CN115369318B

CN115369318B - Low-cost high-strength fire-resistant steel for building structure and production method thereof

Info

Publication number: CN115369318B
Application number: CN202210893054.XA
Authority: CN
Inventors: 杜鹏举; 白云; 赵乾; 孙宪进; 苗丕峰; 高晓东; 沈斌; 马菁; 诸建阳; 许峰; 范海东
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2022-07-27
Filing date: 2022-07-27
Publication date: 2023-06-16
Anticipated expiration: 2042-07-27
Also published as: CN115369318A

Abstract

The invention relates to a low-cost high-strength steel for a fireproof building structure and a production method thereof, wherein the steel is a steel plate with the thickness not more than 30mm, a bainitic ferrite and a majoram are taken as target microstructures, and the sizes and the shapes of the bainitic ferrite and the majoram structures are accurately adjusted through the matching design of a finishing rolling temperature, a water inlet temperature and a finishing cooling temperature so as to simultaneously meet the requirements of low yield ratio, impact toughness and fireproof performance. The yield strength of the steel plate is more than 500MPa, the tensile strength is more than 625MPa, the yield ratio is less than 0.83, and the elongation is more than 16%; the Charpy impact energy at the temperature of minus 40 ℃ is more than 100J; when incubated at 600 ℃ for more than 90 minutes, the yield strength at 600 ℃ is still not lower than 2/3 of the yield strength at room temperature.

Description

Low-cost high-strength fire-resistant steel for building structure and production method thereof

Technical Field

The invention belongs to the technical field of iron-based alloys, and particularly relates to a steel plate for a building structure and a production method thereof.

Background

Development of low-carbon building structure systems such as wood structures, steel structures and the like is beneficial to reduction of carbon emission in the building industry of large energy consumption households. The steel structure has the advantages of high construction efficiency, good quality, light dead weight, high strength, good earthquake resistance and the like. And from the full life cycle of the building structure, the steel structure is easy to recycle, and almost does not produce building rubbish and dust noise pollution during the building and dismantling processes, and belongs to green materials.

The bottlenecks limiting the application of steel structure building structures are mainly high cost, poor fire resistance and poor corrosion resistance, and additional fire and corrosion protection treatments will further increase the construction costs. From the safety perspective, corrosion mainly causes the building steel structure to be slowly thinned to influence the durability, the time span is large, the building steel structure is easy to maintain through subsequent maintenance, the poor fire resistance causes the yield strength of the structural steel to be obviously reduced, and the building suddenly collapses when fire disaster is caused, which is a fatal defect of the building steel structure.

The special performance requirements of earthquake resistance, fire resistance and the like of the building structural steel are increased increasingly, and the technical development of the building structural steel is promoted. The existing refractory building structural steel has higher design standard, and increases weather resistance while guaranteeing the fire resistance, so that elements such as Mo, cr, cu and the like with more content are adopted in the element design. The method clearly increases the material cost of the structural steel, but limits popularization and application. In order to prevent copper embrittlement, ni element is added, so that nickel steam which is harmful to human body is easily generated when the steel structure is welded.

In addition, most of the current products only limit the high temperature performance at 600 ℃, do not relate to the high temperature heat preservation time, and are in accordance with the current standard GB/T228.2 part 2 of the tensile test of metallic materials: high temperature test methods, the incubation time of the sample at high temperature is short, typically 15 minutes. I.e. the performance of the material after a prolonged period of time at high temperature cannot be guaranteed. Whereas actual fire statistics show that 95% of our country fires have a duration of less than 2 hours, only about 80% of which are extinguished within 1 hour, and about 90% of which are extinguished within 1.5 hours. Therefore, in order to obtain a reliable material, the performance of the steel material at high temperature for at least 1.5 hours must be examined, in order to provide a reliable structural steel for the user.

Disclosure of Invention

The invention aims to provide a low-cost high-strength refractory steel for building structures, wherein the low cost refers to that an element system designed by taking expensive elements such as Mo, cr, cu, ni is not adopted, and in order to obtain enough strength and fire resistance and weather resistance, a B, nb, V, mo compatibility is adopted, bainitic ferrite and a small amount of majoram are taken as target microstructures, and through experiments, the yield strength of the steel is more than 500MPa, the tensile strength is more than 625MPa, the yield ratio is less than 0.83, and the elongation is more than 16%; the Charpy impact value (individual value) at-40 ℃ is greater than 100J.

