KR20010060763A - Method of manufacturing hot rolling steel sheet having high strength for linepipe - Google Patents

Abstract

PURPOSE: To manufacture hot rolled steel sheets for high strength line pipe having yield strength of 100ksi(API-x100), molybdenum and vanadium are added to steel. Also, direct quenching process and tempering process are adopted. CONSTITUTION: The hot rolled steel sheet is manufactured by heating a steel slab comprising C 0.05-0.07wt.%, Mn 1.8-2.0wt.%, Si 0.3wt.% or less, Cu 0.2-0.3wt.%, Ni 0.3-0.5wt.%, Mo 0.2-0.3wt.%, V 0.04-0.05wt.%, Ti 0.01-0.02wt.%, P 0.015wt.% or less, S 0.010wt.%, N 0.0050wt.% or less, a balance of Fe, and other inevitable impurities in the temperature range of 1150 to 1250deg.C; hot rolling the steel slab at 900deg.C; directly quenching it the hot rolled steel sheet to ambient temperature in a cooling rate of at least 20°C/sec; tempering it to 600-650deg.C.

Description

Method of manufacturing hot rolled steel sheet for high strength line pipes {Method of manufacturing hot rolling steel sheet having high strength for linepipe}

The present invention relates to a method for producing a yield strength 100ksi (API-x100) class line pipe steel used as a material for petroleum transportation, and more specifically, molybdenum (Mo) and vanadium (V) are added and directly quenched (Direct Quenching) And a tempering process, the present invention relates to a method for manufacturing a hot rolled steel sheet for high strength line pipe.

Conventional yield strength 100ksi line pipe steel is in weight%, C: 0.05 ~ 0.07%, Mn: 1.8 ~ 2.0%, Si: 0.3% or less, Cu: 0.2-0.3%, Ni: 0.3-0.5%, Mo: 1150 steel slab composed of 0.1-0.3%, Nb: 0.03-0.05%, Ti: 0.01-0.02%, P: 0.015% or less, S: 0.010% or less, N: 0.0050% or less, remaining Fe and other unavoidable impurities After heating in the range of -1250 ℃ to control rolling rolling to the end temperature of the rolling 750 ℃, it is manufactured by accelerated cooling treatment in the cooling rate 20-30 ℃ / sec and cooling end temperature 300-350 ℃ range.

The microstructure produced by applying this accelerated cooling condition is a mixed structure of bainite and martensite, and the steel plate with such mixed structure shows continuous yielding during tensile test, so that the yield strength is less than 100ksi required by the standard. This is most often the case.

Therefore, in case of pipe manufacturing, it is necessary to compensate the strength under process hardening in expansion process.

The present invention has a purpose to provide a method for manufacturing a hot rolled sheet that can satisfy the yield strength standard in the hot rolled sheet state, it is to omit the expansion process for the strength compensation in the manufacture of pipes.

Hot-rolled steel sheet manufacturing method of the present invention for achieving the above object, by weight%, C: 0.05-0.07%, Mn: 1.8-2.0%, Si: 0.3% or less, Cu: 0.2-0.3%, Ni: 0.3- 0.5%, Mo: 0.2-0.3%, V: 0.04-0.05%, Ti: 0.01-0.02%, P: 0.015%, S: 0.010% or less, N: 0.0050% or less, remaining Fe and other unavoidable impurities The steel slab is heated in the range of 1150 to 1250 ℃, hot rolled at the end temperature of 900 ℃ rolling, directly quenched to room temperature at a cooling rate of 20 ℃ / sec or more, and then tempered in the temperature range of 600 ~ 650 ℃ It is configured to include.

Hereinafter, the present invention will be described in detail.

In the present invention, by using the method of direct hardening and tempering rather than accelerated cooling method, by using the precipitation strengthening of Mo and V to secure a sufficient yield strength (100ksi = 688MPa) of the standard in the steel sheet state before the pipe manufacturing, its features There is this. This will be described by dividing it into a steel component system and manufacturing conditions.

[Strong component system]

C is the most important element that controls strength and weldability. It should be added more than 0.05% to secure the strength, and when it is added more than 0.07%, it is difficult to secure impact toughness at the weld, so it is desirable to limit it to 0.05-0.07%. Do.

Mn should be added at least 1.8% in order to increase the hardenability when directly quenched, and when added at 2.0% or more, it should be limited to 1.8-2.0% because it promotes nonuniformity of mechanical properties due to central segregation.

Si is an element added as a deoxidizer, and when a large amount is added, scale defects on the surface of the steel sheet are likely to occur, so it is preferably limited to 0.3% or less.

Cu is an element added in order to prevent hydrogen organic cracking occurring in a hydrogen sulfide atmosphere, and it is effective to add 0.2% or more. It is preferable to limit it to 0.2-0.3% because it causes high temperature cracking when a large amount of 0.3% or more is added.

Ni should be added at least 0.3% in order to prevent high temperature cracking of Cu, and it is preferable to limit it to 0.5% or less since it is an expensive element.

