KR100584762B1 - The method of manufacturing hot rolled steels with less anisotropic properties for linepipes - Google Patents

Abstract

The present invention relates to a method for manufacturing a high tensile hot rolled steel sheet for steel sheets used for construction, line pipes and offshore structures, etc. The purpose of the present invention is to use Mo, Cr, V and Ti appropriately and optimize the rolling manufacturing process conditions to precipitate and The present invention provides a method for manufacturing a hot rolled steel sheet which can improve material anisotropy as well as increase in strength due to microstructure.

The present invention having such an object, in weight%, C; 0.03-0.10%, Si; 0.01-0.50%, Mn; 1.2-2.0%, P; 0.03% or less, S; 0.010% or less, Ti; 0.01-0.10%, N; 0.01% or less, Mo; 0.1-0.3%, Cr; 0.01-0.04%, Ni; 0.1-0.5%, Nb; 0.02-0.07%, V; The steel slab consisting of 0.10% or less and the remaining Fe and other unavoidable impurities is reheated at a temperature of 1260-1350 ° C, hot rolled to a finish rolling temperature of 780-820 ° C, and then cooled by water to a temperature of 540-600 ° C. The technical gist of the method for manufacturing a hot-rolled steel sheet for a line pipe having a small material anisotropy formed by winding in.

The present invention is to control the chemical composition of the steel and to control the slab heating temperature, hot rolling and winding conditions, to form a ferrite and acicular ferrite structure and to produce a hot-rolled steel sheet for line pipe with reduced material anisotropy Can be.

Steel Slab, Hot Rolled, Anisotropic Material, Line Pipe, Ferrite

Description

Manufacturing method of hot rolled steel sheet for line pipe with less material anisotropy {THE METHOD OF MANUFACTURING HOT ROLLED STEELS WITH LESS ANISOTROPIC PROPERTIES FOR LINEPIPES}

The present invention relates to a method for producing a line pipe steel having a yield strength of 540 to 570 MPa for use in construction, pipelines, and offshore structures. More specifically, a combination of Mo, Cr, V and Ti is added, and a hot rolling process is appropriately performed. The present invention relates to a method for manufacturing a hot rolled steel sheet which can reduce material anisotropy by controlling microstructure by controlling precipitate formation and controlling distribution and size of precipitate itself.

Crude oil used in the Arctic Ocean, Siberia, etc., or steel materials used in steel pipes or offshore structures for transporting oil or gas are required to have excellent properties due to the harsh environment, and especially low temperature toughness to prevent destruction at low temperatures. Specially required. These steel materials are manufactured by using a hot rolled steel sheet as a spiral pipe for economical use.

Many attempts have been made all over the world to manufacture high strength hot rolled steel sheets with excellent low temperature toughness, and most of these studies have been conducted to improve low temperature toughness by performing hot rolling at most low temperatures. 52107225, 5214486 and Korean Patent No. 2000-0039479.

The gist of the technique proposed in No. 52107225 is C; 0.03-0.8%, Si; 0.6%, Mn; 0.7-2.0%, P; 0.010%, S; 0.008%, Nb; 0.01-0.10%, V; 0.01 It is to secure the low-temperature toughness by lowering the finish hot rolling temperature of the steel containing -0.15%, Mo; 0.50%, Ni; 1.0%, Cr; 1.0% and Cu; 1.0% to a low temperature of 850-600 ° C. However, the hot rolled steel sheet has a tensile strength of only 60 kgf / mm ² , which results in a lack of the maximum effect of alloying elements and the appropriateness of the rolling process.

5214486 is a method of hot rolling, cooling at a rate of 15 ° C / sec or more at an Ar3 temperature or more and winding at about 250 ° C. This method has a disadvantage in that the alloy component system is relatively simple but has a very large capacity of the water cooling system because the coiling temperature is very low.

The above-mentioned prior arts can obtain high strength at the same time as low temperature toughness, but there is a problem that material anisotropy, in particular, the difference in material anisotropy of yield strength is large. Material anisotropy means the difference between the strength in the rolling direction and the strength in the 45 degree direction. In recent years, the most widely used spiral steel pipe requires a material of 45 degrees in the rolling direction. Therefore, when the material anisotropy is large, that is, the yield strength in the rolling perpendicular direction is larger than the 45 degree direction. When the steel pipe is piped, the power of the pipe is increased so that the pipe defective rate increases and the roundness of the steel pipe also decreases.

