KR100514813B1 - A METHOD FOR MANUFACTURING Cr-Mo STEEL FOR HIGH TEMPERATURE APPLICATIONS - Google Patents

Abstract

The present invention relates to a method for producing Cr-Mo steel, which is applied to plant structural steel such as power generation equipment and pressure vessels. The strength of steel by adding Ca in steel, controlling Ca / S, and applying direct quenching after hot rolling The purpose of the present invention is to provide a method for manufacturing a high-temperature Cr-Mo steel that can secure both improvement and weldability.

The present invention for achieving the above object,

In weight%, C: 0.06 to 0.15%, Si: 0.40% or less, Mn: 0.1 to 1.0%, Cr: 1.8 to 3.5%, Mo: 0.8 to 1.6%, Al: 0.02% or less, P: 0.010% or less, Unrecrystallized zone temperature after reheating steel containing N: 0.006% or less, S: 0.005% or less, Ca: 0.05% or less, Ca / S weight ratio of 1-10, balance Fe and other unavoidable impurities After hot rolling at a reduction rate of 5-30% in the range, it is directly quenched at a cooling rate of 5-30 ° C./sec and subjected to a soaking treatment at a temperature range of 650 ° C. to 750 ° C. It is to the technical point.

Description

Manufacturing method of Cr-Meau steel for medium and high temperature {A METHOD FOR MANUFACTURING Cr-Mo STEEL FOR HIGH TEMPERATURE APPLICATIONS}

The present invention relates to a method for manufacturing Cr-Mo steel applied to plant structural steel, such as power generation equipment and pressure vessels. More specifically, it controls the weight ratio of Ca / S in the steel components, and direct quenching and glazing treatment after hot rolling. It relates to a method for producing a high-temperature Cr-Mo steel that can improve the strength and toughness of the base material.

As a Cr-Mo steel applied to plant structural steel such as power generation equipment and pressure vessels, 2.25Cr-1Mo steel has been widely used. This steel has the advantage of satisfying the performance required for medium and high temperature steel, such as oxidation resistance, high temperature strength, creep resistance, which is required in high temperature environment by high Cr and Mo content.

However, in recent years, in order to increase the efficiency of the plant in the plant, especially in the petrochemical industry, there is a tendency to increase the operating conditions at high temperature and pressure and to enlarge the plant, and to reduce the material cost in construction by thinning the members. This is done. Therefore, higher strength and toughness are required than conventional steels.

In addition, the structure is manufactured through a variety of welding process, in order to remove the residual stress caused by welding, heat treatment must be performed, and this causes a re-heat crack, so in the case of conventional 2.25Cr-1Mo steel, the strength of the base material As well as toughness, there was a difficulty in improving weldability at the same time. Here, the reheat crack refers to a crack generated when the heat treatment after welding the steel.

As a related art, Japanese Patent Laid-Open No. 61-223162 discloses a technique for improving the strength of Cr-Mo steel by the minor addition of V and Nb. However, in the above technique, since V and Nb are added, the steel must be heated to a high temperature, thereby requiring a heating facility. In addition, there is also a problem that coarsening of austenite grains is caused and the drop in toughness is remarkable.

Accordingly, the present inventors have repeatedly studied and experimented to solve the above problems, and proposed the present invention based on the results. The present invention adds Ca in steel, controls the weight ratio of Ca / S, and hot rolls. The purpose of the present invention is to provide a method for producing a high-temperature Cr-Mo steel that can secure the strength and weldability of the steel at the same time by applying a direct quenching method.

The present invention for achieving the above object,

In weight%, C: 0.06 to 0.15%, Si: 0.40% or less, Mn: 0.1 to 1.0%, Cr: 1.8 to 3.5%, Mo: 0.8 to 1.6%, Al: 0.02% or less, P: 0.010% or less, Unrecrystallized zone temperature after reheating steel containing N: 0.006% or less, S: 0.005% or less, Ca: 0.05% or less, Ca / S weight ratio of 1-10, balance Fe and other unavoidable impurities In the manufacturing method of the high-temperature Cr-Mo steel, including hot-rolling at a reduction ratio of 5 to 30% in the range, followed by direct quenching at a cooling rate of 5 to 30 ° C./sec and anodizing at a temperature range of 650 to 750 ° C. It is about.