Mo is an important element of the present invention, since it forms a carbide during tempering after direct quenching and prevents a drop in strength due to tempering. Therefore, Mo is added at least 0.2% and limited to 0.3% or less since it is an expensive element. desirable.

V is the most important element that precipitates V carbonitride and increases yield strength during tempering after direct quenching as a key element of the present invention. In order to obtain sufficient precipitation strengthening effect, it must be added at least 0.04% and is less than 0.05% because it is an expensive element. It is desirable to limit.

Ti forms nitrogen compounds to suppress grain growth during slab reheating and decreases the amount of solid solution in welding to improve low temperature toughness. Therefore, when added over 0.01% and when added over 0.02%, Ti causes adverse effects due to solid Ti. It is desirable to limit.

P impairs low temperature impact toughness and limits its content to 0.015% or less.

S is an element that lowers the impact absorption energy, so the content is limited to 0.005% or less.

N is limited to 0.0050% or less because the amount of solid solution in welding damages low temperature impact toughness.

[Production conditions]

The slab formed as described above is heated and hot rolled, and the heating temperature is preferably limited to 1250 ° C. or lower at which coarsening of austenite grains is prevented at 1150 ° C. or higher where the added alloy element can be sufficiently dissolved.

Hot rolling is hot-rolled under the condition of finishing rolling at 900 ° C. or more so that it can be quenched in the austenite state when directly quenched.

Directly quenched immediately after hot rolling as above, and the cooling rate at this time should be such that the structure of the present Myeonggang tempered the mixed structure of bainite and martensite, so that the bainite structure can be formed at 20 ° C / sec or more. It is preferable to cool to room temperature.

Tempering is carried out at 600 to 650 ° C., because the tempering effect should be at least 600 ° C., if it exceeds 650 ° C., the strength is rather deteriorated.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The inventive steel and comparative steel of Table 1 below were prepared under the conditions of Table 2, and then the mechanical properties thereof were measured, and the results are shown in Table 2.

Steel grade C Mn Si P S Cu Ni Mo V Nb Ti N Inventive Steel A 0.05 1.95 0.25 0.010 0.005 0.26 0.4 0.3 0.05 - 0.012 0.004 Inventive Steel B 0.07 1.82 0.23 0.008 0.003 0.26 0.3 0.2 0.04 - 0.011 0.005 Comparative Steel 1 0.07 1.90 0.25 0.012 0.005 0.25 0.3 0.1 0.03 - 0.012 0.004 Comparative Steel 2 0.07 1.93 0.26 0.008 0.004 0.26 0.4 0.2 - 0.04 0.010 0.004 Comparative Steel 3 0.08 1.98 0.25 0.007 0.003 0.25 0.4 0.3 0.04 0.04 0.011 0.004

Inventive steel of Table 1 is a composite additive steel of Mo and V which can cause precipitation hardening by tempering directly after quenching, whereas comparative steel is Nb, etc. Shows that it is added.

division Manufacture conditions Mechanical property Cooling rate (℃ / sec) Cooling end temperature (℃) Tempering temperature (℃) Anti-stiffness (MPa) Tensile Strength (MPa) Elongation (%) Inventive Steel A 25 Room temperature 600 724 779 25 Inventive Steel B 21 Room temperature 650 734 785 24 Comparative Steel 1 25 Room temperature 650 669 726 26 Comparative Steel 2 20 350 - 571 802 24 Comparative Steel 3 25 300 - 596 807 24

As shown in Tables 1 and 2, it can be seen that the inventive steel according to the present invention can satisfy the required strength of the standard in the state of the steel sheet before pipe piping.

The reason is concentrated as follows.

First, the V carbonitride increases the yield strength due to precipitation during direct quenching and the rapid growth of V carbonitride is suppressed by the generation of Mo carbide, so the reinforcing effect is best at the proper tampering temperature range.

Second, in the conventional manufacturing method applying accelerated cooling, a lot of mobile dislocations are generated, resulting in continuous yield increase in the tensile test, but in the present invention applying the direct quenching and tempering process, the flow potential is extinguished by tampering. Therefore, the yield strength is high because the continuous yield does not appear.

As described above, in the present invention, by adding a combination of Mo and V, and by applying a direct hardening and tempering process, it is possible to provide a steel material that satisfies the yield strength of 100 ksi required by the specification in the state of the steel sheet before pipe piping It is.

Claims (1)

By weight%, C: 0.05 to 0.07%, Mn: 1.8 to 2.0%, Si: 0.3% or less, Cu: 0.2 to 0.3%, Ni: 0.3 to 0.5%, Mo: 0.2 to 0.3%, V: 0.04 to 0.05 %, Ti: 0.01 ~ 0.02%, P: 0.015% or less, S: 0.010% or less, N: 0.0050% or less, steel slab composed of remaining Fe and other unavoidable impurities is heated in the range of 1150 to 1250 ° C. and then 900 A method for producing high strength line pipe steel, which comprises hot rolling under a rolling end temperature of not more than 0 ° C., directly quenched to room temperature at a cooling rate of not less than 20 ° C./sec, and tempering in a temperature range of 600 to 650 ° C.