JP2009275238A - High-strength steel and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel in which the hardness of ferrite is equal to or harder than that of bainite, and to provide a manufacturing method therefor. <P>SOLUTION: The high-strength steel contains an appropriate amount of C, Si and Mn and further one or more elements of Ti, Nb, V and Mo in such a range as to satisfy 0.1≤56ä2(Ti/48)+2(Nb/93)+(7/4)×(V/51)+(3/2)×(Mo/96)}≤1; and has a metallographic structure formed of 10 to 60% ferrite by a volume fraction and bainite, in which carbides in ferrite grains have an average size of 0.8 to 3 nm, the number density of the carbides are 1×10<SP>17</SP>to 5×10<SP>18</SP>pieces/cm<SP>3</SP>, and the difference (Hv<SB>F</SB>-Hv<SB>B</SB>) between a Vickers hardness Hv<SB>F</SB>of the ferrite and a Vickers hardness Hv<SB>B</SB>of the bainite is 0 to 40 Hv. The manufacturing method includes: hot-working the steel slab under conditions of heating temperature of ≥1,200°C and final-working temperature FT[°C] of >920°C; primarily cooling it; subsequently retaining it at 580 to 650°C for 3 to 30 seconds; and then secondarily cooling it. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a high-strength steel having a composite structure in which ferrite is strengthened by precipitation, and a method for producing the same.

  To increase the strength of steel, solid solution strengthening by adding elements such as C, Si, and Mn, precipitation strengthening using precipitates such as Ti and Nb, and the metal structure of soft ferrite and hard martensite or bainite It is effective to strengthen the organization as a composite organization consisting of In particular, automobile components are being reduced in weight and improved in safety and durability, and a high strength steel material is required.

  However, solid solution strengthening is less effective than precipitation strengthening and structure strengthening, and it is difficult to increase the strength as required for materials for automobile members. For this reason, a composite steel having a combination of hard phases such as bainite and martensite has been developed. Although this composite structure steel is excellent in uniform elongation, the local ductility is low due to the difference in hardness between the ferrite and the hard phase, and the hole expandability is inferior.

Therefore, it is effective to reduce the difference in hardness between the ferrite and the hard phase in order to improve the hole expandability. Therefore, high-strength steel sheets in which ferrite is strengthened by precipitation to improve the balance of elongation-hole expansion rate have been proposed (for example, Patent Documents 1 to 5).
JP 2004-225109 A JP 2004-250749 A JP 2004-339606 A JP 2006-274318 A JP 2007-009322 A

However, the steel combining conventional precipitation strengthening and structure strengthening has a ferrite hardness smaller than that of a hard phase such as bainite.
An object of the present invention is to provide a high-strength steel in which the Vickers hardness (also referred to as hardness) of ferrite is increased to be equal to or higher than that of bainite and a method for producing the same.

The gist of the present invention is as follows.
(1) By mass%, C: 0.020 to 0.150%, Si: 0.01 to 1.50%, Mn: 0.2 to 3.0%, Ti: 0.03 -0.40%, Nb: 0.01-0.20%, V: 0.01-0.20%, Mo: 0.01-0.20% of 1 type or 2 types or more,
0.10 ≦ 56 {(2 (Ti / 48) +2 (Nb / 93) + (7/4) × (V / 51) + (3/2) × (Mo / 96)}} 1.00
(Ti, Nb, V, and Mo in the formula are the contents [% by mass] of each component in the steel.) The balance is made of Fe and unavoidable impurities. The volume fraction is 10 to 60%, the balance is bainite, the average diameter of the carbides present in the crystal grains of the ferrite is 0.8 to 3 nm, and the number density is 1 × 10 ^{17 to} 5 × 10. A high-strength steel having ¹⁸ pieces / cm ³ and having a difference (Hv _F −Hv _B ) between Vickers hardness Hv _{F of the} ferrite and Vickers hardness Hv _{B of the} bainite of 0 to 40 Hv.
(2) It is a manufacturing method of the high strength steel as described in said (1), Comprising: The steel piece which consists of a component as described in said (1) WHEREIN: Heating temperature> 1200 degreeC, Final processing temperature FT [degreeC]> 920 degreeC Perform hot working under super conditions, perform primary cooling to a stop temperature of 580 to 650 ° C. at a cooling rate of 10 ° C./s or more, and hold for 3 to 30 seconds within the stop temperature range of the primary cooling, then cool A method for producing high-strength steel, characterized by secondary cooling to a stop temperature of 400 to 550 ° C. at a rate of 30 ° C./s or more.

