CN113366138A

CN113366138A - Method for manufacturing high manganese steel cast sheet, high manganese steel sheet, and method for manufacturing high manganese steel sheet

Info

Publication number: CN113366138A
Application number: CN202080011614.3A
Authority: CN
Inventors: 伊藤阳一; 荒牧则亲; 中岛孝一; 木津谷茂树
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-03-19
Filing date: 2020-03-19
Publication date: 2021-09-07
Also published as: WO2020189762A1; JPWO2020189762A1; JP7126077B2; KR20210105418A

Abstract

Provided are a method for producing a high manganese steel sheet and a steel sheet, which can control inclusions and precipitates that determine the crystal grain size and can suppress the occurrence of surface damage, and a method for producing a high manganese steel cast sheet used for the production thereof. In a method for producing a high manganese steel cast piece, the contents of Ca, O and S in molten steel in a tundish in the process of continuously casting the molten steel containing a specific composition satisfy the following formula (1). 0.4. ltoreq. ACR. ltoreq.1.4 1.4 … … (1) ACR of the above formula (1) is calculated from the following formula (2). ACR { [% Ca ] - (0.18+130 [% Ca) x [% O ] }/(1.25 [% S ]) … … (2) The [% Ca ] of the above formula (2) is the content (mass%) of Ca in the molten steel, [% O ] is the content (mass%) of O in the molten steel, and [% S ] is the content (mass%) of S in the molten steel.

Description

Method for manufacturing high manganese steel cast sheet, high manganese steel sheet, and method for manufacturing high manganese steel sheet

Technical Field

The present invention relates to a method for producing steel sheets and steel sheets as a raw material for high manganese steel for structural steels used in extremely low temperature environments, such as mechanical structural members for nuclear fusion facilities, linear motor vehicle foundations, nuclear magnetic resonance scanning rooms, and the like, and containers for storing liquefied gas, and a method for producing high manganese steel cast sheets used for the production thereof.

Background

High manganese steel having an austenite single-phase structure and a nonmagnetic property is increasingly demanded as a steel material which is inexpensive and replaces metal materials for extremely low temperatures such as conventional austenitic stainless steel, 9% nickel steel, 5000 series aluminum alloy, and the like.

Conventionally, steel sheets as a raw material of the high manganese steel are usually produced by producing cast pieces by an ingot casting method and hot cogging-rolling the cast pieces, but recently, from the viewpoint of improvement in productivity and reduction in cost, production from cast pieces produced by a continuous casting method has become indispensable. When steel sheets or steel sheets of high manganese steel are produced from cast sheets produced by a continuous casting method, there are problems that surface cracks of cast sheets during continuous casting, a large number of surface cracks of steel sheets or steel sheets during hot rolling, an increase in repair required for removing crack defects, and a decrease in yield. Therefore, there is a strong demand for the production of cast high manganese steel sheets that can suppress surface cracking during the production of steel sheets or steel sheets.

As a technique for hot rolling a continuously cast slab of high manganese steel without causing surface cracking, patent document 1 discloses a high Mn steel excellent in low temperature toughness containing 0.0002% or more of 1 or 2 of Mg, Ca, and REM in total and containing components satisfying 30C +0.5Mn + Ni +0.8Cr +1.2Si +0.8Mo ≥ 25 and O/S ≥ 1.

Documents of the prior art

Patent document

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

Disclosure of Invention

Problems to be solved by the invention

The high Mn steel disclosed in patent document 1 contains the above components to form excellent fine crystal grains, but has a problem that the control of the type of inclusions and precipitates having a predetermined crystal grain size is insufficient.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high manganese steel sheet and a method for producing a steel sheet, which can control inclusions and precipitates that determine crystal grain sizes and can suppress the occurrence of surface damage, and a method for producing a high manganese steel cast sheet used for the production thereof.

Means for solving the problems

The present invention for solving the above problems has the following features.

[1] A method for producing a high manganese steel cast piece, wherein, in the process of continuously casting a molten steel having the following composition, the contents of Ca, O and S in the molten steel in a tundish satisfy the following formula (1).

0.4≤ACR≤1.4……(1)

ACR in the above formula (1) is calculated from the following formula (2).