Furthermore, the control of low yield ratio and high impact toughness is realized by taking bainitic ferrite as a soft phase and taking dispersed majoram as a hard phase as a composite matrix structure. Meanwhile, the invention adopts two aspects of tissue and high-temperature dynamic precipitation to improve the fire resistance of the steel plate, namely bainitic ferrite and carbide VC dynamic precipitation. The precipitation of carbide VC is closely related to the transformation mechanism of ferrite, which is easily precipitated in polygonal ferrite formed by the diffusion transformation mechanism, but is difficult to be precipitated in bainitic ferrite formed by the non-diffusion transformation mechanism. In order to make more carbide forming elements pre-dissolved in an austenite matrix so as to be beneficial to the dynamic precipitation strengthening of carbide under the high-temperature refractory condition, the invention comprehensively considers the cost and the tissue requirement, and preferentially adopts B element with lower cost to improve the hardenability of the steel plate so as to inhibit the generation of polygonal ferrite, thereby reducing the pre-precipitation of carbide VC. In addition, a small amount of Mo element is used to improve the high temperature stability of the carbide VC. The yield strength of the steel plate at 600 ℃ is not lower than 2/3 of that at room temperature after the steel plate is kept at 600 ℃ for more than 90 minutes.

The specific technical scheme adopted by the invention is as follows: a low-cost high-strength refractory steel for building structures, which comprises the following chemical components in percentage by weight: c:0.05 to 0.1 percent, si:0.1 to 0.5 percent of Mn:1.2 to 1.6 percent, cr:0.2 to 0.6 percent, B:0.001% -0.005%, ti:0.01% -0.02%, nb:0.015% -0.05%, V:0.03 to 0.1 percent of Mo:0.15 to 0.35 percent, cu:0 to 0.2 percent, ni: 0-0.2%, and the balance Fe and unavoidable impurities, wherein Ti/N is more than 3.42, the formation of TiN particles is ensured to be uniform and fine, free N atoms in steel are reduced, BN generation is avoided, and B element can be effectively segregated at grain boundaries.

The product is a steel plate with the thickness not more than 30mm,

the production method of the steel for the low-cost high-strength refractory building structure comprises the following steps:

smelting molten steel conforming to chemical components, and casting the molten steel into a steel billet;

heating the billet until the structure is completely austenitized, homogenized and fully solutionized, and rolling in two stages of rough rolling and finish rolling, wherein: rough rolling is rolling in a recrystallization zone, the rolling temperature is 1150-1000 ℃, the single pass reduction is not lower than 15%, an intermediate billet is obtained, the thickness of the intermediate billet is not lower than 2.5h, and h is the thickness of a finished product; the finish rolling is rolling in a non-recrystallization zone, the finish rolling start temperature is less than Tnr, the finish rolling temperature is greater than Ar3 temperature, the pass reduction rate is 15-20%, the accumulated reduction rate is not less than 60%, and the finish rolling is followed by short relaxation;

and thirdly, adopting laminar flow ACC to accelerate cooling, wherein the cooling temperature is close to or greater than the total solid solution temperature of carbide VC in an austenite matrix, the final cooling temperature is positioned in a bainite transformation area, the cooling rate is 5-15 ℃/s to control the distribution and refinement of a Mao island structure, the Mao island structure is discontinuous long islands or grains and exists between lath bainite and at the prior austenite grain boundary, and finally the microstructure is bainitic ferrite, the Mao island structure and a small amount of carbide, wherein the volume fraction of the Mao island is 3-8%, and the volume fraction of the carbide is less than 1%.

Preferably, in the second step, the temperature of the billet soaking section is 1180-1220 ℃, and the billet soaking section is kept for 2-6 hours.

Preferably, in the second step, the finish rolling start temperature is 820-950 ℃, the finish rolling temperature is 770-850 ℃, and the finish rolling relaxation time is 5-15 s.

