JP2006045644A - METHOD FOR PRODUCING STEEL FOR WELDING HAVING TENSILE STRENGTH OF >=1,150 MPa - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a steel sheet for welding for use in construction machinery, industrial machinery or the like requiring welding and also having a tensile strength of ≥1,150 MPa. <P>SOLUTION: A steel containing, by weight, 0.15 to 0.20% C, 0.15 to 0.50% Si, 0.5 to 2.0% Mn, ≤0.02% P, ≤0.01% S, >0.5 to 2.0% Mo, 0.03 to 0.1% V, 0.0003 to 0.0030% B, 0.01 to 0.1% Al and ≤0.01% N as fundamental components, wherein the values defined by the following expressions satisfy F: 0.9 to 2.0 and Ceq: ≤0.64, respectively is heated, is thereafter hot-rolled, is water-cooled from ≥650°C after the completion of the rolling, and is tempered in the range of 400 to 550°C: F=Mo+10×V, and Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a steel sheet for welding that requires welding of construction machinery, industrial machinery, etc. and has a tensile strength of 1150 MPa or more.

  For the manufacturing method of high-strength steel, especially 780MPa grade steel, the strength toughness is generally improved by quenching (RQ treatment) by heating and cooling the steel sheet to the austenite region and tempering it. Is adopted. Recently, due to the development of thermomechanical processing technology, direct quenching steel that omits quenching after reheating may be applied by cooling immediately after rolling (DQ treatment).

For example, Patent Literature 2, Patent Literature 3, Patent Literature 4 and Patent Literature 7 are examples of RQ-treated steel, and are characterized by combining an alloy composition and RQ treatment conditions.
Patent Literature 1, Patent Literature 6, Patent Literature 10 and Patent Literature 11 are examples of DQ-treated steel. Both are characterized by a combination of alloy composition and heating-rolling-cooling conditions. In many cases, tempering is performed after DQ treatment. However, in order to provide a steel sheet that can be used with water cooling after rolling, Patent Document 10 immediately cools immediately after rolling, and the surface temperature of the steel sheet is 400 to 150. Rapid cooling is stopped when it reaches the temperature range of ° C., and then left to cool.
Patent Document 8 and Patent Document 9 enable production of high-strength steel as it is rolled, and are characterized by utilizing precipitation strengthening of V and Ti by controlling the cooling rate after rolling.
When tempering is applied, most of them are generally applied at a temperature of Ac ₁ point or less, and the temperature conditions are 540 ° C. or more in Patent Document 1 and 500 to 650 ° C. in Patent Document 10. Etc. are disclosed.
JP 05-1323 A Japanese Patent Laid-Open No. 06-65633 Japanese Unexamined Patent Publication No. 06-145787 Japanese Unexamined Patent Publication No. 06-346144 JP 09-31537 A JP 09-31538 A JP 09-67620 A Japanese Patent Laid-Open No. 10-88231 Japanese Patent Laid-Open No. 10-88232 Japanese Patent Laid-Open No. 10-195532 JP 2000-319726 A

  In recent years, the trend toward larger construction machines and improved performance has led to further demands for lighter parts, and as a result, the production of 1150 MPa class steel that exceeds the conventional tensile strength of 780 to 950 MPa has been achieved. It has been demanded. In order to provide such high strength steel that greatly exceeds the conventional level and is easy to be welded, the various production methods invented heretofore have restrictions on the carbon content and the amount of alloying elements, and are difficult to produce. Moreover, it is necessary to add a large amount of alloy elements in order to ensure the tensile strength, and there is a concern about a significant decrease in weldability.

