JP6002779B2 - Non-magnetic high-strength high-manganese steel sheet and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength, high-manganese steel sheet that can be used in heavy electrical equipment such as a distribution board and a transformer and has nonmagnetic properties.

  Materials such as switchboards and transformers usually require excellent nonmagnetic properties as well as high strength.

  In order to satisfy these conditions, stainless steel with a large amount of nickel and chromium has been conventionally used. However, the stainless steel has a problem of low strength and high cost.

  In order to increase the strength, ferritic or martensitic stainless steel can be used. However, since the above ferritic or martensitic stainless steel has high magnetism, power loss due to eddy currents occurs and cost increases. Has the disadvantage of being very expensive.

  Therefore, there is a demand for a material that overcomes the limitations of stainless steel and has high strength and excellent nonmagnetic properties.

  An object of the present invention is to provide a high-strength and high-manganese steel sheet having excellent strength and formability and ensuring excellent nonmagnetic properties, and a method for producing the same.

  According to the present invention, by weight, C: 0.4-0.9%, Mn: 10-25%, Al: 0.01-8.0%, Si: 0.01-2.0%, Ti: 0.05 to 0.2%, B: 0.0005 to 0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: A non-magnetic high-strength high manganese steel sheet containing 0.003-0.01% and the balance Fe and inevitable impurities is provided.

  According to the present invention, a high austenite stability is secured by increasing the austenite stability, ensuring the non-magnetism, and preventing the formation of carbides by carbon by adding Al or the like, thereby providing a high manganese steel that further stabilizes austenite. By doing so, the advantage of having high strength and excellent moldability is obtained. Therefore, it provides sufficient rigidity to be used as a structural member such as a large transformer.

1A is a photograph observing the microstructure of Invention Example 1-7, and FIG. 1B is a photograph observing the microstructure of Comparative Example 1-4. 2 (a) and 2 (b) are XRD graphs obtained by measuring the phase stability of the inventive steel 2-1 and the comparative steel 2-1. FIGS. 3A and 3B are photographs observing the microstructures of the inventive steel 2-1 and the comparative steel 2-1, 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. Since the generation of eddy current increases as the magnetism increases, the loss increases. In general, magnetism is proportional to magnetic permeability (μ). That is, the greater the permeability, the greater the magnetism. The magnetic permeability is defined by the ratio of the induced magnetic field (B) to the magnetic field (H) to be magnetized, that is, μ = B / H. That is, when the magnetic permeability is reduced, the magnetism of the material is reduced. Therefore, when exposed to an electric field, eddy current loss at the surface is prevented and energy efficiency is increased. Therefore, it is advantageous to use energy-free non-magnetic steel plates as materials for switchboards and transformers.

  Therefore, as a result of repeated research, the present inventors have developed a high manganese steel in which austenite stability is increased by adding manganese (Mn) and carbon (C) to the steel in order to provide high strength and nonmagnetic properties. It came to invent. In the present invention, by controlling the content of carbon and manganese, the austenite phase stability is increased, and by adding aluminum, formation of ε-martensite during deformation and slip deformation due to dislocation are suppressed, and excellent It can have excellent nonmagnetic properties as well as strength and stretch ratio (formability).

  Hereinafter, the present invention will be described in detail. First, the steel plate of the present invention will be described in detail. The steel sheet of the present invention satisfies the following composition (hereinafter referred to as “wt%”).

Carbon (C): 0.4-0.9%
C is an element necessary for securing an austenite structure in the steel, and is preferably added in an amount of 0.4% or more in order to sufficiently secure the austenite stability. However, when the amount of C exceeds 0.9%, carbides are excessively precipitated, so that there is a problem that the nonmagnetic characteristics are lowered and castability is lowered. Therefore, the C content is preferably 0.4 to 0.9%.

Manganese (Mn): 10-25%
Mn is an important element that plays a role of stabilizing the austenite structure, and is contained in an amount of 10% or more in the present invention. When the amount of Mn is less than 10%, the α'-martensite phase is present, so the nonmagnetic characteristics are deteriorated. When it exceeds 25%, the production cost is greatly increased. However, there is a problem that internal oxidation is severely generated during heating and the surface quality is deteriorated. Therefore, the Mn content is preferably 10 to 25%.

