WO2017111233A1 - High strength steel and manufacturing method therefor - Google Patents

Abstract

The present invention provides high strength steel and a manufacturing method therefor, the high strength steel comprising, by weight, 0.035-0.07% of C, 0.3% or less (exclusive of 0) of Si, 2.0-3.5% of Mn, 0.3-1.2% of Cr, 0.03-0.08% of Ti, 0.01-0.05% of Nb, and a balance amount of Fe and inevitable impurities, wherein the entire microstructure has a transformed structure containing tempered martensite and bainite at a volume fraction of 90% or more, the tempered martensite structure having an average particle diameter of 2μm or less, the bainite structure having an average particle diameter of 3μm or less, with a volume fraction of 5 % or less of a bainite structure greater than 3μm in particle diameter.

Description

High strength steel and its manufacturing method

The present invention relates to a high strength steel and a manufacturing method thereof, and more particularly, to a high strength steel excellent in high yield ratio and hole expansion properties and a manufacturing method thereof.

Recently, the use of high strength steel sheet is required to improve fuel efficiency and durability by various environmental regulations and energy usage regulations. In particular, as the impact stability regulations of automobiles are expanded, high strength steel with excellent yield strength is adopted for structural members such as members, seat rails, and pillars to improve impact resistance of vehicles. have. Structural member has a characteristic that the higher the yield strength, that is, the higher the yield ratio (yield strength / tensile strength) than the tensile strength, to absorb impact energy. However, in general, as the strength of the steel sheet increases, the elongation decreases, so that a problem arises in that the molding processability is lowered.

Typically, the method of reinforcing steel includes solid solution strengthening, precipitation strengthening, strengthening by grain refinement, transformation strengthening, and the like. However, the reinforcement by solid solution strengthening and grain refinement of the method has a disadvantage that it is very difficult to produce high strength steel with a tensile strength of 490MPa or more.

On the other hand, precipitation-reinforced high-strength steels are formed by adding carbon and nitride forming elements such as Cu, Nb, Ti, and V to precipitate carbon and nitride to reinforce steel sheets or to refine grains by suppressing grain growth by fine precipitates. To secure the technology. The above technique has the advantage of easily obtaining a high strength compared to a low manufacturing cost, but the recrystallization temperature is rapidly increased by the fine precipitate, there is a disadvantage that a high temperature annealing must be performed to ensure ductility sufficient to recrystallize. In addition, the precipitation-reinforced steel which precipitates and strengthens carbon and nitride on a ferrite base has a problem in that it is difficult to obtain high-strength steel of 600 MPa or more.

On the other hand, the transformation hardened high-strength steel is a ferritic-martensitic dual phase steel in which hard martensite is included in the ferritic base, and a transformation induced plasticity (TRIP) steel or a ferritic material using transformation organic plasticity of retained austenite. Various such as CP (Complexed Phase) steels composed of hard bainite or martensite structures have been developed. However, the tensile strength that can be implemented in such an improved high strength steel (of course, the strength can be increased by increasing the amount of carbon, but considering practical aspects such as spot weldability) is limited to the level of about 1200Mpa. In addition, the application to the structural member to secure the safety of the crash, hot press forming steel (Hot Press Forming) to secure the final strength by quenching through direct contact with the die (die) that is water-cooled after molding at high temperature, Due to the excessive investment of facilities, high heat treatment and process costs, the application expansion is not large.

Recently, in order to further improve the stability of passengers during a collision, high strength and light weight of a seat part of a vehicle are simultaneously progressed. These parts are manufactured by two methods, not only roll forming but also press forming. Seat parts are the part that connects the passengers to the body and must be supported with high stress to prevent the passengers from jumping out in the event of a collision. This requires high yield strength and yield ratio. In addition, as most of the parts to be processed require expansion flanges, application of steel having excellent hole expansion properties is required.

A typical manufacturing method for increasing yield strength is to use water cooling during continuous annealing. That is, it is possible to manufacture a steel sheet having a tempered martensite structure in which the microstructure tempered martensite by cracking in an annealing process and then immersing in water to temper. However, this method has very serious disadvantages such as deterioration of workability and deviation of material by position in roll forming application due to inferior shape quality due to width and length temperature variations in water cooling.

As a prior art, Japanese Patent Laid-Open No. 1992-289120 (1992.10.14) is disclosed. The patent is a continuous annealing of 0.18% or more carbon steel, water cooled to room temperature, and then subjected to overaging for 1 to 15 minutes at a temperature of 120 ~ 300 ℃, to develop a martensite steel with a martensite volume ratio of 80 ~ 97% or more will be. As described above, in the case of manufacturing ultra-high strength steel by the tempering method after water cooling, the yield ratio is very high, but the shape quality of the coil is deteriorated due to the temperature deviation in the width direction and the length direction. Therefore, problems such as material defects and workability deterioration according to parts during roll forming processing occur.

