) Not included), S: 0.015 wt% or less (do not include 0 wt%), Al: 0.05 wt% or less (do not include 0 wt%), N: 0.01 wt% or less (do not include 0 wt%) The remainder) preparing a slab comprising Fe and other unavoidable impurities;
Reheating the slab at 1100 to 1300 ° C .;
Rough rolling the reheated slab;
Finishing rolling the rough rolled slab;
Cooling the finished rolled sheet;
Winding the cooled sheet to obtain a hot rolled steel sheet;
Pickling the obtained hot rolled steel sheet; And
Molding the pickled steel sheet to obtain a pressure vessel; Will be a pressure vessel manufacturing method comprising
Finishing rolling the rough rolled slab; in,
The finish rolling is carried out in the temperature range of A ₁ -20 ℃ to A ₁ +20 ℃,
Cooling the finished rolled sheet; Is,
A first cooling step of cooling the finished rolled sheet at a rate of 40 ° C./s to 60 ° C./s; And a second cooling step of cooling the first cooled plate at a rate of 1 ° C./s to 8 ° C./s. Including,
A first cooling step of cooling the finished rolled sheet at a rate of 40 ° C./s to 60 ° C./s; Is,
A ₂ -20 ℃ to A ₂ +20 ℃ temperature range,
Winding the cooled sheet to obtain a hot rolled steel sheet; By,
The plate is wound by cooling to a temperature range of A ₃ -20 ℃ to A ₃ +20 ℃,
Molding the pickled steel sheet to obtain a pressure vessel; Since the,
Heat-treating the pressure vessel; More,
Heat-treating the pressure vessel; Is,
Annealing at a temperature ranging from 600 ° C. to 700 ° C. for 10 to 30 minutes; or
In the temperature range of 800 ° C. to 900 ° C., normalizing for 1 minute to 5 minutes; Pressure vessel manufacturing method comprising a.
A ₁ = 932.1-392.8 [C] -61.9 [Mn] +43.9 [Si] +420.6 [P] +220.0 [Al] -15.5 [Cr] -15.2 [Ni] +31.5 [Mo] -20 [Cu]- --------------------- Formula (1)
(Wherein [C], [Mn], [Si], [P], [Al], [Cr], [Ni], [Mo] and [Cu] refer to the weight percent of the respective component contents. .)
A ₂ = 740.1-35.4 [C] -64.5 [Mn] +29.1 [Si] +16.9 [Cr] -16.9 [Ni] ----------- Equation (2)
(However, the above [C], [Mn], [Si], [Cr] and [Ni] means the weight percent of each component content.)
A ₃ = 693.4-444.5 [C] -80.5 [Mn] -35.0 [Si] -76.0 [Cr] -35.0 [Ni] -85.7 [Mo]-(3)
(However, [C], [Mn], [Si], [Cr], [Ni] and [Mo] refer to the weight percent of each component content.)
The method of claim 4, wherein
The other unavoidable impurities include Cr: 0.3% by weight or less (does not include 0% by weight), Ni: 0.3% by weight or less (does not contain 0% by weight), Mo: 0.2% by weight or less (does not include 0% by weight) Cu) 0.1% by weight or less (does not contain 0% by weight) comprising a pressure vessel manufacturing method.
The method of claim 5,
Finishing rolling the rough rolled slab; By,
The finished rolled slab is a pressure vessel manufacturing method having an austenitic microstructure of 10 to 40 ㎛ size.
The method of claim 6,
A second cooling step of cooling the first cooled plate at a rate of 1 ° C./s to 8 ° C./s; By,
The first cooled plate is a pressure vessel manufacturing method that is cooled for 1 to 8 seconds.
The method of claim 7, wherein
Cooling the finished rolled sheet; By,
Method for producing a pressure vessel in which an equiaxed ferrite (polygonal ferrite) structure is formed.
The method of claim 8,
Cooling the finished rolled sheet; By,
The plate is a pressure vessel manufacturing method having a 75% to 85% by volume equiaxed ferrite tissue and 15% by volume to 25% by volume of austenite structure relative to 100% by volume of the total microstructure volume.
The method of claim 9,
Winding the cooled sheet to obtain a hot rolled steel sheet; in,
The plate is a pressure vessel manufacturing method which is wound by cooling at a rate of 40 ℃ / s to 60 ℃ / s.
The method of claim 10,
Winding the cooled sheet to obtain a hot rolled steel sheet; By,
Method for producing a pressure vessel in which the pearlite structure is further formed.
The method of claim 11,
Yield strength of the hot rolled steel sheet is 295 to 520MPa pressure vessel manufacturing method.
The method of claim 12,
The (yield strength X elongation) value of the hot rolled steel sheet is 11,500 MPa ·% or more and 17,500 MPa ·% or less of the pressure vessel manufacturing method.
The method of claim 4, wherein
Reheating the slab at 1100 to 1300 ° C .; in,
The slab is a pressure vessel manufacturing method that is reheated for 100 to 400 minutes.
The method of claim 4, wherein
Molding the pickled steel sheet to obtain a pressure vessel; in,
The pickled steel sheet is a pressure vessel manufacturing method that is molded by a method by blanking, drawing, tube, welding, or a combination thereof.
The method according to any one of claims 4 to 15,
The pressure vessel is composed of pearlite and ferrite microstructure,
The volume fraction of the ferrite tissue is 75% to 85% relative to the total percentage of the microstructure, the pressure vessel manufacturing method.
The method according to any one of claims 4 to 15,
The pressure vessel is composed of pearlite and ferrite microstructure,
Pressure vessel manufacturing method of the volume fraction of the pearlite tissue with respect to 100% by weight of the total microstructure is 15% to 25%.
The method according to any one of claims 4 to 15,
The tensile strength of the pressure vessel is a pressure vessel manufacturing method of 440 to 650 MPa.
The method according to any one of claims 4 to 15,
The volume expansion ratio of the pressure vessel is 20 to 50% of the pressure vessel manufacturing method.

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