JP2005336548A - Hot rolled steel and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide hot rolled steel having excellent strength, excellent toughness and an excellent balance of strength-ductility and suitable as the stock for a high strength structural member for automobiles and various industrial machineries. <P>SOLUTION: The hot rolled steel has a chemical composition comprising 0.05 to 0.20% C, 0.01 to 2% Ti and 0.001 to 0.5% B, and satisfying the inequality of 2ä(Ti/48)-(N/14)}>(B/11) with each atomic symbol in the inequality as the content in the steel by mass% of the element, and comprises TiB grains with a prismatic structure and TiC grains with a cubic structure. Alternatively, the hot rolled steel has a chemical composition comprising 0.01 to 2.0% Si, 0.5 to 2.0% Mn, 0.002 to 0.05% sol.Al, 0 to 0.1% Nb and 0 to 0.3% V in addition to the above prescription, and the balance Fe with impurities; wherein, the content of N in the impurities is ≤0.01%. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a hot-rolled steel material and a manufacturing method thereof, and more particularly to a hot-rolled steel material suitable as a material for a high-strength member used for automobiles and various industrial machines and a manufacturing method thereof.

  As a technique for increasing the strength of hot-rolled steel, precipitation strengthening by fine precipitates and strengthening by refining crystal grains are well known. In the hot rolling process, Ti-based carbides and carbonitrides precipitate finely and contribute to the suppression of coarsening of crystal grains, in other words, contribute to the refinement of crystal grains. Various hot-rolled steel sheets have been developed.

  However, when TiC is contained in a large amount, there is a problem that toughness and workability deteriorate. For this reason, Patent Document 1 and Patent Document 2 disclose hot-rolled steel sheets with improved toughness and workability.

  Patent Document 1 proposes a technique for realizing high strength by fine precipitation of TiC and improving toughness by making fine ferrite grains. Specifically, the amount of TiC deposited is adjusted by controlling the Ti and C content, and further, the high-speed cooling after hot rolling suppresses the coarsening of TiC, and the addition of a small amount of B and after hot rolling This is a technique for obtaining ferrite having a tensile strength of 950 MPa or more and a crystal grain size of 10 μm or less by high-speed cooling.

  In the case of B-added steel, it is necessary to combine excess N with Ti in order to suppress the precipitation of BN. However, since TiN crystallizes from the liquid phase or precipitates at high temperature, it may coarsen and adversely affect toughness and ductility. Therefore, in order to ensure stable toughness and ductility in the B-added steel, it is essential not to coarsen TiN. However, in Patent Document 1 described above, suppressing the coarsening of TiN is not considered at all.

  In Patent Document 2, a chemical composition containing a specific amount of Ti, an average particle diameter of ferrite as the main phase and the second phase, and a ratio between the hardness of the second phase and the hardness of the ferrite phase are defined. Techniques for improving the workability of steel sheets have been proposed.

  However, in the case of the above-mentioned Patent Document 2, Ti exists as TiC, refines the initial austenite grains in the hot rolling heating stage, and further induces dynamic recrystallization in the subsequent hot rolling process. It is included only for the purpose of obtaining a fine grain structure and is not intended for precipitation strengthening action. For this reason, the TS (tensile strength) of the hot-rolled steel sheet obtained by the technique proposed in Patent Document 2 described above may be only about 740 MPa at most as shown in the examples, This is not a technique that can stably secure a high strength of 750 MPa or more.

JP-A-5-271865

JP 2000-192191 A

  The present invention has been made in view of the above-mentioned present situation, and its purpose is the No. 5 plate tension described in JIS Z 2201 (1998), which is suitable as a material for high-strength members used in automobiles and various industrial machines. When the test piece is used, the tensile strength (hereinafter also referred to as “TS”) is 750 MPa or more, and the product of tensile strength (MPa) and elongation (hereinafter also referred to as “EL”) (%) is “TS × The value of “EL” is 15500 MPa ·% or more, and the fracture surface transition temperature when using a sub-size V-notch test piece with a width of 5 mm described in JIS Z 2202 (1998), that is, the ductile fracture surface ratio is 50%. It is to provide a hot-rolled steel material having a temperature (hereinafter also referred to as “vTrs”) of −15 ° C. or less and a manufacturing method thereof.

  The gist of the present invention resides in a hot-rolled steel material shown in the following (1) to (2) and a method for producing a hot-rolled steel material shown in (3).

