High-strength weldable steel and its variants

 

(57) Abstract:

The invention relates to metallurgy, and more particularly to high strength welded steel. According to this invention a steel with low carbon content and a high content of Mn-Ni-Mo - traces of Ti contains elements such as Cu, V, Cr, Ca, V, etc. the Steel is tempered martensite/bainite mixed structure containing a microstructure of at least 60 vol.% tempered martensite obtained from precrystallization austenite having an average size of austenitic grains (d) not more than 10 μm. The amount of shares specified martensite and share banita equal to at least 90%. The formulas determine the measure of hardenability during hardening steel (P) depending on its chemical composition. Value (R) varies from 2.5 to 4.0. The thus obtained ultra high strength steel having ultimate tensile strength of at least 950 MPa (at least 100 standard AMI) and excellent low-temperature impact strength, impact toughness of the HAZ and weld ability in the field in cold areas. 3 s and 5 C.p. f-crystals, 1 Il., 6 table.

The invention relates to a heavy-duty steel having ultimate tensile strength, mentiroso be used for manufacture of pipelines for transportation of natural gas and crude oil, and as the material of steel, suitable for use when welding various tanks under pressure, and other industrial equipment.

In recent times the required strength of pipelines used to transport crude oil and natural gas over long distances, has increased due to (1) improve the efficiency of transportation by using higher pressure and (2) improve the efficiency styling by reducing the outer diameter and weight of trunk pipelines. Previously practical application received pipelines with the strength to h according to the American petroleum Institute (Americam Petroleum Institute) (at least 620 MPa for tensile strength tensile), however, the need for pipelines higher strength increased.

As steel for pipelines usually known steel with low carbon content, high content of Mn-Nb-(Mo)-(Ni) -traces - traces of Ti, which has a structure containing primarily fine-grained banit, however, the upper limit of the strength of this steel tensile maximum of 750 MPa. In this basic component system does not exist SVERKER the main containing banit, can never be achieved ultimate tensile strength of more than 950 MPa, and, in addition, low-temperature toughness is degraded, if the content of the martensite structure is increased.

Currently, the research method of manufacturing heavy-duty trunk pipelines on the basis of normal production technologies pipelines h (for example, "NKK Engineening Repont", N 138 (1992), page 24-31 and The th Offshore Mechanics and Aretic Engineering (1998), volume V, pages 179-185), however, believe that the production of pipelines X100 (ultimate tensile strength not less than 760 MPa) is the limit on these technologies.

In addition, the known high-strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, nitrogen, boron, copper, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit (JP, 2-250941 A, C, 22 C 38/00, C 21 D 8/02, C 22 C 38/32, 08.10.90.

In order to achieve and in the pipeline, still need to solve many problems, such as the balance between strength and low temperature toughness, ductility of the heat-affected zone (ETVA) when welding, welding in the field UkrAVTO (more X100).

When receiving a heavy-duty main pipes it is necessary to strike a balance between strength and low-temperature viscosity, to allow welding in the field, including welding and installation in cold regions and get a pipe with a tensile tensile strength at least equal to 950 MPa.

Obtaining the specified technical result is ensured according to the invention due to the fact that high-strength weldable steel is a measure of hardenability during tempering in the range from 2.5 to 4.0, depending on its chemical composition with the following formula:

< / BR>
where P is the measure of hardenability during hardening steel.

- conditional parameter, depending on the efficiency of boron B, and when B < 3ppm = 0 , and if B3 ppm = 1, and the microstructure of the steel contains at least about 60. % martensite obtained from precrystallization austenite, having an effective average size d of austenitic grains is not more than 10 μm, and the amount of shares specified martensite and share banita equal to at least 90%, at the following ratio, wt. %:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more the than 0.06

Nitrogen - 0,001 - 0,006

Bor - up 0,0020

Copper - 1.2

Chrome - 0.8

Vanadium 0.10

Iron and inevitable impurities - Rest

Such high-strength weldable steel can contain 6 wt.%: 0,0003 - 0,0020 boron, 0.1 to 1.2 copper, 0,1 - 0,8 chromium and 0.01 - 0.10 vanadium.

