JPH06306458A - Production of steel material by hot charging rolling in austenite region - Google Patents

Abstract

PURPOSE:To produce a steel material, having superior characteristics equal to or higher than those of reheated and rolled materials, by hot charging in austenite region. CONSTITUTION:After casting of a molten steel, working, where the strain necessary to cause recrystallization effective in refining of structure is left, is performed at a slab temp. in the region between the higher temp. between the Ar3 point and the precipitation temp. of precipitation hardening elements and the upper limit temp. where working strain remains. Subsequently, heating is done from a slab temp. in the higher region between the Ar3 point and the precipitation temp. of precipitation hardening elements up to a temp. between the lower limit temp., where recrystallization occurs owing to working strain, and the upper limit temp. where grain coarsening due to grain growth after recrystallization does not occur. After soaking, the slab is extracted, in the stage where the refining of structure due to recrystallization proceeds, from a furnace, by which the steel material can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a steel material by hot charge rolling in an austenite region, and in order to save energy when heating a slab that consumes a large amount of heat energy in the steel material manufacturing process, Even when the heating furnace charging temperature is increased and the heating furnace charging is performed from the austenite (γ) temperature range and hot charge rolling (HCR) is performed, it is as good as or better than the reheating rolled (CCR) material. An object of the present invention is to provide a method for producing a steel material by austenite hot charge rolling (γ-HCR) having characteristics.

[0002]

2. Description of the Related Art In order to make effective use of the thermal energy stored in a slab in recent years, a furnace from a ferrite (α) single-phase temperature range of 200 ° C. to 600 ° C. before the temperature of the cast slab drops to room temperature I put it in and heat it,
Α region hot charge rolling (HCR)
(Α-HCR) is performed. From the viewpoint of thermal efficiency, it is needless to say that γ-HCR in which the slab temperature is higher than the γ temperature range, which is charged into the heating furnace and rolled, is preferable.

However, when a carbon steel / low alloy steel is manufactured by such γ-HCR, γ during slab cooling
Since the α transformation and the α → γ reverse transformation during reheating are not performed, the grain refinement / homogenization due to the transformation is not performed. Therefore, mechanical properties such as strength and toughness, which are important properties required for steel materials used as structural materials, are CCR materials for slabs whose temperature has once decreased to room temperature. Α-HCR
Deterioration is inevitable when compared with wood. Therefore, at present, γ-HCR is scarcely performed for steel materials for which properties such as strength and toughness are important.

However, since the energy saving process at the time of manufacturing is an important issue for structural steel materials,
Various means for improving the properties of the γ-HCR material have been studied. For example, rare earth metals (RE, RE, etc.) such as La and Ce just before the completion of refining, such as “Iron and Steel 1988 p.230”.
M) is added in a small amount, and these fine oxides are used as nucleation sites for the solidification structure to make the solidification structure fine during continuous casting to measure high strength and high toughness. There has been proposed a method of obtaining a fine solidification structure by adding a small amount of these to make these compounds nucleation sites of the solidification structure.

[0005]

The refinement of the solidified structure by the addition of REM or the like is not always sufficient,
At present, it is difficult to obtain mechanical properties equivalent to those of CCR materials and α-HCR materials. In addition, as in Japanese Patent Application No. 58-181408, there has been proposed a means for controlling the temperature range and the rate of reduction in the rolling stage after heating to make the structure finer.
This means is not sufficient for refining the coarse casting structure during solidification, and it is necessary to increase the number of rolling passes, which reduces the production capacity and is uneconomical.

Regarding the dispersion of the compound, even if the amount of the dispersion is increased, there is a limit to the refinement of the solidified structure, and excessive addition acts as inclusions to deteriorate the cleanliness of the steel. However, the characteristics of the steel material are not improved because they cause deterioration of various characteristics.

In the case of steel grades in which a small amount of precipitation hardening microalloys such as Nb and V are added for the purpose of obtaining high strength and high toughness, excessive addition of La, Ce, Ti, B, etc. causes rolling. At times, recrystallization is suppressed, and the effect due to the refinement of the solidification structure is canceled out, and it becomes difficult to manufacture a steel material having both high strength and high toughness by γ-HCR.