In addition, the steel component may further contain 0.5 wt% or less of Cu and / or Ni, and in weight percent, V: 0.10 to 0.40%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.035%, and B : 0.0005% to 0.004% of one or two or more may be contained.

Hereinafter, in the high-temperature Cr-Mo steel of this invention, a steel component and a manufacturing process are demonstrated.

C is an element useful for securing strength and quenching property, and should be contained at least 0.06% to obtain such an effect. However, if excessive addition reduces weldability, it is preferable to set the upper limit to 0.15%.

Si is an effective element for securing strength, but when added in excess of 0.40%, susceptibility to thinning embrittlement and hydrogen erosion increases, and creep rupture stress also tends to decrease. Therefore, the content thereof is preferably set to 0.40% or less.

Mn needs to be contained in an amount of 0.1% or more in order to secure deoxidation, strength and quenchability. However, if the content is 1.0% or more, the soothing embrittlement susceptibility increases and the surface property deteriorates by oxidation during heat treatment, so it is preferable to add it in the range of 0.1 to 1.0%.

Cr is an element effective for improving strength, toughness, corrosion resistance, and hydrogen corrosion resistance. However, if Cr is less than 1.8%, the effect is not sufficient, and if it exceeds 3.5%, the weldability is impaired. Therefore, the content is preferably set to 1.8 to 3.5%.

Mo is effective in improving strength, toughness and creep rupture strength, but less than 0.8% is not sufficient. However, if the content exceeds 1.6%, the above-mentioned effect is saturated and disadvantageous in terms of cost, so the content is preferably set to 0.8-1.6%.

If the content of Al is 0.02% or more, not only the high temperature property and hydrogen erosion resistance of the steel are lowered due to the refinement of grains by AlN precipitation, but also the hot workability is lowered, so the content is preferably limited to 0.02% or less.

P is segregated at grain boundaries, causing soot embrittlement and increasing the reheat cracking susceptibility. Therefore, it is preferable to lower the content as much as possible. .

N is added in order to improve hydrogen erosion resistance, crack resistance and creep rupture strength, but in order to obtain such an effect, the content is preferably set to 0.006% or less.

S is added in order to improve hydrogen erosion resistance and crack resistance, but in order to obtain such an effect, it is preferably added in an amount of 0.005% or less.

Ca is an essential element of the present invention, and binds preferentially with S in steel to form spherical CaS compounds to act to prevent grain boundary segregation of S. In order to obtain such an effect, it is preferable to set the content to 0.05% or less.

On the other hand, in the present invention, considering the content of Ca and S described above, the weight ratio of Ca / S is limited to 1 to 10. The reason is that if the weight ratio is less than 1, there is no S fixing effect by Ca, and if it exceeds 10, This is because coarse Ca compound is excessively precipitated to lower the mechanical properties of the base metal.

In the present invention, Cu and / or Ni may be further added to the steel component as described above. Both Cu and Ni show a great effect on the improvement of the hardenability, but when the content of Cu and / or Ni exceeds 0.5%, the effect is not great compared to the cost and the turbid embrittlement susceptibility is increased. It is preferable to limit to.

In addition, in the steel component of the present invention, one or two or more of V: 0.10 to 0.40%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.035%, and B: 0.0005 to 0.004% are added in weight%. It may be contained as. At this time, when the V content is added at least 0.10%, more preferably at least 0.20%, hydrogen corrosion resistance, high temperature strength, and creep rupture strength are improved, but when the content of V is more than 0.4%, the internal crack cracking sensitivity is increased. The content of is preferably set to 0.10 to 0.40%, more preferably 0.20 to 0.35% range.

When Nb is added to 0.005% or more, it has an effect of improving both room temperature strength and high temperature strength and lowering resistance to thermal cracking, but when the content exceeds 0.1%, since Nb forms non-metallic inclusions and impairs manufacturability, 0.005 ~ 0.1 It is preferable to add in the range of%, especially 0.005 to 0.05%.

The Ti precipitates with TiN and lowers solid solution N in steel, thereby improving hydrogen erosion resistance and internal cracking resistance. In order to play such a role, at least 0.005% or more should be added. However, if the content exceeds 0.035%, the toughness is lowered, so the upper limit is preferably limited to 0.035%.

