KR100311791B1 - METHOD FOR MANUFACTURING QUENCHED AND TEMPERED STEEL WITH SUPERIOR TENSILE STRENGTH OF AROUND 600MPa AND IMPROVED TOUGHNESS IN WELDED PART - Google Patents

Abstract

PURPOSE: Provided is a method for manufacturing quenched and tempered steel with superior tensile strength of around 600MPa and improved toughness in welded part. CONSTITUTION: The quenched and tempered steel is manufactured by (i) hot rolling of a steel slab comprising C 0.06-0.12wt.%, Si 0.1-0.5wt.%, Mn 1.0-1.6wt.%, Al 0.01-0.08wt.%, Ti 0.005-0.02wt.%, B 0.0003-0.003wt.%, N 0.002-0.008wt.% and two or more metals selected from Cu 0.5wt.% or less, Ni 1.0wt.% or less, Cr 0.5wt.% or less, Mo 0.5wt.% or less, Nb 0.1wt.% or less, V 0.1wt.% or less, Ti/N 1.5-2.5, a balance of Fe and other unavoidable impurities wherein carbon equivalent(Ceq) is 0.32-0.38wt.%; (ii) reheating the slab at a temperature range of Ac3 and 950deg.C followed by water-cooling; (iii) tempering at a temperature range of 500deg.C to Ac1.

Description

Tensile strength 600 MPa hardened sheet steel with excellent weld toughness and manufacturing method

The present invention relates to a tensile strength 600MPa grade hardened and polished steel, and more particularly, to a tensile strength 600MPa grade quenched toughened steel excellent in strength and toughness of a base material and excellent in high efficiency welded part toughness and a manufacturing method thereof. .

In general, high tensile strength of 600MPa or more, which is widely used in welded structures such as bridges and storage containers, should be considered at the time of welding. In particular, the ratio of high efficiency welding by automation is gradually increasing in terms of productivity and economic efficiency in construction of welded structures. In this case, as high welding energy is temporarily applied to the steel, grains are coarsened in the weld zone, and a large amount of vulnerable low temperature transformation products are formed, thereby deteriorating the weld performance. Therefore, in spite of the economic advantages of high efficiency welding construction, the amount of welding heat input to be applied must be limited so that weld brittleness does not occur. Therefore, the development of a high-strength steel manufacturing technology that does not have a problem of weld embrittlement even if the welding efficiency is increased by more than two times compared to the ordinary material is strongly demanded in terms of economics and reliability of the welded structure. Tensile strength of 600MPa class high tensile strength steel is caused by rapid toughness deterioration due to coarsening of weld grains or formation of low-temperature transformation products, which is subject to the limitation of the applicable welding heat input.

Until now, steel manufacturing methods for preventing welded embrittlement are largely divided into two types according to the strength level of steel. First, the high strength steel below 500MPa grade has a relatively low level of strength required for the base metal and the welded part, thereby securing strength by applying the recently developed processing heat treatment technology, and forming precipitates such as Ti, REM, and B at the same time as low carbon and low carbon equivalent. Use element to promote tissue transformation and grain refinement during high efficiency welding.

On the other hand, in case of high strength steel of 800MPa grade or more, the hardenability of the trace amount added B is maximized in order to secure the strength of the base metal and improve the toughness of the welded part, and by adding hardenable elements to a certain level or more, by quenching and tempering The proper strength and toughness are harmonized and the welded tissue is designed to be formed of low temperature transformation products such as martensite or lower bainite, just like the hardened structure of the base metal.

Compared to the above two steel grades, the tensile strength of 600MPa class high tensile strength steel is manufactured through hardening and consideration as in the case of 800MPa grade steel. In addition, in order to increase the hardenability and obtain weld toughness as in the case of welding of 800 MPa grade steel, the addition of excessive alloying components is inevitable, and thus the crack susceptibility is sharply increased during welding, and at the same time, it is very disadvantageous in terms of economical manufacturing of steel. Therefore, in this case, it may be effective that measures to improve the toughness of the weld utilize a tissue transformation and grain refining effect applied to the 500 MPa grade steel.

The present invention has been made to solve the above-described conventional problems, to provide a tensile strength 600MPa-class quenched hardened steel and a method of manufacturing the same, which is excellent in strength and toughness of the base material and at the same time excellent in high efficiency welded part toughness. have.

1 is a graph showing the change in absorbed energy according to the re-weld condition of the inventive steel and the comparative steel;

2 is a graph showing a change in absorbed energy values depending on the change in Ti / N ratio in steel;

3 is a graph showing a change in absorbed energy value according to a change in carbon equivalent (Ceq).

