CN117070840A - Economical 235MPa grade hot rolled steel plate and elongation stable control method thereof - Google Patents
Economical 235MPa grade hot rolled steel plate and elongation stable control method thereof Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 138
- 239000010959 steel Substances 0.000 title claims abstract description 138
- 238000000034 method Methods 0.000 title claims abstract description 50
- 238000001816 cooling Methods 0.000 claims abstract description 52
- 238000005096 rolling process Methods 0.000 claims abstract description 37
- 238000010438 heat treatment Methods 0.000 claims abstract description 36
- 238000009749 continuous casting Methods 0.000 claims abstract description 32
- 230000008569 process Effects 0.000 claims abstract description 30
- 238000005266 casting Methods 0.000 claims abstract description 25
- 238000010583 slow cooling Methods 0.000 claims abstract description 25
- 238000003723 Smelting Methods 0.000 claims abstract description 21
- 230000009467 reduction Effects 0.000 claims abstract description 19
- 238000004519 manufacturing process Methods 0.000 claims abstract description 13
- 239000000126 substance Substances 0.000 claims abstract description 9
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 8
- 239000012535 impurity Substances 0.000 claims abstract description 7
- 229910052698 phosphorus Inorganic materials 0.000 claims abstract description 7
- 229910052748 manganese Inorganic materials 0.000 claims abstract description 5
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 4
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 13
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 12
- 239000002994 raw material Substances 0.000 claims description 8
- 239000002893 slag Substances 0.000 claims description 8
- 238000002791 soaking Methods 0.000 claims description 6
- 239000000446 fuel Substances 0.000 claims description 5
- 238000007711 solidification Methods 0.000 claims description 5
- 230000008023 solidification Effects 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 4
- 229910045601 alloy Inorganic materials 0.000 abstract description 11
- 239000000956 alloy Substances 0.000 abstract description 11
- 238000011009 performance qualification Methods 0.000 abstract description 5
- 238000004781 supercooling Methods 0.000 abstract description 4
- 238000005496 tempering Methods 0.000 abstract description 3
- 230000001276 controlling effect Effects 0.000 description 16
- 239000011572 manganese Substances 0.000 description 9
- 229910001563 bainite Inorganic materials 0.000 description 7
- 229910000734 martensite Inorganic materials 0.000 description 7
- 238000005204 segregation Methods 0.000 description 7
- 238000005728 strengthening Methods 0.000 description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 4
- 229910001566 austenite Inorganic materials 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 230000006872 improvement Effects 0.000 description 4
- 230000009466 transformation Effects 0.000 description 4
- 208000012868 Overgrowth Diseases 0.000 description 3
- 238000013461 design Methods 0.000 description 3
- 230000006866 deterioration Effects 0.000 description 3
- 238000011160 research Methods 0.000 description 3
- 239000006104 solid solution Substances 0.000 description 3
- 229910000859 α-Fe Inorganic materials 0.000 description 3
- 230000009286 beneficial effect Effects 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 239000011159 matrix material Substances 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 238000009628 steelmaking Methods 0.000 description 2
- 229910000851 Alloy steel Inorganic materials 0.000 description 1
- 229910000617 Mangalloy Inorganic materials 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- QFGIVKNKFPCKAW-UHFFFAOYSA-N [Mn].[C] Chemical compound [Mn].[C] QFGIVKNKFPCKAW-UHFFFAOYSA-N 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000010923 batch production Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 230000003749 cleanliness Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000005272 metallurgy Methods 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- TWNQGVIAIRXVLR-UHFFFAOYSA-N oxo(oxoalumanyloxy)alumane Chemical compound O=[Al]O[Al]=O TWNQGVIAIRXVLR-UHFFFAOYSA-N 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000012797 qualification Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 230000000930 thermomechanical effect Effects 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
- B22D11/16—Controlling or regulating processes or operations
- B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
- B22D11/225—Controlling or regulating processes or operations for cooling cast stock or mould for secondary cooling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/064—Dephosphorising; Desulfurising
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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- Mechanical Engineering (AREA)
- Metallurgy (AREA)
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- Metal Rolling (AREA)
Abstract
The invention relates to an economic 235MPa hot rolled steel plate and an elongation stable control method thereof, wherein the steel plate comprises the following chemical components in percentage by weight: 0.15 to 0.2 percent of C, 0.1 to 0.2 percent of Si, 0.3 to 0.45 percent of Mn, less than or equal to 0.03 percent of P, less than or equal to 0.02 percent of S, 0.015 to 0.02 percent of Al, 0.015 to 0.03 percent of N, and the balance of Fe and unavoidable impurities. The steel plate production process comprises molten steel smelting, continuous casting, casting blank heating, controlled rolling, cooling, stacking and slow cooling; through each production process, the performance index requirements of the steel plate can be met without subsequent tempering heat treatment, and the problems that the economic steel plate is reduced in head and tail temperature and the cooling process window is narrowed after alloy reduction, so that supercooling occurs at the head and the tail in the length direction of the steel plate, the elongation of the steel plate is lower than the technical standard, and the one-time performance qualification rate of the steel plate is affected are solved.
