JPH08934B2 - Manufacturing method of high-strength steel for low temperature with excellent low temperature toughness - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a steel material excellent in low temperature toughness at a low cost of heat treatment without a hot rolling process and as it is.

[Prior Art] When a low alloy steel is heated to a high temperature of 1250 ° C or higher, austenite crystal grains are coarsened and toughness is remarkably lowered. In the past, in order to improve this toughness, it has been achieved by performing controlled rolling during hot rolling and refining austenite grains by work recrystallization.

Also, as a measure to improve the toughness of coarse-grained austenitic structure steel, the method of refining the weld heat affected zone (hereinafter referred to as HAZ) structure by the formation of intragranular ferrite and improving the toughness by making the effective crystal grains finer is applied. Can be considered.

This is because Ti deoxidation replaces Al deoxidation of molten iron,
The HAZ toughness can be remarkably improved by finely dispersing an oxide and developing an intragranular ferrite transformation structure (hereinafter referred to as IFP) in the HAZ portion during welding. 61-79745, JP-A-61-117245
And JP-A-62-1842.

As a result of applying these methods to steel materials that are heat treated at high temperature, the expected effects were obtained. However, when the relationship between the cooling conditions after high temperature heating and toughness was investigated in detail thereafter, even in the steel in which Ti oxide was finely dispersed in the steel by Ti deoxidation, the toughness could be further improved by the heat treatment conditions. I found a way.

[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. To do so, it is necessary to increase the IFP nuclear deposits and at the same time enhance its own IFP production capacity. To this end, the IF is the first
The P nucleus precipitate was examined in detail by observation with an electron microscope.

As a result, as shown in Fig. 1, the IFP nucleus has a particle size of 0.1.
Approximately 6-10% of Mn in the form of a solid solution with a molecular structure of Ti ₂ O ₃ at 3.0 μm forms a complex with fine MnS attached to the core, and most of the Ti Adhesion of MnS was observed on the oxide.

This MnS deposition occurred during the cooling process from 1400 ° C, and MnS deposition did not occur at all in the sample immediately water-quenched from 1400 ° C. This reprecipitation MnS is used for nucleation of IFP.
Was considered to play an important role, so as shown in Fig. 2, the temperature was kept at 900 ° C for 1000 seconds during the cooling from 1400 ° C, and a heat treatment for forcibly precipitating MnS was added to generate IFP. The effect on quantity was investigated.

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

When expressed in IFP nucleation number, as shown in Fig. 3,
It was found that the amount was about 3000 pieces / mm ² when the holding was not added, and about 9000 pieces / mm ² when the holding was added, and it was found that the heat treatment with the holding increased the number of generations by three times.

By concluding such heat treatment at high temperature and efficiently precipitating MnS, it was concluded that it is possible to develop a structural steel with improved low temperature toughness as it is,
The present invention has been made.

[Means for Solving the Problems] The present invention has been made based on the above findings, and the gist thereof is C: 0.02 to 0.18% by weight% and Si: 0.03 to
0.25%, Mn: 0.4 to 2.0%, S: 0.0007 to 0.0060%, Ti: 0.00
5 to 0.030%, N: 0.0010 to 0.0040%, Al: <0.003
%, P <0.015%, and Ni <3.0 if necessary.
%, Cu <1.5%, Nb <0.05%, V <0.1%, Cr <1.0%, M
Steel containing 1 or 2 or more of o <0.5% and B <0.002% and the balance of Fe and inevitable impurities, and containing Ti oxide particles is heated to a temperature of 1250 ° C. or higher and a melting point or lower. In the next cooling process, the temperature is maintained between 1100 and 800 ° C for 50 to 2000 seconds or cooled at a cooling rate of 0.07 to 6 ° C / sec.
Furthermore, it is a method for producing a high-strength steel for low temperature use, which is excellent in low temperature toughness, characterized by cooling at 800 to 300 ° C at a cooling rate of 30 to 0.1 ° C / sec to generate intragranular ferrite.

 Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the basic composition range of the steel of the present invention will be described.

First, C is an effective component that improves the strength of steel.
If added less than 0.02%, the strength required for structural steel cannot be obtained, and if added in excess of 0.18%,
Weld crackability and HAZ toughness are significantly reduced, so the upper limit was made 0.18%.

Si is necessary for securing the strength of the base metal and pre-deoxidizing molten steel, but if it exceeds 0.25%, high carbon martensite (hereinafter referred to as M ^* ) with a hardened structure is generated in the heat-treated structure and the toughness is significantly reduced. Let On the other hand, if the content is 0.03% or less, the preliminary deoxidation of molten steel necessary for the dispersion of Ti oxide cannot be performed, so the Si content was limited to this range.

When the content of N exceeds 0.0040%, the matrix becomes brittle and the toughness is reduced even under the condition that M ^* does not exist. Also, N is
If less than 0.0010%, almost no nitride is formed in the steel, and IF
The amount of P structure produced decreases and the toughness decreases.

Mn must be added in an amount of 0.4% or more to secure the strength and toughness of the base metal, but the upper limit was made 2.0% within the allowable range of toughness and cracking of the welded part.

For S, 0.000 to precipitate MnS in the composite
7% or more is required, but excessive addition of more than 0.0060% forms coarse sulfide-based inclusions, which leads to a decrease in ductility and anisotropy of the base material, so the content was made 0.0007 to 0.0060%.

Ti is an essential element for forming Ti oxide and Ti nitride,
If it is less than 0.005%, the required amount of Ti oxide and Ti nitride cannot be obtained, and the amount of IFP produced will be reduced, so 0.005% or more must be added, but if it exceeds 0.03%, excess Ti carbide will precipitate. Therefore, the hardness is increased by precipitation hardening and the toughness is lowered, so the content is made 0.03% or less.

P should be reduced as much as possible in order to prevent the deterioration of weld cracking and toughness due to solidification segregation, and the upper limit is 0.015
Limited to%.

Al is a strong deoxidizing element, and addition of 0.003% or more of Ti
The Ti oxide formed by deoxidation is not formed and IF
Since P is not formed and the toughness is reduced, 0.003
Limited to less than%.

The above are the basic components of the present invention steel, Ni, Cu, Nb, V, Cr, M for the purpose of increasing the base metal strength and improving the toughness of the base metal.
One or two or more of o and B can be contained.

First, Ni is an extremely effective element that enhances the toughness of the base metal, but addition of more than 3.0% suppresses the formation of the IFP structure due to the increase in hardenability, and M ^* is generated. The lower the toughness, the upper limit is 3.0%.
And

Although Cu is an effective element for hardening the HAZ in comparison with strengthening the base metal, it is a temper brittleness due to stress relief annealing,
The upper limit was set to 1.5% in consideration of weld crackability.

Nb and V are effective in improving the toughness of the base metal and improving the toughness by suppressing the formation of grain boundary ferrite, but excessive addition of each component exceeding the upper limit is harmful from the viewpoint of toughness and hardenability. , Nb, V, the upper limit is 0.5%, 0.
It was set to 1%.

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

However, excessive addition of each component exceeding the upper limit is harmful from the viewpoint of toughness and hardenability, so the upper limits were made 1.0% and 0.5% for each of Cr and Mo.

B is expected to increase the strength of the base material by improving the hardenability and to improve the toughness of the high temperature heat-treated steel material by suppressing the growth of grain boundary ferrite. However, if it exceeds 0.002%, Fe ₂₃ (C
B) The toughness is reduced due to the precipitation of ₆ and hardening is caused in the quenching treatment, so the upper limit was made 0.002%.