In order to achieve the above object, the hardened and hardened steel of the present invention is C: 0.06-0.12%, Si: 0.1-0.5%, Mn: 1.0-1.6%, Al: 0.01-0.08%, Ti: 0.005- in weight%. 0.02%, B: 0.0003-0.003%, N: 0.002-0.008% and Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.1% or less, V It contains one or two selected from less than 0.1%, and satisfies Ti / N: 1.5-2.5, carbon equivalent (Ceq): 0.32-0.38%, and is composed of the remaining Fe and other unavoidable impurities.

In addition, the manufacturing method of the hardened and trimmed steel of this invention is C: 0.06-0.12%, Si: 0.1-0.5%, Mn: 1.0-1.6%, Al: 0.01-0.08%, Ti: 0.005-0.02% by weight%. , B: 0.0003-0.003%, N: 0.002-0.008% and Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.1% or less, V: 0.1 Slabs containing one or more selected one or less, satisfying Ti / N: 1.5-2.5, carbon equivalent (Ceq): 0.32-0.38%, and are composed of the remaining Fe and other unavoidable impurities. Rolling and reheating to Ac ₃ -950 ° C. followed by water cooling followed by annealing at a temperature of 500-Ac ₁ .

Hereinafter, the present invention will be described in detail.

In the present invention, Ti and B are simultaneously used to simultaneously achieve two opposing goals of securing the hardenability of the base material and the transformation and refinement of the welded part in order to secure the base material mechanical properties and the welded toughness of the 600MPa grade steel. To add a complex, at this time to manage the Ti / N ratio in an appropriate range and at the same time limit the carbon equivalent, there is a feature.

To this end, C is an effective element to increase the strength of steel, and at least 0.06% is required to obtain 600MPa tensile strength. However, the excess C content improves weld hardening and low temperature cracking susceptibility. It is preferable to make the upper limit 0.12% because it causes embrittlement by forming.

In the present invention, Si is an element useful for deoxidation and securing strength, and the toughness due to the increase in strength by Si is relatively small, but the addition amount is preferably 0.1-0.5% because it increases the cracking susceptibility upon excessive addition. .

The amount of Mn is insufficient when the addition amount is less than 1.0%, and when it exceeds 1.6%, the strength increase effect is also saturated and the toughness of the weld is degraded to increase the weld hardenability, so it is preferable to add Mn in the range of 1.0-1.6%.

The Al must be added for deoxidation and by combining with nitrogen to form AlN to refine the structure of the steel and reduce the solid solution nitrogen to improve toughness, and to prevent the binding of B to improve the quenching effect of B If it is less than 0.01% as an element, it is not effective. If it exceeds 0.08%, the effect is saturated, and inclusions are increased to impair toughness. Therefore, it is preferable to add it at 0.01-0.08%.

The Ti is an essential element of the present invention steel to form a single or complex precipitate by the addition of a small amount to inhibit the bonding of B and N during the heat treatment of the base material to maximize the effect of increasing the quenchability of B and microstructure in high efficiency welding And decreases the toughness of welded part by reducing nitrogen solubility, and its content is ineffective below 0.005%, and when it exceeds 0.02%, coarse precipitates are formed and the toughness decreases due to the increase of solid solution Ti. It is preferable to add in the range of%.

The B can increase the hardenability of the steel by adding a trace amount as an essential element of the present invention steel with Ti, and can reduce the amount of reinforcing element harmful to weldability, and forms a complex precipitate at high temperature with Ti during high efficiency welding. Excellent toughness is achieved through tissue metamorphosis and reduced nitrogen solution. When the effect of B is less than 0.0003%, it is difficult to secure effective B. If it exceeds 0.003%, coarse B compounds are formed in the grain boundary and in the mouth, which loses the quenching effect and reduces the toughness. It is preferable to add.

N is an element that effectively works on tissue transformation by combining with Al, Ti, and B, but more than 0.002% is required, but when it exceeds 0.008%, N combines with B when manufacturing steel and impairs the hardenability of B. It is preferable to add it in the range of 0.002-0.008% since it lowers.

Cu, 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.1% or less, V: 0.1% or less The addition of this improves the mechanical properties such as the strength of the steel.

First, Cu is an effective element to increase the hardenability and strength, but if it exceeds 0.5%, the weld cracking sensitivity is increased and the toughness is lowered, so it is preferably added below 0.5%.