  According to the present invention, it is possible to provide a high-strength steel having a ferrite hardness equal to or higher than that of bainite and a method for producing the same. As a result, the hardness difference between ferrite and bainite in the state in which processing strain is introduced is reduced, and the workability, particularly the hole expanding property in which the introduced processing strain is extremely high is improved over conventional steel. There is an effect.

  The present invention is a high-strength steel whose greatest feature is that the hardness of ferrite is increased to be equal to or higher than that of bainite.

  The conventional composite structure steel in which the metal structure is composed of ferrite and bainite and precipitation-strengthens ferrite strengthens the hardness difference between hard bainite and ferrite, but the hardness of ferrite is lower than that of bainite. A stress-strain curve of such a conventional steel is schematically shown in FIG. As shown in FIG. 1, bainite is easier to work harden than precipitation-strengthened ferrite, and the slope of the stress-strain curve above the yield strength is large.

  The Vickers hardness corresponds to a flow stress having a strain of about 8% in the stress-strain curve of FIG. As shown in FIG. 1, in the conventional steel, the stress of ferrite is smaller than that of bainite, and when the strain is about 8%, the difference in stress between ferrite and bainite (ΔHv) is small, but when the strain increases, bainite becomes ferrite. Is harder than that, the difference in stress between ferrite and bainite opens. Therefore, cracks are likely to occur during processing due to hard bainite. In particular, it is considered that the ductility after the stress reaches the maximum value, that is, the local ductility is reduced due to the hardness difference between bainite and ferrite, and the effect of improving the hole expansibility is small.

  In contrast, FIG. 2 schematically shows an example of the stress-strain curve of the steel of the present invention in which the hardness of ferrite is set to be equal to or higher than that of bainite. As in the conventional steel of FIG. 1, when the strain increases due to work hardening of bainite, bainite hardens more than ferrite. However, in the steel of the present invention, the stress of ferrite is large (ΔHv is positive) at a strain of 8% corresponding to Vickers hardness (ΔHv is positive), so the difference in stress between bainite and ferrite ( ΔHv) is small. Therefore, it is considered that the steel of the present invention has a greater effect of improving the hole expansibility than the conventional steel.

  Usually, ferrite is very soft and it is difficult to obtain a hardness equal to or higher than that of bainite. Therefore, the present inventors examined the form of precipitates that can increase the hardness of ferrite to be equal to or higher than that of bainite.

A test piece having a height of 12 mm and a diameter of 8 mm was taken from steel to which Ti, which is a carbide forming element, was added. The test piece was subjected to induction heating and hot working to compress in the height direction, and then Ar gas was blown to perform controlled cooling. In the controlled cooling, after the hot working, the primary cooling was performed, the temperature and the time were changed, and the secondary cooling was performed.
After the test, the Vickers hardness of ferrite and bainite was measured, and the precipitates formed on the ferrite were investigated. The observation of the metal structure was performed using an optical microscope after mirror-polishing the sample and performing nital etching. The hardness in the crystal grains of ferrite and bainite was measured using a sample after etching and a load of 25 gf with a micro Vickers tester. About each of a ferrite grain and a bainite grain, the hardness of 20 or more places was measured, and what was averaged was made into the hardness of each phase.