In addition, as another prior art, Japanese Patent Laid-Open Publication No. 2010-090432 (2010.04.22) uses tempering martensite to simultaneously obtain high strength and high ductility, and also provides a method for manufacturing a cold rolled steel sheet having excellent plate shape after continuous annealing. As high as 0.2% or more, the possibility of induction of furnace dent due to weld inferiority and a large amount of Si is feared.

Therefore, in order to solve the above-mentioned problems, a steel material capable of exhibiting high yield ratio and high porosity while showing high strength is required.

The present invention has been made to solve this problem, the object of the present invention, yield ratio is 0.8 or more in order to secure excellent bending workability and hole expandability, the R / t value of the bending workability index is at most 1 and at the same time hole expansion It is to provide a high-strength steel having a HER value of 50% or more, an index for evaluating the properties and a method of manufacturing the same.

In order to achieve the above object, the high-strength steel according to an embodiment of the present invention, by weight% C: 0.035 ~ 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 ~ 3.5%, Cr: 0.3 ~ 1.2%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05%, residual Fe and unavoidable impurities, and the volume fraction of the metamorphic tissue including tempered martensite and bainite in the total microstructure is 90% or more. The average particle diameter of the tempered martensite structure is 2 μm or less, the average particle diameter of the bainite structure is 3 μm or less, and the volume fraction of the bainite structure in which the particle size exceeds 3 μm is 5% or less.

By weight%, B: 0.0010-0.0050%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.10%, N: 0.010% or less, It is characterized by the above-mentioned.

The average hardness value Hv of the microstructure is 340 or more, and the maximum value of the hardness value is 1.3 times or less of the minimum value.

The distribution density of the nanoprecipitates of 10 nm or less in the metamorphic structure is characterized in that the 150 / μm ² or more.

The tensile strength is 980 MPa or more, and the yield strength is characterized by 780 MPa or more.

On the other hand, high-strength steel manufacturing method, in weight% C: 0.035 ~ 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 ~ 3.5%, Cr: 0.3 ~ 1.2%, Ti: 0.03 ~ 0.08%, Nb: manufacturing a slab containing 0.01 ~ 0.05%, the balance Fe and unavoidable impurities, hot rolling the slab to the finish-rolling exit side temperature Ar ₃ ~ Ar ₃ + 50 ℃ to produce a hot rolled steel sheet, Cold rolling the hot rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold rolled steel sheet, continuously annealing the cold rolled steel sheet in a temperature range of Ac ₃ ± 30 ℃, after the continuous annealing step 650 ~ 700 ℃ The first step of cooling to a cooling rate of 1 ~ 10 ℃ / sec, after the first step of cooling, Ms-100 ~ Ms ℃ includes a second cooling at a cooling rate of 5 ~ 20 ℃ / sec.

After the second step of cooling, further comprising the step of overaging while maintaining a constant temperature.

The overaging step is characterized in that the aging time is adjusted so that the volume fraction of metamorphic tissue consisting of tempered martensite and bainite of the entire tissue is 95% or more.

According to the high strength steel and the manufacturing method according to the present invention has the following effects.

First, steel having a high tensile strength of 980 MPa or more can be manufactured.

Secondly, it is possible to manufacture steel materials having excellent impact absorption due to high yield ratio, and excellent bending property and hole expansion property.

1 is a photograph showing a microstructure according to an embodiment of the present invention,

2 is a photograph showing a nano precipitate according to an embodiment of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite. As used herein, the term "comprising" embodies a particular characteristic, region, integer, step, operation, element, and / or component, and other specific characteristics, region, integer, step, operation, element, component, and / or group. It does not exclude the presence or addition of.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly defined terms used are additionally interpreted to have a meaning consistent with the related technical literature and the presently disclosed contents, and are not interpreted in an ideal or very formal sense unless defined.

Hereinafter, with reference to the accompanying drawings will be described a high strength steel and a method for manufacturing the same according to a preferred embodiment of the present invention.

First, the high strength steel according to the present invention will be described.

To this end, by weight% C: 0.035 to 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 to 3.5%, Cr: 0.3 to 1.2%, Ti: 0.03 to 0.08%, Nb: 0.01 to 0.05 %, Balance Fe and unavoidable impurities, optionally B: 0.0010-0.0050%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.10%, N: 0.010% or less The steel material to further add is manufactured.

At this time, the volume fraction occupied by the metamorphic structure including the tempered martensite and bainite is 90% or more of the entire microstructure of the manufactured steel, the average particle diameter of the tempered martensite tissue is 2 μm or less, It is preferable that the average particle diameter is 3 micrometers or less, and the volume fraction of the bainite structure whose particle diameter exceeds 3 micrometers is 5% or less.

Hereinafter, the reason for numerical limitation of each component will be described.