Aluminum (Al): 0.01 to 8.0%
The Al is an element effective for preventing the formation of carbides, and improves the formability by adjusting the fraction of twins. In the present invention, since carbon acts as an important element for improving the nonmagnetic characteristics by preventing the formation of carbides in order to stabilize the austenite by solid solution, 0.01% or more is contained. However, if it exceeds 8.0%, the manufacturing cost increases, and the surface quality of the product deteriorates due to excessive oxide formation. Therefore, the Al content is preferably 0.01 to 8.0%.

Silicon (Si): 0.01-2.0%
The Si is an element that does not greatly affect the stacking fault energy, and is usually used as a deoxidizer, and is contained by about 0.01% in a general steelmaking process. Moreover, since it will generate many costs when trying to remove this, it is preferable to contain 0.01%. When the content of Si exceeds 2.0%, the manufacturing cost increases, and the surface quality of the product decreases due to the formation of excessive oxides. Therefore, the content of Si is preferably 0.01 to 2.0%.

Titanium (Ti): 0.05-0.2%
The Ti is a component that reacts with nitrogen inside the steel material to precipitate nitrides to form twins, and is added to ensure strength and formability. Further, the Ti content is preferably 0.05% or more in order to form a precipitated phase and increase the strength. When the Ti content exceeds 0.2%, an excessive amount of precipitates are formed and fine cracks are formed during cold rolling, which may deteriorate the formability and weldability. Therefore, the Ti content is preferably 0.05 to 0.2%.

Boron (B): 0.0005 to 0.005%
When B is added in a small amount, it serves to reinforce the grain boundary of the slab, and therefore it is preferably contained in an amount of 0.0005% or more. When the B is excessively included, the cost increases. Therefore, the content of B is preferably 0.0005 to 0.05%.

Sulfur (S): 0.05% or less (excluding 0)
The S needs to be controlled to 0.05% or less in order to control inclusions. If the S content exceeds 0.05%, a problem of hot brittleness may occur. Therefore, the upper limit is preferably 0.05%.

Phosphorus (P): 0.8% or less (excluding 0)
P is an element that is easily segregated, and cracks are generated during casting. Therefore, P is preferably controlled to 0.8% or less in order to prevent this. If the P content exceeds 0.8%, castability may deteriorate. Therefore, the upper limit is preferably 0.8%.

Nitrogen (N): 0.003-0.01%
N is an element that reacts with the atmosphere during the steelmaking process and is always added. If the N content is less than 0.003%, a lot of process costs are generated, and if it exceeds 0.01%, nitrides are formed and formability is lowered, which is not preferable. Therefore, the N content is preferably 0.003 to 0.01%.

  The balance contains Fe and inevitable impurities.

  The steel sheet of the present invention preferably contains 1% by volume or less of carbide in its microstructure. In the present invention, carbon must be dissolved in an atomic state, thereby ensuring austenite stability. That is, when the carbon is present in the steel in the form of carbide, the austenite stability is lowered, the magnetic permeability is increased, and the nonmagnetic properties are inferior. Therefore, in the present invention, it is preferable that the amount of carbide in the steel is as small as possible, and specifically, it is preferably 1% by volume or less. In particular, the carbide is preferably contained at 1% by volume or less even after heat treatment. The heat treatment includes not only heat treatment existing in the manufacturing process of the steel sheet but also heat treatment performed in the process of using the steel sheet.

  On the other hand, the steel sheet of the present invention has an austenite structure and maintains the austenite structure in order to maintain non-magnetism even with external energy such as heat treatment. Therefore, it is preferable that the steel sheet of the present invention has an austenite structure and a part of carbide (1% by volume or less) is formed depending on heat treatment conditions.

The steel sheet of the present invention preferably has a stacking fault energy (SFE) value of 30 mJ / cm ² or more when the Al content is 1.3 to 8.0%. The stacking fault energy is the energy that the interface between the partial dislocations formed in the material has. In the present invention, since the stacking fault energy is controlled by controlling the Al content, the phase stability of the austenite phase is improved.

  When the stacking fault energy has an appropriate value, dislocations and twins harmonize with each other, so the phase stability increases.However, when too low, dislocations cannot be generated / moved, so the phase stability is low. If it decreases and is too high, the deformation proceeds only by dislocations, and the strength increases. Therefore, the present invention has derived the optimum stacking fault energy in order to have an appropriate strength and obtain phase stability.

When the stacking fault energy is less than 30 mJ / cm ² , twins are formed and the strength is increased, but ε-martensite is formed. The ε-martensite has a dense cubic structure and is non-magnetic. However, normally, α-martensite is well formed, so that the magnetism is increased. Therefore, in order to form twins and maintain high strength while maintaining non-magnetism, the stacking fault energy value is preferably 30 mJ / cm ² or more.

