CN111684093A - High Mn steel and method for producing same - Google Patents

Abstract

A method for imparting more excellent ductility to a high Mn steel excellent in low-temperature toughness of a base metal and a welding heat-affected zone is proposed. The high Mn steel has the following composition: contains C: 0.10% -0.70%, Si: 0.05% -1.00%, Mn: 15.0% -30.0%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01 to 0.07 percent of Cr: 2.5% -7.0%, N: 0.0050% to 0.0500% and O: 0.0050% or less, the balance being Fe and unavoidable impurities, and having a microstructure in which austenite is used as a matrix phase, the matrix phase being a polygonal recrystallization domain and a recrystallization recovery delay domain having an area ratio of 10% to 50%, the recrystallization recovery delay domain being composed of a plurality of crystal grains having a diameter of 5 μm or less and having an ellipse having a major axis in a rolling direction of a steel sheet or a shape similar to the ellipse, an aspect ratio of the ellipse being 2.0 or more and the major axis being 10 μm or more.

Description

High Mn steel and method for producing same

Technical Field

The present invention relates to a high Mn steel having excellent toughness particularly at low temperatures, which is suitably used for structural steels that can be used in extremely low temperature environments such as tanks for liquefied gas storage tanks, for example, and a method for producing the same.

Background

When a hot-rolled steel sheet is used for a structure for a liquefied gas storage tank, the steel sheet is required to have high strength and excellent toughness at an extremely low temperature because the use environment is an extremely low temperature. For example, when a hot-rolled steel sheet is used in a tank for liquefied natural gas, it is necessary to ensure that the boiling point of liquefied natural gas: excellent toughness at-164 ℃ or lower. When the low-temperature toughness of the steel is deteriorated, there is a possibility that the safety as a structure for an extremely low-temperature storage tank cannot be maintained, and therefore, there is a strong demand for the steel to be applied to improve the low-temperature toughness.

For this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000-series aluminum alloy, in which austenite that does not exhibit brittleness at extremely low temperatures is used as the structure of the steel sheet, has been used. However, since these materials are expensive in alloy cost and manufacturing cost, a steel material which is inexpensive and excellent in extremely low temperature toughness is desired.

Therefore, as a new steel material replacing the conventional steel for extremely low temperature, for example, patent document 1 proposes that a high Mn steel to which Mn as a relatively inexpensive austenite stabilizing element is added in a large amount is used as a structural steel for an extremely low temperature environment.

Patent document 1 proposes a technique of controlling the austenite grain size to an appropriate size to avoid carbide formed in grain boundaries from becoming a starting point of fracture and a propagation path of cracks. Patent document 2 proposes a technique for improving low-temperature toughness by suppressing Mn segregation to a certain degree or more.

Patent document 1: japanese patent laid-open publication No. 2016-196703

Patent document 2: japanese patent laid-open publication No. 2017-71817.

Disclosure of Invention

In the above-described applications such as structures for liquefied gas storage tanks, the steel material used needs to have high workability, and it is important to ensure ductility in addition to low-temperature toughness. The techniques described in patent documents 1 and 2 do not verify the ductility at all. The thickness of the high Mn steel material described in patent document 1 is about 15 to 50mm, and for example, a thickness of less than 15mm, particularly 10mm or less is required depending on the application. In the method of accelerated cooling after completion of hot rolling as exemplified in patent document 1, when producing such a sheet, the resulting steel sheet is likely to be warped or deformed, and an additional step such as shape correction is required, which hinders productivity. Further, in patent document 2, a long heat treatment is required to alleviate segregation, which is disadvantageous in terms of productivity.

Accordingly, an object of the present invention is to provide an application for providing further excellent ductility to high Mn steel having excellent low-temperature toughness of a base material and a welding heat-affected zone. Another object of the present invention is to provide a method for producing such a thin sheet of high Mn steel without warping or deformation.

Here, the phrase "excellent low-temperature toughness" means that the Charpy impact test at-196 ℃ has an absorption energy vE-196 of 100J or more.

In order to solve the above problems, the present inventors have conducted extensive studies on various factors that determine the composition of steel sheet and the production method for high Mn steel, investigated the relationship with the microstructure, and found the following knowledge.