In the above 2000-0039479, C; 0.03-0.10%, Si; 0.01-0.50%, Mn; 1.2-2.0%, P; 0.03% or less, S ;; 0.010% or less, Ti; 0.01-1.10%, N; 0.01 Steel slabs made of% or less, B; 0.003% or less, Nb; 0.02-0.07%, V; 0.10% or less are reheated at 1100 ~ 1170 ° C, hot rolled to a finish rolling temperature of 800-900 ° C, and water cooled It was wound at a temperature of 550-680 ℃ to produce a high tensile strength hot rolled steel sheet for steel pipe with a low material anisotropy. The inventive steel obtained a steel with less material anisotropy with a reheating temperature of 1150 ° C. compared with the comparative material reheated at 1250 ° C., but yield strength 490 MPa and tensile strength 588 MPa under the above composition and conditions compared to the present invention. In order to be used under the same conditions because the tensile strength is small, there is a disadvantage that a thick steel must be used.

Recently, the material manufacturer is seeking to solve the problem of material anisotropy by adjusting the yield strength characteristics in the 45 degree direction by increasing the amount of alloy addition, but there is a disadvantage in that the manufacturing cost is high. Therefore, the present inventors have been searching for a method for manufacturing high tensile hot rolled steel sheet for steel pipes with small material anisotropy only by adding a small amount of alloying elements and hot rolling control. It is confirmed through experiments that the material anisotropy can be greatly changed by properly managing the distribution and size of the precipitates themselves, and the present invention has been proposed based on the experimental results.

Therefore, an object of the present invention is a hot rolled steel sheet which can reduce the material anisotropy by managing the microstructure by the formation of precipitates and the distribution and size of the precipitates themselves by complex addition of Mo, Cr, V and Ti and appropriately control the hot rolling process To provide a method of manufacturing.

According to one aspect of the invention,

In weight percent C; 0.03-0.10%, Si; 0.01-0.50%, Mn; 1.2-2.0%, P; 0.03% or less, S; 0.010% or less, Ti; 0.01-0.10%, N; 0.01% or less, Mo; 0.1-0.3%, Cr; 0.01-0.04%, Ni; 0.1-0.5%, Nb; 0.02-0.07%, V; The steel slab consisting of 0.10% or less and the remaining Fe and other unavoidable impurities is reheated at a temperature of 1260-1350 ° C, hot rolled to a finish rolling temperature of 780-820 ° C, and then cooled by water to a temperature of 540-600 ° C. Provided is a method for producing a hot rolled steel sheet for a line pipe having a small material anisotropy obtained by winding at

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

C is the most economical and effective element for reinforcing steel, but the weldability, formability and toughness are reduced by the addition of a large amount, and in the present invention, C is limited to 0.03-0.10%. If the added amount is less than 0.03%, it is not economical because other alloy elements must be added in relatively large amounts in order to exhibit the same strength, and it is not preferable because the addition of 0.10% or more lowers the weldability, formability and toughness.

Si is also needed to deoxidize molten steel and is effective as a solid solution element, so an addition in the range of 0.01-0.50% is required. If the addition amount is 0.01% or less, it is difficult to obtain clean steel because it does not sufficiently deoxidize the molten steel, and if it is added more than 0.5%, a red scale formed by Si is formed during hot rolling, so the surface shape of the steel sheet becomes very bad and the ductility decreases. Not.

Mn is an effective element to solidify the steel to be added more than 1.2% can exhibit high strength with the effect of increasing the hardenability. However, the addition of 2.0% or more is not preferable because the segregation part is greatly developed at the center of the thickness when casting the slab in the steelmaking process, and it damages the weldability of the final product.

P is an impurity element present in steel and forms a non-metallic inclusion by combining with Mn or the like, and therefore, it is preferable to reduce it as much as possible because it greatly impairs the toughness and strength of the steel. Therefore, the upper limit is made 0.03%.

S is also an impurity element present in the steel and forms a non-metallic inclusion by combining with Mn and the like, and thus it is desirable to reduce it as much as possible because it greatly impairs the toughness and strength of the steel. Therefore, the upper limit is set to 0.01%.

Ti is a very useful element for refining grains. It exists as TiN in steel and has the effect of inhibiting the growth of grains during heating for hot rolling. Also, Ti remaining after being reacted with nitrogen is dissolved in carbon to bond with carbon. Precipitates are formed and the formation of TiC is very fine, greatly improving the strength of the steel. Therefore, in order to obtain the effect of inhibiting austenite grain growth due to TiN precipitation and increasing the strength due to TiC formation, at least 0.01% of Ti should be added, and when 0.03% or more is added, the steel sheet is welded to the melting point when manufacturing the steel pipe. As the TiN is re-used, the toughness of the weld heat affected zone deteriorates, so the upper limit of Ti addition is made 0.03%.

