JPH01195244A - Manufacture of high tension steel for low temperature use having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To obtain a steel having superior toughness at low temp. as heat- treated by heating a steel having a specified compsn. and contg. Ti oxide grains to a specified temp. and cooling the steel under controlled conditions to form ferrite in the grains. CONSTITUTION:A steel consisting of, by weight, 0.02-0.18% C, 0.03-0.25% Si, 0.4-2.0% Mn, 0.0007-0.0060% S, 0.005-0.03% Ti, 0.0010-0.0040% N, <0.003% Al, 0.015% P and the balance Fe with inevitable impurities and contg. Ti oxide is heated to 1,250 deg.C- the m.p. In the subsequent cooling stage, the steel is held for 50-2,000sec or cooled at 0.07-6 deg.C/sec cooling rate in the temp. range of 1,100-800 deg.C and then the steel is cooled at 30-0.1 deg.C/sec cooling rate in the temp. range of 800-300 deg.C to form ferrite in the grains. The steel may further contain limited amts. of Ni, Cu, Nb, V, Cr, Mo and B.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method of producing a steel material with excellent low-temperature toughness at a low cost without a hot rolling process and in an as-heat-treated state.

[Prior Art] When low-alloy steel is heated to a high temperature of 1250° C. or higher, austenite crystal grains become coarse and the toughness is significantly reduced. Conventionally, this toughness has been achieved by hot controlled rolling and grain refinement by recrystallization of austenite grains.

In addition, as a measure to improve the toughness of coarse-grained austenitic steel, we have applied a method of refining the weld heat-affected zone (HAZ) by generating intragranular ferrite and improving toughness by refining the effective grains. is possible.

This is achieved by finely dispersing T1 oxide in the steel through T1 deoxidation, which replaces Ap deoxidation of molten iron, and developing an intragranular ferrite transformation structure (hereinafter referred to as IFP) in the HAZ area during welding, thereby improving HAZ toughness. JP-A-80-245788 and JP-A-80-79 show that
No. 745, JP-A-61117245, JP-A-82-1
This is shown in Publication No. 842.

When these methods were applied to steel materials subjected to high-temperature heat treatment, the results were as expected. However, when we later investigated the relationship between the cooling conditions after high-temperature heating and toughness, we found that
We have discovered a method that can further improve the toughness of steel in which Ti oxides are finely dispersed through Ti deoxidation by changing heat treatment conditions.

[Problems to be Solved by the Invention] In order to improve the toughness of steel subjected to high-temperature heat treatment, it is necessary to further increase the amount of IFP produced. For this purpose, it is necessary to increase IFP nuclear precipitates and at the same time to increase its own IFP generation ability. To achieve this objective, first, IFP nuclear precipitates were examined in detail by electron microscopy.

As shown in Figure 1, the results show that the IFP nucleus has a particle size of 0.
．． It forms a complex with fine MnS attached to the core of Ti oxide with a molecular structure of Ti2O3 containing 6 to 10% Mn as a solid solution. which T
Adhesion of MnS was observed on the i-oxide.

This MnS adhesion occurred during the cooling process from 1400°C, and no MnS adhesion occurred in the sample that was immediately water-quenched from 1400°C. It was thought that this reprecipitated MnS played an important role in the nucleation of IFP, so MnS was forcibly removed by holding the temperature at 900°C for 1000 seconds during cooling from 1400°C as shown in Figure 2. The effect of heat treatment on the amount of IFP produced was investigated.

As a result, when the temperature is not maintained at 900° C., the amount of IFP produced is small, as shown in FIG. 4, and when the temperature is maintained during cooling, as shown in FIG. 5, a large amount of IFP is produced.

If this is expressed in terms of IFP nucleation number, as shown in Figure 3,
When retention is not included, the number is about 3000/-1, and when retention is added, it is about 9000/mJ, and it has been found that heat treatment with retention increases the number three times.

We reached the conclusion that by devising heat treatment at such high temperatures and efficiently precipitating MnS, it would be possible to develop structural steel with improved low-temperature toughness without undergoing heat treatment.
This is what constitutes the present invention.

