JPH08260061A - Production of dead soft steel - Google Patents

Abstract

PURPOSE: To provide a method for producing a titanium-added dead soft steel sheet industrially stable and excellent in surface properties. CONSTITUTION: This is a method for producing a dead soft steel in which, at the time of producing a dead soft steel in which the content of carbon in the steel is regulated to 0.01 wt.% and added with titanium in the range of 0.005 to 0.150 wt.% slab heating before rolling is executed under the conditions in accordance with the following inequality: a.PO2 0.80 [exp (-Q/RT) t]0.5<50; where (a), coefficient, Q: activation energy of the diffusion of oxygen, R: gas constant, PO2 : oxygen partial pressure in the atmosphere, (t): heating time and T: heating temp. Thus, the dead soft steel small in surface flaws caused by oxidized scales can stably be produced, and the stock for an inexpensive workability steel sheet of good quality can be provided at an improved yield.

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 an ultra low carbon steel sheet having excellent surface properties.

[0002]

2. Description of the Related Art Conventionally, as a material for steel sheets having good workability mainly for automobiles, as disclosed in, for example, Japanese Patent Publication No. 44-18066, "Method for producing cold rolled steel sheet having excellent press formability", A Ti-added ultra-low carbon steel has been widely used in which the contents of carbon (C) and nitrogen (N) therein are reduced as much as possible, and titanium (Ti) forming a compound with C and N is added in an equivalent amount or more.

The purpose of the addition of Ti here is to prevent the formation of wrinkle-like defects called stretcher strain during deep-drawing and strong working, by adding C and N, which are in solid solution from the matrix, to Ti. This is because it is combined with and completely removed to make it harmless.

As a method for producing such a Ti-added ultra-low carbon steel, from the viewpoints of reduction of production cost by mass production, stabilization of high quality of cast slabs, etc., a hollow mold composed of a water-cooled copper plate around the periphery is used. By continuously supplying and solidifying the molten steel, a slab is cast by a so-called continuous casting method of continuously producing slab slabs, and subsequently a method of hot rolling after holding this slab at a high temperature in a heating furnace is used. It has been generally adopted.

[0005]

However, when the Ti-added ultra-low carbon steel is cast and rolled by using the above-mentioned conventional technique, the surface caused by the oxide scale of the surface layer generated at the time of heating the slab slab before rolling. Problems such as frequent defects and a decrease in product yield remained.

The present invention solves the above problems and provides a method for producing an extremely low carbon steel which has few surface defects and is industrially stable and can be produced at a low cost.

[0007]

Therefore, the present inventors have found that T
It has been found that the above problems can be solved by applying the following means by implementing experimental research on a method of improving the surface properties of the i-added ultra-low carbon steel plate, and an ultra-low carbon steel plate having excellent surface properties can be obtained.

According to the present invention, the ultra low carbon steel having a carbon content in the steel of 0.01% by weight or less and titanium added in the range of 0.005 to 0.150% by weight is used before rolling. The slab heating condition is performed according to the following mathematical formula 1.

[0009]

[Formula 2] a · PO ₂ 0.80 · [exp (−Q / RT) t] 0.5 <50 (1)

Here, a: coefficient (= 30907), Q: activation energy of oxygen diffusion, (= 40300 kcal / mol · K), R: gas constant (= 1.987 kcal / mo)
l), PO _2: oxygen partial pressure in the atmosphere (atm), t: heating time (min), T: heating temperature (K)

[0011]

The function of the present invention will be described in detail below. In order to solve the above problems in the prior art, the present inventors have
First, the oxide scale formation behavior occurring when the slab slab was heated was investigated in detail.

FIG. 1 (a) is a Ti-added ultra-low carbon steel (C =
0.0015% by weight, Ti = 0.05% by weight), and the aluminum-killed low carbon steel (C = 0.05% by weight) shown in FIG. 6 (B), which has less surface defects due to the oxide scale of the surface layer compared to this. 1 schematically shows the state of a surface oxide scale after heating at 1200 ° C. for 120 minutes.

The oxygen concentration in the atmosphere in the heating furnace is 3
%. In the case of the aluminum-killed low-carbon steel shown in FIG. 2 (b), in addition to the surface scale, a decarburized layer in which the C concentration in the surface layer on the base steel side is lowered is formed.

On the other hand, in the case of Ti-added ultra low carbon steel shown in FIG.
The formation of the decarburized layer on the surface layer on the ground iron side is not remarkable, and instead of this, the grain boundary of the surface layer and a part of the grain are locally oxidized. As a result of the component analysis of this locally oxidized portion, it was found that these were mainly those in which Ti in the steel was oxidized.

