JP5458641B2 - Carbide dispersion steel - Google Patents

Description

  The present invention is a carbide characterized in that it has both strength and ductility suitable for automobile underbody parts and the like, which are indispensable to achieve processing of complex shape parts and freezing of the shape while maintaining high strength. Concerning dispersion steel.

  In recent years, development of a steel sheet having both strength and workability has been desired, and in particular, it is industrially very useful to provide hot-rolled steel sheets of various strength levels by controlling the precipitation strengthening amount with high accuracy. Therefore, there is a demand for stable production of precipitation-strengthened hot rolled steel sheets using precipitation strengthening of fine precipitates. It is not easy to reconcile conflicting strength and workability.For example, DP steel can achieve high elongation by containing ferrite, but it is an elongation that is a necessary property for undercarriage parts because it has a multiphase structure. Poor flangeability. Here, DP steel means Dual Phase steel, which is a steel having a two-phase structure such as a ferrite structure and a martensite structure.

  On the other hand, bainite single-phase steel containing no ferrite has good stretch flangeability but poor ductility. For this reason, precipitation-strengthened hot-rolled steel sheets have been developed in which the main phase is a ferrite single phase suitable for ensuring high ductility and precipitation strengthening of fine precipitates is used as the main strengthening mechanism. For example, in Patent Document 1 and Patent Document 2, in the production of a hot-rolled steel sheet containing appropriate amounts of Ti and Mo, finish rolling is completed in the austenite region immediately above the Ar3 transformation point, and the coiling temperature is 570 ° C to 700 ° C. By making the following, it is possible to produce hot-rolled steel sheets with a tensile strength exceeding 800 MPa or 950 MPa and excellent ductility or stretch flangeability with a ferrite single phase as the parent phase and dispersed fine precipitates of less than 10 nm. It is disclosed that nano-carbide fine dispersion is achieved by so-called phase interface precipitation by matching the γ → α transformation and MC carbide precipitation process in the winding process after hot rolling. . Here, MC refers to a generic name of carbides having a NaCl-type crystal structure composed of a metal element M and carbon C. For example, TiC, NbC, VC, TaC, (Ti, Mo) C, (Nb, Mo) C, (Ti, W) C, (Nb, W) C, (Ti, V) C, and the like are applicable.

  However, when manufacturing these steels, it is necessary to strictly control the finishing finish temperature and the winding process conditions, and the addition of additions aimed at increasing the precipitates to further increase the strength Since the increase in gold element is strongly affected by the recent rise in alloying elements, a new manufacturing method for industrially sustainable high-strength steel sheets is desired.

  Further, as a method for dispersing fine carbides in steel, for example, Patent Document 3 focuses on the crystallographic consistency between the precipitate and the parent phase, and determines the lattice constant of the precipitate and the lattice constant of the parent phase. A design method that achieves high strength by maximizing precipitation strengthening ability by control is disclosed.

  However, in this method, since the relationship between MC and the lattice constant of the ferrite matrix, which are widely used in general, is limited, it is difficult to obtain even greater strengthening ability.

  In Patent Document 4, the metal element M constituting the MC precipitate is arranged from the viewpoint of electronegativity, and a composite carbide obtained by combining specific elements is used to obtain a particularly excellent dispersion strengthened steel. A method is disclosed.

  However, in this case as well, there is a limit due to the combination of various elements, and in order to obtain higher strength, it is essential to combine other strengthening methods and use the second phase, which deteriorates workability other than strength. Cannot be ignored.

JP 2003-89848 A JP 2002-322539 A JP-A-6-65628 JP 2005-120430 A

    As described above, in particular, a method for achieving high strength by finely dispersing precipitates such as MC in the ferrite phase has already been widely studied, and only in terms of MC type and consistency with the parent phase. With the focused method, it is difficult to further increase the strength.

  In the present invention, based on a ferrite single-phase structure superior in ensuring ductility, focusing on the steel composition and the initial precipitation composition of MC in phase interface precipitation and matched precipitation on the parent phase ferrite, steadily fine carbide dispersion is achieved. The purpose was to realize.

  The present inventors have intensively studied the high-strength steel utilizing the dispersion strengthening of fine carbides having ferrite as a matrix, particularly the initial size of the precipitate. As a result, it was found that the initial size of MC precipitated in a plate-like form was maintained extremely fine by appropriately controlling the amount of P often used as a solid solution strengthening element.

  Furthermore, the inventors also examined the thermal stability of the precipitated MC in the ferrite, and an element group having a very strong carbide forming ability represented by Ti, V and Nb as metal elements constituting the MC. A and element A in the initial composition formed by phase interface precipitation, etc., when A and element group B, which is not single MC type precipitation ability, such as Cr, Mo and W, are combined and used When designed so that the atomic ratio between the group and the B element group is within a certain range, the thermal stability of these composite carbides is ensured, otherwise the carbides grow easily and the effect as a strengthening factor Was found to drop sharply.