As shown above, even if the components of steel are limited, the toughness of steel subjected to high temperature heat treatment cannot be improved unless the manufacturing method is appropriate. First, it is necessary to disperse an appropriate amount of IFP nucleus precipitate (Ti oxide), which is a base for improving the toughness of the high temperature heat treated material, in the steel ingot.

The method of forming the Ti oxide is such that the dissolved oxygen concentration of molten steel is 10
It is obtained by pre-deoxidizing to -60 ppm, deoxidizing Ti, and casting molten steel at a cooling rate during solidification of 20 to 400 ° C / min.

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

The heating temperature was set to 1250 ° C to the melting point temperature in order to temporarily dissolve a part of MnS precipitated in the solidification process and re-precipitate it in Ti oxide in the next cooling process.

This is because the IFP structure is not generated from the Ti oxide unless such treatment is added. This is because the reprecipitation of MnS on Ti oxide forms a dilute Mn region at the boundary between MnS and austenite, which locally raises the γ → α transformation point and accelerates the formation of intragranular ferrite. It is thought to be because.

In the next cooling process between 1100 and 800 ℃, 50 to 2
Holding at 000 seconds or cooling at a cooling rate of 0.07 to 6 ° C / sec means that the Mn dilute region formed by reprecipitation of MnS is kept at high temperature or diffuses and disappears during cooling to produce IFP. Limited to not lose.

Further, according to the research conducted by the present inventors, the IFP is cooled at a cooling rate of 30 to 0.1 ° C./sec between 800 and 300 ° C., which is the case where IFP must be in this cooling rate range. It is not generated, so it is limited to this range.

[Examples] Table 1 shows the chemical composition of experimental steel and the density of Ti oxide. The invention steel contains 10 to 60 Ti oxides / mm ^2, while the comparative steel contains almost no Ti oxide. .

The density of the Ti oxide was determined by subjecting the characteristic X-rays of the Ti and O elements to image analysis processing (CMA device) using a computer.

The trial steel was rolled into a 20 mm steel plate, reheated at 1300 ° C. for 30 minutes under the conditions satisfying the requirements of the method of the present invention, and then cooled to 1100
Cooling rate of 1 ℃ / sec between ~ 800 ℃, 800-300 ℃
The change in toughness when the temperature was cooled at 2 ° C / sec was investigated.

 The results are shown in Table 1.

Since the comparative steels 11 and 12 have Si contents which are outside the limits of the present invention, and the steels 13 and 14 have Ti contents which are outside the limits of the present invention, the toughness is remarkably reduced as compared with the steel 1 of the present invention in which the components other than these limits are almost the same. To do.

In addition, the steel 2 of the invention steel and the steel 15 of the comparative steel are different in the Ti oxide content due to the difference in the Al content. By comparing the toughness, it is apparent that the invention steel containing the Ti oxide has a fracture surface. The transition temperature (hereinafter referred to as vTrs) is as low as 50 ° C, indicating excellent toughness.

Similarly, steel 6 of the invention steel has a lower vTrs of 45 ° C as compared with steel 16 of the comparative steel in which the components other than Al are almost the same, and shows extremely excellent toughness of -95 ° C.

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

As shown in Table 2 and FIG. 6, at 1100 ° C
The retention of 200 to 500 seconds shows the lowest vTrs and the best toughness, and the retention of 500 to 2000 seconds at 900 ° C shows the best toughness. Occurs. On the other hand, if it is kept for 50 seconds or less, a sufficient toughness improving effect cannot be obtained.

Finally, regarding the toughness improving effect of the cooling rate,
This will be explained with reference to the table and FIG.

For steels containing Ti oxides such as Steel 2, 1100-80
When the cooling rate between 0 ° C is 0.07 to 7 ° C / sec, the toughness-improving effect can be clearly obtained.
The amount of P produced is reduced and the desired effect cannot be obtained.