First, high Mn steels do not undergo brittle fracture at very low temperatures, and fracture occurs from grain boundaries. That is, it was judged that the shape of the grain boundary has a great influence on the toughness. In particular, it is known that carbide is formed in grain boundaries, and the distribution of carbide and the morphology of grain boundaries have a great influence on toughness. Specifically, when fine crystal grains are formed, carbide formation sites which are starting points of fracture increase, and the toughness value decreases. Conversely, when coarse grains are formed, the number of carbides is small, so that the starting point is also reduced, but the fracture surface is easily propagated, and the toughness value is lowered.

As a method for suppressing the formation of carbide, rapid cooling (hereinafter, also referred to as rapid cooling) of a steel sheet is effective. However, when the thickness of the steel sheet is 20mm or less, when the steel sheet is rapidly cooled, sheet warpage accompanied by internal stress due to thermal strain may occur. In particular, in the case of high Mn steel, the microstructure is austenite, and therefore the plate warpage tends to be larger than that of ferrite steel. When the warpage of the sheet occurs, it becomes difficult to insert the sheet into a surface smoothing line or the like which is a step after cooling. Further, since the warping of the steel sheet needs to be corrected for shipment, a new process has to be added to the manufacturing line, which leads to an increase in manufacturing cost. The reason why the warpage of austenitic steel becomes large is presumed to be that the thermal conductivity becomes small and the temperature distribution becomes large as compared with ferritic steel, but the detailed reason is not clear.

Table 1 summarizes the results of the situation in which the warpage of the steel sheet imposes a burden on the manufacturing process when the thickness of the steel sheet and the cooling rate of cooling after hot rolling are changed in an actual manufacturing line. As shown in Table 1, it is understood that when the cooling rate exceeds 5 ℃/s, problems occur in the process in steel sheets having a thickness of 20mm or less.

The evaluation criteria and the load on the manufacturing process in table 1 are loads required for inspection, calibration, and the like of the intermediate products and the products in each process. In table 1, the results indicate a case where the transfer to a place where a leveling device (leveler) and a cooling bed are placed as post-production operations can smoothly pass through and be transported on a production line without requiring the maintenance, the correction, and the like, a case where a plate can pass through by performing a slight smoothing process for adjusting the opening degree of the leveler for each production, a case where a slight correction operation (an operation performed by an operator independently and manually) is required for an individual operation that is temporarily off-line, and a case where the correction is not possible in the production and there is a problem in the product shipment itself.

[ Table 1]

[ Table 1]

Very good: the process was carried out without any problem at all

O: making adjustments to pass the sheet at each manufacture

Δ: require slight correction

X: has problems in manufacturing

Therefore, a method of improving toughness by controlling the crystal grain form of the matrix phase even if carbide exists, when the application of quenching treatment effective for suppressing carbide is difficult, has been studied. That is, the influence of the morphology and toughness of the matrix grains in the presence of carbide was studied in depth. As a result, it was found that by combining a region composed of fine crystal grains and polygonal crystal grains, the reduction of the starting point of fracture and the suppression of the propagation of fracture surface are simultaneously performed, and the toughness value is improved.

As a result of analysis of the structure contributing to the improvement of the toughness value, it was found that the region composed of fine crystal grains is a region in which the delay of recrystallization and recovery occurs after rolling, and the polygonal region is a region in which recrystallization and recovery occur at a relatively early stage after rolling. Therefore, it was found that such a structure can be formed by optimizing the temperature conditions of hot rolling and the cooling conditions after further rolling in addition to the composition adjustment. In particular, it was found that the addition of Cr makes it easy to control the region where recovery is suppressed without recrystallization. Further, it was found that the recrystallization recovery-delayed zone is formed by setting the finish rolling temperature to 750 to 850 ℃ and then cooling at a cooling rate of 5 ℃/s or less, and that both strength and toughness can be obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

