The smelting technology of high strength alloy steel
Technical field
The present invention relates to field of metallurgy, be specially a kind of high strength alloy steel smelting technology for drill bit and pump head body material.
Background technology
The petroleum prospecting drill bit has tooth-wheel bit, winged scraping bit, diamond drill machine first-class multiple, and wherein accounting for various countries' petroleum drilling usage quantity more than 90% is tooth-wheel bit, and employed steel generally belong to low carbon high-strength degree alloy carburizing steel, as Ni-Cr-Mo system and Ni-Mo are.
In the oil drilling process, it is bigger influenced by geologic condition, and especially Fu Za landforms require also high to drill rod materials.The U.S. target 4330V material of general employing is made, and is equivalent to domestic trade mark 30CrNiMoV material, and this material also is used for the pump head of multi-cylinder ram pump.Therefore, this material must possess good comprehensive mechanical property, not only will guarantee by chemical ingredients and the specific metallographic structure of strictness, and wants the interior microscopic defective of strict control steel.
After strength grade improved, the Composition Control scope was generally narrow than common structural alloy steel, requires molten steel purity height.For micro impurity element, not only want strict control content, and exist position and form that particular requirement is also arranged to it.As: the content requirement control of sulphur is below 0.015%, and requirement in the past is 0.04%; Because sulfide inclusion causes stress concentration, form formation of crack, so the shape of sulfide also is necessary for the sphere of dispersion.Outside sulphur removal, the phosphorus, gas is all influential to the fatigue resistance of steel and impelling strength etc.Therefore, the smelting of this class high-strength steel generally will be passed through technologies such as electric arc furnace, ladle refining furnace (LF stove), vacuum outgas (VD) and esr.The cost of Ye Lianing is higher like this.
In the steel and alloy smelting process, in the steel-making stage raw material is molten into molten steel, then; In the treating processes of oxidation period, need to add oxygenant (as aerating oxygen, add iron ore or iron scale) with impurity element and foreign gases such as wherein carbon of oxidation removal, phosphorus.Generally, need to add excessive oxygenant, therefore can form oxide inclusion (based on FeO).Oxide inclusion is harmful to steel, and it is hot-short to cause resulting iron and steel to take place easily.Usually need add reductor again and carry out deoxidation treatment, be i.e. reduction period in the smelting iron and steel.
The kind of reductor has a lot, and commonly used have ferrosilicon, manganese steel, manganese-silicon, an aluminium etc., and contains alloying elements such as chromium, boron, phosphorus, nickel, titanium as reductor and alloying material.
The deoxidation treatment technology of carrying out in ladle refining furnace (LF stove) mainly contains bulk deoxidation and diffusive deoxidation etc.
Bulk deoxidation: in molten steel, directly add reductor or alloy deoxidation material, make it to form oxide compound; This oxide compound come-up enters slag.This method deoxidation speed is fast, but the inclusion that produces is more, and inclusion is blended in and is difficult in the molten steel removing.Generally before bulk deoxidation, can in molten steel, add the thin slag that slag making materials manufacturings such as a small amount of fluorite, calcium oxide swim in molten steel surface, be used for the inclusion of absorption molten steel.
Diffusive deoxidation: the content of ferric oxide is higher in slag, can add reductive agents such as carbon dust, silica flour, aluminium powder in slag, is used for reduced steel slag, and the FeO content in the slag is reduced, and the oxygen that is conducive in the molten steel is diffused in the slag.
In the bulk deoxidation process, reductor material commonly used has ferromanganese, ferrosilicon and aluminium, and the price of these three kinds of materials is lower.Generally speaking, reductor is converted into silicon, manganese and aluminium element, with the amount ratio of molten steel be 0.17%~0.37%, 0.3%~0.6% and 0.05%~0.10%.In the existing processes, reductor ferromanganese, ferrosilicon and aluminium add in the molten steel respectively and react; According to the power to oxygen affinity, addition sequence is ferromanganese, ferrosilicon and aluminium, owing to a little less than the avidity of manganese and oxygen, need the long time of reaction, therefore adds at first; Aluminium is comparatively active, and the reaction times is shorter.Because the temperature of molten steel generally about 1600 ℃, adds above-mentioned reductor respectively separately in smelting process, just can not form low-melting liquid macrobead (Al
2O
3, SiO
2Be respectively 2050 ℃, 1713 ℃ and 1785 ℃ with the fusing point of MnO, be higher than liquid steel temperature), these deoxidation productss exist with solid state in molten steel, are blended in the molten steel, are difficult to come-up and enter slag to break away from molten steel.
