JPH04293718A - Production of steel having high strength and high toughness - Google Patents

Abstract

PURPOSE:To obtain a steel of >=60kgf/mm<2> class having high strength and high toughness. CONSTITUTION:A steel having a composition consisting of 0.03-0.15% C, 0.05-0.50% Si, 0.3-2.0% Mn, 0.10-2.0% Mo, 0.01-0.10% Al, <=0.0050% N, and the balance Fe with impurities is heated to 1000-1250 deg.C rolled down at 20-60% reduction of area at a temp. in the recrystallization temp. region, rolled down at 30-80% reduction of area at a temp. in the nonrecrystallizaiton temp. region, subjected to hardening treatment where water cooling is started at a temp. not lower than the Ar, point and stopped at <=150 deg.C, and successively heated to a temp. between 500 deg.C and the Ac1 point at 1-20 deg.C/sec temp. rising rate to undergo tempering.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention has a tensile strength of 60 kgf/m.
The present invention relates to a method for manufacturing high-strength, high-toughness steel that has a strength of m2 or more and excellent low-temperature toughness.

0002

[Prior Art] In recent years, energy demand has been increasing year by year, and high-head penstocks for pumped storage power generation are used for construction of offshore structures and seabed survey work vessels that lead to the development of seabed resources, and for adjusting surplus power at night from thermal power generation. Construction is picking up steam. As these steel structures become larger, there is a desire to develop steel materials that have high structural strength, high toughness, and high weldability.

[0003] Conventionally, there is a quenching and tempering method as a heat treatment for high-strength materials. The transformed structure of martensite or lower bainite is formed by quenching, and supersaturated solid solution carbon is precipitated as carbide by subsequent tempering treatment. This heat treatment method has the disadvantage that the rolled steel plate requires reheating and quenching, which reduces economic efficiency.

However, recently, in order to compensate for the shortcomings of the reheating quenching and tempering method, a direct quenching technique has been developed in which quenching is performed immediately after rolling without cooling, which improves strength without reducing economic efficiency. It has been attracting attention because it can be increased.

[0005] Such a manufacturing method includes, for example, a method for manufacturing boron-containing low-alloy heat-treated high-strength steel sheets disclosed in Japanese Patent Publication No. 60-25494, in which heat is applied to fully exhibit the hardenability improvement effect of B. After inter-rolling, recrystallization is caused before quenching, B is segregated at grain boundaries, and then water quenched,
By subsequently tempering, a high tensile strength steel with excellent strength and toughness can be obtained. In addition, if a small amount of Ti is added to low N steel and the Nb-containing steel is directly quenched and tempered, as in the method for producing high-strength steel sheets with excellent strength and toughness disclosed in Japanese Patent Publication No. 63-66368, Coarsening of heated austenite crystal grains is prevented by Ti, and austenite grains are refined by hot rolling in the recrystallization temperature range, and N during tempering is
By precipitation of b(C,N), a high tensile strength steel with an excellent balance of strength and toughness can be obtained.

[0006]

[Problems to be Solved by the Invention] In all of these manufacturing methods, austenite is processed in a recrystallization temperature range and directly quenched. In other words, if austenite before quenching is processed and quenched in a non-recrystallized temperature range, the hardenability will decrease, and the martensitic structure and lower bainite structure will not be generated, but the upper bainite structure will be formed, resulting in a decrease in strength and toughness. It is. However, if the austenite crystal grains are not sufficiently refined due to recrystallization zone rolling and a martensite structure or upper bainite structure is formed from coarse-grained austenite, it is assumed that sufficient toughness has been obtained. I can't say that.

On the other hand, even when austenite before quenching is processed and quenched in the non-recrystallization temperature range, if the quenched structure is martensite and lower benite, the effective grain size (fracture surface unit) becomes finer. It is known that a good balance of strength and toughness can be obtained. However, in order to compensate for the decrease in hardenability, it is necessary to select and add a large amount of alloying elements, which has the disadvantage of decreasing weldability.