Preferably, in the third step, the opening and cooling temperature is 750-810 ℃, the final cooling temperature is 450-600 ℃,

preferably, in the first step, the thickness of the steel billet is 120-360 mm, and the final product is a steel plate with the thickness not less than 30 mm.

The technical scheme of the application has the following characteristics:

the invention takes bainitic ferrite and majoram as target microstructures, and the sizes and the shapes of the bainitic ferrite and majoram structures are precisely adjusted through the matching design of the finishing temperature, the water inlet temperature and the finishing cooling temperature so as to simultaneously meet the requirements of low yield ratio, impact toughness and fire resistance.

(1) Finish rolling is carried out in a non-recrystallized region to promote the formation of defects such as dislocation, vacancy and the like in the crystal, and short relaxation is carried out after finish rolling, wherein the relaxation time is 5-15 s, so that carbon atoms are diffused to the positions of the crystal defects, thereby facilitating the formation of a carbon-poor region and a carbon-rich region, promoting the formation of hard phase M/A islands in the carbon-rich region and reducing the yield ratio.

(2) In order to enable more carbide VC to be separated out under the high-temperature fire-resistant condition so as to improve the fire resistance performance, the invention designs that the temperature of water entering after rolling is close to or higher than the total solid solution temperature of carbide VC in an austenite matrix based on the characteristic that the solid solubility of carbide forming elements in austenite is reduced along with the temperature reduction so as to reduce or even avoid the separation of carbide VC, and accurately controls final cooling through adjusting laminar cooling water so as to enable the final cooling temperature to be in a bainite phase change region between 450 ℃ and 600 ℃.

(3) inthecoolingprocess(watercooling)ofACCequipment,whenthecoolingspeedislow,thecarbonatomdiffusioncapabilityishigh,enoughtimeisavailableforlong-rangediffusion,acarbon-richregionislarge,theM-Aislandsareeasilylargeinsizeandcontinuouslydistributed,impacttoughnessisdeteriorated,whenthecoolingspeedistoohigh,thelong-rangediffusioncapabilityofthecarbonatomsisinhibited,ahighcarbonconcentrationregioniseasilyformedatalocalphasechangeinterface,long-narrowislandM-Aislandsareformedatalathinterface,thesizeissmall,andtheyieldratioisnoteasytoreduce. accordingtotheinvention,throughadjustmentofthecoolingspeedafterrolling,preferably,thetargetcoolingspeedis5-15℃/s,soastocontrolthedistributionandrefinementofM-Aislands,theM-Aislandsareindiscontinuouslongislandshapesorgranularshapes,andexistbetweenlathbainiteandattheprioraustenitegrainboundaries.

(4) The application adopts bainitic ferrite and a dispersed mao island as microstructures to realize low yield ratio and high toughness at the same time, and utilizes dynamic precipitation strengthening of carbide VC at high temperature to improve the fire resistance of the building steel. Because the solid solubility product of the carbide VC in ferrite is smaller, in order to avoid the early precipitation of the carbide VC in polygonal ferrite, the occurrence of the transformation of the proeutectoid ferrite is restrained, preferably, the B element with lower cost is adopted to improve the hardenability of the steel plate, and the noble alloying elements Cr, cu, ni and the like for increasing the hardenability are reduced or even cancelled. The improvement in hardenability will help to obtain a bainitic ferrite structure at a lower cooling rate. In addition, a small amount of Mo element is used to improve stability of the carbide VC at high temperature, a growth rate of the carbide VC is reduced to improve fire resistance, and Mo is added with formation of Li Maao island structure to reduce yield ratio of the steel sheet. The final steel plate has yield strength of more than 500MPa, tensile strength of more than 625MPa, yield ratio of less than 0.83 and elongation of more than 16%; -40 ℃ Charpy impact value (single value) greater than 100J; and when incubated at 600 ℃ for more than 90 minutes, the yield strength at 600 ℃ is still not lower than 2/3 of that at room temperature.