Additionally, the steel may further contain at least one of the following components, wt.%:

Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

According to the second variant implementation of the invention high strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, nitrogen, boron, copper, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit, characterized in that it has a measure of hardenability during hardening in the range from 2.5 to 4.0, depending on its chemical composition by the following formula P = 2,7 C - 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + 2Mo, where P is the measure of hardenability during hardening steel, the microstructure of the specified steel contains at least 60% of martensite obtained from precrystallization austenite, having an effective average size d of austenitic grains - not more than 10 μm, and the amount of the share of the decree is perod - 0,05-0,10

Silicon is not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0.01

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15-0,60

Niobium - 0,01-0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Nitrogen - 0,001 - 0,006

Bor - 0,0003 - 0,0020

Copper - 1.2

Chrome - 0.8

Vanadium 0.10

Iron and inevitable impurities - Rest

Such high-strength weldable steel may contain, in weight%: 0.01 to 0.10 vanadium, 0.1 to 1.2 copper, 0,1 - 0,8 chrome.

According to the third variant of high-strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, copper, molybdenum, niobium, titanium, aluminum, nitrogen, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit, characterized in that it has a measure of hardenability during hardening in the range from 2.5 to 4.0, depending on its chemical composition by the following formula P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + Mo + V - 1, where P is the measure of hardenability during hardening steel, and the steel microstructure contains at least 60 vol.% martensite obtained from the recrystallized austenite, having an effective average size of the austenite grains is not more than 10 μm, and the amount of the share of the decree is R> Carbon - 0,05-0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,3 - 1,0

Copper - 0,8 - 1,2

Molybdenum - 0,35-0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Nitrogen - 0,001 - 0,006

Chrome - 0.8

Vanadium 0.10

Iron and inevitable impurities - Rest

While steel may contain, in weight% vanadium 0,01 - 0,10, chrome 0,1 - 0,8.

Also, it is expedient if the steel further comprises at least one of the following elements, wt.%:

Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

Used herein, the terms "martensite" and "banit" characterize not only the martensite and banit themselves, but also include the so-called "tempered martensite" and "released banit" obtained by their leave.

In Fig. 1 shows the definition of the effective average size of austenitic grains (d).

The first distinctive feature of the invention is that the steel is of type low carbon steel with high Mn content (at least 1.7 percent), to which the composition is added Ni - Nb - Mo - traces Ti and microstructure contains fine-grained martensite, is m ore than 10 μm, and banit.

Low carbon steel with a high content of M-Nb-Mo was previously well known as steel for pipelines with melkoigolchaty structure, however, the upper limit of its tensile strength is a maximum of 750 MPa. In this basic system, the chemical composition does not exist heavy-duty steel with fine-grained released a mixed structure of martensite/beinit. Thought for tempered martensite/bainite structure Nb-Mo steel never reach the limit of tensile strength higher than 950 MPa, and furthermore that the low-temperature toughness and weldability in the field is also insufficient.

First will be explained the microstructure of the steel according to the present invention.

In order to achieve and in respect of the ultimate tensile strength of at least 950 MPa, the microstructure of the steel material must contain a specified amount of martensite, and its share should be at least 60%. If the proportion of martensite is not more than 60%, sufficient strength cannot be achieved and, moreover, it becomes difficult to provide excellent low-temperature impact strength (most desired proportion martinotti/low-temperature toughness cannot be achieved even when the proportion of martensite at least 60%, if the rest of the structure is not suitable. Therefore, the sum of the fractions of martensite and share banita must be at least 90%.

Excellent low-temperature toughness can be achieved not always even then, when the microstructure is limited as described above. In order to achieve excellent low-temperature toughness, it is necessary to optimize austenitic structure before transformation (prior austenitic structure and effectively improve the final structure of the steel material. For this reason, in the present invention prior austenitic structure is limited precrystallization the austenite, and the average size of the grains (d) does not exceed 10 μm. It was found that excellent balance between strength and low temperature toughness can be obtained even for a mixed structure of martensite and bainite in Nb-Mo steel, which in the past has laid poor low-temperature impact strength as a result of such restrictions.