Furthermore, in any of these techniques, the tempering process remains a conventional technique in which only the tempering temperature is specified without taking into account the temperature increase rate. In other words, the conventional tempering treatment is: (1) removing the internal stress caused by quenching; (2) removing the internal stress caused by quenching; This is done in order to improve the yield strength by decoupling and dispersing elements in the form of carbides in ferrite, and to reduce hardness and improve toughness by eliminating dislocations. Therefore, we place emphasis on the tempering temperature, do not take into account the heating rate during tempering, and instead heat relatively slowly and take a long holding time. From a metallurgical perspective, we can achieve the optimal tempered precipitation-strengthened state from the original structure. It's hard to say that it is. or,
There is also the problem of reduced productivity, and there has been a strong demand for a method for producing steel with even higher strength and toughness.

[0009]

[Means for Solving the Problems] The present invention provides a method for producing high-strength, high-toughness steel that solves the above-mentioned problems.

[0010] The gist of the present invention is that in weight %, C: 0.03
~0.15%, Si:0.05~0.50%, Mn:0
．． 3-2.0%, Mo: 0.10-2.0%, Al: 0
．． 01 to 0.10%, N: 0.0050% or less, or further Ni: 0.3 to 5.0%, Cu: 0.
1-1.5%, Cr: 0.1-1.5%, V: 0.01
~0.10%, Nb: 0.005~0.05%, Ti:
0.005-0.03%, B: 0.0005-0.00
A steel billet containing 20% of a group of strength and toughness improving elements, or one or more of 0.0005 to 0.005% of Ca, which has an effect of controlling inclusion form, and the balance consisting of Fe and unavoidable impurities. After heating to 1000-1250℃,
After hot rolling with a rolling reduction of 20% or more and 60% or less in a temperature range where austenite recrystallizes, and then rolling with a rolling reduction of 30% or more and 80% or less in a temperature range where austenite does not recrystallize, a temperature of Ar3 point or higher. The quenching process starts with water cooling and stops at a temperature of 150°C or lower, followed by 1°C/s to a temperature range of 500°C or higher and Ac1 point or lower.
This is a method for producing high-strength, high-toughness steel, characterized by heating and tempering at a temperature increase rate of ec or more and 20° C./sec or less.

[0011] The present inventors have conducted experiments on various manufacturing methods with the aim of compensating for the shortcomings of conventional methods and developing high-strength and high-toughness steel that takes into consideration high weldability. In the hot rolling process, a low-alloy steel billet with a relatively low carbon content, which has been added with After quenching to form a martensitic structure and a bainite structure (lower bainite structure and upper bainite structure), the temperature increase rate until the tempering temperature is reached is determined in order to prevent the dislocations carried over into the structure after transformation from disappearing. By speeding up the process, solid solution metals (Mo, Cr, V, Nb, etc.) can be removed while leaving many dislocations.
It was found that precipitation strengthening can be increased in the microstructure consisting of a high dislocation density, making it possible to produce high-strength steel with a good balance of strength and toughness that was previously impossible to obtain. .

0012

[Function] Hereinafter, the present invention will be explained in detail together with its function.

First, the reason why the steel applicable to the present invention is limited to the above-mentioned steel components will be described.

C: C is an effective element for improving hardenability and easily increasing strength. However, 0.03%
If it is less than 0.15%, the strength is insufficient, and if it exceeds 0.15%, weldability will be reduced. Therefore, the C content range is 0.0
The content was set at 3% to 0.15%.

Si: Si is effective as a deoxidizing element and in improving the strength of steel, but if it is less than 0.05%, it has no effect. However, if it exceeds 0.50%, toughness decreases. Therefore, the range of Si content was set to 0.05 to 0.50%.

Mn: Mn is effective in improving hardenability and ensuring strength, but if it is less than 0.3%, sufficient effects cannot be obtained. However, if it exceeds 2.0%, temper brittleness increases and toughness decreases. Therefore, the Mn content is 0.3 to 2
．． It was set to 0%.

Mo: Mo is effective in securing strength by improving hardenability and preventing temper brittleness. It is also an important element in the present invention. In other words, Mo has the effect of suppressing the recrystallization of austenite, and also has the effect of suppressing the disappearance of dislocations introduced by processing, so by increasing the heating rate up to the tempering temperature, a large number of dislocations can be generated. Solid solution metals (Mo, Cr, V, Nb, etc.) while remaining
is precipitated as a carbide, and effective precipitation hardening can be obtained without reducing toughness. However, if it is less than 0.1%, the effect is small, and if it exceeds 2.0%, the coarse M
Carbides such as o2C increase and reduce toughness. Therefore, the range of Mo content was set to 0.1 to 2.0%.