The reason for component limitation of N is attributable to the above Ti addition. In general, N is dissolved in the steel and precipitates to increase the strength of the steel, which is much greater than carbon. However, on the other hand, it is known that toughness decreases as more nitrogen exists in steel, and it is a general trend to reduce nitrogen content as much as possible. However, in the present invention, a proper amount of nitrogen is present to react with Ti to form TiN, thereby imparting a role of suppressing grain growth during reheating. However, since part of Ti does not react with N and reacts with carbon in a subsequent step, the upper limit thereof is made 0.01% or less.

Mo and Cr are very effective in increasing the strength of the material, and lowers the yield ratio by promoting the formation of acicular ferrite, which is a low temperature metamorphic tissue. In addition, cementite and carbides are integrated to suppress the formation of pearlite structures exhibiting deteriorated impact characteristics, thereby ensuring good impact toughness. To this end, Mo should be added to 0.1% or more but is an expensive element is preferably limited to less than 0.3%.

When Cr is added in combination with Mo, it is possible to simultaneously obtain resistance ratio and excellent low-temperature toughness by complementary action, so 0.01-0.04% is added.

Ni is an austenite stabilizing element that inhibits the formation of pearlite, and is an element that facilitates the formation of acicular ferrite, which is a low temperature transformation organization, is added at least 0.1% and is an expensive element, so it is preferable to limit it to 0.5% or less.

Nb should be added at least 0.02% because it is a very useful element to refine the grain and at the same time greatly improve the strength of steel, but when 0.07% is added, it is caused by excessive precipitation of Nb carbonitrides Because the material anisotropy increases because the temperature is too high, it is limited to 0.02-0.07%.

V is added as an effective element to increase the strength of the steel by precipitation as carbonitrides, such as Nb, but when the addition amount is too large, the precipitation amount is saturated, so the strength increase is not large and the manufacturing cost is only limited to 0.10%.

Next, the manufacturing method of this invention is demonstrated.

In hot rolling a steel of a component system limited by the above reason, the reheating temperature of the steel slab is 1260-1350 ° C, that is, the slab heating zone temperature is 1260-1350 ° C, and the hot rolling finish temperature (FDT; ) In the range of 780-820 ℃ and water-cooled to wind the odor temperature in the range of 540-600 ℃. Preferably, the crack zone temperature is 1170-1190 ° C.

The anisotropy of the material is determined by the textures and precipitates formed during hot rolling, and the weaker the textures formed, the more the precipitates precipitated out of order. The temperature of reheating the slab is very important in the present invention. If the reheating temperature is set below a temperature at which the precipitate is sufficiently reusable, such as 1255 ° C., precipitates such as NbC are reduced in the process after hot rolling. In addition, as the heating temperature is lowered, the austenite grains are smaller, so that a stronger strain set structure is formed than during hot rolling, and thus the final transformation set structure is strongly developed. As a result, material anisotropy increases. Therefore, the reheating temperature is maintained at 1260 ° C or higher and the crack zone temperature is maintained at 1170 ° C or higher, thereby facilitating the redistribution of precipitates and maintaining the austenite grain size of moderate size, thereby improving the strength level of the material and thereby slowing down the formation of the texture. The material anisotropy of the material is reduced. At this time, if the reheating zone temperature is too high, the strength decreases due to abnormal grain growth of the austenite grains, so the upper limit of the reheating zone temperature is preferably 1350 ° C.

In the present invention, Mo, Cr, V and Ti is a composite addition, hot rolling finish temperature was set in the range of 780-820 ℃ to reduce the strength increase effect and material anisotropy by the precipitation strengthening of the precipitation element. When the hot rolling finish temperature is lower than 780 ° C, a strong deformation set structure is formed in austenite during hot rolling, so that a strong transformation set structure is formed even in the ferrite and acicular ferrite produced by phase transformation. On the other hand, when the hot rolling finish temperature is set higher than 820 ℃, the finish rolling start temperature is high and the grains of the ferrite produced by the transformation is not fine, as a result it is not economical because it does not exhibit the desired strength level.

After the hot rolling is finished, water cooling is performed on the run-out table to suppress austenite grain growth and at the same time to suppress the decrease in toughness by preventing ferrite coarsening. The coiling temperature is suitable for the temperature range of 540-600 ℃. If it is higher than 600 ℃, the microstructure is formed of coarse ferrite and pearlite, and the precipitate is coarse, so that the strength of the material is greatly reduced. If it is lower than 540 ℃, the bainite transformation occurs. Is increased, but toughness of material is greatly reduced and material anisotropy is increased.