Ni is an effective element for improving the strength and toughness of the steel, but if it exceeds 1.0%, the steel manufacturing cost increases, so it is preferably added below 1.0%.

The Cr is an effective element to secure the strength of the steel, but if it exceeds 0.5%, a large amount of Cr carbide is formed to deteriorate the toughness of the steel and at the same time increase the low temperature cracking sensitivity, so the upper limit thereof is 0.5%.

Mo is an element that improves strength and improves brittle embrittlement resistance, but if it exceeds 0.5%, the risk of cracking of welds increases and toughness is lowered, so it is preferably added below 0.5%.

Nb and V are both effective elements for increasing the strength of the base metal and the welded part, but when the content-amount exceeds 0.1%, the effect is saturated and the welding property is impaired.

As shown in FIG. 3, the ratio of Ti / N in the steel component formed as described above shows excellent properties in the appropriate Ti / N range, and the proper Ti / N ratio required for improving the toughness of the weld is usually due to the complex addition of Ti and B. It is limited to 1.5-2.5, which is much lower than the 3.4 level (the stoichiometric ratio of Ti and N), which is an appropriate range for addition to 500MPa grade steels for the purpose of microstructure by adding Ti alone.

In addition, the weld toughness for evaluating the degree of embrittlement during high efficiency welding shows a good correlation with the carbon equivalent (hereinafter referred to as 'Ceq') value of the steel, in which the influence of the reinforcing elements is greater than that of C. For reference, the carbon equivalent is shown in Equation (1) below.

Relationship 1

Until now, only the upper limit of the Ceq value has been regulated to improve weldability, but unlike the low temperature cracking susceptibility, the weld toughness shows excellent properties in the appropriate Ceq range as shown in FIG. 3. That is, when Ceq is less than 0.32% or more than 0.38%, rapid embrittlement occurs, so it is preferable to limit it to 0.32-0.38%. Since the Ceq level is much lower than 0.40-0.45% of the Ceq level of 600MPa high tensile steel in general tensile strength, the steel of the present invention has improved weld toughness and resistance to cold cracking that can occur during welding compared to ordinary steel. It can be seen that.

As described above, the Ti content is limited to 0.005-0.002%, the B content is 0.0003-0.003%, the Ti / N ratio is limited to 1.5-2.5 range, and the Ceq is limited to 0.32-0.38%. By limiting each additive component to the above-mentioned range, excellent base material and weld part characteristics are obtained. That is, by adding Ti and B in combination, the quenchability of B is improved by using the nitrogen fixation effect of Ti, and in the case of high efficiency welding, the above elements form a complex precipitate, so that the solid solution of steel that is detrimental to the weld structure transformation and structure microstructure and the weld toughness The amount of nitrogen is reduced to finally obtain excellent weld toughness.

Steels of the above-described composition are manufactured in a converter, an electric furnace, etc., and then subjected to vacuum degassing, as necessary, to produce slabs by ingot, pulverization, or continuous casting.The slabs thus prepared are usually hot rolled and then the strength of the steel. In order to increase the reheating quenching, the reheating temperature is preferably Ac ₃ -950 ℃ to austenitize. The reason for this is that toughness decreases due to excessive grain coarsening when the reheating temperature exceeds 950 ° C. What is necessary is just to cool to normal temperature after reheating with the above.

After quenching as described above, in order to impart proper toughness to the weakened steel, the soaking temperature is preferably 500 ° C-Ac ₁ . If the soaking temperature is less than 500 ℃, toughness is difficult to secure, and if it exceeds Ac ₁ , ferrite transformation occurs and strength decreases.

Hereinafter, the present invention will be described in detail through examples.

Example 1

As shown in Table 1, after the slab of the present invention steel (1) to which Ti and B are complex added and the comparative steel (1-3) to which Ti and B were not added or added alone were heated to 1150 ° C. for 2 hours, hot rolled Next, after reheating and quenching at 930 ℃, annealing at 650 ℃ was measured for each of the mechanical properties and the results are shown in Table 2 below.