Description
Technical Field
The invention relates to the technical field of hot-rolled steel plate production, in particular to a hot-rolled steel plate with thickness specification of 40-80 mm, economical grade and yield strength of 235MPa grade and a stable control method of elongation thereof.
Background
The 235MPa grade medium plate is a main variety of the medium plate, and is typically represented by low alloy Q235A/B/C steel, the steel is mainly based on carbon manganese steel, and carbon and nitride forming elements such as microalloy elements Al are added into the steel, so that the toughness of the steel plate is improved through solid solution strengthening, precipitation strengthening and fine crystal strengthening. According to statistics, the medium and heavy plates with the strength grade in the hot rolled steel plates produced annually in China account for 20-30% of the total yield. Therefore, development and research of manufacturing techniques of such low-cost steel sheets are significant. For 235MPa grade steel plates with the thickness of more than 40mm, the manganese content is generally designed to be higher (0.6% -0.85%), and a medium carbon component system is adopted, if the low Mn component design is adopted to replace the high Mn component design, the technological processes of steelmaking, rolling, cooling control and the like are optimized, and the alloy cost is obviously reduced.
TMCP (thermo-mechanical control process) is a general term for a technique of performing air cooling or controlled cooling and accelerated cooling on the basis of controlling a heating temperature, a rolling temperature and a rolling reduction in a hot rolling process. The TMCP process can produce high-strength high-toughness steel without adding excessive alloy elements or complex subsequent heat treatment, so the TMCP process is considered as an alloy and energy saving process which is favorable for environmental protection and has become an indispensable technology for producing low-alloy steel plates. As the market demands for TMCP steel continue to increase, the TMCP process itself also continues to develop in application. Research in recent years has focused on controlling cooling, particularly accelerated cooling. By accelerating the cooling rate after rolling, not only the growth of crystal grains can be suppressed, but also the ultra-fine ferrite structure or bainite structure, or even the martensite structure, required for high strength and high toughness can be obtained.
The economic steel plate with 235MPa grade steel plate with reduced alloy cost and reduced process is produced by adopting TMCP process, but the rolling and cooling control process window is narrowed along with the reduction of alloy content. For an economic TMCP steel plate with a thick specification (40-80 mm) and a 235MPa level, the length direction of the steel plate is supercooled at the head and the tail due to the thickness increase, the temperature drop at the head and the tail of the steel plate and the narrowing of a cooling process window after alloy reduction, so that the elongation of the steel plate is lower than the technical standard, and the production and the popularization of the steel plate of the level type are limited.
Therefore, how to solve the problem that the head-to-tail elongation is lower than the technical standard, simultaneously reduce the production cost and improve the performance qualification rate of the steel plate is a key problem to be solved in realizing batch production of the economic 235 MPa-grade hot rolled steel plate steel with the thickness specification of 40-80 mm.
So far, few researches are conducted on a control method for ensuring that the head-tail elongation accords with the technical standard on economical steel plates with the thickness of 40-80 mm and the yield strength of more than or equal to 235MPa at home and abroad. The journal paper "Q235B medium plate elongation unqualified cause analysis and improvement" (Wang Liyun et al, hebei metallurgy "2012, 7 th period) mainly reduces the inclusion content and strip structure of the plate by improving the steelmaking and steel rolling processes, can reduce the number of unqualified plate elongation and improve the comprehensive qualification rate of the plate, but only aims at the steel plate with the thickness of less than 40 mm.
