JPS5920732B2 - Method for heating slabs for producing unidirectional electrical steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized in that the crystal grains constituting the steel sheet have a Miller index of (110
) [Relates to a heating method for a slab for producing a grain-oriented electrical steel sheet having an orientation expressed as 000.

The magnetic properties required for unidirectional electrical steel sheets are high magnetic flux density and low iron loss, but recently there has been a strong demand for smaller transformers and higher performance to eliminate power loss. Inevitably, the design magnetic flux density must be increased.

For this purpose, a material with good magnetization characteristics, that is, a high Blo (magnetic flux density at a magnetization force of 10 AT/cm) value, has been required.

The method of the present invention is particularly effective for producing high magnetic flux density electrical steel sheets with excellent magnetization properties, but it can also be applied to general grain-oriented electrical steel sheets as an economical slab heating method.

Generally, efforts are being made to switch from the ingot method to the continuous casting method (hereinafter referred to as the CC method) as much as possible at the steel manufacturing stage of the iron manufacturing process.

The advantages are improved yield, omission of the blooming process, and cost reduction due to production continuity. However, in the production of unidirectional electrical steel sheets, MnS and At
Although heating is carried out at high temperature in order to dissolve the precipitated dispersed phase such as N,
As a result, the crystal grains of the slab become too coarse, resulting in drawn grains that are not primary recrystallized during hot rolling, and the secondary recrystallization induced by final finish annealing is incomplete (hereinafter, incomplete portions are linear fine grains). (It is called a line mixture because it exhibits grains.)

Therefore, it is disclosed in Japanese Patent Publication No. 54-27820 and Japanese Patent Publication No. 50-3700 that there is a technical drawback of losing the magnetism of the product.
It is disclosed in Publication No. 9. Furthermore, although this is not a problem limited to the CC method, if the slab heating temperature is too low, solid solution of the precipitated dispersed phase of the slab will be insufficient, resulting in incomplete secondary recrystallization (line mixing). Various countermeasures have been proposed to deal with this line mixing problem.

That is, in the method described in Japanese Patent Publication No. 54-27820, a CC slab is first heated at a temperature of 750 to 1200°C.
A method is shown in which blooming is carried out at a rolling ratio of 50%, followed by heating at a high temperature and hot rolling.

Similarly, the method described in Japanese Patent Publication No. 50-37009 describes a method of blooming at a temperature of 1250 to 1300° C. and a rolling reduction of 30 to 70 (fl) (hereinafter referred to as the pre-roll method). All of these are used to prevent coarsening of crystal grains due to high-temperature heating and to take measures against wire mixing by blooming. However, in these methods, the hot rolling process including blooming is carried out twice, which detracts from the process-saving advantage of the CC method. As a technique to compensate for this irrationality, Japanese Patent Application Laid-Open No. 54-120214 discloses rolling crystal grains that have become coarse during high-temperature heating at a rolling temperature of 1190 to 960.
A method is disclosed in which the drawn grains are destroyed and finely recrystallized by performing primary recrystallization rolling at .degree.

Next, Japanese Patent Application Laid-Open No. 52-60216 discloses that in the cold rolling process following the hot rolling process, the temperature during cold rolling is increased to give a thermal effect, thereby changing the cold rolling structure of the coarse grain part. A method is shown in which the structure before final annealing is made uniform and secondary recrystallized grains are stably grown by promoting recrystallization in the next step of continuous annealing. These are technologies that attempt to prevent wire mixing by adjusting hot rolling conditions and cold rolling conditions, and can be said to be measures that approve of coarse grain growth in the hot rolling furnace.
Although no mention is made of the heating technology that induces the wire mixing problem, from the viewpoint of energy saving and scale loss, it is preferable to primarily control the heating conditions to reduce the number of coarse particles.

On the other hand, the optimal grain growth state of the hot rolled slab is determined by the magnetic results, and the grain growth area is at least 30w1 as the surface structure desirable for secondary recrystallization nucleation in the slab cross-sectional direction.
It was found that a two-layer structure having a grain size n (15% of the total thickness) and having at least 5% of the total thickness an ungrained growth structure that is easily eaten by secondary recrystallization nuclei is good magnetically. This content does not refer to specific slab heating conditions. This grain growth state is investigated by hot-rolling the slab, cooling it, cutting it, and polishing it by etching it (hereinafter referred to as macroetch structure). Therefore, it takes several days to conduct the investigation and it is not possible to quickly provide feedback to the heating operation. The magnetic results are correlated with the results obtained after the final process of a slab adjacent to the slab whose macrostructure was investigated. Regarding the hot rolling conditions and the method of imparting a thermal effect during cold rolling, the meaning of the temperature as the hot rolling heating condition is unclear;
This is a simplistic description that simply requires high-temperature heating, which is different from the gist of the present invention, which is to carry out heating operations economically.