The size and number density of the carbides precipitated in the ferrite crystal grains were measured by an electrolytic ion microscope (FIM) and a three-dimensional atom probe measurement method as follows.
First, a needle-like sample is prepared from a sample to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary. The presence or absence of precipitated carbides is observed in a relatively wide field of view by FIM observation, the size of 30 carbides is arbitrarily measured, and the average value is obtained. In addition, the size of the precipitated carbide was the average value (average diameter) of the maximum diameter of the carbide. The maximum diameter of the precipitated carbide was the diameter when the carbide was spherical and the diagonal length when the carbide was plate-like.
In the three-dimensional atom probe measurement, the accumulated data can be reconstructed and obtained as an actual distribution image of atoms in real space. The number density of the precipitated carbide (precipitate density) is obtained from the volume of the three-dimensional distribution image of the precipitated carbide and the number of the precipitated carbide.

As a result, it was found that very fine carbides having a precipitated carbide size (average diameter of carbide) of 3 nm or less are effective in increasing the hardness of ferrite. Furthermore, it was found that the number density must be 1 × 10 ^{17 to} 5 × 10 ¹⁸ pieces / cm ³ in order to increase the hardness of the ferrite to be equal to or higher than that of bainite.
When such precipitates are generated, the difference (Hv _F −Hv _B ) between the Vickers hardness Hv _{F of} ferrite and the Vickers hardness Hv _{B of} bainite becomes 0 to 40 Hv.

Hereinafter, the present invention will be described in detail.
In the present invention, C is an important element that generates fine carbides and contributes to precipitation strengthening, and needs to be added in an amount of 0.020% or more. On the other hand, when the amount of C exceeds 0.150%, cementite is generated, and ductility, particularly local ductility is lowered.
Si is a deoxidizing element, and 0.01% or more is added. Si is an element that contributes to solid solution strengthening, but if the content exceeds 1.50%, the workability deteriorates, so the upper limit of Si content is 1.50% or less.

  Mn is an element effective for deoxidation and desulfurization, and contributes to solid solution strengthening, so 0.2% or more is added. On the other hand, when the amount of Mn exceeds 3.0%, segregation is likely to occur, and ductility, in particular, local ductility decreases.

Ti, V, Nb, and Mo are extremely important elements that precipitate fine carbides in ferrite grains and contribute to precipitation strengthening. One or more of Ti, V, Nb, and Mo are included. Added.
Since Ti increases the hardness of ferrite, it is preferable to add 0.03% or more. On the other hand, when Ti exceeding 0.40% is added, carbides are coarsened, the effect of increasing hardness is reduced, and ductility may be reduced.

  V, Nb, and Mo are elements that precipitate carbide in the ferrite crystal grains in the same manner as Ti. Each of them is preferably added in an amount of 0.01% or more. On the other hand, if the content of each of V, Nb, and Mo exceeds 0.20%, the carbides become coarse, the effect of increasing the hardness is reduced, and the ductility may be reduced.

In the present invention, it is necessary to contain Ti, Nb, V, and Mo so as to satisfy the following formula.
0.10 ≦ 56 {(2 (Ti / 48) +2 (Nb / 93) + (7/4) × (V / 51) + (3/2) × (Mo / 96)}} 1.00
Here, Ti, Nb, V, and Mo are the contents [% by mass] of each component in the steel, and are calculated as 0 when not intentionally added. Also,
56 {(2 (Ti / 48) +2 (Nb / 93) + (7/4) × (V / 51) + (3/2) × (Mo / 96)}} (Formula 1)
Indicates the number of atoms of the alloy component constituting the precipitate when Ti, Nb, V, and Mo form TiC, NbC, V ₄ C ₃ , and Mo ₂ C, respectively, in steel. It is what. When the value of (Formula 1) is smaller than 0.10, there are few alloy components which comprise a precipitate, Therefore Even if it performs a precipitation process at the time of cooling after hot rolling, sufficient precipitation strengthening amount cannot be obtained. On the other hand, when the value of (Equation 1) is larger than 1.00, the alloy components constituting the precipitates become excessive, it is difficult to control the formation of precipitates during cooling, and the precipitates are likely to be coarsened. It may cause deterioration of hole expansibility.