  (1) By mass%, C: 0.05 to 0.20%, Ti: 0.01 to 2% and B: 0.001 to 0.5%, and satisfying the following formula (1) A hot-rolled steel material having a composition, wherein the steel contains TiB particles having an orthorhombic structure and TiC particles having a cubic structure.

2 {(Ti / 48)-(N / 14)}> (B / 11) (1),
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element.

  (2) By mass%, C: 0.05-0.20%, Ti: 0.01-2%, B: 0.001-0.5%, Si: 0.01-2.0%, Mn : 0.5-2.0%, sol. Al: 0.002 to 0.05%, Nb: 0 to 0.1% and V: 0 to 0.3%, the balance consists of Fe and impurities, N in the impurities is 0.01% or less, A hot rolled steel material having a chemical composition satisfying the following formula (1), wherein the steel contains TiB particles having an orthorhombic structure and TiC particles having a cubic structure.

2 {(Ti / 48)-(N / 14)}> (B / 11) (1),
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element.

(3) By mass%, C: 0.05-0.20%, Ti: 0.01-2%, B: 0.001-0.5%, Si: 0.01-2.0%, Mn : 0.5-2.0%, sol. Al: 0.002 to 0.05%, Nb: 0 to 0.1% and V: 0 to 0.3%, the balance consists of Fe and impurities, N in the impurities is 0.01% or less, And it is a method of manufacturing a hot-rolled steel material from a steel ingot or steel slab having a chemical composition satisfying the following formula (1), wherein the steel ingot or steel slab having a temperature of 1200 ° C. or higher is hot. The processing step includes a step of cooling to a temperature T ° C. in a temperature range of 1100 to 900 ° C. at a cooling rate of 20 ° C./s or more and performing a processing with a reduction rate of 10% or more at a strain rate of 100 s ⁻¹ or less. A method for producing a hot-rolled steel material characterized by including orthorhombic TiB particles and cubic TiC particles in steel.

2 {(Ti / 48)-(N / 14)}> (B / 11) (1),
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element.

  The “steel ingot” as used in the present invention refers to one containing “slab” as defined in JIS G 0203 (1984).

Moreover, both said temperature and cooling rate point out the thing in the surface of a steel ingot or a steel piece. Further, the “rolling rate” refers to a “plate thickness reduction rate”, which is a value represented by the following equation (2), where A _{0 is} the plate thickness before processing and A ₁ is the plate thickness after processing.

Reduction ratio (%) = {(A ₀ −A ₁ ) / A ₀ } × 100 (2).

  Hereinafter, the invention related to the hot-rolled steel material of the above (1) to (2) and the invention related to the manufacturing method of the hot-rolled steel material of (3) are referred to as the present invention (1) to the present invention (3), respectively. The present invention (1) to the present invention (3) may be collectively referred to as the present invention.

  The hot-rolled steel material of the present invention can achieve (1) precipitation strengthening by TiC and (2) fine graining and TiN fine dispersion by containing TiB particles having an orthorhombic structure. When using the No. 5 plate-like tensile test piece described in Z 2201 (1998), TS is 750 MPa or more, “TS × EL” value is 15500 MPa ·% or more, and width described in JIS Z 2202 (1998) is 5 mm. Good strength, toughness and “strength-ductility balance” of vTrs of −15 ° C. or lower are obtained when sub-size V-notch test pieces are used. For this reason, it can utilize as a raw material of the high strength structural member for motor vehicles and various industrial machines. This hot-rolled steel material can be manufactured relatively easily by the method of the present invention.

  In order to solve the above-described problems, the present inventors have made various studies on the form of crystallized products and precipitates in Ti-added steel, as well as the stability thereof. As a result, the following findings (a) to (j) were obtained.

(A) In the case of Ti-added steel to which B is not added, TiN that crystallizes from the liquid phase or precipitates at a high temperature becomes coarse, which may adversely affect toughness and ductility. However, in steel to which Ti and B are added in combination, Ti—B-based TiB ₂ and TiN may be combined and crystallized or precipitated at a high temperature. When TiB ₂ crystallizes or precipitates, TiN coarsening is suppressed.

(B) However, TiB ₂ that crystallizes or precipitates at a high temperature is coarse particles having a hexagonal crystal structure, and causes deterioration of toughness and ductility. Therefore, in order to ensure good toughness and ductility, it is necessary to eliminate coarse TiB ₂ in the steel after TiN coarsening is suppressed.

