CN114134405B

CN114134405B - Acicular ferrite/massive ferrite steel plate for ship and manufacturing method thereof

Info

Publication number: CN114134405B
Application number: CN202110569030.4A
Authority: CN
Inventors: 刘朝霞; 罗元东; 许晓红; 刘俊; 周永浩; 戚晓光; 潘继峰; 徐光琴; 高俊; 陈翀
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2021-05-25
Filing date: 2021-05-25
Publication date: 2022-07-22
Anticipated expiration: 2041-05-25
Also published as: CN114134405A

Abstract

The invention relates to an acicular ferrite/massive ferrite steel plate for ships and a manufacturing method thereof, wherein the microstructure is acicular ferrite and massive ferrite, the volume percentage of the acicular ferrite is 40 +/-5%, and the average grain size of the ferrite is 3-6 μm. The transverse tensile property of the yield strength is 390-420 MPa, the tensile strength is 520-550 MPa, the yield ratio is 0.72-0.77, the elongation is more than or equal to 29 percent, the longitudinal impact toughness values at minus 40 ℃ and minus 60 ℃ are more than or equal to 200J, and the impact toughness values at minus 40 ℃ and minus 60 ℃ are not obviously reduced after strain aging. The low carbon content is adopted, and a ferrite phase is formed as much as possible by adding Si-Cr-Nb element, so that the formation of pearlite is inhibited. The higher final cooling temperature is adopted, the higher final cooling temperature ensures that the matrix structure of the steel plate is still ferrite/acicular ferrite, and the upper bainite or granular bainite which can improve the strength but deteriorate the toughness and the plastic structure of the steel plate are avoided.

Description

Acicular ferrite/massive ferrite steel plate for ship and manufacturing method thereof

Technical Field

The invention belongs to the field of ferrous metallurgy, and particularly relates to an acicular ferrite/massive ferrite steel plate for a ship and a preparation method thereof. The thickness of the steel plate is 50-70mm, and the steel plate has good low-temperature impact toughness and strain aging performance.

Background

With the continuous development and utilization of ocean resources in various countries in the world, the ocean shipbuilding industry and the equipment industry are greatly developed, and the technical progress of steel plates for ships and ocean engineering is driven. The steel plate for modern ships and ocean engineering is developing to a larger thickness, and is required to have the characteristics of high strength, high toughness, easiness in welding, easiness in processing, corrosion resistance and the like.

Currently, the most popular steel sheets for high-strength hull structures, such as AH36, DH36, and EH36, having a yield strength of 355MPa or more, are generally delivered by production methods such as hot rolling (AR), Controlled Rolling (CR), Normalizing Rolling (NR), thermomechanical rolling (TMCP), and normalizing (N), and have a mixed structure mainly of ferrite and pearlite in the structure form. For a large-thickness steel plate with the structure of ferrite and pearlite between 50-70mm, because the compression ratio of the continuous casting billet and the steel plate is low, the internal defects caused by the segregation of the continuous casting billet are difficult to be obviously improved by a TMCP process, and the TMCP state is not generally adopted for delivery. When delivered in other states, in order to ensure the strength of the steel plate, sufficient amount of C and other alloy or microalloy elements are required to be added to replace the function of cooling water after TMCP process rolling. The carbon equivalent and the welding cold crack sensitivity index are also high, so that the improvement of the welding performance of the steel plate is not facilitated, and the deterioration of the steel plate structure performance caused by large heat input during the welding resistance of the steel plate influences the implementation of the high-efficiency welding of the thick plate.

Through retrieval, much work is devoted to the production of marine thick plates by the TMCP process, for example, the patent document CN105506253A obtains TMCP ship plate steel through three-stage rolling, wherein the final rolling temperature of the third stage is reduced to be below 800 ℃, and although the performance result of EH36 steel grade is barely achieved, the production rhythm, the shape of a steel plate and the load of a rolling mill are influenced negatively. CN103264052A also obtains 68mm EH40 and 60mm AH36 steel plates by three-stage rolling, the final cooling temperature of 36-stage boat plates is 540-560 ℃, the final cooling temperature is too low to be beneficial to the structural uniformity of the thick plate in the thickness direction, and the cooling rate reaches 14-16 ℃/s, and the upper limit control capability of the common ACC cooling system is reached or exceeded. Patent document CN106756543 relates to a high-strength ship plate steel for TMCP state low-cost large heat input welding, which has many added alloys and too high alloy cost, and does not meet the development trend of cost reduction and efficiency improvement of ship plate steel.