FIG. 2 shows the results of measuring the Vickers hardness of the surface layer on the base metal side. In the case of (b) the aluminum killed low carbon steel, the hardness of the surface layer tends to be lower than that of the inside. It is presumed that this is due to the formation of the decarburized surface layer.

On the other hand, in the case of (a) Ti-added ultra-low carbon steel, on the contrary, the hardness of the surface layer tends to be higher than that of the inside. It is presumed that this is due to the local oxidation at the grain boundary of Ti on the surface layer or in some grains.

From the above, surface flaws due to oxide scales generated during slab heating before rolling tend to occur (a) Ti-added ultra-low carbon steel and few surface flaws due to the same oxide scales (b) low aluminum kill It can be seen that carbon steel has a large difference in the formation behavior of oxide scale during slab heating, especially so-called subscale on the surface layer on the base metal side.

Further, from the above, (a) in the Ti-added ultra-low carbon steel, Ti in the grain boundaries near the surface layer or in some grains is
It is presumed that surface defects due to oxide scale are likely to occur during rolling due to the residual oxide and the increase in the hardness of the surface layer due to the formation of these oxides, that is, the deterioration of workability.

Such an oxidation reaction of the element M in steel is generally expressed by the following equation (2).

[0020]

(Equation 3)

Here, m and n are stoichiometric coefficients.

The change in free energy of formation ΔG per mol of oxygen at this time is expressed by the following equation (3).

[0023]

[Equation 4]

Here, ΔG ₀ is the standard free energy of formation change, R is the gas constant, T is the temperature, aX is the activity of the X component,
PO ₂ is oxygen potential. Further, the larger the negative value of ΔG, the stronger the tendency of oxide formation.

From equation (3), the change in free energy of formation of oxide ΔG is determined by the activity of the components in the steel and the oxygen potential,
In other words, it turns out that it depends on the component concentrations in the steel and the oxygen partial pressure in the atmosphere.

Table 1 assumes that the oxygen partial pressure in the atmosphere is 0.04 atm (oxygen concentration 4%), which is almost the same as that in the slab heating furnace, and the (a) Ti-added ultra-low carbon steel and (b) This is the result of trial calculation of change in free energy of formation of oxide in aluminum-killed low carbon steel. From Table 1, it is inferred that in (b) aluminum-killed low carbon steel, the oxidation tendency of C in steel is stronger than that of base iron, and the decarburization reaction of the surface layer is likely to occur.

On the other hand, in the case of (a) Ti-added ultra-low carbon steel, the oxidation tendency of C in the steel is weaker than the oxidation of the base iron, the decarburization reaction of the surface layer is less likely to occur, and the oxidation of the base iron is Also, it is understood that the tendency of Ti to be oxidized is stronger and that Ti is more likely to be preferentially oxidized.

From the same calculation, the limit concentration at which Ti can be preferentially oxidized over the base iron is estimated to be about 0.0002% by weight, and 0.005 to 0.005 which is industrially added to the ultra low carbon steel is estimated.
In the range of 0.150% by weight, the preferential oxidation of Ti may occur.

[0029]

[Table 1]

FIG. 3 shows the result of trial calculation of the relationship between the C concentration in steel and the change in free energy of CO formation. Here, the oxygen partial pressure is 0.04 atm and the CO partial pressure is 0.01 a.
It was calculated assuming tm. From this, the C concentration is about 0.0
When it is higher than 1% by weight, the oxidation tendency of C is stronger than that of the base iron, and the decarburization reaction is more likely to occur, but when the C concentration is about 0.01% by weight or less, the C content is higher than that of the base iron. It is presumed that the deoxidization tendency is weaker and the decarburization reaction is less likely to occur. This is almost in agreement with the formation tendency of the decarburized layer on the surface when various slab cast pieces having different C concentrations are heated.

As described above, it can be explained semi-quantitatively from the results of thermodynamic examination that the oxide scale, especially the subscale formation behavior during slab heating differs depending on the steel type.

Next, the inventors of the present invention conducted the T after heating the slab.
The relationship between the grain boundary oxidation depth d of i and the occurrence state of scale-based surface flaws after rolling was repeatedly examined. The result is shown in Figure 4.
As shown in FIG. 5, when the grain boundary oxidation depth d of Ti is 50 μm or more, the occurrence of scale-based surface flaws after rolling becomes remarkable,
Below this, it was found that the effect on the surface flaw was almost harmless industrially.

The surface flaw generation rate in FIG. 4 is the ratio of the number of detected surface defects due to oxide scale during slab heating to the total number of samples taken when a 1 m long sample was taken from the cold rolled coil ( %).