  On the other hand, there are various methods for measuring the stacking fault energy, such as an X-ray measurement method, a transmission electron microscope measurement method, and a thermodynamic calculation method. The thermodynamic calculation method using data is most preferable.

  The steel sheet of the present invention preferably has a tensile strength of 800 MPa or more and possesses excellent strength and workability by securing a draw ratio of 15% or more.

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

  A steel slab satisfying the above composition is reheated at 1100 to 1250 ° C. When the heating temperature is too low, a rolling load may be excessively applied during hot rolling, and thus heating at a temperature of 1100 ° C. or higher is preferable. The higher the heating temperature, the easier the hot rolling, but in the case of steel with a high Mn content, internal oxidation may occur severely during high-temperature heating and the surface quality may deteriorate, so the upper limit of the above reheating temperature Is preferably 1250 ° C.

  After the reheating, hot rolling is performed, and hot finish rolling is performed at 800 to 1000 ° C. The higher the hot finish rolling temperature, the lower the deformation resistance and the easier the rolling, but the surface quality may be lowered, so it is preferable to carry out at 1000 ° C. or lower. In addition, when the hot finish rolling temperature is too low, the load increases during rolling.

  After the hot rolling, a winding step is performed. The winding is preferably performed at 400 to 700 ° C. Usually, the cooling rate after the winding is often slow. When the winding start temperature is too low, a large amount of cooling water is required for cooling, and a large load is applied during winding, so the winding start temperature is set to 400 ° C. or higher. When the coiling temperature is high, a reaction between the oxide film on the surface of the plate and the matrix structure of the steel plate occurs during the cooling process after winding, and the pickling property is deteriorated. preferable.

  It is preferable to perform water cooling after the hot rolling and before winding.

  The hot-rolled steel sheet manufactured as described above is cold-rolled to manufacture a cold-rolled steel sheet. The rolling reduction during the cold rolling is usually determined by the thickness of the product. In the present invention, since recrystallization is performed in the heat treatment process after cold rolling, the driving force for recrystallization needs to be well controlled. That is, if the rolling reduction at the time of cold rolling is too low, the strength of the product is lowered, so it is preferable to carry out at a rolling reduction of 30% or more, and when the rolling reduction is too high, it is advantageous for securing the strength. Since the load on the rolling mill increases, the rolling reduction is preferably 60% or less.

  After the cold rolling, continuous annealing is performed. The continuous annealing is preferably performed at 650 to 900 ° C. The continuous annealing is preferably performed at 650 ° C. or more at which recrystallization occurs sufficiently. However, when the annealing temperature is too high, oxides are formed on the surface, and work with the connected product before / after the continuous operation is performed. It is preferable to be performed at 900 ° C. or lower because the properties are reduced.

  Examples of the present invention will be described in detail below. The following examples are for the understanding of the present invention and are not intended to limit the present invention.

Example 1
A steel slab satisfying the composition shown in Table 1 below is reheated to 1200 ° C., hot-finished at 900 ° C., wound at 500 ° C., cold-rolled at a reduction rate of 50%, and continuously at 800 ° C. Annealing was performed to produce a cold-rolled steel sheet.

  In order to investigate the physical properties of the steel sheet, the yield strength, tensile strength, and stretch ratio were measured, and the results are shown in Table 2 below.

  About the manufactured steel plate, the fraction of inclusions, the fraction of carbide according to heat treatment conditions, and the relative permeability at a magnetic field of 25 kA / m were measured. The heat treatment simulates heat treatment in the manufacturing process or heat treatment performed during use of the steel sheet.

On the other hand, the magnetic permeability is displayed as a relative magnetic permeability that is a ratio of the magnetic permeability in a vacuum and the magnetic permeability in a specific atmosphere. In the present invention, it was measured relative permeability mu _r is the permeability ratio of the vacuum and the atmosphere. For measurement, a VSM (Vibrating Sample Magnetometer) equipment is used, the applied magnetic field applied by the Hall probe is recorded using the VSM, and the electromotive force obtained when the sample is vibrated by Faraday's law is recorded. Measure the magnetization value. VSM is a method for detecting an induced electromotive force generated when a vibration is applied to a sample according to the basic operation principle as described above, by a search coil, and measuring the magnetization value of the sample by this electromotive force. The material's magnetic properties can be easily measured as a function of magnetic field, temperature, and time, and a maximum of 2 Tesla magnetic force and a fast measurement in the temperature range of 2 K to 1273 K are possible. In addition, it is widely used as a magnetic measurement method because it has the advantage that most types of samples such as powder, thin film, single crystal, and liquid can be measured.