The reduction of grain size precrystallization austenite to the size of the small grains is especially effective for improving the low temperature toughness of the steel type Nb-Mo of the present invention. For temperature transition in a shock test with a V-shaped incision in Sharpie (Charpy), the average grain size should be less than 10 μm. Here the effective average size of austenitic grains is defined as shown in Fig. 1, and the measurement of the size of austenitic grains take into account the band deformation and twin boundary, which have the same functions as the boundary of the austenite grain. More specifically, to determine d the entire length of the straight line drawn in the direction of thickness of the steel plate, divided by the number of points of intersection with the boundaries of austenite grains existing on this straight line. It was found that the determined average size of austenitic grains are extremely closely correlated with low temperature toughness (the transition temperature at impact test Charpy).

It was also found that when strictly control the chemical composition (adding a large number of Mn-Nb - a large number Mo) steel material and its microscopic structure (without recrystallization of austenite) as described above, the fracture under impact test on Charpy separates, and so on, and can further improve the transition temperature fracture (inaudible). Separation is a phenomenon lamellar exfoliation, occurring Enset degree triaxial stress at the vertex of brittle cracks and improves the properties to stop the distribution of brittle cracks.

The second distinctive feature of the present invention is that (1) steel refers to the type of low carbon steel with a high content of Mn to which the composition is added Ni-Mo-Nb-traces B-traces Ti and (2) its microscopic structure mainly contains fine-grained martensitic structure obtained by transformation from precrystallization austenite having an average size of austenitic grains (d) not more than 10 μm.

The third distinctive feature of the present invention is that (1) steel refers to the type of low carbon steel with high Mn content (at least 1.7 percent) precipitation hardening by allocating Cu which contains from 0.8 to 1.2% Cu and to which the composition is added Ni-Nb-Cu-Mo - traces Ti and (2) its microscopic structure mainly contains fine-grained martensite and banit obtained by transformation from precrystallization austenite having an average size of austenitic grains (d) not more than 10 μm.

Steel precipitation hardening through the allocation of Cu in the past used as high-strength steels (class limit tensile strength 784 MPa) for tanks used under pressure, however, was not found PR is so, because steel precipitation hardening by allocating Cu can easily gain strength, but its low-temperature impact strength is insufficient for the pipeline.

As for the low-temperature toughness, it is extremely important characteristics of halting the spread of cracks with the settings on the appearance of brittle fracture in pipelines. In the case of conventional steel with hardening due to the selection of Cu, the characteristics of the occurrence of brittle fracture parameters sharpies are largely satisfactory, however, the characteristics of the termination of brittle fracture insufficient. This is so because (1) the improvement of the microstructure is not enough and (2) the so-called "separation" that occur on the fracture under impact test on Charpy not used. (Separation is a phenomenon lamellar exfoliation occurring in the fracture under impact test on Charpy, and so forth , parallel to the surface of the plate and believe that it reduces the degree of triaxial stresses at the far end of brittle cracks and improves the properties to stop the distribution of brittle cracks).

However, satisfactory nicotania the AK was described above, in order to achieve excellent low-temperature toughness, it is necessary to optimize austenitic structure before transformation and effectively improve the final structure of the steel material. For this reason, in the present invention prior austenitic structure is limited precrystallization the austenite, and the average size of the grains (d) does not exceed 10 μm. Thus, it was found that extremely excellent balance between strength and low temperature toughness can be obtained even for a mixed structure of martensite and bainite in Nb-Cu steel, for which in the past has laid poor low-temperature impact strength.

Improvement of grain size precrystallization austenite is especially effective for improving the low temperature toughness of the steel type Nb-Cu according to the present invention. In order to achieve proposed low-temperature toughness (junction temperature shock test with a V-shaped cut on the Sharpie is not higher than -80oC), the average grain size should be less than 10 μm. Here the effective average size of austenitic grains is defined as shown in Fig. 1, and the measurement of the size of austenitic grains take in entogo grain. More specifically, to determine d the entire length of the straight line drawn in the direction of thickness of the steel sheet, divided by the number of points of intersection with the boundaries of austenite grains existing on a straight line. It was found that the determined average size of austenitic grains are extremely closely correlated with low temperature toughness (the transition temperature at impact test on Charpy).

