KR20130073736A - High strength non-magnetic steel sheet having excellent austenite stability and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A high intensity non-magnetic steel sheet with an excellent phase stability and a manufacturing method thereof are provided to have the high intensity and an excellent non-magnetic property at the same time; thereby securing the phase stability of an excellent austenite phase. CONSTITUTION: A manufacturing method of the high intensity non-magnetic steel sheet with an excellent phase stability performs the following step. Re-heat the steel slab, which includes C 0.4-0.9wt%, Mn 10-25wt%, Al 1.3-8.0wt%, Si 0.01-2.0wt%, Ti 0.05-0.2wt%, Si 0.01-2.0wt%, B 0.0005-0.005wt%, S less than 0.05wt% (except for 0), P less than 0.8wt% (except for 0), N 0.003-0.01wt%, the remainder Fe and other unavoidable impurity, in 1100-1250°C. Hot-roll the re-heated steel slab, and execute the hot-roll which finishes the rolling at 800-950°C. Reel the hot rolled steel sheet at 400-700°C. Cool and roll at the reduction rate of 30-60%. Consecutively anneal the cooled and rolled steel sheet at 650-900°C. [Reference numerals] (AA) Martensite

Description

High strength non-magnetic steel sheet with superior phase stability and its manufacturing method {HIGH STRENGTH NON-MAGNETIC STEEL SHEET HAVING EXCELLENT AUSTENITE STABILITY AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a non-magnetic steel that can be used in heavy electrical equipment such as switchboards, transformers, and more particularly relates to a high strength steel sheet excellent in austenite phase stability.

Materials such as switchboards, transformers and the like generally require high non-magnetic properties along with high strength.

In order to satisfy these conditions, stainless steels with a large amount of nickel and chromium have been conventionally used. However, there is a problem that the stainless steel is low in strength and high in price. Ferritic or martensitic stainless steels may be applied to increase the strength of the ferritic or martensitic stainless steels. However, since the ferritic or martensitic stainless steels have high magnetic properties, power loss due to eddy currents occurs and the price is very high have.

On the other hand, austenitic steel sheet containing a large amount of manganese may be applied, but a steel sheet containing a large amount of manganese has the advantage of high strength in the case of austenitic steel sheet, but the phase stability is lower than conventional stainless steel Has a problem.

Therefore, development of an austenitic steel sheet having high phase stability while ensuring high strength is urgently required.

One aspect of the present invention is to provide a steel sheet having high strength and excellent nonmagnetic properties, and excellent in austenite phase stability, and a method of manufacturing the same.

The present invention is in weight%, C: 0.4-0.9%, Mn: 10-25%, Al: 1.3-8.0%, Si: 0.01-2.0%, Ti: 0.05-0.2%, Si: 0.01-2.0%, B : 0.0005 ~ 0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: 0.003 ~ 0.01%, the rest contains Fe and unavoidable impurities,

Provided is a high strength nonmagnetic steel sheet having excellent phase stability with a lamination defect energy of 30 mJ / cm 2 or more.

Further, the present invention provides a method for manufacturing a steel slab, comprising: reheating a steel slab satisfying the composition to 1100 to 1250 占 폚;

Subjecting the reheated steel slab to hot rolling and finish rolling at 800 to 950 占 폚;

Rolling the hot-rolled steel sheet at 400 to 700 ° C;

Cold rolling at a reduction ratio of 30 to 60%;

It provides a method for producing a high strength non-magnetic steel sheet excellent in phase stability, including the step of continuous annealing the cold-rolled steel sheet at 650 ~ 900 ℃.

According to the present invention, the austenitic stability is increased to secure nonmagnetic properties, and by adding Al and the like, a high manganese steel is added to prevent a carbide from forming carbon, thereby making the austenite more stable. It has the advantage of excellent properties. Thus, it provides sufficient rigidity for use as structural members such as large transformers.

(A) and (b) of FIG. 1 are XRD graphs measuring the degree of image stability of the inventive steel 1 and the comparative steel 1, respectively.
(A) and (b) of Figure 2 are photographs of microscopic observation of invention steel 1 and comparative steel 1, respectively.

