KR20210105418A - Manufacturing method of high manganese steel slab, manufacturing method of high manganese steel slab and high manganese steel sheet - Google Patents

Abstract

Provided are a method for manufacturing a high manganese steel slab or steel sheet capable of controlling inclusions and precipitates that determine a crystal grain size and suppressing the occurrence of surface flaws, and a method for manufacturing a high manganese steel slab used in their production.
As a manufacturing method of a high manganese steel slab, in the continuous casting of molten steel containing a specific component composition, the content of Ca, O and S of the molten steel in the tundish satisfies the following formula (1): manufacturing method.
0.4 ≤ ACR ≤ 1.4...(1)
ACR of the said (1) formula is computed by following (2) formula.
ACR = {[%Ca] - (0.18 + 130 × [%Ca]) × [%O]}/(1.25 × [%S]) ... (2)
[%Ca] in the formula (2) is the content of Ca in the molten steel (mass%), [%O] is the content of O in the molten steel (mass%), and [%S] is the content of S in the molten steel content (mass %).

Description

Manufacturing method of high manganese steel slab, manufacturing method of high manganese steel slab and high manganese steel sheet

The present invention relates to a method for manufacturing a steel slab or steel sheet, which is a high manganese steel material for structural steel used in a cryogenic environment such as a nuclear fusion facility, a roadbed for a linear motor car, a member for a mechanical structure such as a nuclear magnetic resonance single-layer room, and a tank for storing liquefied gas, and It relates to a method for manufacturing a high manganese steel slab used for their production.

BACKGROUND ART High manganese steel having an austenitic single-phase structure and non-magnetic properties is in high demand as an inexpensive steel material that replaces the conventional austenitic stainless steel, 9% nickel steel, and metal materials for cryogenic use such as 5000 aluminum alloy.

Conventionally, steel slabs used as raw materials for these high manganese steels have been generally manufactured by obtaining slabs by the ingot method and hot ingot-rolling them. is becoming indispensable. When a high manganese steel slab or steel sheet is manufactured from a slab obtained by the continuous casting method, surface cracks of the slab during continuous casting or surface cracks of the steel slab or steel sheet during hot rolling occur frequently, increasing trimming for crack removal and yield degradation is a problem. For this reason, there has been a strong demand for the production of a high manganese steel slab capable of suppressing surface cracking in the production of a steel slab or steel sheet.

As a technique for hot-rolling a continuous cast slab of high manganese steel without causing surface cracks, Patent Document 1 contains 0.0002% or more of Mg, Ca, and REM in a total of 0.0002% or more, 30 C + 0.5 A high Mn steel excellent in low-temperature toughness containing components satisfying Mn + Ni + 0.8 Cr + 1.2 Si + 0.8 Mo ≥ 25 and O/S ≥ 1 is disclosed.

Japanese Laid-Open Patent Publication No. 2016-196703

Although the high Mn steel disclosed in Patent Document 1 achieves excellent fine grains by containing the above components, there is a problem that it is insufficient to control the types of inclusions and precipitates that determine the grain size as intended.

The present invention has been made in view of such a situation, and a method for manufacturing a high manganese steel slab or steel sheet capable of controlling inclusions and precipitates that determine the grain size and suppressing the occurrence of surface flaws, and high manganese steel used in their production An object of the present invention is to provide a method for manufacturing a slab.

The characteristics of this invention for solving such a subject are as follows.

[1] In mass %, C: 0.10% or more and 1.3% or less, Si: 0.10% or more and 0.90% or less, Mn: 10% or more and 35% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% More than 0.1%, Cr: 10% or less, Ca: 0.0001% or more, 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Ti: 0.001% or more and 0.03% or less, N: 0.0001% or more and 0.20% or less, O: 0.0100 % or less, and in the continuous casting of molten steel having a component composition in which the balance consists of iron and unavoidable impurities, the content of Ca, O and S of the molten steel in the tundish satisfies the following formula (1) A method for manufacturing a high manganese steel slab.

0.4 ≤ ACR ≤ 1.4...(1)

ACR of the said (1) formula is computed by following (2) formula.

