Low thermal expansion steel with improved low temperature impact toughness and manufacturing method thereof

Abstract

Provided are low thermal expansion steels having a low thermal expansion coefficient and excellent low-temperature impact toughness, and methods for producing the same. [Solution] A low thermal expansion steel with improved low temperature impact toughness includes the steps of producing a slab containing, by weight, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% and 0.5%, Ni: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, with the remainder being Fe and unavoidable impurities, and hot rolling the slab at 1200 to 1350°C, and hot rolling annealing the slab at 800 to 1000°C to produce a hot rolled steel sheet, wherein the value of the following formula (1) of the slab is 112 or less. Formula (1): 15Cr+3Ni+3Co In formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

Description

[Technical Field]

[0001] The present invention relates to a low thermal expansion steel having improved low temperature impact toughness and a manufacturing method thereof, and more particularly to a low thermal expansion steel having improved low temperature impact toughness, which has a low thermal expansion coefficient and excellent low temperature impact toughness, and a manufacturing method thereof. [Background technology]

[0002] In recent years, as the demand for transporting and storing liquefied natural gas has increased, the demand for container materials capable of storing liquids at cryogenic temperatures has increased significantly.

[0003] The cryogenic environment makes the materials more brittle, which can lead to cracks in the storage container material and potentially leaks.

[0004] On the other hand, cracks in materials can occur due to stress caused by thermal expansion of the material as the temperature changes, or due to a decrease in the impact toughness of the material in an extremely low temperature environment.

[0005] Therefore, there is a need to develop materials that have low thermal expansion and excellent low-temperature impact toughness so that cracks do not occur even in extremely low-temperature environments. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Korean Patent Publication No. 10-1995-0032674 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a low thermal expansion steel having a low thermal expansion coefficient and excellent low-temperature impact toughness by optimizing the steel composition and controlling the manufacturing process, and a manufacturing method thereof. [Means for solving the problem]

[0008] The low thermal expansion steel with improved low temperature impact toughness contains, by weight, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% and 0.5%, Ni: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, with the remainder being Fe and unavoidable impurities, and is characterized by having a value of 112 or less of the following formula (1):

[0009] Formula (1): 15Cr+3Ni+3Co

[0010] In formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

[0011] The low thermal expansion steel having improved low-temperature impact toughness may have a value of 56 or more in the following formula (2).

[0012] Formula (2): 3Cr+2Ni-50Mn-2Co

[0013] In formula (2), Cr, Ni, Mn, and Co represent the content (wt %) of each element.

[0014] Low thermal expansion steel with improved low-temperature impact toughness has a thermal expansion coefficient of 1.0 x 10 from room temperature to 100°C. -6 / °C or less.

[0015] Low thermal expansion steel with improved low temperature impact toughness has an impact toughness of 140 J / cm at -196°C. 2 It may be more than that.

[0016] Low thermal expansion steel with improved low temperature impact toughness has a room temperature impact toughness of 300J / cm 2 It may be more than that.

[0017] A method for producing low thermal expansion steel with improved low temperature impact toughness includes the steps of producing a slab containing, by weight, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% and Cr: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, with the remainder being Fe and unavoidable impurities, and hot rolling the slab at 1200 to 1350°C and hot-rolling annealing the slab at 800 to 1000°C to produce a hot-rolled steel sheet, wherein the slab is characterized in that the value of the following formula (1) is 112 or less.

[0018] Formula (1): 15Cr+3Ni+3Co

[0019] In formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

[0020] The slab may have a value of 56 or more in the following formula (2):

[0021] Formula (2): 3Cr+2Ni-50Mn-2Co

[0022] In formula (2), Cr, Ni, Mn, and Co represent the content (wt %) of each element.

