CN114525455B - Wire rod, preparation method thereof and prestressed concrete steel bar prepared from wire rod - Google Patents

Abstract

The invention discloses a wire rod, which comprises the following elements in percentage by mass: c: 0.28 to 0.33%, Si: 0.75 to 0.85%, Mn: 0.90-1.10%, Cr: 0.01 to 0.20%, Ti: 0.02-0.04%, N: 0.004-0.006 percent of Fe, less than or equal to 0.006 percent of S, less than or equal to 0.010 percent of P, less than or equal to 0.002 percent of O, and the balance of Fe and inevitable impurities. The invention also discloses a preparation method of the wire rod and a prestressed concrete steel bar prepared from the wire rod. The beneficial hydrogen traps are increased by improving the chemical components of the wire rod and controlling the mass ratio of Ti to N; adjusting casting process, limiting soft stirring time, pouring with low superheat degree, and controlling total amount and size of inclusions. The steel bar finally prepared by the method has high tensile strength and elongation after fracture, and has good delayed fracture resistance.

Description

Wire rod, preparation method thereof and prestressed concrete steel bar prepared from wire rod

Technical Field

The invention relates to the field of ferrous metallurgy, in particular to a wire rod, a preparation method thereof and a prestressed concrete steel bar prepared from the wire rod.

Background

The prestressed concrete pipe pile is an important building base material and plays a supporting role for the whole building. In the last thirty years, along with the rapid development of national economy, the application of the concrete pipe pile is greatly increased. The prestressed steel bars are the core framework of the concrete pipe pile, and determine the performance of the concrete pipe pile to a great extent.

Delayed fracture is a core problem that hinders the application of concrete pipe piles. During the manufacturing and using processes of the concrete pipe pile, water is contacted, the water gradually permeates to the surface of steel, a part of hydrogen atoms are formed through a series of electrochemical actions, the hydrogen atoms are diffused into steel and gradually gathered around the hydrogen traps, some large defects in the steel belong to harmful hydrogen traps, the hydrogen atoms gradually weaken the bonding force of the defects and a matrix after being gathered on the surface of the large defects, and catastrophic sudden fracture occurs under the stress lower than the yield strength of the steel after a period of time accumulation under the coupling action of prestress and the environment.

Delayed fracture generally occurs in high-strength steel of 1000MPa or more, and the higher the strength, the greater the tendency to be. The tensile strength of the prestressed steel bar is generally above 1400MPa, the prestressed steel bar belongs to a quenching and tempering structure, delayed fracture is easy to occur, and hidden danger is caused to buildings. In order to solve the problem of delayed fracture, the invention adopts measures of introducing a beneficial hydrogen trap, reducing harmful defects and the like, and obtains remarkable effect.

Chinese patent document CN110747404A discloses a 1570 MPa-level delayed fracture resistant steel bar and a manufacturing method thereof, wherein the steel bar is mainly used for a high-strength delayed fracture resistant PC steel bar and can meet the requirement of delayed fracture resistance under the strength level of 1570MPa by improving the content of Si element, reasonably adding metal element Ti and adjusting the hot rolling and cooling control and steel bar production processes; although the Ti element is added to form an effective hydrogen trap and inhibit the delayed fracture tendency of the steel bar, the mass fraction of the Ti element is 0.05-0.08% or 0.06-0.08%, and the content is slightly higher, so that the coarsening effect of the precipitate cannot be achieved, large-grain TiN inclusion is formed, the wire rod drawing is not facilitated, and the method is not economical. Chinese patent document CN102703804A discloses a method for producing a wire rod for a high-strength prestressed steel rod, which comprises blast furnace smelting, converter smelting, LF ladle refining, and billet rolling, wherein silicon and manganese are used as main strengthening elements, and the emphasis is to improve the strength of the wire rod, and there is no content related to improving the delayed fracture performance of the steel rod. Chinese patent document CN110527917A discloses a 30MnSiBCa hot-rolled wire rod for PC steel rods and a preparation method thereof, wherein B and Ca elements are added into the 30MnSiBCa hot-rolled wire rod, and the proportion of B to Cr, Mn and Si and the content of Ca and B are controlled to improve the hardenability and the delayed fracture resistance of the wire rod.

Disclosure of Invention

Therefore, the technical problem to be solved by the invention is the defect of poor delayed fracture resistance of the existing prestressed steel bar, so that the wire rod, the preparation method thereof and the prestressed concrete steel bar prepared by the wire rod are provided.