Therefore, need be improved existing technology, make reductor can form light specific gravity, fusing point is low, particle is big deoxidation products, form slag in order to break away from the molten steel come-up, thereby molten steel is purified.
Owing to do not find good reductor material, its deoxidation all to adopt conventional reductor, carry out deoxidation respectively separately as ferromanganese, ferrosilicon, aluminium.Can't make molten steel obtain further to purify like this, thereby be difficult to reach and satisfy the client to the requirement of special staple product technical quality,
Summary of the invention
The present invention aims to provide a kind of smelting technology of the high strength alloy steel for the manufacture of drill bit and pump head body material.
Technical scheme is: adjust the order that alloy material adds, the first refining stage of electric arc furnace adds Mo and Ni, and increases decarburized amount; LF stove refining stage uses novel composite deoxidant, and purify hot metal improves mechanical property; At the preceding adding aluminium of vacuum deaeration refining (VD), promote the molten steel alloying, make the molten steel alloying process more abundant, can reduce oxide inclusion again.
A kind of smelting technology of high strength alloy steel comprises the steps:
(1) the clean steel scrap of low-sulfur phosphorus is melted smelting in electric arc furnace and carry out the oxidation and decarbonization dephosphorization, and add Mo and Ni, and dephosphorization; In the clean steel scrap of described low-sulfur phosphorus, iron level 〉=98wt%, carbon content≤0.2wt%, sulphur and phosphorus content difference≤0.05wt%;
The method of decarburization dephosphorization is: add little muck or iron scale at the bottom of electric arc furnace, and add lime, add the clean steel scrap of low-sulfur phosphorus again, carburetting to 0.7%~1.0%, fusing; The amount ratio of the clean steel scrap of the little muck that adds at the bottom of the described electric arc furnace or iron scale and low-sulfur phosphorus is 10~25kg/ ton, and the amount ratio of the clean steel scrap of the lime that adds at the bottom of the electric arc furnace and low-sulfur phosphorus is 20~30kg/ ton; Described little muck is the iron ore that contains 50wt%~70wt% ferric oxide;
Oxygen blast then, and in molten steel, add little muck or iron scale, add lime simultaneously, smelted 1~1.5 hour down at 1530~1600 ℃; The amount ratio of the clean steel scrap of described little muck or iron scale and low-sulfur phosphorus is 10~20kg/ ton, and the amount ratio of the clean steel scrap of lime and low-sulfur phosphorus is 15~25kg/ ton; In the described oxidation and decarbonization dephosphorization step, decarburized amount 〉=0.4%
(2) liquid steel temperature after step (1) dephosphorization and the decarburization rises to 1700~1750 ℃ and pours in the ladle refining furnace, after slag making forms thin slag, adds composite deoxidant and carries out pre-deoxidation, and composite deoxidant and molten steel amount ratio are 4~5 kilograms/ton;
With lime (calcium oxide) and fluorite slag making, with the amount ratio of molten steel be 10~15 kilograms/ton and 5~8 kilograms/ton;
Described composite deoxidant is the alloy that contains manganese, aluminium, silicon, and wherein the content of manganese is 16~20%, and the content of silicon is 8~10%, and the content of aluminium is 8~10%, all is weight percentage;
(3) in molten steel, add Cr and Mn, and add reductor on the slag surface, 1550~1650 ℃ of following refinings 1~1.5 hour, utilize LF furnace bottom blown inert gas to stir simultaneously;
Described reductor is carbon dust and AD powder, perhaps calcium carbide and AD powder, and perhaps carbon dust, calcium carbide and AD powder, the amount ratio of AD powder and molten steel is 3~5 kilograms/ton, carbon dust or calcium carbide powder, perhaps the amount ratio of carbon dust and calcium carbide total amount and molten steel is 1.5~3 kilograms/ton;
(4) in molten steel, add Si, continue to smelt 5~10 minutes;
(5) cast of coming out of the stove is handled in vacuum outgas;
The method of vacuum outgas is: rapidly liquid steel temperature being risen to more than 1680 ℃, then ladle is moved to degassing station, is to handle 20~30 minutes under the state of 4~67pa in vacuum tightness; Described pouring technology condition is: under 1540~1560 ℃ ladle hung degassing station, before cast, uses the air in the replacement dies such as rare gas element such as argon gas, and blanketing with inert gas in casting process;
Wherein, by weight, add Mo, Ni, Mn, Cr, Si in the molten steel after, the weight percent content of each alloying element is respectively Mo0.42%~0.60%, Ni2.40%~2.80%, Mn0.42%~0.80%, Cr0.9%~1.10% and Si0.15%~0.35%.