ACR = {[%Ca] - (0.18 + 130 × [%Ca]) × [%O]}/(1.25 × [%S]) ... (2)

[%Ca] in the formula (2) is the content of Ca in the molten steel (mass%), [%O] is the content of O in the molten steel (mass%), and [%S] is the content of S in the molten steel content (mass %).

[2] The method for producing a high manganese steel slab according to [1], wherein the content of Ti, Mg, and N of the molten steel in the tundish further satisfies the following formula (3).

[%Ti] × [%N] × [%Mg] ≥ 2.0 × 10 ^-8 ... (3)

[%Ti] in the formula (3) is the content (mass%) of Ti in the molten steel, [%N] is the content (mass%) of N in the molten steel, and [%Mg] is the content of Mg in the molten steel content (mass %).

[3] Further, in mass%, Nb: 0.001% or more and 0.01% or less, V: 0.001% or more and 0.03% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Mo: 0.05% or more The method for producing a high manganese steel slab according to [1] or [2], wherein the molten steel having a component composition containing one or two or more selected from the group consisting of 2.00% or less and W: 0.05% or more and 2.00% or less is continuously cast.

[4] A method for producing a high manganese steel slab, wherein the slab produced by the method for producing a high manganese steel slab according to any one of [1] to [3] is hot-rolled to produce the steel slab.

[5] A method for producing a high manganese steel sheet, wherein the steel sheet is manufactured by hot rolling the slab produced by the method for manufacturing a high manganese steel slab according to any one of [1] to [3].

By practicing the present invention, it is possible to manufacture a high manganese steel slab while suppressing the occurrence of surface flaws. In addition, when the high manganese steel slab is hot-rolled onto a steel slab or steel sheet, it is possible to manufacture a high manganese steel slab or steel sheet while suppressing the occurrence of surface flaws during rolling.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which put together the generation|occurrence|production behavior of high manganese steel inclusions and precipitates in a solidification process from molten steel in a prior art example and an invention example.

EMBODIMENT OF THE INVENTION Hereinafter, although embodiment of this invention is described, this invention is not limited to the following embodiment. The high manganese steel according to this embodiment has C: 0.10% or more and 1.3% or less, Si: 0.10% or more and 0.90% or less, Mn: 10% or more and 35% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.1% or less, Cr: 10% or less, Ca: 0.0001% or more and 0.010% or less, Mg: 0.0001% or more and 0.010% or less, Ti: 0.001% or more and 0.03% or less, N: 0.0001% or more and 0.20% or less, O : It contains 0.0001% or more and 0.0100% or less, and has a component composition in which the balance consists of iron and unavoidable impurities. The component composition is Nb: 0.001% or more and 0.01% or less, V: 0.001% or more and 0.03% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Mo: 0.05% or more and 2.00% or less, and W: You may contain 1 type or 2 or more types chosen from 0.05 % or more and 2.00 % or less. Unless otherwise indicated, "%" which shows content of the component in a component composition means "mass %." The content of all components is not a dissolved amount but a total value.

C: 0.10% or more and 1.3% or less

C is added for the purpose of stabilizing the austenite phase and improving the strength. If the C content is less than 0.10%, the required strength cannot be obtained. On the other hand, when the content of C exceeds 1.3%, the amount of precipitation of carbides and cementite becomes excessive, and toughness decreases. For this reason, it is necessary for content of C to be 0.10 % or more and 1.3 % or less, and it is preferable that they are 0.30 % or more and 0.8 % or less.

Si: 0.10% or more and 0.90% or less

Si is added for the purpose of deoxidation and solid solution strengthening. In order to acquire this effect, content of Si needs to be 0.10 % or more. On the other hand, Si is a ferrite stabilizing element, and when it is added in a large amount, the austenite structure of high manganese steel becomes unstable. For this reason, content of Si needs to be 0.90 % or less. Therefore, content of Si needs to be 0.10 % or more and 0.90 % or less, and it is preferable that they are 0.20 % or more and 0.60 % or less.

Mn: 10% or more and 35% or less

Mn is an element that stabilizes the austenite structure and causes an increase in strength. In particular, by setting the Mn content to 10% or more, properties such as non-magnetic and low-temperature toughness expected of austenitic steel are obtained. On the other hand, in general, austenitic steel lacks hot workability, and among them, high manganese steel is known as a material with high cracking susceptibility during continuous casting or hot rolling. In particular, when content of Mn exceeds 35 %, workability will fall remarkably. Therefore, it is necessary for content of Mn to be 10 % or more and 35 % or less, and it is preferable that they are 20 % or more and 28 % or less.

