KR101033394B1 - 440mpa high strengh steel sheet and it's manufacturing method - Google Patents
440mpa high strengh steel sheet and it's manufacturing method Download PDFInfo
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- KR101033394B1 KR101033394B1 KR1020110000447A KR20110000447A KR101033394B1 KR 101033394 B1 KR101033394 B1 KR 101033394B1 KR 1020110000447 A KR1020110000447 A KR 1020110000447A KR 20110000447 A KR20110000447 A KR 20110000447A KR 101033394 B1 KR101033394 B1 KR 101033394B1
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- 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
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
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- 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
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
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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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Abstract
The present invention relates to a high tensile steel sheet manufacturing method, and to a high tensile steel sheet manufacturing method which is excellent in strength and formability and suitable for use as an automotive steel sheet.
The present invention is a weight%, carbon (C): 0.060 ± 0.005%, silicon (Si): 0.010 ± 0.005%, manganese (Mn): 1.600 ± 0.1%, phosphorus (P): more than 0 0.015%, sulfur ( S): more than 0 0.015% or less, niobium (Nb): 0.01 ± 0.005% and a cold rolled steel sheet made from a hot rolled steel sheet consisting of the remaining iron (Fe) and unavoidable impurities, the heating step and annealing temperature 700 ~ 720 ℃ ℃ annealing It provides a high tensile strength steel sheet manufacturing method comprising a heat treatment step consisting of a heat holding step of 10 to 15 hours and a cooling step.
Description
The present invention relates to a high tensile steel sheet manufacturing method, and to a high tensile steel sheet manufacturing method excellent in strength and formability suitable for use as an automotive steel sheet.
The annealing heat treatment manufacturing process of cold steel sheet can be roughly divided into heat treatment of the steel sheet by continuous annealing line (CAL) and box annealing furnace (BAF).
As such, the purpose of annealing the cold rolled steel sheet is not only to improve the workability of the cold rolled steel sheet but is also essential to satisfy the mechanical properties of the final product after processing. The BAF annealing heat treatment process characteristics performed for these two main purposes are as follows.
Box annealing heat treatment process consists of ① steel sheet loading → ② atmosphere homogenization → ③ heating → ④ heating → ⑤ cooling → ⑥ taking out. In this process, the characteristics in terms of materials are composed of recovery in one step, recrystallization in two steps, and grain growth in three steps.
In the present invention, to obtain an annealing cycle method for the optimization conditions such as the heating holding temperature and time in the annealing process to satisfy the mechanical properties of the high tensile steel sheet, and to find the appropriate heat treatment method, for each process of the BAF annealing process It explains in detail.
First, the steel plate is loaded in accordance with the processing capacity, steel type, thickness, and annealing cycle inside the BAF furnace, and the outside of the furnace is shielded. To form a reducing atmosphere and purging. Here, as an atmosphere gas, nitrogen gas (N 2) or hydrogen gas (H 2) is commonly used to form a reducing atmosphere. Purging is a pretreatment step performed to remove the oxidative atmosphere in the treatment space to prevent oxide formation of the steel sheet which is likely to occur after the spheroidizing annealing treatment and to have a decarburization prevention effect. Since the surface of the steel sheet after annealing through purging has little oxide generation, beautiful and clean characteristics can be obtained.
Next, a heating device is operated to heat the steel sheet in the furnace, and is heated step by step to a target holding temperature at an appropriate heating rate. At this time, the holding temperature and time suitable for the use of the steel sheet are set and maintained respectively. Thereafter, the heating device is deodorized according to a normal cooling cycle, and a cooling device with a cooling fan and cooling water is mounted and operated to cool down to the deodorizing temperature, and finally, the material is moved out.
An object of the present invention is to derive a component system of a high tensile strength steel sheet having a tensile strength (TS): 45kg / ㎠ or more, elongation (EL): 34% or more, yield strength (YP): 25 ~ 40kg / ㎜, and to manufacture it It is to provide a manufacturing method.