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

In the above formula (2), [% Ca ] is the content (mass%) of Ca in the molten steel, [% O ] is the content (mass%) of O in the molten steel, [% S ] is the content (mass%) of S in the molten steel, and the composition contains, 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: less than 10%, 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: less than 0.0100%, and the balance of iron and inevitable impurities.

[2] [1] the method for producing a high manganese steel cast piece, wherein the molten steel in the tundish further contains Ti, Mg and N in an amount satisfying the following formula (3).

[％Ti]×[％N]×[％Mg]≥2.0×10^-8……(3)

In the above formula (3), [% Ti ] 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 (mass%) of Mg in the molten steel.

[3] [1] A method for producing a high manganese steel cast piece according to [1] or [2], comprising continuously casting a molten steel having a composition further containing, in mass%, a chemical formula selected from the group consisting of 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: 0.05% to 2.00% of 1 or 2 or more selected from the group.

[4] A method for producing a high manganese steel sheet, comprising hot rolling a cast slab produced by the method for producing a high manganese steel cast slab according to any one of [1] to [3] to produce a steel sheet.

[5] A method for producing a high manganese steel sheet, comprising hot rolling a cast slab produced by the method for producing a cast slab of high manganese steel according to any one of [1] to [3], to produce a steel sheet.

ADVANTAGEOUS EFFECTS OF INVENTION

By implementing the present invention, it is possible to produce a high manganese steel cast piece while suppressing the occurrence of surface damage. Further, when the high manganese steel cast sheet is hot-rolled into a steel sheet or a steel sheet, the high manganese steel sheet or the steel sheet can be produced while suppressing the occurrence of surface damage during rolling.

Drawings

FIG. 1 is a diagram summarizing the behavior of inclusions and precipitates formed in high manganese steel in the solidification process from the start of solidification of molten steel in the conventional examples and the inventive examples.

Detailed Description

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments below. The high manganese steel of the present embodiment contains 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: less than 10%, 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: 0.0001% to 0.0100%, and the balance of iron and unavoidable impurities. The above composition may contain a transition metal selected from the group consisting of 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: 0.05% to 2.00% of 1 or 2 or more selected from the group. Unless otherwise specified, "%" indicating the content of components in the component composition means "% by mass". The total content of all ingredients is the total amount and not the dissolved amount.

C: 0.10% to 1.3%

C is added for stabilization of austenite phase and strength improvement. If the content of C is less than 0.10%, the desired strength cannot be obtained. On the other hand, if the content of C exceeds 1.3%, the precipitation amount of carbide and cementite becomes too large, and the toughness is lowered. Therefore, the content of C needs to be 0.10% or more and 1.3% or less, and preferably 0.30% or more and 0.8% or less.

Si: 0.10% or more and 0.90% or less

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

Mn: 10% or more and 35% or less

Mn is an element for stabilizing the austenite structure and improving the strength. In particular, by setting the Mn content to 10% or more, the characteristics expected for austenitic steels, such as non-magnetic properties and low-temperature toughness, can be obtained. On the other hand, austenitic steel is generally insufficient in hot workability, and in particular, high manganese steel is known to be a material having high crack susceptibility in continuous casting and hot rolling. Particularly, if the Mn content exceeds 35%, the workability is remarkably reduced. Therefore, the Mn content is required to be 10% or more and 35% or less, preferably 20% or more and 28% or less.

P: less than 0.030%

P is an impurity element contained in steel, and causes toughness reduction and thermal embrittlement. Therefore, the smaller the content of P, the better, the allowable range is 0.030%. Therefore, the content of P needs to be 0.030% or less, preferably 0.015% or less.

S: 0.0070% or less

S is an impurity element contained in steel, and sulfides such as MnS act as starting points to lower toughness. Therefore, the smaller the S content, the better, can be tolerated up to 0.0070%. Therefore, the S content needs to be 0.0070% or less, preferably 0.0030% or less.

Al: 0.01% or more and 0.1% or less

Al is added for deoxidation. In order to obtain a desired deoxidation effect, the Al content needs to be 0.01% or more. On the other hand, even if Al is added in an amount exceeding 0.1%, AlN is excessively generated while the deoxidation effect is maximized, and the hot workability is degraded. Therefore, the Al content is required to be 0.01% or more and 0.1% or less, preferably 0.02% or more and 0.05% or less.