Carbon (C) is a very important element added for strengthening metamorphic tissue. Carbon promotes high strength and promotes the formation of martensite in metamorphic steel. As the carbon content increases, the martensite content in the steel increases. However, if the amount exceeds 0.07%, the strength of martensite increases, but the difference in strength from ferrite with low carbon concentration increases. This difference in strength is because the elongation flange property is lowered because the fracture easily occurs at the interface between phases when stress is added. In addition, weldability is inferior and welding defects occur when machining parts. On the other hand, when the carbon content is lowered to less than 0.035%, it is difficult to secure the strength of martensite proposed in the present invention, so the amount is limited to 0.035 to 0.07%.

Silicon (Si) is an element that promotes ferrite transformation and raises the carbon content in the unmodified austenite to form a complex structure of ferrite and martensite, thereby preventing the increase in martensite strength. It is also desirable to limit the possible additions as well as cause surface scale defects in terms of surface properties, as well as degrading chemical conversion. Therefore, in the present invention, the amount of addition was limited to 0.3% or less (excluding 0%).

Manganese (Mn) is an element that refines particles without damaging ductility, precipitates sulfur with MnS, prevents hot brittleness due to the formation of FeS, and strengthens steel, and at the same time, lowers the critical cooling rate at which a martensite phase is obtained. By doing so, martensite can be more easily formed. If the content is less than 2.0%, it is difficult to secure the target strength of the present invention, and if the content exceeds 3.5%, there is a high possibility of problems such as weldability and hot rolling property, so the content of manganese is 2.0 to 3.5%. Limited to the range, it is more advantageous to control in the range of 2.3 to 3.2%.

Phosphorus (P) is a substitution type alloy element having the greatest solid solution strengthening effect, and serves to improve in-plane anisotropy and strength. If the content is less than 0.001%, the effect may not be secured and it may cause a problem in manufacturing cost.However, if the content is added excessively, the press formability may deteriorate and the brittleness of the steel may be generated. It is desirable to limit.

Sulfur (S) is an impurity element in steel and is an element that inhibits the ductility and weldability of the steel sheet. If the content is more than 0.01%, the S content is preferably limited to 0.01% or less because it is highly likely to inhibit the ductility and weldability of the steel sheet.

Soluble aluminum (Sol.Al) combines with oxygen to cause deoxidation and distributes carbon in ferrite like austenite to austenite, such as silicon, and is effective in improving martensite hardenability. If the content is less than 0.01% can not secure the effect, if the content exceeds 0.1%, the effect is not only saturated, but also increases the manufacturing cost, it is preferable to limit the content to 0.01 ~ 0.1%.

Nitrogen (N) is an effective component for stabilizing austenite, and if it exceeds 0.01%, it is preferable to limit the upper limit to 0.01% because the risk of cracking when playing through AlN formation is greatly increased.

Chromium (Cr) is a component added to improve the hardenability of steel and to secure high strength, and is an element that plays a very important role in forming martensite, which is a low temperature transformation phase in the present invention. If the content is less than 0.3%, it is difficult to secure the above-mentioned effects. If the content is more than 1.2%, the effect is not only saturated, but excessive hot-rolling strength increases, resulting in the problem of cold rolling deterioration. Therefore, the content is limited to 0.3-1.2%. It is preferable.

Boron (B) is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is added as an element that suppresses ferrite formation and promotes martensite formation. However, if the content is less than 0.0010%, it is difficult to obtain the above-described effects, and if the content exceeds 0.0050%, the cost is increased according to the excess of ferroalloy, so it is preferable to limit the content to 0.0010% to 0.0050%.

Titanium (Ti) and niobium (Nb) are effective elements for increasing the strength of steel sheets and miniaturizing grains by nano precipitates. Adding these elements combines with carbon to form very fine nano precipitates. These nano precipitates serve to reduce the hardness difference between phases by strengthening the matrix structure. In the present invention, the content of titanium is limited to 0.03 to 0.08%, and the content of niobium is limited to 0.01 to 0.05%. When the content of these elements is less than the minimum value of the present invention, the distribution density of nano precipitates is reduced and the variation in hardness ratio between phases is increased, and when the content exceeds the maximum value of the steel of the present invention, the ductility due to the increase in manufacturing cost and excessive precipitates It can greatly reduce.

Thus, the high-strength steel manufactured according to the present invention has an average hardness value (Hv) of the microstructure of 340 or more, a maximum value of the hardness value of 1.3 times or less of the minimum value, and distribution of nano precipitates of 10 nm or less in diameter in the metamorphic structure It is preferable that the density is 150 pieces / micrometer ² or more, the tensile strength is 980 MPa or more, and the yield strength is 780 MPa or more.

These physical properties will be described in more detail in the description of the manufacturing method to be described later.