When B is added to 0.0005% or more, the quenchability can be improved. However, if the content is 0.004% or more, the ferrite is promoted to lower the strength and toughness. Therefore, the upper limit thereof is preferably limited to 0.004%.

When the steel formed as described above is reheated at a temperature at which alloying elements can be sufficiently dissolved, and then hot rolled, the rolling rate is reduced to 5 to 30% in the unrecrystallized temperature section (above A _r3 temperature below the recrystallization temperature). It is desirable to. The rolling in the non-recrystallized zone is effective in improving the strength and toughness of the steel. When the rolling reduction exceeds 30% in this section, the rolling deformation increases and the anisotropy in the mechanical properties of the steel is remarkably high. It is not suitable for steel for pressure vessels. On the other hand, if the reduction ratio in the unrecrystallized zone is less than 5%, the strength and toughness of the steel cannot be improved. Therefore, the reduction ratio in the unrecrystallized zone temperature range, which is the final stage of hot rolling, is in the range of 5 to 30%. It is preferable to limit.

After the rolling is finished as described above, direct quenching treatment is performed. At this time, since the structure and mechanical properties of the base material vary greatly depending on the cooling rate at the time of quenching, it is necessary to appropriately control the cooling rate. In the present invention, it is preferable to set the cooling rate to 5 ~ 30 ℃ / sec, the reason is that when the cooling rate is less than 5 ℃ / sec it is difficult to ensure sufficient hardenability, when it exceeds 30 ℃ / sec curable This is because this high structure is formed, various cracks occur during welding, and the usability of the steel is reduced.

The steel material quenched in this way is subjected to a soaking treatment at a temperature range of 650 to 750 ° C. which is normal conditions.

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

(Example 1)

Steels prepared as shown in Tables 1 and 2 were hot-rolled at a reduction ratio of 20% in the non-recrystallization zone, and then directly quenched at a cooling rate of 10 ° C./s, and then treated at a temperature of 720 ° C. Thus, the toughness of the base material was evaluated about the steel manufactured, and the result is shown in following Table 3. In addition, in order to evaluate the reheat cracking susceptibility during post-weld heat treatment, the cross section crack incidence rate was measured after heat treatment at 650 ° C. for 12 hours using an inclined Y-type restraint crack test piece, and the results are shown in Table 3 below.

division Chemical composition (% by weight) C Si Mn Cr Mo Al N P S Ca Ca / S Comparative Example 1 0.12 0.40 0.45 2.26 1.01 0.01 0.005 0.010 0.005 － － Comparative Example 2 0.12 0.40 0.45 2.26 1.01 0.01 0.005 0.009 0.005 0.001 0.2 Comparative Example 3 0.112 0.28 0.44 2.18 0.98 0.01 0.004 0.017 0.003 0.045 15 Inventive Example 1 0.12 0.14 0.46 2.22 0.97 0.01 0.004 0.008 0.004 0.01 2.5 Inventive Example 2 0.09 0.24 0.50 2.11 1.12 0.01 0.004 0.008 0.004 0.02 5 Inventive Example 3 0.10 0.09 0.40 2.10 1.01 0.01 0.005 0.009 0.003 0.015 5 Inventive Example 4 0.08 0.24 0.55 2.21 0.98 0.01 0.004 0.008 0.002 0.012 6

division Chemical composition (% by weight) Cu Ni V Nb Ti B Comparative Example 1 － － - - － - Comparative Example 2 - - - - - - Comparative Example 3 0.15 0.13 0.04 - - - Inventive Example 1 － － － - - - Inventive Example 2 - - 0.04 - 0.015 0.0009 Inventive Example 3 0.16 0.21 － 0.01 － － Inventive Example 4 － － 0.04 － - -

division Mechanical properties Base material toughness (wave surface transition temperature, ℃) Reheat crack incidence rate (%) Comparative Example 1 -40 80 Comparative Example 2 -60 50 Comparative Example 3 -50 60 Inventive Example 1 -80 0 Inventive Example 2 -100 0 Inventive Example 3 -95 0 Inventive Example 4 -100 0

As shown in Table 3, in the case of the invention examples (1) to (4) of the present invention in which the Ca / S ratio was adjusted to 1 to 10, it was found that the wavefront transition temperature was very excellent at ductility of -80 ° C or less. Can be. In addition, it can be seen that the crack was not generated at all with the reheat crack incidence rate being 0%.