That is, although the above-mentioned publication indicates a general slab heating temperature, it does not mention the restrictions specific to the heating conditions, which will be described later, such as suppressing coarse grains or improving magnetism. Furthermore, the method of limiting the macroetch structure of the slab requires a long feedback time for practical heating operations, and although it is possible as a rod management check, it is inconvenient as an indicator. The present invention provides an index of the lower limit of heating conditions that satisfies the solid solution of the precipitated dispersed phase of MnS and AtN and prevents fine grains, and the upper limit that prevents line mixing, and as a result, fine heating control is possible). This saves energy in the heating furnace and reduces scale loss while ensuring magnetic stability. Because slabs for unidirectional electrical steel are heated at high temperatures to form a solid solution of MnS and AtN, the fuel consumption of the heating furnace is approximately twice that of normal steel slab processing, and the scale loss is approximately It has increased four times.

A large amount of scale generated in the furnace has an adverse effect on the production capacity of the heating furnace due to the retention limit in the furnace, its processing cost, the repair cost of refractories, and the difficulty of operation.
Therefore, finding the upper and lower limits of heating conditions and carefully controlling the slab so as not to overheat it more than necessary is particularly important from the perspective of saving resources, apart from stabilizing the magnetism well. The feature of the present invention is that in an actual heating furnace where the furnace time is several hours and the insertion length is as long as 30 m,
Continuous temperature measurement from the insertion of the slab to extraction is now possible, and the optimal heating temperature (including temperature gradient) and heating time conditions for the slab can be found, and this can be used as an index for fine-grained heating control. be.

The heating temperature conditions for conventionally common slabs were
As stated in Japanese Patent Publication No. 651 and Japanese Patent Publication No. 54-27820, the temperature is 1260 to 1400°C.

The rationale for this heating temperature is unknown, and the time conditions are not specified, and the slab heating temperature is 1260℃.
In actual operation, even if the heating time is increased, MnS,
It does not satisfy the solid solution conditions for AtN and grain growth does not occur. Furthermore, in the actual temperature values measured in the actual heating furnace described later, the center temperature of the slurp thickness (hereinafter referred to as the center temperature) is, for example, 1280°C, and the temperature 30 m from the surface (hereinafter referred to as the near-surface temperature) is 1.
Even if the temperature gradient reaches 330° C., the same results as above will occur, and wire mixing will occur in the product, resulting in a significant decrease in magnetism.

Also, it is unclear whether the upper limit temperature of 1400°C is the furnace temperature or the slab temperature, but if it is the slab temperature, the slab will be unnecessarily overheated, and the scaling of the slab surface will be proportional to the temperature and time. This increases scale loss and wastes heating energy. From a metallurgical point of view, the rolling temperature after extraction in a heating furnace becomes high, making primary recrystallization difficult, leaving stretched grains and leading to wire mixing problems.
Insufficient precipitation of S also occurs, resulting in deterioration of magnetism. Therefore, the slab heating temperature of 1260 to 1400 DEG C. that has been proposed in the past is not only due to the time factor, but is also inappropriate as the upper and lower limits of the slab heating temperature.

Next, in Japanese Patent Publication No. 50-37009, it states that heating to 1,300°C or higher will cause crystal grains to become too coarse and the secondary recrystallization induced by final annealing will become incomplete. Some heating temperature examples are shown in a comparative example with the hot rolling method.

Although it is unclear what this heating temperature refers to, the time conditions are indicated. However, 1350° C. is a high temperature as described above, and in the examples, the pre-rolling method is good under this heating condition, but the A-slap method of the one-time hot rolling method is described as a result of wire mixing and poor magnetism. Furthermore, it is unclear at what point within the slab the temperature is being referred to, and there is no mention of temperature gradients. Therefore, compared to the heating conditions under which good magnetic results are obtained by the one-step CC hot rolling method of the present invention, the heating conditions are too much and the technical idea is different. Next, JP-A-54-1202
Even in Publication No. 14, the slap heating temperature is only generally expressed as high temperature heating of 1300° C. or higher.