[Means for Solving the Problems] The present invention has been made based on the above findings, and
The gist is that in weight percent C: 0.02-0.18%, S
i: 0.08-0.25%, Mn: 0.4
~2.0%, S:0.0007~0.0060%, Ti
:o, oos~0.030%, N: 0.001
Contains 0 to 0.0040%, Al: <0.003
%, P < 0.015%, and if necessary, N1 < LO%, Cu < 1.5%, Nb < 0.05%
, V < 0.1%, Cr < t, o%, Mo < 0.5
%, B 0.002% containing 18 or two or more,
The remainder consists of Fe and unavoidable impurities, and the steel containing Ti oxide particles is heated to a temperature of 1250°C or higher and below the melting point, and then heated to 5°C between 1100 and 800°C during the next cooling process.
Hold for 0-2000 seconds or cooling rate 0.07
~b Rate at which intragranular ferrite is generated: 30 to 0.1°C/see
This is a manufacturing method for low-temperature high-strength steel with excellent low-temperature toughness.

The present invention will be explained in detail below.

First, the reason for limiting the basic component range of the steel of the present invention will be described.

First, C is added as an effective component to improve the strength of steel, and if it is less than 0.02%, the strength required for structural steel cannot be obtained, and if it is added in excess of 0.18%, it is , weld cracking resistance, HAZ toughness, etc. are significantly reduced, so the upper limit was set at 0.18%.

Si is necessary to ensure the strength of the base metal and to preliminarily deoxidize molten steel, but if it exceeds 0.25%, a hardened structure of high carbon martensite (hereinafter referred to as M*) will be formed in the heat-treated structure, reducing toughness. Significantly lower. In addition, if it is less than 0.03%, preliminary deoxidation of molten steel necessary for dispersing Ti oxides cannot be performed, so S
The t content was limited to this range.

When the content of N exceeds 0.0040%, the matrix becomes brittle even in the absence of M*, reducing toughness.

Furthermore, when N is 0.0010% or less, hardly any nitrides are produced in the steel, the amount of IFP structure produced is reduced, and the toughness is lowered.

Although it is necessary to add Mn in an amount of 0.4% or more to ensure the strength and toughness of the base metal, the upper limit was set to 2.0% within an allowable range such as the toughness and crackability of the welded part.

Regarding S, in order to precipitate MnS of the complex, 0.
0.0007% or more is necessary, but excessive addition of more than 0.0060% will form coarse sulfide inclusions, leading to a decrease in ductility and an increase in anisotropy of the base material, so 0.0007 to 0. .0
060%.

Ti is an essential element for the formation of T1 oxide and T1 nitride, and if it is less than 0.005%, it will not form the necessary T1 oxide and T1 nitride.
0.0 because the amount of nitrides cannot be obtained and the amount of IFP generated is reduced.
It is necessary to add 0.05% or more, but addition of more than 0.03% causes precipitation of excessive Ti carbides, increases hardness due to precipitation hardening, and reduces toughness.
It was set to 3% or less.

P should be reduced as much as possible in order to prevent deterioration of weld cracking properties, toughness, etc. due to solidification segregation, and the upper limit should be set at 0.01.
It was limited to 5%.

Al is a strong deoxidizing element, and if it is added in an amount of 0.00% or more, Ti oxide formed by Ti deoxidation will not be formed, IFP will not be formed, and the toughness will be reduced. It was limited to 0.003% or less.

The above are the basic components of the steel of the present invention.
And for the purpose of improving the toughness of the base material, N1゜Cu, Nb, V
, Cr, Mo, and B.

First, N1 is an extremely effective element that increases the toughness of the base metal, but adding more than 3.0% increases hardenability, suppressing the formation of IFP structure, and forming M*. The upper limit was set at 8 to reduce toughness.
It was set at 60%.

Although Cu strengthens the base material, it hardens the HAZ less.
Although it is an effective element, the upper limit was set at 1.5% in consideration of temper embrittlement caused by stress relief annealing, weld cracking resistance, etc.

Nb and V are effective in improving toughness by strengthening the base material and suppressing the formation of grain boundary ferrite, but excessive addition exceeding the upper limit of each component can be harmful from the viewpoint of toughness and hardenability. Therefore, the upper limits were set to 0.5% and 0.1% for Nb and V, respectively.

Cr and Mo are effective in strengthening the base material by improving hardenability and precipitation hardening. Furthermore, adding an appropriate process such as TMCP is effective in improving the low-temperature toughness of the base material.

However, since excessive addition exceeding the upper limit of each component is harmful from the viewpoint of toughness and hardenability, the upper limit is set to 1.0% for each of Cr and Mo.