In general, when tempering a hardened steel, the yield point and toughness tend to improve as the tempering temperature increases, but the tensile strength tends to decrease significantly at temperatures above 400 ° C due to recovery. . Therefore, in the production of steel with a strength level of about 780 MPa, many of the production methods are appropriately selected in terms of alloy element amount and tempering temperature in order to achieve the desired strength-toughness in tempering after RQ and after DQ. It had been. However, in these methods, in order to suppress the alloy element amount to such an extent that the weldability is not impaired and to secure a tensile strength of 1150 MPa or more, it is necessary to minimize the decrease in strength due to tempering after quenching. In such a case, conventionally, a method of using an alloy element (Cr, Mo, V, etc.) that strengthens precipitation during tempering and a technique of reducing the tempering temperature as much as possible have been known.
However, in order to achieve clear precipitation strengthening, a high tempering temperature range exceeding 550 ° C is required in many cases, and adding a large amount of alloy is unavoidable in order to obtain higher tensile strength. .

  On the other hand, lowering the tempering temperature is advantageous in securing the strength by a small decrease in strength, and the amount of alloying elements may be reduced. However, if the temperature is too low, 0.2% yield strength cannot be obtained. A temperature of 400 ° C or higher is required. However, even at such a low tempering temperature, according to the experiments by the inventors, the hardness is reduced by about 60 points as compared to the time of quenching, so that it is possible to ensure 1150 MPa or more after tempering. In the current technology, it is necessary to increase the initial quenching strength (quenching hardness) in consideration of the above hardness difference. For this purpose, it is necessary to add a large amount of carbon, and the weldability is remarkably increased. There is concern that it will be damaged. Therefore, when tempering is performed at a relatively low temperature of 400 to 550 ° C., a means for reducing the decrease in tensile strength (hardness) from the time of quenching is required.

In order to solve the above-mentioned problems, the present invention is a method for producing a welding steel, which is made in order to provide a steel having a tensile strength of 1150 MPa class or more with the addition of as few alloying elements as possible. Is
(1) In mass%,
C: 0.15-0.20%
Si: 0.15-0.50%
Mn: 0.5 to 2.0%
P: 0.02% or less S: 0.01% or less Mo: More than 0.5, 2.0% or less V: 0.03-0.1%
B: 0.0003 to 0.0030%
Al: 0.01 to 0.1%
N: 0.01% or less is contained as a basic component, and the values defined by the following formulas are F: 0.9 to 2.0, respectively.
Tensile strength of 1150MPa class or higher, characterized by heating steel after Ceq: 0.64 or less, hot rolling, and after cooling, water cooling from 650 ° C or higher and tempering in the range of 400 ° C to 550 ° C Method for producing welding steel having
F = Mo + 10 × V
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
(2) In mass%,
Cr: 0.01-0.5%
Cu: 0.01 to 0.5%
Ni: 0.01-0.5%
Nb: 0.005 to 0.05%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005-0.05%
REM: 0.001 to 0.1%
A method for producing a welding steel having a tensile strength of 1150 MPa class or higher according to (1), characterized by containing one or more of
It is.

  As a result of many experiments, the inventors have found that when a directly quenched steel is tempered at a relatively low temperature of 400 to 550 ° C., a specific alloy element is added in combination, so that the steel after quenching and after tempering are added. It has been found that the difference in hardness can be reduced. FIG. 1 shows the results. As a basic component, 0.17% C-0.27% Si-1.35% Mn-0.025% Al-0.0010% B steel is contained, and V: 0.03-0.07%, Mo: After casting the steel changed in the range of 0.04 to 1.2%, heated to 1150 ° C, rolled, and immediately quenched from a temperature of 760 ° C or higher to produce a 16mm steel plate, followed by 30 to 450-500 ° C. The difference between the hardness after tempering and the hardness at the time of quenching when tempering for a minute is performed is illustrated for Mo + 10 × V.