In the step (2), preferred, in the composite deoxidant, the weight ratio of manganese, silicon and aluminium is 1.9~2.05:0.95~1.05:1; Wherein the weight percent amount of sulphur and phosphorus all is no more than 0.05%, carbon content≤0.2%; Preferred, the content of manganese is 18%, and the content of silicon is 9%, and the content of aluminium is 9%.
The preparation method of this composite deoxidant comprises the steps:
(A) proportioning of pressing manganese, silicon and aluminium is mixed ferromanganese, ferrosilicon, aluminium and soft steel;
The content of manganese is 70%~80% in the described ferromanganese, and the content of silicon is 45%~75% in the ferrosilicon, all is weight percentage the iron level of described soft steel 〉=95%, carbon content≤0.2%, the content of sulphur and phosphorus≤0.05%;
(B) the mixed raw material of step (A) is melted in 1350~1400 ℃ of following mixed smeltings, casting again.
This composite deoxidant is used for deoxidation of molten steel and purification, and deoxidation effect is good, and inclusion is residual few, and molten steel is more pure.Formed deoxidation products fusing point is far below 1270 ℃ after carrying out deoxidation, and the particle diameter of the deoxidation products inclusion particle of generation is big, light specific gravity, is easy to float in the slag.
According to the stoke principle, the ascent rate v calculation formula of deoxidation products is:
V=2/9×g×(P
0-P
1)/η×r
2
Wherein, P
0And P
1Be respectively the proportion (kg/m of molten steel and deoxidation products
3), η is molten steel viscosity, g is universal gravity constant 9.8m/s
2, r is the radius of deoxidation products particle.
This shows that r has the greatest impact for v's; At P
0, P
1, η, g all be defined as under the situation of constant, ascent rate v depends on the particle diameter of deoxidation thing.
This composite deoxidant, formed deoxidation products fusing point was far below 1270 ℃ after the adding molten steel carried out deoxidation, the liquid particle of the form that exists in molten steel for easily reuniting rises to the molten steel top layer easily, rather than is suspended in the fine-powder shape solid particulate in the molten steel; The particle diameter of deoxidation products inclusion particle is big, light specific gravity, and ascent rate is obviously accelerated, and is easy to come-up and enters in the slag, makes molten steel be purified.This composite deoxidant not only deoxidation effect is good, and the inclusion that forms after the deoxidation residual remarkable reduction in molten steel, the product high purity, thus satisfy the client to the requirement of high quality high standard steel.By the steel ingot that this composite deoxidant comes out, its quality in addition can with match in excellence or beauty through the steel ingot of electroslag.
In the steel ingot that the present invention obtains, each component concentration (weight percent) is:
Mn0.42%~0.80%,Si0.15%~0.35%,Cr0.90%~1.10%,Mo0.42%~0.60%,Ni2.40%~2.80%;C0.28%~0.33%,P≤0.005%,S≤0.005%,Cu≤0.15%。Wherein non-metallic inclusion content is all qualified, and A sulfide, B oxide compound, C silicate inclusion, D nodule and the spherical macrobead of DS are mingled with and all reach below 1.5 grades.The UT (Ultrasonic Testing) inspection reaches 1 grade of non-destructive testing standard (JB/T500.15-8), i.e. φ≤1.6mm.All much smaller than 1.5 grades, resulting steel have higher purity for inclusion such as sulfide (being lower than 0.5 grade) and oxide compound (being lower than 0.5 grade).
Mechanical property: yield strength σ
s〉=1050MPa, tensile strength sigma
b〉=900MPa, unit elongation δ
5〉=15%, relative reduction in area ψ 〉=45%, hardness 280~328HB.