Disclosure of Invention

In order to reduce the manufacturing cost of the steel plate for the ship and further improve the obdurability and the weldability of the steel for the ship, the steel for the ship plate with the structure consisting of low-carbon acicular ferrite and massive ferrite is designed, and the strength of the steel plate is ensured by utilizing the characteristic that the hardness of the structures of the acicular ferrite and the massive ferrite is harder than that of the prior eutectoid ferrite and softer than that of pearlite in the traditional steel for the ship body. The interface bonding capability of the acicular ferrite and the massive ferrite is better than that of the proeutectoid ferrite and pearlite phase interface, so that good toughness and brittle failure resistance are achieved.

The components of the invention adopt low carbon design, and design elements for promoting the formation of the massive ferrite/acicular ferrite, and the formation of pearlite is inhibited in the production process by combining TMCP technology, so as to obtain the target structure of the acicular ferrite and the massive ferrite. The steel plate obtained by the invention meets the performance requirements of steel grades with the low-temperature toughness steel of more than or equal to 355MPa and the tensile strength of more than or equal to 490MPa in classification society specifications, and has the characteristics of low yield ratio, good obdurability matching, excellent low-temperature impact toughness, low-temperature strain aging resistance and the like.

The specific technical scheme of the invention is as follows: a steel plate for an acicular ferrite/massive ferrite ship has a microstructure of acicular ferrite and massive ferrite, wherein the volume ratio of the acicular ferrite is 40 +/-5% (the ratio of the structure is the area ratio of the structure on a metallographic picture), and the average grain size of the ferrite is 3-6 μm. The yield strength transverse tensile property is 390-420 MPa, the tensile strength is 520-550 MPa, the yield ratio is 0.72-0.77, the elongation is more than or equal to 29 percent, the longitudinal impact toughness values at minus 40 ℃ and minus 60 ℃ are more than or equal to 200J, and the impact toughness values at minus 40 ℃ and minus 60 ℃ are not obviously reduced after strain aging.

In order to realize the steel plate, the adopted element design is as follows by mass percent: 0.03-0.05%, Si: 0.35 to 0.50%, Mn: 1.40-1.60%, P: less than or equal to 0.0070%, S: less than or equal to 0.0030 percent, Ti: 0.008-0.020%, N: 0.0030-0.0060%, Cr: 0.10 to 0.25%, Nb: 0.040 to 0.060%, Al: 0.005-0.020%, Ca: 0.0005 to 0.0020%, the balance being Fe and unavoidable impurity elements.

The basis for designing the elements is

C: the reduction of the carbon content in the steel plate can inhibit pearlite transformation, increase the ferrite phase fraction, improve the low-temperature impact property and the low-temperature strain aging property of the steel, and improve the welding cold crack sensitivity of the steel plate, and the more the carbon content in the steel is, the more the realization of the target structure of the invention is not facilitated. However, if the carbon design is less than 0.03%, the production cost of converter steelmaking is significantly increased. Therefore, the carbon content is controlled to be 0.03-0.05%.

Si: the present invention is Si-Al deoxidation, and the lower limit of the present invention is set to 0.35% in order to have as much ferrite phase composition as possible, but too much Si seriously deteriorates the weldability of the steel sheet, increases the martensite-austenite (MA) component in the heat affected zone, affects the impact toughness of the heat affected zone, and if more than 0.50% causes segregation in the steel sheet thickness 1/2 and deteriorates the weldability, so the upper limit thereof is defined to 0.50%.

Mn: the steel has the function of delaying the transformation from austenite to ferrite, and is beneficial to refining ferrite and improving strength and toughness. When the content of manganese is low, the effect is not obvious, the strength and the toughness of the steel plate are low, and the like. If the manganese content is too high, the segregation, poor toughness, reduced weldability and the like of the continuous casting billet can be caused, so that the comprehensive addition of the alloy is considered, and the addition amount of the manganese content is regulated to be within the range of 1.40-1.60%.

P: although the corrosion resistance is improved, it is not suitable for structural steel because it reduces low-temperature toughness and hinders weldability, and the present invention provides that the corrosion resistance is controlled to 0.0070% or less.

S: the formation of MnS inclusions also causes center segregation, adversely affects low-temperature toughness, strain aging properties, and weldability, and is controlled to 0.0030% or less.

Ti: by forming Ti₂O₃The particles can promote the generation of intragranular ferrite, increase the formation probability of acicular ferrite, and simultaneously can be used for fixing nitrogen elements in steel, under proper conditions, titanium and nitrogen form titanium nitride, thereby preventing austenite grains from coarsening in the high-temperature process of heating, rolling and particularly welding up to 1350 ℃, improving the extremely-low-temperature toughness of a base material and a welding heat affected zone, and improving the welding performance. When the titanium content is less than 0.008%, the effect is poor, and when it exceeds 0.020%, excessive titanium is compositely precipitated with other elements, and the toughness of the steel deteriorates.