P: 0.030% or less

P is an impurity element contained in steel and causes a decrease in toughness and hot embrittlement. For this reason, although it is so preferable that there is little content of P, up to 0.030 % is permissible. Therefore, content of P needs to be 0.030 % or less, and it is preferable that it is 0.015 % or less.

S: 0.0070% or less

S is an impurity element contained in steel, and a sulfide such as MnS is a starting point to decrease toughness. For this reason, although it is so preferable that there is little content of S, it is permissible up to 0.0070 %. Therefore, content of S needs to be 0.0070 % or less, and it is preferable that it is 0.0030 % or less.

Al: 0.01% or more and 0.1% or less

Al is added for the purpose of deoxidation. In order to acquire a necessary deoxidation effect, content of Al needs to be 0.01 % or more. On the other hand, even if the content of Al exceeds 0.1%, even if it is added, the deoxidation effect reaches a critical point, and at the same time, excess AlN is generated and hot workability deteriorates. Therefore, it is necessary for content of Al to be 0.01 % or more and 0.1 % or less, and it is preferable that they are 0.02 % or more and 0.05 % or less.

Cr: 10% or less

Cr is added for the purpose of solid solution strengthening. On the other hand, when a large amount of Cr is added, the austenite structure of high manganese steel becomes unstable, and coarse carbides that cause embrittlement are precipitated. Therefore, it is necessary for content of Cr to be 10 % or less, and it is preferable that it is 7 % or less.

Ca: 0.0001% or more and 0.010% or less

When Ca is added in an appropriate amount, fine oxides and sulfides are formed, and grain boundary embrittlement caused by precipitation inclusions is suppressed. For this reason, the content of Ca needs to be 0.0001% or more. On the other hand, when content of Ca becomes excessive, precipitation inclusions will coarsen and conversely, grain boundary embrittlement will be accelerated|stimulated. For this reason, the content of Ca needs to be 0.010% or less. It is preferable that content of Ca is 0.0005 % or more and 0.0050 % or less.

Mg: 0.0001% or more and 0.010% or less

Like Ca, Mg is very easy to bond with O and S elements, and from forming fine oxides and sulfides, suppression of grain boundary embrittlement by precipitation inclusions can be expected. For this reason, the content of Mg needs to be 0.0001% or more. On the other hand, when the content of Mg becomes excessive, the reaction with the molten steel becomes intense at the time of addition, and there is a fear that not only the cleanliness of the molten steel is adversely deteriorated, but also there is a fear that the precipitated inclusions are coarsened. For this reason, the content of Mg needs to be 0.010% or less. It is preferable that content of Mg is 0.0005 % or more and 0.0020 % or less.

Ti: 0.001% or more and 0.03% or less

Since Ti bonds with C and N elements under high temperature conditions, it is effective for suppressing the formation of large carbides and suppressing precipitation of Nb and V carbonitrides with high cracking susceptibility. For this reason, content of Ti needs to be 0.001 % or more. On the other hand, when Ti is added in a large amount, a huge carbonitride is produced, and deterioration of low-temperature toughness becomes a problem. For this reason, content of Ti needs to be 0.03 % or less. It is preferable that content of Ti is 0.001 % or more and 0.02 % or less.

N: 0.0001% or more and 0.20% or less

N stabilizes the austenite structure and increases the strength by solid solution and precipitation. For this effect, the content of N needs to be 0.0001% or more. On the other hand, when content of N exceeds 0.20 %, hot workability will fall. For this reason, content of N needs to be 0.0001 % or more and 0.20 % or less. It is preferable that content of N is 0.0050 % or more and 0.10 % or less.

O: 0.0100% or less

The content of O is a value determined depending on the degree of deoxidation and removal of inclusions in the molten steel stage, and it is preferable that the content of O is smaller from the viewpoint of cleanliness. Here, content of O is content of total O containing O as an oxide (inclusion). When the content of O is too large, not only the above-mentioned elements such as Ca and Mg cannot produce sufficient effects, but also solidification defects such as pores tend to occur in the cast steel. For this reason, the content of O needs to be 0.0100% or less. It is preferable that content of O is 0.0030 % or less.