High-strength steel manufacturing method according to the invention, by weight% C: 0.035 ~ 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 ~ 3.5%, Cr: 0.3 ~ 1.2%, Ti: 0.03 ~ 0.08%, Nb: 0.01 ~ 0.05%, the remainder of producing a slab containing Fe and unavoidable impurities, hot-rolled slab to finish the hot rolling outlet temperature of Ar ₃ ~ Ar ₃ +50 ℃ to produce a hot rolled steel sheet Step, cold-rolled hot rolled steel sheet at a reduction ratio of 40 ~ 70% to produce a cold rolled steel sheet, continuous annealing the cold rolled steel sheet in the temperature range of Ac ₃ ± 30 ℃, after the continuous annealing step to 650 ~ 700 ℃ The first step of cooling at a cooling rate of 1 ~ 10 ℃ / sec, After the first step of cooling, Ms-100 ~ Ms ℃ to a second cooling at a cooling rate of 5 ~ 20 ℃ / sec.

In the step of producing the hot rolled steel sheet by hot rolling, the exit temperature, more specifically, the hot rolling is performed so that the exit temperature FDT of the finish rolling mill is Ar ₃ to Ar ₃ + 50 ° C. If the outlet temperature is less than Ar ₃ , the hot deformation resistance is likely to increase rapidly, and the top, tail, and edges of the hot rolled coil become single phase regions, thereby increasing in-plane anisotropy and degrading formability. . On the other hand, if Ar ₃ + 50 ° C. is exceeded, too thick an oxidizing scale may occur, and the microstructure of the steel sheet is likely to coarsen.

After the hot rolling is completed, it is preferable to maintain the temperature in the temperature range of 600 to 750 ° C at the time of winding up. When the coiling temperature CT is less than 600 ° C., excessive martensite or bainite may be generated, resulting in excessive strength increase of the hot rolled steel sheet, which may cause manufacturing problems such as shape defects due to load during cold rolling. On the other hand, if it exceeds 750 ° C., pickling deteriorates due to an increase in the surface scale, and therefore, it is preferable to limit it to the above-described winding temperature.

Hot rolled steel is subjected to pickling and cold rolling. Cold rolling is a step of producing a cold rolled steel sheet, it is preferable to roll at a reduction ratio of 40 to 70%. If the reduction ratio is less than 40%, the recrystallization driving force is weakened, so that there is a big problem to obtain a good recrystallized grain, and shape correction is very difficult. On the other hand, if the reduction ratio exceeds 70%, there is a high possibility of cracking at the edge of the steel sheet, and the rolling load rapidly increases.

Cold rolled steel sheet is provided through the continuous annealing process to provide the basis of the desired microstructure in the present invention. At this time, it is preferable to perform continuous annealing in the temperature (SS) section of Ac ₃ -30 ~ Ac ₃ +30 ℃. If the annealing temperature is lower than Ac ₃ -30 ℃, a large amount of ferrite is generated and the yield strength is lowered, so the yield ratio of 0.8 or more cannot be secured.In particular, the production of a large amount of ferrite increases the hardness difference between phases, The conditions of hardness 340 Hv or more and hardness deviation 1.3 or less presented in the present invention cannot be satisfied. However, when the annealing temperature exceeds Ac ₃ +30 ℃, the size of the martensite phase produced during cooling due to the increase in the austenite grain size due to the high temperature annealing can not satisfy the particle size of the microstructure presented in the present invention.

After continuous annealing, the steel sheet is first cooled to a cooling rate of 1 to 10 ° C / sec to 650 to 700 ° C. This primary cooling step is a process for converting most austenite into martensite by suppressing ferrite transformation.

After the first cooling the second cooling is cooled to a temperature of Ms ~ Ms-100 ℃ at a cooling rate of 5 ~ 20 ℃ / s. This starts to transform the austenite to martensite but does not cool it below the temperature at which the martensite transformation ends. This gives us a tempered martensite structure.

By performing the secondary cooling to such a temperature range, and then overaging while maintaining the temperature, it is possible to secure a high yield strength and a high hole workability (HER) in addition to securing the width direction, the longitudinal shape of the coil.

If the secondary cooling end temperature (RCS) is less than Ms-100 ° C., the amount of martensite is excessively increased during the overaging treatment, which simultaneously increases the yield strength and tensile strength and deteriorates the ductility. In particular, deterioration of shape due to rapid cooling may result in deterioration of workability when processing automotive parts. On the other hand, when cooled to a temperature exceeding Ms, the austenite produced during annealing cannot be transformed into martensite, but is formed as bainite, granular bainite, etc., which is a high temperature transformation, and thus the yield strength deteriorates rapidly. Occurs. The occurrence of such a structure is accompanied by a decrease in yield ratio and deterioration of hole expandability, and thus cannot produce a high yield ratio type high strength steel having excellent elongation flange property.

After the overaging treatment is completed, the skin pass rolling may be performed in an elongation of 0.1 to 1.0%. Typically, the skin pass rolling of the metamorphic tissue steel causes an increase in yield strength of at least 50 MPa with little increase in tensile strength. If the elongation is less than 0.1%, it is difficult to control the shape. If the elongation is 1.0% or more, the operability is greatly unstable due to the high stretching operation, and therefore it is preferable to limit the above range.

In the step of overaging, it is preferable to adjust the aging time so that the volume fraction of the metamorphic tissue composed of tempered martensite and bainite of the whole tissue is 95% or more, which will be described later for the physical properties of Examples and Comparative Examples. This is explained in more detail in the description.

