JP6764510B2 - Low temperature steel sheet with excellent surface processing quality and its manufacturing method - Google Patents

Description

The present invention relates to a low temperature steel sheet having excellent surface processing quality and a method for producing the same.

Steel materials used for storage containers such as liquefied natural gas and liquid nitrogen, marine structures and polar regions structures must be low temperature steel plates that maintain sufficient toughness and strength even at extremely low temperatures. Such a low temperature steel sheet must not only have excellent low temperature toughness and strength, but also have a low coefficient of thermal expansion and thermal conductivity.
Recently, as a steel plate for low temperature, instead of completely eliminating nickel, a large amount of manganese and carbon are added to stabilize austenite, and aluminum is added to make a steel material having excellent extremely low temperature characteristics (Patent Document 1). ), And a steel material having excellent low temperature toughness to obtain a mixed structure of austenite and epsilon martensite by adding manganese (Patent Document 2) has been reported.
In low temperature steel sheets with austenite as the main structure, a large amount of carbon and manganese are added to stabilize austenite, which affects the recrystallization behavior of austenite and partially recrystallizes in the normal rolling temperature section. And non-uniform crystal grain growth causes only a specific small number of austenite crystal grains to grow excessively, resulting in significantly non-uniform size of austenite crystal grains in the microstructure.

Normally, in the case of an austenite structure with a high carbon and manganese content, the deformation behavior is performed by slip and twin, unlike general carbon steel, and is mainly performed by slip, which is a uniform deformation at the initial stage of deformation. After that, twins, which are non-uniform deformations, appear. The larger the crystal grains, the less stress is required to form twins, and the smaller the deformation, the easier it is for twins to occur. If a small number of coarse crystal grains are present in the microstructure, twin deformation occurs in the coarse crystal grains at the initial stage of deformation, causing non-uniform deformation, resulting in inferior surface characteristics of the material and inferior thickness of the final structure. It brings about a big problem in the design and use of the structure, especially in the structure which brings the uniformity and requires the internal pressure resistance which secured the uniform thickness of the steel material such as a low temperature pressure vessel.
Therefore, in the case of a steel material whose microstructure is austenitized by adding carbon and manganese, the surface unevenness due to early twinning deformation of coarse crystal grains is solved, and economical and structural stability is ensured at a low cost and extremely low temperature. The demand for the development of steel materials is urgent.

Korean Patent Application Laid-Open No. 1991-0012277 Japanese Patent Application Laid-Open No. 2007-126715

An object of the present invention is to provide a low temperature steel sheet having excellent surface processing quality and a method for producing the same.

The low temperature steel plate having excellent surface processing quality of the present invention has copper (Mn) in a range satisfying 15 to 35% by mass, carbon (C): 23.6C + Mn ≧ 28, and 33.5C—Mn ≦ 23. Cu): 5% by mass or less (excluding 0% by mass), chromium (Cr): 28.5C + 4.4Cr ≦ 57 (excluding 0% by mass), Ti (titanium): 0.01 to 0 .5% by mass, N (nitrogen): 0.003 to 0.2% by mass, consisting of the remaining iron (Fe), and other unavoidable impurities.
Ti and N satisfy the following relational expression 1 and contain 2.7 × 10 ^{8 to} 5.4 × 10 ⁸ TiN precipitates having a size of 0.012 to 0.025 μm per 1 mm ² .
The microstructure contains austenite in an area fraction of 95% or more, and the number of crystal grains of austenite having a size of 200 μm or more in the microstructure is 5 or less per unit cm ² .

