CN116179939A - Mooring chain steel with high environmental crack resistance and mooring chain - Google Patents

Abstract

The invention relates to a mooring chain steel with high environmental crack resistance and a mooring chain, wherein the mooring chain steel comprises the following components in percentage by mass of 0.20-0.40% of C, 0.10-0.50% of Si, 0.20-1.20% of Mn, 0.50-2.00% of Cr, 1.00-3.50% of Ni, (Mo+V) 0.60-1.60%, 0.005-0.065% of Nb, 0.004-0.020% of N, less than or equal to 0.015% of P, less than or equal to 0.008% of S, and the balance of Fe and impurities; the fluctuation amplitude delta C of the cross section of the object is less than or equal to 0.03 percent. The mooring chain steel and the mooring chain manufactured by the same have the tensile strength of 1000-1250 MPa and cover two strength levels of R5 and R6; adding extremely fine and dispersed Mo and V carbonitrides to generate the multiplication effect of capturing hydrogen in steel, doubly improving the critical hydrogen concentration of cracks not expanding in service of the chain ring, improving the volume fraction of the carbonitrides, simultaneously obtaining finer austenite grains, stabilizing the toughness of the chain ring and ensuring the service reliability of a mooring chain; the fluctuation of the composition of a material chain is reduced, the fluctuation range of the cross section C is reduced from delta C more than or equal to 6% to delta C less than or equal to 3%, and the phenomenon of large fluctuation of the intensity of the existing R5 and R6-grade products is avoided.

Description

Mooring chain steel with high environmental crack resistance and mooring chain

Technical Field

The invention relates to the technical field of metal materials and metal products, in particular to mooring chain steel with high environmental crack resistance and a mooring chain.

Background

The service reliability of the existing R5-grade and R6-grade ultra-high strength mooring chain is a primary problem to be solved while the mechanical performance stability of the existing R5-grade and R6-grade ultra-high strength mooring chain is improved, wherein the problem that the yield ratio is limited by strength is solved. The environmental crack sensitivity (EAC) and resistance thereof caused by the ultrahigh strength of the ocean chain are improved, and the urgency thereof exceeds the improvement of the toughness. Because the higher the strength, the more sensitive the environmental crack. The industry has studied and produced R5 and R6 grade steels and their chain links for many years. For 25 years. More than half of major accidents in ocean floating bodies worldwide are caused by mooring chain problems. A catastrophic event of the gulf of mexico platform toppling in 2011. Namely, hydrogen induced cleavage caused by misuse of the R5 chain.

Based on a comprehensive and deep knowledge of the stress state of the moored links, in recent years the maximum stress spatial distribution and stress concentration coefficient (SCF) of the fatigue or stress corrosion damage of the links has changed significantly. The stress analysis of the original links is limited to the tension state and has now been replaced by a composite state of tension + out-of-plane bending + in-plane bending. The maximum spatial distribution of tension + out-of-plane bending + in-plane bending state links long-term fatigue or stress corrosion damage is increased by approximately a factor of 8 compared to the pure tension state. Accordingly, the american petroleum institute responsible for recommending mooring chain design specifications recently introduced a new standard version 4 (D esign and analysis of station keeping systems for floating structures, API RP 2sk, 4) in place of the 2005 3 rd standard version ^th Editio n)。

The hydrogen induced delayed fracture study of ultra-high strength mooring chains requires consideration of both critical hydrogen concentration and environmental intrusion hydrogen. Li Songjie, yan Jiang, wang Chengduo et al studied the critical hydrogen concentration (Hc) at which a hydrogen-induced delayed fracture occurred in a mooring chain with a tensile strength of 1200MPa for a ni—cr-mo 0.5-microalloy component system using a slow strain rate tensile test (SSRT) in combination with a hydrogen thermal analysis Technique (TDA), and as a result showed that the fracture stress exhibited a significant inflection point at a hydrogen concentration of 2.0wppm (int.j. Electrochem. Sci.,14,2019,2705-2713).