This invention is made | formed based on the above knowledge, The place made into the summary is as follows.
(1) By mass%, C: 0.040-0.060%, N: 0.0020-0.0045%, Mn: 0.50-1.00%, Si + Al: 0.50% or less, P is contained in 0.050-0.100%, Ti as MC forming element And Mo are contained in the range of Ti: 0.05-0.10%, Mo: 0.09-0.40%, the balance is inevitable impurities, and the composition ratio of Ti and Mo constituting the MC precipitate (atomic ratio: Ti / Mo ) Is 0.50 ≦ Ti / Mo ≦ 1.20, and carbide dispersed steel in which the size of MC precipitates in the ferrite phase is controlled to be less than 8 nm.
(2) By mass%, C: 0.040-0.060%, N: 0.0020-0.0045%, Mn: 0.50-1.00%, Si + Al: 0.50% or less, P is contained in 0.060-0.080%, and Ti is used as MC forming element. And Mo are contained in the range of Ti: 0.05-0.10%, Mo: 0.09-0.40%, the balance is inevitable impurities, and the composition ratio of Ti and Mo constituting the MC precipitate (atomic ratio: Ti / Mo ) Is 0.50 ≦ Ti / Mo ≦ 1.20, and the carbide-dispersed steel with excellent strength / ductility balance in which the size of MC precipitates in the ferrite phase is controlled to less than 8 nm.

According to the present invention, it is possible to enhance the dispersion strengthening ability of fine carbides by actively utilizing P, which is often added for solid solution strengthening, for precipitate control. That is, according to the present invention, when the addition amount of the alloy elements constituting the MC carbide is the same, it is possible to obtain a higher strength steel, and an alloy necessary for obtaining a strength equivalent to the conventional steel. The amount added can be substantially reduced.

Details of the present invention will be described below.
First, the reasons for limiting the P addition amount and limiting the atomic ratio of carbide constituent metal elements in the present invention will be described.

The inventors diligently studied the influence of the solid solution strengthening element on the initial precipitation size when the metal element M (Ti, Mo) constituting the MC was made constant. Specifically, the amount of Si, Mn, Al, and P, which is generally added to steel as a solid solution strengthening element in general, is changed in a systematic manner as shown in Table 1. The initial precipitation size was investigated. At this time, the hot-rolling conditions were constant, and the hardness of the air-cooled sample and the size of the MC precipitate were evaluated by a transmission electron microscope after holding at 650 ° C. × 10 min. Showed. Here, the size of the precipitate corresponds to the maximum in-plane length of the plate-like MC precipitate. In general, when depositing in a Baker-Nutting orientation relationship, the projected length at [001] _bcc incidence is √2 It can be regarded as double.

  In addition, the atomic ratio of the metal elements Ti and Mo constituting the MC shown in the table: Ti / Mo is an electron dispersive x-ray spectroscopy (EDS) spectrum obtained by focusing the electron beam to less than 1 nm during TEM observation. Obtained by quantitative analysis.

  Si and Al, which raise the transformation point of steel, promoted MC precipitation, and the size of MC increased under the same winding conditions. On the other hand, with the addition of Mn, which has the effect of lowering the transformation point of steel, the MC precipitation size can be kept small, but MC precipitation is insufficient and sufficient hardness is not obtained. On the other hand, P is an element having a small effect on the transformation point of steel and a very large solid solution strengthening ability.

  However, if added excessively, FeMP precipitates are formed, causing a decrease in the solid solution amount of M to be substantially precipitated as MC, and embrittlement due to P grain boundary segregation becomes a problem. It was found that the size of the plate-like MC precipitates in the ferrite phase can be controlled to be less than 8 nm by limiting the amount to be added in the range of 0.050 to 0.100 mass%. The reason why the size of the initial plate MC becomes fine by the appropriate addition of P is not clear, but the elastic constant of the matrix phase changes due to the solid solution strengthening of P, and the strain environment at the matching interface between MC and matrix phase changes. It is conceivable that metal element diffusion for MC formation is suppressed by the presence of solid solution P.

  If the MC size is less than 8 nm, the tensile strength of 820 MPa or higher can be obtained in this range, and the plate-like MC is up to about 8 nm from the viewpoint of lattice matching with the parent phase. This is because it is possible to grow while maintaining consistency.

  On the other hand, as a result of investigations focusing on the initial precipitation composition of MC, the inventors have determined that the composition of the carbide is stable in the manufacturing process and can suppress long-term growth by precisely controlling the composition of the carbide. I found that there was a condition. In general, the phase interface precipitation phenomenon associated with the γ → α transformation is a non-equilibrium process occurring in a very short time. Supersaturated carbon discharged to the transformation interface forms carbides at the constantly moving interface, but the individual composition does not reach an equilibrium composition.

