JPS5910846B2 - High temperature slab direct rolling method - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for directly rolling hot slabs in which continuous casting is followed by hot rolling without reheating. Specifically, high-temperature slabs, that is, molten steel, are cast in a mold, and the slabs, such as ingots, slabs, blooms, and billets, which are kept at a high temperature after being removed from the mold, are rolled without reheating to produce semi-finished products and products. The purpose is to provide a rolling method that does not cause surface cracks during hot rolling, and its other purpose is to provide hot rolling with good thermal economy. Now, it is well known that when steel ingots are conventionally rolled, if the steel ingots are hot-rolled without first lowering the temperature after solidification, significant hot embrittlement occurs and surface defects occur frequently. Therefore, steel ingots cast in ingots and slabs (slabs, blooms, billets, etc.) cast by continuous casting are generally placed in a soaking furnace and reheated, or once cooled to room temperature and then surfaced. A method has been adopted in which defects are removed, the material is placed in a heating furnace, the temperature is raised to rolling temperature, and hot rolling is performed. Recently, research has been conducted to reduce heat loss during the manufacturing process and to omit the process with the aim of saving energy and labor. It goes without saying that in order to achieve the above purpose, it is better to roll the hot steel ingot or slab immediately after casting without cooling it to produce the desired product, but as mentioned above, this method may cause problems such as surface cracks etc. Due to the problems encountered during hot rolling of cast steel ingots or slabs without reheating, little progress has been made. From operational experience, it has been found that when the temperature of the high-temperature steel ingot or slab drops a little, for example, when it reaches a temperature of about 900°C to 300°C, it is reheated and raised to the rolling temperature, for example, 1.
It has been found that rolling can be carried out while suppressing the occurrence of surface flaws by raising the temperature to 280°C to 1050°C and rolling. The above will be explained in more detail with reference to the drawings. In FIG. 1, the horizontal axis shows time and the vertical axis shows the temperature of the material to be rolled. Line 1 shows the temperature change in a particular steel ingot, such as Bi-Lesotho rolling. Line 1 is an example where the temperature of the steel ingot gradually drops after it reaches the solidification point t1, and rolling stops at the bending point.This is because billets do not have as many problems with surface defects as slabs and galumes. Rolling without reheating, that is, direct rolling, was carried out based on the technical concept of clearly and intentionally suppressing the occurrence of surface defects from the viewpoint of temperature history. isn't it. Next, as mentioned above, the line 2 indicates that the temperature of the steel ingot solidified at the freezing point t2 has decreased to, for example, 900°C, and then the steel ingot is reheated.
This shows an example in which rolling was performed from 200°C, and this has been adopted as an economical method that reduces the occurrence of surface defects and can save energy. However, rolling as shown in line 2 does not necessarily guarantee prevention of surface flaws, and requires a considerable amount of thermal energy for reheating, so it cannot be said to be economical. Next, line 3 is a graph showing temperature changes in the well-known method of reheating and rolling a steel ingot cooled from the solidification point t3 to room temperature to the rolling temperature, and as described above, this method has a very large reheating loss. As a result of research into an economical hot rolling method for steel materials, the inventors of the present invention discovered a method of hot rolling a hot slab continuously obtained by a continuous casting method without reheating (hereinafter referred to as the direct rolling method). ) has been developed to ensure stable operation without surface defects. The gist of the present invention is as follows. (1) In a steel rolling method in which hot rolling is performed without reheating following continuous casting, the flaw recovery temperature of the high-temperature slab after casting is determined in advance for each steel type and cooling rate of the slab surface. 