[0023] The method may further include cold rolling the hot-rolled steel sheet at a reduction of 50% or more and cold-rolling annealing the hot-rolled steel sheet at 800 to 950°C to produce a cold-rolled steel sheet. [Effects of the Invention]

[0024] According to the present invention, by controlling the alloy components and the manufacturing method, it is possible to provide a low thermal expansion steel having a low thermal expansion coefficient and excellent low-temperature impact toughness, and a manufacturing method thereof. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will now be described in detail with reference to the drawings. The following examples are presented to fully convey the spirit of the disclosed invention to those skilled in the art to which the disclosed invention pertains. The disclosed invention is not limited to the embodiments presented herein and may be embodied in other forms. In the drawings, parts irrelevant to the description may be omitted to clarify the disclosed invention, and the sizes of components may be somewhat exaggerated to facilitate understanding.

[0026] Throughout the specification, when a part is said to "comprise" certain elements, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0027] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0028] The reasons for limiting the alloying element contents in the present invention will be explained below. Unless otherwise specified, the units are % by weight.

[0029] The low thermal expansion steel with improved low temperature impact toughness may contain, by weight, C: more than 0% and not more than 0.04%, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% and not more than 0.5%, Ni: 34% or more and 38% or less, Co: more than 0% and not more than 2.0%, with the remainder being Fe and unavoidable impurities.

[0030] The C (carbon) content may be more than 0% and not more than 0.04%.

[0031] Since C is an element effective in increasing strength, it is preferable to add an appropriate amount for use as a thin plate material. However, since C is an element that deteriorates thermal expansion by forming carbides, it is advantageous to control its content low. On the other hand, attempting to control the C content too low increases the processing cost. Therefore, to ensure price competitiveness while maintaining low thermal expansion properties, the C content can be controlled to more than 0% and 0.04% or less. Preferably, the C content can be 0.003% or more and 0.037% or less.

[0032] The content of Si (silicon) may be 0.1% or more and 0.5% or less.

[0033] Silicon facilitates deoxidation during alloy refining and is an effective element for reducing the O content and controlling inclusions. However, because adding silicon increases thermal expansion, the silicon content has traditionally been controlled to 0.1% or less. The disclosed invention can control the silicon content to 0.1% or more and 0.5% or less, so that thermal expansion can be suppressed while facilitating deoxidation. Preferably, the silicon content can be 0.11% or more and 0.42% or less.

[0034] The Mn (manganese) content may be 0.1% or more and 0.4% or less.

[0035] Mn is an element effective in solid solution strengthening and improving hot workability. In particular, Mn can be used as a deoxidizer together with Si during alloy refining. However, in the past, the Mn content was controlled to 0.1% or less to suppress thermal expansion. In the present invention, by adjusting the Cr, Ni, and Co components, the Mn content can be controlled to 0.1% or more and 0.4% or less so that thermal expansion can be suppressed even when the Mn content is increased. Preferably, the Mn content can be 0.12% or more and 0.34% or less.

[0036] The P (phosphorus) content may be 0.003% or less, or more than 0% and 0.003% or less.

[0037] P is an unavoidable impurity contained in steel, and since it causes intergranular corrosion and impairs hot workability, it is advantageous to control its content low. However, attempting to control the P content too low can result in increased process costs. In consideration of this, the P content may be 0.003% or less. Preferably, the P content may be 0.001% or more and 0.003% or less.

[0038] The content of S (sulfur) may be 0.003% or less, or more than 0% and 0.003% or less.

[0039] S is an unavoidable impurity contained in steel, and is an element that segregates at grain boundaries and causes deterioration of hot workability. In particular, S can cause shape defects or cracks during welding, so it is advantageous to control the S content low. In consideration of this, the upper limit of the S content can be limited to 0.003% or less. Preferably, it can be 0.0004% or more and 0.0022% or less.

[0040] The Cr (chromium) content may be more than 0% and 0.5% or less.

[0041] In the typical stainless steel manufacturing process for low thermal expansion steel, Cr can be introduced during the production of other steel types, making it difficult to remove. However, Cr is an element that increases the thermal expansion coefficient, so it is preferable to control it as low as possible. In the present invention, the Ni content can be adjusted to control the Cr content to more than 0% but not more than 0.5%, so that thermal expansion can be suppressed even when the Cr content is increased.

[0042] The Ni (nickel) content may be 34% or more and 38% or less.

