KR100382109B1 - High Tensile & High Toughness carbon steel casting for welded structure purpose and Manufacturing process - Google Patents

Abstract

The present invention relates to a high-strength high toughness alloy cast steel for welded structures and a method of manufacturing the same, the object of which is to improve the chemical composition and heat treatment method lacked in the prior art, excellent weldability and high strength and high strength used in machinery parts of marine equipment and special fields The present invention provides a tough alloy cast steel and a method of manufacturing the same.

The present invention is a high strength high toughness alloy cast steel for welded structures, 0.14 to 0.15wt% C, 0.44 to 0.53wt% Si, 1.26 to 1.33wt% Mn, 0.003 to 0.021wt% P based on 100wt% total alloy steel With 0.008 to 0.017 wt% S, 0.09 to 0.14 wt% Cr, 0.08 to 0,10 wt% Cu, 1.14 to 1.19 wt% Ni, 0.001 to 0.09 wt% Mo, 0.008 to 0.017 wt% Nb , 0.009 to 0.010 wt% N, 0.035 to 0.042 wt% Al, 0.001 wt% Ti, 0.001 to 0.002 wt% V, 0.010 to 0.029 wt% Nb + V, 0.011 to 0.031 wt% Nb + V + Ti and the remainder are made of iron, and cast steel products having a carbon equivalent (CE) of 0.44-0.49% and the manufacturing method thereof are the main points.

Description

High Tensile & High Toughness carbon steel casting for welded structure purpose and Manufacturing process

The present invention relates to an alloy cast steel mainly used for marine equipment and machinery parts in a special field, and has high weldability and high impact strength even at low temperatures (below -20 ° C). will be.

In general, cast steels used for mechanical structures do not require good weldability or high impact values at low temperatures (below -20 ° C).

However, mechanical structural cast steels used in marine equipment, in addition to the mechanical properties and chemical components required for general mechanical structures, require high impact values that are excellent in weldability and withstand low temperatures (below -20 ° C).

Most offshore equipment regulates carbon (C) content to 0.18% or less of chemical components to improve weldability of cast steel products that require welding. The content is also very low and the carbon equivalent (CE) is regulated below 0.50%.

In addition, the mechanical properties require a material with a high impact value at low temperatures because it must be able to withstand low temperatures below -20 ℃.

This is especially necessary for the marine equipment to be able to work without problems even in the polar regions such as the North Sea where the working conditions are very poor.

The required specifications for cast steel used in machinery parts for marine equipment are mechanical properties in terms of tensile strength: at least 540 N / mm2, yield point: at least 400 N / mm2, elongation: at least 20%, section shrinkage: at least 40%, impact value: -20 ° C. 3 specimens are measured at an average of 41J or more and individual specimens are measured at least 21J.The chemical composition is C: 0.18% or less, P: 0.03% or less, S: 0.020% or less, and carbon equivalent (CE): 0.50% or less. We regulate with.

Therefore, as a result of investigating similar materials in general mechanical structural steel, we selected ASTM Al48 90-60, JIS SCW550, JIS SCMnCr3B and compared the mechanical properties and chemical components as shown in Table 1. There was no material suitable for the specification.

Table 1. Table of Mechanical Properties and Chemical Compositions of Similar Materials

ASTM A148 90-60 : Although the tensile strength, yield point, elongation, and cross-sectional shrinkage were satisfactory in mechanical properties, the impact value at -20 ° C was uncertain.

In chemical composition, C. P. S value and carbon equivalent (CE) value are far below the required projection, and there is a problem in weldability.

JIS SCW550 : In mechanical properties, only the tensile strength value satisfies the requirements, and the yield point, elongation, section shrinkage, and impact values all fall short of the requirements. In chemical composition, C, P, S values are slightly exceeded to the requirements, and weldability is judged to be excellent.

JIS SCMnCr3B : Excellent tensile strength and yield point in mechanical properties, but elongation and cross-sectional shrinkage are less than required specifications, and the shock value at -20 ℃ is uncertain. In chemical components, C, P, S and CE equivalents are far below the specifications required, and there is a problem in weldability.

An object of the present invention for solving the above problems is to improve the chemical composition and heat treatment method lacking in the conventional material to provide excellent weldability and high strength high toughness alloy cast steel and its manufacturing method used in machinery parts of marine equipment and special fields To provide.

The purpose of the above is to control the content of C, P, S affecting the weldability during dissolution and chemical composition adjustment, and to improve the strength and toughness of the hardenability improving agent (Ni) and the particle messenger (Al, Ti, Nb, It is achieved by providing an alloy cast steel product having a high strength, high toughness and a method of manufacturing the same by casting the product by adding V) and heat-treating the cast product by a predetermined quenching and tempering heat treatment method.

