Field of the Invention. This invention relates to metallurgy, more specifically, to rolling, and can be used for the production of rolled coils of low- alloy pipe steel possessing high corrosion resistance.

Prior Art. Known is a method of production of cold resistant flat rolled steel (RU 2265067, published 27.11.2005) comprising the production of a steel billet containing (wt.%) C=0.04-0.1 ; Mn=0.60-0.90; Si=0.15-0.35; Ni=0.10- 0.40; Al=0.02-0.06; Nb=0.02-0.06; V=0.03-0.05; balance steel and impurities. Said method implies austenitization of the billet at 1 100-1 150°C, prestraining (roughing down) to a 35-60% total compression at 900-800°C, subsequent cooling of the semifinished billet (prequenching) by 50-70°C, final straining (finishing rolling) to a 65-75% total compression at 830-750°C, accelerated cooling of the flat rolled steel to 500-260°C and slow cooling to a temperature of not higher than 150°C.

However, flat rolled steel produced in accordance with said method has relatively low mechanical properties, in particular, impact toughness at temperatures below 0°C. This is caused by the low cooling rate of finished flat rolled steel under natural ambient conditions to the ambient temperature. Furthermore, the insufficiently high austenitization temperature does not provide for an equilibrium fine-grained structure which is required for attaining high corrosion resistance. The ser of alloying additions used features excessive carbon content and lacks copper which has additional deleterious effect on the corrosion resistance of the steel.

The closest counterpart of the present invention by technical concept is the method of producing rolled strips for pipeline fabrication (RU 2292404, published 27.01.2007). said method comprises heating a continuously cast billet for hot rolling to the austenitic point 1200-1280°C, roughing down to the intermediate thickness, finishing rolling at the preset finishing temperature of rolling and laminar water cooling to the coiling temperature, wherein the finishing temperature of rolling is maintained in the 830-880°C range and the coiling temperature, in the 540-580°C range. The coiled strips are produced from low-alloy steel containing, wt.%: C=0.05-0.09; Mn=T .0-1.4; Si=0.15-0.40; Ni <0.10; A1=0.01-0.06; Nb=0.02-0.06; V=0.01-0.04; Ti =0.01-0.04;Cr<0.10; Cu<0.10; Mo< 0.01 ; P< 0.015; S< 0.006; balance steel and impurities. The ultimate tensile strength, yield stress and percentage of elongation claimed for said method are o _t =390-480 MPa, s=530-660 MPa and 5=31-32%, respectively. This is generally in compliance with the standard requirements to K 52 (X60) strength grade coiled strips for longitudinally welded pipes.

The disadvantages of said method include the insufficient corrosion resistance of the coiled low-alloy steel strips producing using said method, whereas the corrosion resistance requirements are fundamental criteria for longitudinally welded in-field pipelines produced from coiled strips of the abovementioned strength grade. This necessitates the development of a method of high corrosion resistance low-alloy coiled strip production.

Disclosure of the Invention. The technical result of this invention is providing 6-12 mm thick coiled rolled strips exhibiting low corrosion rate while retaining the strength and plasticity performance complying with the requirements to K52 (X60) strength grade.

Said technical result is achieved using a Method of manufacturing low- alloyed coiled strip of higher corrosion resistance comprising heating a continuously cast billet for hot rolling to the austenitic point 1200-1280°C, roughing down to the semifinished rolled stock thickness, finishing rolling at the preset finishing temperature of rolling and laminar water cooling to the coiling temperature, wherein said continuously cast billet is produced from steel having the following ratio of alloying additions, wt.%: Carbon: 0.04-0.07;

Manganese: 0.4-0.9;

Silicon: 0.1 -0.4;

Chromium: 0.2-0.7;

Copper: 0.3-0.6;

Nickel: 0.15-0.60;

Aluminum: max. 0.03;

Molybdenum: max. 0.08;

Sulfur: max. 0.003;

Phosphorus: max. 0.015.

The total content of vanadium, niobium and titanium - not more than 0.15

Iron and inevitable impurities - balance,

wherein said continuously cast billet austenitization before rolling is achieved by tempering at the preset temperature for at least 3 h, subsequent roughing down is carried out at a unit percentage reduction of at least 30% for the first pass and at least 20% for the last pass, the thickness of the semifinished rolled stock is set to 5.5-7.5 thicknesses of the finished rolled strip, and finishing rolling is carried out at a unit percentage reduction of at least 30% for the first pass and max. 10% for the last pass, further wherein said finishing temperature of rolling is preset based on the Tf _r =800*K,°C ratio, where K is the energy coefficient taking ...1, 15 , and said strip is coiled in the 585-670°C range.