Al: Al is an element necessary for deoxidation, and at the same time, it forms nitrides during heating of a steel billet and is effective in refining austenite grains. However, if it is less than 0.01%, the effect is small, and if it exceeds 0.10%, alumina-based inclusions increase and the toughness is reduced. Therefore, A
The l content was set to 0.01 to 0.10%.

N: N combines with Al and Ti to form nitrides and works effectively to prevent coarsening of austenite grains. However, when the amount of N increases, weldability decreases. Therefore, the N content was set to 0.0050% or less.

In the present invention, in addition to the above basic components, Ni,
It is possible to obtain the desired strength and toughness by adding one or more types of strength/toughness improving element group consisting of Cu, Cr, V, Nb, Ti, and B and Ca, which has the effect of controlling inclusion morphology. .

Ni: Ni is an element effective in improving the low-temperature toughness and hardenability of steel to improve its strength. In the present invention, Ni has the effect of suppressing the disappearance of work dislocations during rolling in the non-recrystallized region, and improves toughness and strength by forming martensitic and bainite structures consisting of fine laths. If it is less than 0.3%, there is no effect, and if it exceeds 5.0%, the effect of improving toughness is small compared to the strength, leading to an increase in cost. Therefore, the Ni content was set to 0.3 to 5.0%.

Cu: Cu is effective for ensuring hardenability and toughness. However, if it is less than 0.1%, it has no effect, and if it exceeds 1.5%, the toughness decreases. Therefore, the Cu content was set to 0.1 to 1.5%.

Cr: Like Ni, Cr also produces martensitic and bainite structures consisting of fine laths,
0.1% or more is necessary to ensure strength. but,
If it exceeds 1.5%, weldability deteriorates. Therefore, the Cr content was set to 0.1 to 1.5%.

V: V is effective in increasing the strength of steel through precipitation hardening. However, if it is less than 0.01%, it has no effect, and if it exceeds 0.10%, the toughness is reduced. Therefore, the V content was set to 0.01 to 0.10%.

Nb: Nb forms fine Nb(C,N) and functions effectively in refining austenite grains after rolling and precipitation hardening. However, if it is less than 0.005%, there will be no effect on the material quality, and if it exceeds 0.05%, weldability will deteriorate. Therefore, the Nb content was set to 0.005 to 0.05%.

Ti: Ti forms fine TiN and refines the structure of heated austenite grains and the weld heat affected zone.
In other words, it is effective in improving toughness. However, 0.005
If it is less than 0.03%, the effect will be small, and if it exceeds 0.03%, it will actually reduce the toughness. Therefore, the Ti content is reduced to 0.
．． 0.005 to 0.03%.

B: B is effective in significantly increasing the hardenability of steel and improving its strength and toughness. However, 0.00
If it is less than 0.05%, the effect will not increase, and if it is less than 0.0020
%, B precipitates increase and toughness decreases. Therefore, the B content was set to 0.0005 to 0.0020%.

Ca: Ca is effective in spheroidizing nonmetallic inclusions and has the effect of reducing the anisotropy of toughness. For this purpose, 0.0005% is required, but if it exceeds 0.0020%, the toughness decreases due to an increase in inclusions. Therefore, the Ca content was set to 0.0005 to 0.0050%.

In addition to the above components, unavoidable impurity elements such as P and S are harmful elements that reduce toughness, so it is better to have a small amount of them.Preferably, P is 0.02.
0% or less, and S is 0.01% or less.

Next, the manufacturing method, which is another gist of the present invention, will be described.

Even with the above-mentioned steel components, the manufacturing method must be appropriate in order to fully exhibit precipitation strengthening through tempering and to achieve high strength and toughness. Here, the reasons for limiting the conditions for heating, rolling, cooling and tempering the steel billet will be explained.

[0032] First, a steel piece having the above-mentioned composition was heated to 1,000 to 1
It is heated to 250°C and hot rolled. This heating is performed to refine the heated austenite grains and utilize the solid solution of Mo, Cr, etc., which has the effect of stabilizing the dislocations introduced by rolling in the non-recrystallized region, and the precipitation strengthening during tempering. This is to sufficiently convert carbonitrides such as Mo, Cr, V, and Nb, which exist in this state, into a solid solution. Therefore, 100
At a low temperature below 0° C., the solid solution action of Mo, Cr, V, Nb, etc. becomes insufficient. On the other hand, at temperatures exceeding 1250°C, although carbonitrides such as Mo, Cr, V, and Nb are sufficiently dissolved, the heated austenite grains become coarse and the subsequent austenite grains are difficult to refine, resulting in a decrease in toughness. .

