JPS609824A - Production of tough and hard steel - Google Patents

Abstract

PURPOSE:To produce easily a steel having high strength and excellent toughness at a low cost by subjecting a steel formed by incorporating a specific ratio of C, Si, Mn, Al, Nb and Ti into Fe and regulating the contents of P and S to hot rolling and hardening under specific conditions then subjecting repeatedly the steel to heating and hardening followed by tempering. CONSTITUTION:A steel contg., by wt%, 0.15-0.45 C, 0.05-1.00 Si, 0.3-2.0 Mn, 0.01-0.10 Al and 0.005-0.150 1 or 2 kinds of Nb and Ti and consisting of the balance Fe and unavoidable impurities and regulating the P in the impurities to <=0.025 and S to <=0.015 is prepd. Such steel is subjected to hot rolling at <=1100 deg.C so as to attain <=20% reduction of area (RA) then to hardening directly from the austenite state. The steel is then subjected further repeatedly by >=1 time to a treatment for heating the steel to a temp. region of Ac3 point -[AC3 point+200 deg.C] then hardening followed by a tempering treatment at the temp. of the Ac1 point or below. Cu, Cr, V, Mo, W, Ca, rate earth elements and B may be adequately added to the steel, if necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides high-strength steel, which is suitable as a material for large structures used in low-temperature environments such as extremely cold regions, at low cost and without requiring expensive alloying elements or special equipment. It relates to a method of manufacturing.

In recent years, the progress of the various industries surrounding us has been remarkable, and along with this, various resource development activities, such as the development of underground resources and the development and cultivation of marine resources, have become more and more active. It's coming.

As a support for these activities, we cannot overlook the progress and development of steel structures used in various fields, but we cannot overlook the progress and development of steel structures used in various fields. Due to the necessity of activities, steel structures have begun to show a tendency to become even larger, and at the same time, they have been forced to be used in harsh environments such as extremely cold regions.

By the way, steel is generally known to exhibit a low-temperature embrittlement phenomenon in which toughness rapidly deteriorates at low temperatures.For this reason, steel materials for large structures used in extremely cold regions are There was a strong demand for a material that not only had high strength but also showed excellent toughness even in extremely cold temperatures.

Conventionally, the toughening of steel to meet such demands has been through the use of N
Select steel as the basic component steel and heat treat it to produce a fine tempered martensite structure and fine reverse transformed austenite that precipitates during tempering, or mix fine tempered martensite and fine tempered bainite. This was usually achieved by creating a microstructure and fine reverse transformed austenite.

However, the strong steel obtained in this way has had the extremely disadvantageous problem that it is necessary to add a large amount of N1, resulting in a significant increase in the cost of the steel material.

Therefore, the present inventors first discovered that even if steel does not contain large amounts of expensive elements such as rNi, if the cooling conditions from the austenitic state are limited to specific conditions, high strength and excellent Based on this knowledge, we proposed a manufacturing method for strong steel in Japanese Patent Application Laid-Open No. 57-89424, and have provided strong steel with excellent properties at a low price. However, this Japanese Patent Application Publication No. 5'i'-8942
Regarding the method proposed as No. 4, some people complained that the manufacturing process was somewhat complicated because the cooling conditions had to be changed for each component of the steel.