The balance is Fe and inevitable impurities. In particular, the upper limit of the content of N, P, and S is preferably limited as follows.
N forms TiN and lowers the workability of the steel, so it is preferable to limit it to 0.009% or less. P is preferably limited to 0.1% or less in order to reduce the workability and weldability of steel. S is an element that decreases hot workability, and is preferably limited to 0.005% or less.

  Al may be mixed from the furnace material and may be contained as a deoxidizer, but if added excessively, nitrides are formed and the ductility of the steel is reduced, so it is limited to 0.5% or less. It is preferable.

  In the present invention, in addition to the above basic components, addition of Cr and W is also effective as a solid solution strengthening element for the purpose of improving the strength of the steel sheet, and one or both of these may be added.

Next, the metal structure of the high strength steel of the present invention will be described.
The metal structure of the steel of the present invention is a composite structure composed of ferrite and bainite. When the volume fraction of ferrite is 10% or more, ductility is improved as compared with the case of a bainite single phase. When the volume fraction of ferrite having a work hardening coefficient smaller than that of bainite is 20%, the uniform elongation can be increased. On the other hand, by setting the volume fraction of ferrite to 60% or less, the volume fraction of bainite having a large work hardening coefficient increases, so that the strength after processing can be increased. Further, if the volume fraction of ferrite exceeds 50%, the total amount of precipitates is increased, and in particular, the local ductility is lowered.

  The amount of precipitation strengthening can be considered to be determined by the amount of strengthening per precipitate (the force for pinning dislocations) and the number density of precipitates. If the average diameter of the carbide precipitated in the ferrite crystal grains is less than 0.8 nm, the pinning force per precipitate is weak and the contribution to precipitation strengthening is small. On the other hand, if the average diameter of the carbide exceeds 3 nm, the number density of precipitates generally decreases and the precipitation strengthening amount decreases, so that it is difficult to increase the hardness of ferrite to the same level or higher.

The number density of carbides precipitated in the crystal grains of the ferrite needs to be 1 × 10 ¹⁷ pieces / cm ³ or more in order to increase the hardness of the ferrite to be equal to or higher than that of bainite. On the other hand, when the number density of carbides exceeds 5 × 10 ¹⁸ pieces / cm ³ , the average diameter of the precipitates becomes very small and the precipitation strengthening amount becomes small.
Moreover, the said precipitation carbide | carbonized_material in this invention means what contains not only carbide | carbonized_material but the carbonitride which some nitrogen mixed in carbide | carbonized_material.

The difference (Hv _F −Hv _B ) between the Vickers hardness Hv _{F of} ferrite and the Vickers hardness Hv _{B of} bainite is set to 0 to 40 Hv. When (the hardness of the ferrite Hv _F -the hardness of the bainite Hv _B ) is 0 or more, the difference between the hardness of the bainite after processing and the hardness of the ferrite is reduced, and the ductility, particularly the hole expandability is increased. On the other hand, if (the hardness of the ferrite Hv _F -the hardness of the bainite Hv _B ) is more than 40 Hv, the hardness of the ferrite is too high. There is little improvement effect. In particular, in order to improve the hole expansibility with a large processing strain introduced, it is preferable that (hardness Hv _{F of} ferrite−hardness Hv _{B of} bainite) be 10 Hv or more. In addition, precipitation strengthening of ferrite increases the number of precipitates from which voids are generated. Therefore, in order to improve local ductility, (hardness Hv _{F of} ferrite−hardness Hv _{B of} bainite) should be 30 Hv or less. preferable.