Further, the method for producing a low-temperature steel plate having excellent surface processing quality of the present invention comprises manganese (Mn): 15 to 35% by mass, carbon (C): 23.6C + Mn ≧ 28, and 33.5C-Mn ≦ 23. Satisfied range, copper (Cu): 5% by mass or less (excluding 0% by mass), chromium (Cr): 28.5C + 4.4Cr ≤ 57 (excluding 0% by mass), Ti (titanium): Consists of 0.01-0.5% by weight, N (nitrogen): 0.003-0.2% by weight, remaining iron (Fe), and other unavoidable impurities.
The Ti and N are at the stage of preparing a slab satisfying the following relational expression 1 and
The stage of heating the slab at a temperature of 1050 to 1250 ° C.
Including the stage of hot-rolling a heated slab to produce a hot-rolled steel sheet,
The hot-rolled steel sheet contains 2.7 × 10 ^{8 to} 5.4 × 10 ⁸ TiN precipitates having a size of 0.012 to 0.025 μm per 1 mm ² .
The hot-rolled steel sheet contains austenite as a fine structure in an area fraction of 95% or more.
It is characterized in that the number of crystal grains of austenite having a size of 200 μm or more in the microstructure of the steel sheet is 5 or less per unit cm ² .

[Relationship formula 1]
1.0 ≤ Ti / N ≤ 4.5
(However, Mn, C, Cr, Ti, and N in each formula mean mass% of each component content.)
Furthermore, the means for solving the problem does not list all the features of the present invention. The various features of the present invention and their advantages and effects can be understood in more detail through the following specific embodiments.

According to the present invention, since it has an austenite structure having a uniform particle size, it is possible to provide a low-temperature steel plate having excellent surface quality even after processing and a method for producing the same.

It is a photograph of an optical microscope showing the fine structure of a conventional low-temperature steel sheet. It is a photograph which showed the cross section of the specimen after tensioning of the conventional low temperature steel plate. It is a photograph of an optical microscope showing the fine structure of a low temperature steel sheet according to an embodiment of the present invention. It is a graph which shows the range of carbon and manganese controlled by this invention.

The inventors of the present application have found that austenite structure containing a large amount of carbon and manganese causes partial recrystallization and crystal grain growth of the austenite structure in a normal rolling temperature region, resulting in non-ideally coarse austenite. In general, the critical stress required for twinning is higher than that for slipping, but when the crystal grains are large due to the above reasons, the stress required for twinning decreases and the initial deformation We recognized that there is a problem that twinning deformation occurs in the surface and the surface quality deteriorates due to the discontinuous deformation, and we have been diligently researching to solve these problems.
As a result, it was confirmed that a low-temperature steel material in which fine austenite is uniformly distributed can be obtained by appropriately precipitating Ti-based precipitates by adding Ti in order to suppress excessive coarsening of austenite crystal grains. , The present invention has been completed.
Hereinafter, a low-temperature steel plate having excellent surface quality according to one aspect of the present invention will be described in detail.

The low-temperature steel plate having excellent surface quality according to the present invention is copper (Cu) in a range satisfying manganese (Mn): 15 to 35% by mass, carbon (C): 23.6C + Mn ≧ 28, and 33.5C−Mn ≦ 23. ): 5% by mass or less (excluding 0% by mass), chromium (Cr): 28.5C + 4.4Cr ≤ 57 (excluding 0% by mass), Ti (titanium): 0.01 to 0. It is composed of 5% by mass, N (nitrogen): 0.003 to 0.2% by mass, the remaining iron (Fe), and other unavoidable impurities, and Ti and N satisfy the following relational expression 1.
[Relationship formula 1]
1.0 ≤ Ti / N ≤ 4.5
(However, Mn, C, Cr, Ti and N in each formula mean mass% of each component content.)
First, the alloy composition of the low-temperature steel sheet having excellent surface quality according to one aspect of the present invention will be described in detail. Hereinafter, the unit of each alloy element is mass%.

Manganese (Mn): 15-35%
Manganese is an element that plays a role in stabilizing austenite in the present invention. In the present invention, in order to stabilize the austenite phase at extremely low temperatures, it is preferably contained in an amount of 15% or more. That is, when the manganese content is less than 15% and the carbon content is small, epsilon martensite, which is a metastable phase, is formed and easily transformed into alpha martensite by processing-induced transformation at an extremely low temperature, thus ensuring toughness. If an attempt is made to stabilize austenite by increasing the carbon content in order to prevent this, carbides are deposited and the physical properties are rapidly deteriorated, which is not preferable. Therefore, the manganese content is preferably 15% or more. On the other hand, if the manganese content exceeds 35%, there is a problem that the corrosion rate of the steel material is lowered and the economic efficiency is lowered due to the increase in the content. Therefore, the manganese content is preferably limited to 15-35%.