  For example, even though the original most stable composition is TiC, it can be (Ti, M) C in the very short precipitation nucleus growth (M is a metal element having a carbide forming ability other than Ti). In the winding treatment step for completing the transformation, if a sufficient temperature or long time is maintained, the non-equilibrium initial composition is considered to gradually approach the equilibrium composition. This asymptotic approach to the equilibrium composition is accompanied by the Ostwald growth of precipitates. Therefore, if the asymptotic speed to the equilibrium composition can be suppressed, it is possible to suppress a decrease in strength due to coarsening.

  For this reason, in the fine carbide-dispersed steel utilizing phase interface precipitation, it is not desirable to add Ti, Nb or the like having a particularly high carbide forming ability from the viewpoint of obtaining an initial non-equilibrium composition. Specifically, in addition to the metal element group A such as Ti, Nb, or V, the metal element group B such as Cr, Mo, or W that does not have high carbide forming ability alone is utilized to minimize the initial composition of phase interface precipitation. It is considered important to have an equilibrium composition.

  Actually, a steel containing Ti as an A element and Mo as a B element was investigated. Steel having the composition shown in Table 2 was melted, and after slab heating at 1250 ° C. × 60 min., A hot rolled sheet having a finishing temperature of 950 ° C. and a winding equivalent treatment of 650 ° C. × 30 min. Was prepared. A part of these hot-rolled sheets was held at 650 ° C. for 10 hours, and the MC composition and size immediately after hot-rolling and after post-heat treatment were evaluated by an electron microscope. As a result, as shown in Table 2, when the atomic composition of Ti / Mo is 1.20 or less as the initial composition of MC precipitates after hot rolling, MC Ostwald growth is suppressed, and the strength after post-heat treatment Was found to be maintained.

  As the production conditions for the steel of the present invention, it is only necessary to satisfy the conditions that the finishing temperature does not become a two-phase region and the conditions that do not cause extremely high or low temperature winding. However, in order to ensure the amount of precipitation of MC, it is desirable that the slab heating temperature is 1200 to 1250 ° C., the finish rolling temperature is 900 to 950 ° C., and the winding temperature is 600 to 650 ° C.

Next, the reason regarding limitation of the main suitable components in this invention is demonstrated. In the following description, the content% of component elements means mass%.
"Limited to main ingredients"
C: 0.040-0.060%
C is an essential element for realizing precipitation strengthening by MC type fine carbide. The amount added depends on the amount of metal elements for MC carbide formation, but if it is less than 0.040%, MC precipitation cannot be fully utilized, so the lower limit was made 0.040%. In the present invention, the upper limit of addition is set to 0.060% in order to avoid excessive metal element addition and to suppress formation of hard transformation phases such as bainite and pearlite, but preferably 0.040 to 0.050%.

N: 0.0020-0.0045%
N forms MN-type precipitates such as TiN in the high temperature austenite temperature range. For this reason, when it exists in excess, the amount of effective M falls and MC type precipitation amount cannot be ensured. On the other hand, it is industrially expensive to suppress N to less than 0.0020%. In the present invention, it may be in a range of 0.0020 to 0.0045% that can be reached at a level that is performed in a normal steelmaking process.

P: 0.050 ~ 0.100%
P is an element with high solid solution strengthening ability, and is generally used to adjust the strength level of steel. However, if excessively added, FeMP precipitates are formed, MC precipitation is substantially reduced, and it causes embrittlement cracking due to segregation at grain boundaries. In the present invention, it is important as an essential additive element for obtaining the effect of reducing the initial precipitation size of the plate-like MC, which is the main strengthening factor, but if it is less than 0.050%, the effect cannot be obtained. On the other hand, if the addition exceeds 0.100%, MC precipitation rapidly decreases due to the formation of FeMP and the material softens, so the upper limit was made 0.100%.

Ti: 0.05-0.10%, Mo: 0.09-0.40%
Ti and Mo are elements essential for forming a stable and fine MC in the present invention, and are always added simultaneously. Since the balance of these addition amounts greatly affects the initial composition of MC, it is set so that the atomic ratio of Ti and Mo in the MC composition is 0.5 or more and 1.2 or less. In the range of Ti depletion and Mo excess where the atomic ratio is less than this, MC precipitation is substantially difficult to occur. When the Ti excess component has an atomic ratio exceeding 1.2, the initial composition of MC is close to the Ti-rich thermal equilibrium composition. Therefore, Ostwald growth is promoted in the latter stage of CT treatment, and MC cannot be maintained finely. For this reason, although each addition amount is dependent also on the addition amount of carbon, it controls in the range of Ti: 0.05-0.10%, Mo: 0.09-0.40%.