6- From the above-mentioned viewpoint, the present inventors have developed a structure for large-scale structures used in extremely cold regions, etc., which is made from steel of inexpensive composition without the addition of Ni element, and which does not require any troublesome work. As a result of conducting research to find a method to easily and inexpensively produce strong steel that can be sufficiently applied as . That is, (a) the structure of the steel can be changed to ultra-fine tempered martensite without selecting N] steel as the basic component steel and using the fine reverse-transformed austenite that precipitates during tempering, as in the past. By simply forming a microstructure or a mixed structure of ultra-fine tempered martensite and tempered low-temperature bainite, it is possible to obtain a steel material with appropriate strength and toughness that is sufficiently satisfactory as a steel material for large structures in extremely cold regions. (b) It is generally known that quenching using a rapid heating method such as induction heating is effective for refining the grains of steel. , steel containing a specific amount of Nb and one or more of the T1 components at the same time is subjected to hot rolling under high pressure at a relatively low temperature and direct quenching, followed by heating in an electric furnace] Even if heated at a slow heating rate of less than ℃/see, A, c3 points ~ [A, 03 points + 200℃]
When the quenching process is repeated at least once after heating to a temperature of Or Nb(c
, N) during the next quenching process, the laths of martensite or bainite will not be destroyed at a high temperature just before the transformation to austenite, and a reduction in dislocations will be prevented. is generated from the lath boundaries in addition to the old martensite grain boundaries to form a fine structure, and by quenching, an even finer low-temperature transformed structure (martensite, low-temperature bainite) is produced. And furthermore, N
Since b and T1 prevent coarsening of austenite crystal grains, even if the heating temperature becomes a little high, no particular trouble occurs and stable work can be performed. Therefore, if this is tempered at a temperature below Ac1 point, an extremely fine tempered martensite structure or an extremely fine mixed structure of tempered martensite and tempered low-temperature bainite can be obtained; (C) the above chemical composition; After directly quenching steel with The cross-sectional compression ratio of 20%
(d) where − is an integer of 2 or more, after the (m −1,)th quenching treatment including direct quenching, and before heating for the mth quenching, (e) The steel further contains one or more of Cu, Cr, V, Mo, and W. The addition of Ca and one or more rare earth elements further improves the strength of the steel, while the inclusion of Ca and one or more rare earth elements makes the inclusions in the steel spheroidal and cleans the steel, improving toughness. In addition, by adding a small amount of B, the strength and toughness of the steel are further improved.

This invention was made based on the above findings, and includes: C: 0.15 to 0.45% (hereinafter, the term "weight percentage").

Si: 0.05-1. OO%, Mn: 0.3-2.0%
.

AA: 0.01-0.10%.

One or two of Nb and T1: 0.005 to 0.150
Person in charge.

Contains, and furthermore, 1st category... Cu: 0.05-0.50%.

Cr: 0.05-2. O0%.

V: 0.01-0.15%.

One or two of MO and W.

MO+] 72W, 005-120 section.

Second category... Ca: o, o O1-0.050%.

Rare earth element: O,001-0050%.

Third category... 10- B:O,0O05~0. OO50%.

It also includes one or more of the following: Fe and inevitable impurities: Remaining.

and the content of P and S in the impurities is P: 0.025% or less.

S: 0.015% or less.

After hot rolling the steel so that the cross-sectional compressibility (RA) at 1100°C or lower becomes 20% or more, it is directly quenched from the austenitic state, and then the steel is heated at AC3 points to (AC3
The process of heating to a temperature range of +200°C] and then quenching is repeated one or more times, or the rough tempering is performed at a temperature below point A, c1), and from 3 points of Ac to [3 points of AC +200°C] By repeating the process of heating to a temperature range of 100 degrees and then quenching in this order one or more times, and then tempering again at a temperature below the AC1 point, it is possible to achieve high strength and to withstand low-temperature environments such as extremely cold regions. It is also characterized in that it produces strong steel exhibiting excellent toughness.

Note that the cross-sectional compressibility (RA, ) is expressed by the following formula: RA<m=SO knee-axu-X 100S.

It is expressed as

Tsumashi, this invention [a specific amount of C component, Nb and T]
For steels that simultaneously contain one or more acid components, if the processing temperature and amount of processing (sectional compression ratio) are selected appropriately, hot rolling is performed, and then directly quenched from the austenitic state.
After that, even if heated at a slow heating rate such as electric furnace heating, at least a quenching process is performed after heating to a temperature of AC3 points or higher [A, C3 points + 200°C] or lower (lower than the austenite crystal grain coarsening starting temperature). If repeated one or more times, the steel will become very fine-grained, so if it is tempered at a temperature below AC1, a very fine tempered martensitic structure (
Or, it becomes a mixed structure of tempered martensite and tempered low-temperature bainite), which makes it possible to have both strength and toughness. Of course, it is a matter of course that an even finer grain structure can be obtained by employing rapid heating treatment during quenching after direct quenching.

