JPH0356616A - Production of structural steel stock excellent in cold workability - Google Patents

Abstract

PURPOSE:To improve the desired cold workability under stable conditions by adding specific amounts of Mn-containing iron powder to a molten steel in a tundish at the time when the temp. of the molten steel is at a specific degree of superheat, casting the above molten steel, and then subjecting the resulting cast billet to hot working at a specific forging ratio. CONSTITUTION:Iron powder is added to a steel having a composition consisting of, by weight, 0.40-0.60% C, 0.15-0.35% Si, 0.60-0.90% Mn, <=0.030% P, <=0.025% S, 0.010-0.100% solAl, <=0.0100% N, and the balance Fe with inevitable impuri ties at the time when the degree DELTAT of super heat of the steel in a tundish is 0-40 deg.C. At this time, an iron powder having a composition consisting of 0.10-0.90% C, 0.05-0.85% Si, 0.40-1.10% Mn, and the balance Fe with inevitable impurities and also having 0.8-1.5mm grain size is used as the above iron powder and is added by 2.0-6.0kg per ton of the molte steel. This molten steel is cast, and the resulting cast billet is heated to undergo hot working at >=3S forging ratio and cooled.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing structural steel materials with excellent cold workability. (Conventional technology) Steel materials manufactured by hot processing such as hot rolling and hot forging, and in many cases are used for mechanical structures after being subjected to processing such as forging and cutting, and heat treatment. Examples of carbon steel include S specified in JfS G 4051.
43C, S45C, S48C, S60C, S5
3C, S55C, S58C, etc. are known.

These steel materials are often obtained by subjecting continuously cast slabs to rolling or forging at a forging ratio of 4S or higher.

However, as is well known, in continuously cast slabs, the size of the equiaxed region that appears varies depending on the casting temperature, which has a large effect on the subsequent cold workability of the continuously cast slab.

Therefore, in order to expand the equiaxed product area in the solidification and weave of continuously cast slabs, that is, to improve the macroscopic score of continuously cast slabs, the following methods have been used: Means to reduce ΔT (1 degree difference in molten steel temperature) ■During continuous casting, electromagnetic stirring is applied to the unsolidified part of molten steel to prevent dense segregation and various defects in the center of the slab. However, methods have been used to make the primary dendrites finer by increasing the stirring intensity.
To expand the equiaxed quality range in the solidification and weave of continuous cast slabs, which are materials for machine structural carbon steel materials with a carbon content of more than approximately 0.45%, such as 343C specified in 51. is extremely difficult, and the optimum conditions during manufacturing (ΔT, etc.)
This range was extremely narrow, making it impossible to obtain structural steel materials with the desired cold workability under stable operating conditions.

Here, Masaaki Moto's eleventh objective is to provide a method for manufacturing structural steel materials with excellent four-way workability under stable operating conditions.

(Means for Solving the Problems) In order to solve the above problem of mud, the present inventors added iron powder to the molten steel in the tundish to control the superheating degree ΔT of the melt tunnel in the tundish. We carried out tests to find out what kind of phenomena occur when the amount of water is lowered. The results are shown in FIG. Figure 1 shows the relationship between ΔT and equiaxed product rate when the C content is 0.4% by weight and 0.5% by weight.
As shown in Fig. 1, which is a graph generally shown for bulk steel, it was found that as ΔT decreased, the equiaxed fraction of continuously cast slabs increased. The present inventors also investigated the relationship between ΔT and the amount of iron powder added. The result is 2nd I2I
Shown below. Figure 2 is a graph showing the optimal range in terms of ΔT and the amount of iron powder that can be used to refine the crystal grains. It has been found that by restricting it to a specific range, it is possible to expect finer grains.

Therefore, as a result of further investigation, the inventors of the present invention found that the composition fluctuations that occur in continuously cast slabs when iron powder is added to the molten steel in the tundish can also be suppressed by limiting the composition of the iron powder to an appropriate range. As a result, the present invention was completed based on the knowledge that there was no practical problem at all.