Cr: less than 10%

Cr is added for solid solution strengthening. On the other hand, if Cr is added in a large amount, the austenitic structure of the high manganese steel becomes unstable, and coarse carbides that cause embrittlement are precipitated. Therefore, the Cr content is required to be 10% or less, preferably 7% or less.

Ca: 0.0001% or more and 0.010% or less

When an appropriate amount of Ca is added, fine oxides and sulfides are formed, and grain boundary embrittlement caused by precipitated inclusions is suppressed. Therefore, the content of Ca needs to be 0.0001% or more. On the other hand, if the Ca content is too high, precipitated inclusions become coarse, and grain boundary embrittlement is promoted. Therefore, the content of Ca needs to be 0.010% or less. The content of Ca is preferably 0.0005% or more and 0.0050% or less.

Mg: 0.0001% or more and 0.010% or less

Mg is very likely to bond with O, S element to form fine oxides and sulfides, similarly to Ca, and therefore, it is expected to suppress grain boundary embrittlement caused by precipitated inclusions. Therefore, the Mg content needs to be 0.0001% or more. On the other hand, if the Mg content is too large, the reaction with the molten steel at the time of addition becomes severe, and not only the cleanness of the molten steel may be deteriorated but also precipitated inclusions may be coarsened. Therefore, the Mg content needs to be 0.010% or less. The Mg content is preferably 0.0005% or more and 0.0020% or less.

Ti: 0.001% or more and 0.03% or less

Ti is bonded to C, N element under high temperature conditions, and therefore is effective for suppressing the formation of huge carbides and suppressing the formation of carbonitrides of Nb and V having high precipitation crack sensitivity. Therefore, the Ti content needs to be 0.001% or more. On the other hand, when a large amount of Ti is added, huge carbonitrides are formed, and the low-temperature toughness deteriorates. Therefore, the Ti content needs to be 0.03% or less. The Ti content is preferably 0.001% or more and 0.02% or less.

N: 0.0001% to 0.20%

N stabilizes the austenite structure and increases the strength by solid solution and precipitation. In order to achieve this effect, the content of N needs to be 0.0001% or more. On the other hand, if the content of N exceeds 0.20%, hot workability is deteriorated. Therefore, the content of N needs to be 0.0001% or more and 0.20% or less. The content of N is preferably 0.0050% to 0.10%.

O: 0.0100% or less

The content of O is a value determined by the degree of deoxidation in the molten steel stage and floating removal of inclusions, and is preferably small from the viewpoint of cleanliness. Here, the content of O is the total O content including O in the form of oxides (inclusions). If the content of O is too large, not only the above-mentioned elements such as Ca and Mg cannot exert sufficient effects, but also solidification defects such as a large number of voids are likely to occur in the cast slab. Therefore, the O content needs to be 0.0100% or less. The content of O is preferably 0.0030% or less.

The high manganese steel of the present embodiment may contain 1 or 2 or more kinds of elements selected from the following elements, as necessary.

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

Both Nb and V can generate carbonitride, and the generated carbonitride acts as a trap site for diffusible hydrogen, and therefore has an effect of suppressing stress corrosion cracking. In order to obtain this effect, Nb: 0.001% or more and 0.01% or less, V: 0.001% or more and 0.03% or less. Within the above composition range, the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage, and the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage during rolling when the high manganese steel sheet or the steel sheet is hot-rolled into a steel sheet or a steel sheet.

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

Cu and Ni can stabilize the austenite structure and contribute to suppression of precipitation of carbides. In order to obtain the above effects, it is preferable to contain Cu: 0.01% to 1.00% of Ni: 0.01% or more and 0.50% or less. Within the above composition range, the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage, and the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage during rolling when the high manganese steel sheet or the steel sheet is hot-rolled into a steel sheet or a steel sheet.

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

Mo and W contribute to the improvement of the base metal strength. In order to obtain the above effects, Mo: 0.05% or more and 2.00% or less, W: 0.05% to 2.00%. Within the above composition range, the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage, and the production of the high manganese steel sheet or the steel sheet is not affected while suppressing the occurrence of surface damage during rolling when the high manganese steel sheet or the steel sheet is hot-rolled into a steel sheet or a steel sheet.