  (C) Among the Ti-B-based particles, TiB having an orthorhombic structure is a spherical particle having an average particle diameter of 2 μm or less, and is finely and uniformly dispersed in the austenite region, so that recrystallized austenite grains are refined. It is effective for.

(D) Ti-B-based particles that are stably present in a steel to which Ti and B are added in combination are TiB ₂ having a hexagonal crystal structure. In steels manufactured by conventional general heat treatment or hot rolling, TiB cannot exist stably. However, if hot working is performed on a steel ingot or steel slab containing TiB ₂ under specific conditions, a reaction of “TiB ₂ → TiB + B” occurs to change TiB ₂ particles into TiB particles, and TiB particles are contained in the steel. Since the TiB ₂ particles disappear while being uniformly dispersed, it is possible to prevent a decrease in toughness and ductility due to the presence of coarse TiB ₂ particles.

(E) B produced by the reaction of “TiB ₂ → TiB + B” remains in solid solution in the steel, and the reverse reaction of the reaction takes place to form TiB ₂ particles again, or new Ti—B particles Since Ti is not formed, Ti that forms TiC in combination with C is secured.

(F) Since TiB particles have the effect of inducing dynamic recrystallization in the hot rolling process and refining austenite grains, if a reaction that changes the shape of TiB ₂ particles to TiB particles occurs, toughness and Ductility is significantly improved.

  (G) The strength of steel is greatly increased by finely dispersing TiC having a cubic structure in the hot rolling process and the subsequent cooling process.

  (H) When BN particles are contained in steel, toughness and ductility are lowered.

(I) In order to positively crystallize or precipitate TiB ₂ having an action of suppressing the coarsening of TiN on a steel ingot or steel slab during casting or heating at high temperature, Ti, N in steel It is necessary to satisfy specific conditions for the contents of B and B.

(J) If the content of Ti, N and B in the steel satisfies the specific condition of (i) above, the shape change to TiB by the reaction of “TiB ₂ → TiB + B” in the hot working of (d) above Not only disappears and TiB ₂ particles disappear, but also BN does not occur, so that good toughness and ductility can be secured, and furthermore, TiC having an appropriate amount of cubic structure is precipitated, and high strength can be secured. Incidentally, BN although temperatures to produce the partially overlap the temperature generated by the TiN and TiB _2, because BN has a high solid solubility, in the steel Ti, the content of N and B is the identification of (i) If the condition is satisfied, BN is not generated.

  The present invention has been completed based on the above findings.

  Hereinafter, each requirement of the present invention will be described in detail. In the following description, “%” display of the content of each element means “mass%”.

(A) Particles in steel In order to ensure good toughness and ductility, the hot rolled steel material contains, in the steel, TiB ₂ particles having a hexagonal structure that has a TiN coarsening suppressing action in steel ingots and steel pieces, It is necessary to include TiB particles having an orthorhombic structure whose shape has been changed by the reaction of “TiB ₂ → TiB + B”. As described in the next section “(B) Chemical composition of steel”, when the contents of Ti, N and B in the steel satisfy the above-mentioned formula (1), the reaction of “TiB ₂ → TiB + B”. When the shape change to TiB occurs, TiB ₂ particles disappear, so that the toughness and ductility are not lowered due to the presence of coarse TiB ₂ particles. Further, since BN is not generated, the toughness and ductility are not lowered due to the presence of BN particles.

  In addition to the orthorhombic TiB particles, if the hot-rolled steel material contains cubic TiC particles in the steel, a desired high strength can be obtained.

  Accordingly, in the present invention (1) and the present invention (2), it is necessary that the hot-rolled steel material contains TiB particles having an orthorhombic structure and TiC particles having a cubic structure in the steel.

In addition, it is preferable that the quantity which said TiB particle | grains and TiC particle | grains are contained in steel is the mass%, and is 0.03-2.2% and 0.08-0.65%, respectively.
(B) Chemical composition of steel In order for the hot-rolled steel to contain the orthorhombic TiB particles and the cubic TiC particles in the steel, the steel contains C, Ti and B in its chemical composition. It is necessary to be out.

C: 0.05-0.20%
C is an element effective for increasing the strength of the hot-rolled steel material by two actions of solid solution strengthening and precipitation strengthening by forming TiC having a cubic structure by combining with Ti. C is also an important element governing the “austenite / ferrite” transformation. However, if the content is less than 0.05%, a desired large strength of 750 MPa or more cannot be secured with TS. On the other hand, if it exceeds 0.20%, TiC coarsens, so that toughness and ductility are reduced. Therefore, the content of C is set to 0.05 to 0.20%. The C content is preferably 0.072 to 0.15%.