Drawings

FIG. 1 is a typical microstructure of a steel sheet for a refractory building structure according to an embodiment of the present invention;

FIG. 2 shows microstructure and VC precipitation particles of the steel plate for the refractory building structure of the embodiment of the invention after heat preservation at 600 ℃ for 90 min;

FIG. 3 shows a high temperature (600 ℃) tensile curve of the fire-resistant building structural steel plate of the embodiment of the invention after heat preservation at 600 ℃ for different times, and the abscissa shows engineering strain.

Detailed Description

The invention is described in further detail below in connection with the following examples, which are exemplary and intended to illustrate the invention, but are not to be construed as limiting the invention.

The invention aims to develop a low-cost high-strength building structure steel plate with good fire resistance and earthquake resistance and a preparation method thereof. The sizes and the forms of bainitic ferrite and a Mao island structure are accurately adjusted by utilizing the existing single-stand rolling mill or double-stand rolling mill production line through the matching design of the final rolling temperature, the water inlet temperature and the final cooling temperature so as to simultaneously meet the requirements of yield ratio, impact toughness and fire resistance. The yield strength of the steel plate of the product is more than 500MPa, the tensile strength is more than 625MPa, the yield ratio is less than 0.83, and the elongation is more than 16%; -40 ℃ Charpy impact value (single value) greater than 100J; and the yield strength at 600 ℃ can be not lower than 2/3 of that at room temperature after heat preservation at 600 ℃ for more than 90 minutes.

In order to achieve the above object, the reasons for limiting each chemical element in the embodiment of the present invention are as follows:

c:0.05 to 0.1%

The element C is the cheapest and most effective strengthening element in steel, and the lower limit of this carbon is 0.05%, while an excessively high carbon content causes deterioration of its welding performance and worsens impact performance, and the upper limit of this carbon is 0.1%.

Si:0.1 to 0.5%

Si is a common deoxidizing element, and can improve ferrite strength through solid solution strengthening, so that the lower limit of silicon is 0.1%; too much silicon element will promote the formation of the maolympic structure in the weld heat affected zone, and is unfavorable for the decomposition of the maolympic structure, deteriorating the welding performance, and the upper limit of silicon is 0.5%.

Mn:1.2 to 1.6%

Mn element is a common solid solution strengthening element, can effectively improve strength at room temperature and high temperature, and is beneficial to improving the hardenability of steel, and the lower limit of Mn is 1.2%; too much manganese element tends to cause segregation and deteriorate the impact toughness of the core of the steel sheet, and thus the upper limit of manganese is 1.6%.

Cr:0.2 to 0.6%

The Cr element can improve strength and hardenability, and can suppress formation of coarse carbides at grain boundaries by forming fine Cr carbides, for which the lower limit of Cr is 0.2%; too much Cr affects impact toughness and deteriorates welding performance, for which the upper limit of Cr is 0.6%.

B:0.001 to 0.005%

The B element is an important element of the present invention. B can be biased at the prior austenite grain boundary, can effectively prevent the nucleation of polygonal ferrite, and can strongly improve the hardenability of steel; when the B content is more than 0.005%, the effect cannot be significantly improved, so that the limit range of B is 0.001 to 0.005%.

Ti:0.01 to 0.02%

Ti element is a strong nitrogen fixation element, and preferentially reacts with N in steel to generate TiN, so that the effect of reducing B due to the generation of BN is avoided; too much Ti is liable to precipitate larger-sized TiN during solidification, and therefore the limit range of Ti is 0.01% to 0.02%.

Nb:0.015 to 0.05%

The Nb element is a very effective fine-grain strengthening element, and can effectively control the prior austenite size and refine bainite grains; and the carbide formed by Nb has higher thermal stability, can improve the room temperature and high temperature performance of steel at the same time, and the limit range of Nb element is 0.015% -0.05%.

V:0.03 to 0.1%

The V element is an important element of the present invention. V has a large solid solubility product in austenite. Based on the fact that when the mother phase austenite and the child phase ferrite approximately meet the K-S orientation relation in the phase transformation process, the precipitation density of VC is small, and a large amount of V is dissolved in the child phase ferrite in a solid solution mode. Vanadium dissolved in the ferrite of the sub-phase at room temperature is dynamically separated out at high temperature, so that the high temperature resistance of the steel is improved. Therefore, the limit range of the V element is 0.03% -0.1%.