It was also found that when strictly control the chemical composition of the steel material (adding a large number of Mn-Nb-Mo-Cu) and the form of its microscopic structure (without recrystallization of austenite) as described above, the fracture under impact test on Charpy separates, and so on , and can further improve the transition temperature of a break.

In order to obtain ultra-high strength in relation to the ultimate tensile strength of at least 950 MPa, the microscopic structure of steel must contain a specified amount of martensite, and its share should be at least 90%. If the proportion of martensite is less than 90%, it is impossible to achieve sufficient strength and, moreover, it becomes difficult to provide a satisfactory nicotinoyl so, as described above, there can be obtained a steel material having the expected characteristics. In order to achieve this goal, it is necessary to limit chemical compounds together with microstructure.

Next will be explained the reasons for the restrictions of elements.

The content of C is limited in the range of 0.05-0.10%. Carbon is extremely effective to improve the strength of steel, and to achieve the required strength in the martensitic structure requires at least 0.05% of C. However, if the C content is too large, markedly deteriorate the low-temperature toughness of both the base metal and HAZ, and weld in the field. Therefore, the upper limit of C is as of 0.10%. However, the value of the upper limit is preferably limited to 0.08%.

Si is added for deoxidation and improve durability. However, if the added amount is too large, the HAZ toughness and field weldability conditions are significantly worse. Therefore, set the upper limit of 0.6%. It is possible to achieve a sufficient deoxidation of steel with Al or Ti, and is not always necessary to add Si.

Mn is an indispensable element for turning microscopic structure balance between strength and low temperature toughness, and its lower limit is 1.7%. However, if the added amount of Mn is too large, increase the hardenability of steel, so not only worsen HAZ toughness and weldability in the field, but also increases the Central segregation in slab continuous casting and deteriorating the low-temperature toughness of the base metal. Therefore, the upper limit is set at 2.5 %.

The purpose of the addition of Ni is to improve low carbon steel according to the present invention without deteriorating the low-temperature toughness and weldability in the field. In comparison with addition of Cr and Mo, the addition of Ni leads to less formation of hardened structures and prokatnoe structure (in particular the Central strip segregation in slab continuous casting), which hurts low-temperature toughness, and, in addition, it was found that the addition of trace amounts of Ni, at least 0.1% is also effective for improving the impact toughness of the HAZ. (From the point of view of the impact toughness of the HAZ, especially effective amount of added Ni is at least 0.3 percent). However, if the added amount is too large, then deteriorates not only the economy, but also the level of 1.0%. The addition of Ni is also effective to prevent Cu - cracking during continuous casting and during hot rolling. In this case, the Ni must be added in a quantity of at least 1/3 of the number of Cu.

Mo added to improve the hardenability of steel and to obtain a given structure, mainly containing martensite. In the case of B-containing steels, the influence of Mo on the hardenability increases and the factor in front of Mo removal of the P-value becomes equal to 2 for B-containing steels in comparison with 1 in steel, not B. Therefore, the addition of Mo is especially effective in the case of B-containing steels. In case of joint presence, Nb, Mo suppresses the recrystallization of austenite during controlled rolling, and it is also effective to improve the austenitic structure. To achieve such effects should at least 0.15% of Mo. However, adding excessive amounts of Mo causes deterioration of the HAZ toughness and weldability in the field, and, moreover, eliminates the effect of B on the improvement of the hardenability. Therefore, its upper limit is set at 0.8%.

In addition, the steel of the present invention contains from 0.01 to 0.10% of Nb and 0.005 to 0,030% Ti as mandatory elements. When COMESA thus its structure, but also making a great contribution to the dispersion hardening and increasing the hardenability, but also makes the steel more viscous. The effect of improving the hardenability can be synergistically increased, especially when jointly present Nb and B. However, if the added amount of Nb is too large, it harms the HAZ toughness and weldability in the field. Therefore, its upper limit is set at 0.10%. On the other hand, the addition of Ti leads to the formation of TiN, limits the coarsening of austenite grains during reheating and austenitic grains in the HAZ, improves microscopic structure, and improves the low temperature toughness of both the base metal and HAZ. They also have the function of fixing the solid solution N that separately affect the effect of B on the improvement of the hardenability, in the form of TiN For this purpose it is preferable to add at least a 3.4 N (the weight. %) Ti. When the Al content is low (such as less than 0,005%), Ti forms an oxide acts as the germ of the education of the ferrite inside the grains in the HAZ, and also improves the structure of the HAZ. In order to cause the manifestation of such effects TiN must be added at least 0.005% of Ti. If the Ti content is too large, about the tsya. Therefore, its upper limit is set at the level of 0.03%.