In addition, in the method of this invention, when the quenching treatment at a slow heating rate is repeated two or more times after direct quenching, the heating temperature of the austenitizing lump during the second and nth quenching is set to (n- 1) It is preferable to set the temperature to be lower than the second austenitizing heating temperature. By doing so, the structure of the steel becomes finer and more uniform, and the toughness is further improved.

Furthermore, when carrying out rough tempering to prevent cracks etc., the rough tempering conditions are A1=T(Al
+ tog t ) is calculated by the formula Ax<19. It is preferable to set the temperature so as to satisfy OX I O. By doing this, the collapse of martensite laths and low-temperature bainite laths due to rough tempering can be suppressed to a minimum, and the It is also possible to obtain a fine-grained structure.

Next, in the method for producing strong steel of the present invention, the reason why the chemical composition of the steel and the rolling and heat treatment conditions are limited as described above will be explained.

A. Chemical composition of steel■ C C component is essential for increasing the hardenability and strength of steel, as well as for grain refinement, but if its content is less than 0.1.51 However, if the heating rate is slow after direct quenching, the desired grain refinement cannot be achieved even if the quenching treatment is repeated once or more, resulting in a decrease in trench strength and deterioration of hardenability. On the other hand, if the C content exceeds 0.45%, the toughness deteriorates, so the C content was set at 0.15 to 0.45%.

■ 5I S1 component is effective as a deoxidizing agent for steel, and
Although it has the effect of increasing strength and hardenability,
If the S1 content is less than 0.05%, the desired effect cannot be obtained, and if the S1 content exceeds 100%, the toughness will deteriorate, so the S1 content should be reduced to 0.0%.
It is set at 5% to 1.00%.

(2) Mn The Mn component has the effect of improving hardenability, improving strength and toughness, and deoxidizing steel, but if its content is less than 0.3%, the desired effects cannot be obtained; If the Mn content exceeds 0, the toughness will deteriorate, so the Mn content was set at 0.3 to 2.0.

■ AA AQ acid component, added to stabilize the deoxidation of steel, homogenize it, and make it finer, but its content is o, 0.
If it is less than 11, the desired effect cannot be obtained, and if it is less than 0.
．． If the AC content exceeds 10%, the deoxidizing effect will be saturated and this will also lead to the occurrence of scratches and deterioration of toughness due to an increase in inclusions, so the AC content is set at 0.01 to 0.10%. Ta.

■ Nb and Tl Nb and T1 components have the same effect of increasing the strength of steel, increasing resistance to temper softening, and refining the structure, but one or both of these elements If the total content of seeds is less than 51 O, OO, the desired grain refinement cannot be achieved after direct quenching, even with one or more repeated quenching treatments, especially at slow heating rates; If the total content of one or two of the elements exceeds 0.150%, not only will the above effects not be further improved, but the toughness will also deteriorate. The content of the two types is 0.0 O5~015
It is set at 0%.

■ P and S It is preferable that P and 8 min be as small as possible in order to improve the toughness of the steel, but considering the manufacturing cost of the steel, the upper limit of P is set to 0.025% and S The upper limit of o, 015%
determined respectively.

@Cu, Cr, V', Mo, and W These components have the effect of improving the strength of steel, so one or more of them may be added as needed, but the individual elements will be explained below. The detailed characteristics and reasons for limiting the content will be explained.

i) Cu The Cu component is a preferable element that increases the strength of steel without significantly impairing its toughness, but if its content is 0.
If it is less than 0.05%, the desired effect cannot be obtained;
If Cu content exceeds 50%, hot workability will deteriorate, so the Cu content should be reduced to 0.05 to 0.50%.
%.

ij) Cr The Cr acid component has the effect of increasing the hardenability and temper softening resistance of steel, but if its content is less than 0.05%, the desired effect cannot be obtained in the above effects. On the other hand, if the Cr content exceeds 2.00%, the toughness will deteriorate, so the Cr content should be increased to
It is decided that he will be in charge.

:f Mo and W Both MO and W components have the same effect of increasing hardenability and strength and increasing resistance to temper softening, but W has an atomic weight approximately twice that of Mo. In terms of the dovetail effect, the Mo content is exactly half that of W, making it exactly the same. And M.