Mo:0.15 to 0.35%

Mo element is an important element of the present invention. Mo itself can play a role in precipitation strengthening, improving hardenability and reducing tempering brittleness; and the Mo element can be wrapped around the microalloy carbide, so that the high-temperature thermal stability of the microalloy carbide is improved, and the growth rate of the carbide is reduced. Mo element is a noble metal element, and if it is excessive, the cost increases. Therefore, the limit of the Mo element is 0.15-0.35%.

Cu:0 to 0.2%; ni:0 to 0.2%

Cu and Ni are austenite stabilizing elements, which are favorable for the formation of a Mao island structure and reduce the yield ratio. Cu is easy to segregate in grain boundaries to generate copper embrittlement, and Ni is mainly added to form Cu-Ni alloy with Cu so as to prevent the generation of copper embrittlement. Considering the cost comprehensively, the limit range of Cu and Ni elements is 0-0.2%.

The core of the invention is that the volume fraction of the Mao island in the steel plate is 3-8%, and the volume fraction of carbide is less than 1%.

The embodiment of the invention calculates the total solid solution temperature of V in austenite by a solid solubility product formula of VC in austenite

And determining the water inlet temperature range.

According to relaxation time after finish rolling, temperature Ar of ferrite phase transformation is combined with temperature drop in rolling process and temperature Ar of ferrite phase transformation during cooling ₃ =868-396C-68.1mn+24.6si-36.1 Ni-24.8Cr-20.7Cu and non-recrystallization temperature

And determining the final rolling temperature and the starting rolling temperature. And then cooling to a bainitic transformation zone of 450-600 ℃ at a certain cooling speed by adjusting the volume of the ACC laminar cooling water to obtain the bainitic ferrite mainly in the shape of lath. And finally, air cooling to room temperature on a cooling bed, shearing, sizing and warehousing after the flaw detection is qualified. The selection of the cooling path determines the phase transformation driving force and the diffusion condition of carbon atoms, and the cooling speed is properly controlled, so that the diffusion of the carbon atoms in austenite is inhibited, the carbon atoms are mainly diffused in a short range, the shape of a majoram is mainly in a round grain shape and a long and narrow island shape, and the majoram is mainly distributed at the interface of bainitic ferrite and the prior austenite grain boundary.

In conclusion, the sizes and the forms of bainitic ferrite and a majoram structure are accurately regulated and controlled through the matching design of alloy composition optimization, finish rolling temperature, water inlet temperature, finish cooling temperature and cooling speed, so that the requirements of yield ratio, impact toughness and high-temperature mechanical properties are simultaneously met.

The method of manufacturing the steel sheet is described below. The hot rolling plate comprises heating a plate blank, rough rolling, finish rolling and laminar cooling in sequence, and sizing after the plate blank is qualified through flaw detection. The slab is heated at 1180-1220 ℃ for 2-6 hours.

Further, the rough rolling comprises the steps that the rough rolling temperature is 1150-1000 ℃, the single-pass reduction rate is not lower than 15%, an intermediate billet is obtained, the thickness of the intermediate billet is not lower than 2.5h, and the h is the thickness of a finished product.

Further, the finish rolling comprises the steps of finish rolling into a non-recrystallization zone, wherein the finish rolling initial rolling temperature is 800-950 ℃, the finish rolling temperature is 750-850 ℃, the pass reduction rate is 15-20%, and the accumulated reduction rate is not less than 60%.

Further, the laminar cooling step comprises the step of laminar cooling the slab by using ACC, wherein the speed of a finishing mill is controlled, the proper cooling temperature is matched to be 750-810 ℃, the cooling temperature is not lower than the full solid solution temperature of VC, the precipitation of carbide VC is reduced or even avoided, the final cooling temperature is accurately controlled by adjusting laminar cooling water, the final cooling temperature is in a bainite transformation area, the final cooling temperature is 450-600 ℃, and the cooling rate is 5-15 ℃/s.

