JPH02155548A - Method for continuously casting free cutting steel - Google Patents

Abstract

PURPOSE:To stably manufacture a sulfur series free cutting steel having excellent machinability by making molten steel over-heat degree in a tundish higher than the specific temp. and making cooling velocity in the specific temp. range at lower than the specific temp. in the solidified process lower than the specific value at the specific intermediate point. CONSTITUTION:The molten steel over-heat degree in the tundish 2 arranging an induction heating device 1 is made to >=10 deg.C and the cooling velocity at <=1500 deg.C and at least in the temp. range of 1500-1400 deg.C at the intermediate point between cast slab surface and center in the cast slab cross section is made to <=30 deg.C/min. The cast slab passing through secondary cooling zone 5 is held to heat and heated in heat-holding zone 6 and heating zone 7 to restrain conduction of heat from the cast slab surface. By making the molten steel over-heat degree to >=10 deg.C, quantity of the hard inclusion is drastically reduced and the deterioration of the machinability is prevented. The lowering of the cooling velocity at between 1500-1400 deg.C makes MnS grain large and improves the machinability.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a technique for improving the machinability of continuously cast sulfur-based free-cutting steel in a continuous casting process.

Conventional technology A method of improving the machinability of sulfur-based free-cutting steel produced by continuous casting using a continuous PJ casting process is described in Jikai 62-20754.
7 and Jikai 62-207548. All of the methods described in L are intended to reduce the cooling rate within the slab by continuous casting, and to improve machinability by increasing the size of the Mn3 grains formed in the steel through slow cooling. .

In the above-mentioned JP-A-62-207547, the specific water amount of the secondary cooling water is limited to 0.5 Jl/kg or less, while in JP-A-82-207548, in addition to this limitation on the specific water amount,
A heat insulation zone and a heating zone are installed inside the continuous pi machine to keep the slab warm.
A method is described that achieves this goal by heating. However, these methods only attempt to improve the machinability of sulfur-based free-cutting steel by slow cooling, and the target slow cooling level has not been clarified.

JP-A-58-29Ei58 contains sulfur-based free-cutting steel.
Although an example of manufacturing by a manufacturing method is shown, this method aims to obtain sulfur free-cutting steel with uniform quality and no bubble defects, and is not intended to improve machinability. There wasn't.

An example of installing a heating device in a continuous PJ machine to heat the surface of a slab is shown in JP-A-52-50!333, but the purpose of this is to prevent surface flaws during cooling. , is not intended to improve machinability.

Problems to be Solved by the Invention The machinability of bamboo-based free-cutting steel is determined by the Mr+S generated in the steel.
It is known that the machinability is influenced by the grain size, and the larger the Mn3 grains are, the better the machinability is. On the other hand, the size of these Mn3 grains decreases as the slab cross-sectional size becomes smaller. Therefore, when trying to manufacture sulfur-based free-cutting steel with slabs that are closer to the finished product size, that is, with a smaller cross-sectional size, Mn3
As the grains become smaller, the machinability of the finished product deteriorates.

In order to prevent such deterioration in machinability due to the reduction in slab size in continuous casting, it is insufficient to reduce the flow of secondary cooling water and achieve gradual cooling as in the above-mentioned Japanese Patent Application Laid-Open No. 82-207547. It is thought that.

In addition, in addition to limiting the specific water amount as in the method of JP-A-62-207548, insulating and heating the slab is considered to be an effective means for slow cooling; However, if MnS in free-cutting steel becomes larger, the expected improvement in machinability will be insufficient unless it contributes to sufficiently lowering the cooling rate in the temperature range that is most effective for improving machinability. Conceivable.

In addition, hard inclusions contained in steel cause damage to the tool, shorten the tool life, reduce the roughness of the finished surface of the workpiece, and cause deterioration of machinability. However, none of the methods described above is a method for preventing deterioration of machinability due to these hard inclusions.

Means for Solving the Problems The present invention provides means for solving the above problems, and is the following invention.

The sulfur-based free-cutting steel that is the object of the present invention is a sulfur-based free-cutting steel specified by JIS, and the S-containing agent is 0.08.
It refers to those in the range of ~0.40wt%.

(1) In the continuous casting process of sulfur-based free-cutting steel, the degree of superheating of the molten steel in the tundish is set to 10°C or higher, and the slab surface and the slab cross-sectional center are At least 1 below 1500℃ at the midpoint of
The cooling rate in the temperature range between 500 and 1400℃ is 30
A continuous casting method for free-cutting steel, characterized in that the casting speed is less than ℃/min.