The method for controlling the elongation of the 235 MPa-level steel plate disclosed in the above document solves the problem that the elongation of the steel plate with the thickness specification is unqualified, but is not suitable for controlling the economic type and the yield strength of 235 MPa-level, and the elongation of the steel plate with the thickness specification of 40-80 mm is unqualified.
Disclosure of Invention
The invention aims to overcome the problems and the defects in the prior art and provide a hot rolled steel plate with thickness specification of 40-80 mm, economical grade and yield strength of 235MPa and a stable control method of elongation thereof; by controlling each production process, the performance index requirements of the steel plate can be met without subsequent tempering heat treatment, and the problems that the economic steel plate is reduced in temperature from head to tail and the cooling process window is narrowed after alloy reduction, so that supercooling occurs at the head and the tail in the length direction of the steel plate, the elongation of the steel plate is lower than the technical standard, and the one-time performance qualification rate of the steel plate is affected are solved.
In order to achieve the above purpose, the invention is realized by adopting the following technical scheme:
an economical 235MPa grade hot rolled steel plate, which comprises the following chemical components in percentage by weight: 0.15 to 0.2 percent of C, 0.1 to 0.2 percent of Si, 0.3 to 0.45 percent of Mn, less than or equal to 0.03 percent of P, less than or equal to 0.02 percent of S, 0.015 to 0.02 percent of Al, 0.015 to 0.03 percent of N, and the balance of Fe and unavoidable impurities.
Further, the thickness of the finished steel plate is 40-80 mm.
Further, the yield strength of the transverse stretching of the finished steel plate is more than or equal to 225MPa, the tensile strength is 370-450 MPa, the elongation is more than or equal to 26%, and the transverse Charpy impact energy at minus 20 ℃ is more than or equal to 30J.
The economic stable control method for the elongation percentage of the 235 MPa-grade hot rolled steel plate comprises the steps of molten steel smelting, continuous casting, casting blank heating, controlled rolling, cooling and stacking slow cooling in the steel plate production process; wherein the following processes are controlled:
1) Smelting molten steel: pretreating raw materials by KR molten iron, controlling the S content to be less than or equal to 0.02%, and feeding the raw materials into a converter after slag skimming; p is removed by adopting a double slag method in converter smelting, and the P content is controlled to be less than or equal to 0.030%; controlling the content of C at the smelting end point of the converter to be 0.15-0.2%;
2) Continuous casting; controlling the superheat degree of continuous casting to be 15-25 ℃, the continuous casting blank drawing speed to be 0.9-1.5 m/min, and the secondary cooling specific water quantity to be 1.7-2.2L/kg; putting light reduction into the horizontal sector section, namely the solidification end, wherein the reduction of the continuous casting billet is 8-12 mm;
3) Heating a casting blank: heating the casting blank to 1210-1230 ℃, wherein the total heating time of the soaking section and the heating section is 2-3 h, and the total furnace time is 4-5 h; controlling the air-fuel ratio of the heating furnace to be 1:1.7-1:2.2;
4) And (3) rolling control: during rolling, the reduction rate of each pass of at least the first three passes is more than 15%, the descaling water of the rolling mill is sprayed in at least the first two passes, the descaling time of each pass is 0.5-1 min, and the descaling pressure is 15-20 MPa; after rolling, rapidly casting steel, wherein the casting steel speed is 4-6 m/s, and pre-straightening is carried out;
5) Cooling; laminar cooling is adopted, the cooling temperature is 950-1030 ℃, the final cooling temperature is 830-950 ℃, and the cooling speed is 10-15 ℃/s;
6) Stacking and slow cooling: and (3) carrying out stacking slow cooling on the cooled steel plates, wherein the stacking slow cooling temperature is 150-200 ℃, the number of stacking blocks is not less than 8, and the slow cooling time is more than 8 hours.
Further, the thickness of the continuous casting billet is less than or equal to 300mm, and the continuous casting billet is rolled on a medium plate reciprocating rolling mill.