Carbon (C): Range that satisfies the relationship of 23.6C + Mn ≧ 28 and 33.5C-Mn ≦ 23 Carbon is an element that increases the strength while stabilizing austenite, especially from austenite by the cooling process or processing. It plays a role in lowering Ms and Md, which are transformation points to epsilon or alpha maltensite. Therefore, if the amount of carbon added is insufficient, the stability of austenite becomes insufficient, so that stable austenite cannot be obtained at extremely low temperatures, and processing-induced transformation to epsilon or alpha martensite occurs due to external stress. Since it is easy, the toughness is reduced, and the strength of the steel material is also reduced. On the other hand, if the carbon content is too high, carbides are precipitated and the toughness is rapidly deteriorated, the strength is increased too much, and the workability is deteriorated.
In particular, in the present invention, it is preferable to determine the carbon content while paying attention to the relationship between carbon and other elements added together. The relationship between carbon and manganese for carbide formation found by the inventors of the present application is shown in FIG. As can be seen from FIG. 4, it is clear that carbides are formed by carbon, but carbon does not independently affect the formation of carbides, but acts in combination with manganese to affect its formation tendency. To exert.

FIG. 4 shows the proper carbon content. In order to prevent the formation of carbides, 23.6C + Mn (where C and Mn indicate the content of each component in mass% units) on the premise that other components satisfy the range specified in the present invention. It is preferable to control the value of) to 28 or more. This means the sloping left boundary of the parallelogram region of the drawing. When 23.6C + Mn is less than 28, the stability of austenite decreases and the impact toughness decreases because the process-induced transformation occurs due to the impact at extremely low temperature. If the carbon content is too high, that is, if 33.5C-Mn is larger than 23, carbides are precipitated due to excessive addition of carbon, which causes a problem of lowering low temperature impact toughness. In conclusion, it is preferable to add C so as to satisfy all of Mn: 15-35%, 23.6C + Mn ≧ 28, and 33.5C—Mn ≦ 23. Further, as can be seen from the drawings, the lower limit of the C content within the range satisfying the above formula is 0%.

Copper (Cu): 5% or less (excluding 0%)
Copper has a very low solid solubility in charcoal and diffuses slowly in austenite, so it is concentrated at the interface between austenite and nucleated charcoal, which prevents charcoal growth by interfering with carbon diffusion. It can be effectively delayed, and as a result, has the effect of suppressing charcoal formation. Copper also has the effect of stabilizing austenite and improving cryogenic toughness. However, if the Cu content exceeds 5%, there is a problem that the hot workability of the steel material is lowered, so it is preferable to limit the upper limit to 5%. Further, in order to obtain the above-mentioned effect of suppressing carbides, the copper content is more preferably 0.5% or more.

Chromium (Cr): 28.5C + 4.4Cr ≦ 57 (excluding 0%)
Chromium plays a role in stabilizing austenite to improve impact toughness at low temperatures and solidifying in austenite to increase the strength of steel materials up to an appropriate addition range. Chromium is also an element that improves the corrosion resistance of steel materials. However, chromium is a carbide element, and in particular, it is also an element that forms carbides at the austenite grain boundaries to reduce the impact at low temperatures. Therefore, it is preferable to determine the content of chromium added in the present invention while paying attention to the relationship between carbon and other elements added together. In order to prevent the formation of charcoal, 28.5C + 4.4Cr (where C and Cr indicate the content of each component in mass% units) on the premise that other components satisfy the range specified in the present invention. It is preferable to control the value of) to 57 or less. When the value of 28.5C + 4.4Cr exceeds 57, it becomes difficult to effectively suppress the formation of carbides at the austenite grain boundary due to the excessive chromium and carbon content, and the impact toughness at low temperature is reduced accordingly. There is a problem. Therefore, in the present invention, it is preferable to add the chromium content so as to satisfy 28.5C + 4.4Cr ≦ 57.