Technology of the present invention has increased the toughness of material by adjusting prescription; Optimize smelting technology, adopt molten steel furnace external refining and blown inert gas, impel the impurity come-up; And the use composite deoxidant, deoxidization technique is optimized design, adjust traditional deoxidation step, make the molten steel alloying process more abundant.The present invention has guaranteed that steel ingot has advantages such as degree of cleaning height, surface quality and hardenability characteristic are good, grain fineness number is thin; Steel ingot intensity height, high pressure resistant, corrosion-resistant, satisfactory mechanical property.In this technology, need not the esr operation and can obtain good deoxidation effect, reduce non-metallic inclusion content simultaneously, improve product carrying out flaw detection level, under the prerequisite that guarantees use properties, reduce energy consumption, reduced cost.
Embodiment
Embodiment 1
(1) with 30 tons smelting amount, in the electric arc furnace furnace bottom, adds earlier 0.55~0.6 ton little muck (main component Fe
2O
3Content 60wt%~70wt%) be used for slag making ahead of time with 0.65~0.7 ton of lime (calcium oxide), drop into 28.5~28.7 tons of low-sulfur phosphorus Clean Steels (iron level 〉=98wt% wherein then, carbon content≤0.2wt%, sulphur and phosphorus content are respectively≤0.05wt%), the energising fusing adds carburelant and joins carbon, makes carbon content reach 0.78%; Add 132 kilograms of Mo and 750 kilograms of Ni in melting period;
In the molten steel of fusing, be blown into oxygen, and in molten steel, divide 0.3~0.4 ton of little muck of 2~4 addings or iron scale, and 0.45~0.5 ton of lime, flow slag (changing slag) in the time of slag making, smelt 1~1.5 hour decarburization dephosphorization 1560 ± 20 ℃ of following oxidations; Decarburized amount when finish oxidation period 〉=0.4%;
(2) molten steel after step (1) the dephosphorization decarburization is poured in the LF stove, in the ratio of 10~15kg/ ton (molten steel) and 5~6kg/ ton (molten steel), added calcium oxide and fluorite; After thin slag forms, get composite deoxidant (content of silicon, manganese, aluminium is respectively 9wt%, 18wt% and 9wt%), be broken into the fritter about median size 50mm, in the ratio of 4kg/ ton, join and carry out pre-deoxidation in the molten steel;
(3) replenishing Mn and Cr in the molten steel, to make its content be 0.49% and 0.97%, and divide 3 times on the slag surface, and by the consumption of 3~5kg/ ton (molten steel) and 1.5~3kg/ ton, (contain 12wt%~15wt% aluminium, all the other are Al to adding AD powder
2O
3) and carbon dust, 1620 ± 10 ℃ of following refinings 1~2 hour, utilize LF furnace bottom blowing argon gas to stir simultaneously, to impel the inclusion polymerization better, oxide inclusion is floated in the slag on easier, thereby make molten steel further be purified purification;
(4) in molten steel, replenish Si adjustment silicone content to 0.22%, smelted 5~10 minutes;
(5) liquid steel temperature is risen to 1680~1700 ℃ rapidly again, then ladle is moved to degassing station, carrying out vacuum outgas (degasification) handles, be to handle 20~30 minutes under the state of 4~67pa in vacuum tightness, foreign gas such as hydrogen, nitrogen removes in the molten steel thereby make, until hydrogen richness≤1.5ppm;
Under (6) 1540~1560 ℃ ladle is hung degassing station (VD station) cast, before cast, with the air in rare gas element such as the argon replaces mould, use argon for protecting pouring in the casting process, prevent casting process secondary pollution and oxidation.
Resulting 20 stove finished products carry out non-destructive test(ing)(NDT) and detect, and press the II grade standard examination of JB/T4730.3-2005, and are all qualified.
Products obtained therefrom chemical ingredients and content (weight percent): C0.32%, Mn0.49%, Si0.22%, Cr0.97%, Mo0.44%, Ni2.50%; P0.007%, S0.003%, Cu0.13%.Carry out high power metallographic structure flaw detection data analysis after the gained steel ingot forges, wherein non-metallic inclusion content is all qualified, and A sulfide, B oxide compound, C silicate inclusion, D nodule and the spherical macrobead of DS are mingled with and all reach below 1.5 grades.The UT (Ultrasonic Testing) inspection reaches 1 grade of non-destructive testing standard (JB/T500.15-8),
All much smaller than 1.5 grades, resulting steel have higher purity for inclusion such as sulfide (being lower than 0.5 grade) and oxide compound (being lower than 0.5 grade).