In addition, the high manganese steel which concerns on this embodiment may contain 1 type, or 2 or more types selected from the following elements as needed.

Nb: 0.001% or more and 0.01% or less, V: 0.001% or more and 0.03% or less

Both Nb and V produce carbonitrides, and since the produced carbonitrides act as trap sites for diffusible hydrogen, they have an effect of suppressing stress corrosion cracking. In order to acquire this effect, it is preferable to contain in Nb: 0.001 % or more and 0.01 % or less, and V: 0.001 % or more and 0.03 % or less. Within these component composition ranges, the production of high-manganese steel slabs while suppressing the occurrence of surface flaws, and the hot-rolling of the high-manganese steel slabs into steel slabs or steel plates, while suppressing the occurrence of surface flaws during rolling, It does not affect the production of high manganese steel slabs or steel plates.

Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less

Cu and Ni stabilize the austenite structure and contribute to suppression of precipitation of carbides. In order to acquire these effects, it is preferable to contain in Cu: 0.01 % or more and 1.00 % or less, and Ni: 0.01 % or more and 0.50 % or less. Within these component composition ranges, the production of high-manganese steel slabs while suppressing the occurrence of surface flaws, and the hot-rolling of the high-manganese steel slabs into steel slabs or steel plates, while suppressing the occurrence of surface flaws during rolling, It does not affect the production of high manganese steel slabs or steel plates.

Mo: 0.05% or more and 2.00% or less, W: 0.05% or more and 2.00% or less

Mo and W contribute to the improvement of the strength of the base material. In order to acquire these effects, it is preferable to contain in Mo: 0.05 % or more and 2.00 % or less, and W: 0.05 % or more and 2.00 % or less. Within these component composition ranges, the production of high-manganese steel slabs while suppressing the occurrence of surface flaws, and the hot-rolling of the high-manganese steel slabs into steel slabs or steel plates, while suppressing the occurrence of surface flaws during rolling, It does not affect the production of high manganese steel slabs or steel plates.

The remainder other than the above is iron and unavoidable impurities. As a result of observing the slab obtained by continuously casting molten steel having such a component composition, the following findings were obtained. Since high manganese steel is an austenitic single-phase steel, an austenite structure with a large grain size is generated from the surface of the cast steel. Since the austenite grain size is coarse, in addition to the low ductility at high temperature compared to general steel, impurity elements such as P and S are concentrated at the grain boundaries. The concentration of this impurity element weakens the grain boundaries, and surface cracks occur in the form of propagating the fragile grain boundaries.

In addition, since high manganese steel has a high Mn concentration in the steel and high reactivity with S, the production of MnS becomes more remarkable than that of general steel. M ₂₃ C ₆ carbides (M: Mn, Cr, Fe, Mo) are also produced. These precipitates are likely to precipitate at the grain boundaries of high manganese steel, and even when coarse sulfide or carbide precipitates are concentrated and precipitated at the grain boundaries, the grain boundaries become brittle and surface cracks occur.

Therefore, in order to suppress surface cracking in continuous casting and hot rolling, it becomes important to refine the grain size and to avoid weakening of grain boundaries due to the concentration of impurity elements. In particular, high manganese steel, which is an austenitic single-phase steel, rapidly develops an austenite structure from just below the solidus temperature, so it is important to precipitate inclusions serving as pinning nuclei at least to the solidus temperature. In addition, it is also important to suppress the precipitation of coarse sulfide or carbide, which are harmful to cracks, concentrated at the grain boundaries, and to disperse the distortion concentrated at the grain boundaries by fine precipitation within the grains.

As a method of suppressing the production of MnS, a method of adding Ca to steel is known. Suppression of MnS production by the addition of Ca is being implemented in a steel for a sour-resistant line pipe or the like. As an index of Ca addition, Ca/S and ACR index (atomic concentration ratio index) are known. It is known that adding Ca so as to satisfy Ca/S>2 or adding Ca so as to satisfy the following formula (4) is effective for suppression of MnS production.

1 ≤ ACR < 3...(4)

The said ACR is computed by following (2) Formula.