Also, this publication states that MnS is
, it is said that the grains become fine due to insufficient solutionization of AtN precipitated dispersion, but according to actual measurements, a temperature of 1300°C or higher is not necessarily sufficient; for example, a temperature of 1310°C is not sufficient. In other words, the temperature near the slab surface is 1
The temperature is about 350°C and a longer heating time is required. Next, in the description in JP-A No. 52-60216, there is a description of heating at 1360°C for 40 minutes.
It is unclear which point on the slop is and the temperature is on the verge of overheating.

It is also said that if the cold rolling effect is not provided in the CC one-hot rolling method, wire mixing will occur and the magnetic results will also deteriorate. Of course, this does not refer to the slab temperature distribution and is different from the present invention. As described above, the slab heating conditions disclosed in the conventional patent documents are extremely insufficient to prevent the occurrence of line mixing in products. In view of the current situation, the present invention seeks to provide a slab heating method that can sufficiently avoid the occurrence of wire mixing.

The heating conditions of the present invention will be explained below.

When heating a slab for unidirectional electrical steel sheet, the present inventor
Grain growth rate in the slab thickness direction is minimum 15 (fl) and maximum 9
It was confirmed that the generation of line contamination in the product could be sufficiently prevented by heating the product to have a crystalline structure of 5(f) (5% of ungrown portion remained).

The reason for this is that nucleation at the initial stage of secondary recrystallization occurs near the surface of the steel sheet, and the orientation of the generated nuclei is precisely (110) [000
In order to maintain the orientation (in order to obtain a high magnetic flux density), it is necessary to make the state of the precipitated dispersed phase near the surface of the steel sheet fine and uniform.

This is because when the size and number of precipitated particles in the precipitated dispersed phase are locally unevenly distributed and non-uniform, particles with different orientations also grow and the tendency to converge to the (110)[000 orientation is weakened. This is because it will be done. Therefore, it is the precipitated dispersed phase (MnS, AtN) near the surface that requires a grain growth rate of at least 15%.
It is expressed using the grain growth rate as a medium in order to satisfy the temperature conditions so that the state of the particles is fine and uniform (so that the solid solution conditions are adjusted). In addition, in order for the secondary recrystallization nuclei generated during recrystallization annealing to grow smoothly and complete growth over the entire steel sheet, the crystal structure that is easily eaten by secondary recrystallization nuclei must be located adjacent to the nucleation region on the surface of the steel sheet. It has been found that it is good to have a non-grain growth area of at least 5%. It goes without saying that by suppressing grain growth, the number of drawn grains after hot rolling can be reduced, primary recrystallization can be facilitated, and line mixing can be prevented. Since this grain growth is not the original purpose of heating the slab, but rather has a negative effect, care should be taken to adjust the solid solution state of MnS and AtN to the minimum necessary level.

This leads to reducing heating energy and scale loss. Grain growth rate is important as an index proportional to the solid solution state of MnS and AtN, and we are trying to find slab temperature and time conditions that satisfy the solid solution state and suppress coarse grains by mediating this grain growth rate. It is something to do. The present invention will be explained in detail based on this idea.

Conventionally, continuous temperature measurement of unidirectional electromagnetic steel slurp in a heating furnace could not be completed from insertion to extraction as with ordinary materials due to the durability of thermocouples due to high temperature heating and the generation of large amounts of slag. . By the way, as shown in FIG. 1, continuous temperature measurement has become possible by cooling the thermocouple with N2 while increasing its durability.

In the figure, 1 is a heating furnace, 2 is a slab, 3 is a thermocouple, 4 is a measurement point, and 5 is a flexible hose. Figure 2 shows an example of heating up a slab with a thickness of 200 m, which is inserted into the preheating zone of the heating furnace, passes through the heating zone, and the soaking zone before being extracted.

The temperature measurement point is the center point of the slab thickness, and the difference in heating temperature within the same slab is due to the difference in position within the furnace. The extraction temperature was 1360°C at point a and 1345°C at point b. After the slab was extracted and cooled, the macroetch structure was investigated and the results are shown in Figure 3. This result shows that although the heating furnace extraction temperature differs by only 15°C between points a and b, the grain growth state of the slab is that in part A, grains grow all over the thickness direction, and in part B, part of the grain grows and part of it is CC. The cast structure remains.