On the contrary, in the case of Comparative Examples (1) to (3) that do not contain Ca or the Ca / S ratio is outside the scope of the present invention, the wavefront transition temperature is high, and the reheat cracking incidence rate is 50% or more.

(Example 2)

The steel slab having the component of Inventive Example (4) of Example 1 was heated at a temperature of 1200 ° C. for 2 hours, and then hot-rolled, quenched and considered according to the preparation conditions of Table 4 below. Thereafter, the mechanical properties of the respective base materials were measured, and the results are shown in Table 4 below.

On the other hand, in Table 4, normalizing (normalizing) is a method of cooling by heating to room temperature after hot rolling, then reheating, air-cooling at a cooling rate of 0.5 ℃ / s and then reheated to 910 ℃.

In addition, reheating quenching is a method of cooling to room temperature after hot rolling, then reheating to 910 ° C., quenching at a cooling rate of 40 ° C./s to room temperature, and then reheating and soaking.

division Manufacture conditions Mechanical properties Undetermined backpressure reduction rate (%) Manufacturing method after hot rolling Cooling rate when directly quenched (℃ / s) Yield strength (kg / ㎡) Tensile Strength (kg / ㎡) Elongation (%) Wave Transition Temperature (℃) Comparative Example A － Normalizing － 42.5 52.2 24 -30 Comparative Example B － Reheating Hardening － 57.5 68.2 20 -50 Comparative Example C 2 Direct quenching 10 53.6 65.1 19 -20 Comparative Example D 40 Direct quenching 10 44.3 61.2 22 -50 Comparative Example E 20 Direct quenching 2 40.8 56.3 22 -30 Comparative Example F 20 Direct quenching 40 60.8 70.3 18 -30 Inventive Example A 18 Direct quenching 10 55.1 67.8 24 -100 Inventive Example B 15 Direct quenching 20 55.1 68.4 23 -95

As shown in Table 4, it can be seen that the invention examples (A), (B) directly quenched in accordance with the present invention conditions are excellent in tensile strength and elongation.

On the other hand, in the comparative example (A) to which normal normalization is applied, the comparative example (B) to which reheating quenching is applied, and the comparative example (E) which performs direct quenching but low cooling rate, the tensile strength is low and the wavefront The transition temperature was as high as -30 ° C. In addition, although directly quenched, the comparative examples (C), (D), (F) of the reduction in the unrecrystallized region is outside the scope of the present invention, the elongation was lower than that of the present invention, the wavefront transition temperature is -20 ~- It is high at 50 degreeC, and it turns out that low-temperature toughness is bad.

According to the present invention as described above, not only excellent strength and toughness, but also can produce a high-temperature steel with a low reheat cracking susceptibility, the effect of reducing the number of welding operations or cost in the production of welded structures There is.

Claims (3)

In weight%, C: 0.06 to 0.15%, Si: 0.40% or less, Mn: 0.1 to 1.0%, Cr: 1.8 to 3.5%, Mo: 0.8 to 1.6%, Al: 0.02% or less, P: 0.010% or less, Unrecrystallized zone temperature after reheating steel containing N: 0.006% or less, S: 0.005% or less, Ca: 0.05% or less, Ca / S weight ratio of 1-10, balance Fe and other unavoidable impurities Method for producing a high-temperature Cr-Mo steel, including hot rolling at a reduction ratio of 5 to 30% in the range, followed by direct quenching at a cooling rate of 5 to 30 ° C./sec, and anodizing at a temperature range of 650 to 750 ° C. [Claim 2] The method of claim 1, wherein the steel further contains one or two selected from the group consisting of Cu and Ni in an amount of 0.5% by weight or less. The steel according to claim 1 or 2, wherein the steel is in weight%, V: 0.10 to 0.40%, Nb: 0.005 to 0.1%, Ti: 0.005 to 0.035%, and B: 0.0005 to 0.004%. Method for producing a high-temperature Cr-Mo steel, characterized in that more than one species are further contained