Steel grade Chemical composition (% by weight) C Si Mn Al Ti B N Cu Ni Cr Mo V Ti / N Ceq (%) Comparative steel One 0.14 0.34 1.44 0.04 - - 0.006 - 0.12 - 0.16 0.04 - 0.44 2 0.09 0.30 1.38 0.04 - 0.0017 0.005 - - 0.12 0.14 0.05 - 0.40 3 0.11 0.28 1.38 0.04 0.013 - 0.005 0.15 0.13 - 0.15 0.04 2.6 0.40 Invention steel One 0.09 0.24 1.40 0.04 0.01 0.0022 0.004 - 0.21 - - 0.04 2.5 0.34

Tensile Strength (Mpa) Yield strength (Mpa) Elongation (%) Impact absorption energy (J, at: -10 ℃) Wave Transition Temperature (℃) Comparative steel One 670 580 25 140 -55 2 690 600 28 200 -80 3 630 570 27 150 -40 Invention steel One 690 600 32 230 -90

As shown in Table 2, the present invention steel (1) has a value of 600MPa or more of tensile strength, the wavefront transition temperature is significantly lower than the comparative steel (1-3) and showed a value of impact absorption energy is also large. . Therefore, it can be seen that the inventive steel (1) is superior in mechanical properties of the base material as compared with the comparative steel (1-3).

On the other hand, in order to find out the weld toughness of the invention steel (1) and comparative steel (1-3) by reproducing various welding conditions to measure the absorbed energy according to the cooling rate from 800 ℃ to 500 ℃, the results are shown in Figure 1 As can be seen from Figure 1, the present invention steel (1) containing B and Ti shows an excellent absorption energy of 200J or more regardless of welding conditions, whereas only one or none of B and Ti is contained. Comparative steel (1-3) showed relatively low absorption energy, especially the welding embrittlement phenomenon in which the absorption energy rapidly decreased as the amount of heat input of welding increased.

Example 2

The slab made as shown in Table 3 was heated to 1150 ° C. for 2 hours and then hot rolled, then reheated to 930 ° C. for quenching, and reproduced at 100 kJ / cm equivalent weld, which is a high efficiency welding condition for steel treated at 650 ° C. Toughness, that is, absorption energy is measured and shown in FIG. 2.

Steel grade Chemical composition (% by weight) C Si Mn Al Ti B N Cu Ni V Ti / N Ceq (%) Comparative steel One 0.10 0.33 1.33 0.04 0.007 0.0013 0.0112 0.16 0.21 0.04 0.63 0.34 2 0.10 0.32 0.35 0.04 0.011 0.0011 0.0112 0.16 0.21 0.04 1.0 0.34 3 0.10 0.32 1.34 0.04 0.006 0.0011 0.0049 0.16 0.20 0.04 1.22 0.34 4 0.10 0.30 1.36 0.04 0.014 0.0013 0.0045 0.16 0.20 0.04 3.16 0.34 5 0.10 0.32 1.33 0.04 0.029 0.0012 0.0109 0.16 0.20 0.04 2.62 0.34 6 0.10 0.31 1.34 0.04 0.036 0.0012 0.0114 0.15 0.20 0.04 3.15 0.34 7 0.10 0.34 1.35 0.04 0.032 0.0012 0.0107 0.16 0.20 0.04 2.93 0.34 8 0.10 0.32 1.35 0.04 0.020 0.0011 0.0056 0.16 0.20 0.04 3.60 0.34 9 0.10 0.30 1.35 0.04 0.030 0.0016 0.0077 0.16 0.21 0.04 3.91 0.34 Invention steel One 0.10 0.30 1.30 0.06 0.006 0.0014 0.0039 0.15 0.20 0.04 1.55 0.34 2 0.10 0.31 1.29 0.05 0.007 0.0013 0.0043 0.15 0.20 0.04 1.63 0.34 3 0.10 0.31 1.34 0.04 0.011 0.0012 0.0063 0.15 0.20 0.04 1.75 0.34 4 0.10 0.34 1.34 0.04 0.020 0.0012 0.0109 0.15 0.20 0.04 1.86 0.34 5 0.10 0.32 1.34 0.04 0.017 0.0012 0.0087 0.15 0.20 0.04 1.95 0.34 6 0.10 0.32 1.30 0.04 0.024 0.0011 0.0112 0.15 0.20 0.04 2.14 0.34 7 0.10 0.34 1.36 0.07 0.016 0.0013 0.0067 0.15 0.20 0.04 2.40 0.34 8 0.10 0.33 1.35 0.06 0.018 0.0015 0.0073 0.15 0.20 0.04 2.46 0.34

As shown in Table 3, in the case of the comparative steel (1-9) formed out of the conditions of the present invention, the ratio of Ti / N is less than 1.5 and has a value exceeding 2.5, as shown in FIG. The impact energy value of reproducibility showed the value below 100J. On the contrary, in the case of the inventive steel (1-8) made of a steel component satisfying the conditions of the present invention, and particularly, the ratio of Ti / N satisfying 1.5-2.5, as shown in FIG. It was excellent in high efficiency weld reproducibility.