Ti (titanium): 0.01-0.5%
Titanium (Ti) is an element that combines with nitrogen (N) in steel to form TiN precipitates. In the present invention, excessive coarsening of some austenite crystal grains may occur during high-temperature hot rolling. Therefore, proper precipitation of TiN can suppress the growth of austenite crystal grains. For this purpose, Ti needs to be added at least 0.01% or more. However, if the content exceeds 0.5%, not only the effect is saturated, but also the effect is halved due to the crystallization of coarse TiN, which is not preferable. Therefore, in the present invention, it is preferable to limit the Ti content to 0.01 to 0.5%.

N (nitrogen): 0.003 to 0.2% by mass
In the present invention, nitrogen (N) needs to be added at the same time so that the above-mentioned purpose of adding Ti can be effectively achieved. In particular, for effective precipitation of TiN, it is preferable to add 0.003% or more of N, but since the solid solubility of N is 0.2% or less, it is very difficult to add more. Furthermore, since it is difficult to add 0.2% or less for the precipitation of TiN, the upper limit thereof is preferably limited to 0.2%. In the present invention, the content of N is preferably limited to 0.003 to 0.2%.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities are inevitably mixed from the raw material or the surrounding environment, and therefore this cannot be excluded. Since these impurities are well known to engineers in the normal manufacturing process, all the contents are not specifically mentioned in the present specification.

Further, the mass ratio of Ti to N, that is, Ti / N preferably satisfies the following relational expression 1.
[Relationship formula 1]
1.0 ≤ Ti / N ≤ 4.5
When the Ti / N ratio is controlled to 1.0 or more, the solid solution Ti is combined with nitrogen to precipitate fine TiN, and the TiN precipitated in this way is stably present. , It is very effective in suppressing the grain growth of austenite.
However, if the Ti / N ratio exceeds 4.5, coarse TiN crystallizes in the molten steel, which adversely affects the physical properties of the steel material, makes it impossible to obtain a uniform distribution of TiN, and further precipitates as TiN. Excess Ti is present in a solid solution state, which adversely affects the toughness of the weld heat affected zone. However, if the Ti / N ratio is less than 1.0, the amount of solid solution nitrogen in the base metal increases, which adversely affects the toughness of the weld heat affected zone, so the Ti / N ratio is 1.0. It is preferable to control the above to 4.5 or less.

Further, the low temperature steel sheet according to the present invention preferably contains a TiN precipitate having a size of 0.01 to 0.3 μm.
If the size of the TiN precipitate is less than 0.01 μm, it is likely to be re-solidified in the base metal, and the effect of suppressing crystal grain growth is not sufficient. On the other hand, when the size of the TiN precipitate exceeds 0.3 μm, the pinning effect of the austenite grain boundaries is reduced, and the toughness is adversely affected due to the coarse size. Therefore, the size of the TiN precipitate is preferably 0.01 to 0.3 μm.

Also, low-temperature steel sheet according to the present invention preferably contains 1.0 × ¹⁰ 7 ~1.0 × ^{10 10} per 1 mm ² of TiN precipitates.
When TiN precipitates is ² per 1.0 × 10 below ⁷ 1 mm, the pinning effect of crystal grain boundaries is insignificant, it can not be effectively suppressed the growth of coarse crystal grains. On the other hand, when the number of TiN precipitates exceeds 1.0 × 10 ¹⁰ per 1 mm ² , the size of the precipitates becomes relatively small and unstable, and the impact toughness of the material becomes inferior. .. Therefore, the number of TiN precipitates is preferably 1.0 × 10 ^{7 to} 1.0 × 10 ¹⁰ per 1 mm ² .

Further, in the low temperature steel plate according to the present invention, the number of coarse austenite crystal grains having a size of 200 μm or more in the microstructure is limited to 5 or less per 1 cm ² .
Normally, austenite having a grain size of less than 200 μm has a sufficiently large twinning stress as compared with the slip-generating stress, and is not within the range of the deformation rate of ordinary low-temperature steel materials when manufacturing structures. The size is preferably limited to 200 μm or more so as not to cause uniform deformation. Further, when the density of crystal grains having a size of 200 μm or more exceeds 5 per cm ² , the high density of coarse crystal grains deteriorates to the extent that non-uniform deformation affects the surface quality, so the size is 200 μm or more. It is preferable to limit the density of crystal grains having a grain to 5 or less per cm ² .