Compared with the prior art, the invention has the beneficial effects that:
1) The average carbon segregation index is effectively reduced to inhibit segregation by controlling the continuous casting superheat degree, the blank drawing rate and the secondary cooling specific water quantity, and the secondary cooling strength is limited to inhibit the trend of crack deterioration of the center of a casting blank; the horizontal sector section, namely the solidification end is put into soft reduction, so that the central loose grade and segregation of a casting blank are reduced, austenite grains are refined, and internal tissue defects are reduced;
2) The method has the advantages that the higher heating temperature is adopted, meanwhile, the furnace time of a soaking section and a heating section is ensured, the air-fuel ratio is controlled, the temperature difference between the surface and the core of a continuous casting billet is shortened, the transverse and longitudinal metal flow uniformity of the surface of a steel plate is improved, the temperature uniformity of the upper surface and the lower surface of the steel plate in the subsequent rolling process is ensured, the overgrowth of austenite grains can be effectively restrained by controlling the total furnace time, and the performance of the steel plate is ensured;
3) The three passes before rolling adopt a high-reduction rate and flexible high-pressure water descaling process, so that the tissue from the surface to the core of the steel plate is uniformly distributed, the overgrowth of grains can be restrained, and the plasticity and toughness of the steel plate are improved; the final cooling temperature and cooling speed of the steel plate are controlled by adopting measures such as rapid steel throwing after rolling, ensuring the water inlet temperature of the steel plate, and the number of bainite and martensite phases on the surface of the steel plate and the number of bainite phases in a core structure are inhibited, so that the influence on the toughness of the steel plate due to the occurrence of bainite or martensite phase transformation on the surface is avoided;
4) By adopting a stacking slow cooling process, the slow cooling temperature and time are strictly controlled, the contents of a banded structure, a martensitic structure or a metastable lath-shaped bainitic structure are effectively reduced, and the plasticity and toughness of the steel plate are ensured; the properties of the finished steel plate are as follows: the transverse tensile yield strength is more than or equal to 225MPa, the tensile strength is between 370 and 450MPa, the elongation is more than or equal to 26 percent, and the transverse Charpy impact energy at minus 20 ℃ is more than or equal to 30J.
Detailed Description
The invention relates to an economic 235MPa grade hot rolled steel plate, which comprises the following chemical components in percentage by weight: 0.15 to 0.2 percent of C, 0.1 to 0.2 percent of Si, 0.3 to 0.45 percent of Mn, less than or equal to 0.03 percent of P, less than or equal to 0.02 percent of S, 0.015 to 0.02 percent of Al, 0.015 to 0.03 percent of N, and the balance of Fe and unavoidable impurities.
Further, the thickness of the finished steel plate is 40-80 mm.
Further, the yield strength of the transverse stretching of the finished steel plate is more than or equal to 225MPa, the tensile strength is 370-450 MPa, the elongation is more than or equal to 26%, and the transverse Charpy impact energy at minus 20 ℃ is more than or equal to 30J.
The invention relates to an economic stable control method for the elongation percentage of a 235MPa hot rolled steel plate, wherein the steel plate production process comprises molten steel smelting, continuous casting, casting blank heating, controlled rolling, cooling and stacking slow cooling; wherein the following processes are controlled:
1) Smelting molten steel: pretreating raw materials by KR molten iron, controlling the S content to be less than or equal to 0.02%, and feeding the raw materials into a converter after slag skimming; p is removed by adopting a double slag method in converter smelting, and the P content is controlled to be less than or equal to 0.030%; controlling the content of C at the smelting end point of the converter to be 0.15-0.2%;
2) Continuous casting; controlling the superheat degree of continuous casting to be 15-25 ℃, the continuous casting blank drawing speed to be 0.9-1.5 m/min, and the secondary cooling specific water quantity to be 1.7-2.2L/kg; putting light reduction into the horizontal sector section, namely the solidification end, wherein the reduction of the continuous casting billet is 8-12 mm;
3) Heating a casting blank: heating the casting blank to 1210-1230 ℃, wherein the total heating time of the soaking section and the heating section is 2-3 h, and the total furnace time is 4-5 h; controlling the air-fuel ratio of the heating furnace to be 1:1.7-1:2.2;
4) And (3) rolling control: during rolling, the reduction rate of each pass of at least the first three passes is more than 15%, the descaling water of the rolling mill is sprayed in at least the first two passes, the descaling time of each pass is 0.5-1 min, and the descaling pressure is 15-20 MPa; after rolling, rapidly casting steel, wherein the casting steel speed is 4-6 m/s, and pre-straightening is carried out;
5) Cooling; laminar cooling is adopted, the cooling temperature is 950-1030 ℃, the final cooling temperature is 830-950 ℃, and the cooling speed is 10-15 ℃/s;
6) Stacking and slow cooling: and (3) carrying out stacking slow cooling on the cooled steel plates, wherein the stacking slow cooling temperature is 150-200 ℃, the number of stacking blocks is not less than 8, and the slow cooling time is more than 8 hours.