Next, the thus heated steel billet is hot rolled at a rolling reduction of 20% to 60% in a temperature range where austenite recrystallizes, and then at a rolling reduction of 30% in a temperature range where austenite does not recrystallize. % or more and 80% or less. This is because it is necessary to refine the austenite grains by rolling in the recrystallization zone, and then sufficiently increase work dislocations in the austenite grains by rolling in the non-recrystallization zone. If the reduction rate in the recrystallization temperature range is less than 20%, grain refinement will be insufficient, and if the reduction rate exceeds 60%, the reduction rate in the non-recrystallization temperature range will be insufficient, and the amount of dislocation density will decrease significantly. On the other hand, if the reduction rate in the non-recrystallization temperature range is less than 30%, the increase in dislocation density is insufficient, and if the reduction rate exceeds 80%, the recrystallization reduction rate becomes insufficient, and coarse elongated austenite grains are formed, resulting in poor strength and toughness. The anisotropy of increases.

Next, after this rolling is completed, water cooling is started at a temperature below the Ar3 point, and a quenching process is performed in which the process is stopped at a temperature below 150°C. This means that the austenite that has undergone grain refining and processing is sufficiently martensite and bainite (
This is to improve toughness by transforming the steel into lower bainite and upper bainite, thereby refining the laths of the martensite structure and bainite structure within the grains. Therefore, even if an upper bainite structure is formed, the toughness is improved due to the refinement of austenite grains and laths. However, water cooling from a temperature below the Ar3 point or air cooling causes the formation of ferrite and the disappearance of working dislocations, causing a decrease in strength and toughness. Further, if the water cooling stop temperature exceeds 150°C, the precipitation strengthening effect in the tempering treatment will be insufficient.

[0035] The steel that was water cooled after hot rolling was then heated to 500
1°C/sec or more 2 to a temperature range of ℃ or higher and Ac1 point or lower
The tempering treatment is performed at a temperature increase rate of 0° C./sec or less. This treatment is an important step for the method of the present invention. That is, this is to allow solid solution metals (Mo, Cr, V, Nb, etc.) to precipitate as carbides while allowing many dislocations introduced by the rolling in the non-recrystallized region to effectively increase precipitation strengthening. Here, at a temperature of less than 500°C, M becomes a solid solution.
o, Cr, V, Nb, etc. are not sufficiently precipitated as carbides,
In addition, above the Ac1 point, transformation begins and dislocations are significantly eliminated, causing a decrease in strength. The temperature increase rate during tempering was set to 1°C/sec or more and 20°C/sec or less because, as in the case of conventional heating rates of less than 1°C/sec, solid solution Mo, Cr
, V, Nb, etc., before they precipitate as carbides, dislocations disappear and the precipitated carbides become coarser, resulting in a decrease in strength and toughness. . Also, the temperature increase rate is 20℃/sec
If it exceeds the amount, the amount of precipitation will be insufficient and the strength will decrease. Figure 1 is Table 1,
This study investigated the effects on strength and toughness of steel E shown in No. 2, which were manufactured under the manufacturing conditions within the range of the present invention after rolling and through water cooling, when the temperature increase rate during tempering treatment was varied. When heating and tempering is performed at a temperature increase rate within the range of the present invention, high strength and high toughness can be obtained. However, it can be seen that the strength and toughness decrease at a temperature increase rate outside the range of the present invention.

[0036] The steel plate manufactured by such a manufacturing process is
The martensitic and bainite structures of the steel structure are refined and can ensure excellent mechanical properties, especially high strength without compromising toughness.

[0037]

[Example] Based on the manufacturing conditions of the present invention method and the comparative method shown in Tables 3 and 4, from a steel plate obtained by melting steel having the composition shown in Tables 1 and 2, a plate thickness of 15 to A 50 mm steel plate was manufactured. The mechanical properties of these base materials are shown in Tables 5 and 6.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

[Table 6]

As seen in Tables 5 and 6, examples of the present invention (manufacturing conditions 1, 3, 5, 8, 10, 13, 16, 18 combining the steel composition of the invention and the manufacturing conditions of the invention) ,20,2
2, 24, 26, 28, 30) have sufficiently high values of strength and toughness.