When explaining the configuration and the operation of the embodiment of the present invention to achieve the object as described above and to perform the task for eliminating the conventional defects in detail. The present invention is a wood-making step → molding step → scrap steel (Scrap) Preparation, dissolution and chemical composition adjustment step → Melt injection step → Cooling step → Desalting step → Normalizing heat treatment step → Riser cutting step → Stress relieving step → Primary rough machining step → Non-destructive test (1st step) → High-strength, toughness alloy for welded structures manufactured through the process of repair work → non-destructive test (secondary) → quenching heat treatment → tempering heat treatment → second rough machining → → grinding and finishing In the method for producing a cast steel composition, to produce a cast steel according to the above process, the preparation and dissolution of the scrap metal and chemical composition adjustment step The alloy composition is composed of 0.14 to 0.15 wt% C, 0.44 to 0.53 wt% Si, 1.26 to 1.33 wt% Mn, 0.003 to 0.021 wt% P, 0.008 to 0.017 wt% S, based on 100 wt% of the total alloy steel. , 0.09 to 0.14 wt% Cr, 0.08 to 0,10 wt% Cu, 1.14 to 1.19 wt% Ni, 0.001 to 0.09 wt% Mo, 0.008 to 0.017 wt% Nb, 0.009 to 0.010 wt% N, 0.035 to 0.042 wt% Al, 0.001 wt% Ti, 0.001 to 0.002 wt% V, 0.010 to 0.029 wt% Nb + V, 0.011 to 0.031 wt% Nb + V + Ti, and the remainder of iron Each alloy element is prepared and prepared so that the equivalent (CE) is 0.44-0.49%, and the normalizing heat treatment step is performed at a heating rate (° C./hr) of 50 or less and heated to 940-970 ° C. for 14 hours. After the air-cooled heat treatment step, the quenching (Water Quenching) heat treatment step is heated to 920 ± 15 ℃ and maintained for 14 hours at a heating rate (℃ / hr) or less to 50 and then subjected to a heat treatment step of water cooling Through the tempering column After heating step is maintained for 20 hours, and 620 ~ 680 ℃ to the heating rate (℃ / Hr) to 50 or less so as to mount the heat treatment step of ronaeng features a method for producing a cast steel.

In order to satisfy the specifications required for the alloy steel castings for machinery structures for marine equipment in the manufacturing process, the improvements to the scrap preparation and molten metal injection processes, which are processes for controlling chemical components, will be described. Quenching and tempering heat treatment methods that have a decisive effect will be described in detail.

The alloy cast steel for marine equipment is specifically regulated for C, P, S, and carbon equivalent (CE), which have a significant effect on the strength and weldability among chemical components, and ASTM Al48 90-60, JIS SCW550, which is similar to the alloy cast steel of the present invention. The chemical composition of JIS SCMnCr3B is shown in Table 2 below.

Table 2. C, P, S, CE Chemical Composition Comparison Table

In general, high strength alloy cast steels have a C content of 0.22 to 0.40%, as shown in Table 2 above. High carbon content is advantageous for securing the strength of the steel but deteriorates the toughness and weldability of the product. When the carbon content and the carbon equivalent are managed low to secure a low level, it is difficult to secure the required strength. Therefore, in order to secure strength and toughness simultaneously, the chemical composition control and heat treatment process must be designed to produce a dense microstructure. Control the C, P, S content affecting weldability during preparation, dissolution and chemical composition adjustment, and use hardening agent (Ni) and particle messenger (Al, Ti, Nb, V) to simultaneously secure strength and toughness. It is added to cast the product, and has a high strength and high toughness to heat the cast product by a predetermined quenching and tempering heat treatment method It should provide alloy castings and a method of manufacturing the same.

In order to obtain the physical properties required for the high strength high toughness alloy cast steel of the present invention, twelve test pieces were measured (detailed results are omitted), and alloy elements and residues deliberately added to improve the properties of steel, as well as C, P, and S contents. The range of chemical components that can be adjusted, including traces of residual elements, is introduced as follows.

The present invention includes the following additional elements based on 100 wt% of the total alloy steel.