Therefore, the invention adopts the following technical scheme:

the invention provides a wire rod, which comprises the following elements in percentage by mass:

c: 0.28 to 0.33%, Si: 0.75 to 0.85%, Mn: 0.90-1.10%, Cr: 0.01 to 0.20%, Ti: 0.02-0.04%, N: 0.004-0.006 percent of Fe, less than or equal to 0.006 percent of S, less than or equal to 0.010 percent of P, less than or equal to 0.002 percent of O, and the balance of Fe and inevitable impurities.

Further, the mass ratio of Ti to N is 5-7.

Furthermore, the grades of A, B, C, D, DS inclusions in the nonmetallic inclusions of the wire rod are all less than or equal to 1.0 grade; the sum of the grades of the five types of impurities, namely A + B + C + D + DS, is less than or equal to 3.0 grade, and the length-width product of the largest impurity is less than or equal to 1000 mu m ² The density of inclusions with the equivalent diameter of more than 10 mu m is less than or equal to 10/cm ² 。

The invention also provides a preparation method of the wire rod, which comprises the following steps:

s1: pre-desulfurizing the raw material molten iron to ensure that the sulfur content is less than 50 ppm;

s2: smelting the desulfurized molten iron in a converter;

s3: refining the molten iron smelted by the converter;

s4: continuously casting molten steel square billets obtained after refining;

s5: carrying out high speed wire rolling on the obtained casting blank;

s6: and (4) carrying out stelmor controlled cooling on the obtained crude wire rod to obtain a wire rod finished product.

Further, in step S3, the molten iron is subjected to soft stirring after refining, wherein the pressure of the soft stirring bottom blowing argon is 0.5-0.6MPa, and the time is 30-35min, so as to ensure that the inclusions float up sufficiently.

In step S4, pouring with a tundish is used to ensure low superheat degree pouring and the molten steel temperature is kept at the liquidus temperature (according to the components)Different variation, about 1495 ℃) above 15-25 ℃ to prevent the impurities from gathering and enlarging; adopts a submerged nozzle to carry out full-protection pouring, and the continuous casting is 140 multiplied by 140mm ² Controlling the drawing speed of the small square billet to be 2.6 +/-0.1 m/min during continuous casting; meanwhile, crystallizer electromagnetic stirring and tail end electromagnetic stirring are adopted.

Step S5, before rolling, heating the casting blank, wherein the soaking temperature is 1160 +/-10 ℃, the initial rolling temperature is 1030 +/-10 ℃, and the spinning temperature is 870 +/-10 ℃;

in step S6, the inlet speed of the cooling control roller way is 0.43-0.48m/S, the cooling control environment is room temperature, all fans are closed, the diameter of the finally obtained wire rod is 8-14mm, the tensile strength Rm is more than or equal to 700MPa, and the reduction of area Z is more than or equal to 50%.

The invention also provides a prestressed concrete steel bar which is prepared from the wire rod.

The preparation method of the prestressed concrete steel bar comprises the steps of drawing and quenching a wire rod, and then tempering to obtain the prestressed concrete steel bar.

Further, specifically, the wire rod is drawn into a steel rod with the diameter of 7-13mm and then quenched, wherein the quenching temperature is 900-; then carrying out tempering treatment at the temperature of 440-460 ℃ for 1-3s, wherein the microstructure of the finally obtained product is tempered troostite.

The technical scheme of the invention has the following advantages:

(1) the invention defines the elemental composition in a wire rod, wherein:

c is the most basic strengthening element in steel, the strength of the wire rod is increased by about 10MPa when the content of C is increased by 0.01 percent, but excessive C can reduce the plasticity of the wire rod and deteriorate the processing and welding performance. Therefore, the range of C content in the present invention is strictly limited to 0.28 to 0.33%.

Si is a ferrite strengthening element and can increase the strength of ferrite by solid solution strengthening, and Si is also an important deoxidizer and contributes to reduction of the oxygen content in steel and inclusion reduction. Meanwhile, Si can improve the thermal stability of the structure, weaken the strength reduction degree caused by the rise of the tempering temperature, improve the controllability of the performance of the steel bar and is beneficial to improving the delayed fracture resistance of the steel bar. But too much Si reduces the wire rod plasticity. The Si content range in the invention is selected to be 0.75-0.85%.