The present invention for achieving this object is by weight, carbon (C): 0.060 ± 0.005%, silicon (Si): 0.010 ± 0.005%, manganese (Mn): 1.600 ± 0.1%, phosphorus (P): greater than 0 0.015% or less, sulfur (S): more than 0 and 0.015% or less, niobium (Nb): 0.01 ± 0.005% and a cold rolled steel sheet made of a hot rolled steel sheet consisting of remaining iron (Fe) and unavoidable impurities,
It provides a high tensile strength steel sheet manufacturing method comprising a heating step, an annealing temperature of 700 ~ 720 ℃, an annealing time of 10 to 15 hours, a heat holding step, and a box annealing heat treatment process consisting of a cooling step.
The hot rolled steel sheet, the hot rolling step of roughly rolling the slab plate to RDT (Roughing Delivery Temperature): 950 ~ 850 ℃, FDT (Finishing Delivery Temperature): 890 ℃ ~ A r3 temperature; And the cooling / winding step of winding the hot-rolled sheet to CT (Coiling Temperature): 560 ± 10 ° C .;
The box annealing heat treatment process includes a heating step of 12 hours, a heating and holding step of 10-15 hours, and a cooling step of 23 hours,
The heating step is heated to 300 ± 5 ℃ up to 3 hours, to 670 ± 5 ℃ up to 8 hours, to 710 ± 5 ℃ up to 12 hours,
The cooling step is cooled to 630 ± 5 ℃ to 4 hours after the start of cooling, to 520 ± 5 ℃ to 8 hours, to 350 ± 5 ℃ to 12 hours, to 50 ± 5 ℃ to 23 hours It is preferable.
Through this method, the present invention is in terms of weight%, carbon (C): 0.060 ± 0.005%, silicon (Si): 0.010 ± 0.005%, manganese (Mn): 1.600 ± 0.1%, phosphorus (P): greater than 0 0.015 % Or less, sulfur (S): more than 0 and 0.015% or less, niobium (Nb): 0.01 ± 0.005% and the remaining iron (Fe) and inevitable impurities, tensile strength (TS): 45kg / mm2 or more, elongation (EL ): High strength steel sheet having 34% or more and yield strength (YP): 25 ~ 40kg / mm2.
The present invention derives the component system capable of satisfying the mechanical properties of the high tensile steel sheet, the process conditions for producing the hot rolled material and the annealing heat treatment conditions for producing the same, and the tensile strength (TS) which is a required mechanical property range: 45 kg /
1 is a graph showing a BAF heat treatment condition of an annealing time of 5 hours,
2 is a graph showing a BAF heat treatment condition of an annealing time of 10 hours,
3 is a graph showing a BAF heat treatment condition of an annealing time of 15 hours
4 and 5 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
6 and 7 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
8 and 9 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
10 and 11 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 640 ℃, heat
12 and 13 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 640 ℃, heat
14 and 15 is a microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 640 ℃, heat
Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, and only the embodiments make the disclosure of the present invention complete, and those skilled in the art to which the present invention pertains. It is provided to fully inform the person having the scope of the invention, which is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.
Hereinafter, with reference to the accompanying drawings, a preferred embodiment of the present invention will be described in detail with respect to 440MV class high tensile steel sheet and a method of manufacturing the same.
Manufacture of hot rolled materials
It changes the target composition of carbon (C), silicon (Si), manganese (Mn) and niobium (Nb),
The processing conditions were finishing hot rolling temperature (FDT, Finishing Delivery Temperature) of 890, coiling temperature (CT, Coiling Temperature) of 560 ℃ and 640 ℃. Samples of hot rolled materials were prepared.
The slab plate is roughly rolled to RDT (Roughing Delivery Temperature): 850 to 950 ° C, and finish-rolled to FDT (Finishing Delivery Temperature): 890 ° C to A r3 temperature. The slab plate can be obtained through the continuous casting process after obtaining the molten steel of the desired composition through the steelmaking process.