Hereinafter, examples and comparative examples of the present invention and properties thereof will be described.

Table 1 shows an example (steel numbers 1 to 8) and a comparative example (steel numbers 9 to 17) that satisfy the composition according to the present invention. Table 2 shows an embodiment satisfying the finish rolling temperature (FDT) and the secondary cooling end temperature (RCS) according to the present invention, and a comparative example not satisfied. Table 3 shows the properties of each steel group.

The steel slab formed as shown in Table 1 was vacuum-dissolved, heated in a reheating temperature at 1200 ° C. in a heating furnace for 1 hour, and hot rolled, and then wound up. As shown in FDT of Table 2, the hot rolling was finished in the range of 880 ~ 920 ℃, which is in the range of Ar ₃ ~ Ar ₃ + 50 ℃, and the winding temperature (CT) was controlled at 650 ~ 680 ℃. It was. Pickling was performed using a hot rolled steel sheet, and cold rolling was performed at a cold rolling reduction of 45%. The cold rolled steel sheet was continuously annealed at the temperature of the soaking section (SS) set according to the conditions shown in Table 2, and cooled to the secondary cooling end temperature (RCS). Finally, the skin pass rolling rate was 0.2. Fixed to%.

As shown in Table 3, the martensite + bainite fraction, average particle diameter, microstructure average hardness value, phase hardness ratio, and the distribution density of nanoprecipitates of 10 nm or less in steel were prepared for each steel component and annealing conditions. Investigate. In addition, the JIS No. 5 tensile test piece was produced to measure the physical properties of the material, and the results are shown with the comparative material.

	C	Mn	Si	P	S	Al	Cr	Ti	Nb	B	N	Ac3	Ms	Remarks
One	0.063	2.97	0.137	0.011	0.0034	0.026	1.00	0.047	0.031	0.0021	0.004	865.2	410.0	Invention steel
2	0.062	2.42	0.133	0.011	0.0036	0.024	1.00	0.045	0.031	0.002	0.0047	865.4	427.1	Invention steel
3	0.043	2.94	0.139	0.011	0.0033	0.022	1.00	0.044	0.031	0.002	0.004	874.1	419.3	Invention steel
4	0.043	2.46	0.131	0.011	0.0032	0.023	1.01	0.043	0.031	0.0021	0.0045	873.8	433.8	Invention steel
5	0.053	2.95	0.108	0.011	0.0023	0.031	0.97	0.049	0.032	0.0022	0.0034	868.1	415.2	Invention steel
6	0.051	2.97	0.101	0.011	0.002	0.034	0.69	0.05	0.032	0.002	0.0035	868.7	418.8	Invention steel
7	0.062	2.99	0.125	0.011	0.002	0.033	0.52	0.048	0.032	0.0023	0.0035	865.0	415.6	Invention steel
8	0.062	2.74	0.106	0.011	0.002	0.04	0.71	0.052	0.032	0.0025	0.0035	864.2	420.9	Invention steel
9	0.081	2.52	0.105	0.011	0.002	0.035	0.79	0.051	0.032	0.0025	0.0035	856.9	418.6	Comparative steel
10	0.076	2.65	0.107	0.01	0.002	0.033	0.50	0.05	0.031	0.0023	0.0033	858.8	420.2	Comparative steel
11	0.077	2.63	0.102	0.01	0.002	0.035	0.67	0.049	0.03	0.0025	0.0033	858.2	418.4	Comparative steel
12	0.15	3.1	0.099	0.011	0.003	0.037	0.65	0.051	0.039	0.0035	0.0031	835.8	373.4	Comparative steel
13	0.061	1.5	0.101	0.01	0.004	0.033	0.72	0.04	0.02	0.0029	0.0031	864.4	458.9	Comparative steel
14	0.056	2.91	0.112	0.01	0.003	0.035	0.20	0.04	0.02	0.002	0.0031	867.0	424.4	Comparative steel
15	0.16	2.9	0.1	0.01	0.003	0.03	1.50	0.041	0.04	0.0024	0.0041	833.3	365.0	Comparative steel
16	0.061	2.8	1.2	0.012	0.004	0.033	0.75	0.042	0.036	0.0029	0.0033	913.5	419.0	Comparative steel
17	0.059	2.95	0.101	0.011	0.003	0.036	0.95	0.015	0	0.0031	0.0035	865.2	412.9	Comparative steel