In recent years, the common fracture accidents of the internationally-used R5 < + >, i.e. the tensile strength of the mooring chain reaches the upper limit (1100-1150 MPa) of the tensile strength (1000-1150 MPa) of the R5 < + > chain and the lower limit (1100-1150 MPa) of the tensile strength (1100-1250 MPa) of the R6 < + > chain simultaneously, show that the reliability of the ultra-high strength mooring chain is not only dependent on the mechanical properties, namely the strength-plastic-toughness and the yield ratio, but also more likely to be related to the environmental fracture resistance. Environmental fracture of links when in service in seawater tends to be sensitive with increasing strength. Artificial seawater charging result of specimen prepared from mooring chain object with tensile strength of 1200MPa of Ni 2-3.5-Cr 1-Mo 0.5-microalloy component: the concentration H of the diffuse hydrogen, i.e. the ambient invasive hydrogen, of the invasive sample _E Phase difference from the inflection point of the critical hydrogen studyThere is little, and therefore, a severe risk of environmental fracture. How to raise EAC resistance of mooring chain and ensure service reliability is the primary key of safety application.

Meanwhile, due to macro segregation of elements, the sensitivity of the tensile strength to a heat treatment process tends to be heavy along with the improvement of the physical strength of the existing mooring chain. For example, although narrow-component steel tapping is strictly controlled, the same cross section of the ultra-high strength physical chain ring still has 0.05-0.07 wt% or even higher C fluctuation, so that the fluctuation range of mechanical properties is overlarge, and the fluctuation of tensile strength value of the same cross section is more than 90MPa.

To sum up, existing R5 and R6 grade mooring chain steel link products present a service risk. In comparison with the newly discovered complex stress state, the critical hydrogen concentration at which crack growth does not occur on the surface of the chain ring is often insufficient, and the fluctuation range of the tensile strength of the same cross section is too large, so that the local environmental crack resistance is more difficult to control.

Disclosure of Invention

Aiming at the problems of low critical hydrogen concentration, large fluctuation range of tensile strength of the same cross section, exceeding yield ratio and the like of the prior R5-grade and R6-grade mooring chains, the first aim of the invention is to provide mooring chain steel with high environmental crack resistance.

The mooring chain steel with high environmental crack resistance comprises, by mass, 0.20-0.40% of C, 0.10-0.50% of Si, 0.20-1.20% of Mn, 0.50-2.00% of Cr, 1.00-3.50% of Ni, (Mo+V) 0.60-1.60% of N0.004-0.020%, less than or equal to 0.008% of S, less than or equal to 0.015% of P, and the balance of Fe and impurities.

C is an element effective in forming martensite and improving strength, and C of 0.20% or more is necessary in view of the improvement of the content of the carbonitride forming element Cr, nb, mn, N of the present invention, particularly (Mo+V). C is more than 0.40%, the volume fraction of various carbides is excessively increased, the toughness of the chain ring is reduced, C is regulated to be 0.20-0.40%, and the fluctuation amplitude delta C of the cross section C of the chain steel real object is less than or equal to 0.03%. When DeltaC is more than 0.03%, the sensitivity of the performance of the chain ring to the whole heat treatment becomes difficult to control, the fluctuation range of the tensile strength of the cross section of the chain ring is increased, and uncontrollability is brought to the service performance of the chain ring, such as fatigue life, environmental crack sensitivity and the like.

Si is a deoxidizing element, and is also an element for strengthening ferrite, si is less than 0.1%, deoxidizing effect is not obvious, and Si is more than 0.5%, so that toughness of the chain ring is reduced. The comprehensive consideration prescribes Si 0.10-0.50%.

Mn is a deoxidizing element and is also an effective martensite forming element. Mn < 0.20% has no obvious effect, mn > 1.20%, influences the combination of alloy elements, is unfavorable for improving the environmental crack resistance, and comprehensively considers and prescribes Mn 0.20-1.20%.

Cr is carbide forming element, is favorable for corrosion resistance of atmosphere and seawater environment, participates in formation of quenching structure, cr is less than 0.50%, the effect is not obvious, cr is more than 2.00%, M3C type carbide which is unfavorable for improvement of environmental crack resistance is increased, and Cr0.50-2.00% is comprehensively considered.

Ni is beneficial to the breaking resistance and corrosion resistance of mooring chains, ni is less than 1.00%, the effect is insufficient, ni is more than 3.50%, and the effect tends to be saturated. The design of the target structure for forming the complex bainite is also disadvantageous, and the cost is increased. Ni1.00-3.50% is defined.