The loss due to the eddy current of the material exposed to the electromagnetic field is closely related to the magnetism of the material. The larger the magnetism, the larger the eddy currents are generated and the losses are increased. In general, the magnetism is proportional to the magnetic permeability (μ). That is, the magnetism increases as the magnetic permeability increases. The magnetic permeability is defined by the ratio of the induced magnetic field (B) to the magnetic field (H) to be magnetized, that is, = B / H. Reducing the boehmite permeability reduces the magnetism of the material and increases the energy efficiency by preventing eddy current loss on the surface when exposed to an electric field. Therefore, it is advantageous to prevent energy loss by using a nonmagnetic steel sheet having no magnetism as a material for an electric distribution board and a transformer.

As a result of deep research, the present inventors have come up with the invention of high manganese steel having high austenite stability by adding manganese (Mn) and carbon (C) in the steel in order to have high strength and nonmagnetic properties. In the present invention, by controlling the content of carbon and manganese, the austenite phase stability is increased, and the addition of aluminium suppresses slip deformation due to the formation and dislocation of ε-martensite during deformation, thereby providing excellent strength and elongation (formability). As well as excellent non-magnetic properties.

Hereinafter, the present invention will be described in detail. First, the steel sheet of the present invention will be described in detail. The steel sheet of the present invention satisfies the following composition (hereinafter referred to as " weight%

Carbon (C): 0.4 to 0.9%

C is an element necessary for securing the austenite structure in the steel, and it is preferable to add 0.4% or more to ensure sufficient stability of austenite. However, when the amount of C exceeds 0.9%, carbide is excessively precipitated, resulting in deterioration of the nonmagnetic properties as well as deterioration of the casting composition. Therefore, the content of C is preferably 0.4 to 0.9%.

Manganese (Mn): 10 to 25%

Mn is an important element that stabilizes the austenite structure and is contained in the present invention in an amount of 10% or more. If it is less than 10%, the α'-martensite phase is present and the non-magnetic property is deteriorated. If it is more than 25%, the manufacturing cost is greatly increased and the internal oxidation is seriously generated during heating in the hot rolling step in the process, There is a problem of deterioration. Therefore, the content of Mn is preferably 10 to 25%.

Aluminum (Al): 1.3 ~ 8.0%

The Al is an element effective for preventing the formation of carbide, and improves the formability by controlling the fraction of the twin crystal. In the present invention, carbon is contained in an amount of 1.3% or more because it acts as an important element to prevent carbide formation to improve the nonmagnetic properties in order to stabilize the austenite. However, when it exceeds 8.0%, the manufacturing cost increases, and the surface quality of the product is lowered due to the formation of excessive oxide, so the content is preferably 1.3 to 8.0%.

Silicon (Si): 0.01 to 2.0%

The Si is an element that does not significantly affect the lamination defect energy, and is usually used as a deoxidizer or contained about 0.01% in a general steelmaking process, and if it is desired to remove it, it contains 0.01% because excessive cost occurs. When the content of Si exceeds 2.0%, the manufacturing cost increases, and since the surface quality of the product is reduced due to the formation of excessive oxide, the content is preferably 0.01 to 2.0%.

Titanium (Ti): 0.05 to 0.2%

The Ti is added to the steel to react with nitrogen to precipitate nitrides and form twinning to secure strength and formability. Further, the Ti serves to increase the strength by forming a precipitation phase. For this purpose, Ti is preferably contained in an amount of 0.05% or more, but if it exceeds 0.2%, precipitates are formed excessively and micro-cracks may be formed during cold rolling, which may lead to deterioration of moldability and weldability. Therefore, the content of Ti is preferably 0.05 to 0.2%.

Boron (B): 0.0005 to 0.005%

When B is added in a small amount, B plays a role of strengthening the grain boundary of the main grain, and it is preferable that the B contains 0.0005% or more. However, if it is excessively contained, the cost increases, and therefore the content thereof is preferably 0.0005 to 0.05%.

Sulfur (S): Not more than 0.05% (excluding 0)

It is necessary that the S is controlled to 0.05% or less for control of inclusions. If the content of S exceeds 0.05%, a problem of hot stiffness may occur. Therefore, the upper limit is preferably 0.05%.

Phosphorus (P): not more than 0.8% (excluding 0)

P is an element in which segregation easily occurs, thereby promoting cracking during casting. In order to prevent this, it is preferable to control to 0.8% or less. If the content of P exceeds 0.8%, the main composition may deteriorate, so that the upper limit is preferably 0.8%.