If the value of -4-V2W is less than 0.05%, the desired effect cannot be obtained in the above action, and MO-1-1//2W is 1.20%.
Even if one or more of MO and W is contained in an amount exceeding Mo+1/2 W was set at 0.05 to 1.20.

iv) Component 18- has the effect of increasing the strength of steel as well as its resistance to temper softening, but if its content is less than 0.01%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.15%, it will lead to deterioration of toughness.
■The content was determined to be 001 to ○15%.

■ Ca and rare earth elements These components both spheroidize inclusions in the steel, clean the steel, improve toughness in the direction perpendicular to the rolling direction, and reduce the anisotropy of the steel. If necessary, one or more of them can be added and contained because they have a certain effect, but if the content of any of them is less than 0.001%, the desired effect cannot be obtained. ○ If the content exceeds 0.50%, not only will the toughness improvement effect be saturated, but also non-metallic inclusions such as oxides will increase and the cleanliness of the steel will decrease. -0.050%. Note that it is practically preferable to add the rare earth element in the form of metal.

■ B The B component has the effect of improving the hardenability of steel and improving its strength and toughness, so it is an element that can be added as necessary. However, if its content is less than 0.0005%, the above-mentioned It is not possible to obtain the desired effect in the action, and on the other hand, 0
Since even if the B content exceeds 0.0050%, no further improvement effect will be brought about, the B content should be set at 0.0005 to 0.
It is set at 0050%.

In addition, in the case of B-treated steel, the N content in the steel (@ka(2,5X B(%)-1,5X 10-")
If the following conditions exist, there is a risk that polocarbide will precipitate during tempering below point C1, making it impossible to obtain steel with the desired high toughness.
3XB (%) + 1.2X10””
2.5 X B
(%) -Yu 5X10-"(N (!A<3XB(
It is desirable to set a limit of +1.2 x 10-2.

In this case, if the steel does not contain T1 of 0.005% or more, it is desirable that the quenching heating temperature be 1075° C. or less.

B. Rolling and heat treatment conditions In the method of the present invention, the steel constructed as described above is melted, rolled into thick plates, shaped steel, steel pipes, etc. by a conventional method, and then heat treated. However, the rolling and heat treatment conditions are as follows.

■ Rolling conditions Rolling is carried out in the austenite temperature range of 1100°C or less and with a cross-sectional compressibility of 20% or more.
By doing so, the austenite grains become fine for the first time, and therefore, the low-temperature transformed structure produced by directly quenching the steel of the present invention from the austenitic state immediately after hot rolling with a suitable cooling medium can also be made fine. This also produces the secondary effect of being effective in reducing susceptibility to quench cracking during direct quenching.

If the cross-sectional compressibility in the rolling temperature range of 1100°C or less is less than 20%, the desired fine 21- structure cannot be achieved.

In other words, the cross-sectional compressibility in the temperature range below 1100°C is 20%.
If hot rolling is carried out to obtain the following results, and then quenched directly from the austenite region, even if the heating rate is slow as in electric furnace heating, A, c3 If the treatment of heating and quenching in the temperature range from point to (Ac3 point + 200°C) is performed one or more times, the steel structure can be made extremely fine-grained.

In this way, if the pre-structure during quenching following direct quenching is a fine low-temperature transformed structure, and if it is a fine-grained structure with small lath collapse, the next quenching will produce an even finer-grained structure. You can get it.

Of course, even if a coarse low-temperature transformed structure is produced by direct quenching, a certain degree of fine-grained structure can be obtained by repeating the process of austenitizing and quenching at a temperature between A and C3 points to [Ac3 point + 200℃] one or more times. It is possible, but the effect will be small.

■ Quenching conditions for second quenching after direct quenching: Quenching 1/i, directly quenched material and/or rough tempered material after direct quenching are heated to a temperature of 3 AC to [3 AC + 200°C] to determine the structure. After completely austenitizing, the process of quenching with an appropriate cooling medium to obtain a low-temperature transformed structure is repeated one or more times, but if the heating temperature at that time is less than 3 points of Ac, it is natural that the austenite will not be formed. On the other hand, heating above the Ac3 point +200° C. causes the austenite crystal grains to become coarse, making it impossible to obtain the desired fine structure even by the treatment of the present invention.

