JP7617404B2 - Manufacturing method of hot rolled coil - Google Patents

Description

The present invention relates to a method for manufacturing hot-rolled coils in which the hot-rolled steel sheet after hot finish rolling is wound in a coiler and then the hot-rolled coil is pulled out from the mandrel of the coiler without being significantly deformed by its own weight.

After the hot-rolled steel sheet is finish-rolled, it is wound by a coiler and transported to the next process. When the hot-rolled coil is unwound from the mandrel of the coiler, it may deform under its own weight. If this deformation is large, it may not be possible to smoothly insert the hot-rolled coil into the mandrel of the unwinder in the subsequent process.
In such cases, it is necessary to carry out a coil re-coiling process after passing through a coil refinement process, but if the deformation is significant, it is not possible to re-coil the hot-rolled coil, and there is a problem that the hot-rolled coil must be scrapped.

Conventionally, it has been believed that deformation of hot-rolled coils due to their own weight is caused by volume expansion due to the γ (fcc) → α (bcc) transformation.
For example, in a high-strength hot-rolled steel sheet containing a required amount of alloying elements, the gamma → alpha transformation may not be completed on the runout table after hot rolling, and may continue during and after winding on a coiler, and even after the coil is pulled out from the mandrel. In such cases, the volume expansion of the steel sheet structure accompanying the subsequent gamma → alpha transformation reduces the radial surface pressure of the hot-rolled coil, reducing the rigidity of the entire hot-rolled coil, and the coil, which is a multiple-winding cylinder under tension, behaves like a single-winding cylinder, causing the hot-rolled coil to deform under its own weight.

Therefore, technologies have been proposed to prevent deformation of hot-rolled coils, mainly from the perspective of (a) mechanically maintaining the shape or correcting deformation using a V-shaped skid with a small angle (see, for example, Patent Documents 1 to 4), (b) retaining the coil for a certain period of time in the coiler and cooling it under cooling conditions such as natural cooling, air cooling, mist cooling, and water cooling (see, for example, Patent Documents 5 to 8), and (c) controlling the transformation rate (see, for example, Patent Documents 9 to 12).

However, these techniques have the following problems.
In the mechanical method of (a), since it takes time to place the coil on the V-shaped skid, there is a problem that it is not possible to take measures in time to prevent deformation that occurs immediately after the hot-rolled coil is pulled out from the mandrel.

Method (b) is effective depending on the type of steel, but it is a symptomatic deformation prevention measure based on experience and cannot be said to be a reliable and efficient method of deformation prevention. In addition, it requires a very long residence time, which can significantly reduce work efficiency. Furthermore, when the cooling rate is high, such as with water cooling, the edge parts and several turns at the outermost circumference of the hot-rolled steel sheet can become excessively hardened depending on the type of steel, resulting in strength differences within the hot-rolled coil. This can be dealt with by discarding the outermost circumference, but this reduces the yield, and the hardening of the edge parts can cause breakage during cold rolling, so there is a problem that the types of steel that can be applied are limited.

Method (c) has problems such as a narrow control range and poor robustness, and generally requires hot rolling equipment that performs accelerated rolling to roll at a constant speed with low production efficiency, resulting in reduced efficiency, and requiring major modifications to existing equipment and control logic, making it uneconomical. Method (c) also includes a method in which a certain length of the coil from the tip is wound at a higher temperature than the rest to control the volume expansion rate due to transformation within the coil, but the winding temperature is not selected based on the mechanism that generates coil deformation, so coil deformation is not sufficiently suppressed, and there are problems with the fact that the effect may be almost nonexistent depending on the steel composition.

Japanese Patent Application Laid-Open No. 61-037647 Japanese Utility Model Application Publication No. 01-143609 Japanese Utility Model Application Publication No. 06-034807 Japanese Patent Application Publication No. 07-246422 JP 2001-179335 A JP 2010-089107 A JP 2010-094710 A JP 2012-024793 A JP 2012-055950 A Patent No. 5447744 JP 2014-065077 A Patent No. 5594578

In view of the prior art, the present invention aims to provide a method for manufacturing hot-rolled coils that can prevent deformation of the hot-rolled coils pulled out from the mandrel of a coiler in a simpler manner than conventional means, and that can avoid rewinding the hot-rolled coils and scrapping them.