On the other hand, in the case of steel containing almost no Ti oxide, such as steel 15, even if cooled in the cool speed range where the effect is expected, the toughness improvement effect is not obtained at all, and the toughness is significantly lower than that of steel 2. are doing. The reason for this is that IFP is hardly generated because it does not contain Ti oxide, which has excellent IFP generation ability.

That is, when all the requirements of the manufacturing method of the present invention are satisfied,
VTrs = -95 with heat treatment as Steel 6 shown in Table 1
It is possible to manufacture steel materials for low temperature toughness with excellent low temperature toughness as high as ℃.

[Effects of the Invention] When low-alloy steel is heated at a high temperature of 1250 ° C or higher, the austenite crystal grains become coarse and the toughness is significantly reduced. In the past, in order to improve this toughness, hot rolling was carried out in a controlled manner, and austenite grains were refined by work recrystallization.

The present invention was developed for the purpose of economically producing a high toughness steel by omitting this rolling and directly performing heat treatment.

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

However, it is necessary to further increase the amount of IFP produced in order to improve the toughness with the heat treatment. In order to achieve this purpose, it is necessary to further enhance the IFP-forming ability of the IFP nuclear precipitates, which involves the addition of MnS to Ti oxide during the heating and cooling process.
It is effective to attach the IFP because it dramatically increases the IFP generation ability.

According to the present invention, rolling, forging, etc. due to lack of reduction ratio can be applied to direct steel plate production from molten steel by improving toughness and rolling omission of extremely thick steel plates that cannot be expected to have toughness improving effects by forging, and its industrial economic effect is It is extremely remarkable.

[Brief description of drawings]

Fig. 1 is a schematic diagram of fine MnS reprecipitated on Ti ₂ O ₃ particles during cooling from 1400 ℃, Fig. 2 is a diagram of heat treatment cycle, and Fig. 3
Figure shows IFP vs. retention time, Figures 4 and 5 show micrographs of metal structure, Figure 6 shows retention time and fracture surface transition temperature during cooling, and Figure 7 shows cooling rate and fracture surface. It is a chart with a transition temperature.

Continued Front Page (72) Inventor Shuji Awahara 5-10-1 Fuchinobe, Sagamihara-shi, Kanagawa Inside Nihon Nippon Steel Co., Ltd. 2nd Technical Research Institute (72) Toshinaga Hasegawa 5-10-1, Fuchinobe, Sagamihara-shi, Kanagawa (56) Reference JP 62-214126 (JP, A) JP 62-177120 (JP, A) JP 61-133312 (JP, A) JP-A-53-40621 (JP, A)

Claims (2)

[Claims] 1. By weight%, 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%, Steel containing Ti oxide particles, containing Al, 0.001% and P <0.015%, the balance being Fe and unavoidable impurities, and heating it to a temperature below 1250 ° C and below the melting point. During 1100 ~ 800 ℃ in the cooling process of 50 ~ 2000 seconds, or cooling at a cooling rate of 0.07 ~ 6 ℃ / sec, further 800 ~ 300 ℃ at a cooling rate of 30 ~ 0.1 ℃ / sec , A method for producing high-strength steel for low-temperature use, which has excellent low-temperature toughness, characterized by producing intragranular ferrite. 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%, in weight%. Content of Al <0.003%, P <0.015%, and Ni <3.0%, Cu <1.5%, Nb <0.05%, V <0.1%, Cr <1.0%, Mo <0.5%, B Steel containing Ti oxide particles containing 1 or 2 or more of <0.002% and the balance Fe and unavoidable impurities at 1250 ° C.
Above, heating to a temperature below the melting point, 1100 of the next cooling process
Hold at 50 ~ 2000 seconds between ~ 800 ℃, or cool at a cooling rate of 0.07 ~ 6 ℃ / sec, and further cool at 800 ~ 300 ℃ at a cooling rate of 30 ~ 0.1 ℃ / sec to remove intragranular ferrite. A method for producing a high-strength steel for low temperature, which has excellent low temperature toughness and is characterized by being produced.