Mo and V are elements for improving hardenability and carbide forming elements, so that the production of the large-diameter mooring chain is possible, the conventional water temperature for quenching (less than or equal to 50 ℃) is allowed to be improved, the segregation of impurities such as P and the like at the grain boundary is reduced by Mo, besides the strength of the chain ring is improved, the superfine MC-type nano carbide is more a strong hydrogen trap, (Mo+V) is less than 0.60%, the effects are insufficient, (Mo+V) is more than 1.60%, and Hc values tend to be saturated due to the comprehensive effects of Mo/V, quenching-tempering and the like, the toughness is reduced, and the cost is increased. The content of (Mo+V) is defined to be 0.60 to 1.60%.

Nb acts not only as a deoxidizing element but also as a dragging element to grain boundaries, and NbCN formed has a solubility product of about one percent or less of (Mo, V) C and is a second phase particle that inhibits austenite grain growth. Nb is less than 0.005%, the effect is obviously weakened, nb is more than 0.065%, the physical NbCN coarsens, the particle number is reduced, the effect of refining austenite grains is correspondingly weakened, and the cost is increased. Nb0.005 to 0.065% is defined.

P and S are both unavoidable impurity elements, and P is more than or equal to 0.015%, so that toughness and environmental crack resistance are obviously reduced. S is more than or equal to 0.008 percent, toughness is reduced, and the pit corrosion caused MnS is obviously increased. In order to facilitate control of Hc and stable toughness, P < 0.015% and S < 0.008% are specified.

N forms MCN type carbides that refine the MC carbide and change its solubility product. N is less than 0.004%, the effect is weakened, and the process cost is increased. N is more than 0.020%, the probability of occurrence of coarse NbCN and the like in a weld zone is increased, the toughness of the weld joint is affected, and N is regulated to be 0.004-0.020%

A second object of the present invention is to provide a process for the production of a mooring chain steel having a high environmental crack resistance as described above, comprising the steps of,

s1, primary smelting by adopting an electric furnace or a converter according to a designed tapping target component proportion, wherein metal raw materials used in the primary smelting process are one or more than two of molten iron, pig iron, scrap steel, ferroalloy, sponge iron, metal oxide and ore, and then pouring into steel ingots or continuous casting blanks or electroslag remelting after external refining and vacuum degassing;

s2, heating the steel ingot or the continuous casting billet to 1150-1300 ℃, and forging or rolling the steel ingot or the continuous casting billet into round steel;

s3, straightening the round steel, grinding with a grinding wheel, peeling or turning, and then performing flaw detection, sampling and testing.

Specifically, in step S1, the ingot is cut end to end with a yield of 80% or less, and the continuous casting adopts the measures of full-process electromagnetic stirring, dynamic soft reduction and the like.

Specifically, in the step S2, the ratio of the sectional area of the steel ingot or continuous casting blank to the sectional area of the finished round steel is more than or equal to 7.

Specifically, in step S3, the additional content detected further comprises analysis of the distribution of C over the cross section of the 58-210 mm round bar manufactured for use as a chain link and its accessories.

The third object of the invention is to provide a mooring chain, wherein the mooring chain steel prepared by the preparation process is subjected to blanking, bending, flash butt welding, chain braiding, flaw detection and detection to form an R5 or R6 grade mooring chain.

Heating the material to the temperature of between 850 and 1000 ℃ in sequence, bending the material to the temperature of between 950 and 800 ℃ in sequence, flash butt welding the material to the temperature higher than the solid solution temperature of (Mo, V) CN carbide and lower than the solid solution temperature of NbCN carbide, and heating the material to the temperature of water cooling quenching, tempering air cooling or water cooling; and flaw detection and mechanical property sampling and testing are respectively carried out at one third of the radius of the cross section of the base body and the weld joint after the whole heat treatment of the mooring chain. The weld was tested for impact work only, as standard. Sampling to study the fluctuation of mechanical properties of the cross section is not limited.

Specifically, the braided chain is heat treated in a batch-type or vertical-type continuous tempering furnace, and the accessories thereof are heat treated in the batch-type tempering furnace.

Specifically, the heat treatment steps are: firstly, quenching at least once, wherein the quenching temperature is more than 900 ℃ each time, and water cooling is carried out, and the water temperature is less than 60 ℃; tempering is carried out after quenching treatment, the tempering temperature is 590-650 ℃, and water cooling or air cooling is carried out.