Mn is mainly used for increasing the strength of steel and simultaneously improving hardenability, and can change sulfide components and reduce the harmful effect of S, but excessive Mn can reduce the plasticity and weldability of wire rods and increase the delayed fracture sensitivity of steel. In the invention, the content of Mn is controlled to be 0.90-1.10%.

Cr can further improve the hardenability and the finished product strength and the tempering resistance of the steel bar. But excessive Cr also reduces the plasticity and weldability of the wire rod. The product performance and cost factors are comprehensively considered, and the Cr content is controlled to be 0.01-0.20%.

Ti is an important microalloy element, forms fine Ti (C, N) precipitates in steel, becomes a beneficial hydrogen trap in the steel, prevents hydrogen atoms from aggregating to a harmful hydrogen trap, and improves the delayed fracture resistance of the steel bar. However, too high Ti coarsens precipitates, which is disadvantageous in the smooth continuous casting. In the invention, the Ti content is controlled to be 0.02-0.04%.

N is a gas element in steel, and mainly combines with Ti to form fine precipitates, and the formation of the fine precipitates is not facilitated when the content of N is too high or too low, wherein the content of N is controlled to be 0.004-0.006%.

S and Mn are combined to form MnS large inclusions which are often the starting points of hydrogen induced cracks and can increase the delayed fracture sensitivity of the steel bar; p is a harmful element in steel, is easy to segregate, is enriched at a crystal boundary to cause embrittlement of steel, reduces plasticity and deteriorates welding performance; o is a major source of inclusions in steel and is one of the main causes of the formation of harmful hydrogen traps. Therefore, the content of S, P, O must be strictly controlled so that S is less than or equal to 0.006 percent, P is less than or equal to 0.010 percent and O is less than or equal to 0.002 percent.

(2) The present invention further defines the mass ratio of Ti to N to be 5 to 7, which can secure Ti (C, N) precipitates having a large number and a small size. The Ti (C, N) fine precipitates can pin the grain boundary, prevent austenite grains from growing, increase the area of the grain boundary and hinder the diffusion of hydrogen atoms in steel, and meanwhile, the Ti (C, N) precipitates can become beneficial hydrogen traps in the steel, capture and absorb hydrogen to prevent the hydrogen from diffusing, hinder the hydrogen atoms from gathering to harmful hydrogen traps, reduce the concentration of the hydrogen at the harmful hydrogen traps and achieve the purpose of improving the delayed fracture resistance of the steel rod.

(3) The present invention defines the size of the non-metallic inclusions in the wire rod. Non-metallic inclusions, particularly large inclusions, are important sources of harmful hydrogen traps, hydrogen atoms are highly enriched in local regions of the harmful hydrogen traps, and after the hydrogen concentration of the enriched regions reaches a critical value, the bonding force of the inclusions and steel bases is destroyed, cracks are formed and expanded, and the inclusions are broken under the stress lower than the yield strength of steel. The invention limits the rating of A, B, C, D, DS types of inclusions to be less than or equal to 1.0 grade, the sum of the ratings of five types of inclusions, namely A + B + C + D + DS, is less than or equal to 3.0 grades, and the length-width product of the largest inclusion is less than or equal to 1000 mu m ² The density of inclusions with the equivalent diameter of more than 10 mu m is less than or equal to 10/cm ² The number of harmful hydrogen traps can be effectively reduced, and the delayed fracture resistance of the steel bar is improved.

(4) The invention optimizes the preparation method of the wire rod, the production flow of the wire rod sequentially passes through the technical processes of 'molten iron pre-desulfurization → converter smelting → refining → square billet continuous casting → high-speed wire rolling → stelmor controlled cooling', and the like, particularly the soft stirring time after refining is controlled within 30-35min, so that the inclusions are ensured to float sufficiently, the continuous casting superheat degree is controlled within 15-25 ℃ (the liquidus temperature is about 1495 ℃), the aggregation and the enlargement of the inclusions are prevented, the sizes of the inclusions are controlled, the number of harmful hydrogen traps is reduced, and the delayed fracture resistance of the steel rod is improved.

(5) The steel rod prepared by the wire rod provided by the invention has good delayed fracture resistance on the basis that the tensile strength Rm-b is more than or equal to 1500MPa and the elongation A after fracture is more than or equal to 10%.

Detailed Description

The following examples are provided to further understand the present invention, not to limit the scope of the present invention, but to provide the best mode, not to limit the content and the protection scope of the present invention, and any product similar or similar to the present invention, which is obtained by combining the present invention with other prior art features, falls within the protection scope of the present invention.