The following examples and comparative examples of the present invention are presented to further illustrate the compositional range requirements of the present invention and the rationality of the process path design and to further clarify the features and advantages of the low cost refractory steels described above.

Table 1 shows the main chemical components of each example and comparative example of the present invention.

In terms of components, the room temperature strength is ensured by adopting a low Mn and high Cr component design in the embodiment 1, and the high temperature strength is mainly improved by adopting a low Mo and high V component design when the high temperature strength is ensured. Example 2 uses a high Mn low Cr composition design to ensure room temperature strength and a high Mo low V composition design to ensure high temperature strength by a combination of solid solution strengthening and dynamic precipitation strengthening.

As comparative examples, cr, ni and Cu were selected to have high strength by solid solution strengthening while ensuring weather resistance in comparative example 1, and the material did not contain element V which was able to be dynamically precipitated at high temperature and was mainly used to ensure high temperature strength by solid solution strengthening of Mo element.

Table 1 shows the main chemical components (wt%) of each example and comparative example of the present invention

	C	Si	Mn	Cr	Ni	Cu	B	Nb	V	Mo	Ti	N
													Example 1	0.05	0.2	1.20	0.6	—	0.11	0.003	0.04	0.06	0.15	0.015	0.003
Example 2	0.09	0.25	1.60	0.2	0.08	—	0.001	0.025	0.03	0.35	0.018	0.004
													Comparative example 1	0.04	0.13	1.0	0.25	0.47	0.15	—	0.025	—	0.30	0.012

Note that: representation without addition of

The experiment is carried out by adopting a continuous casting billet with the thickness of 150mm in the embodiment and the comparative example, after the continuous casting billet is subjected to heat preservation for 5 hours at 1200 ℃, phosphorus removal is carried out by high-pressure water, rough rolling is finished at 1150-1100 ℃ to obtain a middle billet with the thickness of 3 hours, and then the middle billet is subjected to TMCP post-rolling to obtain a hot-rolled steel plate, wherein the key technological parameters are shown in the table 2. Among them, examples 1, 2 and comparative example 1 all employ a suitable TMCP hot rolling process to verify the effectiveness and rationality required for the components of the present invention. Comparative example 2 the same composition as example 2, but a different TMCP process was employed to verify the rationality of the TMCP process of the present invention.

From the composition of example 1 in Table 1, it was found that the non-recrystallization temperature of example 1 was calculated to be about 968℃and the total solid solution temperature of VC was calculated to be about 750 ℃. In example 1 with 15mm gauge thickness in table 2, non-recrystallization rolling was adopted, the finish rolling start temperature was 900 ℃, the rolled product was put into ACC cooling equipment after short relaxation, the water inlet temperature was kept slightly higher than the total solid solution temperature 750 ℃ of VC, the rolled product was rapidly cooled to 586 ℃ by ACC, the average cooling rate was about 15 ℃/s, and then the rolled product was air-cooled to room temperature. The non-recrystallization temperature of comparative example 1 was about 930 ℃, the non-recrystallization rolling was used, the initial rolling temperature of finish rolling was 890 ℃, after the finish rolling was briefly relaxed, it was put into ACC cooling equipment at 740 ℃, and it was rapidly cooled to 560 ℃, and it was air-cooled to room temperature.

The structure control ideas of example 1 and comparative example 1 are similar, and the structure control ideas are that the transient relaxation is carried out after finishing finish rolling, so that carbon atom diffusion segregation and defect positions such as grain boundaries, subgrain boundaries, dislocation and the like are facilitated, the carbon atom distribution is uneven, and the formation of M/A island structures in the subsequent cooling process is promoted. In example 1, a certain amount of B element is contained, the hardenability of the experimental steel is good, the microstructure of bainitic ferrite and M/A islands is obtained, the tiny M/A is orderly distributed among bainitic ferrite lath interfaces, and part of M/A islands are distributed on prior austenite grain boundaries. The absence of element B in the chemical composition of comparative example 1 results in comparative example 1 having a relatively slightly inferior hardenability, and finally a microstructure of bainitic ferrite, a small amount of polygonal ferrite and M/A islands, part of M/A being distributed between bainitic ferrite laths, is obtained.