The balance other than the above is iron and inevitable impurities. The following findings were obtained by observing cast pieces obtained by continuously casting molten steel having such a composition. Since the high manganese steel is an austenite single-phase steel, an austenite structure having a large crystal grain size is formed on the surface of the cast slab. Because the austenite grain size is coarse, not only is the high temperature ductility low compared to ordinary steel, but also impurity elements such as P, S are concentrated in the grain boundaries. The grain boundary becomes brittle due to the enrichment of the impurity element, and a surface crack is generated in the form of propagation at the brittle grain boundary.

In addition, since high manganese steel has a high Mn concentration in steel and high reactivity with S, the formation of MnS becomes remarkable compared to ordinary steel. Also generate M₂₃C₆Carbides (M: Mn, Cr, Fe, Mo). These precipitates are easily precipitated at the grain boundaries of the high manganese steel, and even when coarse precipitates of sulfides and carbides are concentrated and precipitated at the grain boundaries, the grain boundaries are weakened and surface cracks are generated.

Therefore, in order to suppress surface cracks in continuous casting and hot rolling, it is important to make the grain size fine and avoid grain boundary embrittlement due to the enrichment of impurity elements. In particular, in high manganese steel, which is an austenite single-phase steel, the austenite structure rapidly develops from below the solidus temperature, and therefore it is important to precipitate inclusions as pinning nuclei at least at the solidus temperature or higher in advance. Further, it is important to suppress the concentration and precipitation of coarse sulfide and carbide precipitates harmful to cracks in grain boundaries, and to disperse strain concentrated in grain boundaries by finely precipitating in the grains.

As a method for suppressing MnS production, a method of adding Ca to steel is known. Ca is added to acid-resistant line pipe steel or the like to suppress MnS formation. As an index of Ca addition, a Ca/S, ACR index (atomic concentration ratio index) is known. It is known that it is effective to add Ca so as to satisfy Ca/S > 2 and to add Ca so as to satisfy the following formula (4) for suppressing the production of MnS.

1≤ACR＜3……(4)

The ACR is calculated from the following formula (2).

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

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

Adding Ca to make Al in molten steel₂O₃The inclusion is controlled to be CaO-Al₂O₃In the form of the oxide, CaS is used to suppress precipitation of MnS during solidification. The method can also be applied to high manganese steel. However, when Ca is added to the molten steel of high manganese steel so as to satisfy the above formula (4), CaS — MnS is produced in the molten steel stage. The CaS-MnS formed in the molten steel stage is easy to be aggregated and merged and is easy to float and remove. Even when the alloy is taken into a cast slab after solidification, the alloy hardly forms pinning nuclei for refining crystal grains.

Therefore, in the method for producing a high manganese steel cast piece of the present invention, Ca is added to the high manganese steel liquid, and the composition is adjusted so that the contents of Ca, O, and S in the high manganese steel liquid in the tundish satisfy the following formula (1). The composition of molten steel can be measured by analyzing the composition of molten steel sampled from the tundish.

0.4≤ACR≤1.4……(1)

The ACR is calculated from the formula (2).

As a result, CaO, MnO, and Al can be made to flow between the liquidus temperature (hereinafter, TLL) and the solidus temperature (hereinafter, TSL) of the high manganese steel₂O₃And (4) precipitating the inclusions. The inclusions precipitated between the TLL and TSL are finely dispersed and precipitated in the grain and grain boundaries in the slab. MnS, CaS and M precipitated after complete solidification₂₃C₆To CaO, MnO, Al₂O₃MnS and M precipitate around the precipitates₂₃C₆And (4) performing fine dispersion. This can suppress the precipitation of coarse sulfides and carbides into grain boundaries, and greatly suppress the occurrence of surface cracks due to grain boundary cracks.

Further, since the precipitates finely dispersed in the crystal grains also function as pinning nuclei for making the crystal grains fine, the crystal grain size is made fine, and the embrittlement of the crystal grain boundary due to the enrichment of the impurity element can be avoided. As a result, the occurrence of surface cracks due to grain boundary cracks can be further suppressed.

On the other hand, if the contents of Ca, O and S in the molten high manganese steel in the tundish are ACR < 0.4, the amount of Ca added is too small, and therefore fine CaO, MnO and Al are not formed₂O₃Inclusions are formed, and MnS is mainly formed on the solidified grain boundaries. When ACR > 1.4, CaS is easily formed in the molten steel stage, and therefore, the function of the pin nuclei is not exerted as a fine crystal grain size, and the crystal grain size cannot be made fine. Preferably, the contents of Ca, O and S in the high manganese steel liquid in the tundish satisfy the following formula (5), whereby the pinning effect by fine MnS is improved and the crystal grain size can be further refined.