The efficiency of said method can be further increased by producing said high corrosion resistance low-alloy coiled strips from steel providing the absence of aluminum-magnesium spinel corrosive nonmetallic inclusions and having the C _c <0.35 equivalent carbon content and the P _cm <0.24 cracking resistance. The concept of the invention is using to the full extent the available properties of the low-alloy steel having the abovementioned chemical composition which is provided by the strain and temperature parameters of its technology. The rolling technology of the steel is adjusted to provide the optimum ferrite/pearlite phase composition and phase morphology, grain refinement in the micro structure, solid solution hardening, precipitation hardening, dislocation and texture hardening which ensure the high corrosion resistance of the material.

A steel billet having the preset chemical composition is initially melted. The carbon content in the low-alloy steel determines its strength. Experiments have shown that providing for carbon contents of below 0.04% at the steel melting stage is a technically difficult task. However, carbon contents of above 0.07% deleteriously affect the corrosion resistance of coiled strips and lead to an inhomogeneous distribution of steel parameters across the strip thickness as a result of zone segregation.

Manganese and nickel additives to the low-alloy pipe steel considered herein favor the solid solution hardening of the metal and hence increase the strength parameters of the finished rolled stock. Industrial experience shows that reducing the manganese content to below 0.4% within the set of alloying additions used herein leads to a decrease of the strength parameters and the low- temperature toughness of the steel to below the acceptable limits. However, increasing the manganese content to above 0.9% accelerates general corrosion and leads to the formation of a bainite structure in the axial zone of the rolled strip thus impairing the cold resistance and the hydrogen cracking resistance of the steel, i.e. deleteriously affects the quality of rolled pipes.

Silicon provides for a higher deoxidation degree of the steel and improves the strength parameters of the rolled strip. Experiments showed that reducing the silicon content to below 0.1% in steels of the chemical composition considered herein significantly complicates the melting technology by deleteriously affecting the fluidity of the steel thus causing an unnecessary increase in the cost of the rolled stock. However, increasing the silicon content to above 0.4% is accompanied by an increase in the quantity of silicon inclusions which reduce the impact toughness and corrosion resistance of the metal and also impair the weldability of the rolled strips.

Chromium and copper alloying increases the strength and corrosion resistance of the metal by forming a protective film on the metal surface which prevents the contact between the metal and the corrosive hydrocarbons transported through the in-field pipelines. There are experimental data suggesting that the minimum chromium content within the set of alloying additions used herein at which chromium affects the corrosion resistance of the coiled strips is 0.2%. Lower chromium concentrations do not provide the required performance. Increasing the chromium content to above 0.7% is inexpedient because then the low-temperature impact toughness decreases and the cost of alloying becomes unjustifiably high. Copper in concentrations of below 0.3% does not have its favorable effect on the corrosion resistance of the coiled strips. On the other hand, the low-temperature impact toughness of steel decreases if the copper content exceeds 0.6%. Within the above concentration ranges, these elements do not deleteriously affect the weldability of the strips during pipe production.

Nickel alloying favors the quality of the strip surface during rolling by preventing metal sticking to the working rolls of the rolling mill and has a positive effect on the corrosion resistance of the coiled strips. It should be bom in mind, however, that nickel concentrations of below 0.15% are insufficient for increasing the corrosion resistance while increasing the nickel content to above 0.6% leads to an unjustified growth of the cost of alloying. Aluminum is used for deoxidation and modifying the steel. Aluminum eliminates the deleterious effect of nitrogen on the properties of the rolled sheets by binding nitrogen to nitrides. However, aluminum has the tendency to form aluminum-magnesium spinel corrosive nonmetallic inclusions which largely control the corrosion resistance of the rolled pipes. This necessitates reducing the aluminum content to below 0.03% for achieving the required level of corrosion resistance.

Molybdenum is an impurity element in this rolled stock composition which enters the steel from metal scrap during melting. However, an increase in its concentration to above 0.08% impairs the weldability of the rolled strips during the production of in-field pipelines and raises the cost of alloying.