Meanwhile, before the hot rolling, the slab plate may further include a slab reheating step of reheating the slab plate to SRT (Slab Reheating Temperature): 1200 to 1250 ° C.
Through reheating of the slab plate, segregation of components segregated during casting may occur in inventory and in inventory of precipitates.
It is preferable that the reheating temperature (SRT) of a slab board material is 1200-1250 degreeC. If the slab reheating temperature (SRT) is less than 1200 ° C., the segregated components during casting are not sufficiently reusable, and there is a problem in that precipitates such as niobium (Nb) are not sufficiently dissolved. On the contrary, when the slab reheating temperature (SRT) exceeds 1250 ° C, the austenite grain size may increase, resulting in coarsening of the ferrite of the final microstructure, which may make it difficult to secure strength, and may increase only the manufacturing cost of the steel sheet due to excessive heating process. .
In the present invention, hot rolling is divided into rough rolling and finishing rolling.
Hot rolled sheet is wound up by cooling to CT (Coiling Temperature): 560 ℃ or 640 ℃.
In the present invention, the cooling process is to cool the rolled plate to the bainite temperature region through a water cooling method, thereby suppressing grain growth of the steel sheet to form a matrix structure having fine ferrite grains, and also fine bainite structure It is carried out to form a high strength and high toughness. The cooling rate may be about 1 to 100 / sec, but is not limited thereto.
Hereinafter, the role and content of each component included in the high tensile steel sheet according to the present invention will be described.
Main six component system (C, Si, Mn, P, S, Nb)
Carbon (C)
Carbon (C) is added to secure the strength.
The carbon is preferably added in 0.055 ~ 0.065% by weight of the total weight of the hot rolled steel sheet according to the present invention, more preferably 0.06% by weight can be presented. If the carbon content is less than 0.055% by weight, it is difficult to secure a tensile strength of 440 MPa or more despite other alloying elements added for strength replenishment, and when the carbon content exceeds 0.065% by weight, the toughness is relatively decreased. have.
Silicon (Si)
Silicon contributes to strength and, in particular, acts as a deoxidizer to remove oxygen in the steel.
The silicon is preferably added in an amount of 0.005 to 0.015% by weight of the total weight of the hot rolled steel sheet according to the present invention in consideration of inherent deoxidation effect and surface quality, and more preferably 0.010% by weight. If the content of silicon is less than 0.005% by weight, the deoxidation effect due to the addition of silicon is insufficient. On the contrary, when the silicon content exceeds 0.015% by weight, the plating property may be inhibited after welding of the steel, and the surface quality may be degraded by generating red scales during hot rolling.
Manganese (Mn)
Manganese (Mn) is very effective as a solid solution strengthening element, and is an effective element for securing strength by improving the hardenability of steel. In addition, manganese is an austenite stabilizing element, which contributes to the refinement of ferrite grains by retarding ferrite and pearlite transformation.
The manganese is preferably added in an amount ratio of 1.5 to 1.7% by weight of the total weight of the high tensile strength steel sheet according to the present invention in consideration of the strength improving effect and the central segregation, and more preferably 1.6% by weight. If the added amount of manganese is less than 1.5% by weight, the effect of strengthening the solid solution is insignificant.On the contrary, when the added amount of manganese exceeds 1.7% by weight, the weldability is greatly reduced, and the ductility of the steel is generated by the formation of MnS inclusions and the occurrence of center segregation. There is a problem that greatly reduces.
Phosphorus (P)
Phosphorus (P) is added to inhibit cementite formation and increase strength.
However, since phosphorus deteriorates weldability and causes final material deviation by slab center segregation, the phosphorus (P) is greater than 0 and less than 0.015% by weight of the total weight of the hot rolled steel sheet according to the present invention. It is preferable to add restrictively within the range of.