Further, in the refinement of the structure by rolling after heating, it is necessary to finely specify the temperature range in which the rolling is performed and the rolling reduction, and the number of rolling passes must be extremely increased, resulting in reduced production efficiency and uneconomical. In addition, it is not sufficient for refining a coarse solidified structure.

[0009]

Means for Solving the Problems The present invention has been studied repeatedly in order to solve the above-mentioned problems, and by redefining the processing conditions before heating of a slab after casting, reheating without lowering thermal efficiency. The present invention succeeds in producing a steel material having excellent properties equal to or better than that of a rolled material by hot charging in the austenite region, and is as follows.

(1) After casting of molten steel, the slab temperature becomes finer in the temperature range below the upper limit temperature at which work strain remains above the Ar ₃ point or the precipitation temperature of the precipitation hardening element, whichever is higher, whichever is higher. Processing is applied to leave the strain necessary for effective recrystallization to occur, and then the slab temperature is regenerated by processing strain from the higher temperature range of Ar ₃ point or precipitation temperature of precipitation hardening element, whichever is higher. Heating above the lower limit temperature where crystals occur and below the upper limit temperature where grains do not coarsen due to grain growth after recrystallization, after soaking, extraction from the furnace at the stage where the refinement of the structure by recrystallization progressed, rolling A method for producing a steel material by hot-charge rolling in an austenite region, which is characterized by being performed.

(2) After the molten steel is cast, the cumulative reduction rate is 15 in the temperature range in which the slab temperature is the Ar ₃ point or the precipitation temperature of the precipitation hardening element, whichever is higher, and 1000 ° C. or less.
% Or more and 50% or less, and then the slab temperature is Ar
_Three points or precipitation hardening element or higher, whichever is higher, whichever is higher, is heated to a temperature of 1000 ° C or higher and 1150 ° C or lower in a heating furnace, and after soaking, refinement of the structure by recrystallization proceeds. A method for manufacturing a steel product by austenite hot charge rolling, which comprises extracting from a furnace and rolling at this stage.

(3) The slab temperature is higher than the Ar ₃ point or higher or the precipitation temperature of the precipitation hardening element, whichever is higher,
The reduction before heating in the temperature range of 1000 ° C. or less is performed by two or more passes, of which the initial pass is a small reduction of 5% or less per pass, and the latter pass is a reduction of 10 per pass. %, And a large reduction is applied. The method for producing a steel material by austenite region hot charge rolling according to the above item (2).

The technical details of the present invention as described above will be explained. In FIG. 1, casting in which the cooling rate during continuous casting is simulated is performed using steel 1 whose chemical composition is shown in Table 1 below. , The slab temperature after casting is 775 ℃ to 120 ℃
In the temperature range of 0 ℃, a small pressure of 5% per initial pass, late 1
After performing a large reduction of 10% per pass and a total reduction of 30%, the slab temperature is set in a heating furnace set to 1050 ° C., soaked for 10 minutes, and then rolled. FIG. 3 shows the relationship between the rolling temperature and mechanical properties of steel products finished at a steel product average temperature of 850 ° C.

[0014]

[Table 1]

That is, Ar ₃ points (767 ° C. for steel 1)
In the case where the reduction was applied below the temperature at which the processing strain remained (1000 ° C for steel 1) and sufficient strain was applied to cause recrystallization of the γ grains, the cast slab as the comparative example was once cooled to room temperature. Rolled material (C
It has the same strength and toughness as CR material). However, the processing temperature is high (1000 ° C for steel 1) where no processing strain remains.
In the above case, the toughness is remarkably deteriorated.

Further, in FIG. 2, after casting the steel 2 shown in Table 1 above, a reduction of 0 to 60% is performed with the slab temperature being 950 ° C., and the slab temperature is immediately set to 1100 ° C. The following shows the relationship between the rolling reduction and the mechanical properties of the steel material which is charged into the above heating furnace, immediately extracted after being soaked without holding and rolled, and finished at the steel material average temperature of 900 ° C. When the processing amount (the cumulative reduction ratio of 15% or more in Steel 2) in which the strain necessary for inducing recrystallization remains is secured, excellent strength and toughness are exhibited, whereas the cumulative reduction ratio is If it is less than 15%, the toughness is deteriorated. The characteristics are almost constant when the cumulative rolling reduction is 50% or more.

Further, in FIG. 3, after casting the steel 3 whose chemical composition is shown in Table 1 above, 30% was obtained in a state where the slab temperature was 920 ° C.
The slab heating temperature is 850 ° C to 13 immediately.
The slab heating temperature and the machine of the steel material which is charged into a furnace whose heating temperature is set so as to change to 00 ° C, is kept for 15 minutes after soaking, is extracted, is rolled, and is finished at a steel material average temperature of 700 ° C. Shows the relationship of the physical properties. That is, when the heating temperature is not sufficient to induce refinement due to recrystallization (less than 1000 ° C in steel 3), or when the temperature at which recrystallized grains coarsen (1150 ° C in steel 3) or more The toughness is deteriorated.

In FIG. 4, after casting the steel 4 whose chemical composition is shown in Table 1 above, the rolling reduction of the first pass was changed to 1 to 20% with the slab temperature kept at 950 ° C. The reduction rate is kept constant at 22% and the total reduction rate before heating the slab is kept constant at 40%. Immediately after that, the material is put into a heating furnace set at 1070 ° C., soaked, extracted and rolled to obtain a steel material. The relationship between the first-pass reduction ratio and the mechanical properties of steel materials finished at an average temperature of 850 ° C. was shown. That is, although excellent characteristics are exhibited under all conditions, the characteristics are remarkably improved when the initial reduction is 5% or less.

Further, in FIG. 5, after casting the steel 5 shown in Table 1 above, the slab temperature was 930 ° C. and the first pass 1
The rolling reduction per pass is constant at 3%, and the rolling reduction of the final pass is 1-
It is changed to 30% and the total reduction rate before slab heating is kept constant at 32%. Immediately after the soaking, the slab temperature is set to 1080 ° C. The relationship between the rolling reduction and the mechanical properties of the final pass of the steel material that has been extracted and rolled in the above manner and finished at the steel material average temperature of 800 ° C is shown. That is, although excellent properties are exhibited under all conditions, the improvement of properties is remarkable when the rolling reduction in the latter period is 10% or more.

Further, FIG. 6 shows the steel 6 in Table 1 above.
After casting 7, when the slab temperature is 970 ° C, the total rolling reduction is 3
A 0% reduction is performed, the slab is immediately charged into a furnace whose temperature is set to be heated to 1090 ° C., the holding time after soaking is changed to 0 to 10 hours, and then extraction and rolling are performed,
The relationship between soaking time and mechanical properties of the steel material finished at the steel material average temperature of 800 ° C. is shown. In both cases, the soaking is completed and the refinement by recrystallization is completed at the same time, so that the excellent characteristics are exhibited even when the holding time is 0, and the characteristics are not changed even if the holding time is increased thereafter.

[0021]

The details of the present invention as described above will be described. The present invention provides the following in the production of carbon steel / low alloy steel:
γ-HCR is a method for producing a steel material having high strength and high toughness equivalent to or higher than that of the reheated rolled material, and the slab temperature after casting is that the slab temperature is ₃ points or higher of Ar or the precipitation temperature of the precipitation hardening element. Either higher temperature or higher, in the temperature range below the upper limit temperature at which processing strain remains, impart processing that residual strain necessary for causing recrystallization effective for refinement of the structure, and then In the state where the slab temperature is in the higher temperature range of Ar ₃ point or the precipitation hardening element precipitation temperature, whichever is higher, it should be higher than the lower limit temperature that causes recrystallization and lower than the upper limit temperature at which the structure does not coarsen due to grain growth. It is characterized in that it is heated and extracted from the furnace at the stage where the refinement of the structure by recrystallization has progressed and rolling is performed.

In the above-mentioned method for producing a steel material by hot charging in the austenite range, the slab temperature range to which the processing is applied, the processing amount, the heating temperature of the slab after the processing, and the slab soaking condition are important factors. The reasons for these limitations are as follows.

(1) Slab temperature range to which processing is applied When the slab temperature exceeds the upper limit temperature at which processing strain remains, the strain recovers immediately after processing and strain-induced recrystallization does not occur in the subsequent heating. Since the structure after rolling showed a mixed grain structure containing coarse grains and satisfactory properties could not be obtained, it was excluded. On the other hand, when processing is given within the scope of the present invention,
The processing strain induces recrystallization during heating, making the coarse solidified structure equivalent to the heated grain size of the slab refined by normal transformation. As shown, the structure after rolling exhibits a fine grain size control structure, and satisfactory properties are obtained.

As for the Ar ₃ point or more and the precipitate formation temperature or less, the formed precipitate does not form a solid solution immediately upon heating,
Precipitation hardening elements are precipitated in a large amount by strain-induced precipitation due to processing strain, and as a result, these precipitates suppress recrystallization of austenite grains that have undergone strain, so that grain refinement of the structure is not promoted, and as a result It was excluded because it has a mixed grain structure containing coarse grains and the characteristics deteriorate. When the number of Ar points is ₃ or less, the structure becomes two phases of ferrite and austenite, and the hot charge does not occur in the austenite region. Further, as in the second invention, the upper limit temperature at which the processing strain remains is set to 1000 ° C. or less, and it is more effective from the viewpoint of the strain remaining.

(2) Processing before heating In the present invention, since the solidified structure is refined by strain-induced recrystallization, processing before heating is a very important factor. That is, if this amount of processing is less than the lower limit value at which the strain necessary for causing recrystallization that refines the structure during heating remains, the recrystallization is not induced and the strain is gradually recovered during heating. It is excluded because it is not refined by crystals. This strain amount is sufficient if the minimum degree of recrystallization induced upon heating remains, but as shown in the second invention, setting the lower limit value to 15% or more makes it possible to reduce the fineness of the structure during heating. It is more effective from the viewpoint of granulation.

If the rolling reduction exceeds 50%, the obtained properties will not change, and the slab length will be twice or more as long as it was cast, which is not a good measure in terms of the heat balance of the heating furnace that heats this. However, since it becomes uneconomical, this is set as the upper limit in the second and subsequent inventions.

Regarding the rolling before heating, it is preferable to determine the initial rolling reduction and the latter rolling reduction as in the third invention. It is known that the solidified structure generally has an equiaxed crystal portion in the central part of the plate thickness and a coarse columnar crystal portion outside the equiaxed crystal portion, and the columnar crystal portion is particularly coarse, so that recrystallization is difficult. Thus, in order to induce uniform recrystallization in the solidified structures with different grain sizes to obtain a fine recrystallized structure, strain is preferentially applied to the rough columnar crystal part on the surface side under the initial low pressure, and It is very effective to give a strain having a distribution to the solidified structure by giving a strain to the whole under a large pressure. Therefore, in the initial stage, a reduction rate of 5% or less where strain is preferentially applied only to the columnar crystal portion, and in the latter stage, a reduction rate of 10% or more where strain is applied over the entire thickness of the slab.

(3) Heating temperature of slab after reduction In the present invention, the strain applied before heating is effectively used to perform refining by recrystallization during heating, so the heating temperature is also an important factor. When the heating temperature is higher than the upper limit temperature at which coarsening does not occur due to grain growth after recrystallization, the structure coarsens due to grain growth, making fine graining impossible in subsequent rolling, and obtaining a steel material with excellent properties. I will not be able to. On the other hand, when the heating temperature is lower than or equal to the lower limit temperature at which recrystallization due to processing strain can occur, recrystallization does not occur because it is not sufficiently activated, and the applied strain gradually recovers a fine recrystallization structure. I can't get it.

From the above, as for the heating temperature of the slab after reduction, excluding below the lower limit temperature at which recrystallization can occur due to processing strain and above the upper limit temperature at which grains are not coarsened due to grain growth after recrystallization. Further, in particular, setting the heating temperature to 1000 ° C. or higher and 1150 ° C. or lower as shown in the second invention is more effective from the viewpoint of miniaturization by recrystallization and suppression of grain growth.

(4) Slab soaking condition The slab soaking condition is an important factor for causing recrystallization and refining the structure. Even in the temperature range where the heating temperature is equal to or higher than the lower limit temperature at which recrystallization occurs due to processing strain and lower than or equal to the upper limit temperature at which grains are not coarsened due to grain growth after recrystallization, the refinement of the structure due to recrystallization is completely achieved. If heating is completed and rolling process is completed before completion, a mixed grain structure containing coarse unrecrystallized grains will result and a fine structure will not be obtained even by rolling. It is necessary to extract from the furnace at the stage when the process has progressed.

[0031]

【Example】

(Example 1) Explaining specific examples of the present invention, the chemical composition of steel used by the present inventors is as shown in Table 2 below, and the manufacturing conditions for such steel and The mechanical properties of the obtained steel material are as shown in Table 3 below.

[0032]

[Table 2]

[0033]

[Table 3]

That is, in the case where the temperature range where the rolling is applied to the slab is 1-3 or 1-5 which exceeds the upper limit temperature where the working strain remains, the mechanical properties of the manufactured steel material, particularly the toughness, are deteriorated. . On the other hand, the temperature range for applying reduction to the slab is Ar ₃
1-1, 1-2 below the upper limit temperature at which the processing strain remains above the point
And 1-4, C of 1-6 shown as a comparative example.
It was confirmed that it has the same characteristics as the CR material.

(Example 2) With respect to the low alloy steel having the chemical composition shown in the following Table 4, the manufacturing conditions of the steel material adopted are as shown in Table 5 described later.

[0036]

[Table 4]

[0037]

[Table 5]

That is, when the temperature range in which the slab shown in Table 4 is subjected to reduction is not less than the Ar ₃ point and not higher than the precipitate formation upper limit temperature shown in Table 5, the precipitate is further excessively precipitated due to strain due to the reduction.
As a result, the refinement of the structure is suppressed, and the mechanical properties are also deteriorated. Further, when the temperature range to which the reduction is applied exceeds the upper limit temperature at which processing strain remains, the structure also becomes rough during heating and the toughness deteriorates. In the case of applying a reduction to the slab at a temperature higher than the precipitate formation upper limit temperature and lower than the upper limit temperature at which the processing strain remains, in comparison with the comparative example,
It was known to have the same characteristics as the CCR material shown in the comparative example.

(Example 3) γ-in the case of using carbon steel / low alloy steel having the chemical composition shown in Table 6 below
The HCR conditions and the mechanical properties of the steel material are as shown in Table 7 below.

[0040]

[Table 6]

[0041]

[Table 7]

That is, in this case, if the rolling reduction applied to the slab is less than the value required to cause recrystallization effective for the refinement of the structure, sufficient recrystallization is not performed during heating, and the structure is Shows a coarse state, and even if it is soaked and rolled, the structure after rolling does not become fine, and it becomes a mixed grain structure, resulting in deterioration of mechanical properties. Further, when the rolling reduction of the slab exceeds 50%, the mechanical properties are the same as when the rolling reduction is 50% or less, but the slab length becomes extremely long, which is not desirable in terms of the efficiency of the furnace. On the other hand, when the rolling reduction ratio of the slab is 15% or more and 50% or less, sufficient strain is applied to induce miniaturization due to recrystallization during heating, and the slab length does not become extremely long, so mechanical properties and efficiency are improved. It is excellent from both sides.

(Example 4) The production conditions of the steel material and the mechanical properties of the steel material when γ-HCR was performed using carbon steel / low alloy steel having the chemical composition shown in Table 8 below are as follows. Further, it is as shown in Table 9 below.

[0044]

[Table 8]

[0045]

[Table 9]

That is, when the heating temperature after the reduction is applied to the slab is less than 1000 ° C., the thermal activation is not sufficient, so that the structure is not refined by the strain-induced recrystallization and the mechanical activation is not performed. The nature is unfavorable. Also, the heating temperature is 11
When the temperature exceeds 50 ° C., the structure is once refined due to strain-induced recrystallization, but since it is thermally activated at a high temperature, this fine structure immediately causes grain growth, resulting in a coarse structure, resulting in mechanical failure. Properties deteriorate. On the other hand, in the case of the present invention in which the heating temperature is 1000 ° C. or more and 1150 ° C. or less, thermal activation is an appropriate amount, so that refinement by strain-induced recrystallization is performed and this is not coarsened. From this, it was confirmed that the structure after rolling also became a fine homogeneous structure and had excellent mechanical properties.

Example 5 Steels having chemical compositions shown in Table 10 below were prepared.

[0048]

[Table 10]

The mechanical properties of the steels produced in the following Table 11 under the γ-HCR conditions for the steels shown in Table 10 are as shown in Table 11 together.

[0050]

[Table 11]

That is, when any one of the slab reduction temperature, the slab reduction rate, the slab temperature at the time of charging the heating furnace, the soaking temperature of the slab, and the soaking time is outside the scope of the present claim, the obtained steel material is obtained. Has deteriorated mechanical properties.

[0052]

According to the present invention as described above, in the method for producing a steel material by hot charging of carbon steel / low alloy steel in the austenite region, the processing temperature and processing rate of the slab before charging in the heating furnace, and after processing By optimizing the heating temperature and the soaking time, it becomes possible to manufacture a steel material having excellent properties and also excellent in energy saving, and it is an invention having a great industrial effect.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between slab reduction temperature and mechanical properties.

FIG. 2 is a chart showing a relationship between a slab reduction ratio and mechanical properties before heating.

FIG. 3 is a chart showing the relationship between heating temperature and mechanical properties after slab reduction.

FIG. 4 is a chart showing characteristic changes associated with a first pass rolling reduction.

FIG. 5 is a table summarizing characteristic changes with the rolling reduction of the final pass.

FIG. 6 is a table summarizing the relationship of characteristic changes with soaking time.

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[Procedure amendment]

[Submission date] February 23, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0033

[Correction method] Change

[Correction content]

[0033]

[Table 3]

Claims (3)

[Claims] 1. After the molten steel is cast, the structure is refined in a temperature range below the upper limit temperature at which the work strain remains at the slab temperature higher than the Ar ₃ point or the precipitation temperature of the precipitation hardening element, whichever is higher. The strain required for effective recrystallization is applied, and then the slab temperature is changed to Ar ₃
From the higher temperature range of the point or the precipitation temperature of the precipitation hardening element, whichever is higher, to a temperature not lower than the lower limit temperature at which recrystallization occurs due to processing strain and not higher than the upper limit temperature at which grains do not coarsen due to grain growth after recrystallization, A method for producing a steel material by austenite hot-rolling, which comprises extracting from a furnace and rolling at a stage where the structure has been refined by recrystallization after heating. 2. After the molten steel is cast, in the temperature range in which the slab temperature is the Ar ₃ point or the precipitation temperature of the precipitation hardening element, whichever is higher, and 1000 ° C. or less, the cumulative reduction is 15% or more and 50% or less. After that, the slab temperature is heated to a temperature of 1000 ° C. or higher and 1150 ° C. or lower in a heating furnace in a state where the slab temperature is higher than the Ar ₃ point or the precipitation temperature of the precipitation hardening element, whichever is higher. A method for manufacturing a steel product by austenite hot charge rolling, which comprises extracting from a furnace and rolling at a stage where the refinement of the structure by crystals has progressed. 3. A slab temperature of Ar ₃ or higher or a precipitation hardening element precipitation temperature or higher, whichever is higher, 10
The reduction before heating in the temperature range of 00 ° C or less is performed by two or more passes, of which the initial pass is a small reduction of 5% or less per pass, and the latter pass is a reduction of 10 per pass. A large reduction of at least 0.1% is used, and the method for producing a steel product by austenite region hot charge rolling according to claim 2.