Example 3

The slab made as shown in Table 4 was heated to 1150 ° C. for 2 hours, then hot rolled, then reheated to 930 ° C. for quenching, and reproduced at 100 kJ / cm equivalent weld, which is a high efficiency welding condition for steel treated at 650 ° C. Toughness, that is, absorption energy is measured and shown in FIG. 3.

Steel grade Chemical composition (% by weight) C Si Mn Al Ti B N Cu Ni Mo V Nb Ti / N Ceq (%) Comparative steel One 0.15 0.24 1.43 0.07 0.014 0.0024 0.0057 - - 0.11 0.044 - 2.46 0.42 2 0.12 0.24 1.39 0.06 0.013 0.0020 0.0064 - - 0.13 0.045 - 2.03 0.40 3 0.13 0.24 1.60 0.05 0.013 0.0016 0.0055 - - 0.13 0.044 - 2.36 0.44 4 0.08 0.25 0.85 0.05 0.012 0.0014 0.0066 - - 0.12 0.040 - 1.82 0.27 5 0.09 0.24 0.86 0.05 0.013 0.0013 0.0065 - - 0.15 0.040 - 2.00 0.28 6 0.08 0.025 1.20 0.05 0.015 0.0020 0.0062 0.15 0.20 - - - 2.42 0.30 7 0.10 0.23 1.17 0.06 0.014 0.0017 0.0066 0.16 0.21 - - - 2.12 0.31 Invention steel One 0.07 0.24 1.41 0.06 0.015 0.0023 0.0062 - - 0.13 0.045 - 2.42 0.35 2 0.11 0.23 1.17 0.06 0.015 0.0017 0.0066 0.16 0.21 - - 0.03 2.27 0.32 3 0.11 0.24 1.46 0.03 0.014 0.0018 0.0066 0.16 0.20 - - - 2.12 0.37 4 0.11 0.23 1.54 0.04 0.015 0.0020 0.0064 0.17 0.20 - - 0.03 2.34 0.38 5 0.10 0.23 1.34 0.05 0.015 0.0023 0.0069 0.16 - - 0.040 - 2.03 0.33 6 0.09 0.24 1.36 0.02 0.014 0.0023 0.0066 0.17 - 0.12 - - 2.12 0.36 7 0.09 0.24 1.40 0.05 0.014 0.0023 0.0062 - 0.20 - 0.044 - 2.26 0.34

As shown in Table 4, in the case of the comparative steel (1-7) prepared outside the conditions of the present invention, Ceq is less than 0.32%, and more than 0.38%, as can be seen in FIG. It does not meet the standard value (ve-10> 47J).

On the contrary, in the case of the inventive steel (1-7) made of a steel component satisfying the conditions of the present invention, and especially Ceq satisfying 0.32-0.38%, as shown in FIG. Efficient weldability reproducibility.

As described above, the steel produced by the present invention can contribute significantly to the reduction of welding labor and cost, and to reduce the amount of alloying elements in steel production, since the steel has excellent mechanical properties and at the same time high efficiency welding can be applied. It can be advantageous in terms of economical manufacturing of steel.

Claims (2)

By weight% C: 0.06-0.12%, Si: 0.1-0.5%, Mn: 1.0-1.6%, Al: 0.01-0.08%, Ti: 0.005-0.02%, B: 0.0003-0.003%, N: 0.002-0.008 % And Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.1% or less, V: 0.1% or less 600MPa-grade hardened and friable steel with excellent tensile strength of welded part, which satisfies Ti / N: 1.5-2.5, carbon equivalent (Ceq): 0.32-0.38% and is composed of the remaining Fe and other unavoidable impurities. By weight% C: 0.06-0.12%, Si: 0.1-0.5%, Mn: 1.0-1.6%, Al: 0.01-0.08%, Ti: 0.005-0.02%, B: 0.0003-0.003%, N: 0.002-0.008 % And Cu: 0.5% or less, Ni: 1.0% or less, Cr: 0.5% or less, Mo: 0.5% or less, Nb: 0.1% or less, V: 0.1% or less , Ti / N: 1.5-2.5, carbon equivalent (Ceq): 0.32-0.38%, the slab composed of the remaining Fe and other unavoidable impurities is hot-rolled, and then reheated to Ac ₃ -950 ℃ water-cooled And then, the method of producing a 600MPa-grade quenched and polished steel sheet having excellent weld toughness, characterized in that it is made by a soaking treatment at a temperature of 500-Ac ₁ .

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