In order to solve the above problems, the present inventors have extensively studied the deformation mechanism of hot rolled coils.
As a result, it was found that the deformation of the hot-rolled coil immediately after it is pulled out from the mandrel or thereafter is not due to loosening of the hot-rolled coil caused by volume expansion accompanying the γ → α transformation, as has been conventionally believed, but is due to a decrease in the rigidity of the entire coil caused by the transformation plasticity phenomenon that occurs during the γ → α transformation.

From this perspective, a method for preventing deformation of the coil was investigated.
Since the strain caused by transformation plasticity increases in proportion to the rate of the γ → α transformation, the idea was reached that by taking advantage of the difference in cooling rate between the outer and inner parts of the coil, a difference in transformation rate could be created between the outer and inner parts of the hot-rolled coil, and the coil could be supported at points where the transformation rate is slow and where strain due to transformation plasticity is less likely to occur, thereby preventing deformation of the coil.
It was found that high-temperature coiling is advantageous for generating a difference in transformation rate between the outer and inner parts of a hot-rolled coil. However, high-temperature coiling has the problem that internal oxidation may occur in the inner part.

As a result of further investigation into means of preventing coil deformation and avoiding the problem of internal oxidation, it was discovered that by changing the coiling temperature to a higher temperature midway when coiling the hot-rolled steel sheet, it is possible to delay the transformation of the part that will become the outer part of the coil (the outer periphery) during the initial stage of drawing, when deformation is likely to occur, and to have the transformation of the outer periphery begin after the transformation rate of the part that will become the inner part (the inside) has slowed down, thereby preventing coil deformation and avoiding internal oxidation of the inner part.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
(1) In a hot-rolled steel sheet winding process, the hot-rolled steel sheet after hot rolling is cooled on a runout table and wound up into a hot-rolled coil by a coiler.
A method for manufacturing a hot-rolled coil, characterized in that the range from the outermost periphery of a hot-rolled coil that is 5% to 30% of the total coil thickness is defined as the outer periphery, and the remaining range inside is defined as the inside, wherein the range corresponding to the inside is coiled at a temperature of 450°C to 650°C, and the range corresponding to the outer periphery is coiled at a temperature that is more than 50°C higher than the temperature of the inside and is 800°C or less.

(2) The method for manufacturing a hot-rolled coil described in (1) above, characterized in that the coiling temperature of the hot-rolled steel sheet is set to the optimum internal coiling temperature for the range from the front end to the inside of the entire length of the hot-rolled steel sheet, and the remaining range corresponding to the outer periphery is set to an outer periphery coiling temperature that is more than 50°C higher than the internal coiling temperature.

(3) A method for manufacturing a hot-rolled coil as described in (2) above, characterized in that a Formaster test is performed in advance, in which a temperature history equivalent to that during cooling of a hot-rolled steel sheet is applied, to examine the time progression of the transformation rate during coil cooling, and the results are used to set the coiling temperatures of the inside and outer periphery.

The present invention provides a method for manufacturing hot-rolled coils that can avoid scrapping and rewinding of hot-rolled coils by simpler means. Furthermore, it is no longer necessary to increase the coiling temperature inside the coil more than necessary, and internal oxidation that occurs when the coiling temperature is high can be avoided.

FIG. 1 is a diagram showing the time progression of the transformation rate from the start of cooling until 300 seconds, calculated by a Formaster test assuming winding at 600° C. FIG. 1 is a diagram showing the time progression of the transformation rate from the start of cooling until 300 seconds, calculated by a Formaster test assuming winding at 690° C. This figure shows the time transition of the transformation rate when the internal winding temperature is 600° C. and the external winding temperature is 690° C., and was created from the time transition of the transformation rate in FIGS. 1 and 2. FIG. 1 is a diagram showing an example of a temperature history assuming winding at 600° C. used in a Formaster test. FIG. 1 is a diagram illustrating a mechanism by which a hot-rolled coil is not deformed when the coiling temperature is increased only at the outer periphery of the coil. FIG. 1 is a diagram showing a schematic diagram of the progress of phase transformation inside the coil and on the outer periphery of the coil during cooling of the hot-rolled coil in the case where the coiling temperature is increased only on the outer periphery of the coil.