1. A method for direct rolling of a hot slab, characterized in that after casting the slab, the slab is rolled at a temperature below the embrittlement recovery temperature without being reheated. (2) In a steel rolling method in which hot rolling is performed without reheating following continuous casting, the embrittlement recovery temperature of the high-temperature slab after casting is calculated in advance for each steel type and cooling rate of the slab surface layer. Asking? The planned rolling temperature after casting of the slab is set at an arbitrary point in the embrittlement recovery temperature range, and the slab is slowly cooled at the optimum cooling rate to reach the scheduled rolling temperature without reheating. A method for directly rolling a hot slab, characterized by starting rolling when a scheduled rolling temperature is reached. Now, as a result of research into the generation mechanism of surface flaws, the present inventors found that the cause of hot embrittlement is low melting point nonmetallic inclusions and
We found that there are solid solution sulfides that precipitate in the temperature range below ℃, and obtained new knowledge that can prevent hot embrittlement caused by them in relation to the temperature of the slab. We succeeded in stably performing rolling at a high temperature range of 900 to 1200°C. Now, in continuous casting, high-temperature slabs are obtained continuously, so
Although it is well predicted that direct rolling would be extremely effective in saving energy, process, and labor, methods and equipment for continuously performing continuous casting and direct rolling are still not practical. At present, only the so-called hot charge method has been proposed, in which a high-temperature slab is cooled to, for example, 900° C. or lower, and then charged into a reheating furnace and then rolled. This is due to the fact that hot embrittlement has not been elucidated as mentioned above.In a process that connects continuous casting and direct rolling in series, the hot slab is rolled at a rolling temperature of, for example, 1200 to 1050°C. Since it is bitten in the rolling mill, it is necessary for the hot slab to be in a state where hot embrittlement does not occur at that temperature.In other words, the cooling, that is, the temperature conditions, must be controlled to achieve this state. . These have been revealed for the first time in the present invention,
In the present invention, for the first time, an effective method in which a continuous casting method and a direct rolling method are combined in series has been realized. A detailed explanation of hot embrittlement will now be provided. Now, as mentioned above, the cause of hot embrittlement at high temperatures is mainly low melting point nonmetallic inclusions (FeS-MnS-S
iO, etc.) and solid solution sulfides. When cooling from a solidified state and performing direct rolling at high temperatures, if low melting point nonmetallic inclusions are present in the melt state,
Surface cracks occur in that area. In a temperature range of 1200° C. or lower, solid solution sulfides further precipitate within austenite grains and at grain boundaries. Precipitates within the grains have a small effect on hot embrittlement, but those precipitated at the grain boundaries reduce the cohesive force of the grain boundaries, causing the aforementioned surface cracks to further expand and new surface cracks to form. Occur. As the temperature during rolling decreases, the solid solution sulfide continues to precipitate, so that surface cracks continue to expand and occur. These phenomena become very noticeable when the cooling rate from solidification is high, and it was confirmed that surface cracking cannot be prevented unless rolling is performed from a surface temperature of 900° C. or lower. This will be explained with reference to FIG. In the figure, the horizontal axis is the surface temperature at the start of rolling ('C), and the vertical axis is the cracking mark (extent of side cracking in flat rolling, taking a high-temperature slab as an example; 0 is no cracking, 5 is the most severe cracking) ), and the range between solid curves 4 and 5 corresponds to normal steel types (C-0.02 to 0.10%, Mn = 0.1
5 ~ 0. 35%) and Mn/S is 7~
10,