ACR = {[%Ca] - (0.18 + 130 × [%Ca]) × [%O]}/(1.25 × [%S]) ... (2)

[%Ca] in the formula (2) above is the Ca content (mass%) in the molten steel in the tundish, [%O] is the O content (mass%) in the molten steel in the tundish, [ %S] is the content (mass %) of S in the molten steel in the tundish.

By the addition of Ca, the Al ₂ O ₃ inclusions in the molten steel is controlled to form CaO · Al ₂ O ₃ oxide, and the MnS precipitated in the time of solidification is suppressed by the CaS. This idea can also be applied to high manganese steels. However, when Ca is added to molten steel of high manganese steel to satisfy Equation (4) above, CaS-MnS is generated in the molten steel step. CaS-MnS generated in the molten steel step tends to aggregate and coalesce and is easily removed by flotation. Even when introduced into the slab after solidification, it is difficult to form pinning nuclei of grain refinement.

For this reason, in the manufacturing method of the high manganese steel slab according to the present invention, Ca is added to the molten high manganese steel, and the content of Ca, O and S of the molten high manganese steel in the tundish satisfies the following formula (1). Adjust the composition. The component composition of molten steel can be measured by component analysis of the molten steel sampled from the inside of a tundish.

0.4 ≤ ACR ≤ 1.4...(1)

Said ACR is computed by said (2) Formula.

Thereby, CaO·MnO·Al ₂ O ₃ inclusions can be precipitated from the liquidus temperature of the high manganese steel (hereinafter referred to as TLL) to the solidus temperature (hereinafter referred to as TSL). The inclusions precipitated between TLL and TSL are finely dispersed and precipitated within the grains in the cast steel and at the grain boundaries. MnS, CaS, and M ₂₃ C ₆ precipitated after complete solidification also precipitate around the CaO·MnO·Al ₂ O ₃ precipitate, so that MnS and M ₂₃ C ₆ are finely dispersed. Thereby, the precipitation of coarse sulfides and carbides at grain boundaries is suppressed, and the occurrence of surface cracks resulting from grain boundary cracks is largely suppressed.

In addition, since the precipitates finely dispersed in the grains also act as pinning nuclei for grain refining, the grain size is reduced, and weakening of grain boundaries due to concentration of impurity elements can also be avoided. As a result, the occurrence of surface cracks resulting from grain boundary cracking is further suppressed.

On the other hand, when the content of Ca, O, and S of the molten high manganese steel in the tundish is ACR < 0.4, since the amount of Ca added is too small, fine CaO·MnO·Al ₂ O ₃ inclusions are not generated, and MnS is mainly generated at grain boundaries after solidification. In the case of ACR > 1.4, since CaS is easily generated in the molten steel step, it does not function as a pinning nucleus for crystal grain size refinement, and the grain size cannot be refined. It is preferable that the content of Ca, O and S of the molten high manganese steel in the tundish satisfies the following formula (5), whereby the pinning effect by fine MnS is increased, and the grain size can be further reduced.

0.4 ≤ ACR ≤ 0.9...(5)

The temperature at which CaO·MnO·Al ₂ O ₃ is deposited is between TLL and TSL, and it is preferable to set the temperature at which the solid phase ratio of the molten steel for high manganese steel is 0.3 or more. This is because the solid phase ratio of 0.3 or more is close to the flow limit solid phase ratio of molten steel, and the precipitated inclusions do not flow into the molten steel and stay at the deposited position. The solid phase ratio is more than the TLL of the steel and is defined as the solid phase ratio = 0, and the solid phase ratio = 1.0 below the TSL of the steel.

Further, Ti and N may be added to the molten steel to finely disperse the precipitates using MgO and TiN as nuclei. Ti and N are added to molten steel, the solubility is reached at a high temperature of 1300 to 1400°C, TiN is precipitated, and crystal grain refinement is achieved by using the precipitate as pinning nuclei. Since fine TiN is stably produced when MgO is present, it is effective to contain Mg in addition to Ti and N. As a result of conducting an experiment in which the contents of Ti, N, and Mg of the molten steel of high manganese steel were changed, the content of Ti, Mg and N of the molten steel of high manganese steel in the tundish satisfies the following formula (3) , it was found that MgO-TiN was finely dispersed and precipitated in the crystal grains immediately after solidification.