River	FDT (℃)	CT (℃)	SS (℃)	RCS (℃)	Remarks
1-1	880	680	820	350	Invention steel
1-2	880	680	810	270	Invention steel
2-1	890	680	820	440	Comparative steel
2-2	880	680	820	270	Invention steel
3	880	680	820	350	Invention steel
4	880	680	820	350	Invention steel
5-1	880	680	810	350	Invention steel
5-2	880	680	820	450	Comparative steel
6-1	880	680	820	300	Invention steel
6-2	880	650	760	350	Comparative steel
6-3	880	680	890	350	Comparative steel
7	880	680	820	350	Invention steel
8	880	680	820	350	Invention steel
9	880	680	820	350	Comparative steel
10	880	680	820	350	Comparative steel
11	880	680	820	350	Comparative steel
12	880	680	820	350	Comparative steel
13	880	680	820	350	Comparative steel
14	880	680	820	350	Comparative steel
15	880	680	820	350	Comparative steel
16	920	680	820	350	Comparative steel
17	880	680	820	350	Comparative steel

River	Transformation fraction (%)	M average particle diameter (㎛)	B average particle diameter (㎛)	Hardness value (Hv)	Hardness ratio between phases	Nano Precipitate Density (piece / μm 2 )	YS (MPa)	TS (MPa)	T-El (%)	R / t	HER (%)	YR	Remarks
1-1	97	1.2	2.5	360	1.3	172	881	1002	9.2	0.4	55.2	0.88	Invention steel
1-2	96	1.3	2.3	390	1.2	182	932	1112	8.0	0.7	63.2	0.84	Invention steel
2-1	90	2.2	3.5	320	2.1	179	620	891	11.6	1.3	39.9	0.70	Comparative steel
2-2	95	1.6	2.6	360	1.3	181	791	982	9.7	One	65.3	0.81	Invention steel
3	97	1.0	2.7	341	1.2	162	812	992	9.5	0.7	65.1	0.82	Invention steel
4	99	1.4	2.8	361	1.2	158	801	1002	14.6	One	68.1	0.80	Invention steel
5-1	100	1.3	2.7	359	1.3	162	862	1081	8.5	0.7	70.2	0.80	Invention steel
5-2	85	2.6	3.8	270	3.2	161	669	945	10.2	One	32.9	0.71	Comparative steel
6-1	99	1.7	2.1	361	1.3	168	786	1001	9.6	0.7	75.3	0.79	Invention steel
6-2	82	1.9	4.1	250	3.5	158	593	922	10.5	1.6	21.5	0.64	Comparative steel
6-3	100	3	3.5	380	1.2	159	751	864	6.1	1.6	42.5	0.87	Comparative steel
7	99	1.5	2.3	370	1.1	161	871	1081	6.5	0.7	68.3	0.81	Invention steel
8	100	1.6	2.5	380	1.2	159	801	993	10.6	0.7	69.5	0.81	Invention steel
9	99	1.2	3.8	390	2.8	160	626	998	9.2	1.6	40.2	0.63	Comparative steel
10	100	1.3	4.5	360	2.5	159	661	1005	12.1	2	35.3	0.66	Comparative steel
11	99	1.5	3.5	370	2.1	153	694	1033	8.6	1.6	33.6	0.67	Comparative steel
12	100	2.5	3.5	360	3.2	158	781	1151	10.5	1.6	21.1	0.68	Comparative steel
13	82	1.8	3.4	250	2.5	159	543	753	13.6	2	46.5	0.72	Comparative steel
14	95	4.2	3.4	340	2.1	158	453	695	14.3	2	38.5	0.65	Comparative steel
15	99	2.2	4.2	360	2.1	159	965	1287	8.6	1.6	32.5	0.75	Comparative steel
16	81	1.5	4.5	250	3.9	158	785	1105	12.1	2	25.1	0.71	Comparative steel
17	93	2.3	4.2	300	2.5	89	592	813	11.5	2	34.2	0.73	Comparative steel

As shown in Tables 1 to 3, the steel plates 1-1, 1-2, 2-2, 3, 4, 5-1, 6-1, 7, 8, which are examples according to the present invention, are all limited in the present invention. You can see that you are satisfied with the numbers you are doing. That is, the fraction of metamorphic tissue, tempered martensite and bainite particle size, hardness value, phase hardness ratio, number of nano precipitates, yield strength and tensile strength and yield ratio, elongation, bending workability (R / t), hole expandability ( HER) etc. are excellent.

On the other hand, the steel sheets 9 to 17, which are unsatisfactory in the composition range of the present invention, not only have a high variation in the hardness ratio between phases, but also do not satisfy the physical properties required by the present invention, such as poor bending workability and poor hole expandability. .

In addition, it is understood that steel sheets 2-1, 5-2, 6-2, and 6-3, which satisfy the composition of the present invention but do not satisfy the continuous annealing temperature or the secondary cooling end temperature, also do not satisfy the physical properties required by the present invention. Can be.

Hereinafter, the physical properties of each steel type will be described in detail.

In the case of the exemplary steel according to the present invention, the martensite transformation rate was at least 95%, the martensite average particle diameter was 1.7 μm or less, and the bainite average particle diameter was 2.5 μm or less. On the other hand, the average hardness of the microstructures of the inventive steels was at least 340 Hv, and satisfies at most 1.3 in the hardness ratio between phases. On the other hand, the nano precipitates of 10 nm or less satisfied 150 or more / 탆 ² or more as proposed in the present invention.