Further, the thickness of the continuous casting billet is less than or equal to 300mm, and the continuous casting billet is rolled on a medium plate reciprocating rolling mill.
The design principle of chemical components in the economic 235MPa hot rolled steel plate is as follows:
c: the steel is the most economical and basic strengthening element in the steel, and the strength of the steel can be obviously improved through solid solution strengthening and precipitation strengthening, but the improvement of the C content has negative effects on the plasticity, the toughness and the weldability of the steel. For this reason, the present invention sets the C content range to 0.15% to 0.2%.
Mn: the strength of the steel is improved by solid solution strengthening while compensating for the loss of strength of the steel sheet due to the decrease in the C content. In addition, the gamma-alpha transformation temperature can be reduced, ferrite grains are further refined, fine low-temperature transformation products are facilitated to be obtained, and the toughness of the products is improved. However, increasing the Mn content can aggravate the center segregation and the tissue deterioration of the continuous casting blank, is unfavorable for the improvement of the low-temperature toughness of the steel plate, and cannot ensure the uniformity of the cross-section tissue of the steel plate. Therefore, the Mn content range is set to 0.3% to 0.45% in the present invention.
Si: has the functions of deoxidizing steel and improving the strength of the matrix. The improvement of Si content can purify ferrite and reduce pearlite content, which is beneficial to reducing the Bactger effect of the matrix material. However, since excessive Si reduces the toughness of the weld heat affected zone of the base material, the Si content is set to a range of 0.10% to 0.20% in the present invention.
N: the N element in the steel has no obvious effect except forming fine TiN grain refined austenite grains, so that the N element needs to be kept at a lower content level, and the N content is set to be 0.015-0.03 percent.
Al: it is usually used as a deoxidizer in steel and has the effect of refining the structure if AlN is formed. When the content of Al exceeds 0.02%, excessive aluminum oxide inclusions may reduce the cleanliness of the steel. However, too low an Al content results in insufficient deoxidation, and oxides are formed from easily oxidizable elements such as Ti, so that the lower limit of the Al content is set to 0.015% in the present invention.
P, S: is an unavoidable impurity element in steel, and theoretically, the lower the concentration is, the better. However, the smelting cost and the process cannot be reduced without limitation. Therefore, the upper limit of the P, S content is set to be 0.03% and 0.02% respectively.
The thickness of the economical 235MPa grade hot rolled steel plate is 40-80 mm, and continuous casting blanks with the thickness below 300mm are used for production on a medium plate reciprocating mill. The production process comprises molten steel smelting, continuous casting, casting blank heating, controlled rolling, cooling and stacking slow cooling. In order to improve the condition that the elongation of the finished steel plate does not reach the standard, the following process is controlled:
1) Smelting molten steel: smelting according to the set components, namely, the weight percentage of chemical components in the steel is C0.15-0.2%, si 0.1-0.2%, mn 0.3-0.45%, P less than or equal to 0.03%, S less than or equal to 0.02%, al 0.015-0.02%, N0.015-0.03%, and the balance of Fe and unavoidable impurities. Raw materials are pretreated by KR molten iron, the S content is controlled to be lower than 0.02 percent, and the raw materials enter a converter after slag skimming; the double slag method is adopted to remove P in the converter smelting, the P content is controlled to be less than or equal to 0.030%, and the C content is controlled to be 0.15% -0.2% at the end point of the converter smelting; and then carrying out slab continuous casting.