1. A high Mn steel having a composition containing, in mass%, C: 0.10% -0.70%, Si: 0.05% -1.00%, Mn: 15.0% -30.0%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01 to 0.07 percent of Cr: 2.5% -7.0%, N: 0.0050% to 0.0500% and O: less than 0.0050%, and the balance of Fe and unavoidable impurities,

and has a microstructure in which austenite is used as a matrix phase, the matrix phase is a polygonal recrystallization region and a recrystallization recovery delay region having an area ratio of 10% to 50%,

the recrystallization recovery-delayed zone is composed of a plurality of crystal grains having a diameter of 5 [ mu ] m or less, and has an ellipse or a shape close to the ellipse, the ellipse having an aspect ratio of 2.0 or more and a major axis of 10 [ mu ] m or more, with the major axis being the rolling direction of the steel sheet.

2. The high Mn steel according to claim 1, wherein the composition further contains, in mass%, a component selected from Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, REM: 0.0010% -0.0200% and B: 1 or more than 2 of 0.0005 to 0.0020 percent.

3. A method for producing a high Mn steel, comprising heating a steel slab having the composition of 1 or 2 to a temperature range of 1100 to 1300 ℃, hot rolling the steel slab to a finish rolling temperature of 750 ℃ or more and less than 850 ℃, and cooling the steel slab at an average cooling rate of 5 ℃/s or less in a temperature range of 650 ℃ from the finish rolling temperature.

4. The method for manufacturing a high Mn steel according to claim 3, wherein the average cooling rate is 3 ℃/s or less.

According to the present invention, a high Mn steel excellent in low-temperature toughness can be provided. In the case of the application of welding the high Mn steel, both the base metal after welding and the welding heat affected zone are excellent in low-temperature toughness. Therefore, the high Mn steel according to the present invention is very useful for improving safety and life of a steel structure used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, and has an industrially significant effect. Further, the manufacturing method of the present invention can manufacture the high Mn steel excellent in low-temperature toughness without causing a reduction in productivity and an increase in manufacturing cost, and thus can provide a manufacturing method excellent in economy.

Drawings

Fig. 1A is a tissue photograph of SEM.

Fig. 1B is a tissue photograph of SEM.

Detailed Description

The high Mn steel of the present invention will be described in detail below.

[ composition of ingredients ]

First, the composition of the high Mn steel of the present invention and the reasons for the limitation thereof will be described. Unless otherwise specified, "%" of a component composition means "% by mass".

C：0.10％～0.70％

C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain this effect, C needs to be contained at 0.10% or more. On the other hand, if the content exceeds 0.70%, Cr carbide is excessively formed, and the low-temperature toughness is lowered. Therefore, C is 0.10% to 0.70%. Preferably 0.20 to 0.60%.

Si：0.05％～1.00％

Si acts as a deoxidizer and is not only essential for steel production but also has the effect of being solid-soluble in steel to increase the strength of the steel sheet by solid-solution strengthening. In order to obtain such an effect, Si needs to be contained by 0.05% or more. On the other hand, if the content exceeds 1.00%, weldability deteriorates. Therefore, Si is 0.05% to 1.00%. Preferably 0.07 to 0.50%.

Mn：15.0％～30.0％

Mn is a relatively inexpensive austenite stabilizing element. The present invention is an important element for achieving both strength and extremely low temperature toughness. In order to obtain this effect, Mn needs to be contained by 15.0% or more. On the other hand, if the content exceeds 30.0%, the effect of improving the cryogenic temperature toughness is saturated, leading to an increase in alloy cost. Further, weldability and cuttability are deteriorated. In addition, segregation is promoted, and the occurrence of stress corrosion cracking is promoted. Therefore, Mn is 15.0% to 30.0%. Preferably 18.0% to 28.0%.

P: less than 0.030%

When P is contained in an amount of more than 0.030%, P segregates at grain boundaries and becomes a starting point of stress corrosion cracking. Therefore, the upper limit is 0.030%, and the lower limit is preferably as small as possible. However, P is 0.030% or less. Preferably 0.028% or less, and more preferably 0.24% or less. Of course, it may be 0%. In order to reduce P to less than 0.002%, refining requires a large cost, and therefore, from the viewpoint of economy, 0.002% or more is preferable.