Nitrogen (N): 0.003 to 0.01%

The N is an element that is indispensably added by reacting with the atmosphere during the steelmaking process. Reducing the amount of N to less than 0.003% causes an excessive cost in the process, and if the content exceeds 0.01%, it forms a nitride and lowers the moldability, which is not preferable. Therefore, the content of N is preferably 0.003 to 0.01%.

The remainder includes Fe and unavoidable impurities.

The steel sheet of the present invention preferably has a Stacking Fault Energy (SFE) value of 30 mJ / cm 2 or more. The stacking fault energy is an energy of an interface between partial potentials formed in the material. In the present invention, the stacking fault energy is controlled by controlling the content of Al, thereby improving the phase stability of the austenite phase.

If the stacking defect energy has an appropriate value, the potential and twin are in harmony to increase the phase stability, but if it is too low, the potential cannot be generated or moved, and the phase stability decreases. Robbery is done. Therefore, in the present invention, the optimum stacking defect energy is derived in order to obtain phase stability with moderate strength.

When the stacking defect energy is less than 30 mJ / cm 2, twins are formed and strength increases. However, ε-martensite is formed. The ε-martensite is non-magnetic in a dense cubic structure, but usually increases the magnetism because it forms α-martensite well. Therefore, in order to form twins while maintaining nonmagnetic properties and to have high strength, the lamination defect energy value is preferably 30 mJ / cm 2 or more.

On the other hand, the method of measuring the stacking defect energy is various, such as X-ray measurement method, transmission electron microscope measurement method, thermodynamic calculation method, etc., thermodynamic calculation method using the thermodynamic data that reflects the influence of the components well, easy to measure is most preferred.

The steel sheet of the present invention has a tensile strength of 800 MPa or more and secures an elongation of 15% or more, thereby having excellent strength and processability.

Hereinafter, the production method of the present invention will be described in detail.

The steel slab satisfying the above composition is reheated at 1100 to 1250 ° C. If the heating temperature is too low, it is preferable to heat at a temperature of 1100 占 폚 or more because the rolling load may be excessive during hot rolling. The higher the heating temperature, the easier the hot rolling. However, the steel having a high Mn content may have severe internal oxidation during high temperature heating, which may lower the surface quality. Therefore, the upper limit of the reheating temperature is preferably 1250 ° C.

After the reheating, hot rolling is performed, and hot rolling is performed at 800 to 1000 占 폚. The hot finish rolling temperature is also easier to roll because the deformation resistance is lower at higher temperatures, but the surface quality may be degraded at higher rolling temperatures, and it is preferable to perform it at 1000 ° C. or lower, the temperature is too low, and the load becomes large during rolling. Therefore, it is preferable to carry out at 800 degreeC or more.

Followed by winding and hot rolling. The winding is preferably carried out at 400 to 700 ° C. The cooling rate after the winding is usually slow. If the winding start temperature is too low, a large amount of cooling water is required for cooling, and a load at the time of winding largely acts and the winding start temperature is set to 400 DEG C or higher. When the coiling temperature is high, the reaction between the oxide film on the plate surface and the steel sheet matrix proceeds during the cooling process after winding, and the acidity is deteriorated.

It is preferable to carry out water cooling before the rolling after the hot rolling.

The hot-rolled steel sheet thus produced is cold-rolled to produce a cold-rolled steel sheet. In the present invention, since the recrystallization proceeds in the heat treatment process after the cold rolling, it is necessary to control the driving force of the recrystallization well. In other words, if the reduction rate during cold rolling is too low, the strength of the product is lowered, and the reduction of the product is carried out at a reduction ratio of 30% or more, and if the reduction ratio is too high, it is advantageous to secure the strength, but the load of the rolling mill increases, in consideration of this, It is preferable to carry out at a reduction ratio of 60% or less.

After the cold rolling, continuous annealing is performed. The continuous annealing is preferably performed at 650 to 900 ° C. Continuous annealing is preferably carried out at 650 ° C. or higher where sufficient recrystallization occurs. However, if the annealing temperature is too high, oxides are formed on the surface, and workability with pre- and post-connected products that are continuously worked is lowered. desirable.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are for the understanding of the present invention, and the present invention is not limited by the examples.