2) Continuous casting: the superheat degree of continuous casting is controlled to be 15-25 ℃, the continuous casting blank drawing speed is controlled to be 0.9-1.5 m/min, and the secondary cooling specific water quantity is controlled to be 1.7-2.2L/kg, so that the average carbon segregation index is reduced, segregation is inhibited, the secondary cooling strength is limited, and the trend of central crack deterioration of a casting blank is inhibited. And (3) putting soft reduction into the horizontal sector section, namely the solidification tail end, wherein the reduction of the continuous casting billet is 8-12 mm, so that the central loosening grade and segregation of the casting billet are reduced.
3) Heating a casting blank: the casting blank (with the thickness of below 300 mm) is sent into a step heating furnace to be heated, the casting blank is heated to 1210-1230 ℃, the total heating time of a soaking section and a heating section is 2-3 hours, and the total furnace time is 4-5 hours. The opening degree of an upper burner and a lower burner in the heating furnace is regulated, and the air-fuel ratio is controlled to be 1:1.7-1:2.2. By adopting higher heating temperature, the furnace time of the soaking section and the heating section is ensured, the temperature difference between the surface and the core of the continuous casting billet is shortened, the transverse and longitudinal metal flow uniformity of the surface of the steel plate is improved, and the aim of controlling the total furnace time is to effectively inhibit the overgrowth of austenite grains and ensure the performance of the steel plate.
4) And (3) rolling control: during rolling, the reduction rate of each pass of the first three passes is preferably more than 15%, the descaling water of the rolling mill is sprayed in the first two passes, the descaling time is 0.5-1 min each time, and the descaling pressure is 15-20 MPa. The steel is rapidly thrown after rolling, the steel throwing speed is 4-6 m/s, the pre-straightening investment is adopted, the upward tilting of the steel plate head in the rolling process is prevented, and the water is stored on the surface due to upward tilting or downward buckling of the steel plate head and tail in the subsequent cooling process, so that the performance uniformity is influenced. The flexible and changeable high-pressure water descaling process ensures that the tissue from the surface to the core of the steel plate is uniformly distributed, and the plasticity and toughness of the steel plate are improved;
5) And (3) cooling: laminar cooling is adopted, the cooling temperature range is 950-1030 ℃, the final cooling temperature range is 830-950 ℃, and the cooling speed is 10-15 ℃/s. In the cooling process, the cooling speed and the reddening temperature are controlled, the formation of bainite and martensite phases on the surface of the steel plate and the number of bainite phases in a core structure are restrained, and the influence on the toughness of the steel plate due to the occurrence of bainite or martensite phase transformation on the surface is avoided.
6) Stacking and slow cooling: and (3) carrying out stacking slow cooling on the cooled steel plates, wherein the stacking slow cooling temperature is 150-200 ℃, the number of stacking blocks is not less than 8, and the slow cooling time is more than 8 hours. The low-temperature stacking is adopted, so that the contents of a strip-shaped structure, a martensitic structure or a metastable lath-shaped bainitic structure are reduced, and the plasticity and toughness of the steel plate are ensured.
The invention combines the chemical components and the production process, and solves the problems that the elongation of the steel plate is lower than the technical standard and the one-time performance qualification rate of the steel plate is affected because the head and tail temperature of the economic 235 MPa-grade hot rolled steel plate is reduced rapidly and the cooling process window is narrowed after alloy reduction, so that supercooling occurs at the head and the tail of the steel plate in the length direction.
The following examples are given by way of illustration of detailed embodiments and specific procedures based on the technical scheme of the present invention, but the scope of the present invention is not limited to the following examples.
[ example ]
Table 1 shows the chemical composition of each example steel, table 2 shows the smelting process parameters of each example steel, and table 3 shows the heating process parameters of each example cast slab; table 4 shows rolling and cooling process parameters of the steels of each example; table 5 shows the stacking slow cooling process parameters and the properties of the finished steel plates of each example.
Table 1 chemical composition (wt.%)
| Examples | C | Si | Mn | N | Al |
| 1 | 0.16 | 0.15 | 0.32 | 0.016 | 0.016 |
| 2 | 0.19 | 0.11 | 0.45 | 0.019 | 0.018 |
| 3 | 0.20 | 0.19 | 0.38 | 0.025 | 0.019 |
| 4 | 0.18 | 0.20 | 0.39 | 0.022 | 0.020 |
| 5 | 0.17 | 0.17 | 0.36 | 0.028 | 0.017 |
| 6 | 0.16 | 0.18 | 0.42 | 0.017 | 0.019 |
Note that: the impurity element P in the steel is less than or equal to 0.03 percent, and S is less than or equal to 0.02 percent.