Mechanical property: the yield strength σ of product
sBe 1100MPa, tensile strength sigma
bBe 1090MPa, unit elongation δ
5Be 18.0%, relative reduction in area ψ=61%, hardness 318HB.
Composite deoxidant in the step (2) prepares by the following method:
Get the raw materials ready: the low carbon manganese steel 54kg(wherein content of manganese is 80wt%, be converted into 43.2kg manganese, carbon content≤0.7wt%), ferrosilicon the 30kg(wherein content of silicon are 72%, be converted into 21.6kg silicon), aluminium block 22kg(purity 〉=98%, be converted into 21.6kg aluminium), clean low S.P steel scrap 134kg(iron level 〉=95wt%, carbon content≤0.2wt%, difference≤0.05wt%), the smelting molten steel total amount is 240kg for sulphur and phosphorus content.
Above-mentioned raw materials is placed the 250kg intermediate frequency furnace, after being molten into molten steel, continue down to smelt 0.5~1 hour at 1380 ± 10 ℃, skim the molten steel surface scum silica frost with iron staff, when temperature is come out of the stove during at 1380 ± 10 ℃, pour in the ladle, under 1330 ± 10 ℃ of temperature, be cast into the roundlet ingot 40kg of no rising head * 6 then, be composite deoxidant.
In the resulting composite deoxidant, press mass ratio, manganese content is 18%, and the content of silicon and aluminium is 9%, and carbon content≤0.5wt%, sulphur and phosphorus content difference≤0.05wt%.
With this composite deoxidant pulverize for behind the particle of particle diameter 1mm in oxygen heated oxide, the melting temperature of product is lower than 1200 ℃.
Reference examples 1
(1) press the proportioning of embodiment 1, low-sulfur phosphorus Clean Steel is added electric arc furnace, join carbon to 0.78%, the energising fusing adds Mo and Ni between melting period; The fusing back is blown into oxygen in molten steel, and adds 0.55~0.6 ton of little muck or iron scale and 0.6~0.7 ton of unslaked lime for slag making.Smelt 1~1.5 hour decarburization dephosphorization down at 1560 ± 20 ℃;
(2) molten steel behind the decarburization dephosphorization is poured in the LF stove, in the ratio of 10~15kg/ ton (molten steel) and 5~6kg/ ton (molten steel), added lime (calcium oxide) and fluorite; After thin slag forms, get ferromanganese and be broken into 50mm left and right sides particle (carbon content≤0.7wt%, manganese content 80wt%,) add molten steel by the consumption of 7~7.1kg/ ton, add in the molten steel by the consumption of 3.5~3.7kg/ ton 0.5 get the particle that ferrosilicon (wherein the content of silicon is 72wt%) is broken into about 50mm after hour, add aluminium powder (purity 98%) by adding 0.4kg/ ton consumption again after 0.5 hour;
(3) divide 3 times on the slag surface, by the consumption of 3~5kg/ ton and 1.5~3kg/ ton (molten steel), (contain 12wt%~15wt% aluminium, all the other are Al to add the AD powder
2O
3) and carbon dust, 1620 ± 10 ℃ of following refinings 1~1.5 hour, utilize LF furnace bottom blowing argon gas to stir simultaneously;
(4) press the method degassing of embodiment 1 step (5), bull ladle cast then.
In the 20 stove finished products of gained, high power inclusion ABCD can't satisfy and all is no more than 1.5 grades requirement, and the UT (Ultrasonic Testing) rank does not all reach the I grade standard of JB/T5000.15-98.
The said products is added the esr operation, and high power inclusion ABCD can be lower than 1.5 grades in the finished product, and the UT (Ultrasonic Testing) rank meets the I grade standard of JB/T5000.15-98.