Ti: 0.01-2%
Ti has the effect of increasing the strength, toughness and ductility of hot-rolled steel by forming Ti-B-based particles, that is, TiB ₂ and TiB, which are borides of Ti, and TiC. That is, by forming TiB ₂ having a hexagonal crystal structure that has the effect of suppressing the coarsening of TiN, it is possible to prevent a decrease in toughness and ductility due to the presence of coarse TiN particles. Further, after that, by causing a reaction of “TiB ₂ → TiB + B”, TiB ₂ is disappeared and has an orthorhombic structure in which spherical particles having an average particle diameter of 2 μm or less are dispersed finely and uniformly in the austenite region. By forming TiB, toughness and ductility can be increased. Further, the strength can be increased by the cubic structure TiC having a precipitation strengthening action. However, when the Ti content is less than 0.01%, the effect of the Ti-B-based particles and TiC particles cannot be ensured. On the other hand, if the Ti content exceeds 2%, the toughness and ductility are reduced. Therefore, the Ti content is set to 0.01 to 2%. The Ti content is preferably 0.01 to 1.5%.

B: 0.001 to 0.5%
B is one of the most important elements in the present invention. That is, B combines with Ti during the casting process or when heated at a high temperature to form TiB ₂ having a hexagonal crystal structure and suppresses the coarsening of TiN, so that the toughness and ductility due to the presence of coarse TiN are reduced. Reduction is prevented. Furthermore, if hot working is performed on the steel ingot or steel slab containing TiB ₂ under specific conditions, a reaction of “TiB ₂ → TiB + B” occurs, and the TiB ₂ particles change into TiB particles having an orthorhombic structure, Since the TiB particles are uniformly dispersed in the steel and the TiB ₂ particles disappear, deterioration of toughness and ductility due to the presence of coarse TiB ₂ particles is also prevented. And since a recrystallized austenite grain refines | miniaturizes by TiB particle | grains, toughness and ductility improve. However, if the B content is less than 0.001%, the effect of the TiB ₂ particles and TiB particles cannot be obtained. On the other hand, if the B content exceeds 0.5%, the toughness and ductility are reduced. Therefore, the content of B is set to 0.001 to 0.5%. The B content is preferably 0.003 to 0.40%.

Formula (1) above for the contents of Ti, B and N:
If the content of Ti, N and B in the steel satisfies the above formula (1), the hexagonal crystal structure has the effect of suppressing the coarsening of TiN in the steel ingot and steel slab during the casting process or during heating at a high temperature. TiB ₂ can be crystallized or precipitated, and the above steel ingot or steel slab containing TiB ₂ is subjected to hot working under specific conditions to cause a reaction of “TiB ₂ → TiB + B”. , TiB ₂ particles can be changed to orthorhombic TiB particles, the TiB particles can be uniformly dispersed in the steel, and the TiB ₂ particles can be lost, and BN that reduces toughness and ductility can be produced. In addition, TiC having an appropriate amount of cubic structure can be generated.

  Therefore, the hot-rolled steel material according to the present invention (1) is in mass%, C: 0.05-0.20%, Ti: 0.01-2%, and B: 0.001-0.5%. And having a chemical composition satisfying the formula (1). Note that N in the formula (1) is an element contained in steel as an impurity.

  The chemical composition of the hot-rolled steel material according to the present invention (2) includes the following amounts of elements from Si to V in addition to the chemical composition rule of the hot-rolled steel material according to the invention of the above (1), and the balance Consists of Fe and impurities, and N in the impurities satisfies the following amount.

  Hereinafter, the reason why the contents of the respective elements from Si to N are defined in the present invention (2) will be described in detail.

Si: 0.01 to 2.0%
Si has the effect of strengthening ferrite and increasing the ductility of ferrite. However, if the Si content is less than 0.01%, the effect of addition is poor. On the other hand, if it exceeds 2.0%, the ductility decreases and the toughness also decreases. Therefore, the Si content is set to 0.01 to 2.0%. A desirable content of Si is 0.01 to 1.0%.