Table 2 parameters of the smelting process for steels
TABLE 3 casting blank heating process parameters
Table 4 parameters of the rolling and cooling process of the steel
TABLE 5 Steel plate Stacking and slow cooling Process parameters and finished Steel plate Properties
Therefore, after the economic 235 MPa-grade hot rolled steel plate elongation stable control method is adopted, the performance index requirements of the steel plate can be met without subsequent tempering heat treatment, and the problems that the economic steel plate is lower than the technical standard and the primary performance qualification rate of the steel plate is affected due to the fact that the head and tail temperature is reduced rapidly and the cooling process window is narrowed after the alloy is reduced, supercooling occurs at the head and the tail in the length direction of the steel plate are solved.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (5)
1. An economical 235MPa grade hot rolled steel plate is characterized by comprising the following chemical components in percentage by weight: 0.15 to 0.2 percent of C, 0.1 to 0.2 percent of Si, 0.3 to 0.45 percent of Mn, less than or equal to 0.03 percent of P, less than or equal to 0.02 percent of S, 0.015 to 0.02 percent of Al, 0.015 to 0.03 percent of N, and the balance of Fe and unavoidable impurities.
2. An economical 235 MPa-grade hot-rolled steel sheet according to claim 1, wherein the thickness of the finished steel sheet is 40-80 mm.
3. The economical 235 MPa-grade hot rolled steel plate according to claim 1, wherein the yield strength of the transverse stretching of the finished steel plate is more than or equal to 225MPa, the tensile strength is 370-450 MPa, the elongation is more than or equal to 26%, and the transverse Charpy impact energy at-20 ℃ is more than or equal to 30J.
4. The stable control method for the elongation percentage of the economic 235MPa grade hot rolled steel plate according to any one of claims 1 to 3, wherein the steel plate production process comprises molten steel smelting, continuous casting, casting blank heating, controlled rolling, cooling and stacking slow cooling; wherein the following processes are controlled:
1) Smelting molten steel: pretreating raw materials by KR molten iron, controlling the S content to be less than or equal to 0.02%, and feeding the raw materials into a converter after slag skimming; p is removed by adopting a double slag method in converter smelting, and the P content is controlled to be less than or equal to 0.030%; controlling the content of C at the smelting end point of the converter to be 0.15-0.2%;
2) Continuous casting; controlling the superheat degree of continuous casting to be 15-25 ℃, the continuous casting blank drawing speed to be 0.9-1.5 m/min, and the secondary cooling specific water quantity to be 1.7-2.2L/kg; putting light reduction into the horizontal sector section, namely the solidification end, wherein the reduction of the continuous casting billet is 8-12 mm;
3) Heating a casting blank: heating the casting blank to 1210-1230 ℃, wherein the total heating time of the soaking section and the heating section is 2-3 h, and the total furnace time is 4-5 h; controlling the air-fuel ratio of the heating furnace to be 1:1.7-1:2.2;
4) And (3) rolling control: during rolling, the reduction rate of each pass of at least the first three passes is more than 15%, the descaling water of the rolling mill is sprayed in at least the first two passes, the descaling time of each pass is 0.5-1 min, and the descaling pressure is 15-20 MPa; after rolling, rapidly casting steel, wherein the casting steel speed is 4-6 m/s, and pre-straightening is carried out;
5) Cooling; laminar cooling is adopted, the cooling temperature is 950-1030 ℃, the final cooling temperature is 830-950 ℃, and the cooling speed is 10-15 ℃/s;
6) Stacking and slow cooling: and (3) carrying out stacking slow cooling on the cooled steel plates, wherein the stacking slow cooling temperature is 150-200 ℃, the number of stacking blocks is not less than 8, and the slow cooling time is more than 8 hours.
5. The stable control method for the elongation percentage of the economic 235MPa hot rolled steel plate according to claim 4, wherein the thickness of the continuous casting billet is less than or equal to 300mm, and the rolling is carried out on a medium and thick plate reciprocating rolling mill.
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