When a hot-rolled coil made by winding hot-rolled steel sheet in which the gamma → alpha transformation has not yet been completely completed is immediately pulled out of the coiler's mandrel after winding is complete, the coil does not deform after the pulling out and can be transported quickly to the next process. Furthermore, it is required from an operational standpoint that the coil does not deform after that until it is cooled.

The deformation accompanying phase transformation is thought to include deformation due to transformation plasticity in addition to deformation due to volume change.
Here, transformation plasticity is a phenomenon in which, when stress acts during phase transformation, deformation occurs in the direction in which the stress acts. In the case of hot-rolled coils, this phenomenon is expected to manifest itself as deformation progressing in the direction that relieves the stress due to its own weight, i.e., vertically downward, during γ→α transformation, resulting in deformation that causes the coil to collapse.

In order to clarify the deformation mechanism of a hot-rolled coil, the present inventors performed an FEM analysis of deformation caused by the weight of a steel sheet formed into a cylindrical body.
As a result, it was found that when the transformation plasticity phenomenon is not taken into consideration, even in a simulation in which a 4 mm thick steel plate is used as a single-wound cylinder, almost no deformation due to its own weight occurs, and even less deformation occurs in a hot-rolled coil, which is a cylinder with multiple turns.
On the other hand, a simulation that took into account the transformation plasticity phenomenon revealed that a single-wound cylinder made of 4 mm thick steel plate deformed significantly, and that a coil, which is a cylinder with multiple turns, also deformed.

Since deformation does occur in a coil, which is a multiple-wound cylinder that is more resistant to deformation due to its own weight than a single-wound cylinder, it was predicted that the deformation of the coil is largely related to a decrease in rigidity due to the transformation plasticity phenomenon, rather than loosening of the coil caused by volume expansion accompanying the γ→α transformation.

It is known that the magnitude of strain caused by transformation plasticity increases in proportion to the transformation rate of the phase transformation. Therefore, the inventors performed a Formaster test to simulate the temperature history from the start of cooling on hot-rolled coils that were cooled on a runout table and coiled at various coiling temperatures, and calculated the magnitude of the transformation rate at the center and outer periphery of the coil for each coiling temperature, and investigated the relationship with the deformation of the coil.

For the Formaster test, a steel slab containing, by mass, 0.15% C, 0.50% Si, and 2.6% Mn was first produced, hot-rolled at a finishing exit temperature of 900°C, cooled, and then coiled at 600°C and 690°C. After coiling was completed (76 seconds after cooling of the coil tip began), the hot-rolled coil was pulled out from the mandrel and the deformation state of the coil after pulling was confirmed. In addition, a sample was taken from the obtained hot-rolled coil and cooled on the runout table. A Formaster test was performed to simulate the temperature history of the coil after it was coiled on a coiler, and the time progression of the transformation rate of the coil after cooling began was calculated.

Here, the transformation rate in the Formastor test was calculated as follows.
The sample steel plate was heated from room temperature to a temperature equivalent to the exit temperature of the finishing rolling mill at a heating rate of 10°C per second, and held for 30 seconds. Thereafter, a temperature history equivalent to that experienced by an actual hot-rolled steel plate during cooling was applied, and the transformation rate and transformation speed from austenite (γ: fcc) to ferrite (α: bcc) (which indicate the percentage of transformation that progresses per second, expressed in "% per second") were calculated from the expansion behavior of the steel plate during this period.

The temperature history used in the Formaster test was equivalent to that of an actual steel sheet, with cooling starting from a temperature equivalent to the finish rolling exit temperature. The hot-rolled steel sheet after finish rolling was cooled on a runout table and wound up on a coiler, after which the temperature history was determined for the center of the coil thickness direction (50% of the thickness, with the outermost circumference being 100%) and the outer circumference of the coil thickness direction (95% of the thickness). Figure 3 shows an example of the temperature history when the finish rolling exit temperature is 900°C and the winding temperature is 600°C.

1 and 2 show the time course of the transformation rate obtained by the Formastor test.
FIG. 1 shows the time progression of the transformation rate in the case where the wire was wound at 600° C., and FIG. 2 shows the time progression of the transformation rate in the case where the wire was wound at 690° C., up to 300 seconds after the start of cooling.