0.4≤ACR≤0.9……(5)

Preferably, CaO, MnO, and Al are used₂O₃The temperature of precipitation is between TLL and TSL, and the solid fraction of the high manganese steel liquid is 0.3 or more. This is because, when the solid phase ratio is 0.3 or more, the solid phase ratio is close to the fluidity limit solid phase ratio of the molten steel, and the precipitated inclusions stay at the precipitated position without flowing into the molten steel. The solid fraction is defined as the solid fraction equal to 0 at TLL or more of steel and equal to 1.0 at TSL or less of steel.

Further, Ti and N may be added to the molten steel, and the precipitates may be finely dispersed with MgO and TiN as nuclei. Ti and N are added to molten steel, and a solution product is formed at a high temperature of 1300 to 1400 ℃ to precipitate TiN, and the grains are refined by using the precipitate as a pinning core. The presence of MgO is effective for containing Mg in addition to Ti and N because fine TiN is stably produced. As a result of conducting experiments to change the contents of Ti, N, and Mg in the molten steel of high manganese steel and investigating the results, it was found that when the contents of Ti, Mg, and N in the molten steel of high manganese steel in the tundish satisfy the following formula (3), MgO — TiN was finely dispersed and precipitated in the grains upon solidification.

[％Ti]×[％N]×[％Mg]≥2.0×10^-8……(3)

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

MgO-TiN and CaO-MnO-Al₂O₃Similarly, the precipitates are in the form of MnS and CaS-MnS precipitates around them. Therefore, the MgO-TiN precipitates are finely dispersed in the grains, and the precipitation of coarse MnS precipitates in the grain boundaries can be suppressed. The MgO-TiN precipitates function as pinning nuclei, and therefore the crystal grain size can be further reduced. This can further suppress the occurrence of surface cracks due to grain boundary cracks.

FIG. 1 is a diagram summarizing the behavior of inclusions and precipitates formed in high manganese steel in the solidification process from the start of solidification of molten steel in the conventional examples and the inventive examples. The inclusions in the molten steel of the high manganese steel are Al as oxides₂O₃MnO inclusions exist in the molten steel at 1500-1600 ℃.

In the conventional example (comparative example) in which Ca was not added, CaO. MnO. Al as pinning nuclei having a fine crystal grain size was not generated₂O₃Inclusions did not achieve refinement of the crystal grain size. In addition, MnS, M₂₃C₆The precipitates are coarse precipitates and precipitated on the solidified grain boundaries. As a result, the grain boundaries are weakened and surface cracks are conspicuous.

In contrast, in the present invention example, Ca was added so that the contents of Ca, O, and S in the high manganese steel liquid in the tundish satisfy the above formula (1) and the composition was adjusted.Thus, Al₂O₃The inclusion of MnO changed to CaO. MnO. Al between TLL and TSL₂O₃The inclusions are finely dispersed and precipitated in the grains and in the grain boundaries.

In the present invention, Ti, Mg, and N are added and the composition is adjusted so that the contents of Ti, Mg, and N in the high manganese steel liquid in the tundish satisfy the above formula (3). As a result, MgO-TiN inclusions are finely dispersed and precipitated in the grains and in the grain boundaries between TLL and TSL.

Then precipitated MnS, CaS and M₂₃C₆In the presence of CaO, MnO and Al₂O₃The precipitates and MgO-TiN precipitates are precipitated around the grains, so that MnS precipitates are dispersed in the grains, and the coarse MnS precipitates are prevented from being concentrated and precipitated in the grain boundaries.

Further, fine precipitates dispersed in the crystal grains function as pinning nuclei for making the crystal grain size fine, and therefore the crystal grain size is also made fine. The surface cracks in the production of high manganese steel cast pieces are suppressed by the suppression of the precipitation of coarse MnS precipitates into grain boundaries and the refinement of crystal grains, and the surface cracks in the continuous casting of high manganese steel cast pieces and in the production of steel pieces or steel sheets by hot rolling the cast pieces are suppressed.