Compared with the prior art, the invention has the following advantages:

according to the invention, the Mo+V content is increased, the volume fraction of the fine and dispersed carbonitride is improved, because the superfine and dispersed MCN is a strong hydrogen trap, the Mo and V are added to generate a multiplication effect, the trap energy for capturing hydrogen is greatly increased, and meanwhile, the problem of toughness reduction caused by the increase of the brittle transition temperature is also generated; in order to prevent the increased carbide volume fraction from reducing the toughness of the chain ring, the invention increases the grain size of quenched austenite from finer than 5 to finer than 8 of the gauge; the invention obtains stable obdurability by refining austenite grains, including refining sub-grains, solves the contradiction between the obdurability and the environmental crack resistance, thereby greatly improving the marine environmental crack resistance of the chain ring and ensuring the service reliability of the mooring chain.

Drawings

FIG. 1 is a graph of the distribution of Mo and V atoms in MC-type carbides of a mooring link according to the invention as measured using a three-dimensional atom probe;

FIG. 2 is a sample of SSRT circular notch for Hc study;

FIG. 3 is CLT and H _E Analyzing the sample.

Detailed Description

The invention is further described below with reference to the drawings and specific examples.

Examples 2 to 6 and comparative examples 2 to 6 were cast into 2.33 ton square ingots using 20 ton electric furnace + external refining + vacuum degassing; example 1 after forging a steel ingot to a 300mm round billet, redissolving the steel ingot by using protective atmosphere electroslag; comparative example 1 is a commercial product.

The components of examples 1 to 6 and comparative examples 1 to 6 are shown in Table 1 in mass percent:

TABLE 1

Sequence number	C	△C	Si	Mn	S	P	Cr	Ni	Mo	V	Nb	N	Remarks
														Example 1	0.22	0.02	0.15	0.25	0.001	0.007	0.55	3.25	1.51	0.05	0.035	0.006	ESR
Example 2	0.24	0.03	0.45	0.45	0.003	0.011	0.73	2.85	0.81	0.12	0.027	0.005
														Example 3	0.36	0.03	0.17	0.31	0.002	0.014	0.81	2.04	0.83	0.10	0.007	0.018
Example 4	0.29	0.03	0.21	1.15	0.003	0.009	1.90	1.07	1.02	0.06	0.026	0.007
														Example 5	0.23	0.02	0.15	0.25	0.001	0.007	0.62	3.05	0.81	0.16	0.036	0.006
Example 6	0.21	0.02	0.12	0.23	0.003	0.010	1.07	3.45	0.45	0.17	0.061	0.005
														Comparative example 1	0.26	0.06	0.20	0.58	0.003	0.011	0.79	3.05	0.47	0.06	0.020	0.006	Goods commodity
Comparative example 2	0.42	0.07	0.21	0.54	0.006	0.014	0.57	0.95	1.09	0.16	0.015	0.008
														Comparative example 3	0.38	0.05	0.56	0.39	0.002	0.008	2.06	2.02	0.70	0.14	0.002	0.005
Comparative example 4	0.18	0.06	0.19	0.15	0.002	0.016	1.21	1.08	0.43	0.08	0.068	0.018
														Comparative example 5	0.20	0.07	0.21	1.24	0.006	0.012	0.47	2.35	0.61	0.03	0.006	0.008
Comparative example 6	0.32	0.07	0.21	0.54	0.009	0.010	0.45	1.35	1.56	0.07	0.010	0.008

According to the designed tapping target component proportion, primary smelting is carried out by adopting an electric furnace or a converter, and the metal raw materials used in the primary smelting process are one or more than two of molten iron, pig iron, scrap steel, ferroalloy, sponge iron, metal oxide and ore. And then casting into steel ingots or continuous casting billets after external refining and vacuum degassing. In order to improve the uniformity of the components of the ingot and the blank, the ingot is cut end to end with the yield less than or equal to 80 percent, and continuous casting is carried out, and measures such as whole-course electromagnetic stirring, dynamic light pressing and the like are adopted, or the ingot and the blank are taken as electrodes and then are thermally processed through electroslag remelting or other special smelting methods. Ensuring that the segregation delta C of the cross section of the object is less than or equal to 0.03 percent.

The ratio of the cross section area of the steel ingot or continuous casting blank to the cross section area of the finished round steel is more than or equal to 7.