The examples do not show the specific experimental steps or conditions, and can be performed according to the conventional experimental steps described in the literature in the field. The reagents or instruments used are not indicated by manufacturers, and are all conventional reagent products which can be obtained commercially.

Examples

The embodiment provides a preparation method of a wire rod, which comprises the following specific steps:

(1) pre-desulfurizing the raw material molten iron to ensure that the sulfur content is less than 50 PPM;

(2) smelting the desulfurized molten iron in a converter;

(3) refining the molten iron after the converter, and carrying out soft stirring on the molten iron after the refining, wherein the pressure of argon blowing at the bottom of the soft stirring is 0.5MPa, and the time is 30min, so that impurities are ensured to float sufficiently;

(4) continuously casting the refined molten iron square billet, and adopting tundish induction heating to ensure low superheat degree pouring, so that the temperature of the molten iron square billet is 20 ℃ above the solidification point of the molten iron, and the aggregation and enlargement of inclusions are prevented; adopts a submerged nozzle to carry out full-protection pouring, and the continuous casting is 140 multiplied by 140mm ² Controlling the drawing speed to be 2.6m/min during continuous casting of the small square billet; meanwhile, crystallizer electromagnetic stirring and tail end electromagnetic stirring are adopted.

(5) And (3) carrying out high-speed wire rolling on the obtained casting blank, heating the casting blank before rolling, wherein the soaking temperature is 1160 ℃, the initial rolling temperature is 103 ℃, and the spinning temperature is 870 ℃.

(6) And (3) carrying out stelmor controlled cooling on the obtained coarse wire rod, wherein the inlet speed of a controlled cooling roller way is 0.45m/s, the controlled cooling environment is room temperature, all fans are closed, and the diameter of the finally obtained wire rod is 10 mm.

The chemical composition of the wire rod obtained in the examples is shown in table 1:

TABLE 1 wire rod chemical composition

Ingredient/wt%	C	Si	Mn	P	S	Cr	Ti	O	N	Ti/N
											Example 1	0.31	0.82	0.95	0.0090	0.0045	0.06	0.028	0.0018	0.0045	6.22
Example 2	0.30	0.79	0.98	0.0079	0.0050	0.07	0.031	0.0010	0.0048	6.46
											Example 3	0.29	0.80	1.01	0.0082	0.0042	0.07	0.032	0.0014	0.0052	6.15
Example 4	0.32	0.80	0.99	0.0084	0.0047	0.05	0.030	0.0008	0.0048	6.25

Comparative example 1

This comparative example is different from the examples in that the final chemical composition of the wire rod was changed so that Ti/N was 2.74.

Comparative example 2

This comparative example is different from the examples in that the final chemical composition of the wire rod was changed so that Ti/N was 8.63.

Comparative example 3

This comparative example is different from the examples in that the soft stirring time in refining was changed to 10 min.

Comparative example 4

Compared with the embodiment, the difference of the comparative example is that the superheat degree during continuous casting is changed, and the temperature of the molten steel is 30 ℃ above the liquidus.

The chemical compositions of the wire rods obtained in comparative examples 1 to 4 are shown in table 2:

TABLE 2 wire rod chemical composition

Ingredient/wt%	C	Si	Mn	P	S	Cr	Ti	O	N	Ti/N
											Comparative example1	0.30	0.81	0.95	0.0093	0.0056	0.06	0.017	0.0017	0.0062	2.74
Comparative example 2	0.29	0.78	0.99	0.0084	0.0045	0.07	0.044	0.0015	0.0051	8.63
											Comparative example 3	0.30	0.80	0.98	0.0095	0.0051	0.05	0.030	0.0016	0.0045	6.67
Comparative example 4	0.31	0.79	0.97	0.0089	0.0046	0.07	0.030	0.0018	0.0044	6.82

Test example 1

The inclusions in the wire rods obtained in the examples and the comparative examples are detected, the detection standard is GB/T10561-2005, and the detection results are shown in the following table 3:

TABLE 3 wire rod inclusions

As can be seen from the above table, in the embodiment of the invention, the grades of the A, B, C, D, DS types of inclusions are all less than or equal to 1.0 grade, the sum of the grades of the five types of inclusions A + B + C + D + DS is less than or equal to 3.0 grade, and the length-width product of the largest inclusion is less than or equal to 1000 mu m ² The density of inclusions with the equivalent diameter of more than 10 mu m is less than or equal to 10/cm ² . Comparative examples 1 and 2 are similar to the present application in inclusion status, while comparative examples 3 and 4 are clearly unsatisfactory in inclusion status because the conditions defined in the present application are not satisfied in soft stirring time and degree of superheat, respectively.