Examples

High manganese steel was melted by using a 150-ton converter, an electrode heating type ladle refining furnace and an RH vacuum exhaust apparatus, and cast pieces having a cross-sectional dimension of 1250mm wide by 250mm thick were continuously cast by using a tundish having a capacity of 30 tons and a bending continuous casting machine having a bending radius of 10.5mR after adjusting the molten steel composition and temperature. The casting speed is 0.7-0.9 m/min, and the cooling water amount for 2 times is 0.3-0.6L/kg. Thereafter, the cast slab is temporarily cooled to a cold slab by slow cooling, and is charged into a heating furnace for a predetermined time so as to have a predetermined target temperature, and thereafter, hot rolled at a total rolling reduction of 48% to produce a steel sheet. The presence or absence of surface cracks in the rolled steel sheet was examined by the Penetrant Test (PT).

Table 1 shows the composition of the molten steel sampled from the tundish, the calculated values of the formulas (1) and (3), and the results of the surface crack investigation in the invention examples 1 to 39.

[ Table 1]

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 sheets after hot rolling were slight or no. In the high manganese steels of invention examples 1, 2, 5 to 15, 27, 29 to 39 satisfying the formulas (1) and (3), the hot rolled steel sheets had no surface cracks. In invention examples 1 to 39, the solidification structure of the cast piece after casting was examined, and it was confirmed that the crystal grains were finer than those in the normal case.

The composition of the molten steel sampled from the tundish, the calculated values of the above equations (1) and (3), and the results of the surface crack investigation in comparative examples 1 to 70 are shown in tables 2 and 3.

[ Table 2]

[ Table 3]

As shown in tables 2 and 3, in the high manganese steels of comparative examples 1 to 70 which did not satisfy the above formula (1), surface cracks were generated in the hot-rolled steel sheets, and deep repair work was required using a grinder, and the cost and the load on the process were large.

Based on the above results, it was confirmed that surface cracks of the hot-rolled steel sheet can be suppressed by allowing the composition of the molten steel in the tundish to satisfy the above formula (1). The above confirmation was performed on a steel sheet manufactured from a cast sheet, and also on a steel sheet manufactured from a cast sheet, surface cracks of the steel sheet can be suppressed similarly. The rolled steel sheet had no or slight surface cracks, and thus it was found that there were no or slight surface cracks in the continuously cast sheet. As described above, it was confirmed that surface cracking of the steel sheet after hot rolling can be further suppressed by allowing the composition of the molten steel in the tundish to satisfy the above expression (1) and expression (3). As described above, if surface cracks of the rolled steel sheet and steel plate can be suppressed, the direct-feed manufacturing process of continuous casting → heating furnace → main rolling can be realized, and the energy cost can be significantly reduced.

Claims

1. A method for producing a high manganese steel cast piece, wherein, in the continuous casting of a molten steel having the following composition, the contents of Ca, O and S in the molten steel in a tundish satisfy the following formula (1),

0.4≤ACR≤1.4……(1)

ACR of the above formula (1) is calculated from the following formula (2),

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

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

the composition of the components comprises the following components in percentage by mass

C: 0.10% to 1.3%,

Si: 0.10% to 0.90%,

Mn: more than 10% and less than 35%,

P: less than 0.030%,

S: less than 0.0070 percent of,

Al: 0.01% to 0.1%,

Cr: less than 10 percent of,

Ca: 0.0001% to 0.010%,

Mg: 0.0001% to 0.010%,

Ti: 0.001% to 0.03%, b,

N: 0.0001% to 0.20%,

O: less than 0.0100%, and the balance of iron and inevitable impurities.

2. The method for producing a high manganese steel cast piece according to claim 1, wherein the molten steel in the tundish further contains Ti, Mg and N in an amount satisfying the following formula (3),

[％Ti]×[％N]×[％Mg]≥2.0×10^-8……(3)

3. The method for producing a high manganese steel cast piece according to claim 1 or 2, wherein a molten steel having a composition further containing, in mass%, from

Nb: 0.001% to 0.01%,

V: 0.001% to 0.03%, b,

Cu: 0.01% to 1.00%,

Ni: 0.01% to 0.50%,

Mo: 0.05% to 2.00%,

W: 0.05% to 2.00% of 1 or 2 or more selected from the group.

4. A method of manufacturing a high manganese steel sheet, wherein a cast sheet manufactured by the method of manufacturing a high manganese steel cast sheet according to any one of claims 1 to 3 is hot-rolled to manufacture a steel sheet.

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