On the other hand, the low temperature steel sheet according to the present invention preferably contains an austenite structure in an area fraction of 95% or more. Austenite, which is a typical soft structure showing ductile fracture even at low temperature, must contain 95% or more in area fraction as an essential microstructure for ensuring low temperature toughness, and less than 95% has sufficient low temperature toughness. That is, since it is not sufficient to secure an impact toughness of 41 J or more at -196 ° C., the lower limit thereof is preferably limited to 95%.
The carbides present at the austenite grain boundaries are preferably 5% or less in terms of surface integral. In the present invention, carbide is a typical structure that can exist other than austenite, but this is because it precipitates at the austenite grain boundaries and causes grain boundary fracture, resulting in inferior low temperature toughness and ductility. The upper limit is preferably limited to 5%.

Hereinafter, the method for producing a low-temperature steel sheet having excellent surface processing quality of the present invention will be described in detail.
The method for producing a low-temperature steel sheet having excellent surface processing quality of the present invention includes a step of preparing a slab satisfying the above-mentioned alloy composition, a step of heating the slab at a temperature of 1050 to 1250 ° C., and a step of heating the heated slab. Includes a step of rolling between to obtain a hot-rolled steel sheet.

Steps to prepare slabs Prepare slabs that satisfy the alloy composition described above. The reason for controlling the alloy composition is also as described above.

Heating Stage The slab is heated at a temperature of 1050 to 1250 ° C.
This is for the casting structure, segregation and solid solution and homogenization of the secondary phase produced during the slab production stage. Below 1050 ° C, the homogenization is insufficient or the temperature of the heating furnace is low. Therefore, there is a problem that the deformation resistance becomes large during hot rolling, and if the temperature exceeds 1250 ° C., partial melting and deterioration of surface quality occur in the segregation zone in the cast structure, TiN crystallizes, and austenite becomes finer. It cannot contribute and may lead to deterioration of physical properties. Therefore, the heating temperature of the slab preferably has a range of 1050 to 1250 ° C.

Hot-rolling stage The heated slab is hot-rolled to obtain a hot-rolled steel sheet.
In the present invention, a low-temperature steel sheet having excellent surface processing quality can be obtained by satisfying the above-mentioned alloy composition and heating temperature of the slab. Therefore, it is not particularly necessary to control the conditions of the hot rolling stage, and hot rolling can be performed by a general method.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it should be noted that the examples described later are examples for explaining the present invention in more detail, and are not for limiting the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by the matters described in the claims and the matters reasonably inferred from the matters.
After producing a slab satisfying the component system shown in Table 1 under the conditions shown in Table 2, the microstructure, yield strength, elongation, Charpy impact toughness at -196 ° C., etc. were measured, and each was measured in Table 2 or It is shown in Table 3.
In Table 3, the surface non-uniformity (surface unevenness) was evaluated by visually observing the surface of the low temperature steel plate.

Examples 1 to 5 are steel grades satisfying the component system and composition range controlled by the present invention, and the density of coarse austenite crystal grains is controlled to 5 or less per 1 cm ² unit area by fine precipitation of TiN. It was found that an excellent quality low-temperature steel material with no surface unevenness can be obtained after processing, and the austenite fraction in the microstructure is controlled to 95% or more, and the carbide is controlled to less than 5%. Since austenite is obtained, it is shown that excellent toughness can be obtained at extremely low temperatures.
On the other hand, in Comparative Examples 1 to 3, it was found that TiN could not be precipitated because Ti was not added, coarse crystal grains were generated, and surface unevenness occurred after processing.
In particular, Comparative Example 4 did not satisfy the component system and composition range controlled by the present invention, and it was found that the impact toughness was extremely inferior due to the formation of ferrite. Further, it was found that the size and number of TiNs controlled by the present invention were not satisfied, the number of coarse crystal grains increased, and surface unevenness occurred.
On the other hand, in Comparative Examples 5 to 6, Ti and N are added within the range controlled by the present invention, but the range in which the mass ratio of Ti and N and the size and number of TiN precipitates are controlled by the present invention is defined. It was found that it was not satisfied and coarse TiN was precipitated to excessively generate coarse crystal grains, and surface unevenness occurred after processing.