Titanium is a strong carbide-forming element which strengths steel. Fine grained titanium carbides precipitating during hot rolling and laminar water cooling of rolled strips are highly resistant to overheating. Welding does not dissolve titanium carbides causing weld seam zone strength degradation. Laminar cooling of rolled strips the niobium microalloying of the steel favors the formation of a cellular dislocation structure of the steel which provides for the required combination of the strength and plastic properties of the metal. Fine-grained niobium carbides hinder the growth of the austenite grains during heating thus favoring the grain refinement during rolling under the process conditions suggested herein. Vanadium and niobium separately and jointly control microstructure grain refinement and increase the strength and impact toughness of the hot-rolled strips. However, if the overall content of these elements exceeds 0.15% the metal exhibits a reduction of its low-temperature impact toughness. Furthermore, an excess of these elements deleteriously affects the weldability of the metal without providing any further improvement of the mechanical of the hot-rolled strips. The steel of the composition disclosed herein may contain max. 0.015% phosphorus and max. 0.003% sulfur. If these maximum concentrations of the above elements are not exceeded the presence of these elements in the hot-rolled strips of the steel of the composition provided herein do not have tangible deleterious effect on the mechanical properties of the strips; however, their complete removal from the melt significantly increases the production costs and complicates the technology. An increase in the concentrations of these detrimental impurities, especially sulfur, to above the above disclosed levels significantly impairs the corrosion resistance of the rolled strips and, particularly, the low-temperature impact toughness.

By and large, the element contents disclosed hereinabove provide for the required phase composition as well as the required level of the mechanical properties and the corrosion resistance of the coiled strips during the implementation of the technology provided herein.

The method of high corrosion resistance low-alloy coiled strip production is implemented as follows. The continuously cast billet for hot rolling is heated to 1200-1280°C and exposed to at least 3 h tempering which is required for steel austenitization in the entire bulk. The sulfides, phosphides, nitrides, alloying and impurity compounds and carbonitride hardening particles dissolve completely in the austenitic matrix. This increases the processing plasticity and deformability of the billet during rolling. Furthermore, since rolling is accompanied by a continuous reduction of the metal temperature, heating the billet to the abovementioned temperature provides for the achievement of the preset finishing temperature of rolling and the coiling temperature.

Roughing down is the preparatory stage of straining which provides for the formation of the required homogeneous structure of the rolled strip due to the refinement of the austenite grains as a result of static recrystallization. Multiple-pass roughing down provides for the intense refinement of the austenite grains to 30-70 mpi. The use of unit percentage reduction of at least 30% during the first pass of the multiple-pass roughing down and at least 20% during the last pass of the multiple-pass roughing down provides for the thorough treatment of the structure of the continuously cast billet and grain refinement over the entire thickness of the billet.

Cooling down of the semifinished rolled stock between the roughing down and the finishing rolling at a thickness of 5.5 -7.5 thicknesses of the finished rolled strip provides for texture development and subgrain formation during the subsequent controlled finishing rolling in the two-phase region, in addition to the precipitation hardening and grain refinement to 11-12 grades. The forming subgrains, along with increasing the hardness, prevent the brittle fracture and fatigue of the metal. At the semifinished rolled stock thickness chosen herein, the subgrain hardening has a significant effect on the formation of the mechanical properties of the rolled strips.

An important feature of the coiled strip hardening during finishing multiple-pass straining in the two-phase region with impeded austenite recrystallization is that the first rolling passes provide for intense hardening of the surface layers of the billet where the strain is the largest. During the further hardening of the surface layers during finishing rolling, the strain starts to penetrate to the depth of the metal and finally propagates to the entire thickness of the semifinished rolled stock at a unit percentage reduction of at least 30% during the first pass of the multiple-pass finishing rolling and not more than 10% during the last pass of the multiple-pass finishing rolling. Since the abovementioned levels of unit percentage reduction during finishing rolling are sufficient for the complete treatment of the structure over the entire thickness of the semifinished rolled stock, the grains are refined and the corrosion resistance and the mechanical properties of the finished rolled strip are improved. Increasing the corrosion resistance of the coiled strip requires the coiled strip be cooled down gradually on the discharge rolling table from the finishing rolling temperature to the coiling temperature so that the residual inner stresses be relieved and a fine-grained equilibrium metal structure be obtained. For this purpose the finishing rolling temperature T _ir is preset based on the T _f. =800*K,°C ratio, where K is the energy coefficient taking on K=l,02, ... l, 15. At said finishing rolling temperature, strip coiling in the T _c =585-670°C range provides for the required laminar cooling rate after rolling and for the formation of the steel phase composition as is required for achieving high corrosion resistance.