As described above, the inventive steels 1-9, which satisfy the characteristics suggested by the present invention steel, have a yield ratio in the range of 0.8 to 0.87, and have an excellent yield ratio with R / t 0.3 to 1.0 and HER value of 55% to 70%. It can be seen that and the elongation flange properties.

1 and 2 shows the microstructure of invention steel No. 5-1. As shown in FIG. 1, the average particle size of martensite is 2 μm or less, and the average particle diameter of bainite is 3 μm or less. In addition, even in the results showing the precipitates in FIG. 2, very large nano precipitates of 10 nm or less, such as TiC and NbC, were distributed in large quantities. As a result, a high hardness value of the microstructure and a low hardness deviation between phases resulted in a yield strength of 0.8 or higher and R /. t 0.3 ~ 1.0, it was possible to manufacture a high strength steel sheet having a HER value of more than 50%.

In addition, even if the component satisfies the present invention steel, the annealing condition does not satisfy the conditions of the present invention steel, or the comparative material does not satisfy the present invention steel in the component did not satisfy the material properties required by the present invention steel.

In Comparative Steels 2-1 and 5-1, the components satisfy the conditions of the present invention, but the secondary cooling end temperature of the RCS temperature is 440 ° C and 450 ° C, which does not satisfy the cooling rate below the Ms temperature suggested by the present invention. The austenite produced during annealing due to overaging could not be transformed into martensite, but was formed by high temperature transformation bainite, granular bainite, etc., resulting in coarse transformation. These coarse metamorphic phases resulted in low yield ratio and low HER value due to low hardness and high hardness ratio between phases.

Comparative steel 6-2 was annealed in the ideal region because the annealing temperature (SS) is very low, and the martensite transformation fraction was 82%, which fell short of the target of the invention steel. The formation of ferrite caused a decrease in the hardness value of the microstructure and an increase in the deviation of the hardness ratio between phases, resulting in a low yield ratio and a deterioration of the HER value.

Comparative steel 6-3 has an annealing temperature (SS) of 890 ° C., which increases the size of the martensite structure produced during cooling due to an increase in austenite grain size due to high temperature annealing. The average particle diameter of was not able to manufacture below 3㎛, resulting in degradation of yield ratio and HER value.

Comparative steels 9 to 12 exceeded the carbon content ranges of carbon presented in the present invention steel. This increase in carbon serves to increase the strength of martensite produced in the quenching process after annealing. However, after quenching and overaging, all martensite remains tempered rather than tempered. In this case, the tempered martensite is reduced in strength due to precipitation of carbon, but the non-tempered lattice type martensite is very stable martensite and has a very high strength due to the added carbon. Therefore, when the carbon content exceeds the component suggested by the present invention steel, the HER value and the yield ratio do not satisfy the criteria suggested by the present invention due to the increase in the strength difference between the rat martensite and the tempered martensite produced in the overaging treatment. I can't.

Comparative steels 13 to 15 did not satisfy the carbon content or Mn, Cr content of the present invention steel. That is, comparative steels 13 and 14 did not have sufficient transformation of martensite due to the low Mn or Cr content, and comparative steel 15 had a high carbon content but a low Cr content, yielding a high ratio between phases and yielding coarse martensite. The ratio and HER value deteriorated.

Comparative steel 16 had a very high Si content. In general, Si is a ferrite forming element, and when the amount is increased, it promotes ferrite formation upon cooling. Steel No. 16 produced 81% martensite due to the high Si content, which did not satisfy the criteria suggested by the present invention. The yield ratio was low and the HER value was deteriorated due to a decrease in hardness value and an increase in hardness ratio between phases.

Comparative steel 17 is a case where Ti and Nb do not satisfy the conditions of the inventive steel. As mentioned above, Ti and Nb combine with carbon to form nano precipitates, and these nano precipitates serve to reduce the hardness difference between phases by strengthening the matrix structure. However, Comparative Steel 17 did not have enough Ti and Nb to form sufficient precipitates, resulting in deterioration of yield ratio and HER value due to the distribution of nano precipitates and increasing the hardness ratio between phases.

Hereinafter, the measuring method and the effect of the physical properties of the steel.

The hardness of the metamorphic tissue was measured by using a Nano-Indenter (NT110) device to measure 100 points in a square with a load of 2g, excluding maximum and minimum values. In addition, bainite, martensite and nano precipitates were evaluated by FE-TEM. In particular, the size and distribution density of the nano precipitates were evaluated by using an image analyzer equipment for the texture photograph of the precipitates measured by FE-TEM. In addition, the fraction of metamorphic tissue was observed by SEM and the image analysis equipment was used.