[0]〉200ppn] shows the crack gauge results obtained when hot slabs were directly rolled.
If o3>2ooppm, the range between focal curves 8 and 9 is Mn/S)15,[0)-50-60pPlH
(However,

[0] is indicated by the amount of oxygen in the sample taken after rolling). As a result of repeating these tests for each type of steel through numerous experiments, it was found that direct rolling can be performed without surface cracking if the rolling start temperature is lowered to approximately 900°C or below. In this case, the rolling power becomes too large and is not suitable for actual operation. Therefore, the rolling start temperature is set to 900°C or higher. It is clear from the example in Figure 2 that the chemical composition of the steel should be taken into consideration as a condition for achieving this. In other words, since the influence of the amount of S and the amount of oxygen is especially large, it can be seen that a decrease in the amount of S (including an increase in the Mn/S ratio) and a decrease in the amount of oxygen are desirable conditions. As a means of solving these various problems related to hot embrittlement, we conducted a detailed study of the thermal history from solidification to rolling temperature, and found that the relationship between the cooling rate and the embrittlement recovery temperature at which rolling is possible is We found that there is a very deep relationship between This relationship is shown in FIG. In Figure 3, the horizontal axis shows the cooling time of the steel ingot, and the vertical axis shows the temperature (℃
), which shows the relationship between the cooling rate and the embrittlement recovery area based on a number of tests in which high-temperature slabs were directly rolled for specific steel types. Curves 8, 9 and 10 starting from the solidification point 11 are cooling curves, and points 11a to 11c are boundary points at which embrittlement does not occur when the hot slab is cooled and directly rolled along the cooling curve, i.e. This is the embrittlement recovery temperature, and the curve 12 connecting points 11a to 11c will therefore be referred to as the embrittlement recovery curve. As is clear from this example, the slower the cooling, the higher the embrittlement recovery temperature, but there is a tendency for it to become saturated. Therefore, by setting the rolling temperature appropriately, it is possible to determine cooling conditions that match the rolling temperature, and when installing new equipment, it is more appropriate to design the cooling equipment for high-temperature slabs and set the rolling process for each target steel type. It will be possible to do this. Such an effect could not be obtained before the present invention. In FIG. 3, the temperature region higher than the embrittlement recovery curve is the embrittlement region, and the lower temperature region is the embrittlement recovery region. Prior to the present invention, there was no means for quantitatively displaying the embrittlement recovery temperature, and therefore, in the present invention, the index is expressed as a crack gauge, which will be described later. The embrittlement recovery temperature is the temperature at which the cracking point is 1.5 even after starting rolling.
In actual operation, if the crack mark is 1.5 or less, direct rolling can be carried out without worrying about product surface defects. A specific method for determining the cracking point is as follows. Cracking point O: No cracking Cracking point 1: 1/8 to 1/8 of slab thickness (width of short side)
Small shallow cracks with a length of 1/4 or less, the number of occurrences is very small.Gage 2: Shallow cracks with a length of 1/4 to 1/2 of the slab thickness.CracksGage 3: 1 of the slab thickness. Shallow crack of /2 or moreCrack mark 4: A crack that is long enough to reach the thickness of the slab and is deepCrack mark 5: A large crack that cannot be rolled In the present invention, the crack mark was used as an indicator, but in the present invention Other indicators may be used as appropriate as long as they do not deviate from the gist. Now, when cooling to the embrittlement recovery range, the restrictions on chemical composition are greatly relaxed. For example, in carbon steel (0.10% C), the oxygen content is 200 ppm or less, the Mn/S ratio is 7 or more, and the surface area is It is possible to prevent cracking caused by low-melting point nonmetallic inclusions and austenite grain boundary cracking caused by solid solution sulfides that occur at temperatures below 1200°C. In addition, as the cooling rate from solidification decreases, the embrittlement recovery temperature increases, so as mentioned above, if this embrittlement recovery curve is determined for each steel type, it will be possible to avoid reheating after casting and solidification.