N: the invention is an important element, which is different from the prior harmful element control, but the nitrogen content in the steel plate is ensured to be certain, when the ratio of Ti atoms to N atoms in the steel is 1: 1, the ratio of the weight of Ti to the weight of N is 3.42, TiN particles are most fine and dispersed, the refining effect on high-temperature austenite grains is strongest, excellent toughness can be obtained, and the content of N is controlled to be 0.0030-0.0060% according to the addition amount of Ti, the oxygen content in steel and other nitrogen-fixing elements.

Cr: is an element for improving the hardenability of steel, and improves the strength of a steel sheet. In addition, Cr is a ferrite forming element and is beneficial to forming ferrite, and compared with Ni and Cu alloys, the same addition amount can obviously reduce the alloy cost. However, the content is too high, which increases the carbon equivalent and the weld cold crack sensitivity index, resulting in poor surface quality of the steel sheet. The Cr content of the invention is controlled between 0.10 and 0.25 percent in consideration of the specification requirements of classification society.

Nb: the solute dragging effect of Nb and the pinning effect of Nb (C, N) on austenite grain boundaries both inhibit the recrystallization of deformed austenite, expand the austenite non-recrystallization interval and reduce the production temperature waiting time of extra-thick plates. And precipitates are formed during cooling or tempering, so that the strength and toughness are improved, and the probability of forming an acicular ferrite phase can be increased. When the addition amount is less than 0.040%, the formation phase of the acicular ferrite is not enough, and when the addition amount is more than 0.060%, the surface crack of the continuous casting billet is easily generated, and the cost is increased. Therefore, the present invention provides that the content of niobium should be in the range of 0.040 to 0.060%.

Al: is an excellent deoxidizer of steel, effectively refines grain elements and improves the strength and the toughness of the steel. In combination with the amount of Si added, the present invention provides that the content of niobium should be in the range of 0.005 to 0.020%.

Ca: the calcium treatment is an essential treatment link of the steel grade, large inclusions can be formed when the content of Ca is too high, and the effect cannot be achieved when the content of Ca is too low, and the content of Ca is regulated to be 0.0005-0.0020%.

Another object of the present invention is to provide a method for producing the above steel sheet, comprising

Smelting molten steel: adopting molten iron pretreatment and KR deep desulfurization to enable the sulfur content in molten steel to be lower than 0.0020 percent; converter tapping adopts premelted refining slag and active lime for slagging; the converter end point C is less than or equal to 0.02%, argon and nitrogen are bottom blown for 15-20 min in the tapping process, and the tapping time of a 150-ton converter is controlled to be 4-6 min; the LF and RH refining of the molten steel is carried out, the molten steel is heated to melt slag after reaching an LF furnace, alloy is added, and finally Ti wires and Ca wires are fed, so that the yield of Ti and Ca is improved, Ti is prevented from being precipitated at an overhigh temperature, and the formation of large titanium oxide inclusions is avoided.

Continuous casting: and casting the molten steel into a continuous casting billet by a continuous casting process.

The rolling process comprises the following steps: the compression ratio of the thickness of the continuous casting blank to the thickness of the finished product is more than or equal to 4, TMCP technology is adopted, the reheating temperature of the continuous casting blank is 1180-1220 ℃, the heating time is 1.2-1.5 min/mm, the rolling is controlled by adopting two stages of rough rolling and finish rolling, the reduction rate of a single pass of rough rolling is 12-20%, the finish rolling temperature of the rough rolling is 1000-1080 ℃, and the thickness of the intermediate blank obtained after the rough rolling is 1.7-2.5 times of the thickness of the finished product; the initial rolling temperature of the finish rolling is 850-880 ℃, the reduction rate of the last two rolling passes of the finish rolling is less than or equal to 5%, accelerated cooling is adopted after rolling, the cooling rate is 8-12 ℃/s, the final cooling temperature is 600-640 ℃, then air cooling is carried out to 400 ℃, and the steel plate is subjected to off-line stacking cooling.

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

1. the low carbon content is adopted, and a ferrite phase is formed as much as possible by adding Si-Cr-Nb element, so that the formation of pearlite is inhibited.