Mn: 0.5 to 2.0%
Mn has an effect of strengthening ferrite by solid solution strengthening. Mn is an important element governing the “austenite / ferrite” transformation. However, if the content is less than 0.5%, a sufficient effect cannot be obtained. On the other hand, if the Mn content exceeds 2.0%, the toughness and ductility are reduced. Therefore, the Mn content is set to 0.5 to 2.0%. A desirable Mn content is 0.8 to 1.5%.

sol. Al: 0.002 to 0.05%
Al has a deoxidizing action and has an effect of suppressing the formation of Ti oxide. However, the content of Al is sol. If it is less than 0.002% with Al, sufficient effects cannot be obtained. On the other hand, the content of Al is sol. Even if it exceeds 0.05% with Al, the above effect is saturated and the cost is increased. Therefore, the content of Al is sol. The content of Al was 0.002 to 0.05%. Preferred sol. The amount of Al is 0.01 to 0.04%. Note that sol. Al refers to so-called “acid-soluble Al”.

Nb: 0 to 0.1%
Nb may not be added. If added, it combines with C to form fine NbC, thereby improving the strength of the steel and suppressing the coarsening of TiB. In order to reliably obtain this effect, the Nb content is preferably 0.005% or more. On the other hand, even if the Nb content exceeds 0.1%, the above effects are saturated and the cost is increased. Therefore, the Nb content is set to 0 to 0.1%.

V: 0 to 0.3%
V may not be added. If added, it combines with C to form fine VC, improving the strength of the steel, and suppressing the coarsening of TiB. In order to reliably obtain this effect, the V content is preferably 0.1% or more. On the other hand, even if the content of V exceeds 0.3%, the above effect is saturated and the cost is increased. Therefore, the content of V is set to 0 to 0.3%.

N: 0.01% or less N is an element contained in steel as an impurity, and may combine with Ti to form coarse TiN, and may combine with B to form BN. Since both the above coarse TiN and BN lower the toughness and ductility, it is desirable that the content of N as an impurity is small. In the present invention (2), the upper limit content is 0.01%. It was. A more desirable upper limit of the N content is 0.005%.

The hot-rolled steel material according to the present invention (1) includes, in mass%, C: 0.05 to 0.20%, Ti: 0.01 to 2%, and B: 0.001 to 0.5%. And it can manufacture by giving the following hot working to the steel ingot or steel piece which has a chemical composition which satisfy | fills said (1) Formula, for example. That is, in the hot working process of the above steel ingot or steel slab having a temperature of 1200 ° C. or higher, it is cooled to a temperature T ° C. in the temperature range of 1100 to 900 ° C. at a cooling rate of 20 ° C./s or more and 100 s. By including a step of processing at a reduction rate of 10% or more at a strain rate of ⁻¹ or less, it can be produced relatively easily.

  The hot-rolled steel material according to the present invention (2) can be manufactured relatively easily, for example, by the method of the present invention (3).

  Hereinafter, the present invention (3) which is an example of the method for producing a hot-rolled steel material according to the present invention (2) will be described.

(C) Hot working of steel ingot or billet (C-1) Temperature of steel ingot or billet in hot working
The material of the hot-rolled steel material may be a steel ingot or a steel slab. As long as the steel ingot or steel slab has a temperature of 1200 ° C. or higher, it may be cast or hot worked, It may be reheated after the temperature becomes less than 1200 ° C. If the temperature of the steel ingot or slab is less than 1200 ° C., the reaction “TiB ₂ → TiB + B” does not occur during hot working, and therefore TiB ₂ having a hexagonal crystal structure still exists. Therefore, the toughness and ductility are reduced due to the presence of coarse TiB ₂ particles. Further, since the reaction does not occur and TiB does not exist, the effect of TiB having the orthorhombic structure described above cannot be obtained.

  In addition, as above-mentioned that the said temperature points out the temperature in the surface of a steel ingot or a steel piece. And in order to reheat a steel ingot or a steel piece in order to ensure the said 1200 degreeC or more temperature, it is preferable that the upper limit of heating temperature shall be 1350 degreeC from a cost surface.

(C-2) Hot working temperature T (° C.) for inducing TiB and cooling rate to that temperature By causing a reaction of “TiB ₂ → TiB + B” by hot working, that is, from TiB ₂ to TiB In order to prevent the deterioration of toughness and ductility due to the presence of coarse TiB ₂ particles by causing TiB ₂ to disappear, and to obtain the effect of TiB already described, 1100 to 900 ° C. It is necessary to perform hot working at a temperature T (° C.) in the temperature range, and the steel ingot or steel slab having a temperature of 1200 ° C. or higher is 20 ° C./s or higher up to the temperature T (° C.). It is necessary to cool at a cooling rate. If the cooling rate in the said cooling is 20 degrees C / s or more, since it is good also as a value of the upper limit which can be implement | achieved with cooling equipment, there is no upper limit in particular. As described above, the above temperature and cooling rate refer to those on the surface of a steel ingot or steel slab.