Mn: 0.50 to 1.30%
Mn is an element with high solid solution strengthening ability, and is used to adjust the strength level of steel and lowers the transformation temperature of steel. In the steel of the present invention, there is an effect of adjusting the transformation time from the austenite phase to the ferrite phase and the MC precipitation time to the coiling temperature in the hot rolling process, and 0.50% to realize coiling around 600-650 ° C. The above addition is necessary. However, if the addition exceeds 1.30%, the transformation point decreases, and MC precipitation during winding may not occur sufficiently, so the upper limit was made 1.30%.

Si + Al: 0.50% or less
Si is an element with high solid-solution strengthening ability, and is used to adjust the strength level of steel and raises the transformation temperature of steel. Al is an element commonly used in the deoxidation process. Like Si and Mn, Al is an element with a high solid solution strengthening ability, and is also used to adjust the strength level of steel. Moreover, like Si, it has the effect of raising the transformation temperature of steel. In the steel of the present invention, it is necessary to adjust the precipitation time of MC from the austenite phase to the ferrite phase to the coiling temperature in the hot rolling process, but as already mentioned, Si and Al are effective in promoting MC precipitation. In addition, since it becomes difficult to secure the two-phase region temperature after rolling, the upper limit is 0.50% in total for the Si and Al contents.

Examples of the present invention will be described below.
Steel having the composition shown in Table 3 was melted in a 50 kg vacuum melting furnace. These slabs were reheated at 1250 ° C. × 30 min., Hot-rolled to a sheet thickness of 3.0 mm at a finishing temperature of 950 ° C., and subjected to a treatment equivalent to winding at 650 ° C. × 60 min. JIS No. 5 test specimens were collected from the L direction of these hot-rolled sheets, subjected to a tensile test, and measured for tensile strength (TS) and elongation (EL). A thin film sample for a transmission electron microscope was prepared from the same sample, and the outer shape and composition of fine MC precipitates in the sample were investigated.

Regarding the outer shape of the precipitate by electron microscope, observation was performed at a magnification of 200,000 times or more under the diffraction conditions near the [001] _bcc incident direction, and the maximum longitudinal size of the plate-like precipitate observed at high density was observed. The average value was obtained. For these compositions, energy dispersive X-ray spectroscopic analysis (EDS) with an electron probe of 1 nm was performed, and the average value of the atomic ratio of Ti and Mo was determined by quantitative analysis.

  Table 3 shows the material, the outer shape of the precipitate, and the composition ratio (Ti / Mo). Steels 1 and 2 have a low strength compared to the invention steels, although exhibiting high elongation, because MC precipitation is not promoted because the Ti addition amount is insufficient. In Steel 3, since the addition amount of P is insufficient, the Ti / Mo ratio is within the scope of the invention, but the size of MC cannot be suppressed.

  On the other hand, steel 8 has a P exceeding the range of the invention, and steel 9 has a balance of Ti excess, so the size and composition of MC are out of the range of the invention, and sufficient strength is obtained. It is not done. On the other hand, for steels 4 to 7, in which the addition amounts of P, Ti and Mo are within the scope of the invention, the size and composition ratio of MC are appropriately controlled, and the strength and elongation balance are highly stable. ing.

  In particular, from the viewpoint of weight reduction and production cost reduction, while it is desired to stably produce a high strength and high formability steel sheet, the fine carbide dispersed steel provided by the present invention has strength and formability especially in the automotive field and the like. It is promising as a material for required members.

Claims (2)

In mass%, C: 0.040 ~ 0.060%, N: 0.0020 ~ 0.0045%, Mn: 0.50 ~ 1.30 %, Si + Al: 0.50% or less, P contains 0.050 ~ 0.100%, and Ti and Mo as MC forming elements , Ti: 0.05 to 0.10%, Mo: 0.09 to 0.40%, with the balance being Fe and inevitable impurities, composition ratio of Ti and Mo constituting MC precipitates (atomic ratio: Ti / Mo) Is a fine carbide-dispersed steel, which is a hot-rolled steel sheet having a ferrite single-phase structure in which 0.5 ≦ Ti / Mo ≦ 1.2 and the size of MC precipitates in the ferrite phase is controlled to be less than 8 nm. In mass%, C: 0.040 ~ 0.060%, N: 0.0020 ~ 0.0045%, Mn: 0.50 ~ 1.30 %, Si + Al: 0.5% or less, P is contained in 0.060 ~ 0.080%, and Ti and Mo as MC forming elements , Ti: 0.05 to 0.10%, Mo: 0.09 to 0.40%, with the balance being Fe and inevitable impurities, composition ratio of Ti and Mo constituting MC precipitates (atomic ratio: Ti / Mo) Is a hot-rolled steel sheet having a ferrite single-phase structure in which the size of MC precipitates in the ferrite phase is controlled to be less than 8 nm , and is a fine carbide-dispersed steel excellent in strength and ductility balance.