On the other hand, Comparative Example 32, in which the manufacturing conditions were within the scope of the present invention, but the steel composition deviated from the present invention;
No. 33 and 34 have low strength and toughness.

Furthermore, even with the steel composition of the present invention, under the manufacturing conditions of the comparative method, in Comparative Examples 2 and 19, the heating temperature was high and the austenite grains were not sufficiently refined, resulting in a decrease in toughness. . In Comparative Example 9, since the heating temperature was low, the solid solution of the precipitates was insufficient, resulting in a decrease in strength and toughness. In Comparative Examples 6 and 21, the reduction ratio in the recrystallized region is high and the reduction ratio in the non-recrystallized region is low, so that the introduction of working dislocations and the refinement of the microstructure are insufficient, resulting in a decrease in strength and toughness. In Comparative Examples 7 and 23, the reduction ratio in the recrystallization region is low and the reduction ratio in the non-recrystallization region is high, so coarse elongated austenite grains are generated, anisotropy in strength and toughness occurs, and a decrease in toughness is observed. In Comparative Examples 14 and 25, the water cooling start temperature after rolling was Ar3.
Because it is below the transformation point, hardenability is significantly reduced,
Strength and toughness are reduced. In Comparative Examples 11 and 27, the water cooling stop temperature was high, and dislocations introduced by rolling disappeared,
Significant decrease in strength. In Comparative Examples 4 and 15, the temperature increase rate during tempering was slow, so dislocations disappeared, resulting in a large decrease in strength. In Comparative Examples 12 and 29, the temperature increase rate during tempering was extremely fast, so precipitation strengthening was small and the strength was reduced. In Comparative Examples 17 and 31, the tempering temperature was heated to the Ac1 point or higher, which caused dislocations to disappear and carbides to coarsen, resulting in a decrease in strength and toughness.

[0047]

[Effects of the Invention] According to the present invention, high tensile strength steel with an improved balance of strength and toughness can be manufactured economically.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing the influence of the temperature increase rate during tempering on the strength and toughness of Steel E.

Claims (2)

[Claims] Claim 1: In weight %, C: 0.03-0.15%, Si: 0.05-0.50%, Mn: 0.3-2.0%, Mo: 0.10-2. 0%, Al: 0.01-0.10%, N: 0.0050% or less, with the balance consisting of Fe and unavoidable impurities. After heating the steel piece to 1000-1250°C, it is hot-rolled. Reduction rate is 20% in the temperature range where austenite recrystallizes.
60% or more, then rolling at a reduction rate of 30% or more and 80% or less in a temperature range where austenite does not recrystallize, and then a quenching process in which water cooling is started at a temperature of Ar3 or higher and stopped at a temperature of 150°C or lower. and then heated to 500℃
1°C/sec or more to a temperature range of Ac1 point or less 20
A method for producing high-strength, high-toughness steel, characterized by heating and tempering at a temperature increase rate of ℃/sec or less. 2. In weight percent, C: 0.03-0.15%, Si: 0.05-0.50%, Mn: 0.3-2.0%, Mo: 0.10-2. 0%, Al: 0.01 to 0.10%, N: 0.0050% or less, and further contains Ni: 0.3 to 5.0%, Cu: 0.1 to 1.5%, Cr: 0.1 to 1.5%, V: 0.01 to 0.10%, Nb: 0.0
A group of strength and toughness improving elements consisting of 0.05 to 0.05%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.0020%, or Ca having an inclusion form control effect: 0.0005 to After heating a steel billet containing one or more of 0.005% and the remainder consisting of Fe and unavoidable impurities to 1000 to 1250°C, it is hot rolled at a rolling reduction rate of 20 in the temperature range where austenite recrystallizes. % or more and 60% or less, followed by rolling at a reduction rate of 30% or more and 80% or less in a temperature range where austenite does not recrystallize, followed by quenching in which water cooling is started at a temperature of Ar3 or higher and stopped at a temperature of 150°C or lower. treatment, followed by temperature range of 500℃ or higher and Ac1 point or lower.
A method for producing high-strength, high-toughness steel, characterized by heating and tempering at a temperature increase rate of ℃/sec or more and 20℃/sec or less.