Embodiment 2
(1) with 30 tons smelting amount, in the electric furnace furnace bottom, adds earlier 0.5~0.55 ton little muck (main component Fe
2O
3) and 0.7~0.8 ton of lime be used for slag making ahead of time, drop into the clean steel scraps of 28.4~28.6 tons of low-sulfur phosphorus (iron level 〉=98wt% wherein, carbon content≤0.2wt% then, sulphur and phosphorus content are respectively≤0.05wt%), the energising fusing adds carburelant and joins carbon, makes carbon content reach 0.85%; Add 153 kilograms of Mo and 795 kilograms of Ni in melting period;
Add in the molten steel and be blown into oxygen, and divide 2~4 times in the molten steel to add 0.3~0.4 ton of little muck or iron scale, and 0.45~0.5 ton of lime, flow slag (changing slag) in the time of slag making, smelt 1~1.5 hour decarburization dephosphorization down at 1560 ± 20 ℃; Decarburized amount when finish oxidation period 〉=0.4%;
(2) molten steel behind the decarburization dephosphorization is poured in the LF stove, in the ratio of 10~15kg/ ton (molten steel) and 6~7kg/ ton (molten steel), added lime and fluorite; After thin slag forms, get composite deoxidant (content of silicon, manganese, aluminium is respectively 9wt%, 18wt% and 9wt%), be broken into the fritter about median size 50mm, in the ratio of 4.5kg/ ton, join and carry out pre-deoxidation in the molten steel;
(3) replenishing Mn and Cr in the molten steel, to make its content be 0.55% and 1.02%, and divide 3 times on the slag surface, and (contain 12wt%~15wt% aluminium, all the other are Al by the consumption adding AD powder of 3~5kg/ ton (molten steel)
2O
3), add calcium carbide by the consumption of 1.5~3kg/ ton, 1620 ± 10 ℃ of following refinings 1~1.5 hour, utilize LF furnace bottom blowing argon gas to stir simultaneously, to impel the inclusion polymerization better, oxide inclusion is floated in the slag on easier, thereby make molten steel further be purified purification;
(4) in molten steel, replenish the content to 0.18% that Si adjusts content silicon, continue to smelt 5~10 minutes;
(5) liquid steel temperature is risen to 1680~1700 ℃ rapidly again, then ladle is moved to degassing station, carrying out vacuum outgas (degasification) handles, be to handle 20~30 minutes under the state of 4~67pa in vacuum tightness, foreign gas such as hydrogen, nitrogen removes in the molten steel thereby make, and makes molten steel obtain better to purify to purify;
Under (6) 1540~1560 ℃ ladle is hung degassing station (VD station) cast, before cast, with the air in rare gas element such as the argon replaces mould, use argon for protecting pouring in the casting process, prevent casting process secondary pollution and oxidation.
Resulting 20 stove finished products carry out non-destructive test(ing)(NDT) and detect, and press the II grade standard examination of JB/T4730.3-2005, and are all qualified.
In the step (2), the preparation method of composite deoxidant is with embodiment 1.
Products obtained therefrom chemical ingredients and content (weight percent): C0.30%, Mn0.55%, Si0.18%, Cr1.02%, Mo0.51%, Ni2.65%; P0.008%, S0.002%, Cu0.13%.Wherein non-metallic inclusion is all much smaller than 1.5 grades, and resulting steel have higher purity.
The yield strength σ of product
sBe 1097MPa, tensile strength sigma
bBe 1093MPa, unit elongation δ
5Be 17.8%, hardness 316HB.Forge the back test of flaw detection waste product appearance does not take place, product is all qualified.
Reference examples 2
(1) proportioning of pressing embodiment 2 is prepared the clean steel scrap of low-sulfur phosphorus, Mo and Ni, and the energising fusing adds carburelant and joins carbon, makes carbon content reach 0.85%; In molten steel, add 0.5~0.55 ton little muck (main component Fe
2O
3) and 0.7~0.8 ton of lime, smelting process is with reference examples 1;
(2) ferromanganese and ferrosilicon consumption are 7.8~8kg/ ton and 3~3.28kg/ ton, and the aluminium powder consumption is 0.45~0.48kg/ ton, and all the other are with reference examples 1;
(3) and (4) with reference examples 1.
In the 20 stove finished products of gained, high power inclusion ABCD can't satisfy and all is no more than 1.5 grades requirement, and the UT (Ultrasonic Testing) rank does not all reach the I grade standard of JB/T5000.15-98.
Traditional method deoxidation and vacuum outgas are added the esr operation after handling, and high power inclusion ABCD can be lower than 1.5 grades in the finished product, and the UT (Ultrasonic Testing) rank meets the I grade standard of JB/T5000.15-98.