Examples 2 to 6 and comparative examples 2 to 6 were cast into 2.33 ton square ingots using 20 ton electric furnace + external refining + vacuum degassing. Example 1 after forging a steel ingot to a 300mm round billet, electroslag remelting was performed in a protective atmosphere. Comparative example 1 is a commercial product.

And heating the steel ingot, the continuous casting billet or the remelted ingot to 1150-1300 ℃, and forging or rolling the steel ingot, the continuous casting billet or the remelted ingot into round steel. And annealing the round steel, straightening, grinding with a grinding wheel, peeling or turning, and performing flaw detection, sampling and testing.

The test steel was forged into a 135mm round bar and the distribution of C was analyzed on the cross section. After turning to 133mm, the materials are subjected to blanking, heating at 850-1000 ℃, bending at 950-800 ℃, temperature measurement, flash butt welding, chain braiding with the diameter of 130mm, flaw detection, car bottom furnace heating, quenching water cooling and tempering water cooling in sequence. And after the whole mooring chain is subjected to heat treatment, sampling and testing mechanical properties and impact energy are respectively carried out at one third of the radius of the cross section of the matrix and the weld joint. The weld was tested for impact work only, as standard.

Conventionally, the braided chain is heat treated in an intermittent or vertical continuous quenching and tempering furnace, and the accessories are heat treated in the intermittent quenching and tempering furnace. The heat treatment steps are as follows: firstly, quenching treatment is carried out at least once, wherein the quenching chain temperature is more than 900 ℃ each time, namely, the quenching chain temperature is higher than the (Mo, V) CN solid solution temperature and lower than the NbCN solid solution temperature, water cooling is carried out, and the water temperature is lower than 60 ℃; quenching and tempering. According to the ship specification, the tempering temperature of the continuous furnace is more than or equal to 590 ℃, and water cooling or air cooling is performed. And after the whole mooring chain is subjected to heat treatment, sampling and testing mechanical properties, namely tensile properties and impact energy, are respectively carried out at one third of the radius of the cross section of the matrix and the weld joint. According to the specifications, the weld joint only tests the impact energy. The fracture toughness project can be increased during the quality acceptance test of finished chain links.

The 5T protective atmosphere electroslag process of the steel example 1:

1. basic information

1 electrode composition: example 1

2 electrode ingot type: phi 300

3, crystallizer: phi 400

4 slag system: 81 slag.

5 slag amount: the weight of the ingot is less than or equal to 1.2 tons, and the slag amount is as follows: 50kg; the ingot weight is more than 1.2 tons, and the slag amount is as follows: 60kg

The 6 slag material baking system is used after being baked for more than 6 hours at 680 DEG C

7 electrode bar baking schedule: is baked for more than 4 hours at 400 DEG C

8 electrode rod shot blasting or barreling

9 dummy ingot plate: iron plate

10-piece steel base cushion

11 furnace cooling for 1h

Air cooling after 12 ingot stripping

13 big and small head sample, analyze C, mn, si, fe

2. Smelting process (constant melting speed control)

1. Slag melting stage: auxiliary electrode weight 280kg:

2. smelting stage

3. Heat capping stage

Note that the above parameters are set values.

The arcing (slagging) stage is mainly controlled by current and voltage, other parameter references, actual current and voltage allowing fluctuation to be controlled within +/-20% of a set value, and particularly is mainly controlled by an actual value.

The melting speed of key parameters in the melting stage (steady-state stage). The actual melting speed is allowed to be controlled within +/-10% of the set value, and other parameters such as current, voltage, swing, weight and the like are automatically adjusted by a computer.

The heat sealing stage takes current and voltage control as a main mode. The actual current and voltage fluctuation is controlled within +/-20% of the set value, and the melting speed is used as a reference.

The local critical hydrogen concentration was determined by SSRT using an annular notched sample as shown in FIG. 2, and the cathode of the sample was charged with hydrogen for 200hr. The SSRT of the test piece loaded thereafter was carried out on a WDML-300kN slow tensile tester with a constant crosshead speed of 0.005mm/min, corresponding to a nominal strain rate of 8.3X10 ^-7 s ^-1 . Fracture stress σf=fmax/Amin, where Fmax is the maximum tensile force and Amin is the cross-sectional area at the notch. Before and after 6 hours of SSRT, the samples were broken and 10mm long sections were immediately cut from the sample breaks and stored in liquid nitrogen. Hydrogen was measured by Thermal Desorption Analysis (TDA). The corresponding diffused hydrogen is the critical hydrogen content of hydrogen embrittlement.