(2) A continuous casting method for free-cutting steel, in which the tundish is provided with an induction heating neck in order to control the degree of superheating of molten steel in the tundish to 10° C. or higher, and the method described in item 1 is applied.

(3) A method of combining a method of installing a heating zone or a heat-insulating zone and a heating zone after the secondary cooling zone inside the continuous casting machine to heat the slab, keep it warm, or heat the slab to reduce the cooling rate inside the slab. The method described in Section 1.

(4) Temperature change of molten steel superheating degree (or molten steel temperature), mold cooling water amount and mold cooling water, secondary cooling water! 11 primary and secondary cooling operating conditions.

Combining the method of performing solidification calculations based on process information consisting of ambient temperature, slab size, and casting speed, and controlling the amount of secondary cooling water and the casting speed so that the cooling rate described in item 1 is 30°C/min or less. The method according to paragraph 1.

(5) Primary and secondary cooling operation conditions such as the degree of superheating of molten steel (or molten steel temperature) in the tundish, the amount of mold cooling water and the temperature change of mold cooling water, the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the operation of the heating zone Solidification calculations are performed based on process information including conditions, ambient temperature, slab size, and casting speed, and secondary cooling water melting, casting speed, and heating zone operation are performed so that the cooling rate described in Section 1 is 30°C/min or less. 4. The method according to paragraph 3, which combines methods of controlling conditions.

It is in.

The invention described above prevents the deterioration of machinability due to hard inclusions and sufficiently reduces the cooling rate in a specific temperature range during the solidification process when manufacturing low-sulfur free-cutting steel using the continuous casting method. The present invention aims to provide a method that makes it possible to manufacture sulfur-based free-cutting steel with excellent machinability by effectively and reliably increasing the size of Mr+S grains. The sulfur-based free-cutting steel mentioned here refers to low-order sulfur free-cutting steel with a C concentration of 0.2% or less, and this low-order sulfur free-cutting steel containing Pb, Bi,
Contains composite free-cutting steel to which elements such as Te and Ca that improve machinability are added.

FIG. 1 is an explanatory diagram showing an embodiment of the present invention, in which 1 is an induction heating device, 2 is a tundish, 3 is a mold, and 4 is an "r"
rL magnetic wire stirrer, 5 is a secondary cooling zone, 6 is a heat insulating zone, 7 is a heating zone, 8 is a vinyl roll zone, and 9 is a slab.

The invention as set forth in claim 1 makes the degree of superheating of the molten steel in the tundish 10°C or higher, prevents the entrainment of hard inclusions due to low-temperature casting, and improves the cross-section at the center of the slab width as shown in Fig. 6. At least 1,500 to 1,400°C below 1,500°C at the midpoint between the slab surface and the center of the slab's cross section (marked ・ and O) within the cross section at the center of the thickness of the slab.
The aim is to improve the machinability of continuously cast free-cutting steel by increasing the size of the Mn5 grains by slow cooling at a cooling rate of 30° C./min or less in the temperature range between 1 and 2.

In order to more reliably reduce the occurrence of hard inclusions, the degree of superheating of the molten steel in the tundish should be set to 1 throughout the entire casting process, as described below.
It is necessary to keep the temperature above 0℃, but i! The degree of superheating of the molten steel in the tundish may fall below 10° C. due to variations in temperature control in the process before li casting or a decrease in the molten steel temperature during casting. The invention of the second precept, the precept, was devised as a measure to solve these problems.

The invention described in item 2 installs an induction heating device in the tundish and adds a molten steel heating function to prevent the temperature of the molten steel in the tundish from decreasing, or more actively raises the temperature, thereby controlling the degree of superheating of the molten steel throughout casting. over 10
℃ or more, and after more reliably preventing the entrainment of hard inclusions, it is intended to further improve machinability by slow cooling.

On the other hand, in the continuous casting process, in order to reduce the cooling rate in the temperature range of at least 1500 to 1400°C within the slab to 30°C/min or less, it is necessary to increase the size of MnS. The third aspect of claim 3, wherein it is preferable to suppress surface cooling and maintain the slab surface at a higher temperature.
The invention described in section 1 provides a heating zone or a heat-insulating zone and a heating zone after the secondary cooling zone, and by combining methods of heating, keeping warm, and heating the slab, it is possible to realize gradual cooling that is effective for increasing the size of MnS. This provides a method for achieving this goal.