In other words, the grain growth rate defined in Figure 4 is 100% in part A;
Part B is 72%. Of course, as mentioned above, the condition of section A is poor with no ungrown grains and excessive growth of coarse grains, while the condition of section B is good, and this slab and adjacent slabs are processed into finished products through post-processing. Looking at the magnetic results, there was some wire mixing in part A, but in part B, it was good and within the specifications. From this fact, it was found that when the center temperature exceeds 1,350°C, too much coarse grain growth occurs, and that there is no need to heat above 1,350°C.

It was also found that the influence of heating time and history is a more important factor than the difference in slap heating temperature. That is, the heating time after the grain growth start temperature is relevant. This temperature at which grain growth starts is influenced by components and the like. For example, the grain growth initiation temperature of a slab for producing unidirectional electrical steel sheets as shown below is approximately as shown below. However, the chill crystal of CC casting structure is 1
Although it occurs even at very low temperatures of about 240°C, it is so thin that it is not considered. Strictly speaking, the time required to exceed the grain growth start temperature should be considered, but due to component variations etc.
℃ shown in Figure 2 as an index of heating operation, and the difference in magnetic properties and component system is used as A group and B group, which represents the temperature at one point (for example, the center) in the slab. is convenient for heating control. For reference, the slab temperature measurement example in Figure 2 belongs to group A, and the time in the furnace at 1300℃ or higher until extraction is 111 at point a.
minute, point b was 67 minutes.

Next, FIG. 5 shows an example in which a three-point thermocouple was embedded in the thickness direction of a slab belonging to group B.

As a result, the extraction temperature was 1320°C at the center of the slab, and 1330°C and 1335°C at a depth of 30 Trm from the surface. Further, the furnace time during which the center temperature exceeded 1300°C was 24 minutes, and the grain growth rate of the macroetch of the slab shown in FIG. 6 was 29(f) in the thickness direction. Figure 7 shows the temperature gradient in the thickness direction of the slab obtained from these three slab temperature measurement points and shows the change over time. As a result, there is no particular lowest temperature point in the thickness direction, and the center temperature is 1320° C., which exceeds the grain growth start temperature of group B slabs. However, as shown in FIG. 6, ungrown portions remain. This shows that the formation does not occur instantaneously when the temperature of a certain minute portion exceeds the grain growth starting temperature, but that a certain amount of time is required. At the same time, although the temperature near the surface is below 1350°C, a satisfactory grain growth rate is obtained, so it is understood that a temperature above 1350°C is unnecessary. The magnetic properties of products from adjacent slabs were also within the specifications. Next, as an example showing the lower limit of the heating conditions, FIG. 8 shows an example of temperature measurements of slabs of group A.

The results showed that the extraction temperature was 1285°C at the center, 1330°C at the top near the surface, and 1285°C at the bottom. The macroetch structure of this slab is shown in Figure 9, and the grain growth rate is
Only a very small amount of chill crystals on the surface have grown, which is 4%.
Furthermore, the magnetic properties of the adjacent slabs were also poor due to the occurrence of wire mixing. From this fact, the center temperature needs to exceed 1,300℃, and even if the temperature near the surface is 1,330℃, the time at 1,300℃ or higher is only about 10 minutes, so the entire slab is not heated enough and the finished product is not finished. It can be seen that line confusion occurs.

From the above slab temperature measurement examples-1, 2, and 3,
It was found that the lower limit temperature of 1260° C. shown in Publication No. 51 was insufficient, and the upper limit of 1400° C. was too much.

In addition, as an optimal heating condition, the temperature near the surface should be 1350°C or less (at least the center temperature can be 1350°C or less), and if the heating time is set so that the center temperature exceeds 1300°C, coarse grains will not grow. It has been found that it is possible to suppress the overflow and bring the magnetism of the finished product within a good range.

The significance of finding these precise temperature conditions is as described above. In addition, when heating a slab on one side, the temperature should be represented by a temperature 1/3 to 1 thick from the high temperature side.Based on this knowledge, we collected experimental data for each slab in Group A and Group B in an actual heating furnace. FIG. 11 shows the relationship between the time during which the slab center temperature exceeds 1300° C. and the grain growth rate.