It was set to 0.5%.

B is expected to increase the strength of the base metal by improving hardenability and improve the toughness of high-temperature heat-treated steel materials by suppressing the growth of grain boundary ferrite.
The upper limit was set at 0.002% because the precipitation of (CB)e causes a decrease in toughness and hardening during rapid cooling treatment.

As shown above, even if the components of steel are limited, it is not possible to improve the toughness of steel subjected to high-temperature heat treatment unless the manufacturing method is appropriate. First, it is necessary to disperse an appropriate amount of IFP nuclear precipitates (Ti oxides), which serve as a base for improving the toughness of the high-temperature heat-treated material, into the steel ingot.

The method for producing Ti oxide is to reduce the dissolved oxygen concentration of molten steel to 1
It is obtained by preliminary deoxidation to 0 to 60 ppm, T1 deoxidation, and casting of molten steel at a cooling rate of 20 to 400° C./win during solidification.

Next, the reasons for limiting the heat treatment conditions, which are the basis of the method of the present invention, will be explained.

The reason why the heating temperature was set from 1250° C. to the melting point temperature is to once dissolve a part of MnS precipitated during the solidification process and cause it to re-precipitate into Ti oxide during the next cooling process.

This is because if such treatment is not applied, the IFP structure will not be generated from the T1 oxide. This is because Mn to T1 oxide
Due to the redeposition of S, a Mn-poor region is formed at the boundary between MnS and austenite, which locally reduces γ→
This is thought to be due to raising the α transformation point and promoting the formation of intragranular ferrite.

In addition, between 1100 and 800°C in the next cooling process,
Hold for 50-2000 seconds or cooling rate 0.07
The reason for cooling at ~6''C/see was to prevent the Mrl-poor region formed by MnS reprecipitation from being maintained at a high temperature or diffusing and disappearing during cooling, thereby preventing loss of IFP production ability.

Furthermore, the reason why we decided to cool between 800 and 300 degrees Celsius at a cooling rate of 30 to 0.1'C/see, which generates intragranular ferrite, is that according to the research of the present inventors, IFP is in this 6-speed range. Otherwise, it will not be generated, so I limited it to this range.

[Example] Table 1 shows the chemical composition and Ti oxide density of the prototype steel. The invention steel contains 10 to 60 Ti oxides/mA, while the comparative steel contains almost no T1 oxide. .

The density of the Ti oxide was determined by image analysis processing (CMA device) of characteristic X-rays of the T1,0 element using a computer.

The prototype steel was made into a 20W steel plate by rolling, and after being reheated for 30 minutes at 1300°C under conditions that meet the requirements of the method of the present invention, it was heated at a cooling rate of 1°C/see between 00 and 800°C during cooling.
Furthermore, changes in toughness when cooling at 2°C/see between 800 and 300°C were investigated.

The results are shown in Table 1.

Comparative steel 11.12 has a Si content, and steel 13.14 has a Ti content.
Since the content is outside the limits of the present invention, the toughness is significantly lower than the steel of the present invention, which has substantially the same components other than these limits.

In addition, the invention steel Steel 2 and the comparison steel Steel 15 have a difference in T1 oxide content due to the difference in AN content, and when comparing their toughness, it is clear that the invention steel containing Ti oxide has a fracture surface. The transition temperature (hereinafter referred to as vTrs) is as low as 50°C,
Shows excellent toughness.

Similarly, the invention steel Steel 6 is 45°C lower in vTrs than the comparative steel Steel 16, which has almost the same components other than Ag, and -9
It exhibits extremely excellent toughness of as much as 5°C.

Next, using Steel 2, the effect of improving toughness by holding during cooling will be described.

As shown in Table 2 and Figure 6, at 1100°C, vTrs is the lowest (
, exhibiting the best toughness, and at 900°C, 500
Holding for ˜2000 seconds shows the best toughness, but each holding time above these results in a decrease in toughness.

On the other hand, if the holding time is 50 seconds or less, a sufficient effect of improving toughness cannot be obtained.

Finally, the effect of improving toughness due to cooling rate will be explained with reference to Table 3 and FIG.

In steel containing T1 oxide like Steel 2, 1100
When the cooling rate is in the range of 0.07 to 800° C. to b, the effect of improving toughness can clearly be obtained, but in a range exceeding this, the amount of IFP produced decreases and the desired effect cannot be obtained.