The steel used is a steel that has traditionally been problematic due to its tendency to deform. Deformation was observed in the coil wound at a winding temperature of 600°C, but not in the coil wound at 690°C.
Therefore, the time transition of the transformation rate shown in Figures 1 and 2 was examined.

In the case of Figure 1, in which the coil was wound at 600°C, the difference in transformation rate between the outer periphery and the center of the coil was small, and the transformation rate immediately after the coil was pulled out (time elapsed after the start of cooling: 76 seconds) was high. Therefore, it is considered that the entire coil was in a state that was susceptible to deformation due to transformation plasticity immediately after pulling out, and the rigidity of the entire coil was reduced, leading to deformation.
In contrast, in the case of Figure 2, in which the coil was wound at 690°C, the transformation of the coil was delayed and the difference in the transformation rate between the outer periphery and the center of the coil at the time the coil was pulled out from the mandrel became large, so that the entire coil did not simultaneously become susceptible to deformation due to transformation plasticity. Furthermore, even if the transformation rate of the outer periphery of the coil became faster immediately after pulling out, causing a decrease in rigidity, it was thought that the shape of the coil was maintained due to the rigidity of the center of the coil.

From the above results, it was confirmed that in order to delay the transformation in the outer periphery of the coil, thereby slowing the transformation rate and maintaining the rigidity of the coil during cooling and suppressing deformation, it is effective to increase the coiling temperature from the current level. However, on the other hand, if the coiling temperature is increased, the inside of the coil, which has a slow cooling rate, will be exposed to high temperatures for a long period of time, which will cause internal oxidation and deterioration of the surface properties, which may result in problems such as a decrease in pickling ability, deterioration of the surface appearance, and platability.
Therefore, it is necessary to simultaneously suppress deformation of the coil and avoid deterioration of the surface properties.

As a result of further investigation into means for preventing deformation of the coil and avoiding the problem of internal oxidation, the inventors came up with the idea of increasing the winding temperature only on the outer periphery of the coil, noting that internal oxidation becomes an issue inside the coil, where the cooling rate is slow, and that internal oxidation is not particularly a problem on the outer periphery of the coil, where the cooling rate is faster than the inside, even at a relatively high winding temperature.
FIG. 3 shows the time progression of the transformation rate, which was created based on the time progression of the transformation rate in FIGS. 1 and 2, when the internal coiling temperature was 600° C. and the outer peripheral coiling temperature was 690° C. After cooling began, the internal transformation rate had sufficiently decreased at the time of coil withdrawal, and it was predicted that sufficient rigidity of the coil could be ensured even when only the outer peripheral portion was coiled at a high coiling temperature.

As a result of the above, it was found that it is possible to suppress deformation of the coil while avoiding deterioration of the surface properties by winding the inside of the coil at a winding temperature where internal oxidation is not an issue, and winding only the outer periphery of the coil at a higher winding temperature, thereby reducing the transformation rate of that part and ensuring sufficient rigidity to suppress deformation of the entire coil.

The reason why deformation can be suppressed by increasing the winding temperature only at the outer periphery of the coil is believed to be as follows.
In the hot-rolled coil 1 in which the hot-rolled steel sheet is wound around the mandrel 2, the coil is prevented from being deformed by the physical support provided by the mandrel 2 (see FIG. 5(a)).

After the start of coiling, cooling progresses inside the coil and at the outer periphery of the coil as shown in FIG. 6(a). In conjunction with this, for example, in the chemical composition of a high-tensile steel sheet, the γ → α transformation progresses as shown in FIGS. 6(b) and 6(c).
That is, since the outer peripheral portion is coiled at a higher temperature than the inner peripheral portion, the γ→α transformation proceeds first in the inner portion 3 of the hot rolled coil 1, reducing the rigidity and making the coil more susceptible to transformation plasticity. On the other hand, in the outer peripheral portion 4 of the hot rolled coil 1, the progression of the γ→α transformation is delayed due to high temperature coiling, so the rigidity is maintained.
Therefore, even if the rigidity of the hot-rolled coil after drawing is reduced in the internal zone where the transformation rate is high, it is held in place by the rigidity of the zone where the transformation is delayed on the outer periphery of the hot-rolled coil 1 and does not deform (see Figure 5 (b)).