  From the results of Table 3 above, when heat treatment is performed at 400 ° C. for 1 hour, which is the heat treatment condition, when the carbide fraction is 1% by volume or less, the magnetic permeability is 1.05 or less and excellent nonmagnetic characteristics are obtained. I understand that. It can also be seen that when the heat treatment is performed at 600 ° C. for 5 hours, which is a more severe heat treatment condition, the magnetic permeability does not exceed 1.10 when the carbide fraction is 1% by volume or less.

  On the other hand, the microstructures of Invention Example 1-7 and Comparative Example 1-3 were observed and shown in FIGS. 1 (a) and 1 (b), respectively. Referring to FIG. 1, the present invention example has a very small fraction of carbide, but the comparative example outside the scope of the present invention has a non-magnetic property inferior because the fraction of carbide exceeds 1% by volume. I understand that.

  Therefore, it can be seen that when the carbide fraction is 1% by volume or less, excellent nonmagnetic characteristics can be secured.

(Example 2)
A steel slab satisfying the composition (% by weight) shown in Table 4 below was reheated to 1200 ° C., subjected to hot finish rolling at 900 ° C., wound at 500 ° C., and then cold-rolled at a reduction rate of 50%. Continuous annealing was performed at 800 ° C. to produce a cold-rolled steel sheet.

  With respect to the cold-rolled steel sheet, the yield strength (YS), tensile strength (TS), and stretch ratio were measured, and the results are shown in Table 5. In addition, stacking fault energy (SFE) and relative permeability were measured, and the results are shown together in Table 5. The relative permeability was measured with a magnetic field of 50 kA / m, and the others were performed under the same conditions as in Example 1 above.

From the results of Table 5 above, it can be seen that all the inventive examples satisfying the scope of the present invention have a stacking fault energy (SFE) of 30 mJ / cm ² or more and a low relative magnetic permeability. That is, it can be seen that excellent non-magnetic properties are ensured 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 ensure any one of the stacking fault energy and the relative permeability.

  On the other hand, FIG. 2 is a graph obtained by analyzing the XRDs of the invention example 2-1 and the comparative example 2-1. FIGS. 2 (a) and 2 (b) show the effects of stacking fault energy by specifying the phase stability of the inventive example and the comparative example, respectively. FIGS. The microstructures of 2-1 and Comparative Example 2-1 were observed. 2 and 3, it can be seen that the invention examples satisfying the scope of the present invention have excellent phase stability because uniform twins are generated over the entire range. On the other hand, in the comparative example, the stacking fault energy is low, and the generation of twins increases after deformation, and it can be seen that some crystals do not generate twins.

Claims (4)

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%, B: 0.0005-0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: 0.003-0. 01%, and the balance Fe and inevitable impurities, the fraction of carbide in the microstructure is 1% by volume or less, non-magnetic high strength high manganese steel sheet,
The stacking fault energy of the steel sheet is 30 mJ / cm ² or more,
The relative permeability of the steel sheet is 1.10 or less in a magnetic field of 25 kA / m .
Non-magnetic high strength high manganese steel sheet. The non-magnetic high-strength high-manganese steel sheet according to claim 1 , wherein the steel sheet has a relative magnetic permeability of 1.05 or less in a magnetic field of 50 kA / m. The non-magnetic high-strength high-manganese steel sheet according to claim 1 or 2 , wherein the steel sheet has a tensile strength of 800 MPa or more and a draw ratio of 15% or more. 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%, B: 0.0005-0.005%, S: 0.05% or less (excluding 0), P: 0.8% or less (excluding 0), N: 0.003-0. Reheating the steel slab consisting of 01% and the balance Fe and inevitable impurities to 1100 to 1250 ° C .;
Hot rolling the reheated steel slab and finishing rolling at 800-950 ° C .;
Winding the hot-rolled steel sheet at 400 to 700 ° C .;
Cold rolling at a rolling reduction of 30-60%,
A step of continuously annealing the cold-rolled steel sheet at 650 to 900 ° C., and a method for producing a non-magnetic high-strength high-manganese steel sheet ,
The manufacturing method of the nonmagnetic high intensity | strength high manganese steel plate whose said steel plate is the nonmagnetic high intensity high manganese steel plate of any one of Claims 1-3 .