As mentioned above, it is very important to control the microstructure and precipitates in order to produce a steel having excellent elongation flange of R / t 1 or less and HER 50% or more and yield ratio of 0.8 or more. As a method of simultaneously increasing bending workability, elongation flangeability and yield ratio, it is necessary to secure a low temperature single phase structure. In general, the tissue having the highest strength among the low-temperature tissues is martensite, and as is well known, the method for preparing martensite is the process of maintaining water for sufficient time to form austenite during annealing, followed by water cooling and tempering treatment. will be. However, the water-cooling method is intended to secure martensite by controlling alloy elements in the present invention due to deterioration in productivity due to problems such as material deviation and shape defects. In other words, by adding a certain amount of hardenable elements such as Mn, Cr, etc., martensite is secured even at a low cooling rate. However, this method may cause problems such as deterioration of weldability due to the addition of high alloying elements. Therefore, in the present invention, it is intended to minimize the carbon content that has the greatest impact on the weldability. For this reason, in the present invention, the carbon content is limited to 0.07% or less.

In order to secure a high yield ratio under cooling conditions such as the present invention, an alloy element should be added as much as possible. However, such an attempt causes additional problems such as deterioration of weldability and increase in hot rolled strength. However, when the martensite size and nano precipitates are controlled without excessive addition of alloying elements, it is possible to satisfy the stretch flangeability and yield ratio proposed in the present invention.

Steel according to the present invention, the percentage of metamorphic tissue should be controlled to 90% or more, wherein the metamorphic tissue is composed of bainite and temper martensite. In order to increase the R / t, HER and yield ratio, 100% of the metamorphosis is possible. In order to increase the strength, it is desirable to make the size of the metamorphic tissue as small as possible. When the average particle diameter of martensite and bainite is outside the scope of the present invention, the bending workability and the elongation flange and the yield ratio could not be satisfied.

In order to secure a yield ratio of 0.8 or more, the average hardness of the structure formed in the steel should be 340 Hv or more. On the other hand, in terms of bending workability and elongation flangeability, it is very important to control the phase-to-strength ratio, so that the hardness ratio of the soft phase and the hard phase within the microstructure must be controlled to 1.3 or less to secure HER 50% or less at the same time. When the hardness ratio between phases and the hardness ratio between phases were not satisfied, it was impossible to secure the HER value below R / t 1, more than 50%, and yield ratio above 0.8.

Although embodiments of the present invention have been described above with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may implement the present invention in other specific forms without changing the technical spirit or essential features thereof. I can understand that.

Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. The scope of the present invention is shown by the following claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included in the scope of the present invention. .

Claims (8)

1. Weight% C: 0.035 ~ 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 ~ 3.5%, Cr: 0.3 ~ 1.2%, Ti: 0.03 ~ 0.08%, Nb: 0.01 ~ 0.05%, balance Fe and inevitable impurities,

   The volume fraction of metamorphic tissues including tempered martensite and bainite is 90% or more, the average particle diameter of tempered martensite tissue is 2 µm or less, and the average particle diameter of bainite tissue is 3 µm or less. And a volume fraction of bainite structure having a particle diameter of more than 3 µm is 5% or less.
2. The method according to claim 1,

   High-strength steel, characterized by further adding B: 0.0010-0.0050%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.10%, N: 0.010% or less.
3. The method according to claim 1 or 2,

   High-strength steel, characterized in that the average hardness value (Hv) of the microstructure is 340 or more, and the maximum value of the hardness value is 1.3 times or less of the minimum value.
4. The method according to claim 1 or 2,

   High-strength steel, characterized in that the distribution density of the nano-precipitates of 10 nm or less in the metamorphic structure is 150 / 탆 ² or more.
5. The method according to claim 1 or 2,

   A high strength steel, characterized in that the tensile strength is 980 MPa or more, and the yield strength is 780 MPa or more.
6. Weight% C: 0.035 ~ 0.07%, Si: 0.3% or less (excluding 0), Mn: 2.0 ~ 3.5%, Cr: 0.3 ~ 1.2%, Ti: 0.03 ~ 0.08%, Nb: 0.01 ~ 0.05%, balance Preparing a slab comprising Fe and inevitable impurities;

   Manufacturing a hot rolled steel sheet by hot rolling the slab to a finish rolling outlet temperature of Ar ₃ to Ar ₃ + 50 ° C .;

   Cold rolling the hot rolled steel sheet at a reduction ratio of 40 to 70% to produce a cold rolled steel sheet;

   Continuously annealing the cold rolled steel sheet in a temperature range of Ac ₃ ± 30 ° C;

   First cooling after the continuous annealing step at a cooling rate of 1˜10 ° C./sec to 650˜700 ° C .;

   After the first step of cooling, the second step of cooling at a cooling rate of 5 ~ 20 ℃ / sec to Ms-100 ~ Ms ℃; comprising, high strength steel manufacturing method.
7. The method according to claim 6,

   After the secondary cooling step, the step of overaging while maintaining a constant temperature; characterized in that it further comprises, high strength steel manufacturing method.
8. The method according to claim 7,

   The overaging step is characterized in that the aging time is adjusted so that the volume fraction of metamorphic tissue consisting of tempered martensite and bainite of the entire tissue is 95% or more, high strength steel manufacturing method.

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