2. By adding a higher amount of Nb, on one hand, part of Nb and Ti are compositely precipitated in the austenite recrystallization (rough rolling) process to prevent austenite grains from coarsening, and then in the cooling process after rolling, Nb is further precipitated to promote the nucleation of acicular ferrite in the grains and ensure that the steel obtains the acicular ferrite with a certain proportion (the volume percentage of the acicular ferrite in the invention is about 40 percent), thereby improving the toughness of the large-thickness steel plate. This structure is significantly different from the ferrite + pearlite structure (some bainite will also appear in the thick plate) of conventional 355MPa grade steel.

3. By adding ferrite forming element Cr, on one hand, the hardenability of the steel is improved, and alloy such as Ni, Cu, Mo and the like is not added, so that the alloy addition cost is saved. And the strength of the ultra-thick steel plate is ensured by adjusting the strength of the cooling water and controlling the cooling speed, the formation of low-temperature structures such as bainite is delayed, the final cooling temperature is controlled at 600-640 ℃, the formation of block-shaped ferrite is ensured, and the occurrence of bainite is avoided. The bulk ferrite has higher strength than the general proeutectoid ferrite and the forming temperature is slightly lower than that of the proeutectoid ferrite.

4. Through the control of the compression ratio of the continuous casting billet, in the TMCP rolling process, the pass reduction is increased, the total number of passes is controlled, and the problem of excessive performance nonuniformity in the thickness direction is solved at a high cooling speed. Meanwhile, the average grain size of the ferrite is 3-6 mu m in order to increase the phase change driving force of acicular ferrite/massive ferrite, increase nucleation points and refine the final grain size.

5. Although the cooling rate is high and is between 8 and 12 ℃/s, the high final cooling temperature (600 to 640 ℃) is adopted, and the high final cooling temperature ensures that the matrix structure of the steel plate is still ferrite/acicular ferrite, so that the phenomenon that the toughness and the plastic structure of the steel plate are deteriorated while the strength of upper bainite or granular bainite can be improved is avoided. In addition, the higher final cooling temperature is beneficial to reducing the internal stress of the thick steel plate, is beneficial to controlling the shape of the steel plate, and prevents the TMCP steel from being reduced in strength in the using process due to stress release.

6. The TMCP process window is an interval range in which large-scale production is easy to control, and can be realized without additionally increasing production equipment. The requirement of the four times compression ratio can be met by the blank design of most steel mills. In addition, the invention can also be popularized and applied to the manufacture of other large and thick steel plates such as steel for high-rise buildings, steel for bridges and other TMCP delivered steel.

Drawings

FIG. 1 shows the reduction ratios at each rolling pass in example 4 of the present invention.

FIG. 2 shows the structure of 1/4 mm in the thickness of the steel sheet in example 3 of the present invention.

Detailed Description

The present invention is described in further detail below with reference to examples, which are intended to be illustrative and not to be construed as limiting the invention.

The chemical compositions of the acicular ferrite/bulk ferrite marine steel sheet corresponding to each example are shown in table 1, in which the data is the mass percentage content of each element, and the balance is Fe and unavoidable impurity elements.

TABLE 1

Examples	Furnace number	C	Si	Mn	P	S	Cr	Ca	Al	Ti	N	Nb
													Example 1	S22007231	0.03	0.36	1.58	0.004	0.0014	0.10	0.0010	0.014	0.016	0.0035	0.047
Example 2	S22004775	0.04	0.48	1.46	0.003	0.0007	0.18	0.0012	0.008	0.012	0.0045	0.050
													Example 3	S11904874	0.05	0.40	1.53	0.005	0.0013	0.22	0.0020	0.018	0.015	0.0050	0.048
Example 4	S11904883	0.04	0.45	1.56	0.003	0.0016	0.15	0.0008	0.012	0.018	0.0036	0.056

The production process of the steel plate comprises the following steps:

adopting molten iron pretreatment and KR deep desulfurization, wherein the sulfur content is lower than 0.0020%; converter tapping adopts premelted refining slag and active lime for slagging; the end point C of the converter is less than or equal to 0.02 percent, argon and nitrogen are bottom blown for 15-20 min in the tapping process, and the tapping time of the 150-ton converter is controlled to be 4-6 min. And then LF and RH refining are carried out, the temperature is raised to melt slag after the molten steel reaches an LF furnace, alloy is added, finally Ti wires and Ca wires are fed, and then the molten steel is cast into a continuous casting billet.