On the other hand, with steel that contains almost no T1 oxide, such as Steel 15, even if it is cooled in the 6th speed range where the effect is expected, no toughness improvement effect can be obtained at all, and the toughness is significantly lower than Steel 2. are doing. The reason for this is T, which has excellent IFP generation ability.
This is due to the fact that almost no IFP is generated because it does not contain monooxide.

That is, when all the requirements of the manufacturing method of the present invention are satisfied, vTrs −
It becomes possible to manufacture steel materials for low-temperature toughness that have excellent low-temperature toughness of -95°C.

Table 3 *Toughness values were determined by cooling at a cooling rate of 2°C/see between 800°C and 300°C and a 2mmV Notch Charpy test.

[Effects of the Invention] When low alloy steel is heated to a high temperature of 1250° C. or higher, austenite crystal grains become coarse and the toughness is significantly reduced. Conventionally, this toughness has been achieved by hot controlled rolling and grain refinement through recrystallization of austenite grains.

The present invention was developed for the purpose of economically producing high-toughness steel without this rolling process and with heat treatment as is.

As a measure to improve the toughness of steel with a coarse-grained austenitic structure, Ti oxide is finely dispersed in the steel, and intragranular ferrite is generated using the oxide as a nucleus to refine the structure and improve toughness by refining the effective grains. It is possible to apply methods to improve this.

However, to improve the toughness without heat treatment,
It is necessary to increase the amount of IFP produced.

To achieve this objective, the IFP of the IFP nuclear precipitate
It is necessary to further increase the production ability, and for this purpose, attaching MnS to Ti oxide during the heating and cooling process dramatically increases the IFP production ability and is effective.

This invention can be applied to improve the toughness of extra-thick steel plates for which rolling, forging, etc. cannot be expected to improve the toughness due to insufficient reduction ratio, and to manufacture steel plates directly from molten steel by omitting rolling, resulting in economical effects on the sheep industry. is extremely remarkable.

[Brief explanation of the drawings] Figure 1 is a schematic diagram of fine MnS reprecipitated on Ti2O3 particles during cooling from 1400°C, Figure 2 is a diagram of heat treatment cycles, Figure 3 is a diagram of IFP and retention time, and Figure 3 is a diagram of IFP and retention time. Figure 4, 5th
The figure is a micrograph of the metallographic structure, FIG. 6 is a graph of retention time during cooling and fracture surface transition temperature, and FIG. 7 is a graph of cooling rate and fracture surface transition temperature. Agent Patent Attorney Tate Chanogi Mistake 1! -7e13? M [950°C) Fig. 4 Fig. 5 vTrx (mc)

Claims (1)

[Claims] 1. C: 0.02-0.18%, Si: 0.03-0.25%, Mn: 0.4-2.0%, S: 0.0007-0.0007% by weight. Contains 0.0060%, Ti: 0.005~0.030%, N: 0.0010~0.0040%, Al: <0.003%, P <0.015%, and the remainder is Steel containing Ti oxide particles consisting of Fe and unavoidable impurities is heated to a temperature of 1250°C or higher and below the melting point, and held at 1100 to 800°C for 50 to 2000 seconds in the next cooling process, or Cooling rate 0.0
Cool at a rate of 7-6°C/sec, and then cool between 800-300°C at a cooling rate of 30-0.1°C to generate intragranular ferrite.
A method for manufacturing high-strength steel for low-temperature use with excellent low-temperature toughness, which is characterized by cooling at a speed of /sec. 2. C: 0.02-0.18%, Si: 0.03-0.25%, Mn: 0.4-2.0%, S: 0.0007-0.0060%, Ti :0.005~0.030%, N:0.0010~0.0040%, Al:<0.003%, P<0.015%, Ni<3.0%, One or more of the following: Cu<1.5%, Nb<0.05%, V<0.1%, Cr<1.0%, Mo<0.5%, B<0.002%. The remainder consists of Fe and unavoidable impurities, and the steel containing Ti oxide particles is
Heating to a temperature of 50°C or higher and lower than the melting point and holding it for 50 to 2000 seconds between 1100 and 800°C in the next cooling process, or cooling at a cooling rate of 0.07 to 6°C/sec and further cooling to 800°C A method for producing a high-strength steel for low temperature use having excellent low-temperature toughness, characterized by cooling between 300°C and 300°C at a cooling rate of 30 to 0.1°C/sec to generate intragranular ferrite.