Al is usually contained in steel as a deoxidizer, and he also has the effect of improving the structure. However, if the Al content exceeds 0.06% of the increase of non-metallic inclusions of the type of alumina, and they spoil the purity of the steel. Therefore, its upper limit is set at the level of 0.06%. The deoxidation can be made by Ti or Si, and Al need to add not always.

N forms TiN, limits the coarsening of austenite grains during re-heating of the slab and austenitic grains in the HAZ, and also improves the low temperature toughness of both the base metal and HAZ. The minimum quantity required for this purpose, is 0.001%. However, if the N content is too large N leads to surface defects on the slab, worsening the impact toughness of the HAZ and the reduction effect In improving the hardenability. Therefore, its upper limit should be limited 0,006%.

In the present invention, the content of P and S as impurity elements are installed at the level of 0.015% and 0.003%, respectively. The main reason is to further improve the low temperature impact strength as the base metal and HAZ. The decrease with the Ren and improves the low temperature toughness. The reduction in the content of S reduces the content of MnS, which is lengthened by hot rolling, and improves ductility and toughness.

Next will be explained the purpose of the addition of B, Cu, Cr and V

The main purpose of adding these elements in addition to basic chemical compounds is to further improve the strength and toughness, as well as to increase the size of the steel material, which can be obtained without deteriorating the excellent characteristics of the present invention. So, naturally, the added amount of these elements should be limited.

An extremely small number of B dramatically improves the hardenability of steel. Therefore, B is essentially a necessary element in the steel of the present invention. It has the effect corresponding to the value of 1 in the value of P, which will appear hereinafter, that is, 1% Mn. In addition, B enhances the effect of Mo on improving the hardenability and synergistically improves the hardenability in the joint presence with Nb. To achieve such effects should at least 0,0003% B. on the other hand, adding in excessive quantities, it not only affects the low-temperature impact strength, but also in some cases destroys the effect the effect of Cu is to improve the strength of low carbon steel according to the present invention without deteriorating the low-temperature toughness. In comparison with the addition of Mn, Cu and Mo, the addition of Cu does not lead to the formation of utverzhdenii patterns, which are harmful for low-temperature toughness in the rolled structure (in particular, in the Central zone of the segregation of the slab), and it was found that it increases strength. However, adding excessive Cu deteriorates the weldability in the field and toughness of HAZ. Therefore, its upper limit is set at the level of 1.2%.

Cu increases the strength as the base metal and the welding part, however, when the added amount is too large, the HAZ toughness and field weldability conditions are significantly worse. Therefore, the upper limit of Cu content is 0.8%.

V has basically the same effect as Nb, but its effect is weaker than that of Nb. However, the effect of addition of V in a heavy-duty steel great, and composite addition of Nb and V makes the excellent characteristics of the steel according to the present invention even more noticeable. From the point of view of the HAZ toughness and weldability field added valid number will produce explained the purpose of adding Ca, REM and Mg.

Ca and REM regulate the form of sulfide (MnS) and improve the low temperature impact strength (increase the absorbed energy in the test according to Charpy and so on ). However, if the content of Ca or REM does not exceed 0,6001%, the practical effect may not be achieved, and if the Ca content exceeds 0,006% or the content of REM exceed 0.02%, produces large quantities of CaO-CaS and REM-CaS and they turn into large clusters and large inclusions and not only spoil the cleanliness of the steel, but also have an adverse effect on weldability in the field. Therefore, the upper limit of the added amount of Ca is limited to About, 006% and the upper limit of the added amount of REM is limited to 0.02%. By the way, heavy-duty trunk pipelines is especially effective to reduce the content of S and About 0.001% and 0.002%, respectively, and set the ratio of the ESSP = (Ca) [1 - 124(O)] /1,25 S to 0.5 ESSP 10,0.