For the 30mm steel sheet, example 2 and comparative example 2 use the same chemical composition, but the TMCP process is different. The metallographic structure of example 2 is bainitic ferrite and an M/a island structure, the M/a island structure being distributed between bainitic ferrite laths in a discontinuous linear arrangement. In contrast, in comparative example 2, when the final cooling temperature is 610 ℃, the final cooling temperature is higher, and under the condition of thicker specification, the cooling speed of air cooling to room temperature is slower, and the carbon diffusion distance is longer, so that M/A island structures with different sizes and irregular shapes are distributed on the matrix structure of bainitic ferrite, and part of M/A island structures are arranged in a linear and continuous manner.

Table 2 shows the main process parameters of the examples and comparative examples of the present invention

Mechanical properties of the low cost refractory steel provided in the examples were tested and the results of the experiments are shown in tables 3 and 4, wherein tensile properties were transverse samples and Charpy impact was longitudinal samples. Room temperature tensile property, impact property and high temperature property detection were performed respectively in the first part of the standard GB/T228.1 Metal Material tensile test: room temperature test method, charpy pendulum impact test method for GB/T229 Metal Material, section 2 of tensile test for GB/T228.2 Metal Material: high temperature test methods.

As shown in Table 3, the mechanical properties at room temperature are good in both the examples 1 and 2, the yield ratio is lower than 0.8, and the impact energy at-40 ℃ is above 120J. Comparative example 1 has good room temperature mechanical properties, but its yield strength is slightly lower due to the presence of polygonal ferrite. While comparative example 2 has good tensile properties, but has a low impact energy at-40 ℃. For analysis reasons, the final cooling temperature of comparative example 2 is higher and exceeds the requirement of the invention, so that M/A island tissues which are arranged linearly and continuously exist in the microstructure, and the impact energy is obviously reduced.

As can be seen from Table 4, examples 1 and 2 of the present invention have good mechanical properties at high temperature (600 ℃ C.), and even after they are kept at 600 ℃ for 90 minutes, they maintain a yield strength at 600 ℃ greater than 0.7 of that at room temperature. While comparative example 1 had been incubated at 600℃for 90 minutes, its yield strength at 600℃was reduced to 0.64 of that at room temperature. The reasons for the analysis are that, firstly, although a small amount of polygonal ferrite in the comparative example is favorable for reducing the yield ratio, the high-temperature mechanical property of the polygonal ferrite is inferior to that of bainitic ferrite, and secondly, the comparative example 1 has no reinforced particles which can be dynamically precipitated at high temperature so as to compensate the strength attenuation caused by the recovery of high-temperature dislocation.

Table 3 shows the structure types and the conventional mechanical properties of the examples and comparative examples of the present invention

	Specification/mm	Yield strength/MPa	Tensile strength/MPa	Yield ratio	Elongation/%	Charpy impact-40 ℃/J
							Example 1	15	620	812	0.76	17	180
Example 2	30	550	780	0.71	19	213
							Comparative example 1	15	490	635	0.77	21.5	146
Comparative example 2	30	505	758	0.66	19.4	58

Table 4 shows the mechanical properties at 600℃of each example and comparative example according to the invention

As is clear from the mechanical property data in Table 3, the steel sheet having the composition of the present invention has the advantages that the steel sheet obtained by the production process of the present invention has a low alloy composition, a low yield ratio, a high impact toughness, and a small decrease in high-temperature yield strength after a long-time heat preservation at 600 ℃. Specifically, under conditions that yield strength of more than 500MPa, tensile strength of more than 625MPa, yield ratio of less than 0.83 and elongation of more than 16% are achieved, it is possible to achieve a Charpy impact value (single value) of more than 100J at-40 ℃, especially when it is kept at 600 ℃ for more than 90 minutes, its yield strength at 600 ℃ is still not lower than 2/3 at room temperature. The comprehensive technical index is far superior to the prior art.

By analyzing the microstructure of the steel, the microstructure of the steel comprises a composite structure of lath distributed bainitic ferrite and majoram, and polygonal ferrite is not present, and the microstructure comprises 3-8% of majoram and less than 1% of carbide by volume.