FIG. 1 is a photograph of the fine structure of a conventional steel material in which austenite crystal grains are coarsened to form non-ideal coarse crystal grains, and FIG. 2 is a photograph of the surface of the steel material after tensioning the steel material of FIG. It is a photograph in which non-uniformity occurred. As described above, when the austenite crystal grains in the fine structure of the steel material are coarsened to form non-ideal coarse crystal grains, it can be confirmed that the surface quality is deteriorated as shown in FIG. 2 after processing. However, in FIG. 3 in which the microstructure in the example of the invention is photographed, since uniform crystal grains without non-ideal coarse austenite crystal grains are formed, the surface processing quality may be excellent even after processing. You can check.

Although the above description has been made with reference to Examples, skilled artisans in the art of the present invention can use the present invention in various ways within the scope of the idea and domain of the present invention described in the claims below. You can see that it can be modified and changed.

Claims (4)

Manganese (Mn): 21.7 to 28.6% by mass, carbon (C): 23.6C + Mn ≧ 28, and 33.5C-Mn ≦ 23, and 0.39 to 1.1% by mass. , Copper (Cu): 5% by mass or less (excluding 0% by mass), Chromium (Cr): 28.5C + 4.4Cr ≦ 57 (excluding 0% by mass), and 0.55 to 3.45 From the range satisfying% by mass , Ti (titanium): 0.01 to 0.5% by mass, N (nitrogen): 0.003 to 0.2% by mass, remaining iron (Fe), and other unavoidable impurities. Become
The Ti and the N satisfy the following relational expression 1 and contain 2.7 × 10 ^{8 to} 5.4 × 10 ⁸ TiN precipitates having a size of 0.012 to 0.025 μm per 1 mm ² .
Surface processing characterized by containing 95% or more of austenite as a microstructure in an area fraction and having 5 or less crystal grains of austenite having a size of 200 μm or more in the microstructure per unit cm ^2. High quality low temperature steel sheet.
[Relationship formula 1]
1.13 ≤ Ti / N ≤ 2.38
(However, Mn, C, Cr, Ti, and N in each formula mean mass% of each component content.) The low-temperature steel sheet having excellent surface processing quality according to claim 1, wherein the carbides present at the grain boundaries of austenite are 5% or less in terms of surface integral. The low-temperature steel sheet having excellent surface processing quality according to claim 1, wherein the impact toughness of the steel sheet is 41 J or more at -196 ° C. Manganese (Mn): 21.7 to 28.6% by mass, carbon (C): 23.6C + Mn ≧ 28, and 33.5C-Mn ≦ 23, and 0.39 to 1.1% by mass. , Copper (Cu): 5% by mass or less (excluding 0% by mass), Chromium (Cr): 28.5C + 4.4Cr ≦ 57 (excluding 0% by mass), and 0.55 to 3.45 From the range satisfying% by mass , Ti (titanium): 0.01 to 0.5% by mass, N (nitrogen): 0.003 to 0.2% by mass, remaining iron (Fe), and other unavoidable impurities. Become
The Ti and the N are at the stage of preparing a slab satisfying the following relational expression 1 and
The step of heating the slab at a temperature of 1050 to 1250 ° C. to form a fine structure of austenite, and
Including a step of hot-rolling the heated slab to produce a hot-rolled steel sheet.
The hot-rolled steel sheet contains 2.7 × 10 ^{8 to} 5.4 × 10 ⁸ TiN precipitates having a size of 0.012 to 0.025 μm per 1 mm ² .
The hot-rolled steel sheet contains austenite as a fine structure in an area fraction of 95% or more.
A method for producing a low-temperature steel sheet having excellent surface processing quality, wherein the number of crystal grains of austenite having a size of 200 μm or more in the fine structure of the steel sheet is 5 or less per unit cm ² .
[Relationship formula 1]
1.13 ≤ Ti / N ≤ 2.38
(However, Mn, C, Cr, Ti, and N in each formula mean mass% of each component content.)