S: 0.0070% or less

S deteriorates the low-temperature toughness and ductility of the base material, and therefore, it is preferable to set the upper limit to 0.0070% as low as possible. Therefore, S is 0.0070% or less. Preferably 0.0050% or less. Of course, it may be 0%. In order to reduce S to less than 0.0005%, refining requires a large cost, and from the viewpoint of economy, 0.0005% or more is preferable.

Al：0.01％～0.07％

Al functions as a deoxidizer and is most commonly used in a molten steel deoxidizing step of a steel sheet. In order to obtain such an effect, Al needs to be contained by 0.01% or more. On the other hand, if the content exceeds 0.07%, the content is 0.07% or less because the content is mixed into the weld metal portion during welding and the toughness of the weld metal is deteriorated. Preferably 0.02 to 0.06 percent.

Cr：2.5％～7.0％

Cr is an element which stabilizes austenite by adding an appropriate amount and is effective for improving the extremely low temperature toughness and the base material strength. And is an element effective for forming a recrystallization recovery-delaying region described later. In order to obtain such an effect, Cr needs to be contained at 2.5% or more. On the other hand, if the content exceeds 7.0%, the low-temperature toughness and the stress corrosion cracking resistance are reduced by the formation of Cr carbide. Therefore, Cr is 2.5 to 7.0%. Preferably 3.5 to 6.5 percent.

N：0.0050％～0.0500％

N is an austenite stabilizing element and is an element effective for improving the extremely low temperature toughness. In order to obtain such an effect, N needs to be contained by 0.0050% or more. On the other hand, if the content exceeds 0.0500%, the nitride or carbonitride becomes coarse, and the toughness is lowered. Therefore, N is 0.0050% to 0.0500%. Preferably 0.0060% to 0.0400%.

O: 0.0050% or less

O forms oxides that degrade the very low temperature toughness. Therefore, O is in the range of 0.0050% or less. Preferably 0.0045% or less. Of course, it may be 0%. In order to reduce O to less than 0.0005%, refining requires a large cost, and therefore 0.0005% or more is preferable from the viewpoint of economy.

The balance other than the above components is iron and inevitable impurities. Examples of the inevitable impurities include Ca, Mg, Ti, Nb, and Cu, and a total of 0.05% or less is acceptable.

In the present invention, the following elements may be contained as necessary in addition to the above essential elements for the purpose of further improving the strength and the low-temperature toughness.

May be added a material selected from Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, REM: 0.0010% -0.0200%, B: 1 or more than 2 of 0.0005 to 0.0020 percent.

Mo, V, W: respectively less than 2.0%

Mo, V and W contribute to stabilization of austenite and to improvement of base material strength. In order to obtain such effects, Mo, V, and W are preferably contained in an amount of 0.001% or more, respectively. On the other hand, if the content exceeds 2.0%, coarse carbonitrides are formed, which may become starting points of fracture and may impose a pressure on the production cost. Therefore, when these alloying elements are contained, the content is 2.0% or less, respectively. Preferably 0.003% to 1.7%, more preferably 1.5% or less.

REM：0.0010％～0.0200％

REM is an element useful for controlling the form of an inclusion, and may be contained as needed. The term "morphology control" of inclusions means that the stretched sulfide-based inclusions are made into granular inclusions. The ductility, toughness and sulfide stress corrosion cracking resistance are improved through the morphological control of the inclusions. In order to obtain such an effect, REM is preferably contained at 0.0010% or more. On the other hand, if the content is excessively increased, the amount of non-metallic inclusions may be increased, and ductility, toughness, and sulfide stress corrosion cracking resistance may be rather decreased. Therefore, the amount of REM is preferably 0.0015% to 0.0200%.

B：0.0005％～0.0020％

B segregates at grain boundaries, contributing to improvement of toughness due to the grain boundary strength of the material. However, when the amount is excessively added, coarse nitrides and carbides are formed, and the addition amount is preferably 0.0005% to 0.0020%.

Next, the structure form for achieving low-temperature toughness will be described.