The rolling process comprises the following steps: the compression ratio of the thickness of the continuous casting billet to the thickness of a finished product is more than or equal to 4, a TMCP (thermal mechanical control processing) process is adopted, the continuous casting billet is reheated, rolling is controlled in two stages of rough rolling and finish rolling, and the temperature of the rough rolling and the finish rolling is 1000-1080 ℃; the initial rolling temperature of finish rolling is 850-880 ℃, the reduction rate of the last two times of rolling is less than or equal to 5%, accelerated cooling is adopted after rolling, the final cooling temperature is 600-640 ℃, the cooling rate is 8-12 ℃/s, then air cooling is carried out to 400 ℃, and the rolling is carried out after line stacking cooling. The specific rolling process is shown in table 2, the typical rolling pass reduction distribution is shown in fig. 1 as example 4, and the specific cooling process is shown in table 3.

TABLE 2

TABLE 3

The mechanical properties of the steel sheets manufactured by the smelting, rolling and water cooling processes of the above examples are shown in Table 4. From the tensile property of the embodiment, the transverse tensile property of the yield strength of the steel plate is 397-412 MPa, the tensile strength is 523-545 MPa, the yield ratio is 0.72-0.77, and the elongation is better than or equal to 29%. The longitudinal impact toughness value at-40 ℃ and-60 ℃ is more than or equal to 200J.

TABLE 4

The 5% plastic deformation is carried out on the examples, the aging impact toughness values at 250 ℃ for 1 hour and at-40 ℃ and-60 ℃ are not obviously reduced, and the low-temperature aging resistance is excellent, and the concrete results are shown in Table 5.

TABLE 5

In addition to the above embodiments, the present invention also includes other embodiments, and all technical solutions formed by equivalent transformation or equivalent replacement should fall within the protection scope of the claims of the present invention.

Claims

1. An acicular ferrite/massive ferrite steel plate for a ship, characterized in that: the microstructure of the steel plate is acicular ferrite and massive ferrite, wherein the volume percentage of the acicular ferrite is 40 +/-5%, and the average grain size of the ferrite is 3-6 mu m;

the method for manufacturing the steel plate comprises the following steps:

smelting molten steel: adopting molten iron pretreatment and KR deep desulfurization to enable the sulfur content in molten steel to be lower than 0.0020%; converter tapping adopts premelted refining slag and active lime for slagging; the converter end point C is less than or equal to 0.02%, argon and nitrogen are bottom blown for 15-20 min in the tapping process, and tapping is controlled according to the tapping time control of a 150-ton converter within 4-6 min; LF and RH refining is carried out on the molten steel, the molten steel is heated and slagged after reaching an LF furnace, alloy is added, and finally Ti wires and Ca wires are fed;

continuous casting: casting the molten steel into a continuous casting billet by a continuous casting process;

the rolling process comprises the following steps: the compression ratio of the thickness of the continuous casting blank to the thickness of the finished product is more than or equal to 4, TMCP technology is adopted, the reheating temperature of the continuous casting blank is 1180-1220 ℃, the heating time is 1.2-1.5 min/mm, the rolling is controlled by adopting two stages of rough rolling and finish rolling, the reduction rate of a single pass of rough rolling is 12-20%, the finish rolling temperature of the rough rolling is 1000-1080 ℃, and the thickness of the intermediate blank obtained after the rough rolling is 1.7-2.5 times of the thickness of the finished product; the initial rolling temperature of finish rolling is 850-880 ℃, the reduction rate of two times of rolling at the end of finish rolling is less than or equal to 5%, accelerated cooling is adopted after rolling, the cooling rate is 8-12 ℃/s, the final cooling temperature is 600-640 ℃, then air cooling is carried out to 400 ℃, and the rolling is carried out by off-line stacking cooling.

2. The steel sheet according to claim 1, wherein: the steel plate comprises the following element components in percentage by mass: 0.03-0.05%, Si: 0.35 to 0.50%, Mn: 1.40-1.60%, P: less than or equal to 0.0070%, S: less than or equal to 0.0030 percent, Ti: 0.008-0.020%, N: 0.0030-0.0060%, Cr: 0.10 to 0.25%, Nb: 0.040 to 0.060%, Al: 0.005-0.020%, Ca: 0.0005 to 0.0020%, the balance being Fe and unavoidable impurity elements.

3. The steel sheet according to claim 1, wherein: the yield strength transverse tensile property is 390-420 MPa, the tensile strength is 520-550 MPa, the yield ratio is 0.72-0.77, the elongation is more than or equal to 29 percent, the longitudinal impact toughness values at minus 40 ℃ and minus 60 ℃ are more than or equal to 200J, and the impact toughness values at minus 40 ℃ and minus 60 ℃ are not obviously reduced after strain aging.

4. The steel sheet according to claim 1, wherein: the production thickness of the steel plate is 50-70 mm.