In addition, in order to reliably improve machinability through slow cooling, we always check whether slow cooling that is effective for increasing the size of MnS is achieved during casting, and if slow cooling is not achieved, 2. When a heating zone is provided, it is preferable to quickly reduce the cooling rate to a target level by adjusting the secondary cooling conditions, casting speed, and heating zone operating conditions.

The method according to claims 4 and 5 is the first method.
When applying the method described in Section 3 or the method described in Section 1, in which a heat insulation zone or a heat insulation zone and a heating zone are installed, solidification calculations are performed based on process information related to cooling of slabs. The cooling rate at a specific location within the slab is always estimated, and the amount of secondary cooling water and casting speed, if a heating zone is provided, is further adjusted so that the cooling rate is below the target of 30°C/min. The aim is to stabilize and ensure the machinability improvement effect of slow cooling by controlling operating conditions.

Each limiting condition in the present invention will be explained below.

First, the reason why the degree of superheating of molten steel in the tundish is set to 10° C. or higher will be explained. As mentioned above, hard inclusions cause deterioration of the machinability of free-cutting steel, and it is important to prevent the inclusion of hard inclusions when casting free-cutting steel.
bawl.

It is generally well known to those skilled in the art that the lower the degree of superheating of the molten steel in the tundish, the worse the level of inclusions in the continuously cast material. Therefore, the present inventors investigated the relationship between the degree of superheating of molten steel in a tundish and the level of hard inclusions when producing low-coal sulfur-based free-cutting steel by a continuous casting method.

The hard inclusions that cause problems in free-cutting steel are high-melting-point oxide-based inclusions represented by M and 03 series, and are caused by deoxidizing elements such as M contained in various alloys added for composition adjustment. The main components are oxides that are mixed in due to oxidation or erosion of various refractories.

As is clear from the graph of the above investigation results shown in Figure 4, it is recognized that the level of hard inclusions in free-cutting steel tends to improve as the degree of superheating of the molten steel in the tundish increases, especially when the degree of superheating of the molten steel is 10 Low-temperature casting below ℃ significantly deteriorates the level of hard inclusions. Therefore, in continuous casting of free-cutting steel, operations where the molten steel superheat in the tundish is less than 10°C should be avoided, and if possible, operations at higher molten steel superheats without operational problems such as breakouts should be avoided. It is preferable to do so.

By adopting the method of adding a heating function to the tundish as described in claim 2, it is possible to not only prevent the degree of superheating of molten steel in the tundish from dropping below 10°C, but also to increase the degree of superheating to a higher degree. This makes it possible to cast while holding it in place.

The amount of hard inclusions that are harmful to machinability can be significantly reduced.

Next, in the invention described in item 1, at a midpoint between the surface of the slab and the center of the slab cross section in the cross section at the center of the slab width and at the center of the slab thickness, at least 1,500 to 14
Explain the reason for adopting slow cooling in which the cooling rate is 30°C/min or less in the temperature range between 00°C and 0°C.
In a low-sulfur free-cutting steel equivalent to UN23, we conducted a unidirectional solidification experiment to investigate the change in Mn3 grain size during solidification, and investigated the effect of the cooling rate inside the slab on the Mn3 grain size in the slab, and found the following findings. I got it.

Figure 2 shows the change in MnS size during solidification investigated in the unidirectional solidification experiment.
It is clear that MnS is generated and grows in the temperature range of 00°C, and the smaller the cooling rate, the larger the Mn3 grain size. Moreover, this result shows that the size of Mn3 grains of sulfur free-cutting steel is determined by the cooling rate between 1500 and 14QQ°C.

Investigations on the effect of cooling rate on actual slabs were conducted on slabs with three different cross-sectional sizes: 1 part 8 thick part (5 parts), 1 part 4 thick part (Q part) and 1 part 8 thick part (part 5) in the cross section at the center of the slab width. The relationship between the two was investigated at three locations: part 1, part 2, and thick part (part 0). The survey results are shown in Figure 3.

From Figure 3, the MnS size in the low carbon sulfur free-cutting steel slab is:
1 during solidification, regardless of the cross-sectional size of the slab or its position within the slab.
500~! It can be seen that it depends almost on the cooling rate (R') between 400° C., and the smaller the cooling rate (R'), the larger the MnS becomes.