[%Ti] × [%N] × [%Mg] ≥ 2.0 × 10 ^-8 ... (3)

In the formula (3) above, [%Ti] is the content (mass%) of Ti in the molten steel in the tundish, [%N] is the content (mass%) of N in the molten steel in the tundish, [ %Mg] is the content (mass %) of Mg in the molten steel in the tundish. Considering the lower limit of analysis, even when Ti is not added, Ti is 0.0001 mass %, and Mg is 0.0001 mass % even when Mg is not added.

MgO-TiN takes a form in which MnS and CaS-MnS are precipitated around the CaO·MnO·Al ₂ O _{3 precipitate, similarly to the CaO·MnO·Al 2 O 3 precipitate.} For this reason, it is possible to suppress the precipitation of coarse MnS precipitates at grain boundaries by finely dispersing the MgO-TiN precipitates in the crystal grains. Since the MgO-TiN precipitates function as pinning nuclei, the crystal grain size is further refined. Thereby, generation|occurrence|production of the surface crack resulting from grain boundary cracking is further suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which put together the generation|occurrence|production behavior of high manganese steel inclusions and precipitates in a solidification process from molten steel in a prior art example and an invention example. And inclusions in molten steel of manganese steel is, an oxide of Al ₂ O ₃ -MnO inclusions, and is present in the molten steel of 1500 ~ 1600 ℃.

In the conventional example (comparative example) in which Ca is not added, the CaO·MnO·Al ₂ O ₃ inclusions serving as pinning nuclei for crystal grain size refinement are not generated, and the grain size refinement is not achieved. Moreover, MnS and M ₂₃ C ₆ become coarse precipitates and precipitate at the grain boundaries after solidification. As a result, the grain boundaries become brittle and surface cracks become conspicuous.

On the other hand, in the example of this invention, Ca is added and the component composition is adjusted so that content of Ca, O, and S of the molten high manganese steel in a tundish may satisfy said formula (1). Thereby, Al ₂ O ₃ -MnO inclusions become CaO·MnO·Al ₂ O ₃ inclusions between TLL and TSL, and are finely dispersed and precipitated in grains and at grain boundaries.

In addition, in the example of this invention, Ti, Mg, and N are added, and the component composition is adjusted so that content of Ti, Mg, and N of the molten high manganese steel in a tundish may satisfy said formula (3). As a result, the MgO-TiN inclusions are finely dispersed and precipitated in the grains and at the grain boundaries between TLL and TSL.

MnS and CaS and M ₂₃ C _{6 that} are precipitated thereafter are precipitated around the CaO·MnO·Al ₂ O ₃ precipitate and the MgO-TiN precipitate. Concentration in the precipitation is suppressed.

In addition, since the fine precipitates dispersed in the crystal grains function as pinning nuclei for refining the crystal grain size, the crystal grain size is also reduced. By suppressing the precipitation of these coarse MnS precipitates into grain boundaries and refining the grains, surface cracks during the production of high manganese steel slabs are suppressed, and high manganese steel slabs are continuously cast, and the slabs are hot-rolled to obtain steel slabs or steel sheets. The surface cracking at the time of manufacturing is suppressed.

Example

High manganese steel is melted using a 150-ton converter, an electrode-heated blowing and refining furnace, and an RH vacuum degasser, and after adjusting the molten steel composition and temperature, using a tundish with a capacity of 30 tons, a curved continuous casting machine with a bending radius of 10.5 mR was used to continuously cast a slab with a cross-sectional size of 1250 mm in width x 250 mm in thickness. The casting speed was made into the range of 0.7-0.9 m/min, and the secondary cooling water amount was made into the range of 0.3-0.6 L/kg. Thereafter, the slab was once lowered to a cold slab by slow cooling, and the slab was charged into a heating furnace for a predetermined time so as to reach a predetermined target temperature, and then hot-rolled at a total reduction ratio of 48% to manufacture a steel slab. The presence or absence of surface cracks of the steel piece after rolling was investigated by the penetration liquid test (PT).

Table 1 shows the component composition of the molten steel sampled from the tundish in Invention Examples 1-39, the calculated values of the formulas (1) and (3), and the investigation result of surface cracks.