C: 0.14-0.15 wt% Si: 0.44-0.53 wt% Mn: 1.26-1.33 wt%

P: 0.003-0.021 wt% S: 0.008-0.017 wt% Cr: 0.09-0.14 wt%

Cu: 0.08-0.10 wt% Ni: 1.14-1.19 wt% Mo: 0.001-0.09 wt%

Nb: 0.008 to 0.017 wt% N: 0.009 to 0.010 wt% Al: 0.035 to 0.042 wt%

Ti: 0.001 wt% V: 0.001-0.002 wt% Nb + V: 0.010-0.029 wt%,

Nb + V + Ti: 0.011-0.031 wt%, carbon equivalent (CE): 0.44--0.49% The reason for the numerical limitation on the alloying elements as described above is because the object of the present invention can be achieved within the above numerical range. Among the properties of cast steel, carbon equivalents (numbers of reinforcing alloy elements other than carbon multiplied by the weight of carbon as a single carbon content: high values increase strength and decrease toughness) are important and individual alloys. Among the elements, the content ranges of Ni, Al, Ti, Nb, and V (aka particle refinement elements) having a particle refining function, which improve the curing ability of the alloy are important. Yield strength (400 MPa or more) and impact to be obtained in the present invention are important. Cast steel that satisfies toughness (average shock absorption energy above 41 Joule at minus 20 ℃) is 1.14 to 1.19 wt.% Of Ni as individual element and half of particle refiner such as Al, Ti, Nb and V. The chemical composition is characterized by having a carbon equivalent of 0.44% to 0.49%, which is contained in the sample.In general, when the carbon equivalent is lower than the lower limit, the yield strength is insufficient, and when the upper limit is exceeded, the impact toughness is insufficient. The addition of Ni and the addition of particle refiner can improve both strength and toughness in the range of carbon equivalents presented. On the other hand, other elements (e.g., C, Mn, Cr, Mo without addition of Ni) can be improved. Only by adjusting the carbon equivalent by), it is not possible to obtain a predetermined quenching characteristic and ultimately yield strength and impact toughness is limited as described above.

The alloy steel of the present invention having the above chemical composition has improved mechanical and physical properties of high strength, high toughness alloy steel for marine equipment, thereby greatly improving impact resistance, weldability, and toughness at low temperatures (below -20 ° C) than conventional steel. (See Table 6. Test Piece Measurements).

Next, the results obtained through the measurement of 12 test specimens for the Quenching Tempering process, which is the most important process that determines the mechanical properties of the alloy cast steel of the present invention, are described as follows.

(Example) The alloy composition is 0.14 to 0.15 wt% C, 0.44 to 0.53 wt% Si, 1.26 to 1.33 wt% Mn, and 0.003 to 0.021 wt% P, 0.008 to 0.017 wt% S, 0.09 to 0.14 wt% Cr, 0.08 to 0,10 wt% Cu, 1.14 to 1.19 wt% Ni, 0.001 to 0.09 wt% Mo, and 0.008 to 0.017 wt% Nb, 0.009 to 0.010 wt% N, 0.035 to 0.042 wt% Al, 0.001 wt% Ti, 0.001 to 0.002 wt% V, 0.010 to 0.029 wt% Nb + V, 0.011 to 0.031 wt% Nb + V + Ti and the remainder are made of iron, and each alloy element is prepared so as to have a carbon equivalent (CE) of 0.44-0.49wt%, and the heating rate (℃ / hr) is 50 or less at 940-970 ℃. Normalizing heat treatment step of heating to and holding for 14 hours and then air-cooling, and heating up to 920 ± 15 ℃ with heating rate (℃ / hr) below 50, holding for 14 hours, and water quenching (Water Quenching) Heat treatment step and heating rate (℃ / hr) below 50 The steel casting of the present invention is completed through a tempering heat treatment step of heating to 620 to 680 ° C. and holding for 20 hours, followed by quenching. For example, the quenching heat treatment step is performed for 13 hours as a previous step. After heating up to 650 ℃ with heating rate (℃ / Hr) 50 for 4 hours and maintaining it for 4 hours, and then heating to 920 ± 15 ℃ for 5.5 hours and maintaining it for 14 hours, the water temperature is 20 ~ 50 ℃ After quenching in a cooling tank within 5 minutes, cooling was performed for 2 hours while maintaining a water temperature. The tempering heat treatment step was performed at a heating rate (° C / hr) of 50 to 620 ° C for 13 hours as a previous step. After heating for 20 hours, the cooling step was performed by cooling to 300 ° C. for 8 hours at a cooling rate (° C./hr) of 50, followed by air cooling after cooling.

The heat treatment conditions to determine the mechanical properties of the material together with the chemical composition were obtained through 12 test specimen measurements (detailed results are omitted), as shown in Table 3.Normalizing heat treatment → Water Quenching heat treatment → Tempering heat treatment Determined by a three-step heat treatment of, the test block was injected to perform a mechanical property test.

At this time, the reference size of the test block was selected as thickness 100mm x 300mm x 300mm.

Table 3. Heat Treatment Procedure for Securing Properties of High Strength High Toughness Materials

Table 4. Q-T heat treatment curve for securing the properties of high strength, high toughness materials

Finally, the results obtained by performing heat treatment (quenching and tempering) are shown in Table 5, and satisfy all mechanical properties required for high strength, high toughness alloy cast steel for marine equipment.

Table 5. Mechanical Property Test Result

* The above table shows the results of mechanical properties measured after performing heat treatment according to the final heat treatment method (Table 4 Q-T heat treatment curve) after 12 test specimen tests.