It becomes possible to roll the slab by keeping the temperature of the slab below the embrittlement recovery temperature in advance. Next, as mentioned above, the method of the present invention can be used extremely effectively in a process that combines continuous casting and direct rolling. It has significant benefits when applied to various processes. That is, to control the slab width in continuous casting for the purpose of producing hot rolled sheets, currently two methods are used: changing the mold and continuously changing the width during casting. However, there are drawbacks such as the time it takes to convert the mold and change the slab width. When controlling slab width with a conventional rolling mill,
Although it is highly efficient, as mentioned earlier, if the cooling rate from solidification and rolling temperature are not taken into consideration, surface cracking will occur due to hot embrittlement during rolling, so conventional slab sizing by direct rolling is not practical. However, it was not standardized.
The present invention has made it possible to realize a continuous process in which continuous casting is followed by slab width sizing rolling and product rolling. Next, specific examples will be described. Table 1 shows that for five types of steel, high-temperature slabs are cooled at various cooling rates from solidification after the start of pouring, the relationship between the sizing rolling start temperature of the slabs and the cracking gauge is determined, and then the cracking gauge is determined. The temperature at which is between 1 and 1.5 (hereinafter referred to as the embrittlement recovery temperature) was determined. Table 1 clearly shows that as the cooling rate decreases, the embrittlement recovery temperature increases. Also, low carbon-low Mn/S system (Mn/S=7 to 9
) and a low-carbon, medium-Mn/S system (Mn/S = 10 to 15), the low-carbon, medium-Mn/S system with a large Mn/S ratio has a strong embrittlement recovery temperature and is able to withstand higher temperatures. It can be seen that rolling is possible. Furthermore, as a result of considering the amount of P, as shown in the low carbon-low P system, the amount of P was increased from 0.020 to o. When the oi is reduced to 1%, recovery from embrittlement occurs in a short time, and the apparent effect is the same as increasing the embrittlement recovery temperature, with only a decrease in the amount of S (or an increase in the Mn/S ratio) in direct rolling. Not, but
It turns out that reducing the amount of P is also very important. Figure 4 shows low carbon (C0.02-0.12) one low'Mn
An example of the embrittlement recovery curve of /S system is shown. In Figure 4, the horizontal axis represents the elapsed cooling time (self), and the vertical axis represents the temperature (Q). Solid line 18 shows the steel type shown in Example 1 in Table 2 when cooled at 0.54°C/sec.
Solid line 19 is 0.25℃. The case of cooling at /'sec is shown. Moreover, dotted lines 20 and 21 are 0.0 for the steel type of Example 2, respectively.
The case of cooling at 75°C/sec and 0.18°C/sec is shown. Thus, the embrittlement recovery curves 22 and 23 correspond to the steel types of Examples 1 and 2. It can be seen from FIG. 4 that when the cooling rate is lowered, the embrittlement recovery temperature increases. Next, FIG. 5 shows an example of the embrittlement recovery curve for medium carbon (C=0.13 to 0.25) and low Mn/S steel, from the injection start temperature of 24 to the embrittlement recovery temperature of 25 to Up to 28 are shown by cooling rate. Solid line 29. 30 is for the steel types shown in Example 3 in Table 3; 0.4 0℃/sec and 0.2 3℃/s, respectively.
When cooled by EC, the dotted line 31. 32 is 0.36℃/sec, 0.16℃/sec for the steel type shown in Example 4.
Curve 33.34 shows the case of cooling in Example 3,
3 shows an embrittlement recovery curve for the steel type shown in Example 4. Next, Figure 6 shows an example of the embrittlement recovery curve for high carbon (C = 0.26-0.50%), high Mn/S steel, and shows the steel types shown in Table 4 from the injection starting temperature of 35. Embrittlement recovery temperature for 39. Up to 40 are illustrated by cooling rate,
Solid line 36. 37 has a cooling rate of 0.28℃/s.
This is an example of cooling at ec, 0.16°C/see. This clearly shows that slow cooling has the effect of increasing the embrittlement recovery temperature to the high temperature side. According to the findings of the present inventors, better results are obtained when P is lowered for medium-coal, high-coal, and steel types with a higher carbon content. Further, as the Mn/S decreases, the embrittlement recovery curve 22 shifts toward the higher temperature side (in other words, shifts toward the shorter time side), so rolling from a higher temperature becomes possible. Next, we will discuss the effect of oxygen content on the embrittlement recovery temperature. Figure 7 shows the oxygen content (ppII1) on the horizontal axis and the cracking point on the vertical axis, and shows the results for Mn/S10 low carbon steel and high-temperature slabs with the same molten steel temperature.