Sulfur (S)
Sulfur (S) is an element that inhibits the toughness and weldability of steel, and combines with manganese to form MnS non-metallic inclusions to generate cracks during processing of steel.
Therefore, the content of sulfur (S) is preferably limited to more than 0 0.015% by weight of the total weight of the hot rolled steel sheet according to the present invention.
Niobium (Nb)
Niobium (Nb) is a precipitate forming element and is one of the elements that has the greatest influence on the strength of steel, and is an element that enhances the strength of steel through precipitation of carbonitride in steel or solid solution strengthening in Fe. In particular, niobium-based precipitates are solid-solution precipitated during hot rolling after solid solution in a heating furnace of 1200 ℃ or more when reheating the slab effectively increases the strength of the steel.
The niobium is preferably added in an amount of 0.005 to 0.015% by weight of the total weight of the hot rolled steel sheet according to the present invention. It has a sufficient niobium addition effect when the content of niobium is 0.005% by weight or more. On the other hand, when the content of niobium exceeds 0.015% by weight, due to excessive precipitation it may reduce the playability, rolling properties and elongation.
According to the composition, it was classified into four types of H1, H2, H3, and H4, and divided into two groups according to the winding temperature.
Table 1 shows the composition of four types of specimens (H1, H2, H3, H4),
Table 2 shows the compositions and BAF heat treatment cycles of the four specimens wound at a coiling temperature of 640 ° C.
Table 3 shows the composition of the four specimens wound at the winding temperature of 560 ° C and the BAF heat treatment cycle.
Cold Rolling Heat Treatment Condition
The BAF simulation test was used to measure the change in physical properties and the change in heat treatment temperature and holding time.
The specimen was prepared, loaded into BAF (Batch Annealing Furnace), and the inside of the annealing furnace was vacuumed through a purging process, followed by heat treatment.
The size of the specimen loaded in BAF was 45mm * 250mm.
Annealing temperature was set to 710 ℃, the annealing time was changed to 5, 10, 15 hours.
FIG. 1 is a graph showing BAF heat treatment conditions for annealing time of 5 hours, FIG. 2 is a graph showing BAF heat treatment conditions for annealing time of 10 hours, and FIG. 3 is a graph showing BAF heat treatment conditions for annealing time of 15 hours. .
The graph is shown based on the target temperature, and the test temperature shows the actual test temperature set at the hot spot.
Referring to Figure 1, the entire heat treatment process consists of a 40-hour cycle, consisting of a heating step (setp1 ~ 3) of 12 hours, a holding step (step4) of 15 hours, and a cooling step (steps 5-8) of 23 hours.
First, the heating step is performed by heating to 300 ° C. for 3 hours after the start of heating (step 1), heating to 670 ° C. for 8 hours (step 2), and heating to 710 ° C. for 12 hours (step 3). Wherein each temperature may have an error in the range of ± 5 ° C.
After maintaining the temperature of 710 ℃ for 5 hours in the holding step (step4), the cooling step of 23 hours is carried out.
The cooling stage is cooled to 630 ° C. for up to 4 hours after the start of cooling (step 5), to 520 ° C. for up to 8 hours (step 6), to 350 ° C. for up to 12 hours (step 7) and to 50 ° C. for up to 23 hours. (Step8).
In the 45-hour cycle heat treatment of FIG. 2, the heating step and the cooling step are the same as the 40-hour cycle heat treatment, and the maintenance step is performed for 10 hours.
3 and the 50-hour cycle heat treatment, the heating step and the cooling step is the same as the 40-hour cycle heat treatment, the difference is that the holding step is carried out for 15 hours.
Each hot rolled material was classified into two groups according to the winding temperatures (560 ° C. and 640 ° C.), and the mechanical properties were measured after the BAF heat treatment conditions were changed and tested.
The yield strength (YP), tensile strength (TS) and elongation (EL) were measured as mechanical properties.