Here, the gist of the present invention is, in weight %, C: 0.40-0.60%, Si: 0.15
~0.35%, Mn: 0.60~0.90%, P
: 0.030% or less, S: 0.025% or less,
sol. Eight Q: 0.010 ~ 0.100%, N:
When the degree of superheating ΔT in the kundish of a molten steel having a steel composition consisting of 0.0100% or less, the balance consisting of Fe and unavoidable impurities is 0°C & l40°C or less, in the molten steel, C: 0 in weight%. .10-0.90%, Si: 0.05
~0.85%, Mn: 0.40-1.10%, balance Fe and unavoidable impurities, and the particle size is 0.8-1.5%. After adding 2.0 to 6.0 kg per ton, VT forming is performed to obtain a steel billet, after which the above-mentioned billet is heated, further hot worked with a forging ratio of 3S or more, and then cooled. This is a method for producing structural steel materials with excellent cold workability.

In the present invention, hot working refers to rolling, forging, forging,
It refers to ILL or a combination of these, and specifically refers to physical training, embedding training, expansion training, hollow training, hole expansion training, etc. specified in JIS G 0701.

(Function) Hereinafter, the present invention will be explained in detail along with the function and effect. In this specification, "%" means "% by weight" unless otherwise specified.

First, in the present invention, the reason why the composition of the molten steel in the tundish is limited will be explained.

C: C is an essential element for ensuring the strength of steel materials, and in particular, steels with a tensile strength of 58 kgf/m or more, such as S43C specified in JIS G 4051,
In order to have a strength with a yield point of 35 kgf/mm" or more, it is necessary to contain 0.40% or more. On the other hand, if the content exceeds 0.60%, the strength of the resulting steel increases too much. Cold workability deteriorates.Therefore, the C content is limited to 0.40% or more and 0.60% or less.

Si: St is an element that effectively deoxidizes steel, and in order to achieve this effect, it is effective to contain it in an amount of 0.15% or more. However, if the content exceeds 0.35%, oxide-based inclusions will be generated and the ductility, toughness, etc. of the steel material will deteriorate. Therefore, we decided to increase the Si content to 0.15% or more.
．． Limited to 35% or less.

Mn: Mn is an extremely important element for deoxidizing steel and ensuring the strength of w4Ilt, tensile strength of steel material: 58 kgf/mm” or more, yield point: 35 kgf
/11 or more, it is necessary to add 0.60% or more. However, if the content exceeds 0.90%, the strength increases too much and cold workability deteriorates. Therefore, the Mn content is limited to 0.60% or more and 0.90% or less.

P, S: In steel materials, P and S are impurities that deteriorate various properties of the steel material.In particular, S causes red heat brittleness in relation to the Mn content, so its content is It is desirable that the amount be as small as possible.

However, since extreme reduction would require costs, the respective contents should be reduced to P≦0.030% and S≦0.
．． It is limited to 0.025%.

sol. 8Q: Is 8Q for deoxidizing sea bream, for example in a ladle? It is an element added to No. 8 steel, and from the viewpoint of ensuring a deoxidizing effect, it is effective to contain it in an amount of 0.010% or more. However, since excessive addition results in high costs, the upper limit is set to o. Let it be too%. Therefore, the AQ content is 0.010% or more. Limit it to 100% or less. N: N is a gas component dissolved in molten steel because steel is generally hammered in the atmosphere, and it causes deterioration in weldability, so it is desirable to reduce it as much as possible. ．． However, in order to prevent coarsening of crystal grains, o. oto
Since it is necessary to add 0.0% or less, preferably about 0.0030 to 0.0100%, the N content is 0.0100% or less.
% or less.