Table 6. Test Piece Measurement Results

Specimen measurement in Table 6 was performed in the presence of a DNV inspector and issued a DNV Certificate for the material.

The present invention is not limited to the above-described specific preferred embodiments, and various modifications can be made by any person having ordinary skill in the art without departing from the gist of the present invention claimed in the claims. Of course, such changes will fall within the scope of the claims.

As described above, the alloy cast steel for machine construction for marine equipment according to the present invention is excellent in weldability and can be manufactured alloy cast steel having high strength and toughness at low temperature (-2O ℃), there is no need to rely on external materials It can be manufactured at half the price, increasing competitiveness.

In addition, the weldability is superior to the conventional alloy steel and the mechanical properties are greatly improved, so that it can be used for special mechanical parts, and its practicality is very high.

Claims (3)

In the high strength high toughness alloy cast steel for welding structure, 0.14 to 0.15 wt% C, 0.44 to 0.53 wt% Si, 1.26 to 1.33 wt% Mn, 0.003 to 0.021 wt% P, 0.008 to 0.017 wt% S, 0.09 to 0.14 based on 100 wt% of the total alloy steel wt% Cr, 0.08-0.10 wt% Cu, 1.14-1.19 wt% Ni, 0.001-0.09 wt% Mo, 0.008-0.017 wt% Nb, 0.009-0.010 wt% N, 0.035-0.942 wt % Al, 0.001 wt% Ti, 0.001 to 0.002 wt% V, 0.010 to 0.029 wt% Nb + V, 0.011 to 0.031 wt% Nb + V + Ti, and the rest are composed of iron, and carbon equivalent (CE) A high strength, high toughness alloy cast steel for welded structures, characterized by 0.44 to 0.49% cast steel. Sculpting step → Molding step → Scrap preparation and dissolution and chemical composition step → Melt injection step → Cooling step → Desalting step → Normalizing heat treatment step → Riser cutting step → Stress relieving step → Primary roughing machine Machining stage → Non-destructive inspection (1st stage) → Maintenance work stage → Non-destructive inspection (second stage) → Quenching heat treatment stage → Tempering heat treatment stage → Second rough machining process → Grinding and finishing In the manufacturing method of the high strength high toughness alloy cast steel composition for welded structure produced through the process, According to the above process to manufacture a cast steel, In the scrap preparation, melting, and chemical composition adjustment, the alloy composition was 0.14 to 0.15 wt% C, 0.44 to 0.53 wt% Si, 1.26 to 1.33 wt% Mn, and 0.003 to 100 wt% based on 100 wt% of the total alloy steel. 0.021 wt% P, 0.008 to 0.017 wt% S, 0.09 to 0.14 wt% Cr, 0.08 to 0,10 wt% Cu, 1.14 to 1.19 wt% Ni, 0.001 to 0.09 wt% Mo, and 0.008 to 0.017 wt% Nb, 0.009 to 0.010 wt% N, 0.035 to 0.042 wt% Al, 0.001 wt% Ti, 0.001 to 0.002 wt% V, 0.010 to 0.029 wt% Nb + V, 0.011 to 0.031 wt% Nb + V + Ti and the remainder are made of iron, and each alloying element is prepared to prepare a carbon equivalent (CE) of 0.44-0.49%, In the normalizing heat treatment step, the heating rate (℃ / hr) is 50 or less, heated to 940 to 970 ° C. and maintained for 14 hours, followed by a heat treatment step of air cooling. The quenching (Water Quenching) heat treatment step is a heating rate (℃ / hr) to 50 or less to heat up to 920 ± 15 ℃ and maintained for 14 hours to undergo a heat treatment step of water cooling, The tempering (Tempering) heat treatment step is to heat the heating rate (℃ / hr) to 50 or less to 620 ~ 680 ℃ and maintained for 20 hours and then subjected to a heat treatment step of cooling to produce a cast steel product Method for producing a high strength, high toughness alloy cast steel composition for welded structures. The method of claim 2, The quenching (Water Quenching) heat treatment step, the heating step (heat / ℃ / Hr) for 13 hours as a previous step to heat up to 650 ℃ to 4 hours after the temperature is maintained for 4 hours and then again heated to 920 ± 15 ℃ over 5.5 hours 14 hours After holding for a while, the cooling step consists of quenching in a cooling bath of water temperature 20 ~ 50 ℃ within 5 minutes and then cooling for 2 hours, The tempering heat treatment step was carried out by heating up to 620 ° C. at a heating rate (° C./hr) of 50 for 13 hours as a previous step and maintaining it for 20 hours, followed by cooling at 50 ° C. (° C./hr) for 8 hours. A method of producing a high strength, high toughness alloy cast steel composition for welded structures, characterized by consisting of a step of cooling down to 300 ° C. followed by air cooling.