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

Example

Invented steel having a chemical composition as shown in Table 1 was dissolved in a slab, and hot rolled to prepare a plate.

At this time, the invention steel was rolled (compared or rolled invention) under the conditions as shown in Table 2 and subjected to a tensile test. Steel grades A and B are rolled comparatively using the invention steel, and steel grades C and D are rolled invention using the invention steel.

Steel grade C Si Mn P S Nb V Ni Cr Mo Ti N Remarks A 0.059 0.19 1.59 0.008 0.001 0.056 0.056 0.24 0.02 0.25 0.018 0.0054 Invention steel B 0.061 0.22 1.55 0.011 0.001 0.053 0.053 0.25 0.03 0.24 0.020 0.0072 Invention steel C 0.063 0.21 1.57 0.011 0.001 0.052 0.056 0.23 0.02 0.25 0.017 0.0046 Invention steel D 0.061 0.20 1.57 0.010 0.001 0.056 0.052 0.25 0.01 0.23 0.017 0.0056 Invention steel

Steel grade Furnace temperature (℃) Finish rolling temperature (℃) Winding temperature (℃) Remarks Heating table Crack A 1256 1151 792 560 Inventive Steel-Comparative Rolling B 1255 1155 788 565 Inventive Steel-Comparative Rolling C 1261 1175 783 554 Inventive Steel-Invention Rolling D 1266 1173 787 555 Inventive Steel-Invention Rolling

Table 3 below shows the yield strength, the tensile strength and the deviation of the comparative rolling material of the invention steel and the invention rolling material of the invention steel in the 30 and 90 degree directions.

Steel grade Yield strength in 30 ^o direction (MPa) Yield strength in 90 ^o direction (MPa) 30 ^o direction tensile strength (MPa) Yield strength in 90 ^o direction (MPa) Yield strength difference (MPa) Tensile Strength Difference (MPa) Remarks A 503 605 670 703 102 33 Inventive Steel-Comparative Rolling B 498 584 689 734 86 45 Inventive Steel-Comparative Rolling C 550 602 693 750 52 57 Inventive Steel -Invention Rolling D 562 607 744 725 45 19 Inventive Steel -Invention Rolling

Looking at the case of changing the manufacturing conditions using the composition steel of the present invention are as follows. In the case of steel grades A and B comparatively rolled using the present invention steel, the difference in yield strength in the 30 degree direction and the 90 degree direction showing material anisotropy is very large, more than 85 MPa, and the yield strength in the 30 degree direction is 503 MPa or less. Can be. On the other hand, in the case of steel grades C and D manufactured under the invention rolling conditions using the inventive steel, the difference in yield strength in the 30-degree direction and the 90-degree direction is within 52 MPa, which is very excellent compared to that of the comparative material. It can be seen that the yield strength of 550MPa or more.

As described above, when rolling under the conditions of the present invention, excellent hot-rolled steel sheet for line pipe having less material anisotropy of 550 MPa or more and a material anisotropy of 52 MPa or less in a 30 degree direction can be produced.

As described above, the present invention can form ferrite and acicular ferrite structures by controlling the chemical composition of the steel and controlling the slab heating temperature, hot rolling and winding conditions, and by adjusting the rolling conditions, fine precipitates are obtained. At the same time, the material anisotropy decreases. In addition, it is possible to improve the rolling productivity by using a relatively accurate hot rolling conditions in the manufacturing conditions, and even in the use of the steel, the material anisotropy is small, thereby improving the error rate and productivity in the manufacture of parts, the production cost according to the present invention It can be said that the effect is very large.

Claims (1)

In weight percent C; 0.03-0.10%, Si; 0.01-0.50%, Mn; 1.2-2.0%, P; 0.03% or less, S; 0.010% or less, Ti; 0.01-0.10%, N; 0.01% or less, Mo; 0.1-0.3%, Cr; 0.01-0.04%, Ni; 0.1-0.5%, Nb; 0.02-0.07%, V; The steel slab consisting of 0.10% or less and the remaining Fe and other unavoidable impurities is reheated at a temperature of 1260-1350 ° C, hot rolled to a finish rolling temperature of 780-820 ° C, and then cooled by water to a temperature of 540-600 ° C. A method for producing a hot rolled steel sheet for a line pipe having a small material anisotropy formed by winding in.