By causing the reaction of "TiB ₂ → TiB + B" (C-3) TiB at a strain rate and rolling reduction thermal processing for inducing, i.e., cause morphological change from TiB ₂ to TiB, the TiB ₂ In order to obtain the effect of TiB already described by eliminating, the temperature was suppressed to 100 s ⁻¹ or less at the temperature T (° C.) in the temperature range of 1100 to 900 ° C. described in the previous section (C-2). It is necessary to apply a reduction with a reduction rate of 10% or more at the strain rate. The lower the strain rate, the better. However, from the viewpoint of productivity, the lower limit is desirably 5 s ⁻¹ . On the other hand, the rolling reduction may be 10% or more, and may be the upper rolling reduction of the hot working equipment.

As already described, the “rolling rate” refers to the “plate thickness reduction rate”, and is expressed by the above equation (2), where A _{0 is} the thickness before processing and A ₁ is the thickness after processing. Value. However, the reduction ratio of the processing performed at the temperature T (° C.) at a strain rate suppressed to 100 s ⁻¹ or less is 10% or more. For example, the reduction ratio in one processing such as one-pass hot rolling is 10% or more. It does not indicate the total reduction ratio in the case of multipass rolling.

If the steps described in the above items (C-1) to (C-3) are included in the hot processing step, a reaction of “TiB ₂ → TiB + B” occurs regardless of the steps before and after that, Since TiB ₂ disappears, the deterioration of toughness and ductility due to coarse TiB ₂ can be prevented, and the effect of TiB already described can be obtained.

Therefore, in the said invention (3), the temperature of the temperature range of 1100-900 degreeC with the cooling rate of 20 degrees C / s or more in the hot processing process of the steel ingot or steel piece which has a temperature of 1200 degreeC or more. It was decided to include a step of cooling to T ° C. and performing processing with a reduction rate of 10% or more at a strain rate of 100 s ⁻¹ or less.

  Hereinafter, the present invention will be described in more detail with reference to examples.

The steel slab having the chemical composition shown in Table 1 was subjected to low-speed rolling of one pass as shown in detail in Table 2 using an experimental rolling facility, that is, the slab was heated to 1000 ° C. and then at that temperature for 10 s. 1-pass rolling at a strain rate of ^{-1 and} a rolling reduction of 15%, or after heating the slab to 1250 ° C, it is cooled to 850-1000 ° C at a cooling rate of 5-50 ° C / s, and 10-200 s 1-pass rolling was performed at a strain rate of ^{−1 and} a rolling reduction of 15 to 30%. For any of the above conditions, after the one-pass low-speed rolling, 5-pass hot rolling is performed by a normal method, and cooling and winding are performed to obtain a hot-rolled steel sheet having a thickness of 5.2 mm. It was.

  Various test pieces were collected from the steel sheet having a thickness of 5.2 mm obtained in this way, and the identification and measurement of the particles (precipitates and inclusions) in the steel, the tensile properties and the Charpy impact properties were investigated. .

  The particles in the steel were identified by observing with a transmission electron microscope using an extracted replica sample obtained by polishing the central part of the thickness of the hot-rolled steel sheet, followed by nital corrosion and carbon deposition.

  Further, the amount obtained by electrolyzing the hot rolled steel sheet with “10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte” and quantitatively analyzing the obtained residue was defined as “total amount of particles in steel”. .

  Of the particles in the steel, the amount of TiB and the amount of TiC were determined as follows.

That is, first, an X-ray diffraction experiment of the extraction residue was performed to investigate the generation status of TiB, TiB ₂ , TiC, TiN, and BN. As a result, it was found that it can be classified into the following 4 groups.

Group (1): TiB, TiC and TiN are all present, but TiB ₂ and BN are both absent.

Group (2): TiB ₂ , TiC and TiN are all present, but TiB and BN are both absent.

Group (3): TiB, TiC, TiN and BN are all present, but TiB ₂ is not present.

Group (4): TiC and TiN are all present, but none of TiB, TiB ₂ and BN are present.

  Next, chemical analysis of the extraction residue was performed, and for each of the above groups, the amounts of Ti, C, N and B in the extraction residue (hereinafter referred to as “Ti”, “C”, “N” and ("B").