[ microstructure with austenite as matrix phase ]

When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in a low-temperature environment because the steel material may cause brittle fracture in a low-temperature environment. Here, it is assumed that when the steel material is used in a low-temperature environment, the crystal structure of the matrix phase of the structure of the steel material must be austenite having a face-centered cubic structure (fcc). The phrase "austenite is used as a matrix phase" means that the austenite phase may be 90% or more, or 100% in terms of the area ratio of the microstructure. On the other hand, the remainder other than the austenite phase is made up of ferrite, martensite phase, inclusions, and precipitates having a BCC structure, and the ratio thereof is preferably 5% or less. The austenite fraction can be determined by observation with EBSD, analysis by XRD, magnetic permeability, and the like.

[ microstructure morphology ]

The present invention improves low-temperature toughness by controlling microstructure, particularly austenite structure, in hot rolling and cooling thereafter. Therefore, it is important to control the morphology of the microstructure. In particular, it is important that a region having polygonal crystal grains, i.e., a polygonal region, where recrystallization and recovery are rapidly performed and recovery are performed, and a region having a large amount of strain therein, i.e., a recrystallization recovery delayed region, where recrystallization and recovery are delayed exist appropriately during cooling during hot rolling and after hot rolling, so that the starting point of a fracture surface is reduced and the progress of the fracture surface is suppressed, thereby improving toughness. The form of each region will be discussed in detail below.

[ recrystallization recovery delay zone ]

The recrystallization recovery-delayed region is a region in which recrystallization and recovery from strain introduction by hot rolling are delayed and which is composed of a plurality of crystal grains containing a large amount of strain therein. The region is a region in which each crystal grain has a size of 5 μm or less, substantially reacts to a rolled structure, extends in a rolling direction, and has an ellipsoidal shape in which a plurality of crystal grains are aggregated. That is, in the recrystallization recovery-delayed zone, a cross section (hereinafter referred to as an L cross section) orthogonal to the rolling direction of the steel sheet is observed, and the cross section has an ellipse having a major axis in the rolling direction or a shape similar to the ellipse, and the ellipse has an aspect ratio of 2.0 or more and the major axis of 10 μm or more. As a method for identifying this region, tissue control for forming the above-described shape is performed as described later.

[ area ratio of recrystallization recovery-delayed region: 10% -50% ]

The recrystallization-recovery-delaying region is formed in combination with the polygonal region, and when the area ratio of the recrystallization-recovery-delaying region is high, the entire structure becomes a structure with a large strain, which is disadvantageous from the viewpoint of ductility. In addition, the increase in carbides formed at grain boundaries and the like in the recrystallization recovery-delaying region increases the starting points of fracture surfaces, and is also disadvantageous in toughness. Therefore, the upper limit of the ratio of the recrystallization recovery-delaying region in the microstructure is 50% by area ratio. On the other hand, if the area ratio is less than 10%, the other portions are formed of polygonal crystal grains, and thus the strength of the material is reduced. In particular, when the area ratio is less than 10%, the fracture surface unit in the toughness test increases and the fracture surface easily progresses, and therefore, the recrystallization recovery-retarded region must be a fraction of 10% or more. The area ratio is preferably 20% to 40%.

[ polygonal region ]

The polygonal region is a region where the deformed region introduced by hot rolling is sufficiently recrystallized and recovered to be polygonal crystal grains. The grains also undergo recovery of deformation, effectively promoting improvement of ductility. Further, since the grain boundary of the polygonal region is relatively large, the formation density of carbide is reduced, and the starting point of the fracture surface is reduced, thereby effectively improving the toughness. In the case where the crystal grains are approximated to be elliptical and the maximum diameter of the crystal grains is defined as the major axis of the ellipse in the L-section observation of the steel sheet, the polygonal crystal grain preferably has an aspect ratio of 1.0 to 1.8. This is because an aspect ratio exceeding 1.8 is affected by stretching due to rolling, and is disadvantageous in suppressing propagation of a fracture surface. The particle diameter is preferably 5 to 100 μm in terms of the major axis of the ellipse. When the particle size is less than 5 μm, the grain boundary, which is a carbide formation site, increases, and thus the starting point of fracture is disadvantageously reduced. On the other hand, when the particle diameter exceeds 100. mu.m, the unit of the cross section becomes large, the cross section easily progresses, and the toughness is lowered. The proportion of the polygonal region in the entire microstructure is preferably 40% to 90% in terms of area ratio. More preferably 60% to 80%.

