JP2002346602A - Production method of billet without crack defect - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a crack in the billet at the time of edge rolling. SOLUTION: The production method of the billet which prevent crack defects at the time of edge rolling is characterized in that, when a width of carbon steel billet produced by continuous casting is reduced by edge rolling, the billet temperature when the edge rolling is finished is prescribed so that the deposit amount in the billet satisfies the formula mentioned below. W<=1000×d<-2.5> where: W (ppm): Deposit amount to be determined by a concentration and temperature of composing elements of a deposit in the billet. d (mm): Average radius of grain particle in the billet before edge rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field 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 more particularly to a method for producing a steel slab having no surface defects.

[0002]

2. Description of the Related Art For mass production of steel, a process in which a continuous casting machine and a width reduction mill are combined is known. In this process, in continuous casting, the width of the slab is changed by reducing the width of the slab by reducing the width of the slab in a continuous rolling mill, and the time loss associated with changing the width of the mold in the continuous casting machine It can be expected to improve productivity and increase the yield of the width changing part due to the absence of the above.

[0003] However, Al, Nb, V,
When N or the like is contained, there is a problem that cracks occur at the time of width reduction, leading to surface defects. This embrittlement is 70
It has been found that embrittlement occurs at a temperature close to 0 to 1000 ° C. due to a precipitate at a γ grain boundary. It is known that as a means for preventing such embrittlement, it is effective to keep the slab temperature before deformation strain is higher than the embrittlement temperature range, that is, to reduce the amount of precipitates.

Further, as described in Japanese Patent Application Laid-Open No. 55-14173, cooling-reheating is performed in a secondary cooling zone such that the surface layer temperature of the slab is Ar1 or less and the recuperation temperature is in the range of Ar3 + 100 ° C. It is disclosed that by repeating at least two times, the austenite grain size is refined, thereby preventing grain boundary precipitation of carbonitride and preventing precipitation of film-like ferrite, thereby preventing cracking. Further, JP-A-5-161948
In the publication, after passing through the final straightening point in the continuous casting machine, the surface of the slab surface portion within 10 mm is once reduced to a temperature indicated by a specific formula, and then heated to 850 ° C. or more to perform hot rolling. Thus, a method of preventing cracking during width reduction is disclosed.

However, in order to specify these embrittlement temperature ranges, it is necessary to carry out a tensile test for each steel component, and it is practically impossible to specify the embrittlement zones of all steel types. is there. In particular, when the carbon concentration of steel is different, there is a phenomenon that embrittlement does not occur even with the same amount of precipitate. Further, the method of controlling the temperature history and making the crystal grains fine, which is disclosed in the above two patent publications, is a method that can be realized only with a steel type of a specific component. It is difficult if it is over 15%. Therefore, a general rule for specifying the embrittlement temperature caused by the precipitate, which can be applied to any steel type, has been desired.

[0006]

SUMMARY OF THE INVENTION The present invention provides a general condition under which embrittlement does not occur even when the concentration of carbon or precipitate constituent elements changes, that is, cracks do not occur. An object of the present invention is to provide a production method capable of preventing surface cracks even when a steel containing a causal element is reduced in width.

[0007]

In order to achieve the above object, the present invention has the following features. (1) When the width of a carbon steel slab obtained by continuous casting is reduced, the temperature at the time of completion of the width reduction is defined so that the amount of precipitates in the slab satisfies the following equation. This is a method for producing a steel slab which does not cause cracking defects due to the width reduction. W ≦ 1000 × d ^−2.5 where, W (ppm): Precipitation amount determined by the concentration of constituent elements of precipitates in the slab and temperature d (mm): Average γ particle size in the slab before width reduction

(2) The composition of steel is C: 0.001-0.
5% by mass, Mn: 0.1 to 3.0% by mass, Si: 0.0
05 to 2.0% by mass, P: 0.001 to 0.1% by mass,
S: 0.001 to 0.05% by mass, N: 0.002 to
0.015% by mass oxygen: 0.0005 to 0.0050% by mass, and further contains one or more of Al, Nb, Ti, and V, and each component range is Al: 0.0
01 to 0.1% by mass, Nb: 0.01 to 0.1% by mass T
i: 0.005 to 0.1% by mass V: 0.01 to 0.1% by mass, and is a slab consisting of iron and unavoidable impurities. This is a method for producing a steel slab that does not cause cracking defects due to rolling.

(3) The components of steel are Cr, Mo, Ni, B,
(2) The method for producing a steel slab free from cracking defects caused by width reduction as described in (2) above, wherein one or more of Zr, Mg, and Ca are contained, and each component is 0.1% by mass or less. It is.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors first set A together with N
Focusing on the fact that the embrittlement of steel containing l, Nb, V and the like is all embrittlement of austenite (γ) grain boundaries, as a result of examining the conditions under which these embrittlements occur, We came to the idea of determining whether or not cracks occurred by a relational expression between the γ particle size and the amount of precipitates.