  As is apparent from FIG. 1, it can be seen that even when tempering at a low temperature, the addition of V and Mo reduces the decrease in hardness during quenching. Therefore, in order to obtain a tensile strength of 1150 MPa or more (in terms of hardness, Hv: about 370), if the quenching hardness at the time of quenching is given, the required alloying element addition amount can be obtained from FIG. it can. It is known that the hardness at the time of quenching mainly depends on the amount of C. In consideration of the addition of the amount of C in a range that does not impair the weldability (in the present invention, 0.15 to 0.20%), ΔHv is − Since it is necessary to be 55 or more, it becomes clear that the amount of V and Mo to be added is Mo + 10 × V (F value), and 0.9 or more is necessary.

In practicing the present invention, the strength and toughness as a steel material for welding, and the provision of the alloy composition are very important for ensuring weldability.
C: The most important element for improving the strength. In order to ensure quenching hardness, addition of 0.15% or more is necessary. However, if it exceeds 0.20%, the weldability is significantly impaired. 0.20%.
Si: Acts as a deoxidizing material and a strengthening element, and its effect is observed when added in an amount of 0.15% or more. However, if added over 0.50%, the toughness may be impaired, so the upper limit is made 0.5%.
Mn: Mainly improves hardenability and is effective in improving the strength. Addition of 0.5% or more is necessary, but if added over 2.0%, the toughness and weldability may be lowered. Is 2.0%.
P: When present in a large amount, the toughness is lowered, so the smaller one is desirable. The upper limit content is 0.02%. It is preferable to reduce the content inevitably mixed in as much as possible.
S: If present in a large amount, the toughness is lowered, so the smaller one is desirable. The upper limit content is 0.01%. S, like P, should have the unavoidable amount of contamination as small as possible.
Mo: An element that improves quenching and contributes to strength improvement by precipitation strengthening during tempering. Addition over 0.5% is required, but if over 2.0% is added, weldability is impaired, so the upper limit is made 2.0%.
V: An element that contributes to strength improvement by precipitation strengthening during tempering. Addition of 0.03% or more is required, but if added over 0.1%, the toughness is impaired, so the upper limit is made 0.1%.
B: An element that suppresses the formation of ferrite and significantly improves the hardenability. Addition of 0.0003% or more is necessary, but if the added amount exceeds 0.0030%, nitride is formed and the hardenability is lowered. Since there is a tendency, the upper limit is set to 0.003%.
Al: added to steel as a deoxidizing element and needs to be 0.01% or more, but addition exceeding 0.1% tends to inhibit toughness, so the upper limit is made 0.1%.
N: When added in a large amount of steel sheet, the toughness is lowered, so the smaller one is desirable, and the upper limit content is 0.01% or less.

The above is the basic component according to the present invention, but in the present invention, Cr, Cu, Ni, Nb and Ti as elements for improving the strength of the base material, further as elements for improving ductility and toughness, One or more of Ca, Mg and REM can be added.
Cr, Cu, Ni: All are elements effective in improving the hardenability and improving the strength. However, a large amount of addition impairs the weldability, so the range is 0.01% or more and 0.5% or less.
Nb, Ti: elements that can improve the yield strength and toughness by refining the crystal grains of the base material, both of which can be effective with 0.005% addition, but significant addition may impair the toughness, The addition amount is set to a range of Nb: 0.005 to 0.05% and Ti: 0.005 to 0.03%.
Ca, Mg, REM: All of these elements are effective as elements for preventing the reduction of ductility due to the extension of sulfide during hot rolling. Ca, Mg are 0.0005% or more, respectively. The effect is exhibited by addition of 0.001% or more, but excessive addition may cause coarse oxides at the time of melting simultaneously with the coarsening of sulfides. Therefore, the range of the addition is set to Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, and REM: 0.001 to 0.1%, respectively.

Based on the above component ranges, the present invention further constrains the component ranges by the following two equations.
F = Mo + 10 × V
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
As described above, the F value defines the amount of addition of V and Mo effective for suppressing the strength reduction due to tempering in the range of the low tempering temperature, and is the strength of the 1150 MPa steel which is the object of the present invention. In order to obtain the above, 0.9 or more is necessary. However, if it exceeds 2.0, the weldability is impaired, so the range is set to 0.9 to 2.0.
Ceq is also called a carbon equivalent and is known as an indicator of weldability. The larger the value, the greater the curability during welding and the lower the weld cracking property. In the present invention, when the welding conditions of the conventional 780 MPa class steel are significantly exceeded, the upper limit is set to 0.64 or less because the environment deteriorates due to welding preheating and the work efficiency decreases.