Test example 2

The steel wire rods obtained in the examples and the comparative example 1 are used for preparing prestressed concrete steel rods, and the preparation method comprises the following steps:

and drawing the wire rod into a steel rod with the diameter of 8mm, quenching at 920 ℃ for 1s, and then tempering at 450 ℃ for 1s to finally obtain the prestressed concrete steel rod.

The properties of the wire rod and the steel bar prepared by the wire rod are detected according to GB/T21839-2019, and the results are shown in the following table 4:

TABLE 4 Main Properties of wire rod and Steel bar

Obviously, as can be seen from the above table, the critical stress values of the steel bar in the embodiment of the invention when the steel bar is constantly loaded in the ammonium thiocyanate solution for 5 hours are all more than or equal to 0.80Rm-b, and the requirement that the prestressed product is loaded with tensile stress of 0.80Rm-b and does not break when the prestressed product lasts for 5 hours in the ammonium thiocyanate solution is met; the steel bars in the comparative examples are different from the steel bars in the comparative examples 1 and 2, the Ti/N ratio of which does not meet the requirements of the application, or the steel bars in the comparative examples 3 and 4, the soft stirring time of which is too short, and the comparative examples 4, the critical stress value of which is less than 0.80Rm-b when the steel bars are constantly loaded in an ammonium thiocyanate solution for 5 hours, show that the tensile stress loaded on the steel bars in the comparative examples 0.80Rm-b lasts for less than 5 hours in the ammonium thiocyanate solution, and the stress corrosion resistance of each pair of the comparative examples is poor.

The above examples are merely illustrative for clarity and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications therefrom are within the scope of the invention.

Claims (1)

1. A preparation method of a prestressed concrete steel bar is characterized in that a wire rod is drawn into a steel bar with the diameter of 7-13mm and then quenched at the temperature of 900-;

the wire rod comprises the following elements in percentage by mass:

c: 0.28 to 0.33%, Si: 0.75 to 0.85%, Mn: 0.90-1.10%, Cr: 0.01 to 0.20%, Ti: 0.02-0.04%, N: 0.004-0.006 percent of the total weight of the alloy, less than or equal to 0.006 percent of S, less than or equal to 0.010 percent of P, less than or equal to 0.002 percent of O, and the balance of Fe and inevitable impurities;

the mass ratio of Ti to N is 5-7;

among the nonmetallic inclusions of the wire rod, the grades of A, B, C, D, DS types of inclusions are all less than or equal to 1.0 grade; the sum of the grades of the five types of impurities, namely A + B + C + D + DS, is less than or equal to 3.0 grade, and the length-width product of the largest impurity is less than or equal to 1000 mu m ² The density of inclusions with the equivalent diameter of more than 10 mu m is less than or equal to 10/cm ² ；

The preparation method of the wire rod comprises the following steps:

s1: pre-desulfurizing the raw material molten iron;

s2: smelting the desulfurized molten iron in a converter;

s3: refining the molten iron smelted by the converter;

s4: continuously casting molten steel square billets obtained after refining;

s5: carrying out high speed wire rolling on the obtained casting blank;

s6: carrying out stelmor controlled cooling on the obtained coarse wire rod to obtain a wire rod finished product;

in step S3, soft stirring is carried out on the molten iron after refining, wherein the pressure of argon blowing at the bottom of the soft stirring is 0.5-0.6MPa, and the time is 30-35 min;

in step S4, low superheat degree casting is adopted, and the continuous casting is carried out for 140 multiplied by 140mm ² Controlling the drawing speed of the small square billet to be 2.6 +/-0.1 m/min during continuous casting;

step S5, before rolling, heating the casting blank, wherein the soaking temperature is 1160 +/-10 ℃, the initial rolling temperature is 1030 +/-10 ℃, and the spinning temperature is 870 +/-10 ℃;

in step S6, the entrance speed of the controlled cooling roller path is 0.43-0.48m/S, the controlled cooling environment is room temperature, all fans are closed, and the diameter of the finally obtained wire rod is 8-14 mm.