Mg forms a finely dispersed oxide limits the enlargement of the grains in the heat-affected zone during welding and improves toughness. If the amount of additive is less than about 0.001%, it is impossible to observe the improvement of impact strength, and if it exceeds 0,006%, formed of coarse oxides, and the toughness deteriorates.

In addition to ogranichevatsya P, namely, by the Way, takes the value O when B < 3 ppm and a value of 1 when B 3ppm. This is done in order to implement the proposed balance between strength and low temperature toughness. The reason why the lower limit value P is set to 1.9, is that you need to get the strength of at least 950 MPa and excellent low-temperature impact strength. The upper limit on the value of P is limited to 4.0 in order to preserve the excellent HAZ toughness and mounting weldability.

When you get high-strength steel with excellent low temperature toughness according to the present invention preferably uses the following production method.

After the steel slab with a chemical composition according to the present invention was re-heated to a temperature in the range from 950oC to 1300oC, it is subjected to hot rolling so that the total compression during rolling at a temperature not higher than 950oC were at least 50%, and the final temperature of hot rolling was not below 800oC. Then carry out cooling at a rate not lower than 10oC/s to an arbitrary temperature below 500oC. If necessary, osuwa steel slab so determine, to sufficiently formed a solid solution of the elements, and the upper limit defined by condition, with no noticeable coarsening of crystal grains.

Temperature below 950oC characterizes the temperature zone without recrystallization, and in order to obtain the desired size of the small grains, the value of the total compression during rolling at least 50%. The final temperature of hot rolling is limited by the fact that it must be not lower than 800oC, when not formed beinit. Then carry out the cooling speed of at least 10oC/sec, to form martensite and bainite structure. Since the transformation is basically ends at 500oC, produces cooling to a temperature below 500oC.

In addition, you can leave the steel of the present invention at a temperature below the point Ac1. This leave may be appropriate to restore ductility and toughness. Vacation does not change the proportion of the microstructure as such, does not impair the excellent characteristics of the present invention and has the effect of narrowing the width of the softening in the heat-affected zone during welding.

Next will be the op who were by melting at laboratory scale (50 kg ingot, thickness 120 mm) or BOF method of continuous casting, the thickness of 240 mm). These slabs were subjected to hot rolling, the steel plate having a thickness of from 15 to 28 mm in various conditions. Investigated the mechanical properties of each of the rolled so plates and their microscopic structure.

Mechanical properties (yield strength: YS, ultimate tensile strength: TS, absorbed energy at -40oC impact test on Charpy: VE-40and the transition temperature vTrS) steel plates was measured in the direction orthogonal to the direction of rolling. Plasticity HAZ (absorbed energy at -20oC impact test on Charpy: VE-20) was evaluated using specimens with simulated HAZ (maximum temperature: 1400oC, the cooling time from 800 to 500oC [t800-500] 25 seconds). Weldability in field conditions was estimated as the lowest temperature pre-heating is necessary to prevent low-temperature cracking in HAZ method tests the tendency to form cracks for seam with the Y-gap (JIS C3158) (method of welding: metal arc welding electrode, shielding gas, welding electrode: ultimate tensile strength of 100 MPa, dive torches etc is emery. Steel plate obtained by the present invention have an excellent balance between strength and low temperature toughness, toughness HAZ and weld in the field. In contrast, comparative examples were substantially worse in their characteristics, as the chemical composition or their microscopic structures were inappropriate.

As in steel N 9 C content was too high, the absorbed energy in Charpy for the base metal and HAZ was low, and was also high temperature pre-heating during welding. As in steel No. 10 was not added Ni, low-temperature toughness of the base metal and HAZ was worse. Because steel N 11 added amount of Mn and P-value were too large, the low temperature toughness of the base metal and HAZ was worse, and preheating temperature during welding was also too high.

As in steel No. 12 was not added Nb, strength was insufficient, the size of the austenite grains was large, and toughness of the base metal was worse.