Therefore, the austenite phase forming the matrix phase (parent phase) of the steel sheet is mainly defined by the polygonal region and the recrystallization recovery-delayed region. However, although there may be regions that do not satisfy these predetermined conditions, for example, crystal grains having an aspect ratio of 1.0 to 1.8 and less than 5 μm, regions that are identified as recrystallization recovery-delaying regions by the following observation method but have an aspect ratio of less than 2.0, and the like, these regions are suppressed to 5% or less in terms of the area ratio of the microstructure, and most of the austenite phase needs to be formed as either of the polygonal region and the recrystallization recovery-delaying region. That is, the matrix phase is a polygonal recrystallization region and a recrystallization recovery delay region having an area ratio of 10% to 50%.

Next, the method of identifying these regions is described below.

The above-described regions can be identified by optimizing the method of adjusting the SEM observation sample. Specifically, when the surface of the steel sheet is subjected to mirror polishing with colloidal silica and then ion etching with ion milling, fine irregularities are formed on the surface layer in the recrystallization recovery delay region, and therefore, the surface layer can be identified by incident mirror texture observation and reflected electron image observation by a low-acceleration SEM of 5kV or less. In addition, even by performing mirror polishing using electrolytic polishing, the recrystallization recovery-delayed region can be identified. The factors that cause the contrast difference in the matrix phase (matrix phase) are not known in detail, considering the difference in hardness and strain, the distribution of trace elements, and the like. The analysis is to define the area that can be identified by the above-described binarization as an area ratio by image processing.

Furthermore, each region may be identified by using an image quality equivalent value using ebsd (electron Back Scattered diffraction). However, in this case, since strain is sometimes introduced to the sample surface by polishing in preparing the sample, care must be taken in preparing the sample, and it is necessary to reliably remove the strain in the surface layer by electrolytic polishing, ion polishing, or the like.

As a form of the recrystallization recovery-delayed zone, a small form is constituted by a plurality of 2 or more crystal grains, a size (long axis) of an aggregate of the plurality of crystal grains is about 10 μm, and on the other hand, there is a zone having a band structure (band-like structure extending in the rolling direction of the plate) in a large zone, a width (lamination) in the plate thickness direction) of 50 μm, and a length (long side direction (long axis) of the extending band-like structure) of about 500 μm. In fig. 1, for 3 cases, as shown in the structural photograph of the reflected electron image, the recrystallization recovery-delayed region can be clearly distinguished from the polygonal region as indicated by the surrounding line in the figure. That is, fig. 1A is a photograph of the structure at 200 magnifications, and the structure (recrystallization recovery-delayed region) extending in the rolling direction can be observed. Fig. 1B is a 500-fold photograph of the structure, and it can be confirmed that unrecrystallized regions (regions in which recrystallization recovery is delayed) of various forms are formed in the observation region.

In consideration of the above, in the SEM texture observation, the magnification (200 to 5000 times) is appropriately adjusted for each visual field of about 300 × 500 μm at a depth position (hereinafter referred to as 1/4t portion) from the steel sheet surface thickness 1/4, the area of the recrystallization recovery retardation region in the visual field is measured, and the area ratio of the visual field is calculated. This operation was performed at least at 10 positions, and the average value was calculated as the area ratio of the recrystallization recovery-delayed zone.

The polygonal crystal grains (polygonal regions) were observed at 1000 times by SEM, and 100 or more crystal grains were identified. In this case, the measurement of the size of crystal grains in a region excluding the recrystallization recovery-delayed region identified by SEM observation may be performed in combination with EBSD.

The high Mn steel of the present invention can be produced by melting molten steel having the above-described composition by a known melting method such as a converter or an electric furnace. Further, refining may be performed 2 times in a vacuum degassing furnace. Then, a steel material such as a slab having a predetermined size is produced by a known casting method such as a continuous casting method or an ingot-cogging rolling method.