Next, we will explain the reason why the degree of superheating ΔT of molten steel having the above structure in the tandy soybean is limited to more than 0°C and less than 40"C. If the degree of superheating is less than 0°C, solidification will occur before the addition of iron powder. This makes it difficult to uniformly add iron powder to i8 steel, making it impossible to achieve sufficient grain refinement, making it difficult to impart the desired cold workability to the resulting steel. This is because the degree of superheat is 40″C.
This is because if the amount is too high, the amount of iron powder to be added will increase, resulting in an increase in cost. Therefore, the degree of heat conductivity is
Limit to below 0°C. In addition, from the viewpoint of stability of actual operation, the more desirable range of superheating degree is 10°C or more and 40°C.
The range is below °C. In order to limit the degree of superheating within this range in the actual manufacturing process, techniques such as ■controlling the tapping temperature and ■controlling the temperature in the ladle can be used. The present invention adds a certain amount of iron powder having a certain composition and particle size to the molten steel in the tundish having such a limited composition and degree of superheating. This section explains the limited ranges of iron powder composition, particle size, and amount added, and the reasons for these limits. Ikki Tanane The steel material with excellent cold workability can be obtained by the present invention because:
This is thought to be because the addition of iron powder makes the crystal grains finer because it affects nucleation during solidification. However, it is a problem if the addition of iron powder changes the composition of the molten steel and changes the performance of the molten steel, but according to the findings of the present inventors,
C: 0. 10-090%, Sl: 0.05-0.
When using iron powder having a composition of 85%, Mn: 0.40-1.10%, and the balance consisting of Fe and inevitable impurities,
C: ±0.01 for the molten steel having the above 1J1
%, Si: -0.01% to +0.02%, Mn±0.0
The fluctuation only occurs within a range of 2%, which poses no practical problem. Therefore, the composition of the iron used) is C: 0.10
~0.90%, Si:0.05 ~0.85%, Mn
: 0.40 to 1.10%, the balance being limited to Fe and unavoidable impurities.

One pillar: If the average particle size of the iron powder is less than 0.81, when the iron powder is added to the molten steel in the tundish, if the equipment becomes clogged, scattering from the molten steel will occur after the addition. Moreover, if it exceeds 1.5 mm, unmelted iron powder will be significantly generated and will be unevenly dispersed in the steel. Therefore, the average particle size of iron powder was set to 0.8 mm.
Limit it to 1.5-1 or less. Attachment 1: The optimal range of the amount W (kg) of iron powder added varies depending on the value of the degree of superheating ΔT (°C) of the molten steel in the tundish. That is, when ΔT is large, the time until solidification increases and the amount of melted iron powder increases, so W should be increased. By the way, for the reasons mentioned above, in the present invention, ΔT is limited to more than O'C and less than 40°C. In other words, when ΔT is 40°C, 6 k per ton of molten steel
When g and ΔT are approximately 0°C, 2k per ton of molten steel
By adding g, it is possible to achieve the desired refinement of crystal grains. Therefore, the addition of iron powder f
fiW (kg) 2.0 kg or more per ton of molten steel6
．． Limit it to 0 kg or less. As mentioned above, from the viewpoint of ensuring stability during actual operation, when 10≦ΔT≦40, 10 15
It is further desirable to carry out the addition of iron powder in three ranges.

As shown in FIG. 3, the fluctuation in the composition of molten steel caused by the addition of iron powder is C: ±0. Ol%,
Si: 0.01~+0.02%, Mn=±0.0
2% range, and the amount of iron powder added is 2.0 to 6.0 k
Even if the composition of molten steel varies within the range of
It fully satisfies the range specified in G0321, and there is no problem in practical use.

An example of such ferrous rice is that it is produced by atomizing using gas or water. Further, it goes without saying that there are no particular restrictions on the means for adding iron powder, and it is sufficient to use a method of adding iron powder using Ar gas or N2 gas as a carrier gas.