The product organization and performance data after preparation are shown in table 2, each data being an average of three sets of data.

TABLE 2

TABLE 3 Artificial seawater+CP (-1000 mV, SCE)

No.	Rm,MPa	Hc	HE	Hc/HE	SCF≥2
						Rm level	1230～1250		1.6
Example 2	1230	3.8		2.4	＞
						Comparative example 6	1235	3.6		2.3	＞
Rm level	1200～1205		1.4
						Example 3	1204	3.5		2.5	＞
Comparative example 1	1201	2.1		1.5	＜
						Example 4	1165	3.3	1.2	2.8	＞
Rm level	1115～1130		1.0
						Example 1	1115	4.1		4.1	＞
Example 5	1130	4.3		4.3	＞
						Example 6	1120	4.0		4.0	＞

The composition and performance data according to the above examples and comparative examples illustrate the features of the present invention.

The austenitic grain size of the mooring chain steel and chains required by the ship's gauge is a fine grain grade of ISO643 standard grain size, i.e. grade 5-8. And adopts the presentThe inventive technique relies on Nb and NbCN to retard austenite grain growth the average austenite grain diameter of the primary or secondary quenched examples is all finer than 19.7 μm, i.e. finer than grade 8. The austenite grain size of example 1 was the finest, reaching grade 10. The examples are all finer grains, thereby doubling the environmental crack resistance when increasing the carbide element Mo+V (the invention is applied at a strain rate of 8.3X10 ^-7 s ^-1 The hydrogen concentration when the breaking stress appears obvious inflection point in the pull-down elongation test is the critical hydrogen concentration when the crack does not propagate, hc _TS And the TS is GPa value of tensile strength of a sample), and the problem of impact power reduction is solved.

As MC is increased and stability is kept, all the toughness and yield ratio of each embodiment are qualified, hc _TS And also greatly improves. From Table 3, hc TS/H of the examples _E All greater than the highest safety factor of the design criteria (scf=2). Hc with tensile strength of 1200MPa _TS / _HE Example 3 is 1.7 times that of comparative example 1. The SCF of examples 1, 5 and 6 was greater than 4. While comparative example 1, i.e. the existing R6 commodity chain SCF is even less than a low safety factor of 1.67, does not exclude possible risks of service. (Mo+V) > 1.60% of comparative example 6, i.e., exceeding the upper limit of the present invention. Comparison of Hc with example 2 _1.25 Near saturation.

Designing artificial seawater with cathode potential-1000 mV (SCE) according to the ship gauge, and measuring environmental intrusion hydrogen H by Constant Load Test (CLT) with load of 0.9Rm/240H _E (FIG. 3). This is a severe loading condition.

Considering the increase of the thermal deformation resistance of the steel, the invention improves the heating temperature before bending and the bending temperature to offset the increase of the breaking force and ensure the quality of the welding seam.

FIG. 1 is a distribution of Mo atoms and V atoms in MC-type carbides of mooring links of the present invention as determined using a three-dimensional atom probe. Comparing the equivalent particle size, M2C is about 4 times that of MC. As is well known, in the heat-treated state, the hydrogen trapping energy of M2C type carbide of the hexagonal lattice is lower than that of MC type carbide (Mo, V) C of the face-centered cubic lattice.

FIG. 2 is a cathode pre-charged Slow Strain Rate Test (SSRT) cyclic notch test specimen for studying hydrogen embrittlement. SSRT of the loaded test piece was performed on a WDML-300kN slow tensile tester with a constant crosshead speed of 0.005mm/min, corresponding to a nominal strain rate of 8.3X10 ^-7 s ^-1 . Fracture stress σf=fmax/Amin, where Fmax is the maximum tensile force and Amin is the cross-sectional area at the notch. Before and after 6 hours of SSRT, the samples were broken and 10mm long sections were immediately cut from the sample breaks and stored in liquid nitrogen. Hydrogen was measured by Thermal Desorption Analysis (TDA). The corresponding diffused hydrogen is the critical hydrogen content of hydrogen embrittlement.