In oreder to increase the efficiency of the method provided herein, high corrosion resistance low-alloy coiled strips are made from low-carbon steel providing the absence of aluminum-magnesium spinel corrosive nonmetallic inclusions and having the C _c <0.35 equivalent carbon content and the P _cm <0.24 cracking resistance. The elimination of said aluminum-magnesium spinel corrosive nonmetallic inclusions provides for higher corrosion resistance of the coiled strip material in hydrogen and hydrogen sulfide media in which said aluminum-magnesium spinel corrosive nonmetallic inclusions act as the main local corrosion initiation sites. The use of steel with an equivalent carbon content corresponding to the parameters claimed herein provides for stable quality of the weld seam during pipe production from coiled strips obtained in accordance with the method provided herein. The cracking resistance determines the formation probability of surface defects during rolled stock straining. Adherence to the parameter values disclosed herein prevents crack initiation during the production of longitudinally welded pipes from coiled strips on a profile-bending machine. These conditions can be achieved subject to the use of the set of alloying additions disclosed herein. Said content of alloying additions provides for the required C _c <0.35 equivalent carbon content and the P _cm <0.24 cracking resistance and prevents the formation of aluminum-magnesium spinel corrosive nonmetallic inclusions.

Embodiments of the Invention. The use of the method provided herein is exemplified by its industrial application on a wide-strip rolling mill 2000 for the production of a 8>< 1400 strip (coil) having the K52 (X60) strength grade. The billets are made from steel containing, wt.%: C=0.052%; Mn=0.67%; Si=0.23%; Cu=0.35%; Ni=0.179%; Nb=0.034%; Cr=0.433%; Mo=0.073%; Al=0.022%; V=0.044%; Ti=0.021%; S=0.0014%; P=0.0072%, balance iron and impurities with contents of max. 0.002% for each impurity element. The contents of the alloying additions is in full compliance with the chemical composition claimed herein. The overall content of niobium, vanadium and titanium is Nb+V+Ti=0.099%, i.e., complies with no more 0.15 wt.% ratio disclosed herein.

Heating the continuously cast 250x 1450x6900 mm billets to 1230°C and tempering for 3 h provide for the austenitization of the low-alloy steel for the dissolution of the carbonitride hardening particles. After output from the oven the billet is roughed down to the semifinished rolled stock thickness of 38 mm which is 4.75 thicknesses of the finished rolled strip, this being in compliance with the specifications of the technical solution provided herein. The unit percentage reduction is 32% for the first roughing-down pass and 29% for the last roughing-down pass which is in compliance with the unit percentage reduction range disclosed herein.

The next step is the finishing rolling of the semifinished rolled stock to the coiled strip size 8 ^c 1390 mm with a unit percentage reduction of 31% for the first finishing rolling pass and 6% for the last finishing rolling pass. The finishing rolling temperature is 7V=898°C. The strain and temperature conditions of the finishing rolling are fully in compliance with their respective claimed ranges. The rolled strip is laminar water-cooled on the discharge rolling table of the wide-strip rolling mill followed by coiling at T _c = 592°C. The accelerated cooling of the metal after the finishing rolling leads to the refinement of the structural components and the formation of a ferrite-bainite structure.

The mechanical properties of the coiled strip produced as described above were tested for standard specimens. The strain and temperature conditions of rolling provided for a fine-grained ferrite-bainite structure with a noticeable longitudinal anisotropy of the grains. The static tensile tests were carried out for flat specimens as per GOST 1497 Russian Standard, and the impact bending tests were carried out for V-notched specimens as per GOST 9454 Russian Standard at -50°C. The mechanical properties of transverse specimens were as follows: yield strength a _y =560-570 N/mnT; yield stress a _t =500-510 N/mnT; percentage of elongation 8=23-23,5%; impact toughness KCV _-5 o= 190-235 J/cm . The abovementioned mechanical properties are fully in compliance with the requirements for coiled strips of the K52 (X60) strength grade. This level of mechanical properties of the coiled strips is provided by the penetration of the plastic strain zone from the billet surface to the entire billet depth at relatively low finishing rolling temperatures that ensure the intense treatment of the structure and grain refinement.