In addition, from the results of investigating the formation and growth mechanism of MnS in low-sulfur free-cutting steel, it was found that HnS in wooden steel forms l1ln during solidification, especially in the liquid and solid phases at temperatures below 1500°C.
The concentration product of has become clear.

Therefore, NnS becomes larger in low-coal i-yellow free-cutting steel due to a decrease in the cooling rate (R') between 1500 and 1400°C. This is considered to be because, as a result, the diffusion of Mn in the vicinity of MnS is promoted.

For the reasons mentioned above, in order to increase the size of MnS, it is necessary to
Especially at 1500 to 140 below 1500℃ where nS is generated.
It is considered important to reduce the cooling rate (R') between 0°C.

In addition, the reason why the above cooling rate (R') is set to 30°C/min or less at the midpoint between the slab surface and the slab cross-sectional center in the cross section at the center of the slab width and the center of the slab thickness is as follows. That's right. The size of MnS in the 1/4 thickness part of the product that satisfies the machinability required for low-sulfur free-cutting steel products is 50 mm.
It has been found that the Mn
Although S is stretched in the rolling direction, its size does not change, so it is necessary to obtain MnS with a size of about 50 pieces or more at the slab stage.

For this purpose, the cooling rate (R') shown in Figure 3 and the MnS
Due to the size, at the midpoint between the slab surface and the center of the slab cross section in the cross section at the center of the slab width and the cross section at the center of the slab thickness, which approximately corresponds to the 1/4 thickness part of the finished product, 1500 to 1
Cooling rate (R゛) in the temperature range between 400℃ and 30℃
/minute or less.

Function Figure 1 shows a conceptual diagram of the present invention and an outline of the curved continuous test casting machine used in the examples described below. 6 and 7 are a heat-retaining zone and a heating zone, respectively, which are installed to reduce the cooling rate in the slab. The slab that has passed through the secondary cooling zone 5 is kept warm and heated in the heat insulation zone 6 and the heating zone 7, and heat loss from the surface of the slab is suppressed in these areas.

As is clear from the relationship between the molten steel superheating degree of the tundish and the hard inclusion rating in low-sulfur free-cutting steel shown in Figure 4, by setting the molten steel superheating degree of the tundish to 10°C or higher, the solidified shell The amount of hard inclusions trapped in the steel can be significantly reduced, and deterioration of machinability due to hard inclusions can be prevented.

MnS has an effective effect on machinability in low-sulfur free-cutting steel
The diffusion rate of Mn in the solid phase, which controls the production and growth of Mn, increases rapidly as the temperature increases. Therefore, in the continuous casting solidification process of low-order sulfur-based free-cutting steel,
The cooling rate decrease between 0 and 1400°C is caused by Nn near MnS.
This contributes to improving machinability by promoting the diffusion of MnS and increasing the size of MnS.

Installing an induction heating device in the tundish and adding a molten steel heating function absorbs variations in temperature adjustment in the previous process, prevents the tundish molten steel temperature from decreasing during casting, and maintains the molten steel heating level throughout casting. It is possible to guarantee a temperature of 10°C or higher.

Heating or heat retention of the slab after the secondary cooling zone suppresses heat loss on the slab surface and cools the inside of the slab to below 1500℃,
In particular, it is possible to significantly reduce the cooling rate in the temperature range of 1500 to 1400°C.

In applying the method described in paragraph 1 or the method described in paragraph 1,
When applying the method described in Section 3 of installing a heat insulation zone or a heat insulation zone and a heating zone, perform solidification calculations based on process information related to cooling of the slab, and always estimate the cooling rate described in Section 1. In order to keep the cooling rate below the target of 30°C/min, as mentioned above, by combining methods to control the amount of secondary cooling water, casting speed, and if a heating zone is installed, the operating conditions of the heating zone. As described above, stable and reliable slow cooling can be achieved, making it possible to manufacture low-order sulfur-based free-cutting steel with highly stable machinability.

Examples Examples will be given and explained below. Curved test shown in Figure 1? ! ! Using a casting machine, a slow cooling test was conducted on low-lying sulfur free-cutting steel (steel equivalent to 90M23), in which the slab was kept warm and heated using a heat insulation zone and a heating zone provided after the secondary cooling zone. The slab size was 182 m thick and 162 mm layer width, and the hard inclusions rating of the test material was measured using a tanditsu induction heating device and the degree of superheating of the molten steel was controlled within the range of 30 to 40°C. Shown in Figure 4.