As time passes, the γ → α transformation progresses further. However, since the cooling history of the outer periphery and the center of the coil is different as shown in Figure 6 (a), by the time the transformation rate increases and the rigidity decreases in the outer periphery 4 of the hot-rolled coil 1, the transformation rate in the inner region 3 has already decreased sufficiently and the rigidity has increased, preventing deformation of the coil (see Figure 5 (c)).
As a result, it is possible to prevent deformation of the hot-rolled coil after the hot-rolled coil is cooled (see FIG. 5(d)).

Based on the above basic concept, specific manufacturing conditions were examined, and as a result, the inventors arrived at the present invention, which is characterized in that, when coiling the steel sheet after hot rolling, as specified in (1) above, the range corresponding to the inside of the coil is coiled at a temperature of 450°C or more and 650°C or less, and the range corresponding to the outer periphery is coiled at a temperature that is more than 50°C higher than the internal temperature and is 800°C or less.
Such requirements and preferred requirements of the present invention will be further explained below.

(Chemical Composition)
This method is effective for manufacturing hot-rolled coils using steel types that have a component composition that causes problems with coil deformation and that cannot be coiled at high temperatures due to internal oxidation and other reasons.
Examples of such steel types include high-tensile steels with compositions as shown in Table 1 of the Examples. In particular, C, Si, and Mn are listed as components that have an effect, and steels containing 0.05% or more of C, 0.2% or more of Si, and 1.3% or more of Mn are targeted. The higher the content of these elements, the more likely it is that the gamma → alpha transformation will be delayed on the runout table, and coil deformation will become a problem. Mo and B also have the effect of delaying the gamma → alpha transformation. Furthermore, when the Si content is 0.2% or more and the Si/Mn value, which is the component ratio with Mn, is 0.13 or more, internal oxidation will occur when the steel is wound at a high temperature, so the entire length of the coil cannot be wound at a high temperature.

(Hot rolling conditions)
The hot rolling conditions may be in accordance with conventional methods and are not particularly limited. For example, the conditions include heating a steel slab to 1150°C or more and less than 1280°C, and using a tandem rolling mill, hot rolling is performed so that the finish rolling exit temperature is in the γ range, that is, for example, 850°C or more and less than 980°C, and then cooling on a runout table and coiling at a temperature of 450°C or more.

(Cooling and coiling conditions after hot rolling)
In the hot-rolled steel sheet winding process in which the hot-rolled steel sheet after hot rolling is cooled on a runout table and wound up by a coiler to form a hot-rolled coil, the cooling is weakened or stopped midway and the winding temperature of the outer circumferential part is made higher than the previous temperature.
In other words, if the range from the outermost periphery of the hot-rolled coil that is 5% to 30% of the total coil thickness is defined as the outer periphery, and the remaining range inside is defined as the inside, the range corresponding to the inside is wound at a temperature of 450°C to 650°C, and the range corresponding to the outer periphery is wound at a temperature that is more than 50°C higher than the temperature of the inside and not more than 800°C.

The reason why the outer periphery is set to 5% or more of the total coil thickness is to ensure a thickness necessary to ensure the rigidity to resist deformation of the entire coil immediately after the coil is pulled out. In order to ensure even greater rigidity, 10% or more is preferable.
The wider the outer peripheral region where the temperature is high, the better, but if it is too wide, the problem of internal oxidation will occur. To avoid internal oxidation, it must be 30% or less of the total coil thickness. To further avoid internal oxidation, it is preferable to keep it 25% or less.

The reason why the internal coiling temperature is set to 450°C or higher and 650°C or lower is that below 450°C, the temperature passes through the transition boiling region of water, resulting in a mixture of film boiling and nucleate boiling, which causes extremely large temperature variations in the steel plate being cooled and has a detrimental effect on the mechanical properties, and above 650°C, internal oxidation becomes a problem. The internal coiling temperature is preferably 500°C or higher, which can better avoid the transition boiling region.