FIG. 3 is CLT and H _E Analyzing the sample. Immediately after the CLT test, a 10mm long section was cut from the specimen fracture. The corresponding diffused hydrogen is the content of the environmental invading hydrogen in the 10mm section.

The amplitude of C segregation on the cross section is controlled, deltaC is less than or equal to 0.03%, and the fluctuation amplitude DeltaRm of the tensile strength on the cross section is small. Example 1 was subjected to electroslag remelting, the link material object Δc=0.02% and Δrm is less than or equal to 26MPa, i.e. the fluctuation range was 2.3% of the tensile strength. The steel ingot of example 6 was cut out at 25% by weight of the head and tail, and the chain ring real object Δc=0.02% and Δrm is not more than 18MPa, that is, the fluctuation width of the tensile strength was 1.6%.

The steel ingots of examples 2,3,4 were cut off 25% by weight from the head to the tail, the link entity Δc=0.03, wherein Δ R m of example 2 is the largest of all examples, and Rm is less than or equal to 40MPa, and the fluctuation range of tensile strength is only 3.3%.

And comparative examples 1,2,4 to 6 were shown to have a real object ΔC.gtoreq.6% and comparative example 3 was shown to have a real object ΔC=5%. Wherein Δrm=140 MPa of comparative example 6, the fluctuation range of the tensile strength against force is 11.4%, which is the largest among all the comparative examples. Δrm=94 MPa of comparative example 4, which is the smallest of all comparative examples, also exceeds 8.6 in the fluctuation range of tensile strength. The comparative examples were 3.7 to 7.1 times as large as the examples in comparison with the percentage of the fluctuation range of tensile strength. That is, the invention greatly reduces the difficulty of controlling the strength of the chain ring and the resistance to environmental cracks, and can ensure the service reliability of the chain ring.

The toughness of the comparative example was lower or lower due to measures to refine the austenite grains and thermal history of the loops in the chain. In comparative example 1, which is a commercial material, the composition of the components does not meet the present invention, and therefore the tensile strength exceeds 1200MPa and the yield ratio exceeds the upper limit.

The amount of C in comparative example 2 exceeds the upper limit specified in the present invention, and the amount of Ni is below the lower limit specified in the present invention. As a result, the impact energy of both the link matrix and the weld is below the lower limit.

Comparative example 3, where the nb content was below the present technique, the average austenite grain size was coarser than grade 5, and the impact energy of both the matrix and the weld joint was below the lower limit. Comparative example 3 the Si content and Cr content, which reduce the ferrite toughness, both exceed the upper limit specified in the present invention. Cr increases the volume fraction of M3C type carbide, and moreover, the volume fraction of MC type carbide is also increased significantly due to the higher content of V. As a result of the combination of several aspects, the toughness of the link is below the lower limit.

Comparative example 4, although finer, showed insufficient quenching temperature because the content of P was higher than the upper limit specified in the present invention, and C and Nb were lower and higher than the present technology, respectively, and NbCN tended to coarsen. The result is a decrease in tensile strength below the upper limits of R6 and R5. The impact energy of the welding line is lower than the lower limit.

The Mn of comparative example 5 is higher than the upper limit prescribed in the present invention while Cr is reduced. The Cr content is below the lower limit specified in the present invention, and the tensile strength is still below the upper limit of R6 or R5 level, although the tempering temperature is reduced to near the lower limit specified.

The contents of both comparative examples 6 (Mo+V) and S exceeded the present technique. S not only increases the corrosion rate of the link, but also, in combination with (Mo+V), reduces the toughness of the link matrix and the weld.

The impact energy of comparative examples 2,3, 6 was not only double below the lower limit, but also below 70% of the lower limit. 70% of the standard value of the average impact energy is the test value of the single sample allowed by the ship's gauge.

In conclusion, the invention increases the (Mo+V) content and improves the volume fraction of the fine and scattered carbide. Extremely fine, dispersed MCN is a strong hydrogen trap, adding Mo and V produces a multiplicative effect. However, the trap energy for trapping hydrogen is greatly increased, and the problem of toughness reduction caused by the increase of the brittle transition temperature is also caused. To prevent the increased carbide volume fraction from decreasing the toughness of the links. The invention improves the grain size of quenched austenite from less than 5 grades of the ship gauge to less than 9 grades (example 2 is less than 8 grades). The invention obtains stable toughness by refining austenite grains and reducing thermal deformation resistance, solves the contradiction between the toughness and environmental crack resistance, greatly improves the marine environmental crack resistance of the chain ring, and can ensure the service reliability of the mooring chain.