The details of the present invention will be described below. The inventors have
First, the conditions under which the steel slab obtained when the slab was reduced in width were examined based on the data of 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 carried out focusing on the γ grain size and the amount of precipitates in the slab. The result is shown in FIG. From FIG. 1, γ
If the particle size is large, cracks will occur unless the amount of precipitates in the slab is very small, but if the γ particle size is small, cracks will occur even if the amount of precipitates in the slab is large. Does not occur.

From FIG. 1, it is found that the region where cracks occur and the region where cracks do not occur can be divided by the following equation. W ≦ 1000 × d ^−2.5 where, W (ppm): the amount of precipitation determined by the concentration (% by mass) of the constituent elements of the precipitate and the temperature d (mm): average γ particle 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, the equation is defined, but as described above, it is obtained from the result of analyzing the actual manufacturing conditions and data on the occurrence of cracks. As a specific application method, first, it is necessary to determine the γ grain size of the slab before the width reduction. This is determined in advance from the γ crystal grain structure revealed 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 at the time of continuous casting, and the temperature conditions of the heating furnace before the width reduction, so if we investigate some representative steel types and manufacturing conditions, It is not necessary to investigate every steel grade. The area for measuring the γ particle size is approximately 10
It is sufficient to measure 10 or more points in the width direction at a depth of mm, convert to a circle equivalent diameter, and then take an average value.

As a method of positively reducing the γ grain size, the slab is strongly cooled in a continuous casting machine, and once transformed from γ to α, the slab is heated in the continuous casting machine or in the continuous casting machine. A method of reverse transformation to γ again is known, but in this case, if the γ particle diameter obtained by the reverse transformation and becoming fine is substituted into the formula of the present invention, the amount of precipitates that do not cause cracking can be reduced. Since the number of components may be increased, the degree of freedom of components and manufacturing conditions is increased.

Next, a method of estimating the amount of precipitation in a slab is described. Generally, a formula for the solubility product of precipitates in steel is known, and the calculation is performed using these formulas. Hereinafter, regarding the precipitate AlN generated by Al and N,
A specific example will be described. For Al and N, the following equations are known as typical ones. (Darken's formula) log {(% Al) × (% N)} = − 7400 / T +
1.95 where (% Al) and (% N): each in steel
l concentration (% by mass), N concentration (% by mass), T: absolute temperature (K)

The precipitate of AlN at a certain temperature is represented by the following equation. log [{(% Al) -α)} × {(% N) -β}] =
−7400 / T + 1.95 Here, α and β: Al and N amounts as AlN precipitates Also, from the relationship of the binding mass ratio of Al and N, α: β = 27: 14 When solved, the amount of precipitation at that temperature can be calculated.

For the precipitate NbN involving Nb and N, the following equation was used. (Narita's formula) log {(% Nb) × (% N)} = − 8500 / T +
2.89 where (% Nb) and (% N): N in steel
b concentration (% by mass), N concentration (% by mass), T: absolute temperature (K) The following equation was used for the precipitate VN involving V and N. (Narita's equation) log {(% V) * (% N)} =-8700 / T + 3.
63 Here, (% V) and (% N): V concentration (% by mass) and N concentration (% by mass) in steel, respectively, T: Absolute temperature (K)

When Nb and V are simultaneously contained in a predetermined amount or more, the calculation is first performed for NbN, and then, except for the N used for NbN, this time is calculated for VN, and the sum of the respective deposition amounts is calculated. It may be used. In addition, the influence of Ti needs to be considered as follows. That is, Ti also combines with N to form TiN, but TiN does not significantly affect grain boundary embrittlement. Since TiN precipitates at a relatively high temperature, the amount of TiN deposited at that temperature is first calculated, and the N content contained therein is subtracted.
The amount of bN or VN deposited is calculated.

As an application to an actual manufacturing process, the upper limit of the amount of precipitation is determined from the given γ particle size so as to satisfy the formula of the present invention, and the component and width reduction are determined so as to be less than that value. By changing the temperature at the time, manufacturing conditions that do not cause cracks are required. The target steel type may be any type of carbon steel, particularly those containing Al and N, or those containing Nb and N, and those containing V and N. The effect of the present invention is remarkable. Considering the steel composition range of the steel material actually used, the following composition ranges are obtained.