Finally, for each of the above groups, the following (simultaneous) equations were solved to determine the amount of TiB and the amount of TiC. Note a in (simultaneous) equations, b, c, d and e, respectively, it means the amount of TiB, the amount of TiB _2, the amount of TiC, the amount of amount and BN of TiN. Here, a (that is, the amount of TiB) in group (2) and group (4) is 0 (zero).
・ Group (1):
a × {11 / (48 + 11)} = “B”
c × {12 / (48 + 12)} = “C”,
・ Group (2):
c × {12 / (48 + 12)} = “C”,
・ Group (3):
a × {11 / (48 + 11)} + e × {11 / (14 + 11)} = “B”
c × {12 / (48 + 12)} = “C”,
d × {14 / (48 + 14)} + e × {14 / (14 + 11)} = “N”
a × {48 / (48 + 11)} + c × {48 / (48 + 12)} + d × {48 / (48 + 14)} = “Ti”
・ Group (4):
c × {12 / (48 + 12)} = “C”.

  Tensile properties were investigated using No. 5 plate-like tensile test pieces described in JIS Z 2201 (1998).

  Further, the Charpy impact characteristics were evaluated by the fracture surface transition temperature (vTrs) when a sub-size V-notch test piece having a width of 5 mm described in JIS Z 2202 (1998) was used.

Table 3 shows the results of the above various investigations. In Table 3, particles marked with “#”, that is, TiB ₂ (#) and TiN (#) are coarse particles.

  FIGS. 1 and 2 respectively show the relationship between “total amount of particles in steel” and “vTrs” and the relationship between “total amount of particles in steel” and “TS × EL”.

  As is apparent from Table 3, the hot rolled steel sheets of Test Nos. 1 to 11 having the chemical composition defined in the present invention and the particles in steel have a high TS exceeding 750 MPa. Moreover, despite the fact that the total amount of particles in the steel is 0.20% by mass or more, vTrs has an excellent toughness of −15 ° C. or less, and the value of “TS × EL” exceeds 15500 MPa ·%. The “strength-ductility balance” is also excellent.

  On the other hand, although the chemical composition satisfies the conditions defined by the present invention, when the test particles 12 to 15 have a particle number in the steel that deviates from the regulations defined by the present invention, the desired toughness (vTrs) and “strength— The “ductility balance” (“TS × EL”) is not obtained.

That is, in test number 12, the slab heating temperature is as low as 1000 ° C. For this reason, the temperature of the slab is naturally below 1200 ° C., so that even when hot-worked, the morphology change from hexagonal structure TiB ₂ to orthorhombic structure TiB due to the reaction of “TiB ₂ → TiB + B” does not occur. Coarse TiB ₂ particles remain in the hot-rolled steel sheet, and the toughness and ductility are lowered, and the desired toughness (vTrs) and “strength-ductility balance” (“TS × EL”) are not obtained. .

Test No. 13 has a low cooling rate of 5 ° C / s from 1250 ° C to 1000 ° C rolled in one pass, so the morphology change from TiB ₂ to TiB due to the reaction of "TiB ₂ → TiB + B" even after hot working And coarse TiB ₂ particles remain in the hot-rolled steel sheet, toughness and ductility are reduced, and the desired toughness (vTrs) and “strength-ductility balance” (“TS × EL”) are obtained. It is not done.

Test No. 14 has a low one-pass rolling temperature of 850 ° C. For this reason, there is no morphological change from TiB _{2 having} a hexagonal structure to TiB having an orthorhombic structure due to the reaction of “TiB ₂ → TiB + B”, and coarse TiB ₂ particles remain on the hot-rolled steel sheet. The toughness and ductility are reduced, and the desired toughness (vTrs) and “strength-ductility balance” (“TS × EL”) are not obtained.

In test number 15, the strain rate during one-pass rolling is as large as 200 s ⁻¹ . For this reason, the shape change from TiB ₂ to TiB due to the reaction of “TiB ₂ → TiB + B” does not occur, coarse TiB ₂ particles remain in the hot-rolled steel sheet, the toughness and ductility are lowered, and the desired Toughness (vTrs) and “strength-ductility balance” (“TS × EL”) are not obtained.

  In Test No. 16 and Test No. 17, the steel does not contain B and deviates from the chemical composition conditions defined in the present invention, so that the desired toughness (vTrs) and “strength-ductility balance” (“TS × EL”) are obtained. It is not done.