While the preferred embodiments of the present invention have been illustrated and described, the present invention is not limited to the embodiments, and various equivalent modifications and substitutions can be made by one skilled in the art without departing from the spirit of the present invention, and these equivalent modifications and substitutions are intended to be included in the scope of the present invention as defined in the appended claims.

Claims (9)

1. The mooring chain steel with high environmental crack resistance is characterized by comprising, by mass, 0.20-0.40% of C, 0.10-0.50% of Si, 0.20-1.20% of Mn, 0.50-2.00% of Cr, 1.00-3.50% of Ni, (Mo+V) 0.60-1.60%, 0.004-0.020% of N, less than or equal to 0.008% of S, less than or equal to 0.015% of P, and the balance of Fe and impurities.

2. The high environmental crack resistance mooring chain steel according to claim 1, wherein Δc of the mooring chain steel cross section is 0.03% or less, wherein Δc = C _max -C _min 。

3. A process for producing a high environmental crack resistance mooring chain steel as defined in claim 1 or 2, comprising the steps of,

s1, primary smelting by adopting an electric furnace or a converter according to the designed tapping target component proportion. The metal raw materials used in the primary smelting process are one or more than two of molten iron, pig iron, scrap steel, ferroalloy, sponge iron, metal oxide and ore, and then are cast into steel ingots or continuous casting billets or electroslag remelting after external refining and vacuum degassing;

s2, heating the steel ingot or the continuous casting billet to 1150-1300 ℃, forging or rolling into round steel, or rolling into an intermediate billet, and then rolling into round steel;

s3, straightening the round steel, grinding with a grinding wheel, peeling or turning, and then performing flaw detection, sampling and testing.

4. A process for the production of a mooring chain steel with high environmental crack resistance according to claim 3, characterized in that in step S1, the ladle is soft-blown argon followed by casting; the cast ingot is cut into head and tail parts with the yield less than or equal to 80 percent. The tundish is heated by plasma; in the continuous casting process, a crystallizer and a terminal electromagnetic stirring and soft reduction process are adopted; the continuous casting blank which does not accord with the normal heat history, in particular to the continuous casting blank of each first furnace section is cut off uniformly.

5. A process for the production of a mooring chain steel with high environmental crack resistance according to claim 3, characterized in that in step S2 the ratio of the cross-sectional area of the ingot or slab to the cross-sectional area of the finished round is not less than 7.

6. A process for the production of a mooring chain steel with high environmental crack resistance according to claim 3, characterized in that in step S3 the additional content detected further comprises analysis of the distribution of C over the cross section of the 58-210 mm round bar manufactured for use as a chain link and its accessories.

7. A mooring chain with high environmental crack resistance, which is characterized in that the mooring chain steel prepared by the preparation process of claims 3-6 is subjected to blanking, heating, bending, flash butt welding, chain braiding, heat treatment, flaw detection and detection to form an R5 or R6 grade mooring chain;

wherein, after blanking, induction heating is carried out at 850-1000 ℃ and bending is carried out at 950-800 ℃.

8. A mooring chain with high environmental crack resistance according to claim 7, characterized in that the braided chain is heat treated in a batch or vertical continuous tempering furnace, the accessories of which are heat treated in the batch tempering furnace.

9. A high environmental crack resistance mooring chain according to claim 8, wherein heat treating: heating, water-cooling quenching, tempering air cooling or water cooling at a temperature higher than the solid solution temperature of (Mo, V) CN carbonitride and lower than the solid solution temperature of NbCN carbonitride; the heat treatment steps are as follows: firstly, quenching at least once, wherein the quenching temperature is more than 900 ℃ each time, and water cooling is carried out, and the water temperature is less than 60 ℃; tempering is carried out after quenching treatment, the tempering temperature is 590-650 ℃, and water cooling or air cooling is carried out; and flaw detection and mechanical property sampling and testing are respectively carried out at one third of the radius of the cross section of the base body and the weld joint after the whole heat treatment of the mooring chain. The weld was tested for impact work only, as standard.

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