Further, the results of investigating the variation of f@#t< in the radial direction of the steel slab are shown in a graph in FIG. 4, which shows that there is no practical problem at all.

In this way, molten steel to which iron powder has been added is continuously cast into a mold to produce a continuously cast slab. There are no particular restrictions on the dimensions of such steel pieces, and (200
~600) X (400~1600) x(2000
~9000) A steel piece (billet, slab, etc.) may be used.

The thus obtained steel billet was heated and further heated to 8i1! It is made into steel by hot working with a shape ratio of 3S or higher.

This heating is carried out for rolling in the next step, and from this point of view it is desirable to heat to a temperature of 1160°C or more and 1360°C or less. Furthermore, the reason why the forging forming ratio is limited to 3S or more is to refine the crystal grains and promote recrystallization, thereby regularizing the crystal grains and improving the cold workability of the obtained steel material. This is because if the aha form ratio is less than 3S, this goal cannot be achieved. After this hot working, the steel material is cooled to obtain a structural steel material with excellent cold workability.

As a cooling method, it is preferable to perform gradual cooling or cooling. This is because if cooling other than slow cooling or standing cooling is performed, for example, rapid cooling, recrystallization may become insufficient and desired cold workability may not be obtained.

The crystal grain size of the plume or billet thus obtained is shown in Fig. 5 using a metallographic photograph. It is clear that the crystal grain size of the bloom or billet obtained from the molten iron powder added is smaller, and the crystal grains are refined.

Further, the present invention will be explained in detail using examples, but these are illustrative of the present invention and are not intended to limit the present invention.

Example After adding iron powder having the composition, particle size and addition amount also shown in Table 1 to the molten steel in Tandynoyu having the composition and degree of superheat shown in Table 1, continuous casting is performed to obtain a continuously cast slab, After that, the continuously cast slab was heated to 1260°C, further rolled at a forging ratio of 1m shown in Table 1, and then allowed to cool, thereby forming a structural steel sample Nll.
Sample Nα7 was obtained.

These samples were investigated for their mechanical properties (elongation, reduction and bending). The results are also shown in Table 1. In addition, in the bending test, the sample was bent 180'' at 1511,
Those with no cracks were evaluated as ○, those with cracks were evaluated as ×, and those with slight cracks were evaluated as Δ.

(The following is a blank space) As is clear from Table 1, the samples according to the present invention are found to have excellent cold workability.

(Effects of the Invention) As detailed above, according to the present invention, structural steel materials with excellent cold workability can be stably manufactured.

The significance of the present invention having such effects is extremely significant.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the degree of superheating and equiaxed crystallinity; Figure 2 is a graph showing the relationship between the degree of superheating and the amount of iron powder added; Figure 3 is a graph showing the relationship between the molten steel composition and the check analysis value. Graph showing the relationship between: FIG. 4 is a graph showing the variation of check C in the radial direction of the particle; and FIG. 5 is a photograph showing the metal &llllii of crystal grains. Fig. 1 IO 20 30 40 AT (0 this Fig. 2 O /0 20 30 40 A (゜C) Fig. 3?

Claims (1)

[Claims] In weight%, C: 0.40 to 0.60%, Si: 0.15 to 0.35
%, Mn: 0.60 to 0.90%, P: 0.030% or less, S: 0.025% or less, sol. Al: 0.010
~0.100%, N: 0.0100% or less, the balance is Fe and unavoidable impurities. , in weight%, C: 0.10-0.90%, Si: 0.05-0.85
%, Mn: 0.40 to 1.10%, balance Fe and unavoidable impurities, and iron powder having a particle size of 0.8 to 1.5 mm at a rate of 2.0 to 6.0 mm per ton of molten steel. After adding 0 kg, casting is performed to obtain a steel billet, and then the steel billet is heated,
A method for producing a structural steel material with excellent cold workability, further comprising performing hot working at a forging forming ratio of 3S or more and then cooling.