Further, production conditions for producing a steel material having excellent low-temperature toughness from the above-described steel material will be described.

[ heating temperature of steel blank: 1100 ℃ -1300℃)

The heating temperature before hot rolling is 1100 ℃ or higher in order to coarsen the grain size of the microstructure of the steel material. However, since there is a possibility that a part of the melt starts to melt when the temperature exceeds 1300 ℃, the upper limit of the heating temperature is 1300 ℃. The temperature control at this time is based on the surface temperature of the steel blank.

[ finish rolling finish temperature: 750 ℃ -850℃)

The hot rolling finish temperature and the cooling conditions thereafter are important in controlling the recrystallization recovery-delayed region. When the temperature is higher than 850 ℃, recrystallization proceeds during and immediately after the final rolling, formation of polygonal crystal grains is promoted, grain boundaries become large, and toughness becomes excessively high. When the rolling temperature is less than 750 ℃, the formation of polygonal crystal grains due to recrystallization is suppressed, and a large amount of strain is introduced into the recrystallization recovery delay region, so that the strength is increased and the toughness is deteriorated.

[ Cooling Rate from finish Rolling finish temperature to 650 ℃: 5 ℃/s or less ]

In order to achieve both the formation of polygonal grains by recrystallization and recovery and the remaining of a recrystallization recovery delay region, it is important to control the cooling at 650 ℃ from the rolling completion temperature to the recovery and recrystallization. At this time, if the cooling rate is too high, the structure after rolling is frozen, sufficient polygonal crystal grains are not formed, and toughness deteriorates, so the upper limit of the cooling rate is set to 5 ℃/s. Preferably 3 ℃/s or less. In particular, in the case of a thin material, since the sheet warps as described above and becomes a problem in the process, it is preferable to cool the sheet at a rate of 3 ℃/s or less. Since cooling in a temperature range of less than 650 ℃ does not affect recrystallization and recovery of the matrix phase (matrix phase), the regulation of the cooling rate is in a temperature range from the finish rolling temperature to 650 ℃. On the other hand, the cooling in the temperature region of less than 650 ℃ may be performed arbitrarily as follows.

Here, since the cooling rate varies depending on the thickness of the sheet, it is advantageous to appropriately perform adjustment by water cooling or the like. The cooling treatment here is performed based on the thickness center temperature of the steel sheet. The center temperature can be determined by heat transfer calculation from the surface temperature of the steel sheet measured by a radiation thermometer.

The lower limit of the cooling rate is not particularly set, and the use of a holding furnace or the like is disadvantageous in terms of the cost of the furnace, the process cost, and the production time, and therefore can be within the range of air cooling.

[ Cooling at less than 650 ]

The present invention achieves an improvement in toughness at low temperatures by the combination of the polygonal region and the recrystallization recovery-delaying region in a state where carbide is formed at grain boundaries. Therefore, there is no particular limitation on the cooling at less than 650 ℃. However, since carbide suppression is effective for toughness and the influence of the sheet warpage is reduced from a temperature range of less than 650 ℃, it is preferable to perform rapid cooling of 10 ℃/s or more from the viewpoint of carbide formation suppression.

Further, the above-mentioned cooling treatment (cooling at less than 650 ℃) is performed, and then, a treatment of heating to a temperature range of 300 to 650 ℃ and cooling is added, as necessary. That is, the tempering treatment may be performed for the purpose of adjusting the strength of the steel sheet.

Examples

The present invention will be described in detail below with reference to examples. The present invention is not limited to the following examples.

(1) Steel plate

Billets having the composition shown in table 1 were prepared by vacuum melting. Then, the obtained slab was charged into a heating furnace and heated to 1250 ℃, and then hot rolled while varying the finish rolling temperature, and then cooled while varying the cooling rate in a temperature range from the finish rolling temperature to 650 ℃, thereby producing a steel sheet 5 to 20mm thick. In hot rolling, a thermocouple was provided in the central portion of the thickness of the steel sheet, and the temperature of the steel sheet was detected to measure the finish rolling temperature. The cooling rates in the temperature ranges from the finish rolling temperature and the finish rolling temperature to 650 ℃ are shown in Table 2.