Since C is an indispensable 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 sheet material. Also, Mn is necessary for obtaining strength, and the lower limit is set to 0.1% by mass in order to obtain the effect, and the upper limit is set to the maximum value of 3.0% by mass when used for special applications. Although Si may be unnecessary depending on the use, it is inevitably mixed, so the lower limit is 0.005% by mass, and the upper limit is 2.0% by mass used for special applications.

Since P is an element harmful to steel, its upper limit is preferably set to 0.1% by mass and is as small as possible. However, since it is inevitably mixed, the lower limit of 0.001% by mass is practical. Similarly, S often causes harm to the product characteristics, and is desirably as low as possible. However, since it is inevitably mixed, the lower limit of 0.001% by mass is practical. The upper limit is set to 0.05% by mass to prevent cracking during continuous casting. Oxygen causes the formation of nonmetallic inclusions. Therefore, it is desirable that oxygen is as low as possible. However, the lower limit is 0.001% by mass, which is inevitably mixed, and the upper limit is 0% because too much inclusion causes product defects. 0.050 mass%.

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 in order to obtain the effects of the present invention. The lower limit was defined as a value at which embrittlement did not occur. That is, the present invention does not need to be used as long as it is at or below the lower limit. From this perspective, N:
It becomes 0.002-0.015 mass%. Al is generally used as a deoxidizing element as needed, but is sometimes used for the purpose of increasing the strength of the material because it combines with N to produce AlN. From this viewpoint, the lower limit is set to 0.001% by mass inevitably mixed, and the upper limit is set to 0.1% by mass in order to increase the strength of the material.

Nb, V, and Ti are used to increase the strength and toughness of the material as needed, but the upper limit is limited in order to obtain the effects of the present invention. The lower limit was defined as a value at which embrittlement did not occur. That is, if it is below the lower limit,
It is not necessary to use the present invention. From this perspective,
Nb: 0.01 to 0.1% by mass, V: 0.01 to 0.1
% By mass Ti: 0.005 to 0.1% by mass. In addition, Cr, Mo, Cu, Ni, Zr,
One, two or more of B, Mg, and Ca may be contained in an amount of 0.1% by mass or less.

[0024]

EXAMPLES Continuous casting of carbon steel having the components shown in Table 1 under the production conditions shown in Table 2 was carried out, and the obtained slab was reduced in width. As shown in Table 3, the upper and lower surfaces of the steel slab were subjected to scarf cutting by 2 m.
m to 10 mm, and the surface was visually observed. Further, a sample was cut out from the steel slab and the state of cracks in the cross section was examined by color check. Table 4 shows the results.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

From the table, it can be seen that the case of the present invention (Test No. 1,
When the conditions of 3, 5, 7, 9, 11, 13, 15) were satisfied, no cracks were detected by both visual observation and color check. On the other hand, all of the comparative examples (Test No. 2 and
4, 6, 8, 10, 12, 14, 16), the actual amount of precipitation exceeds the allowable precipitation calculated from the formula of the present invention. Was observed.

[0030]

As described above, according to the present invention, N and A
Even in steel containing 1, NbV, and Ti, cracking during width reduction does not occur, and a good steel slab without surface flaws can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embrittlement occurrence region based on a γ particle size and a precipitation amount.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyuki Taya 1 Nishinosu, Oita-shi, Oita Pref. F-term in Nippon Steel Corporation Oita Works (reference)

Claims (3)

[Claims] When the width of a carbon steel slab obtained by continuous casting is reduced, the temperature at the time of completion of the width reduction is defined so that the amount of precipitates of the casting satisfies the following equation. A method for producing a steel slab which does not cause cracking defects due to width reduction. W ≦ 1000 × d ^−2.5 where, W (ppm): Precipitation amount determined by the concentration of constituent elements of precipitates in the slab and temperature d (mm): Average γ particle size in the slab before width reduction 2. The steel composition comprises: C: 0.001 to 0.5% by mass, Mn: 0.1 to 3.0% by mass, Si: 0.005%.
To 2.0% by mass, P: 0.001 to 0.1% by mass, S:
0.001 to 0.05% by mass, N: 0.002 to 0.0
15% by mass, oxygen: 0.0005 to 0.0050% by mass
And one or more of Al, Nb, Ti, and V, and each component range is Al: 0.001
To 0.1% by mass, Nb: 0.01 to 0.1% by mass Ti:
The steel sheet according to claim 1, wherein V is 0.005 to 0.1% by mass V: 0.01 to 0.1% by mass, and is a steel slab composed of a balance of iron and unavoidable impurities. A method for producing a billet without defects. 3. The steel component is Cr, Mo, Ni, B, Z.
3. The method for producing a steel slab according to claim 2, wherein one or more of r, Mg, and Ca are contained, and each component is 0.1% by mass or less. .