The reason why the coiling temperature of the outer periphery is set to be more than 50°C higher than the inner coiling temperature is because it is necessary to shift the period when the transformation rate of the outer periphery is high from the period when the transformation rate of the inner portion is high. The reason why the upper limit is set to 800°C or less is because the cooling rate of the outer periphery is higher than that of the inner portion, and although the time held at high temperature is short, internal oxidation is very likely to occur at temperatures above 800°C. In addition, the higher the coiling temperature, the more likely it is that a thicker outer oxide layer (black scale) will form between finish rolling and coiling by the coiler, which will lead to a reduction in the effective plate thickness after removal by pickling and a deterioration in yield. For this reason, the upper limit of the coiling temperature of the outer periphery is set to 800°C or less, preferably 750°C or less.

(How to select the winding temperature)
A method for selecting the coiling temperatures of the outer periphery and the inside of the hot-rolled steel sheet is as follows: within the entire length of the hot-rolled steel sheet to be coiled, a range from the front end to the inside is set as an internal coiling temperature optimum for the steel type of the hot-rolled steel sheet, and the remaining range corresponding to the outer periphery is set as an outer periphery coiling temperature that is more than 50° C. higher than the internal coiling temperature.

The outer coiling temperature can be set within the above range by trial and error, but it can also be set in advance by examining the time progression of the transformation rate during coil cooling as shown in Figures 1 and 2 through a Formaster test.
Specifically, for example, a temperature history when winding is performed at various winding temperatures is set, and the temperature history is used to determine the time progression of the transformation rate when winding is performed at various winding temperatures. A combination of winding temperatures is then selected within the winding temperature range for the inner and outer periphery such that the transformation rate does not simultaneously take on large values in the outer periphery and inner part, or a winding temperature is selected for the inner part such that the transformation rate transitions at a low value, for example, not exceeding 55 x ^10-3 % per second.
(Other uses)
In the manufacturing method of the present invention, since the coiling temperature at the end of the coil is different, there are some steel types that cannot be applied to applications in which the product is a hot-rolled steel sheet that is not cold-rolled. However, since a cold-rolled steel sheet is annealed after cold rolling, it can be used without restrictions.

Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions.

A slab having the composition shown in Table 1 was smelted, and a hot-rolled coil was produced under the conditions shown in Table 2. The hot-rolled coil was then pulled out from the mandrel in order to be transported to the next process.
The hot rolled coils thus produced were examined for the rate of defective coils due to deformation (NG rate) and the degree of internal oxidation. These results are also shown in Table 2.

As shown in Table 2, in the invention examples (production Nos. 3, 5, 7, 9, 10, 13, and 16) whose production conditions were within the range of the present invention, the NG rate due to coil deformation was 0%, and no internal oxidation was observed.
On the other hand, in the comparative examples (Production Nos. 1, 2, 4, 6, 8, 11, 12, 14, and 15) in which the production conditions were outside the range of the present invention, the NG rate due to coil deformation exceeded 0%, or internal oxidation was observed.

As described above, the present invention provides a hot-rolled coil that can avoid scrapping and recoiling of the hot-rolled coil. Therefore, the present invention has a high applicability in the steel industry.

1 Hot rolled coil 2 Mandrel 3 Inside 4 Outer periphery

Claims (1)

In the hot-rolled steel sheet winding process, the hot-rolled steel sheet after hot rolling is cooled on a runout table and wound up into a hot-rolled coil by a coiler.
When the range from the outermost periphery of a hot rolled coil to 5% to 30% of the total thickness of the coil is defined as the outer periphery, and the remaining range inside is defined as the inside,
In advance, a Formaster test was conducted in which the same temperature history as that of a hot-rolled steel sheet was applied to investigate the time transition of the transformation rate during coil cooling, and using the results,
A range from the front end to the inside of the entire length of the hot-rolled steel sheet is set as an optimal internal coiling temperature for the hot-rolled steel sheet,
The range corresponding to the outer periphery of the remaining part is set to an outer periphery winding temperature that is higher than the internal winding temperature by more than 50° C.
The area corresponding to the inside is heated to a temperature of 450°C or higher and 650°C or lower.
A method for manufacturing a hot rolled coil, comprising coiling a region corresponding to the outer periphery at a temperature that is more than 50°C higher than the internal temperature and not exceeding 800°C.

Images