Example 2. Slabs with different chemical composition were obtained by melting at laboratory scale (bars 50 to the rolling of the steel plate, having a thickness of from 15 to 25 mm in various conditions. Investigated various properties of rolled thus plates and their microscopic structure. Mechanical properties (yield strength: VS, ultimate tensile strength: TS, absorbed energy at -40oC test on Charpy: VE-40and the transition temperature of 50% destruction VTrS) were measured in the voltage orthogonal to the direction of rolling. The impact toughness of HAZ (absorbed energy at -40oC test on Charpy: VE-40was estimated by HAZ reproduced by a playback device heat cycle (maximum temperature: 1400oC, the cooling time from 800 to 500oC: [t800-500] : 25 seconds). Weldability in field conditions was assessed by the low temperature pre-heating is necessary to prevent low-temperature cracking in HAZ method tests the tendency to form cracks for seam with the Y-gap (JIS C3158) (method of welding: metal arc welding electrode, shielding gas, welding electrode: ultimate tensile strength of 100 MPa, heat input: 0.3 kJ/mm, the hydrogen content in the weld metal: 3 cm3/100 g of metal).

In table. 1 and 2 are examples. Steel is DN strength and low temperature toughness, impact HAZ toughness and weldability in the field. In contrast, the comparative steel were clearly and significantly worse on all characteristics, as chemical components or their microscopic structures were inappropriate.

Example 3. Slabs with various components of the steel were obtained by melting at laboratory scale (50 kg, thickness 120 mm) or BOF method of continuous casting (thickness 240 mm). These slabs were subjected to hot rolling, the steel plate having a thickness of from 15 to 30 mm under various conditions. Investigated various properties of rolled thus plates and their microstructure.

Mechanical properties (yield strength: VS, ultimate tensile strength: TS, absorbed energy at -40oC impact test on Charpy: VE-40and the transition temperature VTrS ) were investigated in a direction orthogonal to the direction of rolling.

The impact toughness of HAZ (absorbed energy at -20oC impact test on Charpy: VE-20was assessed by HAZ reproduced by a playback device heat cycle (maximum temperature: 1400oC, the cooling time from 800 to 500oC [t800-500] : 25 seconds).

In table. 1 and 2 are examples. Steel plate obtained by the present invention showed an excellent balance between strength and toughness, toughness HAZ and weld in the field. In contrast, the comparative steel were significantly worse in all its characteristics, as chemical components or microscopic structures were inappropriate.

As in steel N 9 the content of S was too large, the absorbed energy in Charpy for the base metal and HAZ was low, and the temperature of pre-heating during welding was high. As in steel N 10 the content of Mn and P were too high, lower the temperature of the base metal and HAZ was worse, and preheating temperature during welding was high.

As in steel N 11 the content of S was too large, the absorbed energy in the metal-based and HAZ was low.

Industrial applicability.

In accordance with the present invention, it becomes possible to steadily produce large if the 50 MPa and greater than the X100 standard NAFA), which have excellent low-temperature toughness and weldability in the field. The result can significantly improve the security of the pipelines, and the effectiveness of transportation pipeline and work efficiency can be improved radically.

1. High-strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, nitrogen, boron, copper, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit, characterized in that it has a measure of hardenability during hardening in the range from 2.5 to 4.0, depending on its chemical composition by the following formula where P is the measure of hardenability during hardening steel, is a conditional parameter that depends on the efficiency of boron B, and when B < 3 ppm = 0, and when B is 3 ppm = 1, and the microstructure of the steel contains at least 60 vol.% martensite obtained from precrystallization austenite, having an effective average size d of austenitic grains is not more than 10 μm, and the amount of shares specified martensite and share banita equal to at least 90% at the following ratio, wt.%:

Carbon - 0,05 - 0,10

Silicon is Not sick - 0,15 - 0,60

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Nitrogen - 0,001 - 0,006

Bor - Up 0,0020

Copper - 1.2

Chrome - 0.8

Vanadium - 0.1

Iron and inevitable impurities - Rest

2. High-strength weldable steel under item 1, characterized in that it contains, in weight. %: 0,0003 - 0,0020 boron, 0.1 to 1.2 copper, 0,1 - 0,8 chromium and 0.01 - 0.1 vanadium.