[0] was changed to investigate its effect, and the rolling start temperature was 1120℃,
The cooling rate was 0.25°C/sec. As is clear from Figure 7

It can be seen that the lower [0] is, the more preferable it is. As is clear from the above explanation, when continuous casting is followed by direct rolling without reheating, the rolling temperature is selected in advance to be in the embrittlement recovery region for the intended steel type, and the optimal cooling is performed to reach the intended rolling temperature. If the high temperature slab is gradually cooled at a high speed, there is no need to worry about surface cracking when the temperature is reached.
Rolling can begin. Furthermore, as detailed above, Mn/S is preferable depending on the component system, and

[0] It is clear that there is a limit to the amount, and therefore, in the present invention, the above-mentioned suitable range of Mn/S is referred to as "setting Mn/SJ,"

If the upper limit of the [0] amount is referred to as the "limit [O] amount," the feature of the present invention is that the rolling start temperature is set in advance in the direct rolling method following continuous casting, and the Mn set for the steel type to be rolled is set in advance. /S and limit

It can also be said that this is a technology that selects a steel type with an amount of [0] or less, starts rolling at the above-mentioned rolling start temperature, and makes it possible to obtain the desired hot-rolled product without causing surface cracks. Mn/S will be explained in more detail. FIG. 8 is an example in which the crack gauge was investigated for low carbon steel by changing the Mn/S and rolling start temperature. In this example

Although no correspondence is shown for [0], [P] and cooling rate, it clearly shows a tendency for cracking to decrease as Mn/S increases. In the figure, solid line 42, dotted line 43, and chain line 44 each indicate Mn/S of 6.
-10, 25-30, 40-45. Next, the temperature of the high temperature slab in the present invention will be explained in detail. When a relatively large slab is cooled from a solidified state, a temperature difference occurs between the surface layer and the inside. In order to prevent surface cracking when such a temperature difference occurs, the cooling curve at each position in the cross section is calculated or measured in advance. By the start temperature, the critical depth to reach the embrittlement recovery region is determined by the depth of the columnar crystal part in the cross section or 5 from the surface.
The inventors have learned that the smaller value of 0 round or approximately 1/4 of the thickness (meaning the width of the short side) is sufficient. In this case, as mentioned above, there is still a part of the embrittlement area inside the slab, but if the surface layer has reached the embrittlement recovery area, there is no cracking in this part. It was confirmed that cracks that occurred inside did not open on the surface because there were no cracks and cracks did not propagate. FIG. 9 schematically shows the embrittlement recovery temperature range in the cross section of a high-temperature slab 45 cast in a continuous casting machine.
indicates the embrittlement recovery temperature range, 47 indicates the embrittlement temperature range, and the shortest distance between the embrittlement temperature range 47 and the surface of the hot slab,
That is, the distance L1, L1' in the width direction and the shortest distance L2, L2' in the thickness direction are 50rr as an example, as described above.
Good results were obtained when the temperature was set to an or more. However, since the shortest distance varies depending on the type and size of the hot slab, an appropriate value should be determined depending on each actual case. The example shown in FIG. 9 uses values determined by actual surface temperature measurements and cross-sectional thermal calculations. As explained in detail above, the present invention provides a rolling method that does not generate cracks or surface flaws in the production of hot rolled steel products using the continuous casting-direct rolling method, and is energy saving and
This is a useful way to ensure low costs.

[Brief explanation of drawings]

Figure 1 is a graph showing the thermal history of the slab from casting to rolling, Figure 2 is a graph showing the relationship between cracking points by component and rolling start surface temperature during direct rolling, and Figure 3 is embrittlement recovery. Figures 4, 5, and 6 are graphs showing the embrittlement recovery curves for low, medium, and high carbon steels, respectively, and Figure 7 shows the relationship between [01 and the crack gauge point. FIG. 8 is a graph showing the relationship between Mn/S and cracking point, and FIG. 9 is a schematic diagram regarding the embrittlement recovery temperature range of a high-temperature slab. L2, 3... Curves showing the thermal history from solidification to rolling, 4-9... Curves showing the relationship between rolling start surface temperature and cracking mark.

Claims (1)

[Scope of Claims] 1. In a steel rolling method in which hot rolling is performed without reheating following continuous casting, the embrittlement of the surface of the slab is determined in advance for each steel type by cooling rate for the high-temperature slab after casting. A method for directly rolling a hot slab, characterized in that the recovery temperature is determined, and after the slab is cast, the slab is rolled at a temperature below the embrittlement recovery temperature without being reheated. 2. In a steel rolling method in which hot rolling is performed without reheating following continuous casting, the embrittlement recovery temperature of the high-temperature slab after casting is determined in advance for each steel type and cooling rate of the slab surface layer. The planned rolling temperature after casting of the slab is arbitrarily set in the embrittlement recovery temperature range, and the slab is slowly cooled at the optimum cooling rate to reach the scheduled rolling temperature without reheating. A method for directly rolling a hot slab, characterized by starting rolling when a scheduled rolling temperature is reached.