From the results shown in Figure 11, the lower limit value is 15% grain growth rate.
At this time, the solid solution conditions for the precipitated dispersed phase of MnS and AtN were satisfied and the magnetic properties of the product were good, so the A group slabs had a temperature of 1300°C or higher for which the slab was maintained for 15 minutes, and the B group slabs had a temperature of 10 minutes. It will be a minute. Also, from the perspective of not allowing coarse grains to grow, the upper limit is set at 50! ) Since the magnetic properties of the finished product were good when left, the retention time for the slab center temperature of 1300° C. or higher was 70 minutes for the A group slabs and 50 minutes for the B group slabs. Group B slabs undergo two annealing and cold rolling processes, and the required magnetic properties of the finished product are low, so there is less of the problem of wire contamination due to overheating as with Group A.
Since heating more than necessary is wasteful in terms of energy saving and scale loss, the holding time is an important index like the A group slabs.

This temperature-time index intentionally recognizes the slab temperature distribution, but the heating furnace type is a walking beam furnace, there are few skid marks, and the rolling shape is not a problem under high-temperature heating conditions.

The temperature near the surface of the slab and the center temperature can be adjusted by the furnace temperature, the time the slab is in the furnace, and the heating method. Next, specific details of controlling the heating furnace will be explained based on the heating conditions described above. The temperature control method of the slurp of the heating furnace can be carried out by incorporating the management index of the present invention into various disclosed known control methods, but an outline thereof will be explained below.

In reality, the heating furnace is a multi-zone furnace, but here it is shown as one zone in FIG. 12 and the basic concept will be described. Slab 13 whose initial temperature is now measured with thermometer 12 in heating furnace 11
Slap 1, which had been at the tip until now, was inserted into the furnace.
3't) S is extracted.

At the same time, the insertion time signal, slab conditions (thickness, width, composition, physical property values such as slab thermal conductivity, specific heat, target value of heating time of 1300°C or more), etc. are given to the computer 14, and each slab in the furnace is The in-furnace position is calculated from the slab dimensions and stored. 15 is a flue.

The i-1+n slap that stays in the furnace is determined by the following 1 for each arbitrary time.
Calculated by the formula. Next, using this temperature as an initial condition, the temperature at which each slab is extracted and the time in the furnace above 1300°C are calculated from the remaining time in the furnace determined from the extraction pitch.

Here, among the i-1+n slabs, the slab that is least likely to be heated based on the furnace time of 1300° C. or higher is selected as the reference slab.

If the time in the furnace at 1300°C or higher for this reference slab is insufficient, first increase the furnace temperature, and if the time exceeds the furnace temperature, lower the furnace temperature. If this furnace temperature range becomes impossible due to equipment operation constraints, or if it deviates from the heat pattern that increases the thermal efficiency of the entire furnace (for multi-zone furnaces), change the extraction pitch. Adjust furnace time. 13th
The figure shows this control flow. In this way, the lower limit heating time that does not cause wire mixing is strictly observed, and the heating is controlled so as not to exceed the necessary level, resulting in an energy saving rate of 10.
(f) The scale loss is reduced by 1%, and the magnetism of the finished product can be stabilized. The slab heating conditions of the present invention can be applied as a control index for slabs for producing unidirectional electrical steel sheets, regardless of any heating control method.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the slab temperature measurement method, Fig. 2 is a chart showing slab temperature measurement example 1 in a heating furnace, Fig. 3 is a metal cross-sectional photograph showing the slab macroetch structure in slab temperature measurement example 1, Fig. 4 is an explanatory diagram of the definition of grain growth rate, Fig. 5 is a chart showing slab temperature measurement example 2 in a heating furnace, and Fig. 6 is a metal cross-sectional photograph showing the slap macroetch structure in slap temperature measurement example 2. , Fig. 7 is a chart showing the change in temperature gradient over time in slab temperature measurement example 2, Fig. 8 is a chart showing slab temperature measurement example 3 in a heating furnace, and Fig. 9 is a chart showing slab macro etching in slab temperature measurement example 3. Metal cross-sectional photograph showing the structure, No. 10
The figure is a chart showing the change in temperature gradient over time in Slab Temperature Measurement Example 3, Figure 11 is a chart showing the relationship between the time in the furnace above 1300°C and the grain growth rate, and Figure 12 is a conceptual diagram of heating furnace control. , FIG. 13 is a diagram showing the slab heating control flow.

Claims (1)

[Claims] 1. In a method for producing a grain-oriented electrical steel sheet in which a continuously cast slab containing 2.5 to 4.0% Si as a main component is hot-rolled in one hot rolling process and then cold-rolled and annealed, the slab before hot-rolling is A slab for producing unidirectional electrical steel sheets, characterized in that the heating conditions include a slab center temperature of 1350°C or lower, and a heating time for which the slab thickness center temperature is 1300°C or higher set within the time range shown below. Heating method.