  In the method of the present invention, the steel slab having the above-described component system is used as a starting material and is manufactured through a heating / rolling process and a heat treatment. The steel slab is manufactured as a steel slab by a continuous casting method, an ingot-making / splitting method, or the like after the components are adjusted and melted by a converter or an electric furnace.

  Next, after heating the steel slab, it is rolled to the target thickness by hot rolling. At this time, the heating temperature of the steel slab and the rolling conditions may be those generally used. For example, the slab heating is 1000 ° C. or more and 1250 ° C. or less. In hot rolling, the finishing temperature, the cumulative rolling reduction in finishing rolling is 30% or more, and the finishing temperature is 850 ° C. or more. In particular, under the finish rolling conditions, there is no problem in the present invention even if the controlled rolling is performed for the purpose of refining crystal grains.

Further, in the present invention, after rolling, the steel sheet is cooled with water from a temperature of 650 ° C. or higher, cooled to room temperature, and then tempered at 400 to 550 ° C. The conditions for starting water cooling are necessary to obtain an appropriate quenched structure, and if it is less than 650 ° C., an appropriate hardened structure (almost 100% martensite structure) cannot be obtained by water cooling. Therefore, the lower limit temperature is set to 650 ° C.
The tempering temperature of the present invention is set as a range in which the yield point and toughness are restored while suppressing the decrease in strength due to tempering as much as possible. In steels to which a predetermined amount of V and Mo are added, the temperature is 400 ° C. or higher. The effect appears at 550 ° C. However, when the temperature exceeds 550 ° C, the yield point increases due to precipitation, but the increase in strength is not observed so much and the toughness may decrease. Therefore, it is set as the range of 400-550 degreeC.

Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the test steel used in the examples. Each test steel was produced as a steel piece by the ingot-making or continuous casting method. In the table, A to H steels have chemical components within the scope of the present invention, I -P steel is manufactured out of the chemical component range of the present invention.

  The steel slabs shown in Table 1 were manufactured as steel plates under the manufacturing conditions shown in Table 2. For the base material, tensile test and impact test specimens were taken in the rolling direction at a thickness of 1/4 t, Tensile strength and absorbed energy at −20 ° C. were measured. As for weldability, 20 mm thick x 300 x 300 mm steel plates are cut from each test steel plate by cutting, etc., and after processing a V-shaped groove in the center of the steel plate, deformation during welding is prevented. After fixing both ends, manual welding with a heat input of 2.3 kJ / mm was performed with a preheating of 125 ° C. The welding environment at that time was room temperature and 70% humidity. Thereafter, after 72 hours, five macro test pieces were cut out at intervals of 50 mm at the center portion (250 mm length) of the weld line, and the presence or absence of cracks in the weld heat affected zone was observed. The results are shown in Table 2.

In Table 2, Steel 1 to Steel 8 are within the scope of the present invention. In either case, the tensile strength of the base material was 1150 MPa or more, the toughness was 40 J or more, and no weld cracks were observed.
On the other hand, Steel 9 to Steel 25 are examples in which one or both of the chemical components and / or the manufacturing conditions of the steel plate depart from the scope of the present invention.