3. High-strength weldable steel according to any one of paragraphs.1 and 2, characterized in that it further comprises at least one of the following elements, wt.%:

Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

4. High-strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, molybdenum, niobium, titanium, aluminum, nitrogen, boron, copper, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit, characterized in that it has a measure of hardenability during hardening in the range from 2.5 to 4.0, depending on its chemical composition by the following formula P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + 2Mo, where P is the measure of hardenability during hardening steel, the microstructure of the specified steel contains at least about 60. % martensite obtained from precrystallizer what about the martensite and share bainite is at least 90% in the following ratio of components, wt.%:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0.01

Sulfur is Not more than 0.003

Nickel - 0,1 - 1,0

Molybdenum - 0,15 - 0,60

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Nitrogen - 0,001 - 0,006

Bor - 0,0003 - 0,0020

Copper - 1.2

Chrome - 0.8

Vanadium 0.10

Iron and inevitable impurities - Rest

5. High-strength weldable steel under item 4, characterized in that it contains, wt%: 0.01 to 0.10 vanadium, 0.1 to 1.2 copper, 0,1 - 0,8 chrome.

6. High-strength weldable steel containing carbon, silicon, manganese, phosphorus, sulfur, Nickel, copper, molybdenum, niobium, titanium, aluminum, nitrogen, chromium, vanadium, iron and unavoidable impurities, having a microstructure containing martensite and banit, characterized in that it has a measure of hardenability during hardening in the range from 2.5 to 4.0, depending on its chemical composition by the following formula P = 2,7 C + 0,4 Si + Mn + 0,8 Cr + to 0.45(Ni + Cu) + Mo + V - 1, where P is the measure of hardenability during hardening steel, the microstructure of the specified steel contains at least about 60. % martensite obtained from precrystallization austenite, having an effective average size of austenitic grains not benii components, wt.%:

Carbon - 0,05 - 0,10

Silicon is Not more than 0.6

Manganese - 1,7 - 2,5

Phosphorus - Not more than 0,015

Sulfur is Not more than 0.003

Nickel - 0,3 - 1,0

Copper - 0,8 - 1,2

Molybdenum - 0,35 - 0,50

Niobium - 0,01 - 0,10

Titanium - 0,005 - 0,030

Aluminum is Not more than 0,06

Nitrogen - 0,001 - 0,006

Chrome - 0.8

Vanadium 0.10

Iron and inevitable impurities - Rest

7. High-strength weldable steel under item 6, characterized in that it contains, wt%: vanadium 0,01 - 0,10, chrome 0,1 - 0,8.

8. High-strength weldable steel according to any one of paragraphs.4 to 7, characterized in that it further comprises at least one of the following elements, wt.%:

Calcium - 0,001 - 0,006

REM IS 0.001 - 0.020

Magnesium - 0,001 - 0,006

Priority points:

06.02.95 - PP.1, 2, 3, 6, 8;

26.01.95 - PP.4, 5.

 

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FIELD: ferrous metallurgy.

SUBSTANCE: invention provides round-profiled iron smelted from alloyed steel composed of, wt %: carbon 0.06-0.11, manganese 0.30-0.9, silicon 0.001-0.15, boron 0.0005-0.0050, vanadium 0.005-0.08, aluminum 0.02-0.06, titanium 0.01-0.04, sulfur 0.005-0.020, nitrogen 0.005-0.015, calcium 0.001-0.010, iron and unavoidable impurities - the balance. When following relationships are fulfilled: Ti/48+Al/27-N/14 ≥ 0.6 x 10-3; Mn+5.0C ≥ 0.80; Ca/S ≥ 0.065, rolled iron has following characteristics: maximum degree of pollution with nonmetal inclusions, in particular sulfides, oxides, silicates, and nitrides, does not exceed 3 points for each type of inclusions; longitudinally uniform spheroidized structure composed of at least 60% grainy perlite; effective grain size 5-10 points; diameter 10-16 mm; carbon-free layer not exceeding 1.0% of diameter; cold setting value at least 1/3 height; throughout hardenability in circles up to 16 mm in diameter; point of maximum load not higher than 500 MPa; relative elongation at least 22%; and relative contraction at least 70%.

EFFECT: ensured optimal conditions for cold die forging of high-strength geometrically complex fastening members and simultaneously improved steel hardenability characteristics.

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