The coiled strip produced as described above did not contain aluminum- magnesium spinel corrosive nonmetallic inclusions that deleteriously affect the corrosion resistance of steel. The equivalent carbon content was C _c =0.31 and the cracking resistance was P _cm =0.15, i.e., these parameters were within their respective claimed ranges.

Thus, the use of the rolling method provided herein provides for the desired result, i.e., the production of coiled strip with a high corrosion resistance for the production of longitudinally welded pipes of the K52 (X60) strength grade on a wide-strip rolling mill. The optimum process conditions for the implementation of the method were determined experimentally. The experiments showed that billet heating to below 1200°C does not homogenize the austenitic structure which hinders the achievement of the required properties of the finished rolled stock. On the other hand, increasing the heating temperature to above 1280°C leads to an intense growth of the austenite grains and thus a reduction of the strength of the finished rolled sheets. If the austenitization duration is less than 3 h, the billets has not enough time for homogeneous heating, this causing a significant inhomogeneity of the strain and the formation of surface defects in the finished product.

The experiments showed that if the unit percentage reduction during the first pass of roughing-down is less than 30%, the structure is insufficiently treated in the axial zone of the billet and a segregation streamer is retained, this impairing the low-temperature impact toughness of the finished rolled strips. However, if the unit percentage reduction during the last pass of roughing-down is less than 20%, the structure formation in the low-alloy steel cannot be controlled to a sufficient extent, and the required mechanical properties are not provided.

The experiments showed that if the semifinished rolled stock thickness is less than 5.5 thicknesses of the finished rolled strip, finishing rolling is not capable of providing the low-temperature strain required for treating the metal structure and achieving the sufficient grain refinement in the finished rolled stock. However, if the semifinished rolled stock thickness is more than 7.5 thicknesses of the finished rolled strip, the excessive overall strain during finishing rolling reduces the toughness of the metal.

It should be noted that if the unit percentage reduction during the first pass of roughing-down is less than 30%, the structure of the semifinished rolled stock is insufficiently treated over its thickness which deleteriously affects the impact toughness of the finished rolled strips. However, if the unit percentage reduction during the last pass of roughing-down is more than 10%, the maximum allowed rolling force for the last stand of the rolling mill used can be exceeded, i.e., there is the risk of emergency.

If the finishing rolling temperature 7} _r is below the temperature designed for the technical solution provided herein the laminar cooling rate in the finishing rolling / coiling temperature range is insufficient for achieving the required strength. If the finishing rolling temperature T _fr is higher the designed level, the low-alloy steel of the chemical composition disclosed herein is in a temperature range which is unfavorable for straining, and this may impair the mechanical properties of the finished product.

The laminar cooling of the finished rolled strip on the discharge rolling table of the wide-strip rolling mill to the coiling temperature T _c = 585°C does not provide for the sufficiently high corrosion resistance and low-temperature toughness due to the excessive bainite content. However, coiling temperatures of above T = 670°C may lead to insufficient strength of the rolled stock.

The above analysis suggests that for the implementation of the technical solution provided herein, the required quality of rolled strips for longitudinally welded pipes is achieved by choosing the optimum process conditions and chemical composition of the steel and by adjusting the strain and temperature parameters of coiled strip rolling on the wide-strip rolling mill. However, if the variable process parameters fall beyond the limits specified for the implementation of said method, it may become impossible to provide the compliance of the finished rolled strips with the corrosion resistance and mechanical properties requirements as set forth above. Thus, the test data confirm the correctness of the technical solutions provided herein from the viewpoint of setting the acceptable limits of the process parameters for the method of high corrosion resistance low-alloy coiled strip production for longitudinally welded pipes. The technical and economical advantages of the invention disclosed herein are in that the temperature and strain conditions of rolling stock production disclosed herein provide for using to the full extent all the available hardening mechanisms for the low-alloy steel of the chemical composition disclosed herein, i.e., microstructure grain refinement, dislocation hardening, precipitation hardening and anisotropy of the structure and properties. The use of the method provided herein for the production of K52 (X60) strength grade 6- 12 mm thick coiled rolled stock with a high corrosion resistance will allow new products on wide-strip rolling mills.