As shown in Table 1, in the high manganese steels of Invention Examples 1 to 39 satisfying the formula (1), the surface cracks of the steel pieces after hot rolling were slight or no surface cracks. Moreover, in the high manganese steels of Invention Examples 1, 2, 5-15, 27, 29-39 which satisfy Formula (1) and Formula (3), there was no surface cracking of the steel piece after hot rolling. In Invention Examples 1 to 39, as a result of examination of the coagulation structure of the cast slab after casting, it was confirmed that the crystal grains were smaller than in the normal case.

Table 2 and Table 3 show the component composition of the molten steel sampled from the tundish in Comparative Examples 1-70, the calculated values of the formulas (1) and (3), and the results of surface cracking.

As shown in Tables 2 and 3, in the high manganese steels of Comparative Examples 1 to 70 that do not satisfy the above expression (1), surface cracks occurred in the steel pieces after hot rolling, and all of them were moderately polished with a grinder. As this was necessary, the cost and process load were large.

From these results, when the component composition of the molten steel in the tundish satisfies the formula (1), it was confirmed that the surface cracking of the steel piece after hot rolling could be suppressed. Although the said confirmation is based on the steel slab manufactured from a slab, even if it is a steel plate manufactured from a slab, the surface crack of a steel plate can be suppressed similarly. From the fact that the steel slab after rolling had no or slight surface cracks, it can be seen that there were no surface cracks or surface cracks were slight even in the slab continuously cast. Thus, when the component composition of the molten steel in a tundish satisfy|fills said (1) Formula and said (3) Formula, it was confirmed that the surface crack of the steel piece after hot rolling can be suppressed further. In this way, if the surface cracks of the steel slab and steel sheet after rolling can be suppressed, a direct feed manufacturing process such as continuous casting → heating furnace → main rolling becomes possible, and a significant reduction in energy cost becomes possible.

Claims (5)

in mass %,
C: 0.10% or more and 1.3% or less;
Si: 0.10% or more and 0.90% or less,
Mn: 10% or more and 35% or less;
P: 0.030% or less;
S: 0.0070% or less;
Al: 0.01% or more and 0.1% or less,
Cr: 10% or less;
Ca: 0.0001% or more and 0.010% or less,
Mg: 0.0001% or more and 0.010% or less,
Ti: 0.001% or more and 0.03% or less,
N: 0.0001% or more and 0.20% or less,
In the continuous casting of molten steel containing O: 0.0100% or less and having a component composition in which the balance consists of iron and unavoidable impurities, the content of Ca, O and S of the molten steel in the tundish is as follows (1) A method for producing a high manganese steel slab that satisfies the formula.
0.4 ≤ ACR ≤ 1.4...(1)
ACR of the said (1) formula is computed by following (2) formula.
ACR = {[%Ca] - (0.18 + 130 × [%Ca]) × [%O]}/(1.25 × [%S]) ... (2)
[%Ca] in the formula (2) is the content of Ca in the molten steel (mass%), [%O] is the content of O in the molten steel (mass%), and [%S] is the content of S in the molten steel content (mass %). The method of claim 1,
The content of Ti, Mg and N of the molten steel in the tundish further satisfies the following formula (3), a method for producing a high manganese steel slab.
[%Ti] × [%N] × [%Mg] ≥ 2.0 × 10 ^-8 ... (3)
[%Ti] in the formula (3) is the content (mass%) of Ti in the molten steel, [%N] is the content (mass%) of N in the molten steel, and [%Mg] is the content of Mg in the molten steel content (mass %). 3. The method according to claim 1 or 2,
In addition, in mass %,
Nb: 0.001% or more and 0.01% or less,
V: 0.001% or more and 0.03% or less,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 0.50% or less,
Mo: 0.05% or more and 2.00% or less,
W: A method for producing a high manganese steel slab in which molten steel having a component composition containing one or two or more selected from among 0.05% or more and 2.00% or less is continuously cast. A method for producing a high manganese steel slab, wherein the steel slab is manufactured by hot rolling the slab produced by the method for manufacturing a high manganese steel slab according to any one of claims 1 to 3. A method for manufacturing a high manganese steel sheet, wherein the steel sheet is manufactured by hot rolling the slab produced by the method for manufacturing a high manganese steel slab according to any one of claims 1 to 3.

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