Next, the manufacturing method of the high strength steel of this invention is demonstrated.
The high-strength steel of the present invention is a steel material produced by hot working, for example, hot rolling or hot forging. Since the control of the cooling rate is important after hot working, hot rolling is preferable. In particular, hot rolling for producing steel plates and steel strips having a relatively thin plate thickness used for automobiles and building materials is suitable.

Steel is melted and cast by a conventional method to produce a steel slab. From the viewpoint of productivity, continuous casting is preferable.
The heating temperature for the hot working is set to 1200 ° C. or higher in order to sufficiently decompose and dissolve the carbide forming element and carbon in the steel material. In addition, you may hold | maintain the steel slab after casting at the temperature of 1200 degreeC or more as it is, and may start hot processing. When the steel slab is heated to 1200 ° C. or higher, it is preferable to hold it for 1 hour or longer.

The hot working finish temperature (final working temperature FT) is over 920 ° C. This is to suppress coarsening of the carbide at high temperature, and it is necessary to start cooling immediately after hot working.
After the hot working, after the primary cooling, it is held (retained), and further secondary cooling is performed.

The cooling rate of the primary cooling is 10 ° C./s or more. This is to suppress carbide precipitation and growth at high temperatures and ferrite transformation during cooling. The upper limit of the cooling rate of the primary cooling is not specified, but if it exceeds 100 ° C./s, it becomes difficult to control the stop temperature of the primary cooling.
When the primary cooling stop temperature exceeds 650 ° C., carbides become coarse, and the hardness of the ferrite decreases. On the other hand, if the primary cooling stop temperature is lower than 580 ° C., the precipitation of carbides becomes insufficient.

  After the primary cooling is stopped, the mixture is held for 3 to 30 seconds within the primary cooling stop temperature range of 580 to 650 ° C. This is because if the residence time is less than 3 s, the precipitation of carbide becomes insufficient, and if it exceeds 30 s, the carbide becomes coarse and the hardness of the ferrite decreases. In addition, residence time is time which makes the temperature of steel within the temperature range of 580-650 degreeC, and may cool slowly and may use heating means, such as a heating furnace and induction heating.

Further, secondary cooling is performed. The cooling rate of the secondary cooling is set to 30 ° C./s or more in order to suppress an increase in volume fraction of ferrite and generation of pearlite. The upper limit of the cooling rate of the secondary cooling is not particularly specified, but if it exceeds 100 ° C./s, it becomes difficult to control the stop temperature.
The secondary cooling stop temperature is set to 550 ° C. or lower in order to suppress an increase in the volume fraction of ferrite and generation of pearlite. Moreover, in order to suppress the production | generation of a hard martensite, the minimum of the stop temperature of secondary cooling shall be 400 degreeC or more.

  Steel having the component composition shown in Table 1 was melted in a laboratory to obtain a steel slab. A test piece having a height of 12 mm and a diameter of 8 mm was taken from the obtained steel piece, the test piece was subjected to induction heating, subjected to hot working for compression in the height direction, and controlled cooling was performed by blowing Ar gas. Hot working (heating temperature, final working temperature FT (end temperature)), controlled cooling ((primary cooling: cooling rate, stop temperature) (residence time after stopping primary cooling) (secondary cooling: cooling rate, Table 2 shows the conditions of the stop temperature)).

  The specimen was mirror-polished, subjected to nital etching, the metal structure was observed with an optical microscope, the area ratio of ferrite was measured by the point count method, and the volume fraction was obtained. The hardness in the crystal grains of ferrite and bainite was measured using a sample after etching and a load of 25 gf with a micro Vickers tester. About each of a ferrite grain and a bainite grain, the hardness of 20 or more places was measured, and what was averaged was made into the hardness of each phase.