Table 4 shows each composition of the 640 ℃ winding material and the mechanical properties according to BAF heat treatment conditions,
Table 5 shows each composition of the 560 ℃ winding materials and the mechanical properties according to BAF heat treatment conditions.
4 and 5 show the microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
6 and 7 show the microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
8 and 9 show the microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 560 ℃, heat
10 and 11 show the microstructure photograph of the cold rolled steel sheet manufactured by the winding temperature of 640 ℃, heat
12 and 13 show the microstructure photograph of the cold rolled steel sheet produced by the winding temperature of 640 ℃, heat
14 and 15 show a microstructure photograph of a cold rolled steel sheet manufactured by a winding temperature of 640 ° C., a heat treatment holding temperature of 710 ° C., and a holding time of 15 hours.
As to the physical properties of the annealing holding time, the yield strength and the tensile strength decrease and the elongation tends to increase as the annealing holding time increases.
To summarize the results, in the case of component system, it is weight%, carbon (C): 0.060 ± 0.005%, silicon (Si): 0.010 ± 0.005%, manganese (Mn): 1.600 ± 0.1%, phosphorus (P): 0.015% Sulfur (S): 0.015% or less, niobium (Nb): 0.01 ± 0.005%, and
Winding temperature is carried out at low temperature winding at 560 ℃,
When performing the box annealing heat treatment process including the heating and holding step of the annealing temperature 700 ~ 720 ℃, annealing time 10 ~ 15 hours was able to produce a high tensile strength steel sheet that satisfies the target properties.
In the above description, the embodiment of the present invention has been described, but various changes and modifications can be made at the level of those skilled in the art. Such changes and modifications may belong to the present invention without departing from the scope of the present invention. Therefore, the scope of the present invention will be determined by the claims described below.
Claims (8)
A high strength steel sheet manufacturing method comprising a box annealing heat treatment step comprising a heating step, an annealing temperature of 700 to 720 ° C., an annealing time of 10 to 15 hours, and a cooling step.
The hot rolled steel sheet,
Hot rolling step of slab slab plate rough rolling to RDT (Roughing Delivery Temperature): 850 ~ 950 ℃, FDT (Finishing Delivery Temperature): 890 ℃ ~ A r3 temperature; And
The hot-rolled sheet material CT (Coiling Temperature): a high tension steel sheet manufacturing method characterized in that it is manufactured by a process comprising a; cooling / winding step of winding by cooling to 560 ± 10 ℃.
The box annealing heat treatment process includes a heating step of 12 hours, a heat holding step of 10-15 hours, and a cooling step of 23 hours.
The heating step is a high tensile steel sheet manufacturing method characterized in that the heating to 300 ± 5 ℃ to 3 hours, to 670 ± 5 ℃ to 8 hours, and to 710 ± 5 ℃ to 12 hours.
The cooling step is cooled to 630 ± 5 ℃ to 4 hours after the start of cooling, to 520 ± 5 ℃ to 8 hours, to 350 ± 5 ℃ to 12 hours, to 50 ± 5 ℃ to 23 hours High tensile steel sheet manufacturing method characterized in that.
The cooling / winding step
High tensile steel sheet manufacturing method characterized in that the shear cooling method is applied.
A high tensile strength steel sheet having a tensile strength (TS) of 45 kg / mm 2 or more and an elongation (EL) of 34% or more.
The high tensile steel sheet
Yield strength (YP): A high tensile strength steel sheet having 25 to 40 kg / mm 2.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105624556A (en) * | 2016-01-23 | 2016-06-01 | 山西太钢不锈钢股份有限公司 | Hot-rolled pole sheet steel and manufacturing method thereof |
| KR101685838B1 (en) * | 2015-08-19 | 2016-12-12 | 현대제철 주식회사 | Cold worked steel sheet with batch annealing furnace thermal treatment and method for manufacturing the same |
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