In this way, by limiting the heating temperature for the second quenching treatment as described above, even at a slow heating rate such as electric furnace heating, ultra-fine particles can be obtained by repeating the heating quenching process one or more times. It is possible to realize a low-temperature transformed structure,
Toughness can be significantly improved.

As mentioned earlier, it is preferable that the heating temperature for the second and subsequent quenching be lower than the temperature for the previous quenching.This will realize a finer grained structure and improve the performance of the steel material. can be improved.

(2) Tempering conditions If the fine #1 structure obtained by the above-mentioned quenching treatment is finally tempered at a temperature below the Ac1 point, the desired strength and toughness will be imparted to the steel.

In this case, if the tempering temperature exceeds the A and C4 points, the strength of the steel material will change significantly and the toughness will deteriorate;
The score is set at 1 point or less.

As mentioned earlier, the temperature of rough tempering is Al"" (A2 + Iogt) Al calculated by the formula is A, x < 19. OX 10” It is preferable to set it so that it satisfies the following.

Next, the present invention will be specifically explained by comparing it with Examples and Comparative Examples.

Example 1 First, steels 1 and 2 having compositions within the range of the present invention as shown in Table 1 were melted and made into steel billets, which were then heated to 1200
After soaking at a temperature of 1,100° C., the material was hot-rolled using a hot rolling machine so that the cross-sectional compressibility at 1100° C. or lower took various values.

Subsequently, water quenching was immediately carried out at 930°C, and then 0.
It was heated to 920° C. at a slow heating rate of 75° C./sec, held for 20 minutes, and then water quenched again.

Next, this was further tempered at about 540 to 580°C to adjust the tensile strength of each steel material to approximately 90 kgf/mi, and this was subjected to a Charpy impact test.

The relationship between the Charpy fracture transition temperature and the area compressibility (RA) thus obtained is shown in FIG.

As is clear from Fig. 1, if the cross-sectional compressibility (RA) at 25 - below 1100C is about 20% or more, a good fracture surface transition temperature can be secured, and the effect of the method of the present invention can be improved. You can see that it's big.

Example 2 Steels 3 and 4 having compositions within the range of the present invention as shown in Table 1 were melted into steel slabs, and heated at 1220°C.
After soaking, the sheets were hot-rolled using a hot rolling machine so that the cross-sectional compressibility (RA) at 1100° C. or lower took various values.

Subsequently, water quenching was immediately performed at 920°C, and then water quenching was performed at 0.
heating to 9oO<0>C at a slow heating rate of 5[deg.]C/sec;
After holding for 20 minutes, water quenching was performed again.

Next, this was further tempered at about 540 to 580°C to adjust the tensile strength of each steel material to approximately 90 kgf/ma, and this was subjected to a Charpy impact test.

The relationship between the Charpy fracture surface transition temperature and the area compressibility (RA) thus obtained is shown in FIG.

From Figure 2, the cross-sectional compressibility (RA) below 1100℃
It is clear that a good fracture surface transition temperature can be ensured if the ratio is about 20% or more.

Example 3 Steels 1 to 61 having the chemical compositions shown in Table 2 were melted by a conventional method.

After forming these into steel slabs, they were soaked at 1200°C and put through a hot rolling machine, and the cross-sectional compressibility (RA, ) at 1100°C or less was
It was hot rolled so that it had a thickness of about 35 inches. Subsequently, this was left to cool in the atmosphere and directly quenched at the temperature shown in Table 3, and then quenched and tempered under the conditions shown in Table 3.

The strength and toughness of these materials were measured, and the results are also shown in Table 3.

The results shown in Table 3 also show that if the chemical composition and rolling/heat treatment conditions of the steel are within the range of the present invention, a steel material with an excellent balance of strength and toughness can be obtained.
It is clear that if even one of the above conditions deviates from the scope of the present invention, a steel material with satisfactory properties cannot be obtained.

Example 4 Steels 45 and 46, which are the steels subject to the present invention in Table 2 above, were
After soaking at 1200℃, each
Hot rolling was performed so that the cross-sectional compressibility (RA) at 100° C. or lower was approximately 30%. After that, it was left to cool in the atmosphere and directly quenched at the temperature shown in Table 4, and then further quenched and tempered under the conditions shown in Table 4 to measure its strength and toughness.The results are also shown in Table 4. Indicated.