[ Table 2]

[ Table 3]

The obtained steel sheet was evaluated for tensile test properties and low-temperature toughness in the following manner, and the structure was analyzed.

(2) Tensile test Properties

Tensile tests were carried out on each of the obtained steel sheets using tensile test specimens of JIS5 under the provisions of JIS Z2241 (1998) to examine the tensile test characteristics. In the present invention, a yield strength of 400MPa or more and a tensile strength of 800MPa or more are determined as tensile properties. Further, the elongation of 30% or more was judged as excellent ductility.

(3) Low temperature toughness

From a position 1/2 away from the surface plate thickness of each steel sheet, Charpy V notch test pieces were taken in the direction perpendicular to the rolling direction in accordance with the specification of JIS Z2202 (1998), and each steel sheet was subjected to Charpy impact test 3 times in accordance with the specification of JIS Z2242 (1998) to obtain the absorption energy at-196 ℃ to evaluate the toughness of the base material. In the present invention, the average value of the 3-time absorption energy (vE-196) is 100J or more, and it is assumed that the base material toughness is excellent. Then, a Charpy V notch test piece of half size (5mm) was prepared and tested on a steel plate having a plate thickness of 10mm or less, and it was determined that the absorption energy was 50J or more.

(4) Tissue analysis

For tissue analysis, tissue observation was performed using a scanning electron microscope (FE-SEM) having a field emission gun and a lens type detector. That is, a sample prepared by embedding a steel sheet in a resin was mirror-polished by diamond polishing and colloidal silica, and then surface sputtering was performed by an Ar ion beam. The structure observation was performed at an accelerating voltage of 5kV, the morphology of the recrystallization recovery delay region was evaluated, and the area ratio was calculated. That is, the unrecrystallized region is extracted from each SEM image, and the region is followed. The area of the tracked region is obtained by using image analysis software or the like, and the area ratio is calculated. The observation area is an area of 500 × 500 μm at 1 position from 1/4 positions on the surface of the steel sheet, and the average value was obtained by performing the observation at 10 positions.

The results of the evaluation and observation obtained as described above are shown in table 3.

As shown in Table 3, it was confirmed that the high Mn steel of the present invention satisfies the above-mentioned target properties (yield strength of the base material is 400MPa or more, and low temperature toughness is 100J or more in terms of average value of absorption energy (vE-196)). On the other hand, any one or more of the yield strength and the low-temperature toughness in the comparative examples outside the scope of the present invention cannot satisfy the above-described target properties.

[ Table 4]

Claims (4)

1. A high Mn steel having the following composition: contains, in mass%, C: 0.10% -0.70%, Si: 0.05% -1.00%, Mn: 15.0% -30.0%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01 to 0.07 percent of Cr: 2.5% -7.0%, N: 0.0050% to 0.0500% and O: less than 0.0050%, and the balance Fe and inevitable impurities,

and, has the following microstructure: the steel sheet is characterized in that austenite is used as a matrix phase, the matrix phase is a polygonal recrystallization region and a recrystallization recovery delay region with an area ratio of 10% to 50%, the recrystallization recovery delay region is composed of a plurality of crystal grains with a diameter of 5 [ mu ] m or less and has an ellipse with a rolling direction of the steel sheet as a major axis or a shape similar to the ellipse, the aspect ratio of the ellipse is 2.0 or more, and the major axis is 10 [ mu ] m or more.

2. The high Mn steel according to claim 1, wherein the composition further contains, in mass%, a component selected from Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, REM: 0.0010% -0.0200% and B: 1 or more than 2 of 0.0005 to 0.0020 percent.

3. A method for producing a high Mn steel, comprising heating a steel slab having the composition of claim 1 or 2 to a temperature range of 1100 to 1300 ℃, hot rolling the steel slab at a finish rolling temperature of 750 ℃ or more and less than 850 ℃, and cooling the steel slab at an average cooling rate of 5 ℃/s or less in a temperature range of 650 ℃ from the finish rolling temperature.

4. The method for manufacturing a high Mn steel according to claim 3, wherein the average cooling rate is 3 ℃/s or less.

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