The hard inclusion score of this test material is within the range that shows the relationship between the conventional tandish molten steel superheat degree and the hard inclusion score, and the hard inclusion score is significantly improved compared to when the molten steel superheat degree is less than 10°C. There is.

In addition, the size measurement results of nS in the 1/4 thick part of the slab produced in the Honzumi cooling test are shown in Figure 3 as a fist.As is clear from Figure 3, the heat retention after the secondary cooling zone, Cooling rate (Ro) between 1500 and 1400℃ by heating is 30℃
In this test material, the MnS size decreased to less than 65 min.
The size has increased to about 1 ml per pot, which is significantly larger than the average size of 45 rugs without slow cooling, as indicated by O.

Furthermore, one test material was rolled to 80φ, and the results of the machinability investigation of the finished product were shown as follows:
Fig. 5 shows a comparison with the machinability of non-slow cooling material cast at ℃.
The best machinability was obtained with this test material, which was cooled slowly and controlled within the temperature range of It is clear that machinability can be improved.

Effects of the Invention As detailed above, the present invention reduces hard inclusions harmful to machinability and improves the cooling rate in a specific temperature range during the solidification process in the production of low-order sulfur free-cutting steel by continuous casting. By sufficiently reducing the MJIS grain size and effectively increasing the size of the MJIS grains, it becomes possible to stably produce sulfur-based free-cutting steel with excellent machinability.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram of the present invention and a schematic diagram of a curved test series PJ machine used in an example. Figure 2 shows Mn3 during cooling.
FIG. 3 is a diagram showing the relationship between changes in grain size and MnS size. Figure 3 shows the 1,500~
FIG. 3 is a diagram showing the relationship between the cooling rate (Ro) at 1400° C. and the MJIS size, and showing the effect of increasing the size of MnS grains according to the present invention, which was confirmed in Examples. FIG. 4 is a diagram showing the relationship between the degree of superheating of molten steel and the hard inclusion rating, and the hard inclusion level in the example of the present invention is shown in the diagram. FIG. 5 is a diagram showing the machinability improvement effect of the present invention. FIG. 6 is an explanatory diagram showing the position of the slab that defines the cooling rate of claim 1. 1φ...induction heating device, 2-fist/tandish, 3-
-Mold, 4...Electromagnetic stirrer, 5...Secondary cooling zone, 6...Heating zone, 7. φ/heating zone, 8--Vinch roll zone, 9...-Slab.

Claims (5)

[Claims] (1) In the continuous casting process of sulfur-based free-cutting steel, the degree of superheating of the molten steel in the tundish is set to 10℃ or higher, and the slab surface and the slab cross-sectional center within the cross section at the center of the slab width and the cross section at the center of the slab thickness. At least 1 below 1500℃ at the midpoint of
The cooling rate in the temperature range between 500 and 1400℃ is 30
A continuous casting method for free-cutting steel, characterized in that the casting speed is less than ℃/min. (2) The continuous casting method for free-cutting steel according to claim 1, wherein the tundish is provided with an induction heating device for controlling the degree of superheating of the molten steel in the tundish to 10° C. or higher. (3) A request for combining a method of installing a heating zone or a heat-retaining zone and a heating zone after the secondary cooling zone inside the continuous casting machine to heat the slab, keep it warm, or heat the slab to reduce the cooling rate inside the slab. Item 1
Method described. (4) Primary and secondary cooling operating conditions such as tundish molten steel superheating degree (or molten steel temperature), mold cooling water amount and mold cooling water temperature change, secondary cooling water amount, ambient temperature, slab size, and casting speed 2. The method according to claim 1, further comprising performing a solidification calculation based on process information consisting of the following: and controlling the amount of secondary cooling water and the casting speed so that the cooling rate is 30° C./min or less. (5) Degree of superheating of the molten steel in the tundish (in other words, molten steel temperature), the amount of mold cooling water and temperature changes in the mold cooling water, the primary and secondary cooling operating conditions such as the amount of secondary cooling water, the heat retention capacity of the heat insulation zone, and the temperature change of the mold cooling water. Solidification calculations are performed based on process information consisting of tropical operating conditions, ambient temperature, slab size, and casting speed, and secondary cooling water volume, casting speed, and heating zone operating conditions are controlled so that the cooling rate is 30°C/min or less. The method according to claim 1, which combines the methods of: