JP4593006B2 - Method for producing a billet free from crack defects - Google Patents

Description

【００２６】
【表２】

【００２７】
【表３】

【００２８】
【表４】

【００２９】
表より、本発明の場合（試験Ｎｏ．１，３、５，７，９，１１，１３，１５）の条件を満たす場合には、目視観察およびカラーチェックとも割れは検出されなかった。
一方、いずれの比較例（試験Ｎｏ．２，４，６，８，１０，１２，１４，１６）においても、実際の析出量が、本発明の式から計算される許容析出量を越えており、鋳片の目視やカラーチェック検査で割れが観察された。
【００３０】
【発明の効果】
以上のように本発明により、Ｎと共にＡｌ、ＮｂＶ及びＴｉを含む鋼においても幅圧下時の割れが発生しなくなり、表面疵のない良好な鋼片が得られることが可能となる。
【図面の簡単な説明】
【図１】γ粒径と析出量から脆化発生領域を表した図[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a steel slab by reducing the width of a slab produced by a continuous casting method, and particularly to a method for producing a steel slab having no surface defects.
[0002]
[Prior art]
In order to mass-produce steel, a process combining a continuous casting machine and a width reduction rolling mill is known. This process involves casting with a constant slab width in continuous casting, and then reducing the width of the steel slab by reducing the width with a rolling mill. It can be expected to improve productivity and to improve the yield of the width changing part.
[0003]
However, when Al, Nb, V, N, or the like is contained in the slab, there is a problem that cracks occur during width reduction, leading to surface defects. This embrittlement has been found to be due to precipitates at the γ grain boundaries that occur at temperatures near 700-1000 ° C.
As a means for preventing such embrittlement, it is known that it is effective to keep the slab temperature before deformation strain higher than the embrittlement temperature range, that is, to reduce the amount of precipitates deposited.
[0004]
Further, as described in JP-A-55-14173, cooling-recuperation is performed twice or more in the secondary cooling zone so that the slab surface layer temperature is Ar1 or less and the recuperation temperature is in the range of Ar3 + 100 ° C. By repeating, it is disclosed that the austenite grain size is refined to prevent cracking by suppressing grain boundary precipitation of carbonitride and preventing precipitation of film-like ferrite.
Furthermore, in JP-A-5-161948, after passing through the final correction point in the continuous casting machine, the slab surface layer within 10 mm is quickly lowered to a temperature indicated by a specific formula and then reheated to 850 ° C. or higher. A method of preventing cracking during width reduction by performing hot rolling is shown.
[0005]
However, in order to identify these embrittlement temperature regions, it is necessary to perform a tensile test for each steel component, and it is virtually impossible to identify the embrittlement regions of all steel types. In particular, when the carbon concentration of steel is different, there is a phenomenon that the steel does not become brittle even with the same amount of precipitates.
In addition, the method of controlling the temperature history and making the crystal grains fine as shown in the above two patent publications is a method that can be realized only with the steel type of a specific component, and the carbon concentration is 0 in the actual process. It is difficult at 15% or more.
Therefore, a general rule for specifying the embrittlement temperature caused by precipitates that can be applied to any steel type has been desired.
[0006]
[Problems to be solved by the invention]
The present invention provides a steel containing an element that causes embrittlement by presenting general conditions in which embrittlement does not occur even when the concentration of carbon and precipitate constituent elements changes, that is, cracks do not occur. The present invention also provides a production method capable of preventing surface cracking even when the width is reduced.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is characterized by the following configurations.
(1) When the width of the carbon steel slab obtained by continuous casting is reduced, the temperature at the completion of the reduction is defined so that the amount of precipitates in the slab satisfies the following formula: It is a manufacturing method of the steel slab which does not produce the crack defect by width reduction.
W ≦ 1000 × d ^−2.5
here,
W (ppm): Precipitation amount determined by the concentration and temperature of the constituent elements of the precipitates in the slab d (mm): Average γ grain size in the slab before width reduction
(2) The component of steel is
C: 0.001 to 0.5 mass%,
Mn: 0.1 to 3.0% by mass,
Si: 0.005 to 2.0 mass%,
P: 0.001 to 0.1% by mass,
S: 0.001 to 0.05 mass%,
N: 0.002 to 0.015 mass%
Oxygen: 0.0005 to 0.0050% by mass,
Further, one or more of Al, Nb, Ti, and V are included, and each component range is
Al: 0.001 to 0.1% by mass,
Nb: 0.01 to 0.1% by mass
Ti: 0.005 to 0.1% by mass
V: 0.01 to 0.1% by mass,
It is a steel slab consisting of the remaining iron and unavoidable impurities, and is the method for producing a steel slab in which crack defects due to width reduction do not occur as described in (1) above.
[0009]
(3) The steel component is one or more of Cr, Mo, Ni, B, Zr, Mg, and Ca, and each component is 0.1% by mass or less (2) It is a manufacturing method of the steel piece in which the crack defect by the described width reduction does not arise.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
First, the inventors of the present invention paying attention to the fact that all of the embrittlement of steel containing Al, Nb and V together with N is embrittlement of austenite (γ) grain boundaries. As a result of studying the resulting conditions, the inventors have come up with the idea of determining the presence or absence of embrittlement, that is, cracking, using the relational expression between the γ grain size and the amount of precipitates.
[0011]
Details of the present invention will be described below.
The inventors first examined the conditions under which the steel slab cracked during width reduction of the slab occurred based on data from actual machine tests. For various steel types, the temperature during width reduction was changed and the presence or absence of cracks was investigated. In particular, the analysis was conducted by paying attention to the γ grain size in the slab and the amount of precipitates deposited. The result is shown in FIG.
From FIG. 1, when the γ grain size is large, cracks occur unless the precipitation amount of the precipitate in the slab is made very small. However, if the γ particle size is reduced, the precipitation amount of the precipitate in the slab is reduced. No cracks occur at most.
[0012]
From FIG. 1, it was found that the area where the crack occurs and the area where the crack does not occur can be divided by the following expression.
W ≦ 1000 × d ^−2.5
Here, W (ppm): Precipitation element concentration (% by mass) and temperature determined by temperature d (mm): Average γ grain size before width reduction
Next, the reason for defining the conditions of the present invention and the specific application method of the present invention will be described.
First, as to the definition of the formula, as described above, it is obtained from the result of analyzing the actual manufacturing conditions and the data on the presence or absence of cracks.
As a specific application method, first, it is necessary to obtain the γ grain size of the slab before width reduction. This is obtained in advance from the γ grain structure that appears by corroding the cross section of the slab of the component. The γ crystal grain size is almost determined by the components, especially the carbon concentration and the temperature history during continuous casting, and the temperature conditions of the heating furnace before the width reduction, so if you investigate about some representative steel types and production conditions, It is not necessary to investigate all steel grades.
The region for measuring the γ grain size may be an average value after measuring at least 10 points in the width direction at a depth of about 10 mm from the slab surface layer and converting to a circle equivalent diameter.
[0014]
As a method of actively reducing the γ particle size, the slab is strongly cooled in a continuous casting machine, and after γ to α transformation, the slab is heated again in the continuous casting machine or in the continuous casting machine and then again γ However, in this case, if the γ grain size that has been refined by reverse transformation is substituted into the formula of the present invention, the amount of precipitates that cause no cracking increases. Therefore, the degree of freedom of components and production conditions is increased.
[0015]
Next, there is a method for estimating the amount of precipitation in the slab. In general, formulas for solubility products of precipitates in steel are known, and calculation is performed using these formulas.
Hereinafter, the precipitate AlN produced by Al and N will be described with specific examples. As for Al and N, the following formulas are known as typical ones. (Darken's formula)
log {(% Al) × (% N)} = − 7400 / T + 1.95
here,
(% Al) and (% N): Al concentration (% by mass), N concentration (% by mass) in steel, T: Absolute temperature (K)
[0016]
The precipitate of AlN at a certain temperature is represented by the following formula.
log [{(% Al) −α)} × {(% N) −β}] = − 7400 / T + 1.95
here,
α and β: Al and N amounts as AlN precipitates, and α: β = 27: 14 from the relationship between the Al and N bond mass ratio.
By solving these two equations simultaneously, the amount of precipitation at the temperature can be calculated.
[0017]
For the precipitate NbN involving Nb and N, the following equation was used. (Narita formula)
log {(% Nb) × (% N)} = − 8500 / T + 2.89
Here, (% Nb) and (% N): Nb concentration (% by mass) and N concentration (% by mass) in steel, respectively, T: Absolute temperature (K)
For the precipitate VN involving V and N, the following equation was used. (Narita formula)
log {(% V) × (% N)} = − 8700 / T + 3.63
Here, (% V) and (% N): V concentration (mass%) and N concentration (mass%) in steel, respectively, T: Absolute temperature (K)
[0018]
When Nb and V are included at a predetermined amount or more at the same time, NbN is calculated first, except for N used for NbN, this time VN is calculated, and the sum of the respective precipitation amounts may be used. .
Moreover, it is necessary to consider the influence of Ti as follows. That is, Ti combines with N to produce TiN, but TiN does not significantly affect grain boundary embrittlement. Since TiN precipitates at a relatively high temperature, first, the amount of TiN deposited at that temperature is calculated, and the amount of NbN or VN deposited is calculated by subtracting the N content contained therein.
[0019]
As an application to the actual manufacturing process, the upper limit value of the precipitation amount is determined from the obtained γ particle size so as to satisfy the formula of the present invention, and the components and the temperature at the time of width reduction are set to be equal to or less than that value. By changing, manufacturing conditions that do not cause cracking are required.
The target steel type may be any carbon steel as long as it is carbon steel, particularly those containing Al and N, and / or Nb and N, and / or V and N. Thus, the effect of the present invention is remarkable. Considering the steel component range of the steel material actually used, the following component range is obtained.
[0020]
Since C is an essential element for imparting the strength of steel, the lower limit is set to 0.001% by mass, and the upper limit is set to 0.5% by mass as the maximum amount of carbon used in the plate material.
Further, Mn is also necessary for obtaining strength. In order to obtain the effect, the lower limit is set to 0.1% by mass, and the upper limit is set to 3.0% by mass when used for special purposes.
Although Si may be unnecessary depending on the application, since it is inevitably mixed, the lower limit is set to 0.005 mass%, and the upper limit is set to 2.0 mass% used for special applications.
[0021]
Since P is an element harmful to steel, the upper limit is preferably 0.1% by mass and is preferably as small as possible. However, since it is inevitably mixed, the lower limit is 0.001% by mass.
S is also harmful to the product characteristics in the same way, and it is desirable to make it as low as possible. However, since it is inevitably mixed, the lower limit of 0.001% by mass is practical. The upper limit was set to 0.05% by mass to prevent cracking during continuous casting.
Oxygen causes non-metallic inclusions to be generated, so it is desirable that it be as low as possible. However, the lower limit is unavoidably 0.001% by mass, and the upper limit is 0% because too much inclusion causes product defects. 0.050 mass%.
[0022]
N is an element related to the present invention. Although used to increase the strength and toughness of the material, the upper limit is limited to obtain the effects of the present invention. Further, the lower limit was defined as a value at which embrittlement does not occur. That is, if it is below a lower limit, it is not necessary to use this invention. From this viewpoint, N: 0.002 to 0.015 mass%.
Al is generally used as a deoxidizing element as needed. However, since it combines with N to produce AlN, it may be used for the purpose of increasing the strength of the material. From this viewpoint, the lower limit is inevitably mixed 0.001% by mass, and the upper limit is 0.1% by mass in order to increase the strength of the material.
[0023]
Nb, V, and Ti are used to increase the strength and toughness of the material as necessary, but the upper limit is limited to obtain the effects of the present invention. Further, the lower limit was defined as a value at which embrittlement does not occur. That is, if it is below a lower limit, it is not necessary to use this invention. From this point of view, Nb: 0.01 to 0.1 mass%, V: 0.01 to 0.1 mass% Ti: 0.005 to 0.1 mass%, respectively.
In addition, 0.1 mass% or less of one or more of Cr, Mo, Cu, Ni, Zr, B, Mg, and Ca may be included depending on the use of steel.
[0024]
【Example】
The carbon steel having the components shown in Table 1 was continuously cast under the production conditions shown in Table 2, and the obtained slab was subjected to width reduction, and the obtained steel slab was examined for cracks. As shown in Table 3, the crack was evaluated by performing scarf cutting on the upper surface and the lower surface of the steel piece from 2 mm to 10 mm and visually observing the surface. Furthermore, a sample was cut out from the steel piece, and the state of the crack in the cross section was investigated by color check. The results are shown in Table 4.
[0025]
[Table 1]

[0026]
[Table 2]

[0027]
[Table 3]

[0028]
[Table 4]

[0029]
From the table, when the conditions of the present invention (Test Nos. 1, 3, 5, 7, 9, 11, 13, 15) were satisfied, no cracks were detected in both visual observation and color check.
On the other hand, in any of the comparative examples (Test Nos. 2, 4, 6, 8, 10, 12, 14, 16), the actual precipitation amount exceeds the allowable precipitation amount calculated from the formula of the present invention. Cracks were observed by visual inspection and color check inspection of the slab.
[0030]
【The invention's effect】
As described above, according to the present invention, even when steel containing N, Al, NbV and Ti is not cracked at the time of width reduction, it is possible to obtain a good steel piece free from surface flaws.
[Brief description of the drawings]
FIG. 1 is a diagram showing an embrittlement occurrence region from γ particle size and precipitation amount.

Claims (3)

When the width of the carbon steel slab obtained by continuous casting is reduced, the temperature at the completion of the reduction is defined so that the amount of precipitates in the slab satisfies the following formula: A method for producing a steel slab that does not cause cracking defects due to rolling.
W ≦ 1000 × d ^−2.5
here,
W (ppm): Precipitation amount determined by the concentration and temperature of the constituent elements of the precipitate in the slab d (mm): Average γ grain size in the slab before width reduction. Steel composition is
C: 0.001 to 0.5 mass%,
Mn: 0.1 to 3.0% by mass,
Si: 0.005 to 2.0 mass%,
P: 0.001 to 0.1% by mass,
S: 0.001 to 0.05 mass%,
N: 0.002 to 0.015 mass%,
Oxygen: 0.0005 to 0.0050% by mass,
Further, one or more of Al, Nb, Ti, and V are included, and each component range is
Al: 0.001 to 0.1% by mass,
Nb: 0.01 to 0.1% by mass
Ti: 0.005 to 0.1% by mass ,
V: 0.01 to 0.1% by mass,
The method for producing a billet according to claim 1, wherein the billet is a billet made of balance iron and unavoidable impurities, and does not cause cracking defects due to width reduction.   The width according to claim 2, wherein the steel component contains one or more of Cr, Mo, Ni, B, Zr, Mg, and Ca, and each component is 0.1 mass% or less. A method for producing a steel slab that does not cause cracking defects due to rolling.

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