That is, in the case of the test number 16 and the test number 17, since the steel does not contain B, the effect of suppressing the coarsening of TiN by TiB ₂ cannot be obtained. For this reason, TiN coarsens and toughness and ductility decrease, vTrs is high at −8 ° C. and 15 ° C., respectively, and “TS × EL” is small at 14552 MPa ·% and 11089 MPa ·%, both of which reach the target. Not.

  In Test Nos. 18 to 21, since the steel deviates from the definition of the formula (1), the vTrs is −5 to 30 ° C., the toughness is low, and the TS is as low as 552 to 573 MPa. And since TS is low, "TS * EL" is also as low as 7449-14985MPa%.

  That is, in the case of test numbers 18 to 21, since the chemical composition of the steel does not satisfy the above formula (1), excessive B is combined with N to form BN, resulting in decreased toughness and ductility. To do. Further, since only coarse TiC exists and fine TiC does not exist, the strength is low, and all of TS, vTrs, and “TS × EL” have not reached the target.

  As shown in FIG. 1, the toughness tends to decrease (that is, vTrs increases) as the “total amount of particles in steel” increases. However, when the “total amount of particles in the steel” is at the same level, the vTrs of the test numbers 1 to 11 according to the present invention example is 40 ° C. or more lower than the vTrs of the test numbers 12 to 21 according to the comparative example. It is clear that the toughness is excellent.

  Further, as shown in FIG. 2, “strength-ductility balance” (“TS × EL”) tends to deteriorate with an increase in “total amount of particles in steel”, but test number 1 according to the present invention example. In the case of ˜11, deterioration is suppressed, and it is clear that “strength-ductility balance” is excellent.

  The hot-rolled steel material of the present invention has a TS of 750 MPa or more and a value of “TS × EL” of 15500 MPa ·% or more when using the No. 5 plate-like tensile test piece described in JIS Z 2201 (1998), JIS Z 2202 (1998) has a good strength, toughness and "strength-ductility balance" of v-15 when the sub-size V-notch test piece having a width of 5 mm described in (1998) is used. It can be used as a raw material for high strength structural members.

It is a figure which rearranges and shows the relationship between "the total amount of the particle | grains in steel" and "vTrs" (toughness) of the various hot-rolled steel plates in an Example. It is a figure which rearranges and shows the relationship between "the total amount of the particle | grains in steel" of various hot-rolled steel plates in an Example, and "TSxEL" ("strength-ductility balance").

Claims (3)

It has a chemical composition that includes C: 0.05 to 0.20%, Ti: 0.01 to 2%, and B: 0.001 to 0.5% and satisfies the following formula (1) by mass%: A hot-rolled steel material comprising a TiB particle having an orthorhombic structure and a TiC particle having a cubic structure in the steel.
2 {(Ti / 48)-(N / 14)}> (B / 11) (1)
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element. By mass%, C: 0.05 to 0.20%, Ti: 0.01 to 2%, B: 0.001 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.00. 5 to 2.0%, sol. Al: 0.002 to 0.05%, Nb: 0 to 0.1% and V: 0 to 0.3%, the balance consists of Fe and impurities, N in the impurities is 0.01% or less, A hot rolled steel material having a chemical composition satisfying the following formula (1), wherein the steel contains TiB particles having an orthorhombic structure and TiC particles having a cubic structure.
2 {(Ti / 48)-(N / 14)}> (B / 11) (1)
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element. By mass%, C: 0.05 to 0.20%, Ti: 0.01 to 2%, B: 0.001 to 0.5%, Si: 0.01 to 2.0%, Mn: 0.00. 5 to 2.0%, sol. Al: 0.002 to 0.05%, Nb: 0 to 0.1% and V: 0 to 0.3%, the balance consists of Fe and impurities, N in the impurities is 0.01% or less, And it is a method of manufacturing a hot-rolled steel material from a steel ingot or steel slab having a chemical composition satisfying the following formula (1), wherein the steel ingot or steel slab having a temperature of 1200 ° C. or higher is hot. The processing step includes a step of cooling to a temperature T ° C. in a temperature range of 1100 to 900 ° C. at a cooling rate of 20 ° C./s or more and performing a processing with a reduction rate of 10% or more at a strain rate of 100 s ⁻¹ or less. A method for producing a hot-rolled steel material characterized by including orthorhombic TiB particles and cubic TiC particles in steel.
2 {(Ti / 48)-(N / 14)}> (B / 11) (1)
Here, the element symbol in the formula (1) indicates the content in steel in mass% of the element.

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