(Example)

The steel slab that satisfies the composition (weight%) shown in Table 1 below was reheated to 1200 ° C., hot finished rolling at 900 ° C., wound up at 500 ° C., and then cold rolled to 50%, and then rolled to 800 ° C. Continuous annealing was performed to produce a cold rolled steel sheet.

Specimen Type C Mn P S Al Si Ti B N One 0.61 18.0 0.091 0.004 0.01 0.01 0.0662 0.0021 0.0097 2 0.61 18.3 0.087 0.0034 1.49 0.01 0.0857 0.0023 0.0087 3 0.60 18.3 0.087 0.0024 1.93 0.01 0.0856 0.0023 0.0078 4 0.61 18.5 0.090 0.0027 2.68 0.01 0.0833 0.0025 0.0065 5 0.61 14.5 0.097 0.0051 0.02 0.01 0.0766 0.0021 0.0098 6 0.61 15.1 0.094 0.0055 1.51 0.01 0.0854 0.0024 0.0081 7 0.61 15.5 0.094 0.0049 1.97 0.01 0.0846 0.0024 0.0069 8 0.61 11.6 0.101 0.0053 0.01 0.01 0.0684 0.002 0.0095 9 0.61 11.6 0.102 0.0057 1.45 0.01 0.0868 0.0023 0.0039 10 0.61 12.4 0.098 0.0039 1.94 0.01 0.0915 0.0022 0.0069 11 0.61 18.3 0.092 0.0041 0.51 0.01 0.0662 0.0021 0.0097 12 0.62 18.4 0.091 0.0042 1.02 0.01 0.0857 0.0023 0.0087 13 0.61 18.2 0.093 0.0041 1.21 0.01 0.0856 0.0023 0.0078 14 0.61 18.3 0.092 0.0044 4.52 0.01 0.0833 0.0025 0.0065 15 0.61 18.4 0.091 0.0045 6.02 0.01 0.0766 0.0021 0.0098 16 0.62 18.1 0.092 0.0041 7.513 0.01 0.0854 0.0024 0.0081 17 0.61 14.3 0.096 0.0052 0.51 0.01 0.0846 0.0024 0.0069 18 0.61 14.5 0.097 0.0051 1.01 0.01 0.0684 0.002 0.0095 19 0.61 14.4 0.095 0.0053 1.23 0.01 0.0662 0.0021 0.0097 20 0.62 14.5 0.096 0.0052 4.51 0.01 0.0857 0.0023 0.0087 21 0.62 14.4 0.097 0.0054 6.03 0.01 0.0856 0.0023 0.0078 22 0.61 14.2 0.096 0.0052 7.54 0.01 0.0833 0.0025 0.0065 23 0.61 11.4 0.102 0.0053 0.52 0.01 0.0766 0.0021 0.0098 24 0.62 11.6 0.101 0.0052 1.01 0.01 0.0854 0.0024 0.0081 25 0.61 11.3 0.103 0.0054 1.22 0.01 0.0846 0.0024 0.0069 26 0.62 11.3 0.102 0.0055 4.53 0.01 0.0684 0.002 0.0095 27 0.61 11.4 0.101 0.0052 6.01 0.01 0.0868 0.0023 0.0039 28 0.61 11.6 0.101 0.0053 7.51 0.01 0.0915 0.0022 0.0069

For the cold rolled steel sheet, yield strength (YS), tensile strength (TS) and elongation were measured and the results are shown in Table 2. In addition, the result of measuring the stacking fault energy (SFE) is shown, and the relative permeability was measured and the results are shown in Table 2 together.

On the other hand, the magnetic permeability is expressed as the relative magnetic permeability, which is the ratio of the magnetic permeability in a vacuum and the magnetic permeability in a specific atmosphere. VSM (Vibrating Sample Magnetometer) was used for VSM measurement, and the applied magnetic field was recorded by the Hall probe. The magnetization value of the sample was recorded by the Faraday rule, and the electromotive force obtained when the sample was vibrated was recorded. . VSM is a method of detecting the induced electromotive force generated when a sample is vibrated by this basic operating principle in the search coil and measuring the magnetization value of the sample by this electromotive force. VSM can easily measure the magnetic properties of a material as a function of magnetic field, temperature and time, and has the advantage of being able to measure magnetic forces up to 2 Tesla and a quick range of temperatures from 2 K to 1273 K. It is widely used as a magnetic measuring method because it can measure most types of samples such as single crystal and liquid.