It is clear from the results shown in Table 4 that steel materials with excellent strength and toughness can be produced by the method of the present invention.

Example 5 Steel 46, which is the steel subject to the present invention in Table 2 above, was converted to 1250
After soaking at a temperature of 0.degree. C., the material was hot-rolled using a hot rolling mill so that the area compressibility (RA) at 1100.degree. C. or less was 10% and 35%. Subsequently, this was left to cool in the atmosphere, directly quenched from 950°C or 250°C, and further quenched under the conditions shown in Table 5 to obtain an austenite grain size number (AS
TMA) was measured.

The obtained results are also shown in Table 5.

35- It is clear from the results shown in Table 5 that an extremely fine-grained structure can be obtained by the method of the present invention.

Example 6 Steel 45, which is the steel subject to the present invention in Table 2 above, was heated to 120.0°C and then subjected to a hot rolling mill.
Hot rolling was performed so that the cross-sectional compressibility (RA) at 0.0° C. or lower was approximately 40%. Subsequently, this was directly quenched at a temperature of 850°C, then rough tempered under the conditions shown in Table 6, and then quenched and tempered.
Its strength and toughness were measured.

The obtained results are also shown in Table 6.

From the results shown in Table 6, it is clear that steel materials with excellent strength and toughness can be obtained even if rough tempering treatment is performed to prevent cracking, etc., before the quenching treatment performed after direct quenching. Also, the rough tempering conditions at this time are A4<19. It can also be seen that when OX 10 is used, the steel material 37- has an even better balance of strength and toughness.

As described above, according to the present invention, there is no need to add expensive elements such as N1, and there is no need for special equipment or troublesome work. Industrially useful effects such as the ability to produce strong steel easily and at low cost are brought about.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are both in the embodiment of the present invention]
It is a graph showing the relationship between cross-sectional compressibility (%) and fracture surface transition temperature (g) at 100° C. or lower. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and one other person 39-c. ) I thunder jr luck (U jo ψ

Claims (1)

[Claims] (1) C: 0.15 to 0.45% by weight. Si: 0.05 to 1.00. Mn: 0.3-2.0%. AQ: 0.01-0.10%. One or both of Nb and Ti: 0.005 to 0.150%. Contains, if necessary, the first category... Cu: 0.05 to 0.50%. Cr 20,05~2゜00%. v: 0°01-0.15%. One or two of MO and W: l-Mo-1-1/2W at 0.05-1.20%. Second category... Ca: 0.001-0.050%. Rare earth element: O, OO1-0.050%. 3rd division... B: 0.0005~O, OO50 section. Contains one or more of the following: Fe and unavoidable impurities: Residues. The content of P and S in the impurities is P: 0.025% or less. S: 0.015% or less. The cross-sectional compressibility (RA) of steel at 1.100℃ or less
After hot rolling to 20% or more, quenching is performed directly from the austenitic state, followed by Ac 3 points ~ [Ac
3 points + 200°C] and then quenching is repeated one or more times, followed by tempering at a temperature below the ACI point. (→ Weight percentage: C: 0.15-0.45%. Si: O○ 5-1.00%. Mn: 0.3-2.0%. At!: 0.0 1-0.1 0%. One or two of Nb and T: 0.005-0]50%. If necessary, the first category Cu: 005-050%. Cr: 0.05-2 .00%. V: 0.01-0.15%. One or two types of Mo and W: Mo + VW of 0.05-120. Second category... Ca: 0.001-0. 050%. Rare earth elements: 0.001-0.050 ratio. Third category... B: 0.0005-0.OO50 ratio. Contains one or more of the following: Fe and unavoidable impurities: Remaining. and the content of P and S in the impurities is P: 0025% or less, S: 0015% or less.
) is 20% or more, then directly quenched from the austenitic state, followed by tempering at a temperature below AC1 point, A, C3 points ~ (Ac3 point + 200℃
A method for producing strong steel, which comprises heating the steel to a temperature range of 1 and then quenching the steel in this order one or more times, and then tempering the steel again at a temperature below point A and c1.