[0043] Ni is an essential element for reducing thermal expansion and ensuring low thermal expansion. Generally, to ensure low thermal expansion, elements other than Ni and Fe must be kept at extremely low contents, which increases process costs. In the present invention, the Ni content is controlled to 34-38% and the Cr and Co components are adjusted to lower the thermal expansion coefficient. Preferably, the Ni content can be controlled to 34.6% or more and 37.1% or less.

[0044] The Co (cobalt) content may be more than 0% and not more than 2.0%.

[0045] Co is generally known to reduce the thermal expansion coefficient when added at 4% or less, but to increase it when added in excess of 4%. Furthermore, increased Co content can reduce cryogenic impact toughness. In the disclosed invention, the Co content is controlled to more than 0% and less than 2.0%, and the Cr and Ni contents are adjusted to lower the thermal expansion coefficient. Preferably, the Co content is more than 0% and less than 0.5%, and more preferably, the Co content is 0.01% or more and less than 0.5%.

[0046] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of the contents of these impurities will be specifically mentioned in this specification.

[0047] Low thermal expansion steel with improved low-temperature impact toughness may have a value of 112 or less in the following formula (1).

[0048] Formula (1): 15Cr+3Ni+3Co

[0049] In the above formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

[0050] The present invention aims to suppress thermal expansion by optimizing the alloy components to lower the thermal expansion coefficient. Therefore, formula (1) was derived by combining alloy components that can affect the thermal expansion coefficient. When the value of formula (1) exceeds 112, the thermal expansion coefficient from room temperature to 100°C is 1.0 × 10 -6 / °C. That is, when the value of formula (1) exceeds 112, the thermal expansion suppression ability may be poor.

[0051] The value of the above formula (1) can be specifically 102 to 112, more specifically 105 to 112, and even more specifically 108 to 112. Within the above range, the low thermal expansion steel with improved low-temperature impact toughness according to the present invention can be more advantageous in achieving excellent low-temperature impact toughness even with a low thermal expansion coefficient value.

[0052] Low thermal expansion steel with improved low-temperature impact toughness may have a value of 56 or more in the following formula (2).

[0053] Formula (2): 3Cr+2Ni-50Mn-2Co

[0054] In the above formula (2), Cr, Ni, Mn, and Co represent the content (wt %) of each element.

[0055] The present invention aims to improve low-temperature impact toughness by adjusting the component system. Therefore, formula (2) was derived by combining alloy components that can affect low-temperature impact toughness. When the value of formula (2) is less than 56, the impact toughness at -196°C is 140 J / cm 2 That is, when the value of formula (2) is less than 56, the low-temperature impact toughness may be inferior.

[0056] The value of the above formula (2) may be specifically 56 to 72.5 or less, 56 to 70, more specifically 56 to 65, and even more specifically 58 to 60. Within the above range, the low thermal expansion steel of the present invention with improved low temperature impact toughness is advantageous in achieving excellent low temperature impact toughness even with a low thermal expansion coefficient, and can further improve the balance between room temperature impact toughness and -196°C impact toughness, thereby realizing even better impact toughness properties.

[0057] Low thermal expansion steel with improved low-temperature impact toughness has a thermal expansion coefficient of 1.0 x 10 from room temperature to 100°C by adjusting the alloy composition and manufacturing method. -6 / °C or less. That is, low thermal expansion steel with improved low temperature impact toughness has little thermal expansion with temperature changes, so it can be used as steel for storing low temperature liquids.

[0058] In addition, low-thermal expansion steel with improved low-temperature impact toughness has an impact toughness of 140 J / cm at -196°C. 2 The room temperature impact toughness is 300J / cm or more. 2 That is, low-thermal expansion steel with improved low-temperature impact toughness is less likely to crack even in an extremely low-temperature environment, and can therefore be used as steel for storing cryogenic liquids.

[0059] Next, a method for producing the low thermal expansion steel having improved low temperature impact toughness according to the present invention will be described.