The above-described embodiments are exemplary embodiments of the present invention. Various modifications may be made to the above-described embodiments by those skilled in the art without departing from the inventive concepts disclosed herein, without departing from the scope of the invention.

Claims

1. A low cost high strength refractory steel for building construction characterized by: the chemical components in percentage by weight are as follows: c:0.05 to 0.1 percent, si:0.1 to 0.5 percent of Mn:1.2 to 1.6 percent, cr:0.2 to 0.6 percent, B:0.001% -0.005%, ti:0.01% -0.02%, nb:0.015% -0.05%, V:0.03 to 0.1 percent of Mo:0.15 to 0.35 percent, cu:0 to 0.2 percent, ni:0 to 0.2%, and the balance Fe and unavoidable impurities, wherein Ti/N >3.42;

the production method of the steel comprises the following steps:

heating the billet until the structure is completely austenitized, homogenized and fully solutionized, and rolling in two stages of rough rolling and finish rolling, wherein: rough rolling is rolling in a recrystallization zone, the rolling temperature is 1150-1000 ℃, the single pass reduction is not lower than 15%, an intermediate billet is obtained, the thickness of the intermediate billet is not lower than 2.5h, and h is the thickness of a finished product; the finish rolling is rolling in a non-recrystallization zone, the finish rolling start temperature is less than the non-recrystallization temperature Tnr, the finish rolling temperature is greater than Ar3 temperature, the pass reduction rate is 15-20%, the cumulative reduction rate is not less than 60%, and the relaxation is carried out after the finish rolling, and the relaxation time is 5-15 s;

and thirdly, adopting laminar flow ACC to accelerate cooling, wherein the cooling temperature is higher than the total solid solution temperature of carbide VC in an austenite matrix, the final cooling temperature is positioned in a bainite transformation area, and the cooling rate is 5-15 ℃ per second so as to control the distribution and refinement of a Mao island structure, and the Mao island structure is in a discontinuous long island shape or a discontinuous granular shape and exists between lath bainite and at the prior austenite grain boundary.

2. The low cost, high strength, refractory structural steel according to claim 1, wherein: the product is a steel plate with the thickness not more than 30mm, the microstructure is bainitic ferrite and a majoram structure, the yield strength of the steel plate is more than 500MPa, the tensile strength is more than 625MPa, the yield ratio is less than 0.83, and the elongation is more than 16%; the Charpy impact energy at the temperature of minus 40 ℃ is more than 100J; when incubated at 600 ℃ for more than 90 minutes, the yield strength at 600 ℃ is still not lower than 2/3 of the yield strength at room temperature.

3. The low cost, high strength, refractory structural steel according to claim 2, wherein: the volume fraction of the Mao island tissue in the microstructure is 3-8%, and the microstructure also comprises carbide with the volume fraction less than 1%.

4. A method of producing the low cost, high strength, refractory structural steel in accordance with claim 1, wherein: comprising the following steps:

5. The method according to claim 4, wherein: in the second step, the temperature of the billet soaking section is 1180-1220 ℃, and the billet soaking section is insulated for 2-6 hours.

6. The method according to claim 4, wherein: in the second step, according to the relaxation time after finishing rolling, the temperature Ar of ferrite phase transformation starting during cooling is combined with the temperature drop during rolling ₃ =868-396C-68.1mn+24.6si-36.1 Ni-24.8Cr-20.7Cu and non-recrystallization temperature

And determining the final rolling temperature and the starting rolling temperature.

7. The method according to claim 4, wherein: in the second step, the finish rolling temperature is 820-950 ℃ and the finish rolling temperature is 770-850 ℃.

8. The method according to claim 4, wherein: in the third step, the total solid solution temperature of carbide VC in the austenitic matrix

And determining the water inlet temperature, namely the cooling temperature.

9. The method according to claim 4, wherein: in the third step, the opening and cooling temperature is 750-810 ℃ and the final cooling temperature is 450-600 ℃.

10. The method according to claim 4, wherein: in the first step, the thickness of the steel billet is 120-360 mm.