Specimen Type YS (MPa) UTS (MPa) Elongation (%) SFE (mJ / m ² ) Relative permeability Remarks One 484.1 1105.6 60.4 24.57 1.07 Comparative Example 1 2 498.3 960.1 59.3 37.00 1.01 Inventory 1 4 498.8 848.9 49.7 46.68 1.01 Inventive Example 2 5 509.3 1124.1 51.3 20.71 1.06 Comparative Example 2 6 479.5 976 57.6 32.97 1.02 Inventory 3 7 488.2 938.9 58.4 36.94 1.02 Honorable 4 8 485.6 837.8 16.1 19.97 1.08 Comparative Example 3 9 491.9 899.5 30.3 31.34 1.04 Inventory 5 10 477.6 914.6 40.7 35.13 1.02 Inventory 6 11 - - - 29.11 1.04 Comparative Example 4 12 - - - 33.20 1.03 Honorable 7 13 - - - 34.92 1.03 Inventive Example 8 14 - - - 60.82 1.00 Proposition 9 15 - - - 72.50 1.00 Inventory 10 16 - - - 84.07 1.00 Exhibit 11 17 - - - 25.48 1.05 Comparative Example 5 18 - - - 28.51 1.05 Comparative Example 6 19 - - - 30.23 1.04 Inventory 12 20 - - - 56.02 1.00 Inventory 13 21 - - - 67.66 1.00 Inventive Example 14 22 - - - 79.20 1.00 Honorable Mention 15 23 - - - 24.60 1.06 Comparative Example 7 24 - - - 28.67 1.05 Comparative Example 8 25 - - - 29.30 1.04 Comparative Example 9 26 - - - 55.03 1.00 Inventory 16 27 - - - 66.64 1.00 Inventory 17 28 - - - 78.14 1.00 Inventory 18

As can be seen from the results of Table 2, all the invention examples satisfying the scope of the present invention can be seen that the lamination defect energy (SFE) is more than 30 mJ / m ² and the relative permeability is low. That is, it can be seen that excellent nonmagnetic properties can be secured and phase stability is high.

On the other hand, it can be seen that the comparative example has a problem that it is difficult to secure the effect of any one of the lamination defect energy and the relative permeability.

On the other hand, Figure 1 is a graph analyzing the XRD of the invention example 1 and Comparative Example 1, respectively. Figures 1 (a) and (b) specify the degree of stability of the invention and comparative examples, respectively, and confirm the effect of the stacking defect energy, Figure 2 (a) and (b) is compared with the invention example 1 The microstructure of Example 1 was observed. 1 and 2, the invention examples satisfying the scope of the present invention can be seen that evenly generated twins in the entire range it can be confirmed that having excellent phase stability through this. On the other hand, the comparative example can be confirmed that the stack defect energy is low due to the low stack defect energy, twin crystals do not occur in some crystal bill.

Claims (3)

By weight%, C: 0.4-0.9%, Mn: 10-25%, Al: 1.3-8.0%, Si: 0.01-2.0%, Ti: 0.05-0.2%, Si: 0.01-2.0%, B: 0.0005- 0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: 0.003-0.01%, the remainder contains Fe and inevitable impurities,
High-strength nonmagnetic steel sheet with excellent phase stability with a lamination defect energy of 30 mJ / ㎠ or more.
The method according to claim 1,
The steel sheet is a high strength non-magnetic steel sheet having excellent phase stability with a relative permeability of 1.05 or less in a magnetic field of 50 kA / m.
By weight%, C: 0.4-0.9%, Mn: 10-25%, Al: 1.3-8.0%, Si: 0.01-2.0%, Ti: 0.05-0.2%, Si: 0.01-2.0%, B: 0.0005- 0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: 0.003 ~ 0.01%, the rest is reheated steel slab containing Fe and unavoidable impurities to 1100 ~ 1250 ℃ Doing;
Subjecting the reheated steel slab to hot rolling and finish rolling at 800 to 950 占 폚;
Rolling the hot-rolled steel sheet at 400 to 700 ° C;
Cold rolling at a reduction ratio of 30 to 60%;
Continuous annealing the cold rolled steel sheet at 650 ~ 900 ℃
Method for producing a high strength non-magnetic steel sheet excellent in phase stability comprising a.

Images