First, in Steel 9 to Steel 14, the chemical components are within the scope of the invention, but the manufacturing conditions of the steel sheet do not satisfy the scope of the invention. That is, Steel 9 and Steel 12 do not satisfy the water cooling start temperature of 650 ° C. or higher, which is the range of the invention, have a tensile strength of 1150 MPa or less, and an absorbed energy of 40 J or less.
Steels 10 and 14 have tempering temperatures of 625 and 350 ° C., respectively, which are examples that depart from the scope of the present invention. That is, the steel 10 is not high, the tensile strength does not satisfy 1150 MPa, and the steel 14 is not high. On the other hand, the tensile strength is high, but the toughness value is extremely reduced to 12 J.
Steel 11 is an example in which a water-cooled steel sheet is subjected to reheating and quenching before tempering. A decrease in tensile strength is observed.
Steel 13 is an example in which, after hot rolling, after rolling without water cooling, reheating quenching and then tempering were performed. Like steel 11, the tensile strength is low.
Next, steels 15, 17, 18, 19, 20, 22, 23, and 24 are examples in which only chemical components do not satisfy the scope of the present invention. That is, in Steel 15 and Steel 17, C deviates from the lower limit and the upper limit of the range of the present invention, Steel 15 has low tensile strength, and Steel 17 is inferior in weld cracking.
Steel 18 is an example in which Mn is added exceeding the upper limit of the range of the present invention. Thereby, Ceq is as high as 0.69, and the weld cracking property is inferior.
Steel 19 and steel 20 are examples in which Mo does not satisfy the scope of the present invention. Steel 19 is an example in which the amount of Mo is added in excess of the lower limit and steel 20 is added in excess of the upper limit, and F value for steel 19 and F value and Ceq for steel 20 do not satisfy the scope of the present invention. Therefore, steel 19 has a low tensile strength, and steel 20 has a poor toughness and a poor weld cracking property.
Steel 22 is an example in which V is added exceeding the upper limit. Although the tensile strength of the base material is satisfactory, the toughness is low.
Steel 23 and steel 24 are examples in which Ni and Cr were added exceeding the upper limit of the scope of the invention. In any case, Ceq exceeds the upper limit range of the present invention, and the weld cracking property is inferior.
Further, Steels 16, 21 and 25 are examples in which both chemical composition and production conditions do not satisfy the scope of the present invention. That is, the steel 16 is not cooled with water even under manufacturing conditions, and is tempered after reheating and quenching, and the tensile strength does not reach the predetermined 1150 MPa.
Steel 21 was obtained with the tempering temperature exceeding the lower limit. Tensile strength is very high but toughness is very low. Also, the weldability is poor.
Finally, Steel 25 is an example in which the water cooling start temperature deviates from the scope of the present invention. The tensile strength does not satisfy 1150MPa.

  The present invention is an excellent method capable of providing steel having a tensile strength of 1150 MPa or more without reducing expensive alloying elements and without reducing workability during welding, and reducing the manufacturing cost of the steel sheet. Industrial effects such as reduction of manufacturing cost of welded structures and improvement of safety of structures are extremely large.

It is the figure which showed the effect of addition of Mo and V with respect to the fall of the tensile strength (hardness) by tempering.

Claims (2)

% By mass
C: 0.15-0.20%
Si: 0.15-0.50%
Mn: 0.5 to 2.0%
P: 0.02% or less S: 0.01% or less Mo: More than 0.5, 2.0% or less V: 0.03-0.1%
B: 0.0003 to 0.0030%
Al: 0.01 to 0.1%
N: 0.01% or less is contained as a basic component, and the values defined by the following formulas are F: 0.9 to 2.0, respectively.
Tensile strength of 1150MPa class or higher, characterized by heating steel after Ceq: 0.64 or less, hot rolling, and after cooling, water cooling from 650 ° C or higher and tempering in the range of 400 ° C to 550 ° C Method for producing welding steel having
F = Mo + 10 × V
Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 % By mass
Cr: 0.01-0.5%
Cu: 0.01 to 0.5%
Ni: 0.01-0.5%
Nb: 0.005 to 0.05%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005-0.05%
REM: 0.001 to 0.1%
The method for producing a welding steel having a tensile strength of 1150 MPa class or more according to claim 1, comprising one or more of the following.

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