It is presumed that this is because the surface roughness of the rolled steel sheet is usually about several tens of μm, and grain boundary oxidation with a depth equal to or less than this depth does not manifest as a defect.

Therefore, the present inventors have examined the oxygen partial pressure PO ₂ (at) in the atmosphere of the heating furnace as the influence of the slab heating condition on the grain boundary oxidation depth d of Ti after the slab heating.
m), heating holding temperature T (K), heating holding time t
Focusing on (min), the effects of these conditions were examined in detail. As a result, the grain boundary oxidation of Ti is controlled by the diffusion of oxygen from the surface, and if the oxygen diffusion coefficient Do is used, the following (4)
It was found that it is expressed as a formula.

[0036]

[Equation 5] d = a · √ (Do · t) ………… (4)

Here, a is a coefficient and √ (Do · t) represents the diffusion distance of oxygen. Further, Do is expressed as a function of the temperature T as shown in the following equation (3).

[0038]

[Equation 6] Do = Do · exp (−Q / RT) ………… (5)

Here, Do is a coefficient, Q is an activation energy of diffusion, 40300 kcal / mol · K, and R is a gas constant of 1.987 kcal / mol.

According to the research conducted by the present inventors, d is proportional to the oxygen concentration (partial pressure) PO ₂ in the atmosphere in the heating furnace to the 0.80th power, and as the final form of the equation (4), Equation (6) was experimentally derived.

[0041]

## EQU00007 ## d = a.PO ₂ 0.80. [Exp (-Q / RT) t] 0.5 ... (6)

Where a is an experimentally determined coefficient, which is 309
At 07, the value of d could be estimated most accurately. Therefore, if the heating condition of the slab is adjusted by comparing the d obtained by the equation (6) and the result shown in FIG. 4, the value of d in the equation (6) is 50 μm or less, it is almost harmless industrially. An ultra low carbon steel sheet having a good surface can be obtained.

[0043]

[Example] An ultra-low carbon steel slab manufactured by a continuous casting method,
Oxygen partial pressure in heating atmosphere PO ₂ = 0.04,0.07a
tm, heating holding temperature T = 1323, 1373, 142
3,1473,1523K, heating holding time t = 60,9
Under the conditions of 0, 120, 150, and 180 min, the present invention example was appropriately combined with the other conditions, and heating was performed, and hot rolling was performed.

Table 2 shows the typical chemical composition of the ultra-low carbon steel slab used in this example.

[0045]

[Table 2]

Table 3 shows that the coil after hot rolling was cold rolled,
This is a summary of the results of an investigation of the occurrence of surface defects due to scale. In the case of the example of the present invention in which the value of d calculated according to the formula (6) from the table is 50 μm or less, the result of the coil extraction inspection is that the occurrence of surface defects due to scale is slight and the defect occurrence rate is an industrial target level. It satisfied 1% or less, and passed the whole amount.

[0047]

[Table 3]

[0048]

As described above, according to the present invention, the carbon content is 0.01% or less, and titanium is 0.005 to 0.15.
In producing ultra low carbon steel added in the range of 0%,
By performing the slab heating conditions before rolling according to a certain conditional expression, titanium-added ultra-low carbon steel with less surface defects due to oxide scale during slab heating, can be manufactured industrially stably and at low cost. The yield as a product can be improved, and a high-quality, inexpensive workable steel sheet material can be provided.

[Brief description of drawings]

FIG. 1 is a drawing schematically showing the state of generation of surface oxide scale during slab heating in a (a) Ti-added ultra-low carbon steel and (b) Aluminum-killed low-carbon steel.

FIG. 2 is a drawing showing the results of measuring the Vickers hardness of the surface layer on the base steel side.

FIG. 3 is a drawing showing the results of trial calculation of the relationship between the C concentration in steel and the change in free energy of formation of CO.

FIG. 4 is a drawing showing the relationship between the grain boundary oxidation depth and the state of occurrence of scale-based surface defects (defects) after rolling.

Claims (1)

[Claims] 1. A slab before rolling in producing an ultra-low carbon steel having a carbon content of 0.01% by weight or less and titanium added in an amount of 0.005 to 0.150% by weight. A method for producing an ultra-low carbon steel, characterized in that the heating condition is carried out according to the following formula 1. [Number 1] a · PO ₂ 0.80 · [exp (-Q / RT) t] 0.5 <50 where, a: coefficient (= 30907) Q: activation energy of oxygen diffusion (= 40300kcal / mol · K) R: Gas constant (= 1.987 kcal / mol) PO ₂ : Oxygen partial pressure in atmosphere (atm) t: Heating time (min) T: Heating temperature (K)