To measure the size of precipitated carbide, a needle-like sample is prepared from the sample to be measured by cutting and electropolishing, and the presence or absence of precipitated carbide is observed with a relatively wide field of view by FIM observation. The size of the carbide was measured and the average value was obtained. In addition, the number density of precipitated carbides (precipitate density) was determined from the measurement volume and the number of precipitated carbides by three-dimensional atom probe measurement.
The results are shown in Table 3.

It can be seen that the Vickers hardness (Hv _F ) of ferrite is equal to or higher than the Vickers hardness (Hv _B ) of bainite if the components and production conditions are within the scope of the present invention.

On the other hand, Steel No. Since H has a small amount of Ti added and the value of (Equation 1) is also small, the density of precipitates is lowered, and the hardness (Hv _F ) of ferrite is low. Steel No. Since I has a large amount of Ti added and the value of (Equation 1) is also large, the size of the precipitate becomes large and the hardness (Hv _F ) of the ferrite is low.
Steel No. Since J has a small amount of C, the volume fraction of ferrite is large and the density of precipitates is reduced. Since K has a large amount of C, pearlite is generated.

Test No. Nos. 2 and 6 have a long residence time after primary cooling. No. 2 is a ferrite single phase. No. 6 has a high volume fraction of ferrite. Test No. No. 5 has a short residence time after primary cooling and a small precipitate size, so the hardness of ferrite is low.
Test No. No. 8 has a high primary cooling stop temperature and a large precipitate size, so the ferrite hardness is low. Test No. 9 has a low primary cooling stop temperature and is a bainite single phase.
Test No. No. 12 has a slow cooling rate of secondary cooling and pearlite is generated, and test No. 15 has a low secondary cooling stop temperature and generates martensite.
In test No. 17, the cooling rate of the primary cooling is slow and the size of the precipitate is large, so the hardness of the ferrite is low.

  The high-strength steel of the present invention is a material for a strength member such as an automobile, which has excellent strength and ductility, particularly local ductility, and requires burring workability and stretch flangeability because the ferrite has a higher hardness than the hard phase. It can be used as a hot-rolled steel sheet.

It is a schematic diagram of the stress-strain curve of each phase of the conventional precipitation strengthening type composite structure steel. It is a schematic diagram of the stress-strain curve of each phase of the high strength steel of the present invention.

Claims (2)

% By mass
C: 0.020 to 0.150%,
Si: 0.01 to 1.50%,
Mn: 0.2 to 3.0%
In addition,
Ti: 0.03 to 0.40%,
Nb: 0.01-0.20%,
V: 0.01-0.20%,
Mo: 0.01-0.20%
1 type or 2 types or more of
0.10 ≦ 56 {(2 (Ti / 48) +2 (Nb / 93) + (7/4) × (V / 51) + (3/2) × (Mo / 96)}} 1.00
(Note that Ti, Nb, V, and Mo in the formula are the content [% by mass] of each component in the steel.) The balance is comprised of Fe and inevitable impurities,
The volume fraction of ferrite is 10-60%, the balance is bainite,
The average diameter of carbides present in the ferrite crystal grains is 0.8 to 3 nm, the number density is 1 × 10 ^{17 to} 5 × 10 ¹⁸ pieces / cm ³ ,
A high-strength steel characterized in that a difference (Hv _F -Hv _B ) between the Vickers hardness Hv _{F of the} ferrite and the Vickers hardness Hv _{B of the} bainite is 0 to 40 Hv. It is a manufacturing method of the high strength steel according to claim 1,
The steel slab comprising the component according to claim 1 is hot-worked under conditions of a heating temperature ≧ 1200 ° C. and a final processing temperature FT [° C.]> 920 ° C.,
After primary cooling to a stop temperature of 580 to 650 ° C. at a cooling rate of 10 ° C./s or more and retaining for 3 to 30 seconds within the stop temperature range of the primary cooling,
A method for producing high-strength steel, characterized by secondary cooling to a stop temperature of 400 to 550 ° C. at a cooling rate of 30 ° C./s or more.

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