[0060] A method for producing a low thermal expansion steel having improved low temperature impact toughness includes the steps of producing a slab containing, by weight, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% and Cr: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, with the remainder being Fe and unavoidable impurities, and hot rolling the slab at 1200 to 1350°C, and hot-rolling annealing the slab at 800 to 1000°C to produce a hot-rolled steel sheet, wherein the slab may have a value of the following formula (1) of 112 or less:

[0061] Formula (1): 15Cr+3Ni+3Co

[0062] In formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

[0063] The slab may have a value of 56 or more in the following formula (2):

[0064] Formula (2): 3Cr+2Ni-50Mn-2Co

[0065] In formula (2), Cr, Ni, Mn, and Co represent the content (wt %) of each element.

[0066] The component ranges of each alloy composition and the reasons for limiting the values ​​of formulas (1) and (2) have been described above, and each manufacturing step will be described in more detail below.

[0067] After producing a slab satisfying the above alloy composition, formula (1) and formula (2), it can be subjected to hot rolling, hot rolling annealing, and, if necessary, cold rolling and cold rolling annealing steps.

[0068] First, the slab is hot rolled at 1200 to 1350°C, and then hot rolled and annealed at 800 to 1000°C to produce a hot rolled steel sheet.

[0069] If the hot rolling temperature is low, it may be difficult to redissolve the coarse precipitates formed during slab production, but if the hot rolling temperature is too high, the internal grains may become too coarse.

[0070] If the hot rolling annealing temperature is low, segregation formed during casting may remain, resulting in poor elongation, whereas if the hot rolling annealing temperature is too high, grain coarsening may occur, resulting in reduced strength.

[0071] Next, if necessary, the method may further include a step of cold rolling the hot-rolled steel sheet at a reduction of 50% or more and cold-rolling annealing the hot-rolled steel sheet at 800 to 950°C to manufacture a cold-rolled steel sheet.

[0072] If the rolling reduction is less than 50%, recrystallization may decrease during the rolling annealing stage, resulting in coarse crystal grains.

[0073] If the cold rolling annealing temperature is too low, recrystallization may be insufficient, resulting in a low elongation ratio.However, if the cold rolling annealing temperature is too high, the crystal grains may become coarse, and the oxides formed at the grain boundaries may become deep, which may result in poor surface quality after pickling.

[0074] The present invention will be described in more detail below through examples. However, the description of these examples is intended to illustrate the implementation of the present invention, and the present invention is not limited by the description of these examples. The scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom. [Example]

[0075] Slabs were produced in a vacuum induction melting furnace for various alloy composition ranges shown in Table 1 below. The produced slabs were hot rolled at 1250°C and hot rolled and annealed at 900°C to produce hot rolled steel sheets. The hot rolled steel sheets were cold rolled at a reduction of 60% and cold rolled and annealed at 900°C to produce test specimens.

[0076] [Table 1]

[0077] Table 2 below shows the values ​​of formula (1), formula (2), thermal expansion coefficient, room temperature impact toughness, and impact toughness at -196°C. The values ​​of formula (1) were calculated using the following formula (1): Formula (1): 15Cr + 3Ni + 3Co

[0078] In formula (1), Cr, Ni, and Co represent the content (wt %) of each element.

[0079] The value of formula (2) was calculated using the following formula (2).

[0080] Formula (2): 3Cr+2Ni-50Mn-2Co

[0081] In formula (2), Cr, Ni, Mn, and Co represent the content (wt %) of each element.

[0082] The thermal expansion coefficient was measured using a dilatometer. First, the change in length of the test piece due to the temperature change was measured when the temperature was raised from room temperature to 120°C at a rate of 1°C / s. Next, the linear thermal expansion coefficient at temperatures between 25 and 100°C was calculated using the following equation (3).

[0083] Formula (3):

[0084] In equation (3), α m is the thermal expansion coefficient, L0 is the initial length, ΔL is the change in length, and ΔT is the change in temperature.

[0085] The room temperature impact toughness and -196°C impact toughness were measured at low temperatures of 25°C and -196°C using an impact tester manufactured by Zwick Roell. On the other hand, the -196°C impact toughness was evaluated by immersing the test piece in liquid nitrogen for 5 minutes.

[0086] [Table 2]

[0087] As shown in Table 2, Examples 1 to 12 satisfied the alloy components, values ​​of formula (1), values ​​of formula (2), and manufacturing methods proposed in the present invention. Therefore, Examples 1 to 12 had a thermal expansion coefficient of 1.0×10 from room temperature to 100°C. -6 / ℃ or less, and impact toughness at -196℃ is 140J / cm 2 and room temperature impact toughness 300J / cm 2 The above conditions were satisfied. That is, Examples 1 to 12 had little thermal expansion and excellent low-temperature impact toughness. However, Comparative Examples 1 to 6 did not satisfy the requirement that the value of formula (2) be 56 or more. Therefore, Comparative Examples 1 to 6 had an impact toughness of 140 J / cm at -196°C. 2 The above requirements could not be satisfied. That is, Comparative Examples 1 to 6 were inferior in low-temperature impact toughness.

[0088] Comparative Examples 5 to 12 did not satisfy the requirement that the value of formula (1) be 112 or less. Therefore, Comparative Examples 5 to 12 had a thermal expansion coefficient of 1.0×10 -6 / ° C. or less was not satisfied. That is, in Comparative Examples 5 to 12, thermal expansion occurred relatively frequently.

[0089] According to the present invention, it is possible to provide a low thermal expansion steel having a low thermal expansion coefficient and excellent low-temperature impact toughness by controlling the alloy components and manufacturing method, and a manufacturing method thereof.

Claims

1. The steel sheet contains, in weight percent, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% or less, Ni: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, and the remainder being Fe and unavoidable impurities, A low thermal expansion steel having improved low-temperature impact toughness, characterized in that the value of the following formula (1) is 112 or less: Formula (1): 15Cr+3Ni+3Co (In formula (1), Cr, Ni, and Co represent the content (wt%) of each element.)

2. 2. The low thermal expansion steel having improved low-temperature impact toughness according to claim 1, wherein the value of the following formula (2) is 56 or more: Formula (2): 3Cr+2Ni-50Mn-2Co (In formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of each element.)

3. The thermal expansion coefficient from room temperature to 100°C is 1.0 x 10 －６ 2. The low thermal expansion steel having improved low temperature impact toughness according to claim 1, wherein the low thermal expansion strength is 1 / °C or less.

4. Impact toughness at -196°C is 140 J / cm ２ 2. The low thermal expansion steel having improved low temperature impact toughness according to claim 1, wherein:

5. Room temperature impact toughness: 300 J / cm ２ 2. The low thermal expansion steel having improved low temperature impact toughness according to claim 1, wherein:

6. producing a slab containing, by weight, C: more than 0% and 0.04% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.1% or more and 0.4% or less, P: 0.003% or less, S: 0.003% or less, Cr: more than 0.5% or less, Ni: 34% or more and 38% or less, Co: more than 0% and 2.0% or less, with the remainder being Fe and unavoidable impurities; hot rolling the slab at 1200 to 1350°C and hot rolling annealing the slab at 800 to 1000°C to produce a hot-rolled steel sheet; The slab is A method for producing low thermal expansion steel with improved low-temperature impact toughness, characterized in that the value of the following formula (1) is 112 or less: Formula (1): 15Cr+3Ni+3Co (In formula (1), Cr, Ni, and Co represent the content (wt%) of each element.)

7. The slab is 7. The method for producing a low thermal expansion steel having improved low-temperature impact toughness according to claim 6, wherein the value of the following formula (2) is 56 or more: Formula (2): 3Cr+2Ni-50Mn-2Co (In formula (2), Cr, Ni, Mn, and Co represent the content (wt%) of each element.)

8. 7. The method for manufacturing a low thermal expansion steel having improved low temperature impact toughness according to claim 6, further comprising cold rolling the hot rolled steel sheet at a reduction of 50% or more and cold rolling annealing the hot rolled steel sheet at a temperature of 800 to 950°C to manufacture a cold rolled steel sheet.

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